LnCl(CN2) with Ln � La, Ce, and Pr: Synthesis and Structure of a NewLanthanide Chloride Cyanamide Related to the PbFCl-Type Structure

Radhakrishnan Srinivasan, Jochen Glaser, Sonja Tragl, H.-Jürgen Meyer*

Tübingen, Abteilung für Festkörperchemie und Theoretische Anorganische Chemie am IAC der Universität

Received July 27th, 2004.

Professor Arndt Simon zum 65. Geburtstag gewidmet

Abstract. Solid state metathesis reactions between LaCl3 andLi2(CN2) carried out in silica ampoules at 600 °C over two daysresulted in the formation of a light gray powder, later identified tobe lanthanum chloride cyanamide. Single crystals were obtained ascolourless transparent plates using a LiCl-KCl flux. The crystalstructure of LaCl(CN2) was solved and refined from X-ray singlecrystal data (P21/m (no. 11), Z � 2, a � 5.330(1) A, b �

4.0305(8) A, c � 7.545(1) A, V � 159.24(5) A3, β � 100.75(2)°).The structure of LaCl(CN2) is closely related to the tetragonalLaOCl (PbFCl type) structure and contains a [La2Cl2]4� block

LnCl(CN2) mit Ln � La, Ce und Pr: Synthese und Struktur eines neuen, mit demPbFCl-Strukturtyp verwandten, Lanthanoidchloridcyanamids

Inhaltsübersicht. Metathesereaktionen der Festkörper LaCl3 undLi2(CN2), die in Kieselglasampullen bei 600 °C über zwei Tagedurchgeführt wurden, führten zur Bildung eines hellgrauen Pulvers,welches später als Lanthanchloridcyanamid identifiziert wurde.Einkristalle wurden unter Verwendung des Flussmittels LiCl-KClals farblose durchsichtige Plättchen erhalten. Die Kristallstrukturvon LaCl(CN2) wurde mittels Röntgenbeugung an Einkristallen ge-löst und verfeinert (P21/m (no. 11), Z � 2, a � 5,330(1) A, b �

4,0305(8) A, c � 7,545(1) A, V � 159,24(5) A3, β � 100,75(2)°).Die Struktur von LaCl(CN2) ist eng mit der tetragonalen LaOCl-

Introduction

Compounds with the matlockite (PbFCl) structure type aretechnologically interesting materials. For example, Sm2�- orEu2�-doped BaFX (X � Cl, Br, I) crystals are well estab-lished optical luminophores or can be used as X-ray phos-phors in imaging plate systems [1]. The structures of lantha-nide oxide halides such as LnOX [2] (Ln � lanthanide and

* Prof. Dr. H.-Jürgen MeyerUniversität TübingenAbteilung für Festkörperchemie und Theoretische AnorganischeChemieInstitut für Anorganische ChemieAuf der Morgenstelle 18D-72076 TübingenTel.: 07071-29-76226Fax. 07071-29-5702e-mail: juergen.meyer@uni-tuebingen.deWeb-Seite: http://www.uni-tuebingen.de/AK-Meyer

Z. Anorg. Allg. Chem. 2005, 631, 479�483 DOI: 10.1002/zaac.200400323 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 479

layer and two layers of (CN2)2� ions in an alternating sequence.The La3� ion in LaCl(CN2) is situated in a polyhedron composedof four chloride ions on one side and five cyanamide ions on theother side. Cerium and praseodymium analogues were identifiedfrom indexed powder patterns with analogous unit cells, assumingthem to form isostructural compounds.

Keywords: Cyanamide; Lanthanide; Metathesis; PbFCl type;Synthesis

Struktur (PbFCl-Strukturtyp) verwandt. Sie enthält eine alternie-rende Abfolge aus einem Schichtpaket [La2Cl2]4� und zwei Schich-ten (CN2)2�-Ionen. Das Lanthan-Ion in LaCl(CN2) befindet sichin einem Polyeder, das auf einer Seite aus vier Chlorid-Ionen undauf der anderen Seite aus fünf Cyanamid-Ionen besteht. Für dieanalogen Verbindungen mit Cer und Praseodym wurden anhandihrer indizierten Pulverdiffraktogramme analoge Elementarzellenermittelt; es wird angenommen, dass sie isostrukturelle Verbindun-gen bilden.

X � Cl, Br, I), lanthanide fluoride sulfides [3], dysprosiumoxide fluoride sulfide [4], and lanthanum oxide nitrate [5]are some of the examples with structures related to thePbFCl structure type. The majority of the LnOX (X � Cl,Br, I) compounds exists in the tetragonal PbFCl structuretype (Ln � La � Ho) while the heavier lanthanides (Ln �Er � Lu) crystallize in the rhombohedral SmSI type. Thestructural features of all these compounds are their [M2X2]or [M2O2] block layers arranged in an alternating sequencewith anionic layers in different ways. A more general con-cept for the description of such structures has been pro-vided by Sillen [6] for oxide halides. This concept is worth-while when different numbers of individual layers and layersequences are involved. Sillen adopted the notation Xn

(n � 1, 2, 3) to indicate the number of (n) halide layersseparating the [M2O2] block layers, e.g. for the X2-type BiOI[7]. The oxygen atoms can be replaced by fluorine, as is thecase for the X2-type PbFX (X � Cl, Br). The structures ofPbFCl and PbFBr [8] were the first to be characterized by

R. Srinivasan, J. Glaser, S. Tragl, H.-J. Meyer

X-ray diffraction techniques within this structure family.The tetragonal La2O2(CN2) [9] structure also contains ablock layer of [La2O2]2� and one layer of (CN2)2� ions ar-ranged in an alternating sequence, and can be assigned tothe Sillen X1-type.

In this article, the synthesis of LaCl(CN2) and its struc-tural characterization by single crystal X-ray diffractiontechnique, as well as by IR spectroscopy and thermal analy-ses are reported. The employment of a LiCl-KCl flux [10]adopted from the syntheses of LnOCl compounds withLn � Y, Ho, Er, Tm, Yb was considered as being favorablefor crystal growth.

Experimental Section

Preparative Methods

Most reactions were carried out in fused silica ampoules. The silicaampoules were filled with the reaction mixture in an argon filledglove box and then sealed under vacuum. The reaction timesranged from one day to several weeks at temperatures up to 700 °C.Some reactions in the temperature regions of 650 °C and abovewere carried out in tantalum containers, especially when a LiCl-KCl flux was involved. The reaction procedures explained in theprevious literature [11] are applied here similarly. Most of the reac-tions below 700 °C were performed in a self made silica furnaceequipped with an Eurotherm temperature controller. Reactions at700 °C and above were carried out in a commercial (Carbolite)furnace. The samples were heated within one day and then cooledto room temperature within one or two days after the respectivereaction time. The employment of pure LaCl3 was vital to obtainLaCl(CN2) as the major phase. LaCl3 obtained from La2O3

(Ventron, 99.5 %) through the NH4Cl/HCl route [12] was eithersublimed under vacuum or purified with AlCl3 (Alfa Aesar, sub-limed) [13] before the reaction. The same procedure was appliedalso for CeCl3 and PrCl3. Li2(CN2) was synthesized as describedearlier [11].

Synthesis of LaCl(CN2)

The reaction between LaCl3 and Li2(CN2) according to (1) wascarried out in a silica ampoule at 600 °C over two days to obtain alight gray powder, later found to be lanthanum chloride cyanamide.

LaCl3 � Li2(CN2) � LaCl(CN2) � 2 LiCl (1)

The ampoules were opened using a glass cutter inside the glove boxunder Ar. The obtained powder was analyzed by X-ray powderdiffraction and IR measurements. The reactions can be successfullyperformed in the temperature range of 500-700 °C over the periodof 2-3 days. Later on, well developed transparent colourless plate-like single crystals were obtained when a 2:3 molar ratio of LaCl3and Li2(CN2) (164 mg) was treated in the presence of a LiCl-KClflux (29 mg) at 650 °C for seven days in a tantalum ampoule.LaCl(CN2) is air and water stable. LiCl and the added flux canbe washed off with water before drying the residual LaCl(CN2) inan oven.

Powder diffraction and crystal structure determination

All phase analyses were done by X-ray powder diffraction tech-niques. The powder patterns were recorded on a StadiP dif-

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 479�483480

fractometer (STOE), using germanium monochromated Cu-Kα1

radiation (λ � 1.540598 A) and a position sensitive X-ray detector(opening angle: 2θ � 6°). The powdered samples of LaCl(CN2)were washed in ethanol, dried overnight, and finally placed betweentwo mylar foils for the X-ray powder pattern measurements. Rou-tine analyses were done in the 2θ range between 10 and 60°. As anew structure was evident from the powder pattern, a measurementin the 2θ range of 5-130° was performed with step increments of0.2° and exposure periods of 120 seconds. All reflections were inde-xed using Louer’s algorithm (DICVOL) in a monoclinic cell witha � 7.5403(9) A, b � 4.0286(3) A, c � 5.3271(6) A, β �

100.728(7)°, V � 158.99(4) A3 for LaCl(CN2) except for a few un-identified low intensity reflections. The number of single indexedlines obtained was 47 with a figure of merit of 120 for 50 selectedreflections. The measured X-ray powder pattern of LaCl(CN2) isin agreement with the calculated pattern as shown in Figure 1.Suitable colourless transparent single crystals of LaCl(CN2) wereselected under a microscope and mounted on the tip of glass fibersfor X-ray diffraction studies. Single crystal measurements were per-formed on an IPDS (STOE) using graphite monochromated Mo-Kα radiation (λ � 0.71073 A). Selected crystal data and measuringconditions are provided in Table 1. The atomic positions along with

Figure 1 The measured X-ray powder pattern of LaCl(CN2) (above)in comparison to the calculated powder pattern (below).

Table 1 Selected crystal data and measuring conditions.

Space group (no.), Z P21/m (11), 2Lattice constants /A, angle /° a � 5.330(1), b � 4.0305(8), c � 7.545(1),

β � 100.75(2)Cell volume /A3 159.24(5)Density calc. /g·cm�3 4.471Molecular weight /g·mol�1 214.38Crystal appearance transparent colourless platesCrystal size /mm3 0.12 x 0.12 x 0.04Diffractometer STOE, IPDSRadiation, temperature Mo-Kα (λ � 0.71073 A3),

graphite monochromator, 293(2) KRange: θ /° 3.89 to 30.44Index range �7 � h � 7, �5 � k � 5, �10 � l � 10Data correction Lorentz, polarisation and absorptionµ /mm�1 13.98Collected reflections 3021(F0 > 2σ(F0))Unique reflections 513Parameters refined 31 (all atoms refined anisotropically)R indices (all data) R1a � 0.0215, wR2b � 0.0506Final R indices [I > 2σ(I)] R1a � 0.0195, wR2b � 0.0496GooF (all reflections) 0.961Res. peak: max.; min. /e/A3 1.265, �1.235

a R1 � Σ�Fo� � �Fc� / Σ�Fo�; b wR2 � [Σ w(Fo2 � Fc

2)2 / [Σw(Fo2)2]]1/2

LnCl(CN2) with Ln � La, Ce, and Pr

Table 2 Atomic coordinates and isotropic equivalent displace-ment parameters /A2 for LaCl(CN2).

Atom x y z Ueqa

La 0.82426(4) 0.25 0.26333(3) 0.0095(1)Cl 0.2558(2) 0.25 0.0435(2) 0.0136(2)N1 0.8787(7) 0.25 0.6186(6) 0.0124(7)N2 0.4329(7) 0.25 0.6595(6) 0.0130(7)C 0.6521(8) 0.25 0.6412(6) 0.0111(7)

a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Table 3 Selected bond lengths/A and bond angle/° in LaCl(CN2).

La�N1 2.614(3) �2 C�N1 1.251(6) �1La�N1 2.642(4) �1 C�N2 1.202(6) �1La�N2 2.565(2) �2 N2�C�N1 178.8(5)La�Cl 3.0385(9) �2La�Cl 3.077(1) �1La�Cl 3.172(1) �1

Table 4 Cell parameters/A from powder patterns of LnCl(CN2)with Ln � La, Ce, Pr.

Ln a b c β(°) V (A3) No. ofindexed lines

La 5.3297(6) 4.0299(5) 7.545(1) 100.711(9) 159.23(5) 56Ce 5.322(3) 4.022(2) 7.530(5) 100.75(4) 158.3(2) 35Pr 5.296(2) 3.934(1) 7.460(2) 100.43(1) 152.8(1) 21

the isotropic equivalent displacement parameters are given in Table 2.Selected bond lengths and the N-C-N bond angle are shown inTable 3. Details on the crystal structure investigations can be ob-tained from the Fachinformationszentrum Karlsruhe, Germany,D-76344 Eggenstein-Leopoldshafen (fax: (�49)7247-808-666;e-mail: crysdata@fiz-karlsruhe.de) on quoting the depository num-ber 413904 for LaCl(CN2). Cell parameters of the homologousLnCl(CN2) compounds with Ln � La, Ce, Pr are presented inTable 4.

Thermal analysis and infrared spectrum

Thermal analyses (DTA/TG) of the formation and decompositionreactions were performed using a Netzsch thermal analyser STA409 in the temperature range between 25 and 800 °C. In order tostudy the formation reaction of LaCl(CN2), the starting materialsLaCl3 and Li2(CN2) were carefully fused (under cooling with liquidN2) into a tiny self-made silica ampoule. The ampoule was thenmounted on the sample holder of the DTA apparatus. TheDTA/TG study of the decomposition reaction of LaCl(CN2) wasperformed with a corundum container in the presence of N2, usinga heating rate of 5 °C/min. LaCl(CN2) was washed well withethanol to remove LiCl and dried for 12 h before performing themeasurement.Infrared spectra were recorded with a Perkin Elmer FT-IR spec-trometer. The measurements for LaCl(CN2) were done in the rangeof 200-4000 cm�1 using KBr pellets.

Results and Discussion

Recently we have reported the synthesis and structure ofLa2Cl(CN2)N [11] while pursuing reactions between LaCl3

Z. Anorg. Allg. Chem. 2005, 631, 479�483 zaac.wiley-vch.de © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 481

and Li2(CN2), originally designed for the synthesis of lan-thanum cyanamide, La2(CN2)3 [14]. As no such lanthanumcyanamide was obtained, we now investigate the metathesisreactions [15] in the system LaCl3/Li2(CN2) at compara-tively low temperatures to study the reaction pathways. Ourpresent reactivity studies revealed that the (CN2)2� ions re-main intact up to 650 °C, when LaCl3 is heated withLi2(CN2) in a sealed silica ampoule with either 1:1 or 2:3molar ratios. An exothermic DTA peak is obtained near480 °C, when a 1:1 mixture fused in a silica ampoule isslowly heated (5 °C/min). This effect can be safely inter-preted as the formation reaction of LaCl(CN2), followed byan endothermic effect near 600 °C, which is due to the melt-ing of the co-produced LiCl. In the DTA of the decompo-sition of LaCl(CN2) a broad endothermic peak is obtainedin the temperature range of 650-700 °C (measured up to

Figure 2 Perspective views of the crystal structures of LaCl(CN2)(above) and LaOCl (below). The monoclinic unit cell is includedin the drawing for LaCl(CN2) and a non-conventional cell for tetra-gonal LaOCl. Lanthanum atoms are shown black, carbon atomswhite, nitrogen and oxygen atoms dark gray, chlorine atoms lightgray.

R. Srinivasan, J. Glaser, S. Tragl, H.-J. Meyer

800 °C). The X-ray powder diffraction studies on the result-ant dark gray powder showed the reflections ofLa2Cl(CN2)N (orthorhombic, a � 13.367(3) A, b �9.618(3) A, c � 3.966(1) A, V � 509.9(3) A3) as the majorphase along with LaOCl as the minor phase.

As explained in the synthesis part, well developed colour-less transparent plate-like single crystals were grown by aflux route. The crystal structure of LaCl(CN2) turns out tobe related to the tetragonal LaOCl structure (PbFCl type).The LaOCl structure corresponds with a Sillen X2-typestructure with an alternating sequence of one [La2O2]2�

block layer and two layers of Cl� ions. Similarly, the struc-ture of LaCl(CN2) contains one [La2Cl2]4� block layer andtwo layers of cyanamide ions being arranged in an alternat-ing sequence, as shown in Figure 2. In the structure ofLaCl(CN2), the La3� ion is surrounded by four Cl� ionsfrom one side and five (CN2)2� ions from the other side.The coordination number of the La3� ion in LaCl(CN2)and in LaOCl is nine, exhibiting the typical coordinationpattern that is well known for Pb2� in the PbFCl structure,as shown in Figure 3. The coordination environmentaround the (CN2)2� unit with three La neighbours fromone side and two La neighbours from the other side isshown in Figure 4. The variations within the C�N bondlengths and the N-C-N angle of the (CN2)2� ion inLaCl(CN2) are slightly outside the 3σ limit (1.251(6) A and1.202(6) A, and 178.8(5)°, see Table 3), and thus representsa cyanamide rather than a carbodiimide ion. However, we

Figure 3 Coordination arrangement around the La3� ion inLaCl(CN2) (left) and LaOCl (right).

Figure 4 Coordination environment of the (CN2)2� unit inLaCl(CN2). Atom colors are the same as in Figure 2 and 3.

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 479�483482

Table 5 Vibrational frequencies /cm�1 from selected cyanamidesalong with the title compound (a � carbodiimide).

Compound νas (CN2)2� νs (CN2)2� δ (CN2)2�

NCNH2 2218/2270 1152 604α-Sr(CN2) 1989/2023 1251 663/677Ba(CN2) 1947 1238 662/673Ag2(CN2) 1980 1191 633Hg(CN2) 1942/2036 1214 653/666Zn(CN2) 2048 1293 677/694Pb(CN2) 1950/1995 1216/1306 628/641Eu(CN2) 1969/2087 1244 655/666Li2(CN2)a 2152 � 575/653LaCl(CN2) 2033/2178 1252/1296 660La2Cl(CN2)Na 2010 � 625

note that the borderline between both forms of these ionscan be fluent. For a cyanamide we may regard a significantdifference in C�N bond lengths of the (CN2)2� ion, in-ducing the presence of one triple and one single bond,which is not quite the case here.

The presence of (CN2)2� in LaCl(CN2) was confirmed byinfrared spectroscopy. The characteristic infrared signals forthe (CN2)2� ion were observed and compared with someother cyanamide compounds known from the literature[16], listed in Table 5. In spite of the similar C�N bondlengths, the presence of the νs vibration indicates a cyan-amide for LaCl(CN2). In the carbodiimide containingLa2Cl(CN2)N, this mode is not permitted.

After the structural work on LaCl(CN2) was complete,the question arose, why this phase has not been observedin the previous set of reactions that have led to the dis-covery of La2Cl(CN2)N. The synthesis of La2Cl(CN2)N hasbeen already performed from three different starting mix-tures: (a) 2 LaCl3 � Li3N � Li2(CN2), (b) La2NCl3 �Li2(CN2), and (c) 2 LaCl3 � 3 Li2(CN2), all of them variedin stoichiometry and reacted at 750 °C or higher. Inreaction (a) La2NCl3 is being formed from 2 LaCl3 � Li3Nnear 500 °C and is an intermediate that can react furtherwith Li2(CN2) to yield La2Cl(CN2)N at above 750 °C, asobtained according to (b). A fragmentation reaction ofthe (CN2)2- ion is necessary in reaction (c) to yieldLa2Cl(CN2)N at around 800 °C [11].

The formation of LaCl(CN2) has been obtained in reac-tions similar to (c) from approximate 1:1 molar reactionsof LaCl3 and Li2(CN2) at 600 °C. According to its thermaldecomposition, LaCl(CN2) may be suspected to transforminto La2Cl(CN2)N under the release of cyanuric chloride.On the other hand, when Li3N was used along with the1:1 molar LaCl3/Li2(CN2) mixture at 650 °C for one week,La2Cl(CN2)N was obtained as the product without anyLaCl(CN2). Hence, the reactivity between LaCl3 andLi2(CN2) is clearly influenced by the presence of Li3N.

Syntheses of LnCl(CN2) type compounds were also per-formed by adding AlCl3 to a 1:2 molar reaction mixture of(non sublimed) LnCl3 (Ln � La, Ce, Pr) and Li2(CN2) in asilica ampoule at 600 °C over two days. The silica ampoulesused for the reactions were positioned in such a way thatthe cooler end resided outside the Simon-Müller furnace

LnCl(CN2) with Ln � La, Ce, and Pr

to induce a temperature gradient. The lanthanide chloridecyanamides remained at the hot end while AlOCl and AlCl3sublimed to the cold end of the silica tube. LnCl(CN2)phases were obtained as main products in these reactions,as determined by X-ray powder patterns. The indexed cellparameters of LnCl(CN2) with Ln � La, Ce, and Pr arepresented in Table 4. The lanthanide contraction obtainedin the trend of cell parameters suggests that the compoundsare isostructural.

Acknowledgements. This research was supported by DeutscheForschungsgemeinschaft (Bonn) through the project “Nitridocar-bonate“. One of the authors (R. Srinivasan) is thankful for thescholarship provided by the Landesgraduiertenförderung ofBaden-Württemberg.

References

[1] (a) K. Takahashi, J. Miyahara, Y. Shibahara, J. Electrochem.Soc. 1985, 132, 1492. (b) K. Takahashi, J. Luminescence 2002,100, 307.

[2] (a) D. H. Templeton, C. H. Dauben, J. Am. Chem. Soc. 1953,75, 6069. (b) G. Meyer, T. Schleid, Z. Anorg. Allg. Chem. 1986,533, 181. (c) H. Haeuseler, M. Jung, Mat. Res. Bull. 1986, 21,1291. (d) I. Mayer, S. Zolotov, F. Kassierer, Inorg. Chem. 1965,4, 1637.

Z. Anorg. Allg. Chem. 2005, 631, 479�483 zaac.wiley-vch.de © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 483

[3] (a) T. Schleid, H. Grossholz, Z. Anorg. Allg. Chem. 2001, 627,2693. (b) T. Schleid, Z. Anorg. Allg. Chem. 1999, 625, 1700.(c) T. Schleid, Z. Anorg. Allg. Chem. 2000, 626, 2429.

[4] T. Schleid, H. Z. Grossholz, Z. Anorg. Allg. Chem. 2002,628, 1012.

[5] J. M. Haschke, Inorg. Chem. 1974, 13, 1812.[6] L. G. Sillen, Z. Anorg. Allg. Chem. 1939, 242, 41.[7] L. G. Sillen, Svensk. Kem. Tidskr. 1941, 53, 39.[8] (a) W. Nieuwenkamp, J. M. Bijvoet, Z. Kristallogr. 1932, 82,

157. (b) W. Nieuwenkamp, J. M. Bijvoet, Z. Kristallogr. 1932,81, 469.

[9] Y. Hashimoto, M. Takahashi, S. Kikkawa, F. Kanamaru, J.Solid State Chem. 1995, 114, 592.

[10] E. Garcia, J. D. Corbett, J. E. Ford, W. J. Vary, Inorg. Chem.1985, 24, 494.

[11] R. Srinivasan, M. Ströbele, H.-J. Meyer, Inorg. Chem. 2003,42, 3406.

[12] (a) F. L. Carter, J. F. Murray, Mat. Res. Bull. 1972, 7, 519. (b)G. Meyer, Inorg. Synth. 1989, 25, 146.

[13] (a) H. Gunsilius, W. Urland, R. Kremer, Z. Anorg. Allg. Chem.1987, 550, 35. (b) D. Hake, W. Urland, Z. Anorg. Allg. Chem.1990, 586, 99.

[14] (a) H. Hartmann, W. Eckelmann, Z. Anorg. Allg. Chem. 1948,257, 183; (b) H. Hartmann, G. Dobek, Z. Anorg. Allg. Chem.1953, 271, 138.

[15] K. Gibson, M. Ströbele, B. Blaschkowski, J. Glaser, M.Weisser, R. Srinivasan, H.-J.Kolb, H.-J. Meyer, Z. Anorg. Allg.Chem. 2003, 629, 1863.

[16] O. Reckeweg, A. Simon, Z. Naturforsch. 2003, 58b, 1097.