Dual-Layered Quasi-Two-Dimensional Organic Conductors with Presumable Incoherent Electron Transport

MICROREVIEW

DOI:10.1002/ejic.201402041

CLUSTERISSUE

Dual-Layered Quasi-Two-Dimensional OrganicConductors with Presumable Incoherent ElectronTransport

Rimma Lyubovskaya,*[a] Elena Zhilyaeva,[a] Gena Shilov,[a]

Alain Audouard,[b] David Vignolles,[b] Enric Canadell,[c]

Sergei Pesotskii,[a] and Rustem Lyubovskii[a]

Keywords: Conducting materials / Electron transport / Fermi surfaces / Molecular electronics

We have prepared a series of BEDT-TTF or BETS-based radi-cal-cation salts (D)4MBr4(C6H6–nXn) with tetrahedral anions[MIIBr4]2– (D = BETS, BEDT-TTF; X = F, Cl, Br; n = 1, 2). Thesalts (D)4MBr4(C6H6–nXn) are characterized by the differentstructures of neighboring conducting layers, structural phasetransitions, and different types of conductivity, namely, met-allic and semiconducting, along and across the conductinglayers, respectively. The Fermi surface of the metallic layerof (D)4MBr4(C6H6–nXn) with alternating crystallographically

1. Introduction

The conventional classical concept of electron transportin metallic crystals is based on coherent electron motion ina conductivity band and is described by the semiclassicalBoltzmann transport theory. Within this concept, the be-havior of charge carriers coincides well with the propertiesof Fermi liquids.[1,2] However, new materials have recentlybeen developed that cannot be interpreted by using the sim-ple Boltzmann theory, and to explain the experimental re-sults a more complex transport mechanisms have to be in-voked.[3,4] These materials, which include superconductingcuprates,[5] colossal magnetoresistance materials,[6] quasi-one- and -two-dimensional organic metals,[7,8] and cobaltoxides,[9] evidence strong correlations between quasiparti-cles and a considerable decrease in effective dimensionalityto reveal non-Fermi liquid properties. In some of these ma-terials, for example, on-site Coulomb repulsion (U) may in-hibit or completely block intersite electron hopping (t) ortunneling to a neighboring level or layer. Depending on theU/t ratio, material can be a metal or an insulator. The corre-

[a] Institute of Issues of Chemical Physics, RAS,142432 Chernogolovka, MD, RussiaE-mail: [email protected]

[b] Laboratoire National des Champs Magnetiques Intenses (UPR3228 CNRS, INSA, UJF, UPS),143 avenue de Rangueil, 31400 Toulouse, France

[c] Institut de Ciencia de Materials de Barcelona, CSIC,Campus de la UAB, 08193 Bellaterra, Spain

Eur. J. Inorg. Chem. 2014, 3820–3836 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3820

different conducting layers (dual-layered salts) provides aroadmap for interpreting the transport properties of quasi-two-dimensional organic conductors. This review reports onthe structures of these compounds, the effect of MBr4 anionsand solvent on the occurrence of phase transitions, differentFermi surface topologies in connection with the differentstructures of neighboring conducting layers, and the possibil-ity of the incoherent transport of charge carriers in such sys-tems.

lation effects increase with a decrease in the number of al-lowed dimensions. In 3D systems, low-energy electronicstates behave as quasiparticles, whereas in 1D systems, evenweak interactions break the quasiparticles into collectiveexcitations.[10] Dimensionality is particularly important fora class of new low-dimensional materials. Quasi-two-dimen-sional organic layered conductors are characterized as co-herent or incoherent,[11,12] or weakly incoherent,[4] provid-ing specific signatures for inter- and intralayer transport.The main feature of the conductivity of such materials isthat the temperature dependence of resistance measuredalong and across the conducting layers behaves very dif-ferently.[13] This is in contradiction of the results expectedfor anisotropic Fermi liquids for which longitudinal andtransversal conductivity always show the same temperaturedependence, determined by the velocity of charge carrierswithin the layer (τ–1

�). A coherent regime in Q2D organicmetals is characterized by the fact that the rate of dissi-pation of τ–1

� within the layers is significantly less than thespeed of an electron jump between the layers: τ–1

� = t�/R,in which t� is the transition energy between the layers, thatis, τ–1

� �� τ–1�. In this case, all electrons in one layer jump

coherently to many layers in the perpendicular directionwithout losing momentum and phase. The resistance of thecrystal falls as the temperature is lowered, that is, dρ/dT�0.If τ–1

� ��τ–1�, that is, if an electron in the layer dissipates

inside many times, for example, because of impurities, be-fore it tunnels or jumps to the next layer, then this regimeis called incoherent.[11,14] In this case, cross resistance in-creases with decreasing temperatures, that is, dρ�/dT� 0.

http://onlinelibrary.wiley.com/doi/10.1002/ejic.v2014.24/issuetoc

www.eurjic.org MICROREVIEW

There are crystals in which transverse resistance (ρ�) be-haves as an insulator at high temperatures and as a metal atlow temperatures. Therefore a crossover occurs.[14–18] This

Rimma Lyubovskaya received her Doctor of Chemical Sciences degree in 1991 and became a Full Professor in 1996. Hercurrent position is head of the Laboratory of Synthesis of Organic Conducting and Magnetic Materials at the Institute ofProblems of Chemical Physics RAS (IPCP RAS) in Chernogolovka. Her current research interests are low-dimensionalorganic conductors, metals, and superconductors, complexes of fullerenes, charge-transfer salts, and π-conjugated electronicmaterials. She has published more than 300 original papers and several reviews.

Elena Zhilyaeva is a senior researcher at the Laboratory of Synthesis of Organic Multifunctional Compounds, IPCP RAS,Chernogolovka, Russia. She received a degree in chemistry from the Moscow State University and a Ph. D. degree inphysical chemistry from IPCP RAS (1985). Her research interests include molecular metals and superconductors based oncharge-transfer salts and molecular magnets. In 1992, together with her co-workers, she was awarded Premium of theRussian Academy of Sciences and of the Polish Academy of Sciences. She is the co-author of 100 research papers

Gena Shilov is a leading researcher in the Laboratory of Structural Chemistry, IPCP RAS, Chernogolovka, Russia. Hereceived a degree in physics from the Moscow Physical Technical Institute and a Ph. D. degree in chemical physics fromIPCP RAS. His research interests are the X-ray investigation of different crystals and phase transitions in crystal systems.He is the co-author of more than 300 papers

Alain Audouard, after his thesis (doctorat d�état) obtained at the University of Nancy I in 1982, entered the CNRS wherehis main interest was the physics of defects and disordered systems (ion irradiation and implantation effects, electronicproperties of amorphous materials). He moved to Toulouse in 1989 where he progressively shifted his area of interest tothe physics of organic conductors. His current research interests at the Laboratoire National des Champs MagnétiquesIntenses include quantum oscillations study of low-dimensional systems in high magnetic fields and the field-dependentproperties of iron-based superconductors.

David Vignolles gained his Ph. D., devoted to the study of Bechgaard salts in high magnetic fields, at the Institut Nationaldes Sciences Appliquées of Toulouse in 1999. Now he is Professor at this school of engineering and Director of the AppliedPhysics Department. He conducts his research, devoted to low-dimensional correlated systems in high magnetic fields, atthe Laboratoire National des Champs Magnétiques Intenses, where he has developed many cutting-edge experimentaltechniques.

Enric Canadell was educated at the Universities of Barcelona and Autónoma de Madrid, conducted research at the Universi-ties of Barcelona, Chicago, and Paris-Sud (Orsay), and since 1996 is Research Professor at the Institut de Ciència deMaterials de Barcelona (CSIC). He is mostly interested in the development of ideas relating to the structures and propertiesof solids, and has co-authored more than 300 papers and 2 books. In 1995 he was awarded the Rochat-Julliard Prize of theAcadémie des Sciences de Paris for work on the electronic structure of low-dimensional solids, and since 2013 is a memberof Academia Europaea.

Sergei Pesotskii is a senior researcher at IPCP RAS, Chernogolovka, Russia. He received his Ph. D. degree in1988 (solid-state physics). His scientific interest during the last 25 years has been in magnetic oscillations in low-dimensional organicmetals.

Rustem Lyubovskii is a leading researcher at the Institute of Issues of Chemical Physics RAS in Chernogolovka, Russia,having finished at the Moscow Physical-Technical Institute. He received his Ph. D. in 1972 and Doctor of Science in 2011.His main research interest is the physics of quasi-two-dimensional organic conductors and superconductors, their transportand magnetic properties. He is interested in preparing scientific sets for measuring the transport properties of small organiccrystals at ambient and high pressure.


crossover can be triggered by temperature, magnetic field,or a certain concentration of impurities. At low tempera-tures such compounds behave as highly anisotropic 3D


Fermi liquids in which the electron movement between lay-ers can be described by weak one-electron tunneling.[11,19,20]

The theory of such strongly correlated processes is now farfrom being completely understood.[4,17]

The majority of quasi-two-dimensional (Q2D) organicmetals and superconductors synthesized up to now haveonly one type of conducting monolayer in the lattice, andelectrons can jump to the next layer coherently without los-ing momentum and phase, provided the above reportedconditions are fulfilled. These compounds are typical Q2Dconducting systems with coherent electron transport withinthe layers and in the transverse direction. Only some ofthem contain alternating layers with two different packingmotifs (so-called dual-layered conductors).[21–30] Thesedual-layered compounds can contain the same neighboringconducting layers but with differently oriented stacks insideeach layer. Thus, either both layers or only one of themmay be metallic, whereas its neighbor is semiconducting.Compounds in which neighboring layers have differenttypes of cation/anion packing may also exist. Dual-layeredcompounds with two metallic neighboring layers may alsohave different Fermi surfaces. The symmetry of cations andanions can play an important role in the synthesis of suchcompounds. Some of these dual-layered compounds arepresented below:

(TMET-STF)2BF4, based on the unsymmetrical TMET-STF [trimethylene(ethylenedithio)diselenadithiafulvalene]donor molecule, has two different metallic layers A and B,and hence a different Fermi surface (FS) topology: LayerA provides a two-dimensional FS, whereas layer B providesa quasi-one-dimensional FS. This compound is a supercon-ductor with Tc = 4.5 K.[21]

The radical-cation salt (EDOEDT-TTF)4Hg3Br8 is basedon the unsymmetrical EDOEDT-TTF (4,5-ethylenedioxy-4�,5�-ethylenedithiotetrathiafulvalene) donor molecule andis composed of different types of EDOEDT-TTF layers:κ(4 �4)- and α��-type layers.[22]

α-β��-(BEDT-TTF)4NH4Fe(C2O4)3(solvent), in whichthe solvent is PhN(CH3)CHO, PhCH2CN, or PhCOCH3,are constructed of both α- and β��-type layers. In these com-pounds, the stacks from the neighboring layers differ inboth structure and orientation.[23]

The radical-anion salt (Me-3,5-DIP)[Ni(dmit)2]2 (Me-3,5-DIP = N-methyl-3,5-diiodopyridinium, dmit = 1,3-di-thiole-2-thione-4,5-dithiolate,) contains two kinds of layersof Ni(dmit)2 anions, one of which shows 2D metallic con-ductivity and the other is a Mott insulator.[24]

In the Q2D radical-anion salt (MDABCO+)(C60·–)(TPC),

(MDABCO+ = N-methyldiazabicyclooctane cation, TPC =triptycene), the existence of two kinds of hexagonal fuller-ene layers is related to the peculiarity of its orientationalordering.[25]

The compounds κ-α�-(BEDT-TTF)2Ag(CF3)4(1,1,2-C2H3Cl3) and κ-β��-(BEDT-TTF)2(PO-CONHC2H4SO3)contain layers of κ-type-packed BEDT-TTF dimers alter-nating with layers of stacks of α�- and β��-type packing,respectively[26] (PO = 2,2,5,5-tetramethyl-3-pyrrolin-1-oxylradical). The former is a superconductor with Tc =


9.5 K[26a] whereas the latter shows metallic behavior downto at least 1.7 K.[26b]

α-“Pseudo-κ”-(BEDT-TTF)4H3O[Fe(C2O4)3](C6H4Br2)contains conducting layers A of the α-type and insulatinglayers B composed of charged dimers and neutral mono-mers of BEDT-TTF of the “pseudo-κ” type.[27]

In contrast to the above compounds, the superconductorβ��-(BEDT-TTF)4(H3O)M(C2O4)3(PhCN)[31] and organicmetals θ-(BETS)4HgBr4(C6H5Cl)[32,33] and θ-(BETS)4-MnBr4(C2H5OH)2

[34] contain the same neighboring con-ducting layers but with differently oriented stacks insideeach of them. The structures of the cation layers of θ-(BETS)4HgBr4(C6H5Cl)[32,33] and θ-(BETS)4MnBr4-(C2H5OH)2

[34] are similar at room temperature.We shall review herein the structures of the family of

compounds D4(MBr4)(C6H6–nXn) (D = BETS, BEDT-TTF;M = Hg, Cd, Zn, Co; X = Cl, Br) and the effect of the[MBr4] anions and solvents on the formation of conductinglayers and phase transitions. Some peculiarities of the be-havior of these dual-layered compounds with different ori-entations of neighboring conducting layers and correspond-ingly with different views of Fermi surface layers will bediscussed.

2. Synthesis of Compounds of the D4(MBr4)-(C6H6–nXn) Family (D = BETS, BEDT-TTF)

The synthesis and crystal structure of the tetragonalphase of (BETS)4HgBr4(C6H5Cl)x was described for thefirst time in ref.[32] Further investigation revealed that thiscompound has the composition (BETS)4HgBr4(C6H5Cl)and shows metallic intralayer conductivity behavior andnonmetallic interlayer conductivity behavior.[33] A struc-tural phase transition was observed at T = 240–246 K.[33]

It was shown that the neighboring layers have a differentstructure below the transition temperature and hence dif-ferent Fermi surfaces above and below the transition tem-perature.[35] The metallic character of the electron bandstructure of the layers was proved by the observation ofquantum oscillations of magnetoresistance.[33,35]

To understand the formation of layers with different con-ductivities and the effect of the structure of the anionic lay-ers, comprising tetrahedral anions and solvent, on the for-mation of organic layers requires a series of compounds ofsimilar structure. To this end, we synthesized, identified,and characterized by electrical resistivity measurements aseries of novel BETS- and BEDT-TTF-based radical-cationsalts of stoichiometry (D)4MBr4(C6H6–nXn) with tetra-hedral anions of divalent metals [MIIBr4]2– (MII = Hg, Cd,Co, Zn; n = 1, 2; X = Cl, Br; Table 1). The compounds wereprepared by the electrochemical oxidation of the BETS,BEDT-TTF, and [D8]BEDT-TTF donors. The tetraalk-ylammonium or tetraphenylphosphonium salts, [Bu4N]2-MBr4 and [Ph4P]2MBr4, were used as supporting electro-lytes and mono-, di-, or trisubstituted halobenzenes wereused as solvents. The crystals were obtained as rectangularplates when monosubstituted halobenzenes were used as


solvents. When 1,2-disubstituted halobenzenes were used assolvents, electrocrystallization afforded plates of either oc-tagonal, hexagonal, rectangular, or triangular habits. Theuse of 1,3-dichlorobenzene resulted in thin needles of unde-termined composition. When 1,2,4-trichlorobenzene wasused as solvent, plate-shaped crystals of (BEDT-TTF)3-HgBr4 were formed that did not contain 1,2,4-tri-chlorobenzene. The use of a solution of solid 1,4-dichloro-or 1,4-dibromobenzene in 1,2,4-trichlorobenzene resulted incrystals of halobenzene-free (BEDT-TTF)3HgBr4 and(BEDT-TTF)2HgBr4. Analysis of the compositions of com-pounds prepared from different halobenzenes showed thatthe dimensions of the halobenzene molecule have an influ-ence on the formation of the phase. The size of the 1,2-

Table 1. Conditions for synthesis, crystal systems, room-temperature conductivities and phase-transition temperatures for (D)4-MBr4(C6H6–nXn).

Compound[a] Supporting electrolyte; Tsynthesis [°C] Crystal system/T [K] of σRT [S/cm]; Ea [eV][b] Tp [K][c] Ref.crystal structuredetermination

ET4HgBr4·DCB [Bu4N]2HgBr4/Bu4NBr; 5 tetragonal/293 σ� = 15, metal, 284 [36]

σ� = 2�10–3; Ea = 0.01ET4CdBr4·DCB [Ph4P]2CdBr4/Ph4PBr; 23 σ� = 30, metal, 284 [36]

σ� = 10–3; Ea = 0.005ET4ZnBr4·DCB [Bu4N]2ZnBr4; 27 triclinic/293 σ� = 10, metal, 320 [37]

σ� = 3�10–3

ET4CoBr4·DCB [Bu4N]2CoBr4; 18 triclinic/300 σ� = 20, metal, 320 [38]

tetragonal/343 σ� = 2�10–3

([D8]ET)4HgBr4·DCB [Bu4N]2HgBr4/Bu4NBr; 30 σ� = 15, TM–I ≈ 45, 284 [39]

σ� = 10–3

BETS4HgBr4·DCB [Bu4N]2HgBr4/Bu4NBr; 50 tetragonal/293 σ� = 10, metal, 288 [39]

σ� = 3� 10–3

BETS4CoBr4·DCB Metal [40]

BETS4ZnBr4·DCB Metal [41]

ET4HgBr4·DBB [Bu4N]2HgBr4; 40 tetragonal/293 σ� = 8, TM–I ≈ 70, 256 [36]

Minor phase σ� = 10–3; Ea = 0.002ET4CdBr4·DBB [Ph4P]2CdBr4/Ph4PBr; 23 σ� = 15, TM–I ≈ 63, 259 [36]

σ� = 2�10–3; Ea = 0.014BETS4HgBr4·PhCl HgBr2/Bu4NBr; 49 tetragonal/300 σ� = 80, metal, 240–246 [32,33,35]

monoclinic/200 σ� = 8�10–3

BETS4CdBr4·PhCl [Ph4P]2CdBr4/Ph4PBr; 49 Metal ≈ 245 [42]

ET4HgBr4·PhCl [Bu4N]2HgBr4/Bu4NBr; 23 σ� = 30, TM–I ≈ 50, 235–250 [36]

σ� = 5�10–3; Ea = 0.02ET4CdBr4·PhCl [Ph4P]2CdBr4/Ph4PBr; 23 σ� = 60, TM–I ≈ 90, 238–250 [36]

σ� = 4�10–3; Ea = 0.01ET4ZnBr4·PhCl [Bu4N]2ZnBr4; 27 σ� = 20, TM–I ≈ 72, [37]

σ� = 2�10–3

([D8]ET)4HgBr4·PhCl [Bu4N]2HgBr4/Bu4NBr; 30 σ� = 10, TM–I ≈ 100 – [39]

σ� = 5� 10–3

BETS4HgBr4·PhBr HgBr2/Bu4NBr; 49 σ� = 20, metal, 225–230 [42]

σ� = 3�10–3

BETS4CdBr4·PhBr [Ph4P]2CdBr4/Ph4PBr; 49 tetragonal/293 σ� = 50, metal, 225–230 [42]

σ� = 6�10–3

ET4HgBr4·PhBr [Bu4N]2HgBr4; 18 σ� = 8, TM–I ≈ 100, 220–235 [36]

σ� = 2�10–3; Ea = 0.01ET4CdBr4·PhBr [Ph4P]2CdBr4/Ph4PBr; 19 σ� = 30, TM–I ≈ 95, 225–240 [36]

σ� = 2�10–3; Ea = 0.02ET4ZnBr4 PhBr [Bu4N]2ZnBr4; 27 σ� = 10, TM–I ≈ 80, [37]

σ� = 3�10–3

([D8]ET)4HgBr4·PhBr [Bu4N]2HgBr4/Bu4NBr; 30 σ� = 20, TM–I ≈ 50, [39]

σ� = 4�10–3

ET4ZnBr4 PhF [Bu4N]2ZnBr4; 23 σ� = 20, TM–I ≈ 183, 240–254 [37]

σ� = 3 �10–3

[a] ET = BEDT-TTF; DCB = 1,2-dichlorobenzene; DBB = 1,2-dibromobenzene. [b] σ� = conductivity along the layers; σ� = conductivityperpendicular to the layers; Ea = activation energy of conductivity. [c] Tp = phase-transition temperature.


C6H4Br2 molecule is maximal, and the crystal lattice of(BEDT-TTF)4MBr4(C6H6–nXn) cannot accommodate 1,3-and 1,4-dihalobenzenes. The radical-cation salts preparedare listed in Table 1. The salts are divided into two largegroups depending on the solvent: (D)4MBr4(C6H4X2), con-taining 1,2-disubstituted halobenzenes C6H4X2 (X = Cl,Br), are presented at the top and (D)4MBr4(C6H5X), con-taining monosubstituted halobenzenes C6H5X (X = Cl, Br,F; D = BETS, BEDT-TTF, [D8]BEDT-TTF; M = Hg, Cd,Zn, Co), are presented at the bottom of Table 1.

Crystal quality is known to be an extremely importantparameter with regard to the atomic structure determi-nation of molecular organic conductors. More specifically,as developed in the following, crystals of these compounds


become twinned as a result of a metal-to-metal phase tran-sition, which complicates the structure determination. Crys-tal twinning also affects the properties of compounds at lowtemperatures and, in particular, the reproducibility of elec-trical resistivity and magnetic-field-dependent properties.Conductivity measurements show that, when the phase-transition temperature is not surmounted even during re-peated heating/cooling cycles, the data for the crystal arerepeatedly reproduced. Therefore it is important to keep theprepared crystals below the phase-transition temperature. Itwas particularly important to find compounds for whichthe phase transition occurs above room temperature. Thehigh quality of such crystals remains unaffected by heating/cooling cycles across a wide temperature range. We suc-ceeded in synthesizing (D)4MBr4(1,2-C6H4Cl2) (D =BEDT-TTF, BETS; M = Co, Zn) with phase-transitiontemperatures higher than 293 K, which allowed us to deter-mine their structures above and below the phase-transitiontemperature (Table 1).

3. Conducting Properties of (D)4MBr4-(C6H6–nXn)

The conductivity of (D)4MBr4(C6H6–nXn) was measuredon single crystals by using the standard four-probe tech-nique. For all (D)4MBr4(C6H6–nXn) (X = Cl, Br; M = Cd,Hg, Co, Zn; n = 1, 2), the room-temperature conductivitymeasured parallel to the conducting layers (σ�) is 5–80 S/cm, and the conductivity perpendicular to the conductinglayers (σ�) is (2–8)� 10–3 S/cm. The anisotropy of the con-ductivity is therefore 103–104 (Table 1).

3.1. Conductivity of the BETS-Based Conductors

For all the BETS-based conductors, metallic behavior ofthe in-plane resistivity (R�) down to around 4.3 K and semi-conducting growth of the interlayer resistivity (R�) wereobserved with decreasing temperature.[33,35,39–42] ForBETS4HgBr4(C6H5Cl), the metallic behavior of R� changedto semiconducting behavior at T = 246 K (Figure 1),[33]

which at T2 = 240 K transformed again to metallic behav-ior. For (BETS)4MBr4(C6H5Br) (M = Cd, Hg; Figure 2,a),[42] the metallic behavior of R� changed to semiconduct-ing behavior at T = 230 K, which at T2 = 225 K trans-formed again to a metallic state. The temperature intervalbetween these two metallic states increased with the coolingrate (Figure 1, onset, v1 = 0.3 K/min, v2 = 0.6 K/min, v3 =7.0 K/min) and a hysteresis was observed. In contrast toC6H5X-containing BETS salts, which show a slight increasein R� (230–246 K) on cooling, a sharp (within 0.5 K) de-crease in R� resistivity is observed in (BETS)4HgBr4(1,2-C6H4Cl2) at around 288 K (Figure 2, b).[39] One more ad-ditional feature of the C6H4Cl2-containing BETS salts (M= Hg, Co, Zn) is the nonmetallic behavior of R� observeddown to 29–11 K only. Below 29–11 K, R� decreases (Fig-ure 2, c).[39–41] This change of regime may be related to acrossover from incoherent to coherent conductivity in theseBETS-based compounds. The position of the crossover


peak depends on the quality of the crystal. For comparison,nonmetallic behavior of R� in the C6H5X-containing BETSsalts is observed down to liquid helium temperature.[33,42]

Figure 1. Temperature dependence of the relative resistivity of(BETS)4HgBr4(C6H5Cl). Inset: Cooling-rate-dependent resistivitybehavior at temperatures close to the phase-transition temperature.Reprinted from ref.[33] with permission from Elsevier.

Figure 2. Temperature dependence of the intra- and interlayer rela-tive electrical resistivities of (a) (BETS)4HgBr4(C6H5Br) [Reprintedfrom ref.[42] with permission from Elsevier.], (b) (BETS)4HgBr4(1,2-C6H4Cl2) [Reprinted from ref.[39] with permission from the RussianAcademy of Sciences.], and (c) (BETS)4CoBr4(1,2-C6H4Cl2). Re-printed from ref.[40] with permission from the Russian Academy ofSciences.

3.2. Conductivity of the BEDT-TTF-Based Compounds

In contrast to the BETS-based compounds, most of theBEDT-TTF-based salts undergo a metal–insulator (M–I)transition with decreasing temperature, and only (BEDT-TTF)4MBr4(C6H4Cl2) (M = Cd, Hg, Co, Zn), which con-


tain solvated molecules of 1,2-dichlorobenzene, retain met-allic behavior of R� down to low temperatures (Figure 3,a).[36–38,43] Interestingly, in crystals of deuteriated ([D8]-BEDT-TTF)4HgBr4(C6H4Cl2), R� shows metallic behaviordown to only 40 K,[39] and below 40 K R� increases.

Figure 3. Temperature dependence of the intra- and interlayer rela-tive electrical resistivities of (a) (BEDT-TTF)4CdBr4(C6H4Cl2),(b) (BEDT-TTF)4CdBr4(C6H4Br2), (c) (BEDT-TTF)4CdBr4(PhCl),and (d) (BEDT-TTF)4CdBr4(PhBr). Reprinted from ref.[36] withpermission from Elsevier.

The interlayer resistivity of crystals of (BEDT-TTF)4-MBr4(C6H4X2) (M = Cd, Hg, Co) exhibits nonmetalliccharacter down to liquid helium temperature (Figure 3, a).An exception is (BEDT-TTF)4ZnBr4(C6H4Cl2), whichshows metallic behavior of resistivity below 30 K.[37]

On heating, the intralayer resistivities of (D)4MBr4-(C6H4Cl2) crystals, which contain solvated molecules of 1,2-dichlorobenzene, grow sharply at phase-transition tempera-tures (Tp) lying in the 284–320 K range, which correlateswith the anion volume (Tables 1 and 2). Upon further heat-ing, the temperature dependence shows metallic character(see parts a and b in Figures 2 and 3). In (BEDT-TTF)4-HgBr4(C6H4Br2) and (BEDT-TTF)4CdBr4(C6H4Br2),which contain larger 1,2-dibromobenzene solvent mol-ecules, the transition temperature (Tp = 256–259 K) is no-ticeably lower (Figure 3, b).[36] Upon reverse crystal cool-ing, the resistivity shows a hysteresis with a width ΔT = 1–2 K, and the longitudinal resistivity drops by 10–14 %. Thestepwise changes in resistivity in a narrow temperaturerange and the observed hysteresis allow one to assume thata metal–metal phase transition occurs (e.g., see inset in Fig-ure 3, b). In deuteriated ([D8]BEDT-TTF)4HgBr4-(C6H4Cl2), a phase transition is observed at a temperaturesimilar to that observed for the nondeuteriated analogue.An applied pressure of 0.3 kbar does not affect the transi-tion temperature (Figure 4, a).[39]

Note that the R� resistance minima and its increase atlow temperatures (Figures 3 and 4) may be connected withimpurities, defects, and the rate of cooling of the crystalsduring experiments.[33] Furthermore, some additional de-fects may appear as twins during phase transitions at hightemperatures (Figure 3, b at T = 258 K and c at T ≈ 240 K).


Table 2. Crystal lattice parameters and temperature ranges of theexistence of tetragonal phases (D)4MBr4(C6H6–nXn).

Compound[a] Space a [Å] c [Å] T [K][b] θ [°][c] Ref.group

ET4CoBr4·C6H4Cl2 I41/a 9.742 74.42 �320 104.0 [38]

ET4HgBr4 C6H4Cl2 I41/a 9.728 74.73 �284 104(1) [36]

ET4HgBr4·C6H4Br2 I41/a 9.760 75.06 �256 104(1) [36]

BETS4HgBr4·C6H4Cl2 I41/a 9.755 75.84 �287 105.6 [39]

BETS4HgBr4·C6H5Cl I41/a 9.774 75.73 �246 106.1 [33]

BETS4CdBr4·C6H5Br I41/a 9.764 76.03 �230 105.1 [42]

[a] ET = BEDT-TTF. [b] Temperature range in which the phaseexists. [c] θ = dihedral angle between donor molecules of the adja-cent stacks.

Figure 4. Temperature dependence of the intra- and interlayer rela-tive electrical resistivities of (a) ([D8]BEDT-TTF)4HgBr4(C6H4Cl2)and (b) ([D8]BEDT-TTF)4HgBr4(PhCl). Reprinted from ref.[39]

with permission from the Russian Academy of Sciences.

For (BEDT-TTF)4MBr4(C6H5X) (M = Hg, Cd, Zn),which contain monosubstituted halobenzene (Table 1), lon-gitudinal resistivity decreases as the temperature decreasesdown to around 70, 80, or 185 K, respectively, dependingon the anion and solvent (C6H5Cl, C6H5Br, or C6H5F). Be-low these temperatures, a metal–insulator transition is ob-served.[36,37] The behavior of the interlayer resistivity (R�)of the θ-(BEDT-TTF)4MBr4(PhX) salts is sample-depend-ent (e.g., see Figure 3, c,d). In most crystals R� shows non-metallic character because it increases as the temperaturedecreases below room temperature.

In addition to the M–I transition, a phase transitionfrom one metallic state to another was observed in (BEDT-TTF)4MBr4(C6H5X) at around 260–220 K. Longitudinalresistivity increases with decreasing temperature in an ex-tended temperature range (ca. 15–20 K).[36,37,39] A similarphase transition is observed for (BETS)4MBr4(C6H5X) ataround 6 K.[33,42]

No phase transition is detected in deuteriated ([D8]-BEDT-TTF)4HgBr4(C6H5Cl) and ([D8]BEDT-TTF)4-HgBr4(C6H5Br) at around 260–220 K according to the res-istivity data along the conducting layers. Nevertheless, anapplied pressure of 0.3 kbar on crystals of ([D8]BEDT-TTF)4HgBr4(C6H5Cl) restores the phase transition (Fig-ure 4, b). These facts and the extended phase transition al-low one to assume that phase transitions in crystals of(BEDT-TTF)4HgBr4(C6H5X) containing C6H5X could bedue to conformational rearrangements of terminal ethylenegroups.[39] Moreover, we evidenced that disordered mol-ecules of C6H5Cl are retained in the structure of (BETS)4-HgBr4(C6H5Cl) below the transition, hence the phase-tran-sition temperature is not related to solvent.[35]


4. Crystal Structures of (D)4MBr4(C6H6–nXn)

Of particular interest and importance are the factors in-fluencing the formation of phase transitions in the com-pounds analyzed and the origin of the observed differencesin the temperature dependence of conductivity along andacross the layers. We solved and analyzed the crystal struc-tures above and below the phase-transition temperature forseveral compounds of the (D)4MBr4(C6H4X2) and (D)4-MBr4(C6H5X) families, which do not have the same halo-benzene molecule. The compounds of both families showlayered crystal structures. The cation layers are formed fromstacks of BEDT-TTF or BETS donor molecules, and theanion layers involve [MBr4]2– anions and mono- (C6H5X)or disubstituted (C6H4X2) halobenzene molecules.

4.1. Crystal Structures of the Tetragonal Phase of(D)4MBr4(C6H6–nXn) (above the Phase-TransitionTemperature)

Above Tp, the compounds (D)4MBr4(C6H6–nXn) (D =BEDT-TTF, BETS; M = Cd, Hg, Co, Zn; X = Cl, Br, n =1, 2) have a tetragonal structure. Their crystal lattice param-eters, the temperature ranges in which the tetragonal phaseexists, and the dihedral angles (θ) between donor moleculesin adjacent stacks are listed in Table 2.

The crystal structures of the tetragonal phases of (D)4-MBr4(C6H6–nXn) are similar and consist of cation andanion layers alternating along the longest unit cell axis (e.g.,see Figure 5). The structures of the cation and anion layersin the tetragonal phase of θ-(BEDT-TTF)4CoBr4(C6H4Cl2)are shown in Figure 6. There is one independent radical-cation layer with two types of stacks (A and B) in the (D)4-MBr4(C6H6–nXn) tetragonal phases. The neighboring layersare rotated 90° relative to each other in four donor layersper unit cell.[33,36,38,42]

The modes of intermolecular overlap in the stacks of alltetragonal phases are similar (Figure 7). In stack A, radicalcations are shifted along the shorter axis of the donor mole-cule and in stack B they are shifted along the shorter andlonger axes, the averaged planes of the radical cations beingparallel in every stack. The dihedral angles (θ) between thedonor molecules of the adjacent stacks, which predeterminethe overlapping of the π orbitals,[44] are very similar forboth the BETS- and BEDT-TTF-based tetragonal phases

Figure 5. Crystal structure of the tetragonal phase of θ-(BETS)4CdBr4(C6H5Br). Reprinted from ref.[42] with permission from Elsevier.


Figure 6. Structures of the cation (left) and anion (right) layers inthe tetragonal phases: (a) θ-(BEDT-TTF)4HgBr4(C6H4Cl2) (repro-duced from ref.[36] with permission of Elsevier) and (b) θ-(BEDT-TTF)4CoBr4(C6H4Cl2). Reproduced from ref.[38] with permissionof The Royal Society of Chemistry.

of (D)4MBr4(C6H6–nXn) (see Table 2). The structure of thecation layers of the organic metal θ-(BETS)4MnBr4-(EtOH)2, synthesized and analyzed in ref.[34], in which con-ducting layers are also differently oriented, is similar to thatof the tetragonal phases of θ-(BETS)4MBr4(C6H5X). Un-fortunately, there are no data available regarding its inter-layer resistivity.

The anion layers of the tetragonal phases are constructedof [MBr4]2– anions and disordered solvent molecules. Fig-ure 6 (right) shows disordering of the 1,2-C6H4Cl2 moleculein four positions in the structure of the tetragonal phase ofθ-(BEDT-TTF)4CoBr4(C6H4Cl2).[38] The distances betweenthe [MBr4]2– anions in the anion layers are equal to thecrystal lattice parameter a (see Table 2).


Figure 7. Structures of stacks A and B in the tetragonal phase of θ-(BETS)4CdBr4(PhBr). Reprinted from ref.[42] with permission fromElsevier.

4.2. Crystal Structures of (D)4MBr4(C6H6–nXn) below thePhase-Transition Temperature

4.2.1. (D)4MBr4(C6H4X2) Conductors with DisubstitutedHalobenzenes

As the temperature decreases, the (D)4MBr4(C6H4X2)crystals comprising disubstituted halobenzene undergotransition from the tetragonal to the triclinic phase. In thisfamily of compounds, the phase-transition temperatures(Tp) correlate with the volumes of different anions and sol-vent molecules. Smaller anions and solvent molecules pro-vide higher Tp (Table 3). The crystal structures of the tri-clinic phases of (BEDT-TTF)4CoBr4(C6H4Cl2)[38] and(BEDT-TTF)4ZnBr4(C6H4Cl2)[37] were determined. Thestructures of the salts are fully ordered. The terminal ethyl-ene groups of the BEDT-TTF molecules in the cation layersand the solvent molecules in the anion layers are ordered.

Table 3. M–Br distances and dihedral angles θ between the donormolecules of adjacent stacks in (BEDT-TTF)4MBr4(C6H6–nXn)phases below the phase-transition temperature Tp.

Compound[a] Tp [K] R[b] [Å] Layer A Layer B Ref.θ1 [°] θ2 [°] θ1 [°] θ2 [°]

ET4ZnBr4·C6H4Cl2 320 2.41–2.43 104.8 113.6 98.7 99.4 [37]

ET4CoBr4·C6H4Cl2 320 2.42 104.8 113.5 97.3 99.6 [38]

ET4CdBr4·C6H4Cl2 284 2.58 [36]

ET4HgBr4·C6H4Cl2 284 2.61 [36]

ET4CdBr4·C6H4Br2 259 2.58 [36]

ET4HgBr4·C6H4Br2 256 2.61 [36]

BETS4HgBr4·C6H5Cl 240–246 2.61 108.4 102 [35]

[a] ET = BEDT-TTF. [b] R = M–Br distance.

In contrast to the tetragonal phase described by one in-dependent radical-cation layer, there are two independentradical-cation layers A and B in the triclinic phase (Fig-ure 8). Radical-cation stacks run along the a direction inlayer A and along the b direction in layer B. Two types ofstacks were found in each cation layer. Figure 9 (a,b) showsthe structures of two neighboring radical-cation layers andthe anion layer of the triclinic phase of (BEDT-TTF)4-CoBr4(C6H4Cl2).


Figure 8. Crystal structure of θ-(BEDT-TTF)4CoBr4(C6H4Cl2)projected along the a direction. Two different radical-cation layersare labeled A and B. Reprinted from ref.[45] with permission fromthe European Physical Society.

Figure 9. Structures of neighboring radical-cation layers of the(BEDT-TTF)4CoBr4(C6H4Cl2) triclinic phase: (a) Layer A and(b) layer B. Reproduced from ref.[38] with permission of The RoyalSociety of Chemistry.

The stacks of the first type (a1 for layer A and b1 forlayer B) are formed by two different BEDT-TTF radicalcations located one above the other without shifting alongthe longer molecular axis. The stacks of the second type (a2for layer A and b2 for layer B, Figure 9, a,b) are formed byequivalent BEDT-TTF cations in which the radical cationsare shifted along the longer axis of BEDT-TTF in a zigzagmanner to form cavities in which the anions are allocated(as in stack B, Figure 7).

In cation layer A, the radical cations of stacks a2 areparallel, whereas in stacks a1 the angle between the planesof neighboring BEDT-TTF molecules is 8.8°.[37,38] There-fore there are two dihedral angles (θ) between the planes ofBEDT-TTF of stacks a1 and a2 with respect to the twodifferent molecules in stack a1 (Table 3). Similar dihedralangles were found for the θ-(BEDT-TTF)2CsM(SCN)4 andθ-(BEDT-TTF)2RbM(SCN)4 metals, which show metal–in-sulator transitions at 20 and 190 K, respectively.[46] In cat-ion layer B presented in Figure 9 (b), the angle between theneighboring radical cations of stack b1 is 2.3° for (BEDT-TTF)4CoBr4(C6H4Cl2) and 0.7° for (BEDT-TTF)4-ZnBr4(C6H4Cl2). The angles (θ) between the planes ofBEDT-TTF in stacks b1 and b2 are around 98° (Table 3),which is close to the dihedral angle in the structure of theθ-(BEDT-TTF)2I3 superconductor.[46] Thus, neighboringradical-cation layers A and B differ in stack direction, theperiod along a conducting stack, and the dihedral angle (θ)between radical cations of neighboring stacks (Table 3).

The anion layers of the triclinic phases of (BEDT-TTF)4-MBr4(1,2-C6H4Cl2) are constructed of [MBr4]2– dianionsand 1,2-dichlorobenzene molecules arranged chequerwise.


In contrast to the tetragonal phases, in which both chloro-benzene and dichlorobenzene molecules are disordered, thesolvent molecules are ordered in the triclinic phases of(BEDT-TTF)4MBr4(C6H4Cl2), both chlorine atoms of the1,2-dichlorobenzene molecule being directed nearly alongthe b axis (Figure 9, a,b). Such an arrangement of solventmolecules results in parameter b being higher than param-eter a, and the stacking period in the stacks in the b direc-tion (in layer B) being greater than in the stacks in the adirection (in layer A). It is most probable that an asymmet-rical ordered solvent molecule in the anion layers is themain factor leading to symmetry reduction in the crystalstructure of the triclinic phase.

4.2.2. (D)4MBr4(C6H5X) Compounds withMonosubstituted Halobenzenes

For compounds of this family, the phase-transitionwidths increase to 5–6 K for the BETS salts[33,35,42] and toaround 15–20 K for the BEDT-TTF species.[36] The phase-transition temperatures correlate with the volume of thesubstituent in the halobenzene. Larger substituents provideslightly lower values of Tp (Table 1).

The structures of (D)4MBr4(C6H5X), which containmonosubstituted chloro- or bromobenzene, have been at-tributed to a monoclinic system below the phase-transitiontemperature (Table 1). In this structure, crystallographicallydifferent layers formed by radical cations alternate with thelayers formed by [MBr4]2– anions and monosubstitutedhalobenzene molecules, similarly to those of the triclinicphases of compounds containing C6H4Cl2. For example, inthe structure of (BETS)4HgBr4(C6H5Cl) determined at200 K (phase transition temperature is 246–240 K), thereare two independent conducting radical-cation layers A andB with differently oriented stacks, as in the triclinic phasesof compounds containing dichlorobenzene.[35] Neighboringradical-cation layers A and B differ in stack direction, theperiod along a conducting stack, and the dihedral angle (θ)between the radical cations of neighboring stacks. The an-gle θ of 108.4° between averaged BETS planes in neigh-boring stacks in layer A and of 102.0° in layer B are verydifferent to the values found for the triclinic phases of com-pounds containing C6H4Cl2 (Table 3).

The anion layers of the monoclinic phases of (D)4-MBr4(C6H5Cl) are formed by [MBr4]2– anions and chloro-benzene molecules. In contrast to the triclinic phases, inwhich solvent molecules are ordered, two crystallographi-cally independent types of chlorobenzene molecules werefound in the phases of (D)4MBr4(C6H5Cl).[35] Because achlorobenzene molecule remains disordered after themetal–metal transition, one could suppose that the effect ofchlorobenzene in the anion layer of (D4)MBr4(C6H5Cl) onthe formation of the new structure below the phase-transi-tion temperature is weaker than in dichlorobenzene-con-taining compounds. Analysis of phase transitions in the or-ganic metals (BEDT-TTF)4HgBr4(C6H5Cl) and ([D8]-BEDT-TTF)4HgBr4(C6H5Cl) allowed one to expect thatthe effect of the cationic part of the structure on the transi-tion dominates in (D)4MBr4(C6H5Cl).[39] However, one


should consider the correlation of the phase-transition tem-peratures with the substituent volume in a halobenzene mo-lecule.

4.3. Temperature Dependence of Lattice Parameters

Analysis of the temperature dependence of lattice param-eters and X-ray analysis of the profiles of peaks across awide temperature range show that during multiple transi-tions at temperatures above and below the transition tem-perature, single peaks at the transition from the tetragonalto the triclinic phase are split into four components, whichindicates that the crystal becomes twinned.[38] Analysis ofthe changes in lattice parameters of the compounds acrossa wide temperature range reliably established the ranges ofexistence of the triclinic and tetragonal phases of (BEDT-TTF)4CoBr4(C6H4X2).

Figure 10 shows the changes in crystal lattice parametersof the single-domain crystal of (BEDT-TTF)4MBr4-(C6H4X2) (M = Co) as the temperature changes in the 200–360 K range. The phase transition in the 320–325 K rangeis accompanied by drastic changes in the parameters a andb, namely, parameter a increases and parameter b decreases.Above the transition, a = b and the unit cell is tetragonal.With further temperature increases, parameters a and bgrow linearly. Parameter c in the tetragonal phase also in-creases linearly with temperature. With a decrease in tem-perature in the triclinic phase in the range 320–290 K, pa-rameter a decreases and parameter b increases. Parameter cin the triclinic phase remains almost unchanged with tem-perature. The angles α, β, and γ of the unit cell of the tri-clinic phase tend to 90° when approaching the transitiontemperature.

Figure 10. Temperature dependence of the unit cell parameters aand b in (BEDT-TTF)4CoBr4(C6H4Cl2) (9 Å was subtracted fromthe values of parameters a and b). Reproduced from ref.[38] withpermission of the Royal Society of Chemistry.

4.3.1. Twinning in Crystals of (BEDT-TTF)4CoBr4(1,2-C6H4Cl2) at the Phase Transition

Analysis of the changes in the lattice parameters at thephase transition gives an insight into the process of crystaltwinning below the phase transition and the concomitantdeterioration of their quality.[38] The appearance of twincomponents is detected by a single diffraction peak split-tinginto several components when a crystal transformsfrom the tetragonal to the triclinic phase.


At the phase transition to a low-temperature modifica-tion, the solvent can be located in one of four possible posi-tions in the crystal (Figure 6, right) As a result, the crystalcan be either a single or twinned domain. For example, aninitial single-domain (BEDT-TTF)4CoBr4(1,2-C6H4Cl2)crystal remained in the symmetrical tetragonal phase for along time during the experiment and became twinned afterthe transition to the triclinic phase. At the transition fromthe tetragonal (symmetrical) to the triclinic (unsymmetrical)phase, the domains become strongly distorted, each domainshrinking along the a axis and elongating along the b axisof the unit cell. Because the directions of the unit cell axesare different for different domains, domain distortions canresult in the formation of cracks in a single crystal. Thiseffect is more and more pronounced with decreasing tem-perature because the difference b–a increases. On transitionfrom the triclinic to the tetragonal phase, twinning disap-pears.

4.4. Effect of Solvent on the Occurrence of PhaseTransitions in (D)4MBr4(C6H6–nXn)

The key role in phase transitions in (D)4MBr4(C6H6–nXn)is played by a unique combination of the donor molecule,the tetrahedral tetrabromometallate dianion, [MBr4]2–, andan aromatic halobenzene molecule. The structures of thesecompounds are specified by the alternation of conductinglayers differing in the direction of radical-cation stacks. Thestacking direction in one conducting layer is perpendicularto those of the nearest-neighboring layers. The divalent[MBr4]2– anion is characterized by the tetrahedral configu-ration of the M–Br bonds. The two Br atoms of the[MBr4]2– dianion imbedded in one conducting layer of thetetragonal phase form four short Br···H contacts with theradical cations from three stacks, A, B, and A�, surroundingthe anion. The two other Br atoms of the same [MBr4]2–

dianion imbedded in the next conducting layer similarlyform four short Br···H contacts with three radical cationsof this layer, enforcing them to stack in the direction per-pendicular to the stacking direction of the previouslayer.[36,38] These H···Br contacts are likely responsible forthe specific molecular packing of the crystal structure.

A comparative analysis of the structures of (BEDT-TTF)4MBr4(1,2-C6H4Cl2) above and below the transitiontemperature shows that a transition provides ordering inboth the cation and anion layers of the structure. A solventmolecule is ordered in the anion layers, and the terminalethylene groups of the donor molecules are ordered in thecation layers. The transition temperature is affected bychanges in the anion layer: An increase in both the anionsize and volume of C6H2X2 provide lower Tp (Table 3). Incontrast, changes in the cation layer, for example, the sub-stitution of BEDT-TTF by deuteriated [D8]BEDT-TTF, donot affect Tp (Table 1).[39] This allows one to conclude thatthe phase transition in (BEDT-TTF)4MBr4(C6H4X2) origi-nates from ordering in the anion layers.

Unlike C6H4Cl2-containing compounds, the chloroben-zene molecule in C6H5Cl-containing compounds remains


disordered below the phase-transition temperature. Thechanges in the anion layer, namely, the increase in the anionand C6H5X volume affect very weakly the phase-transitiontemperature range (Table 1). In contrast, the substitution ofBEDT-TTF by deuteriated [D8]BEDT-TTF in the cationlayers of the (BEDT-TTF)4HgBr4(C6H5Cl) conductor re-sulted in no transition in the deuteriated analogue ([D8]-BEDT-TTF)4HgBr4(C6H5Cl). The application of 0.3 kbarpressure on the crystals restores the phase transition (seeFigure 4, b). These facts together with the extended charac-ter of the phase transition allowed us to suppose that thephase transitions in crystals of (BEDT-TTF)4-HgBr4(C6H5X) (X = Cl, Br) could be related to conforma-tional rearrangements of the terminal ethylene groups.

Thus, the number and arrangement of substituents in thearomatic solvents, halobenzenes, used in the synthesis of the(BEDT-TTF)4MBr4(C6H6–nXn) conductors play a key rolein the character of the metal–metal phase transitions. InC6H5X-containing compounds, the transition is affectedmainly by ordering in the cationic part of the structure,whereas in compounds containing 1,2-C6H4Cl2, phase tran-sitions relate to ordering in the anionic part.

5. Some Peculiarities of the Fermi Surface andConductivity in Zero and High Magnetic Fieldsof Dual-Layered Compounds with DifferentlyOriented Conducting Layers

5.1. θ-(BETS)4HgBr4(C6H5Cl) � A Dual-LayeredCompound with Two Conducting Layers

Band structure calculations of the Q2D organic metal θ-(BETS)4HgBr4(C6H5Cl) based on room-temperature X-raydata yield the Fermi surface reported in refs.[33,35] It is com-posed of two hole tubes and a single electron tube. Providedthe magnetic breakdown gaps between electron- and hole-type tubes, which are otherwise compensated, are smallenough, the Fermi surface (FS) should yield a network ofcompensated orbits. However, for this compound, a phasetransition observed at around 240 K is liable to modify theFS.

To gain a reliable basis for band structure calculations atlow temperature, diffraction measurements were performedat 200 K.[35] As expected, the X-ray data evidence a lower-ing of the crystal symmetry from tetragonal to monoclinic.As reported above, it should be noted that as the tempera-ture decreases, the above-mentioned phase transition yieldslower-quality single crystals at a macroscopic level. Morespecifically, the X-ray peaks are broadened and some ofthem are split, that is, a single crystal becomes twinned pre-venting reliable X-ray experiments. The crystal with thelowest twin components was analyzed, however, the crystalwas far from ideal, as evidenced by its poor R factor (R =0.1235). Because of this, the experimental massif of reflec-tions obtained were somewhat distorted. Nevertheless, wewere able to analyze the X-ray data quite correctly and, inaddition, clear quantum oscillations were observed.


The independent part of the low-temperature crystalstructure involves four BETS radical cations, one HgBr4

2–

anion, and one solvent molecule, which can occupy twosites each with a probability of 50%. Figure 11 shows theprojection of the crystal structure at 200 K on the ab plane.As is the case at room temperature,[33] the unit cell containsfour layers of radical cations. The [HgBr4]2– anions and thesolvent (C6H5Cl) molecules are located between these lay-ers, as is the case for all tetragonal phases. In contrast tothe room-temperature data,[33] two different kinds of radi-cal-cation layers, referred to as A and B in the following,are evidenced at 200 K (Figures 11 and 12).[35] Each of the

Figure 11. Crystal structure of θ-(BETS)4HgBr4(C6H5Cl) at 200 Kprojected along the c axis. The labels A and B refer to two differentcation layers. Reprinted from ref.[35] with permission from Elsevier.

Figure 12. Conducting radical-cation layers A and B of θ-(BETS)4HgBr4(C6H5Cl) at 200 K. Reprinted from ref.[35] with permission fromElsevier.

Figure 13. Electronic band structures (in which the dashed lines refer to the Fermi level) and Fermi surfaces of the radical-cation layersA and B of θ-(BETS)4HgBr4(C6H5Cl) at 200 K. Reprinted from ref.[35] with permission from Elsevier.


two cation layers, which are both of the θ type, are com-posed of two different stacks labeled A1 (B1) and A2 (B2)in layer A (B). In summary, the phase transition at around240 K corresponds to some degradation of the crystallo-graphic order.

Tight-binding band structure calculations based on theextended Hückel method[47] were used to calculate the elec-tronic band structures and FSs corresponding to layers Aand B (Figure 13).

There are four BETS molecules per repeat unit of thelayers, and thus the band structures contain four bandsmainly constructed from the HOMOs (highest occupiedmolecular orbital) of the BETS molecules. Because theaverage charge of the BETS molecules is +0.5, these bandsshould house two holes. The two FSs are almost identicalbut turned by approximately 90°. These FSs can be seenfrom a superposition (and hybridization) of a closed loop,which, as for the usual θ phases,[35] can be described eitheras a rounded rectangle or as an elongated ellipse. The resultis a Fermi surface with a topology observed in many com-pounds containing closed and open portions with verysmall hybridization gaps. From the viewpoint of the re-


sulting orbit network, both of them can be regarded as alinear chain of coupled orbits, roughly turned by around90° with respect to each other. This picture is very differentfrom the FS deduced from calculations at room tempera-ture, which yield a network of compensated electron andhole orbits.

In terms of the electronic band structure, it can be seenthat two upper HOMO bands cross the Fermi level. There-fore layers A and B should be metallic. Figure 14 showsthe temperature dependence of the relative inter- (R�) andintralayer (R�) resistances of the single crystal. It can beseen that the intralayer resistance shows metallic behavior(dR�/dT�0) and that the interlayer resistance shows a non-metal (dR�/dT �0) behavior across the whole temperaturerange. This is the first time that such resistance behaviorhas been seen for organic conductors[33] and led to the con-clusion that the studied compound can be characterized asan incoherent electron transport material.

Figure 14. Temperature dependence of the relative resistivity of the(BETS)4HgBr4(C6H5Cl) crystal. Upper and lower solid lines arethe data for interlayer and in-plane resistivity, respectively. The ar-rows mark the first-order transition. The thick black line representsinterlayer resistance of the single crystal from another synthesis.Reprinted from ref.[33] with permission from Elsevier.

Good-quality Shubnikov-de Haas (SdH) quantum oscil-lation spectra were measured in the temperature range of1.6 to 4.2 K in magnetic fields up to 54 T,[35] despite thedisorder induced by the phase transition at 240 K. The Fou-rier spectra of the oscillatory part of the magnetoresistanceare displayed in Figure 15. The spectra evidence eight fre-quencies. According to the FSs of Figure 13, spectra com-posed of linear combinations of closed tubes (α orbit) andmagnetic breakdown orbits (β) with an area equal to thatof the first Brillouin zone should have been observed.[45]

At variance with this statement, the Fourier componentsobserved in Figure 15 cannot be analyzed on this basis.

Alternatively, other orbits could be considered.[35] For ex-ample, magnetic breakdown-like orbits induced by coherent


Figure 15. Fourier spectra of the oscillatory part of magnetoresis-tance. The field range is (a) 10–54, (b) 18–54, and (c) 45–54 T. Thecurves are shifted down from each other for clarity. Reprinted fromref.[35] with permission from Elsevier.

carriers that jump from one layer to another could accountfor the appearance of additional frequencies. Such a phe-nomenon would be similar to the bilayer splitting phenome-non already proposed in the case of cuprates with highTc.[48] Clearly, such a hypothesis requires both experimentaland theoretical justification. The possibility of an ad-ditional phase transition at temperatures below 200 K, suchas density wave or solvent molecule ordering, could also beconsidered. Such modulation could, for example, induce afolding of the FS, which results in the appearance of ad-ditional small closed orbits and accounts for the observeddiscrepancy between the SdH spectra and the results of thepresent band structure calculations.

According to the above definition of coherent/incoherenttransport, this Q2D metal, (BETS)4HgBr4(C6H5Cl), can bereferred to as a dual-layered organic compound with twoconducting layers and incoherent electron transport.

5.2. θ-(BEDT-TTF)4CoBr4(C6H4Cl2) � A Dual-LayeredCompound with One Conducting Layer

As was shown above, it is impossible to synthesize high-quality Q2D organic metals for analysis at helium tempera-ture when there is a phase transition below room tempera-ture (because of twinning). The synthesis of θ-(BEDT-TTF)4-CoBr4(C6H4Cl2), which undergoes phase transition aboveroom temperature and, as a result, is stable over a widetemperature range below the phase transition, provides agood possibility for analyzing magnetoresistance and quan-tum oscillations with high-quality crystals.[45] Figure 8shows the projection of the crystal structure at 295 K alongthe a direction. This compound shows no phase transitionfrom room temperature down to 100 K, with a first-orderphase transition at 320 K from triclinic to tetragonal syn-gony.

The main peculiarity of this compound is the fact thatits unit cell contains two different donor layers, labeled Aand B. In contrast to layer B, one of the stacks in layer A


contains two BEDT-TTF units with different inclinationswith respect to the stack direction, as in most α-typeBEDT-TTF salts. The crystal structures and electronicband structures of both layers are shown in Figure 16.

Figure 16. Arrangement of the BEDT-TTF molecule planes andcalculated band structures relevant to metallic layer B and insulat-ing layer A of the triclinic structure of θ-(BEDT-TTF)4-CoBr4(C6H4Cl2). The dashed lines mark the top of the HOMOband and the Fermi level for layer A and layer B, respectively. Lab-els Δ0 and Δ1 represent the band extremes. Γ, X, Y, M, and S referto (0, 0), (a*/2, 0), (0, b*/2), (a*/2, b*/2), and (a*/2, –b*/2), respec-tively. Reprinted from ref.[45] with permission from the EuropeanPhysical Society.

As shown in Figure 16 for layer B, the two upper bandsoverlap and the Fermi level cuts both bands leading to com-pensated holes and electrons and consequently this layer ismetallic, at variance with layer A. (As reported in the cap-tion, the dashed line in the band structure of layer A is notthe Fermi level but the top of the HOMO band.) Thereforelayer A should be a small-gap semiconductor.

The FS of metallic layer B (Figure 17) is composed ofone closed tube centered at Y with an area of 18% of thefirst Brillouin zone (FBZ) area (yielding an α orbit in themagnetic field) and two Q1D sheets with a rather small gapin between. Therefore it is expected that the magneticbreakdown (MB) orbit β, which corresponds to the holetube centered at Γ with an area equal to that of the FBZ,is observed in moderate fields, giving rise to a linear chainof coupled orbits introduced by Pippard[49] in the early sixt-ies.

Figure 17. Fermi surface for layer B of the compound θ-(BEDT-TTF)4CoBr4(C6H4Cl2). Reprinted from ref.[45] with permissionfrom the European Physical Society.

Magnetic torque (dHvA) oscillations and magnetoresis-tance (SdH oscillations) were measured in pulsed magneticfields up to 55 T in the temperature range of 1.4 to 4.2 K.The magnetic torque measurements were performed with a


piezoresistive microcantilever[50] and the magnetoresistancewas measured by using a contactless radio frequency mea-surement technique based on a tunnel diode oscillator(TDO).[51] The Fourier analysis of these measurements isshown in Figure 18. Besides Fourier components linked tothe closed orbit α and the MB orbit β, predicted by bandstructure calculations of Figure 17, additional linear combi-nations of α and β are observed in the high-field range ofthe torque spectra. As for TDO spectra, numerous combi-nations are observed (Figure 18, b). Detailed analysis of thefield and temperature dependences of the torque and TDOspectra is given in ref.[45]

Figure 18. Fourier analysis in the field range 35–55.3 T of (a) themagnetic torque data (dHvA oscillations) and (b) the tunnel diodeoscillator data (SdH oscillations) reported in the respective insets.Solid triangles represent calculations with Fα(θ = 0°) = 0.944 kTand Fβ(θ = 0°) = 4.60 kT. Data are shifted down from each otherby a constant value. Reprinted from ref.[45] with permission fromthe European Physical Society.

In contrast to θ-(BETS)4HgBr4(C6H5Cl), all the fre-quencies obtained by this Fourier analysis (Figure 18) arelinear combinations of the main closed orbit α and MBorbit β. All these frequencies correspond to the Fermi sur-face relevant to layer B (Figure 17) and there is no effect ofneighboring layer A in these spectra. Because layers A areinsulating, the conducting planes are well separated fromeach other, which suggests a strong 2D behavior.

Figure 19 shows the temperature dependence of the in-ter- and intralayer resistances normalized to room tempera-ture. At this temperature, the anisotropy of resistivity, R(in-terlayer)/R(intralayer), is 104.[38] As the temperature is re-duced to liquid helium temperature, the anisotropy furtherincreases to R(interlayer)/R(intralayer) = 106. This aniso-tropy is high and is linked to the fact that the shortest dis-tance between the nearest conducting layers is around 26 Å.A kink is seen for R� near the helium temperature (Fig-ure 19), and although the origin of the kink is so far un-known, it does not prevent a perfect coincidence betweenquantum oscillation and FS data calculated at high tem-peratures.[45]

In summary, dual-layered θ-(BEDT-TTF)4CoBr4-(C6H4Cl2) with one conducting layer demonstrates metallicintralayer resistance (dR�/dT� 0) and nonmetal interlayerresistance (dR�/dT � 0) across the whole temperature rangeand can be characterized as an incoherent electron trans-port material.


Figure 19. Temperature dependence of zero-field intralayer (lowercurve) and interlayer (upper curve) resistance normalized to thevalues at 290 K for θ-(BEDT-TTF)4CoBr4(C6H4Cl2). Reprintedfrom ref.[45] with permission from the European Physical Society.

5.3. Compensated Q2D Organic Metals

To compare the dual-layered Q2D organic conductorsdiscussed above with layered Q2D organic conducting sys-tems with compensated orbit networks we selected the(BEDT-TTF)8Hg4X12(C6H5Y)2 (X,Y = Cl, Br) family.[52]

Substitution of Cl and Br yields four isostructural salts thatdiffer considerably in their physical properties. The com-pound with X = Y = Cl is metallic down to 1.4 K, whereasthe compound with X = Y = Br undergoes a metal–insu-lator phase transition at 160 K. Figure 20 shows a unit cellin the bc plane, connected to a conducting donor layer forcompound X = Y = Cl. This unit cell contains eight BEDT-TTF molecules linked pairwise to the inversion center fi-nally resulting in four independent BEDT-TTF moleculesA, B, C, and D (Figure 20, right).

The unit cell can be defined by a number of parallelchains in different directions: Along the α direction (slippedchain), along the γ direction (σ chain), and along the β di-rection (π chain). Estimation of the transfer integrals forthe short contacts shown in Figure 20 for the α, β, and γdirections permitted the energy dispersion to be calculatedfor (BEDT-TTF)8Hg4Cl12(C6H5Cl)2 in terms of the ex-tended Hückel tight-binding band structure approxi-mation.[53] Figure 21 shows the distribution of the HOMOlevels close to the Fermi level (dashed line). It can be seenthat the sixth and seventh energy levels cross the Fermilevel. Therefore the compound (BEDT-TTF)8Hg4Cl12-

Figure 20. Left: View of the BEDT-TTF molecules in the conducting layer of (BEDT-TTF)8Hg4Cl12(C6H5Cl)2. Every molecule is orientedapproximately along the central C=C bond. Right: Schematic representation of the left side, showing different types of molecules anddonor–donor intermolecular interactions. Reproduced from ref.[53] with kind permission of the European Physical Journal (EPJ).


(C6H5Cl)2 has to be at least a semimetal. The calculatedenergy spectrum yields the Fermi surface shown in Fig-ure 22.

Figure 21. Dispersion relationship of the two HOMO bands calcu-lated for the donor layer of (BEDT-TTF)8Hg4Cl12(C6H5Cl)2. Thedashed line represents the Fermi level. Reproduced from ref.[53]

with kind permission of the European Physical Journal (EPJ).

Figure 22. Fermi surface according to band structure calculations[reproduced from ref.[53] with kind permission of the EuropeanPhysical Journal (EPJ)]. In addition to the elongated electron andhole closed orbits (a orbits), δ and Δ are indicated, which corre-spond to the basic frequencies of SdH oscillation spectra. E1 andE2 represent the gaps between the hole and electron orbits. Γ, Z,Y, S, and M are parameters of the first Brillouin zone.

The room-temperature Fermi surface of (BEDT-TTF)8-Hg4Cl12(C6H5Cl)2 results from the hybridization of twopairs of hidden quasi-1D sheets. The final Fermi surfaceobtained after raising the degeneracy of these sheets is con-structed from one hole and one elongated electron tube cen-tered around Z and M (or S), respectively, in the first


Brillouin zone (FBZ). Although the cross-sectional area ofboth the electron and hole tubes is 13% of the FBZ area,the resulting orbits do not share the same topology and areseparated from each other by two unequal gaps labeled E1

and E2.[53]

Provided that these gaps are not too large, magneticbreakdown between electron and hole orbits can occur in amagnetic field. In addition to quantum oscillations linkedto the electron and hole closed orbits, this breakdown maygive rise to many oscillation frequencies that can be ac-counted for either by the coupled orbits model ofPippard[49] and Falicov and Stachowiak[54] or by quantuminterference.[55]

Interlayer magnetoresistance of the Q2D organic metal(BEDT-TTF)8Hg4Cl12(C6H5Cl)2 was analyzed in pulsedmagnetic fields extending up to 36 T in the temperaturerange of 1.6 to 15 K.[52] A very complex oscillatory spec-trum, constructed of linear combinations of only three ba-sic frequencies, is observed. These three frequencies arisefrom the compensated closed hole and electron orbits (a),a “forbidden” orbit δ, and the quantum interference path bwith an area equal to that of the FBZ. The Fourier spectrain Figure 23 evidence more than 15 frequencies that, inagreement with the 2D nature of the FS, follow the cosinelaw F(θ) ≈1/cosθ, in which θ is the angle between the mag-netic field direction and the normal to the conductingplane.

Within the framework of the coupled orbits model ofFalicov and Stachowiak,[54] the field and temperature de-pendences of the amplitude of various oscillation series re-veal that they result from the contribution of either the con-ventional Shubnikov–de Haas effect (SdH) or quantum in-terference (QI), both of these being induced by magneticbreakdown. Nevertheless, there are some discrepancies be-tween the experimental and calculated parameters that indi-cate that these phenomena alone cannot account for all ofthe data. One of the most fascinating frequencies is associ-ated with the FBZ area and corresponds to the quantuminterference path b, which can be subscribed by more thanfive different trajectories. All these interferometers with dif-ferent topologies have different magnetic breakdown damp-ing factors. Because the effective mass relevant to a quan-tum interference path is equal to the difference in the effec-tive mass of each arm,[55] the effective mass connected withthe b orbit is very small. For this reason, the b frequency isclearly observed up to 13 K. All the peculiarities of theFS of the Q2D organic metals (BEDT-TTF)8Hg4Cl12-(C6H5X)2, (X = Cl, Br), related to the magnetic field, tem-perature, effective masses, Dingle temperature, and pressure,are described in refs.[52,56,57]

Concerning the FS topology and magnetic oscillations, itis necessary to remark that the metallic ground state is sta-bilized in (BEDT-TTF)8Hg4Cl12(C6H5Cl)2 down to thelowest temperatures. Indeed, the investigated intralayer (R�)and interlayer (R�) resistances exhibit metallic behaviordown to T = 1.4 K with a residual resistivity ratio equal toaround 100 and without any sign of nesting of either elec-tron or hole tubes.[58] Thus, this typical Q2D organic com-


Figure 23. a) Fourier transform spectra deduced from the data ofthe oscillatory part of the magnetoresistance at around 1.7 K fordifferent magnetic field orientations. θ is the angle between themagnetic field direction and the normal to the conducting plane.The magnetic field window is 10–30 T. b) Fourier transform spectraat different temperatures for θ = 0° in the magnetic field range 18–35.7 T. Frequency combinations involving a, b, and δ orbits areindicated in the figure. Full triangles represent calculations with Fa

= 241.5 T, Fδ = 149 T, and Fb = 2185 T. Note that the Fouriercomponent b = 2a + δ + Δ and corresponds to the area of theFBZ. Reproduced from ref.[52] with permission from the AmericanPhysical Society.

pound, with a monoconducting layer and both dR�/dT �0and dR�/dT� 0 across the whole temperature range, can beassigned to a coherent transport system.

6. Conclusions

In this microreview we have presented a family of layeredorganic conductors (D)4MBr4(C6H6–nXn) in which metallicbehavior of resistivity is observed within organic layers withnonmetallic behavior observed perpendicular to the layers.Some of these conductors, namely, (BEDT-TTF)4-MBr4(C6H4Cl2) and all the BETS-based salts, show intra-


layer metallic behavior of resistivity across the whole tem-perature range. In the other salts, intralayer resistivityshows metallic behavior down to 70–100 K, and belowthese temperatures R� becomes semiconducting in behavior.

A remarkable feature of these conductors is the alter-nation of conducting layers with differently oriented stacksinside each layer (i.e., different stacking directions in neigh-boring organic layers). This structure is realized due to aunique combination of a donor (BEDT-TTF or BETS), atetrahedral [MBr4]2– anion, and a planar halobenzene mo-lecule. During (D)4MBr4(C6H6–nXn) crystal growth, the di-valent [MBr4]2– anion, with a tetrahedral configuration ofthe M–Br bonds, plays a dominant role. Two of the Bratoms of [MBr4]2– form four short Br···H contacts with theradical cations of one conducting layer, whereas the othertwo Br atoms of the same [MBr4]2– anion similarly formfour short Br···H contacts with radical cations of the neigh-boring layer. As a result, the direction of stacks in one con-ducting layer is perpendicular to that of the nearest-neigh-boring layer. These H···Br contacts are likely responsible forthe specific molecular packing shown by the crystal struc-ture.

Another remarkable feature of this family of conductorsis their structural phase transitions. Above the phase-transi-tion temperature (Tp), the compounds (D)4MBr4(C6H6–n-Xn) exhibit tetragonal syngony, are isostructural, and arecomposed of the same neighboring conducting layersturned by 90° with respect to each other. Below the phase-transition temperature, the compounds show triclinic ormonoclinic syngony.

For C6H5X-containing compounds, the differences be-tween neighboring layers are not great below Tp and bothlayers show metallic resistance, as deduced from Fermi sur-face calculations, for example, for (BETS)4HgBr4(G6H5Cl).These dual-layered compounds have two conducting layers.The effect of the cationic part of the structure on the transi-tion dominates in these conductors.

Fermi surface calculations for C6H4Cl2-containing com-pounds, for example, for (BEDT-TTF)4CoBr4(C6H4Cl2),have shown that half of the cation layers are metallicwhereas the other half are small-gap semiconductors. Thus,these dual-layered compounds have only one conductinglayer. The phase transition in (BEDT-TTF)4-MBr4(C6H4Cl2) originates from ordering in the anion lay-ers.

For dual-layered compounds with two conducting layers,for example, θ-(BETS)4HgBr4(C6H5Cl), magnetoresistanceand magnetization measurements at the helium temperaturereveal rather complex Fourier spectra. The spectra can berelated to frequencies arising from quasiparticles jumpingfrom one layer to the next. For a dual-layered compoundwith one conducting layer, θ-(BEDT-TTF)4CoBr4-(C6H4Cl2), the quantum oscillations are in agreement withthe Fermi surface calculations, which show that the unit cellof this compound includes one metallic and one insulatinglayer. In that respect, this compound provides a textbookFermi surface for quantum oscillations physics. For com-parison, (BEDT-TTF)8Hg4Cl12(C6H5Cl)2, a typical mono-


layered Q2D organic metal, demonstrates a 2D network ofcompensated electron and hole orbits that give Fourierspectra with more than 15 frequencies that can be describedby a linear combination of three basic frequencies.

The fact that the resistance of the dual-layered organicmetals (D)4MBr4(C6H6–nXn) measured along the layer (R�)decreases with decreasing temperature but the resistancemeasured across the layer (R�) increases at the same timeimplies that the resistance of these salts has incoherentcharge transport (dR�/dT �0 and dR�/dT� 0), whereas theresistance of the monolayered compound has coherent elec-tron transport (dR�/dT�0 and dR�/dT �0).

To the best of our knowledge, two other layered organicconductors based on electron-accepting molecules, Ni-(dmit) and C60, with metallic behavior of resistance in theorganic layers and nonmetallic behavior perpendicular tothe layers down to 80 and 200 K, respectively, have beendescribed. Both of them have a dual-layered structure.[24,25]

In the radical-anion salt (Me-3,5-DIP)[Ni(dmit)2], I···S in-teractions between the asymmetric cation of Me-3,5-DIPand the Ni(dmit)2 anions play a dominant role in the for-mation of two kinds of layers of Ni(dmit)2 anions.[24] In theQ2D dual-layered organic metal (MDABCO+)(C60

·–)(TPC)(MDABCO+ = N-methyldiazabicyclooctane cation, TPC =triptycene), the existence of two kinds of hexagonal fuller-ene layers is related to the peculiarity of its orientationalordering.[25] The theory of strongly correlated processes isfar from being completely understood. Probably, a com-parison of the properties of dual-layered organic (based onBEDT-TTF and alternative molecules) and inorganic con-ductors is a way to gaining a better understanding of themechanism of interlayer charge carrier transport.

Acknowledgments

This work was supported by the Program of Fundamental Re-search of Presidium of RAS No. 8, by the CNRS-RAS inter-national project of scientific cooperation PICS (grant 5708); Euro-MagNET II under the European Union (EU) (contract number228043). Work at Bellaterra was supported by Ministerio de Econ-omía y Competitividad (MINECO) (grant numbers CSD2007-00041 and FIS2012-37549-C05-05).

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Received: February 11, 2014Published Online: April 28, 2014

Dual-Layered Quasi-Two-Dimensional Organic Conductors with Presumable Incoherent Electron Transport

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