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Fe centers and charge compensation in ZnWO4 single crystals characterized by ESR and i.r. spectroscopy
OOZZ-3697/92
3. Phys. Chem. Solids Vol. 53, No. 7, pp. 889-895, 1992
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Fe CENTERS AND CHARGE COMPENSATION IN ZnWO, SINGLE CRYSTALS CHARACTERIZED BY ESR AND I.R. SPECTROSCOPY-f A. WATTERICH,~ M. W~HLECKE,~ H. M~~LLER,§K. RAKSANYIJ A. BREITKOPF~ and B. ZELEI~~ SResearch Laboratory for Crystal Physics, Hungarian Academy of Sciences, Budaiirsi Qt 45, H-l 112 Budapest, Hungary $University of Osnabriick, FB Physik, P.O.B. 4469, D-4500 Osnabriick, Germany IlResearch Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budaiirsi tit 45, H-l 112 Budapest, Hungary (Received 2 December
1991; accepted 24 January 1992)
Abstract-In ZnWO,:Fe one of the numerous Fe3+ centers with reduced symmetry (C,) was identified as an Fe)+ ion compensated probably by a Zn*+ vacancy not very close to the Fe’+ ion. The lack of a proton superhypefine interaction of the different Fe3+ complexes with C, symmetry and the missing correlation between the absorption coefficient of the i.r. band at 3445 cm-’ and the ESR intensities of the Fe)+ complexes leads to a modified model for the i.r. active center, namely an OH- ion substituting for O*- associated with a Zn vacancy and designated as V,. Keyworcis:
ZnWO,:Fe, crystal, OH, i.r., ESR, point defects, V& centre.
1. INTRODUCTION ZnWO, has been shown to be a promising material for scintillator detectors [l-3]. However, impurities with optical absorption in the emission range reduce the light emission efficiency. The optical absorption band at 460 nm was found to be related to Fe2+ [4] and therefore the undesirable self-absorption was attributed mainly to the iron impurity. By infrared (i.r.) spectroscopy a narrow (13 cm-‘) band at 3445cn-’ was assigned to the OH stretch mode of Fe3+-OH complexes. The OH- defect was assumed to be in an interstitial position [S]. In undoped ZnWO, Fiildvari et al. observed at room temperature (RT) an i.r. band at 3420cm-’ with a halfwidth of 65 cm-’ as well, and ascribed it to substitutional OHwithout any other disturbance nearby [S]. The Fe3+ ion substituting for Zn has already been investigated by electron spin resonance (ESR). The majority of Fe3+ retains the original C, site symmetry of Zn because the charge compensation is non-local [6]. One Fe3+ center with (C,) symmetry was also reported, but, not studied in detail [7]. Fe is always present even in undoped ZnWO,. FeWO, crystallizes with the same structure as ZnWO, and forms a solid solution with it. Therefore the effective segregation coefficient of iron is close to unity. Since Fe seems to be a persistent and unwanted impurity in ZnWO, we carried out ESR and i.r. t This work is dedicated to Prof. I. Tarjan on the occasion of his 80th birthday.
studies in order to clarify the structure of Fe complexes with C, symmetry and the related charge compensation mechanism. 2. CRYSTAL STRUCTURE ZnWO, belongs to an isostructural series of divalent NiWO,-type monoclinic tungstates containing small cations like Mg, Mn, Fe, Co, Zn and Ni. It has the space group C:, (P2/c) with two formula units per unit cell [8,9]. We have described the crystal structure and its consequences with respect to ESR at length in an earlier paper [lo&The following summarizes only the most important features. Both types of cations are surrounded by six oxygens that are pairwise equidistant resulting in C, point symmetry for the cation sites. Substitutional paramagnetic impurities that replace the Zn ions without other lattice defects nearby yield a single ESR spectrum for the symmetrically equivalent sites at an arbitrary orientation; likewise for the W ion replacements. However, for reduced symmetry C, the number of transitions is multiplied by two (except for orientations in the (010) plane) if the impurity is locally charge-compensated by a vacancy or another impurity ion. 3. EXPERIMENTAL Single crystals of ZnWO, were grown in air by a balance-controlled Czochralski technique using a platinum crucible [ll]. The raw materials were
889
A.
890
WATTERlCH
analytical grade ZnO (REANAL, Hungary) and WO, chemically produced from analytical grade Na,WO, [12]. The crystals were doped by adding 10-5-10-3 mol/mol Fe,O, powder to the melt. Cr, Al, Na and Li were introduced by adding Cr,O,, A1,03, NaNO, and LiOH, respectively. The crystal samples were oriented by X-ray diffraction, cut perpendicularly to the [OOl] and [IOO] directions, and cleaved in the (010) plane. The ESR meas~ements were performed using a Bruker ER 200 D-SRC X-band sp~trometer with a rectangular TE,, 1 cavity (lOO-kHz mod~ation). The microwave power was 20 mW and the frequency N 9.4 GHz. All ESR measurements were made at RT except where indicated. The infrared absorption spectra were measured using either a Bruker IFS 113 CV Fourier spectrometer or a Perkin-Elmer model 1710 Fourier IR spectrometer with a DTGS detector. The u.v.-vis absorption spectra were recorded with a PerkinElmer model 554 spectrophotometer. All absorption spectra were measured at RT. 4. EXPERIMENTAL RESULTS 4.1.
ESR
results
In iron-doped ZnWO, single crystals, numerous different ESR lines were found around the strongest transition of the Fe3+ center with C, symmetry for B 11[OlO] (Fig. 1). We shall refer to this center as Fe3+(C,). The Mn*+ impurity has been identified by its characteristic hyper&e (HF) structure (Fig. 1). This ion is usually present in ZnWO,. Additionally to these spectra, the angular variation of seven sets of lines bears a resemblance to that of the Fe3+fC2) spectrum. We designated these spectra Fe(l)-Fe(7) in the sequence of their position in increasing magnetic field for B /I[OlO](Fig. 1). The intensity of these ESR
et al.
3
160
z!
ZnWOPFe
1o-5
1 o-4
1 o-3
Fe nominal cont. (mol/mof)
Fig. 2. The relative ESR intensities of the seven different Fe centers indicated in Fig. 1 vs nominal concentration of the Fe content of the crystals. The smallest Fe concentration corresponds to the undoped crystal determined by atomic absorption. The ESR spectra were recorded at RT, for B 11 [OlO]. The errors are standard deviations calculated from values of different samples cut from different boules. Only the largest errors are indicated, corresponding to the steepest slopes.
lines is correlated with the amount of Fe in the crystal (Fig. 2). The energy levels for Fe3+(C,), which is a high spin 3& ion, as a function of external magnetic field for B // [OlO] are shown in Fig. 3, and have been calculated for the parameters given in [6]. Transitions at 9.42 GHz are indicated, and occur at calculated fields 175, 389 and 868 mT. The levels corresponding to the m, quantum numbers -S/2, -3/2, - l/2, l/2, 3/2 and 5/2 are numbered (increasing energy) from 1 to 6. The Fe’+(C,) line in Fig. 1 corresponds to the The numbered lines 3 tr 4 transition (m,: -1 -f).
I
4
f
1
Fe3+(C2) in ZnWo4
I
,,-4
---.-===;-== =z-----___
-3
1.. -4-~
I f 0
2 .. 1‘ i _-.__
I
500 magnetic
I
1000
I
1500
field (mT)
Fig. 3. The energy levels for Fe3+(C,) as a function of external magnetic field for B 11[OlO] calculated with the parameters given in [6]. Transitions at 9.42 GHz are Fig. 1. ESR spectrum of Fe’+ centers in single crystal indicated, and occur at calculated fields 175, 389 and ZnWO,:Fe measured at RT and 9.42 GHz for B 11 [IOO]. 868 mT. The levels corresponding to the m, quantum The Fe’+(C,) line corresponds to the 3 ++4 (m,: -f c* i) numbers - 512, - 312, - l/2, l/2, 312 and 512 are numbered in order (increasing energy) from 1 to 6. transition (see Fig. 3).
Fe centers and charge compensation
5 9 2
400
*g 300 if! F 200 p 500 1 1001, 0
,
,
15
30
rotation
,
,
,
45
60
75
,, 90
angle (degree)
Fig. 4. Angular variation of the Fe(l) center in ZnWO,: Fe measured at room temperature and 9.43 GHz in the (100) plane. 0” and 90” correspond to B 11 [OOl]and [OlO],respectively. Experimental data are shown by asterisks and crosses (and x correspond to 5 t* 6 and 3 ct 4 transitions, respectively), and solid lines represent results calculated from the best-fit spin-Hamiltonian parameters.
891
None of the Fe(l)-Fe(7) spectra showed a splitting due to a superhyperfine (SHF) structure indicating the presence of a nearby hydrogen. Although in [4] slightly split Fe lines were reported, careful investigations showed that the splitting was due to sub-grain boundaries. One of the Fe centers (Fe(l)) had a concentration large enough to follow the angular variation of the ESR lines of two different transitions (3 -4 and 5 c* 6). The angular variations of the Fe( 1) center lines were recorded in three (100) planes. The angular variations of two different transitions are shown for RT in Fig. 4 for the (100) plane, where asterisks and crosses represent experimental data. The line positions can be described by the spinHamiltonian given in [13] with S = 5/2 for C, symmetry:
+ B:O: + B:O: + C$; + C:fl: + C$2;. due to Fe complexes may also be attributed to the same transition because of their similar angular variation. (The separation of the different Fe lines from each other was even smaller for any other orientations differing from B /I [OlO].) One small line between the sixth Mn*+ and Fe(6) lines shows very different anisotropy behaviour. Thus this line is probably due to another transition of one of the above Fe centers. Unfortunately it was too weak for its angular variation to be followed completely. Between Fe(l) and Fe(S) some other small lines can be distinguished as well. On rotating the crystal they disappear and therefore their paramagnetic features could not be studied. For arbitrary orientations in the (100) and (001) planes each of the seven numbered ESR lines splits into two, indicating two inequivalent sites and, therefore, C, site symmetry.
For C, symmetry, the quantity B;O; + B:O; + B:O: + C$; + C$: + C#
Bi in
Fe’+(C,) - 6987 4935 3.26
Fe3+(C,) -6971 k 50 6210 1.8 + k 0.5 50
5:
- 17.3 -1.78
- 1.8 k 0.5
C”: C:
-
-60 9+1 f 10 -0.9 + 0.3 Additional parameters for C, symmetry
Eigenvectors
Reference
- 700 * 200 - 140 + 30 2.0019 & 0.0005
2.0019 0.707 0.000 0.707
0.000 1.000 0.000
M
-0.707 0.000 0.707
(2)
should be added to eqn (1). In Table 1 the fitted spin-Hamiltonian parameters and the eigenvectors of the principal axes are presented in comparison with the parameters given for Fe’+ with C, symmetry. The solid lines in Fig. 4 represent calculated results based on the computer-fitted spin-Hamiltonian parameters listed in Table 1. In the computation, allowance was made for a slight sample misorientation. For Fe( 1) signals two symmetrically arranged satellite lines are observed (Fig. 5). The “Fe isotope has nuclear spin I = l/2 with a natural abundance of 2.2%. Most of the Fe isotopes have I = 0 and a natural abundance of 97.8%. From their natural abundances the expected intensity ratio of the main
Table 1. Spin-Hamiltonian parameters of Fe3+ tons in ZnWO, single crystals. The non-zero fine structure components are listed in units of MHz. The direction cosines of the dimensionless eigenvectors are defined with respect to the crystallographic axes [loo], [OlO]and [OOl]in downward order, respectively. The estimated errors for eigenvectors are +0.003. Measurements were made at RT
B: C: g, = & = gi
(1)
0.701 -0.205 0.683
0.079 - 0.709 0.974 -0.095 0.211 0.699 [Present work]
892
Fe(l)
A. WAITERICHet
al.
resolved structure was observed for B 11[lOO], while for Fe( 1) it was still unresolved and similar to that for B 11[OlO]. Since the Fe(l) center has a reduced symmetry the environment of the Fe ion must be different compared to the Fe(C,) center, therefore complete agreement concerning the lg3W SHF structure is not expected. After annealing the sample in air at 840°C for 90 h, the Fe( 1) spectrum completely disappears.
in ZnWO,
4.2. Optical absorption results 57Fe
1
1
Fig. 5. One part of the spectrum of the Fe(l) center, 5” away from B (1lOlO]. The HF lines due to the 57Fe isotones are indicatedihe~unresolved superhyperfine (SHF) structure of the main line is explained by the interaction with the surrounding r8’W6+ ions (I = l/2, natural abundance = 14.4%) similar to that of Fe’+(C,) [14]. The spectrum was taken at 25 K.
line and each of the satellite lines is 89, in good agreement with the observed ratio of 95. The angular variation of the weak HF lines could not be followed because of superposition of other lines. The HF separation is 1.06 + 0.02 mT for B (([OlO]. The Fe(l) main lines showed an unresolved superhyperfine (SHF) interaction close to B 11[OlO]. This line shape cannot be explained by a SHF interaction with a hydrogen ion (I = l/2, natural abundance = 100%) or another ion with I = 312 and natural abundance loo%, such as Na or Li. However, it is very similar to that of the Fe3+(C,) in the same orientation, which was attributed to the superposition of surrounding ‘r3W6+ ions (I = l/2, natural abundance = 14.4%) [14]. For Fe3+(C,) a
The i.r. band at 3445 cm-’ is always present in undoped and Me3+ doped samples, and shows up at the same position and with the same halfwidth. In undoped and some heavily doped (10-3mol/mol Fe,O,) samples the broader i.r. band at 3420cm-’ (attributed to substitutional OH- [S]) could be detected, as well. In Fig. 6 the absorption coefficient of the i.r. band at 3445 cm-’ is plotted vs the nominal concentration of Fe. The absorption coefficient did not correlate over the full concentration range with the Fe content and any of the Fe(l)-Fe(7) ESR lines (Fig. 2). As mentioned above the same i.r. band (with neither position nor halfwidth changed) was found when ZnWO, was doped by other triply charged ions like Cr and Al or even in undoped crystals; however, it was very weak for Na- and Li-doped crystals (Table 2). Since it was shown in [4] that a band at 350 nm is due to Fe3+ (here the band probably involves the absorption of Fe3+(C,) and that of the Fe3+complexes, as well), the Fe content can be checked by observing the U.V. absorption (Table 2). Me3+ ions
Fe nominal cont. (mol/mol) Fig. 6. The i.r. absorbance of the band at 3445 cm-’ vs the nominal Fe content of the samples. The smallest Fe concentration corresponds to the undoped crystal determined by atomic absorption [4]. The i.r. spectra were measured at room temperature.
Fe centers
and charge
893
compensation
Table 2. Comparison of uv. absorption coefficients (at 350nm, proportional to the Fe’+ concentration); the ESR intensity (proportional to the Fe)+(C,) center concentration) and i.r. absorption coefficients (at 3445 cm-’ proportional to the OH center concentration) in different ZnWO, samples. All data were taken at RT. The Fe U.V. absorption could not be determined in ZnWO,: Cr samples because of the superposition of the Cr absorption band. The given concentrations are nominal ones except those marked with asterisks measured by atomic absorption
Dopant
Concentration (mol/mol)
Undoped Fe’+ Fe’+ Cr’+ Cr’+ Al’f Na+ Li+
7 x 10m6(Fe)* 10m4(Fe)* 9 x 10-4(Fe)* 10m4(Cr) 10m3(Cr) 4 x 10-S(Al)* 7 x 10m5(Na)* IO-‘(Li)
U.V. absorption coefficient at 350nm
Rel. ESR intensity of Fe’+(C,) (a.u.) 1 6.7 13.2 0 0 0.45 3.6 2
3 14 113 2.2 4.9 4.1
in small concentrations generally enhance, and Me+ ions suppress the intensity of the band at 3445 cm-’ while the Fe3+ concentration is decreased in the former case, contrary to Me+ enhancing it. This means that the Me3+ ions compete with Fe3+ ions, while Me+ ions charge-compensating the Fe3+ ions increase the Fe3+ concentration and lead to a simultaneous decrease in the OH band at 3445 cm-‘.
For ing to giving to the
i.r. absorption coefficient at 3445cm-’ 0.42 f 0.08 0.81 + 0.09 0.33 * 0.03 0.44 0.23 0.73 <0.05
both i.r. bands we found D - 12, correspondx = + 16”, where the estimated error is f4”, the transition moment orientation with respect X axis. These angles correspond to +6” and
-26” with respect to the crystallographic [loo] axis, respectively. Assuming that the transition moment and the OH bond orientations are parallel one may get information
about the OH positions.
Looking
at
The orientation of the electric dipole moment can be determined from the angular dependence of the
the projection of the 0 octahedron around the Zn ion in the (010) plane one can find an 0-O connecting
polarized absorption. In monoclinic ZnWO,, with the two-fold axis along [OlO], the dichroic ratio D of the maximum and minimum absorption for light polarization in the (010) plane yields the orientation of the transition moment with respect to the dielectric axis X. The angular dependence of the absorbance can be expressed by the following equation:
line (the 0 ions are at a distance of 0.297 nm) and a Zn-O bond with an angle of + 16.7” and - 31.5” with respect to the crystallographic [ 1001 axis, respectively (Fig. 7). These values are in good agreement with the experimental ones, taking into consideration that the calculated values refer to the undistorted lattice while
a(O) = -ln(e-“rdcos20
+ e-““sin20),
may occur,
(3)
where 0 is the angle of the electric vector with respect to the X dielectric axis, a, and a, are absorbances at the maximum and minimum, i.e. parallel to X and Z dielectric axes, respectively. The dichroic ratio D can be expressed
around the defects some lattice relaxation thus changing the ion positions.
I
WI
A
by D = ax/a2 = m cot2X,
(4)
where x is the angle between the transition moment and the X axis [lS]. We have set m = n,/n, - 1 as a good approximation [ 161. Fiildvari et al. [S] had measured the angular dependences for both i.r. bands, but the reported dichroic ratios do not refer unambiguously to the (010) plane. Therefore we repeated the measurements at RT and found a maximum absorbance for light polarization parallel to X. Precise measurements of the wavelength dependent optical constants yield the dielectric X axis in the (010) plane with an angle of N - 10” with respect to the crystallographic [loo] axis [16].
* r1001
a = - 31.50 B = 16.7”
Fig. 7. Projection of a Zn (full circle) and its six neighboring 0 (open circles) ions in the (010) plane. The long (I), medium (m) and short (s) Zn-O bonds (0.216, 0.210 and 0.205 nm, respectively) and the crystallographic axes are indicated. a represents the angle between a Zna bond and the [loo] crystallographic axis, while /I is the angle of an @O connecting line with respect to the [lOO] axis.
5. DISCUSSION
The spin-Hamiltonian parameters obtained for the Fc(1) centers differ only slightly from those of the Fe3”(C,) (non-locally compensated) center given in [6] (Table 1). Some additional defect is believed to perturb the Fe3+ ion leading to the reduced symmetry. This defect can be neither a nearby OH-ion because of the lack of a proton SHF interaction and correlation with the i.r. bands, nor a charge-compensating Na+ or Li+ ion nearby because of the lack of an appropriate SHF interaction. Therefore the defect is assumed to be a Zn vacancy serving as a partial charge compensator, since a vacancy is the most common charge compensator in oxide crystals, as well. A first-neighbor vacancy is expected to provide a strong crystalline electric field similar to that reported for Fe3 + associated with the oxygen vacancy in SrTiO,, causing an approximate doubling of the Bi fine structure parameter [17]. Although the Fe3+-nearest neighbour Zn vacancy distance is larger than that of the Fe’+-0 vacancy, we believe that the vacancy must be at a larger distance since none of the parameters including the eigenvectors change by much. If a Zn vacancy really perturbs the Fe environment, annealing of the crystal should destroy the Fe-Zn vacancy complex. This was in fact observed. The further Fe complexes are also supposed to be due to Zn vacancy-perturbed Fe centers where the vacancies may be at different distances and directions. As mentioned above, the attribution of the i.r. band at 3445 cm-’ to the Fe3+-OH complex does not seem to be very likely. Additionally one has to mention that two i.r. bands were attributed to Pt-OH complexes at 2852 and 2920 cm-’ [S]. In [lo] the ESR spectrum of Pt in ZnWO, did not show any SHF structure, indicating the presence of a nearby OH. (Moreover the same bands could be observed in the case of other host materials like TeO,, LiNbO, and NaCl. These bands seem to be artifacts or surface contaminations.) One can point out from these findings that even the Pt3+ charge surplus is not compensated locally by a nearby OH- ion. From the optical absorption results we conclude that the i.r. band at 3445 cm-’ is due to centers which compensate the surplus charge of Me3+ ions if they are present in small concentration. Since Me3+ ions do not change the i.r. band position and halfwidth, therefore the impurity ions cannot be near to the i.r. active center. This conclusion agrees with the ESR results, according to which the paramagnetic Fe centers did not correlate with the i.r. band, and the ESR lines did not show a SHE: splitting due to a nearby OH ion. Thus the charge compensation occurs non-locally. All these findings lead us to
suggest a modified model for the OH center. An OHion associated with a Zn vacancy explains the experiments correctly. This type of center has been considered in simple oxides like MgO and CaO and designated as V6u with i.r. bands at 3296 and 3418 cm-i, respectively [18]. From the point of view of charge compensation of Van center, having a net charge -e with respect to the perfect crystal, satisfies the requirement for compensation of a Me3+ ion having a net charge +e with respect to the perfect lattice. It acts as a charge compensator when the iron concentration is small (Fig. 6) and the Fe3+(C,) (i.e. unperturbed; nonlocally compensated) center is predominant compared to Fe complexes, like Fe(l) which is overcompensated by a Zn*+ vacancy. When these Fe complexes become dominant at higher nominal dopant concentrations (Fig. 2), the V&u center loses its importance, the charge compensation occurs via Zn vacancies and the concentration of the Voi, centers decreases. The i.r. band at 3420cm-’ with a halfwidth of 65cm-’ at RT was attributed to substitutional OH without any other perturbation nearby. In fact one of the transition moments was oriented approximately along one of the O-O bonds (Fig. 7). In the case of a substitutional OH- the net charge is +e; electrostatically it should be oriented in the direction of a negative ion, i.e. 02-. When a cation vacancy is present the OH- ion should be oriented along the OH-Zn vacancy connecting line as in MgO [I@ In fact, our polarization
%
3
0 1
Fig. 8. Projection of a Zn (full circle) and its six neighboring 0 (open circles) ions in the plane determined by the O(l), O(3) and O(5) ions. The long (I), medium (m) and short (s) ZnXI bonds (0.216, 0.210 and 0.205 nm, respectively) and the crystallographic axes are indicated. The 0-Zn-0 bonds are nearly linear, the angles and distances are as follows: for O(3)-Zn-O(4) 172” and 0.418nm, for O(6)-Zn-O(2) and O(5)-Zn-O(l) 166” and 0.419 nm, respectively.
Fe centers and charge compensation
results allow that one of the i-r, active centers is oriented approximately in the direction of an 0-Zn connection line (Fig. 7). Taking into account these orientations and assuming the above models the shape of the Lr. bands can be interpreted in the same way as for MgO [ 191.It has been shown in fl9,20] that for the linear system of the absorption line deOH . f . 0, the half~d~ pends on the separation of the 0 ions. For as large as 0.42 nm distances the predicted linewidth is very small, while for a distance of 0.297 nm it is expected to be of the order of 100 cm-‘. The latter case is in good agreement with substitutional OH. Although ZnWO, is monoclinic the distortion of the oxygen octahedron is not very large around a Zn ion and the possible 0-Zn-0 bonds are nearly linear (Fig. 8). Calculating the angles and distances they are as follows:O(3)-Zn~(4)(~7~*)~.41~ nm,O(6)-Zn-O(2) and O(S)-Zn-U(1) (166”) 0,419nm. For the Vo, center the observed iinewidth of 13 cm-’ [S] agrees with the expectations. From our polarized absorption results the latter two orientations seem to be the most probable orientations for the V& center, 6. CONCLUSIONS A number of Fe3+ complexes with reduced symmetry (C,) were found in ZnWO, doped by Fe. One of these centers was ascribed to Fe3+ ton . associated probably with a Zn2+ vacancy not very close to the Fe’+ ion. The further Fe compiexes are also supposed to be due to Fe centers perturbed by Zn vacancies where the vacancies may be in different distances and orientations from the Fe ions. The surpXus charge of Me3* ions are consequently compensated by Zn vacancies when the Me3+ concentration is > 10m4mol/mol. If the Fe3+ (or Me’+) concentrations was < IO-” mol/mol the charge compensation occurred non-locally by means of V& centers. This center is associated with an i.r. band at 3445 cm-‘. The absorption parameters, the polarization behavior and the dependence of this band on Me-dopants ied to the identification of the VG, center.
895
Acknowledgements-This research was supported jointly by DFG, SFB 225 and the National Science -Research Foundation KYI’KA~ of Hunnarv. The authors wish to express their gratitude to Mrs x. ‘P&er and Dr L. Kovacs for helpful discussions, and to Mrs Zs. Tbth, Mr I. Cravero, E. Possenriede and A. Griine for assistance.
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3. Phys. Chem. Solids Vol. 53, No. 7, pp. 889-895, 1992
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Fe CENTERS AND CHARGE COMPENSATION IN ZnWO, SINGLE CRYSTALS CHARACTERIZED BY ESR AND I.R. SPECTROSCOPY-f A. WATTERICH,~ M. W~HLECKE,~ H. M~~LLER,§K. RAKSANYIJ A. BREITKOPF~ and B. ZELEI~~ SResearch Laboratory for Crystal Physics, Hungarian Academy of Sciences, Budaiirsi Qt 45, H-l 112 Budapest, Hungary $University of Osnabriick, FB Physik, P.O.B. 4469, D-4500 Osnabriick, Germany IlResearch Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budaiirsi tit 45, H-l 112 Budapest, Hungary (Received 2 December
1991; accepted 24 January 1992)
Abstract-In ZnWO,:Fe one of the numerous Fe3+ centers with reduced symmetry (C,) was identified as an Fe)+ ion compensated probably by a Zn*+ vacancy not very close to the Fe’+ ion. The lack of a proton superhypefine interaction of the different Fe3+ complexes with C, symmetry and the missing correlation between the absorption coefficient of the i.r. band at 3445 cm-’ and the ESR intensities of the Fe)+ complexes leads to a modified model for the i.r. active center, namely an OH- ion substituting for O*- associated with a Zn vacancy and designated as V,. Keyworcis:
ZnWO,:Fe, crystal, OH, i.r., ESR, point defects, V& centre.
1. INTRODUCTION ZnWO, has been shown to be a promising material for scintillator detectors [l-3]. However, impurities with optical absorption in the emission range reduce the light emission efficiency. The optical absorption band at 460 nm was found to be related to Fe2+ [4] and therefore the undesirable self-absorption was attributed mainly to the iron impurity. By infrared (i.r.) spectroscopy a narrow (13 cm-‘) band at 3445cn-’ was assigned to the OH stretch mode of Fe3+-OH complexes. The OH- defect was assumed to be in an interstitial position [S]. In undoped ZnWO, Fiildvari et al. observed at room temperature (RT) an i.r. band at 3420cm-’ with a halfwidth of 65 cm-’ as well, and ascribed it to substitutional OHwithout any other disturbance nearby [S]. The Fe3+ ion substituting for Zn has already been investigated by electron spin resonance (ESR). The majority of Fe3+ retains the original C, site symmetry of Zn because the charge compensation is non-local [6]. One Fe3+ center with (C,) symmetry was also reported, but, not studied in detail [7]. Fe is always present even in undoped ZnWO,. FeWO, crystallizes with the same structure as ZnWO, and forms a solid solution with it. Therefore the effective segregation coefficient of iron is close to unity. Since Fe seems to be a persistent and unwanted impurity in ZnWO, we carried out ESR and i.r. t This work is dedicated to Prof. I. Tarjan on the occasion of his 80th birthday.
studies in order to clarify the structure of Fe complexes with C, symmetry and the related charge compensation mechanism. 2. CRYSTAL STRUCTURE ZnWO, belongs to an isostructural series of divalent NiWO,-type monoclinic tungstates containing small cations like Mg, Mn, Fe, Co, Zn and Ni. It has the space group C:, (P2/c) with two formula units per unit cell [8,9]. We have described the crystal structure and its consequences with respect to ESR at length in an earlier paper [lo&The following summarizes only the most important features. Both types of cations are surrounded by six oxygens that are pairwise equidistant resulting in C, point symmetry for the cation sites. Substitutional paramagnetic impurities that replace the Zn ions without other lattice defects nearby yield a single ESR spectrum for the symmetrically equivalent sites at an arbitrary orientation; likewise for the W ion replacements. However, for reduced symmetry C, the number of transitions is multiplied by two (except for orientations in the (010) plane) if the impurity is locally charge-compensated by a vacancy or another impurity ion. 3. EXPERIMENTAL Single crystals of ZnWO, were grown in air by a balance-controlled Czochralski technique using a platinum crucible [ll]. The raw materials were
889
A.
890
WATTERlCH
analytical grade ZnO (REANAL, Hungary) and WO, chemically produced from analytical grade Na,WO, [12]. The crystals were doped by adding 10-5-10-3 mol/mol Fe,O, powder to the melt. Cr, Al, Na and Li were introduced by adding Cr,O,, A1,03, NaNO, and LiOH, respectively. The crystal samples were oriented by X-ray diffraction, cut perpendicularly to the [OOl] and [IOO] directions, and cleaved in the (010) plane. The ESR meas~ements were performed using a Bruker ER 200 D-SRC X-band sp~trometer with a rectangular TE,, 1 cavity (lOO-kHz mod~ation). The microwave power was 20 mW and the frequency N 9.4 GHz. All ESR measurements were made at RT except where indicated. The infrared absorption spectra were measured using either a Bruker IFS 113 CV Fourier spectrometer or a Perkin-Elmer model 1710 Fourier IR spectrometer with a DTGS detector. The u.v.-vis absorption spectra were recorded with a PerkinElmer model 554 spectrophotometer. All absorption spectra were measured at RT. 4. EXPERIMENTAL RESULTS 4.1.
ESR
results
In iron-doped ZnWO, single crystals, numerous different ESR lines were found around the strongest transition of the Fe3+ center with C, symmetry for B 11[OlO] (Fig. 1). We shall refer to this center as Fe3+(C,). The Mn*+ impurity has been identified by its characteristic hyper&e (HF) structure (Fig. 1). This ion is usually present in ZnWO,. Additionally to these spectra, the angular variation of seven sets of lines bears a resemblance to that of the Fe3+fC2) spectrum. We designated these spectra Fe(l)-Fe(7) in the sequence of their position in increasing magnetic field for B /I[OlO](Fig. 1). The intensity of these ESR
et al.
3
160
z!
ZnWOPFe
1o-5
1 o-4
1 o-3
Fe nominal cont. (mol/mof)
Fig. 2. The relative ESR intensities of the seven different Fe centers indicated in Fig. 1 vs nominal concentration of the Fe content of the crystals. The smallest Fe concentration corresponds to the undoped crystal determined by atomic absorption. The ESR spectra were recorded at RT, for B 11 [OlO]. The errors are standard deviations calculated from values of different samples cut from different boules. Only the largest errors are indicated, corresponding to the steepest slopes.
lines is correlated with the amount of Fe in the crystal (Fig. 2). The energy levels for Fe3+(C,), which is a high spin 3& ion, as a function of external magnetic field for B // [OlO] are shown in Fig. 3, and have been calculated for the parameters given in [6]. Transitions at 9.42 GHz are indicated, and occur at calculated fields 175, 389 and 868 mT. The levels corresponding to the m, quantum numbers -S/2, -3/2, - l/2, l/2, 3/2 and 5/2 are numbered (increasing energy) from 1 to 6. The Fe’+(C,) line in Fig. 1 corresponds to the The numbered lines 3 tr 4 transition (m,: -1 -f).
I
4
f
1
Fe3+(C2) in ZnWo4
I
,,-4
---.-===;-== =z-----___
-3
1.. -4-~
I f 0
2 .. 1‘ i _-.__
I
500 magnetic
I
1000
I
1500
field (mT)
Fig. 3. The energy levels for Fe3+(C,) as a function of external magnetic field for B 11[OlO] calculated with the parameters given in [6]. Transitions at 9.42 GHz are Fig. 1. ESR spectrum of Fe’+ centers in single crystal indicated, and occur at calculated fields 175, 389 and ZnWO,:Fe measured at RT and 9.42 GHz for B 11 [IOO]. 868 mT. The levels corresponding to the m, quantum The Fe’+(C,) line corresponds to the 3 ++4 (m,: -f c* i) numbers - 512, - 312, - l/2, l/2, 312 and 512 are numbered in order (increasing energy) from 1 to 6. transition (see Fig. 3).
Fe centers and charge compensation
5 9 2
400
*g 300 if! F 200 p 500 1 1001, 0
,
,
15
30
rotation
,
,
,
45
60
75
,, 90
angle (degree)
Fig. 4. Angular variation of the Fe(l) center in ZnWO,: Fe measured at room temperature and 9.43 GHz in the (100) plane. 0” and 90” correspond to B 11 [OOl]and [OlO],respectively. Experimental data are shown by asterisks and crosses (and x correspond to 5 t* 6 and 3 ct 4 transitions, respectively), and solid lines represent results calculated from the best-fit spin-Hamiltonian parameters.
891
None of the Fe(l)-Fe(7) spectra showed a splitting due to a superhyperfine (SHF) structure indicating the presence of a nearby hydrogen. Although in [4] slightly split Fe lines were reported, careful investigations showed that the splitting was due to sub-grain boundaries. One of the Fe centers (Fe(l)) had a concentration large enough to follow the angular variation of the ESR lines of two different transitions (3 -4 and 5 c* 6). The angular variations of the Fe( 1) center lines were recorded in three (100) planes. The angular variations of two different transitions are shown for RT in Fig. 4 for the (100) plane, where asterisks and crosses represent experimental data. The line positions can be described by the spinHamiltonian given in [13] with S = 5/2 for C, symmetry:
+ B:O: + B:O: + C$; + C:fl: + C$2;. due to Fe complexes may also be attributed to the same transition because of their similar angular variation. (The separation of the different Fe lines from each other was even smaller for any other orientations differing from B /I [OlO].) One small line between the sixth Mn*+ and Fe(6) lines shows very different anisotropy behaviour. Thus this line is probably due to another transition of one of the above Fe centers. Unfortunately it was too weak for its angular variation to be followed completely. Between Fe(l) and Fe(S) some other small lines can be distinguished as well. On rotating the crystal they disappear and therefore their paramagnetic features could not be studied. For arbitrary orientations in the (100) and (001) planes each of the seven numbered ESR lines splits into two, indicating two inequivalent sites and, therefore, C, site symmetry.
For C, symmetry, the quantity B;O; + B:O; + B:O: + C$; + C$: + C#
Bi in
Fe’+(C,) - 6987 4935 3.26
Fe3+(C,) -6971 k 50 6210 1.8 + k 0.5 50
5:
- 17.3 -1.78
- 1.8 k 0.5
C”: C:
-
-60 9+1 f 10 -0.9 + 0.3 Additional parameters for C, symmetry
Eigenvectors
Reference
- 700 * 200 - 140 + 30 2.0019 & 0.0005
2.0019 0.707 0.000 0.707
0.000 1.000 0.000
M
-0.707 0.000 0.707
(2)
should be added to eqn (1). In Table 1 the fitted spin-Hamiltonian parameters and the eigenvectors of the principal axes are presented in comparison with the parameters given for Fe’+ with C, symmetry. The solid lines in Fig. 4 represent calculated results based on the computer-fitted spin-Hamiltonian parameters listed in Table 1. In the computation, allowance was made for a slight sample misorientation. For Fe( 1) signals two symmetrically arranged satellite lines are observed (Fig. 5). The “Fe isotope has nuclear spin I = l/2 with a natural abundance of 2.2%. Most of the Fe isotopes have I = 0 and a natural abundance of 97.8%. From their natural abundances the expected intensity ratio of the main
Table 1. Spin-Hamiltonian parameters of Fe3+ tons in ZnWO, single crystals. The non-zero fine structure components are listed in units of MHz. The direction cosines of the dimensionless eigenvectors are defined with respect to the crystallographic axes [loo], [OlO]and [OOl]in downward order, respectively. The estimated errors for eigenvectors are +0.003. Measurements were made at RT
B: C: g, = & = gi
(1)
0.701 -0.205 0.683
0.079 - 0.709 0.974 -0.095 0.211 0.699 [Present work]
892
Fe(l)
A. WAITERICHet
al.
resolved structure was observed for B 11[lOO], while for Fe( 1) it was still unresolved and similar to that for B 11[OlO]. Since the Fe(l) center has a reduced symmetry the environment of the Fe ion must be different compared to the Fe(C,) center, therefore complete agreement concerning the lg3W SHF structure is not expected. After annealing the sample in air at 840°C for 90 h, the Fe( 1) spectrum completely disappears.
in ZnWO,
4.2. Optical absorption results 57Fe
1
1
Fig. 5. One part of the spectrum of the Fe(l) center, 5” away from B (1lOlO]. The HF lines due to the 57Fe isotones are indicatedihe~unresolved superhyperfine (SHF) structure of the main line is explained by the interaction with the surrounding r8’W6+ ions (I = l/2, natural abundance = 14.4%) similar to that of Fe’+(C,) [14]. The spectrum was taken at 25 K.
line and each of the satellite lines is 89, in good agreement with the observed ratio of 95. The angular variation of the weak HF lines could not be followed because of superposition of other lines. The HF separation is 1.06 + 0.02 mT for B (([OlO]. The Fe(l) main lines showed an unresolved superhyperfine (SHF) interaction close to B 11[OlO]. This line shape cannot be explained by a SHF interaction with a hydrogen ion (I = l/2, natural abundance = 100%) or another ion with I = 312 and natural abundance loo%, such as Na or Li. However, it is very similar to that of the Fe3+(C,) in the same orientation, which was attributed to the superposition of surrounding ‘r3W6+ ions (I = l/2, natural abundance = 14.4%) [14]. For Fe3+(C,) a
The i.r. band at 3445 cm-’ is always present in undoped and Me3+ doped samples, and shows up at the same position and with the same halfwidth. In undoped and some heavily doped (10-3mol/mol Fe,O,) samples the broader i.r. band at 3420cm-’ (attributed to substitutional OH- [S]) could be detected, as well. In Fig. 6 the absorption coefficient of the i.r. band at 3445 cm-’ is plotted vs the nominal concentration of Fe. The absorption coefficient did not correlate over the full concentration range with the Fe content and any of the Fe(l)-Fe(7) ESR lines (Fig. 2). As mentioned above the same i.r. band (with neither position nor halfwidth changed) was found when ZnWO, was doped by other triply charged ions like Cr and Al or even in undoped crystals; however, it was very weak for Na- and Li-doped crystals (Table 2). Since it was shown in [4] that a band at 350 nm is due to Fe3+ (here the band probably involves the absorption of Fe3+(C,) and that of the Fe3+complexes, as well), the Fe content can be checked by observing the U.V. absorption (Table 2). Me3+ ions
Fe nominal cont. (mol/mol) Fig. 6. The i.r. absorbance of the band at 3445 cm-’ vs the nominal Fe content of the samples. The smallest Fe concentration corresponds to the undoped crystal determined by atomic absorption [4]. The i.r. spectra were measured at room temperature.
Fe centers
and charge
893
compensation
Table 2. Comparison of uv. absorption coefficients (at 350nm, proportional to the Fe’+ concentration); the ESR intensity (proportional to the Fe)+(C,) center concentration) and i.r. absorption coefficients (at 3445 cm-’ proportional to the OH center concentration) in different ZnWO, samples. All data were taken at RT. The Fe U.V. absorption could not be determined in ZnWO,: Cr samples because of the superposition of the Cr absorption band. The given concentrations are nominal ones except those marked with asterisks measured by atomic absorption
Dopant
Concentration (mol/mol)
Undoped Fe’+ Fe’+ Cr’+ Cr’+ Al’f Na+ Li+
7 x 10m6(Fe)* 10m4(Fe)* 9 x 10-4(Fe)* 10m4(Cr) 10m3(Cr) 4 x 10-S(Al)* 7 x 10m5(Na)* IO-‘(Li)
U.V. absorption coefficient at 350nm
Rel. ESR intensity of Fe’+(C,) (a.u.) 1 6.7 13.2 0 0 0.45 3.6 2
3 14 113 2.2 4.9 4.1
in small concentrations generally enhance, and Me+ ions suppress the intensity of the band at 3445 cm-’ while the Fe3+ concentration is decreased in the former case, contrary to Me+ enhancing it. This means that the Me3+ ions compete with Fe3+ ions, while Me+ ions charge-compensating the Fe3+ ions increase the Fe3+ concentration and lead to a simultaneous decrease in the OH band at 3445 cm-‘.
For ing to giving to the
i.r. absorption coefficient at 3445cm-’ 0.42 f 0.08 0.81 + 0.09 0.33 * 0.03 0.44 0.23 0.73 <0.05
both i.r. bands we found D - 12, correspondx = + 16”, where the estimated error is f4”, the transition moment orientation with respect X axis. These angles correspond to +6” and
-26” with respect to the crystallographic [loo] axis, respectively. Assuming that the transition moment and the OH bond orientations are parallel one may get information
about the OH positions.
Looking
at
The orientation of the electric dipole moment can be determined from the angular dependence of the
the projection of the 0 octahedron around the Zn ion in the (010) plane one can find an 0-O connecting
polarized absorption. In monoclinic ZnWO,, with the two-fold axis along [OlO], the dichroic ratio D of the maximum and minimum absorption for light polarization in the (010) plane yields the orientation of the transition moment with respect to the dielectric axis X. The angular dependence of the absorbance can be expressed by the following equation:
line (the 0 ions are at a distance of 0.297 nm) and a Zn-O bond with an angle of + 16.7” and - 31.5” with respect to the crystallographic [ 1001 axis, respectively (Fig. 7). These values are in good agreement with the experimental ones, taking into consideration that the calculated values refer to the undistorted lattice while
a(O) = -ln(e-“rdcos20
+ e-““sin20),
may occur,
(3)
where 0 is the angle of the electric vector with respect to the X dielectric axis, a, and a, are absorbances at the maximum and minimum, i.e. parallel to X and Z dielectric axes, respectively. The dichroic ratio D can be expressed
around the defects some lattice relaxation thus changing the ion positions.
I
WI
A
by D = ax/a2 = m cot2X,
(4)
where x is the angle between the transition moment and the X axis [lS]. We have set m = n,/n, - 1 as a good approximation [ 161. Fiildvari et al. [S] had measured the angular dependences for both i.r. bands, but the reported dichroic ratios do not refer unambiguously to the (010) plane. Therefore we repeated the measurements at RT and found a maximum absorbance for light polarization parallel to X. Precise measurements of the wavelength dependent optical constants yield the dielectric X axis in the (010) plane with an angle of N - 10” with respect to the crystallographic [loo] axis [16].
* r1001
a = - 31.50 B = 16.7”
Fig. 7. Projection of a Zn (full circle) and its six neighboring 0 (open circles) ions in the (010) plane. The long (I), medium (m) and short (s) Zn-O bonds (0.216, 0.210 and 0.205 nm, respectively) and the crystallographic axes are indicated. a represents the angle between a Zna bond and the [loo] crystallographic axis, while /I is the angle of an @O connecting line with respect to the [lOO] axis.
5. DISCUSSION
The spin-Hamiltonian parameters obtained for the Fc(1) centers differ only slightly from those of the Fe3”(C,) (non-locally compensated) center given in [6] (Table 1). Some additional defect is believed to perturb the Fe3+ ion leading to the reduced symmetry. This defect can be neither a nearby OH-ion because of the lack of a proton SHF interaction and correlation with the i.r. bands, nor a charge-compensating Na+ or Li+ ion nearby because of the lack of an appropriate SHF interaction. Therefore the defect is assumed to be a Zn vacancy serving as a partial charge compensator, since a vacancy is the most common charge compensator in oxide crystals, as well. A first-neighbor vacancy is expected to provide a strong crystalline electric field similar to that reported for Fe3 + associated with the oxygen vacancy in SrTiO,, causing an approximate doubling of the Bi fine structure parameter [17]. Although the Fe3+-nearest neighbour Zn vacancy distance is larger than that of the Fe’+-0 vacancy, we believe that the vacancy must be at a larger distance since none of the parameters including the eigenvectors change by much. If a Zn vacancy really perturbs the Fe environment, annealing of the crystal should destroy the Fe-Zn vacancy complex. This was in fact observed. The further Fe complexes are also supposed to be due to Zn vacancy-perturbed Fe centers where the vacancies may be at different distances and directions. As mentioned above, the attribution of the i.r. band at 3445 cm-’ to the Fe3+-OH complex does not seem to be very likely. Additionally one has to mention that two i.r. bands were attributed to Pt-OH complexes at 2852 and 2920 cm-’ [S]. In [lo] the ESR spectrum of Pt in ZnWO, did not show any SHF structure, indicating the presence of a nearby OH. (Moreover the same bands could be observed in the case of other host materials like TeO,, LiNbO, and NaCl. These bands seem to be artifacts or surface contaminations.) One can point out from these findings that even the Pt3+ charge surplus is not compensated locally by a nearby OH- ion. From the optical absorption results we conclude that the i.r. band at 3445 cm-’ is due to centers which compensate the surplus charge of Me3+ ions if they are present in small concentration. Since Me3+ ions do not change the i.r. band position and halfwidth, therefore the impurity ions cannot be near to the i.r. active center. This conclusion agrees with the ESR results, according to which the paramagnetic Fe centers did not correlate with the i.r. band, and the ESR lines did not show a SHE: splitting due to a nearby OH ion. Thus the charge compensation occurs non-locally. All these findings lead us to
suggest a modified model for the OH center. An OHion associated with a Zn vacancy explains the experiments correctly. This type of center has been considered in simple oxides like MgO and CaO and designated as V6u with i.r. bands at 3296 and 3418 cm-i, respectively [18]. From the point of view of charge compensation of Van center, having a net charge -e with respect to the perfect crystal, satisfies the requirement for compensation of a Me3+ ion having a net charge +e with respect to the perfect lattice. It acts as a charge compensator when the iron concentration is small (Fig. 6) and the Fe3+(C,) (i.e. unperturbed; nonlocally compensated) center is predominant compared to Fe complexes, like Fe(l) which is overcompensated by a Zn*+ vacancy. When these Fe complexes become dominant at higher nominal dopant concentrations (Fig. 2), the V&u center loses its importance, the charge compensation occurs via Zn vacancies and the concentration of the Voi, centers decreases. The i.r. band at 3420cm-’ with a halfwidth of 65cm-’ at RT was attributed to substitutional OH without any other perturbation nearby. In fact one of the transition moments was oriented approximately along one of the O-O bonds (Fig. 7). In the case of a substitutional OH- the net charge is +e; electrostatically it should be oriented in the direction of a negative ion, i.e. 02-. When a cation vacancy is present the OH- ion should be oriented along the OH-Zn vacancy connecting line as in MgO [I@ In fact, our polarization
%
3
0 1
Fig. 8. Projection of a Zn (full circle) and its six neighboring 0 (open circles) ions in the plane determined by the O(l), O(3) and O(5) ions. The long (I), medium (m) and short (s) ZnXI bonds (0.216, 0.210 and 0.205 nm, respectively) and the crystallographic axes are indicated. The 0-Zn-0 bonds are nearly linear, the angles and distances are as follows: for O(3)-Zn-O(4) 172” and 0.418nm, for O(6)-Zn-O(2) and O(5)-Zn-O(l) 166” and 0.419 nm, respectively.
Fe centers and charge compensation
results allow that one of the i-r, active centers is oriented approximately in the direction of an 0-Zn connection line (Fig. 7). Taking into account these orientations and assuming the above models the shape of the Lr. bands can be interpreted in the same way as for MgO [ 191.It has been shown in fl9,20] that for the linear system of the absorption line deOH . f . 0, the half~d~ pends on the separation of the 0 ions. For as large as 0.42 nm distances the predicted linewidth is very small, while for a distance of 0.297 nm it is expected to be of the order of 100 cm-‘. The latter case is in good agreement with substitutional OH. Although ZnWO, is monoclinic the distortion of the oxygen octahedron is not very large around a Zn ion and the possible 0-Zn-0 bonds are nearly linear (Fig. 8). Calculating the angles and distances they are as follows:O(3)-Zn~(4)(~7~*)~.41~ nm,O(6)-Zn-O(2) and O(S)-Zn-U(1) (166”) 0,419nm. For the Vo, center the observed iinewidth of 13 cm-’ [S] agrees with the expectations. From our polarized absorption results the latter two orientations seem to be the most probable orientations for the V& center, 6. CONCLUSIONS A number of Fe3+ complexes with reduced symmetry (C,) were found in ZnWO, doped by Fe. One of these centers was ascribed to Fe3+ ton . associated probably with a Zn2+ vacancy not very close to the Fe’+ ion. The further Fe compiexes are also supposed to be due to Fe centers perturbed by Zn vacancies where the vacancies may be in different distances and orientations from the Fe ions. The surpXus charge of Me3* ions are consequently compensated by Zn vacancies when the Me3+ concentration is > 10m4mol/mol. If the Fe3+ (or Me’+) concentrations was < IO-” mol/mol the charge compensation occurred non-locally by means of V& centers. This center is associated with an i.r. band at 3445 cm-‘. The absorption parameters, the polarization behavior and the dependence of this band on Me-dopants ied to the identification of the VG, center.
895
Acknowledgements-This research was supported jointly by DFG, SFB 225 and the National Science -Research Foundation KYI’KA~ of Hunnarv. The authors wish to express their gratitude to Mrs x. ‘P&er and Dr L. Kovacs for helpful discussions, and to Mrs Zs. Tbth, Mr I. Cravero, E. Possenriede and A. Griine for assistance.
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