Optical Materials 47 (2015) 244–250

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Optical Materials

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Electron Spin Resonance study of charge trapping in a-ZnMoO4 singlecrystal scintillator

http://dx.doi.org/10.1016/j.optmat.2015.05.0320925-3467/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: buryi@fzu.cz (M. Buryi).

M. Buryi a,⇑, D.A. Spassky b,c, J. Hybler a, V. Laguta a, M. Nikl a

a Institute of Physics AS CR, Cukrovarnicka 10, 162 00 Prague, Czech Republicb Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estoniac Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia

a r t i c l e i n f o

Article history:Received 19 February 2015Received in revised form 6 May 2015Accepted 19 May 2015Available online 3 June 2015

Keywords:Electron Spin ResonanceScintillatorCharge trapsZinc molybdate

a b s t r a c t

The origin and properties of electron and hole traps simultaneously appearing in a-ZnMoO4 scintillatorafter X-ray irradiation at low temperatures (T < 35 K) were studied by Electron Spin Resonance (ESR).ESR spectrum of the electron type trap shows pronounced superhyperfine structure due to the interactionof electron spin with nuclear magnetic moments of 95,97Mo and 67Zn lattice nuclei. Considering the nearlytetragonal symmetry of the center this allows us to identify the electron trap as an electron self-trappedat the (Mo(1)O4)2� complex. Nearly 60% reduction of the spin–orbit coupling at the Mo(1) ion is causedby the overlap of the Mo and ligand oxygen orbitals indicating an essential delocalization of the electronover the complex. Holes created by the X-ray irradiation form the O� type defects. Superhyperfinestructure of their ESR spectra shows the contributions from two groups of 95,97Mo nuclei and from25Mg nucleus as an uncontrolled impurity. It proves that namely the O(3) regular oxygen site istransformed into the O� center after the X-ray irradiation. Spectral parameters of the traps have beenanalyzed in the framework of the crystal field theory.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Zinc molybdate, namely the a-ZnMoO4 insulator, hassufficiently high Mo mass concentration in the host and no otherradioactive isotopes except the 100Mo. Consequently it fits therequirements for cryogenic phonon-scintillating detector applica-tions [1]. However, the light yield of this material estimated atcryogenic temperatures shows an unexpectedly low value of550 ph/MeV [2]. One of the reasons of the low light yield is thepresence of shallow traps active at cryogenic temperatures. Inthe recent publication [2] these traps were identified as: (i)(MoO4)3� complex created by an electron trapped at regular lattice(MoO4)2� tetrahedron; (ii) O� defect created by a hole trapped at aregular lattice oxygen anion. Important role of the charge traps inthe process of energy transfer was stressed. Preliminary ElectronSpin Resonance (ESR) data of the mentioned defects are also pro-vided in [2]. To the best of our knowledge, no other ESR study ofthe ZnMoO4 has been reported so far and the actual structureand spectral parameters of these defects are not known. Low crys-tal symmetry of ZnMoO4 (space group P�1) makes determination ofcorrect spectral parameters complicated.

In preceding years many works were dedicated to the study ofvarious irradiation-induced centers (charge traps) in promisingscintillating materials such as CaWO4, CdWO4, PbWO4 (PWO)[3–5], ZnWO4 [6], aluminum perovskites (YAlO3 (YAP) [7]) and gar-nets (Y3Al5O12 (YAG), Lu3Al5O12 (LuAG) as well as orthosilicates(Y2SiO5 (YSO) [8]). Their spectral parameters determined fromESR studies are summarized in [9]. The charge traps induced byirradiation of these materials were mostly O�-like hole and(WO4)3� and (MoO4)3�-like electron traps. Nevertheless it is worthnoting the presence of the complex (O–O)3� hole defect in theCdWO4 and (WO4–WO4)3�, ðWO�3 Þ

þ2 (divacancy with one electron)

and ðWO�3 Þ�2 (divacancy with three electrons) electron traps found

in the CaWO4 scintillators.Electron and hole trap centers can be formed in a host due to

the charge transfer and localization processes under sufficient irra-diation dose. Charge carriers may become self-trapped at regular(unperturbed) lattice sites. The self-trapped holes (STH) studiedin X-ray or c-irradiated CaWO4, UV irradiated CdWO4, X-ray irradi-ated ZnWO4 [6,9] were stable up to approximately 30–50 K. TheSTHs created in pure Y2SiO5 (YSO) [8] after X-ray irradiation, sur-vived up to approximately 130 K and are gradually transformedinto another, more stable O�-like center. The (WO4)3� polaronsin UV irradiated PbWO4 [3] and Ti3+ polarons in BaTiO3 [10] can

M. Buryi et al. / Optical Materials 47 (2015) 244–250 245

be mentioned as an example of the self-trapped electrons (STE).These STEs are thermally stable up to about 40–70 K.

However, the most of the charge trapped centers are usuallyformed close to impurities, and/or vacancies in the lattice, whichprovide an additional stabilization of the centers. The electronsand holes trapped in such a way demonstrate higher thermal sta-bility and survive at the trap to higher temperatures than theself-trapped charge carriers. For instance, (WO3)� electron trapcenters (W5+ ions in the vicinity of VO) in pure PWO [4] are stableup to room temperature. The (MoO4)3� impurity complexes (elec-tron trapped at Mo6+ ion) in both doubly doped PWO:Mo,La andPWO:Mo,Y withstand heating up to 260 K [5]. One can thus inferthat self-trapped charge carriers have activation energy muchlower than those stabilized by an imperfection nearby. However,in general, it is not easy to distinguish the self-trapped mechanismof charge trapping from that involving a defect. Such a problem canbe usually solved by performing the detailed study of local struc-ture of charge trapping center.

In the present work we provide complete characterization ofthe (MoO4)3� and O� defects created by X-ray irradiation at thecryogenic temperatures (T < 35 K) in the a-ZnMoO4. Experimentaldata were analyzed in the framework of the crystal field theorythat allowed to determine the parameters of electron-nuclei inter-actions in both types of defects and to clarify their local structures.Our data show that the (MoO4)3� centers are created by theself-trapping mechanism while a hole additionally stabilized atO� ion by a Mg impurity ion.

2. Samples and experimental

Zinc molybdate studied in this paper belongs to the a-ZnMoO4

structural type, space group P�1, triclinic system [11]. The unit cellis presented schematically in Fig. 1. It is composed of six ZnMoO4

molecules. The lattice constants are aR = 9.625 Å, bR = 6.965 Å,cR = 8.373 Å, a = 103.28�, b = 96.30�, c = 106.72�. The molybdenumatoms are surrounded by distorted oxygen tetrahedra creatingthree different types of Mo(1–3)O4 complexes (see Fig. 1). Zincatoms are embedded into the octahedral Zn(1) and Zn(2), and pen-tahedral Zn(3) positions (Fig. 1).

Fig. 1. Crystal structure of a-ZnMoO4. Red balls represent oxygen ions and bluedashed lines enclose the margins of one unit cell. The data on the atom positionswere taken from [11]. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

The zinc molybdate initial charge was prepared by a solid-phasesynthesis technique from the MoO3 and ZnO powders (both of99.995% purity). Single crystals were grown in the frameworks ofLUMINEU program from the stoichiometric melt by the low tem-perature gradient Czochralski technique. Since the crystallographicaxes (marked as aR, bR, cR in Fig. 2) do not form the orthogonal basisneeded for ESR measurements, the crystals were cut from the par-ent boule in three orthogonal planes (ab), (bc) and (ac) whichessentially differ from the real lattice directions. In the choice ofthe orthogonal set of axes we took advantage of the cleavage planeexistence. It is perpendicular to the vector c (reciprocal to the crys-tallographic vector cR). Other two axes are located in the planewhich is perpendicular to the c direction as it is shown in Fig. 2.Angular dependencies of ESR spectra were measured by rotationof samples with respect to these orthogonal axes. Continuous waveESR measurements were carried out with standard 3 cm wave-length ESR spectrometer at 9.25–9.28 GHz within the temperaturerange 20–35 K using an Oxford Instruments cryostat. Samples wereexposed to the X-ray irradiation directly in the spectrometer cavityat 30 K (this temperature is far below the traps stability threshold)for 30 min, using an X-ray tube (U = 55 kV, I = 30 mA).

3. Results and discussion

Prior to the X-ray irradiation there are no significant signals(Fig. 3) whereas strong resonance lines arise after the irradiationat 30 K earlier identified as belonged to an O� ion and (MoO4)3�

complex [2]. Both these paramagnetic centers will be consideredin the detail separately.

3.1. Spectral characteristics and local structure of the (MoO4)3�

complex

ESR spectrum corresponding to the molybdenum complex isshown in Fig. 4. The presence of hyperfine and superhyperfine(HF/SHF) structures beside the strong central line (nuclear spinI = 0) at the magnetic field value of 3517 G is clearly visible. TheHF structure originates from the interaction of an electron spinwith the nuclear magnetic moments of two intrinsic 95,97Mo iso-topes (I = 5/2), having nuclear moments ratio 0.98 and abundancesof 15.92%, and 9.55%, respectively (blue sextet in Fig. 4). The group

Fig. 2. Schematic view of how samples were cut from the as grown parent boule.The parallelepiped-shaped samples with specific faces orientation are distinguishedby color as well as numbers assigning sample numbers; a, b, c are the virtualorthogonal axes used for sample rotations in ESR measurements; c = 107.25� is theangle in the real primitive cell between crystallographic directions aR, bR; n = 2.5� isthe deflection angle of a from aR. a, b, aR, bR are vectors which belong to the sameplane. Vector c is reciprocal to the real cR (not shown in figure). (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

Fig. 3. ESR spectra in ZnMoO4 before (a) and after (b) X-ray irradiation at 30 K.

246 M. Buryi et al. / Optical Materials 47 (2015) 244–250

of satellite lines around the central line shown separately in theupper inset of Fig. 4, is the SHF structure originating from twoinequivalent 67Zn nuclei (I = 5/2, abundance 4.1%). To confirm theassignment of the satellite lines to Mo and Zn isotopes the spec-trum in Fig. 4 was simulated with using the spin Hamiltonian con-taining the Zeeman and (S)HF interaction terms:

H ¼ beSgH þX4

i¼1

SAiIi; ð1Þ

where be is the Bohr magneton, g is the g factor which determinesthe position of the central line, H is the magnetic field, S = 1/2 isthe electron spin, Ii and Ai are nuclear spins and HF constants ofMo and Zn isotopes. Intensities of the central line (I = 0) and HFlines (I – 0) were calculated taking into account natural abundancesof different isotopes. The spectrum was almost perfectly approxi-mated using the following HF parameters: A(95,97Mo) = 155.77 and159.05 MHz, A1(67Zn) = 10.52 MHz and A2(67Zn) = 15.78 MHz.

According to crystallographic data [11] the averaged Mo(i)–Odistance (here i = 1, 2, 3) is the longest for the Mo(1)–O bond(1.768 Å). Most probably, the Mo(1) states form the bottom ofthe conduction band. Therefore, we assume that namely Mo(1)ion traps an electron and creates a local electronic level in the for-bidden gap. Further thorough consideration of the SHF structurecoming from Zn nuclei supports this assumption.

Fig. 4. ESR spectrum of the (Mo(1)O4)3� complex (black line). Hyperfine structuredue to one pair of 95,97Mo nuclei (blue sextet) and superhyperfine structure from67Zn nuclei of two inequivalent Zn(2) and Zn(3) sites (shown in upper inset), areemphasized by the theoretical spectrum (red line), simulated for the spectralparameters A(95,97Mo) = 155.77; 159.05 MHz and A1,2(67Zn(2,3)) = 10.52 and15.78 MHz. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

The SHF interaction depends on the distance between anunpaired electron and a nucleus and usually comprises contribu-tions from the isotropic (known as Fermi contact term) and aniso-tropic (mostly dipolar type) parts which are both also proportionalto nuclear magnetic moment. Referring to the crystallographic data[11], the Mo(1)–Zn(3) distance (3.244 Å) is the shortest distancebetween the Mo(1) and neighboring Zn ions. This suggests thatnamely the Zn(3) position is responsible for the stronger 67ZnSHF interaction, giving the A2(67Zn(3)) = 15.78 MHz splitting inthe spectrum. Obviously, the second smaller 67Zn SHF splitting isproduced by Zn isotope in the Zn(2) position marked as ‘‘1’’ inFig. 5. This Zn ion is directly connected to the molybdenum com-plex via O(4) oxygen site, whereas the Zn(2) position marked as‘‘2’’ is unbound to the (Mo(1)O4)3� complex.

To determine the g and HF tensors of the (Mo(1)O4)3� complex,the angular variations of resonance magnetic fields were measuredin three orthogonal planes. Corresponding set of the angulardependencies is shown in Fig. 6. Only the HF lines from the molyb-denum nuclei could be thoroughly distinguished in the spectra dueto large splitting between HF lines. The SHF structure from twoinequivalent 67Zn nuclei was poorly resolved at most orientationsof the crystal with respect to the external magnetic field.Therefore its angular variations were not analyzed. The dependen-cies in Fig. 6 were described by the spin-Hamiltonian of the form

H ¼ beSgHþ SAI; ð2Þ

where S = 1/2 and I = 0 or 5/2 (for the 96Mo and 95,97Mo isotopes,respectively). The g and HF tensors principal values for the(Mo(1)O4)3� complex are collected in Table 1. One can see that boththe g and A tensors of the (Mo(1)O4)3� complex are axial in the prin-cipal axes frame. The fit is perfect for the central line which dependsonly on g tensor. However, there is some disagreement between theexperimental and calculated HF resonances, especially at high mag-netic fields (see Fig. 6). It could be related to the very low crystal

Fig. 5. Spheroidal cluster of ZnMoO4 crystal structure with the Mo(1) in the center(the cluster radius is 5.6 Å). Positions of two Zn(2) sites, marked as ‘‘1’’ and ‘‘2’’, andone Zn(3) site in relation to the Mo(1) site are shown. The O(3) oxygen position isemphasized by the black arrow. The Mo(3)–O(3) bond length is 1.842 Å and theMo(1)–O(3) distance is 2.796 Å [11].

Fig. 6. Angular dependencies of (Mo(1)O4)3� resonance fields taken at T = 35 K.Open circles and solid rectangles represent experimental data on the central linepositions (nuclear spin is zero) and the 95Mo HF lines, respectively. Blue and redsolid lines are theoretical curves calculated in ‘‘Easyspin 4.5.5 toolbox’’ program[12] and fitted to the corresponding experimental data. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 7. (a) (Mo(1)O4)3� complex (tetrahedron) is shown. (b) The scheme of energylevels created by the tetrahedral crystal field undergoing a tetragonal distortion.

M. Buryi et al. / Optical Materials 47 (2015) 244–250 247

symmetry represented by the space group P�1, where crystallo-graphic axes are not perpendicular to each other. This leads to slightmismatch of the principal axes of the g and HF tensors, which wereassumed to be the same for the both tensors in the spinHamiltonian (Eq. (2)). Nevertheless, in the further analysis of theHF and g tensors we will use only principal values of these tensors.The undistorted (Mo(1)O4)3� complex (tetrahedron) has tetrahedralsymmetry, point group Td (Fig. 7a). 2D free ion level of the Mo5+ (4d1

configuration) in the tetrahedral symmetry is split into doublet E(ground level) and triplet T2 (for details see e.g. [13]). The slightD2d tetragonal distortion, supported by the axial symmetry of gand HF tensors, removes degeneration of the doublet E completely,and, partly, of the triplet T2. Since g\ > g|| (see Table 1) the groundstate |0i is the non-degenerate B1, corresponding to the dx2�y2 typeorbital [14]. The other energy states, in accordance with the charac-ter table of D2d group, are A1 (dz2 ), B2 (dxy) and E (dyz;xz).

The g tensor values can be calculated by the following equationsassuming that the ground state is well separated from the excitedstates:

gii ¼ ge � 2kKii;

Kii ¼Xn–0

h0jLijnihnjLij0iEn � E0

; ð3Þ

where En and E0 are the energies of excited and ground states,respectively, D = En � E0 are the energy differences between levels,Li (i = x, y, z) is the orbital moment operator. The spin–orbit coupling(SOC) is represented by Kii and k is the SOC constant.

The excited energy state with non-zero contribution by the SOCto the ground state is determined by the direct product of the irre-ducible representations Cn � CL � C0, which should include atotally symmetric one. Here Cn, C0, and CL are the irreducible rep-resentations of the excited and ground states. Thus, the SOC

Table 1Spin Hamiltonian parameters of the (Mo(1)O4)3� complex and O�(3) defect determined fr

Center Nucleus gxx gyy

(Mo(1)O4)3� 95Mo 1.9275(5) 1.9275(5)97Mo

O� 95Mo(3) 2.0302(5) 2.0136(5)97Mo(3)95Mo(1)97Mo(1)

contribute to the ground state through Lz: B2 – dxy and Lx,y:E – dyz;xz. Constructing the wavefunctions of each level, one shouldaccount for some contributions from the oxygen ligand orbitals aswell by putting into consideration partial coefficients. Finally, thewavefunctions are:

j0i ¼ ajdx2�y2 i þ a0wl;

jn1i ¼ bjdxyi þ b0wl1; ð4Þjn2i ¼ cjdxzi þ c0wl2;

jn3i ¼ hjdyzi þ h0wl3;

where a, a0, b, b0, c, c0, h, h0, are the weight coefficients of the d andligand orbitals, wl are the ligand wavefunctions. Since the g factors(see Table 1) suggest weak tetragonal distortion, we can assumethat a � b � c � h. Then, after tedious algebraic manipulationsand assuming that the angular momentum of the central Mo5+ ionis significant, Eq. (3) becomes:

gjj ¼ ge �8ka4

D1;

g? ¼ ge �4ka4

D2; ð5Þ

where the coefficient a accounts for the reduction of the SOC due tothe overlap of the metal and ligand orbitals. From optical spectrameasured in [15], the cubic crystal field splitting isD0 � 2 eV = 16100 cm�1, D1 ¼ D0 � d1=2þ d2=2 andD2 ¼ D0 þ d1=2þ d2=2 (see Fig. 7b). Substituting g tensor valuesfrom Table 1 into Eq. (5), taking k = 1030 cm�1 and assumingd1 � d2 at weak tetragonal distortion, we obtainD1 = D0 = 16100 cm�1, D1/D2 = 0.82, D2 = 19700 cm�1 and a2 = 0.6.All these crystal field parameters are quite reasonable for Mo5+

ion. The reduction of the SOC is essential and indicates delocaliza-tion of the trapped electron over surrounding ligands. This agreeswell with the SHF structure of (Mo(1)O4)3� spectrum where thecontribution of surrounding Zn isotopes is sizable.

Since the ground level is non-degenerate, the HF tensor compo-nents can be written as follows [16]:

Ajj ¼ �K � P4a2

7� ge þ gjj

� �;

A? ¼ �K þ P2a2

7� ge þ g?

� �; ð6Þ

om the fit of experimental angular dependencies (Figs. 6, 9 and 10) by Eq. (2).

gzz |Ax|, (MHz) |Ay|, (MHz) |Az|, (MHz)

1.8208(5) 100(3) 100(3) 210(3)102(3) 102(3) 214(3)

2.0023(5) 26.5(5) 26.0(5) 25.5(5)27.1(5) 26.6(5) 26.0(5)7.0(3) 6.5(3) 8.0(3)7.1(3) 6.6(3) 8.2(3)

248 M. Buryi et al. / Optical Materials 47 (2015) 244–250

where K is the isotropic Fermi contact term. Using g||, g\, A||, A\ val-ues from Table 1 and the a2 value defined above, and consideringthat the parameter P < 0 for free Mo5+ ion [17] we deduced thatboth A||, A\ > 0. Under these conditions, we have determined fromEq. (6) for the 95Mo isotope: P = �178 MHz and K = �117 MHz.The isotropic contact term K determined above can be used for cal-culation of the parameter v, which characterizes the density ofunpaired spin at a nucleus [18]:

v ¼ 4pS

wX

i

dðriÞszi

����������w

!¼ �3

2hca3

0

gegnbebnK; ð7Þ

here a0 = 0.528 � 10�8 cm is the Bohr radius, h is the Planck constant,c is the speed of light in vacuum, ge = 2.0023, gn(95Mo) = �0.3656,be, bn are the Bohr and nuclear magnetons, respectively. Thenv = �7.02, which is in a value range obtained for the Mo5+ oxygencomplexes in other crystals (see, e.g. [17]).

Note that the value of P is about 11% smaller than that expectedfor the free Mo5+ ion, P = �(201�204) MHz [17]. This implies theoxidation state of the central Mo5+ ion to be a little smaller thanthe formal charge state 5+ due to the charge transfer in the Mo–O bonding orbitals. Note that both HF and SHF structures do notshow presence of any perturbing impurities in the vicinity of the(Mo(1)O4)3� complex. We can thus conclude that an electron isself-trapped. Possible stabilizing intrinsic defect, like oxygenvacancy, is excluded as well as it would produce a lattice distortionwith symmetry lower than the tetragonal one observed for the(Mo(1)O4)3� complex.

3.2. Spectral characteristics and local structure of the O� center

The ESR spectrum of the O� center (2p5, S = 1/2) is shown inFig. 8. This spectrum is created simultaneously with the(Mo(1)O4)3� one after X-ray irradiation at cryogenic temperatureand characterized by the strong central line surrounded by SHFsatellites. The simulation of the spectrum with a spinHamiltonian similar to the spin Hamiltonian in Eq. (1) shows thatthis SHF structure originates from the interaction of the trappedhole with two groups of 95,97Mo nuclei and one 25Mg nucleus(I = 5/2, abundance 10%). The Mg ion appears as an uncontrolledimpurity and stabilizes the trapped hole at an oxygen lattice ion.The simulated spectrum (see Fig. 8) shows quite good agreementwith the experimental one excluding the presence of some uniden-tified single lines which do not belong to the O� spectrum.

Fig. 8. ESR spectrum of the O� defect (black line) and its simulation (red line). Thesuperhyperfine structure is composed of two inequivalent pairs of 95,97Mo nuclei(hyperfine constants marked as A1 and A2) and one 25Mg nucleus (A3) for thespectral parameters A1(95,97Mo) = 25.50, 26.04 MHz; A2(95,97Mo) = 7.00, 7.15 MHzand A3(25Mg) = 28.00 MHz. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

The g tensor of the O� ion and the HF tensors of two inequiva-lent groups of 95,97Mo nuclei (it will be shown below that they cor-respond to Mo(1) and Mo(3) ion positions) were determined byfitting the experimental angular dependencies of resonance mag-netic fields presented in Figs. 9 and 10 by the spin Hamiltonianin Eq. (2) in the ‘‘Easyspin 4.5.5 toolbox’’ program [12].

The g tensor values of the O� center was interpreted assumingthat the pz orbital forms a ground state since one of the g factors(gzz = 2.0023) amounts to the free electron value. The angularmomentum is quenched in that case. The other two g factors differfrom the free electron value due to the admixture of px,y orbitalsinto the ground state by spin–orbit coupling. The energy states ofthe O� ion could be the following:

j0i ¼ jpzi;jn1i ¼ sjpxi þ mjpyi;jn2i ¼ mjpxi � sjpyi; ð8Þs2 þ m2 ¼ 1;

where s and m are the weight coefficients of the |pxi and jpyiorbitals.

The g tensor components are given by:

gxx ¼ ge � 2km2

DEn1þ s2

DEn2

� �;

gyy ¼ ge � 2ks2

DEn1þ m2

DEn2

� �; ð9Þ

where k = �150 cm�1 is the O� spin–orbit coupling constant, andDEni are the energy separations between the higher p states andthe ground state pz. Assuming DEn2

DEn1� gxx�ge

gyy�ge¼ 2:47 and substituting

gxx and gyy from Table 1, one derives from Eq. (9): s2 = 0.21,m2 = 0.79, DEn1 = 6199 cm�1 and DEn2 = 15311 cm�1.

It is hard to infer exactly what oxygen site O(1–12) (Fig. 1)becomes a hole trap under irradiation. Obviously, 2p orbitals ofsuch an ion must be at the top of valence band to create a shallowtrap state. The length of the Mo(3)–O(3) bond, counting 1.842 Å isthe longest distance in the MoO4 tetrahedra. Thus, most probably,the O(3) oxygen sites form the top of the valence band and can pro-duce the O� centers. To answer this question one needs to considerthe SHF structure of the O� ESR spectra and analysis of SHF con-stants. It is useful to expand further the SHF tensors in isotropic(a), axial (b) and rhombic (e) parts, similarly as it was done in [19]:

Fig. 9. Angular dependencies of the O� resonance fields with SHF lines from95Mo(3) isotope. The experimental data are presented for the single central line(I = 0) by the open circles and for 95Mo(3) SHF structure by solid rectangles. Blueand red solid lines are the theoretical curves simulated in ‘‘Easyspin 4.5.5 toolbox’’program [12]. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 10. Angular dependencies of the O� resonance fields with SHF lines from95Mo(1) isotope. The experimental data are presented for the single central line(I = 0) by the open circles and for 95Mo(1) SHF structure by solid rectangles. Redsolid lines are theoretical curves simulated in ‘‘Easyspin 4.5.5 toolbox’’ program[12]. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

M. Buryi et al. / Optical Materials 47 (2015) 244–250 249

Ax ¼ a� bþ e; ð10ÞAy ¼ a� b� e;

Az ¼ aþ 2b:

All components of the SHF tensor have the same sign as its ani-sotropy is small and, thus, |a| >> |b|, |e|.

Substituting the SHF tensor principal values for the 95Mo(3)nucleus in Eq. (10) one derives |a| = 26.0(2), |b| = |e| = 0.3(2) MHz.The sign of the parameter b has to be opposite to the sign of aand e. Only under this assumption the SHF tensor componentscan be reproduced by Eq. (10). For the 95Mo(1) nuclei one canobtain |a| = 7.2(2), |b| = 0.4(2), |e| = 0.3(2) MHz with the same signfor all three components. One can see that both |b| and |e| parts,responsible for the dipolar anisotropic interaction, are very small,comparable in value with uncertainty in determination of SHF ten-sor components. The large difference in the isotropic SHF interac-tion from two Mo nuclei suggests that namely the O(3) oxygen,directly connected to the Mo(3) becomes a trap for a hole afterX-ray irradiation. The second molybdenum ion Mo(1), which con-tributes to SHF structure, is not bonded directly to the O(3) ions(see Fig. 5). It interacts with the O� ion much weaker due to largeMo(1)–O(3) distance of 2.796 Å as compared to the 1.842 Å dis-tance between Mo(3) and O(3) ions in the initial unrelaxedMo(3)O4 tetrahedron.

The O(3) site has in the close vicinity two 67Zn nuclei at Zn(2)and Zn(3) sites at 2.033 Å and 2.106 Å distances, respectively (seeFig. 5). The 67Zn isotope, poorly abundant, makes negligible contri-bution to the SHF which cannot be resolved in spectra of the O�.However, the resolved SHF structure from 25Mg nucleus (Fig. 8)allows us to assume that Mg substitutes for one of these closestto the O(3) ion Zn(2) or Zn(3) sites. In the angular dependenciesof the resonance lines produced by the O� defect, the 25Mg SHFsextets are too intermixed with the 95,97Mo sextets so only theutmost lines could be clearly seen (see Fig. 8). Their shifts in spec-tra under the crystal rotation correspond to almost isotropic HFtensor with the principal values near 28 MHz.

Since the SHF structure from the 25Mg nucleus was notobserved in the spectrum of the self-trapped electron (see Fig. 4),the Mg impurity ions must be positioned far enough from Mo.The oxygen ligand 2p orbitals interfering with the Mo(1) 4d orbi-tals participate in generation of the molecular orbitals of the(Mo(1)O4)3� complex, given empirically by Eq. (4), shortening thusthe effective electron-nucleus distance. The Mg2+ ion is thereforeembedded at the Zn(2) position unbound to the molybdenum com-plex (‘‘2’’ in Fig. 5). But it is connected to the O(3) site directly and

the hole captured at the O(3) site is thus not self-trapped. The 67Znnucleus at Zn(2) position, connected to the (Mo(1)O4)3� complexvia O(4) oxygen (assigned as ‘‘1’’ in Fig. 5), must be involved intothe SHF interaction, giving the SHF component observed in spectra(see Fig. 4, upper inset).

4. Conclusions

In this work we studied the charge traps created in thea-ZnMoO4 single crystals by X-ray irradiation. Detailed analysisof the pronounced superhyperfine structure due to the intrinsic95,97Mo nuclei and other weak superhyperfine components dueto two inequivalent 67Zn nuclei at Zn(2) and Zn(3) sites allowedus to identify the electron trap as an electron self-trapped at the(Mo(1)O4)3� complex. The reduction of the spin–orbit coupling atthe central molybdenum ion caused by the overlap of the Moand ligand oxygen orbitals counts nearly 60% and points to thecharge transfer in the Mo-O bonding orbitals and delocalizationof the electron over the complex. The determined value ofv = �7.02, which characterizes the density of unpaired spin atthe nucleus, is typical for Mo5+–O complexes. The components ofthe SHF structure originated from two inequivalent zinc nuclei atthe Zn(2) and Zn(3) sites are too intermixed with each other to findout corresponding HF tensors separately.

We note that the (Mo(1)O4)3� complexes are thermally stableup to approximately 80 K (see [2] for the details), showing weakbonding of the captured electron with the complex. Such a featureis common for the self-trapped charge carriers, studied in otheroxide materials.

The ESR spectrum of the O� defect is composed of the contribu-tions from three inequivalent nuclei: the two groups of 95,97Mo iso-topes situated at the Mo(3) and Mo(1) regular sites and 25Mg as anuncontrolled impurity. This has allowed us to identify the O(3) reg-ular oxygen site for the O- center creation after the irradiation. Thetrapped holes are stabilized at oxygen anions by magnesium ionsnearby.

Presented results will also be useful for identification anddescription of similar defects in other Mo-containing scintillators.

Acknowledgements

The financial support of the Czech Science Foundation – CzechRepublic (Project No. P204/12/0805) and the Ministry ofEducation, Youth and Sports of Czech Republic (Projects No.LM2011029 and No. LO1409) is gratefully acknowledged. We aregrateful to Dr. V. Shlegel for providing us with the ZnMoO4 samples.D.S. acknowledges financial support from Mobilitas ESF program(grant MTT83) and institutional research funding program IUT(IUT02-26) of the Estonian Ministry of Education and Research.

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