Optical, magneto-optical and EPR study of chromium impurities in Bi4Ge3O12 single crystal

J. Phys. Chem. Solids Vol. 34, No. 7. pp. 8094 16, 1993 Rintcd in Great Britain.

OPTICAL, CHROMIUM

0022-36!97/?33 s6.00 + 0.00 pagamon Pm8 Ltd

MAGNETO-OPTICAL AND EPR STUDY OF IMPURITIES IN Bi,Ge30,, SINGLE CRYSTALS

E. MOYA,? C. ZALJJO,~ B. BRIAT,$$ V. TOPA$ (I and F. J. ~~PEzT[ tInstituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientif~cas, C-IV, Campus Universitario de Cantoblanco, 28049 Madrid, Spain &aboratoire d’Optique Physique, ESPCI, 10 rue Vauquelin, 75005 Paris, France FDepartamento de Fisk de Material=, C-IV, Universidad Aut6noma de Madrid, 28049, Spain (Received 30 October 1992; accepted 12 February 1993) Abstract-The spectroscopic properties of Cr impurities in four Bi,Ge,O,* single crystals of different origins have been investigated in detail with thne complementary techniques. Optical absorption, electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) demonstrate that Cr’+ at the tetrahedral germanium site is by far the dominant species in these crystals. Low temperature MCD is shown to be appropriate for a nondestructive evaluation of the amount of dopant. Lattice defects appear to be present in as-grown crystals. Diamagnetic centers, contributing to the near-u.v. absorption, are also formed by U.V.irradiation at 310 nm. Finally X-ray irradiation at 15 K produces similar but more drastic effects on the absorption spectra. Keywords: Bi,Ge,O,,, BGO/Cr, bismuth germanate, magneto-optical effects, photorefractive materials, electron paramagnetic resonance, lattice defects, photoinduced changes.

1. INTRODUCI’ION

Undoped Bi&e30u (BGO 2:3) single crystals have found practical applications as scintillators in high energy photon and particle detectors [l]. However, these applications are limited by an optical damage (decrease of the luminescence output) induced by the photons or particles to be measured [2,3]. Although it is believed that this damage is related to the presence of impurities [4,5], work performed on intentionally doped BjGe3011 is scarce. As a consequence, the microscopic origin of the damage is still unclear. In Fe-, Gd-, or Mn-doped crystals, it has been found recently [6] that U.V. irradiation below the bandgap(24,000cm-‘~Ed36,000rm-‘)inducesa broad .optical absorption which looks similar to that found in crystals irradiated with photons of higher energy (y - or X-rays). Moreover, the increase of the optical absorption seems to be related to a change of the oxidation state of these doping ions, The addition of C!xQ to Bi,,wO,l single crystals enabled us to write holographic gratings in the material [fl. We have ascribed these gratings to the existence of a photorefractive effect. The room temperature absorption spectrum of Crdoped

#Author to whom correspondence should be addressed. lIPresent address: institute de Physique Atomique, 1 rue Atomistilor. 76900 Magurele-Bucarest, Romania.

Bi.,Ge301z shows [7J a number of components covering the spectal range 6000-36,000 cm-’ (band edge). The origin of these features has not yet been discussed because of our uncertainty regarding the valence state and lattice site of the chromium ions incorporated. More recently [8], we found that the above crystals present a characteristic EPR spectrum which we have ascribed to the presence of Cfl+ ions substituting for Ge’+ ions in the matrix. These ions sit at a site of D, symmetry. In this work, we have investigated four Crdoped single crystals by means of three complementary techniques, i.e. optical absorption, electron paramagnet;: resonance (EPR) and magnetic circular dichroism (MCD). The latter is defined as the difference (Aa) in the absorption .coe&ient (a in cm-‘) for left and right circularly polarized lights when the sample is submitted to a longitudinal magnetic field B. MCD is expected to be very large at liquid helium temperatures for paramagnetic Centers. Our main purpose was to explore the role of preparative conditions and irradiation history. It will be demonstrated that the three samples contain essentially Cr“+ at the germanium site, besides lattice defects created during the growth or/and during U.V. or X-ray irradiation. A preliminary account of this work has been published elsewhere [9, lo] while the interest of MCD to study photochromic effects in the parent material Bi,,GeO~ has been demonstrated recently [ll, 121.

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810 2. EXPERIMENTAL

TECHNIQUES

Several Cr-doped Bi,Ge,O,r single crystals were grown by the Cxochralski technique in the Crystal Growth Laboratory of the University of Madrid and in the Institute of Physics in Bucharest. In Madrid, chromium was incorporated to the melt as CrO, or Cr,Os with no sign&cant difference to the properties of the final product. The crystals (grown in air) look green, the coloration of the hiily-doped ones being inhomogeneous due to a segregation of chromium ions. The Cr-concentration was determined in the Spanish samples with a Perkin Elmer ICP/SSOO plasma emission spectrometer. A BiJZ+O,, crystal doped with 1% Cr,O, in the melt was found to have a Cr concentration of 1800 ppm (62 x 10” at. cmT3) at its top and 6700ppm at its bottom. The samples used in this study (reference SP) have been cut from the least doped part of the crystal. The preparative conditions for the romanian samples in an oxygen atmosphere have been given elsewhere [9]. Using different amounts of Cr,03 as the dopant, we produced three crystals which, under a thickness of roughly 1 mm, are deep green, light green and essentially colorless (samples ROl, R02 and R03, respectively). The experiments conducted in Madrid and in Paris are quite complementary. In Madrid, optical absorption spectra have been recorded with a Varian spectrophotometer model 2415. In these experiments, the temperature of the sample was varied with a closecycle helium cryostat operating between 15 K and room temperature (hereafter RT). EPR spectra have been measured at 80 K, using a Varian spectrometer model E-12. Room temperature U.V. irradiation has been performed in situ both in optical absorption and in EPR experiments. The monochromated light (310 nm) from a 900 W xenon lamp has been guided to the, measuring cavities using a multimode optical waveguide. The irradiation dose was about 120 /AW cmv2. X-ray irradiation has been performed at 15 K. The polychromatic emission of a Siemens AG source, model Kristalloflex 2H, with a tungsten target was filtered with a 2mm Al plate. In Paris, MCD spectra have been measured at T < 4.2 K by placing the samples in an immersion cryostat containing a superconducting magnet producing a magnetic field B of 3 T at the lowest temperature achievable (1.3 K). The principles of such measurements have long been established [13]. Our present dichrometer incorporates a home-made photoelastic modulator [14] and a lock-in amplifier (Stanford Research, SR 510). The magnetic field was oriented along the direction of propagation of the light beam, parallel to a tetragonal axis of

the crystal. MCD spectra were calibrated with a freshly prepared solution of camphorquinone in methanol which shows a natural circular dichroism Aa/a = -0.0105 at 21,55Ocm-‘. Under these conditions, the low temperature MCD of a KC1 crystal doped with potassium ferricyanide is expected [13b, 151 to be positive for the first charge transfer band at 23,500 cm-’ with Ax/a = th (g@/2kT). Such a measurement was repeated in the course of our experiments in order to ascertain the MCD sign. We found g = 1.67, in very good agreement with the value (1.72) reported previously [15]. Note that absorption spectra at 1.4 K have also been taken on the same instrument by using an additional beam splitter placed prior to the sample, and a second photomultiplier collecting the absorbed and reference beams for an electronic comparison. Spectra have been measured in the spectral range 13,000-32,0OO~m-~, with a narrow spectral band width of 0.75 A. Illumination of the crystals in Paris was performed with a 450 W xenon lamp coupled with a broad band U.V. filter (Schott, UG5). 3. EXPERIMENTAL

RESULTS

3.1. Absorption and EPR data Figure 1 shows an overall view of the absorption spectrum of sample SP at 300 K and 15 K, the spectrum of an undoped crystal being also given for comparison. Although the 15 K spectrum is slightly better resolved than the room temperature one, most of the bands do overlap each other. The major features of the spectra are the following: (i) a weak band (A) in the 8500-11,500 cm-’ spectral region, which shows some vibronic structure (see inset); (ii) several resolved absorption peaks (B) covering the 11,500-17,000 cm-’ range; (iii) weak components (C) between 18,000 cm-’ and 21,OOOcm-I; (iv) above 21,000 cm-‘, one observes several bumps around 24,000 (D), 28,000 (E) and 34,000 cm-’ (F). Using a 1 cm thick crystal, we have also seen a very weak band around 78OOcm-‘. Whether it corresponds to an impurity or to a chromium ion is not yet clear and it will therefore not be discussed here. The results of a U.V. or X-ray irradiation of sample SP are displayed in Fig. 2. Samples irradiated at 300 K with 310 nm light (Fig. 2a) show a slight modification of the optical absorption spectrum. The changes saturate after 10 min of light irradiation and they can be reversed by heating the sample for 1 h at 500°C. The intensity of band B decreases by about 7% while that of band A is also slightly lowered. By contrast, band D is enhanced upon U.V. illtination. The behaviour of band C is uncertain in this experiment.

Chromium impurities in Bi@ejOIl single crystals

811

loo -

6600

10066

16000

80600

86000

30000

a6000

Fig. 1. Survey optical absorption spectra of Crdoped Bi,Ge,O,, single crystal (sample SP) measured at 300 K (dashed line) and IS K (solid line). The inset of the 6gure shows a detail of the spectrum at 15 K. The spectrum at I5 K of an undoped crystal (dotted line) is also given for comparison.

As shown in Fig. 2b, the irradiation of SP at 15 K with polychromatic X-rays induces greater changes in the absorption spectrum. Band B suffers now a large decmase (30%) while band D is again enhanced. Band A apparently increases upon X-ray irradiation. This is actually due to the creation of a defect which

is also formed in undoped samples [ 161. This defect is also responsible for the rise of the absorption coefficient between 16,008 and 21,080 cm-‘. The creation of this defect was independently checked on a different sample (R02) by taking an absorption spectrum at 77 K prior to and after X-ray irradiation at this same temperature. It has been mentioned above that a characteristic EPR signal occurs at 80 K for sample SP. This signal is also sensitive to U.V. irradiation. Figure 3 shows the result of a twin experiment of that displayed in Fig. 2a. One observes that the EPR intensity decreases by about 5% after U.V. irradiation. We mention tinally that, under a Xe lamp light excitation of sample SP at 77K, we have not observed any luminescence characteristic of Crj+. In two other experiments performed at 10K with a very sensitive Ge detector on samples ROl and R03 down to 60OOcn~-~, we did not see either the sharp and strong lines found for Cr in forstetite [Il.

eo-

3.2. iUCD and absorption data

lo-

0

I

Fig. 2. Optical absorption spectra of a Crdopad crystal (SP) before (full lines) and aftar (broken lines) irradiation. To obtain a refwsnx state, the sample was heated for 1 h at 5WC. (al 1 h U.V.irradMon (310nm) andmeasurement at room temperature; (b) 17h X-ray irradiation at 15K and optical absorption at the same temperature.

Figure 4 shows survey (fast scan) MCD spectra for three “as received’* samples at 4.2 K and under a magnetic field of 2.5 T. Note that the ordinate scale is only appropriate for sample SP. The two other spectra have been multiplied by an appropriate factor so as to match the SP spectrum in the near i.r. region. It is clear from these data that the same species is responsible for most of the MCD features up to at least 21,080 cm-‘. This point is con8rmed by the fact that, for the three crystals and in the same spectral region, the ratio between the spectra at 1.4 K and 4.2 K is independent of the considered energy. Slight

E. MOYAet al.

812

2oLo Blho

Bsbil

4obo

4l IO

MAGNETIC FIELD/G Fig. 3. Electron pammagnetic resonance spectra (80 K) of Crdoped Bi@e,O,r (SP) before (full line) and after (broken line) 1 h of room temperature irradiation with 310 run light. To obtain a reference state, tbe sample was heated before. illumination for 1 h at 500°C.

deviations

(at most 6% at 3O,OOOcm-‘) occur at

higher energies. Using the same scaling factors, we have plotted the corresponding

ahsoxptiou

spectra in Fig. 5. They

match each other solely in the region of the C band

around 18,000-20,000 cm-‘. Actually, a comparison of the data in Figs 4 and 5 leads us to the firm conclusion that “as received” crystals show absorption bands which do not contribute significantly to the &ED. They are therefore assigned to diamagnetic centres. The least “defectuous” crystal appears to he

ROl since it presents the weakest absorption around 23,000 cm-‘. The effect of U.V. illumination on the MCD has been examined very carefully (low speed scans) for sample SP. After heating it for 30 min at 3OO”C,one obtains the spectrum shown in Fig. 4 (actually, the two peaks at lower energy appear 20% higher because the scanning speed is now appropriate). The inset of Fig. 4 shows the difference between the initial spectrum and that after illumination. Up to 21,000 cm-‘, this difference spectrum is proportional to the initial one and it indicates a 10% reduction of the centre responsible for the MCD features. At higher energies, illumination causes the appearance of

40

R02

0

-20 li

II , XJ

16cHJo

E/cm-’ 2oooo

24ooo

2mlm

32m

Fig. 4. Comparison of the MCD spectra of samples SP, ROl and R02 at 4.2 K and under 2.5 T. The ordinate scale is only appropriate for sample SP (see text). The scaling factors for samples SP, ROl and R02 were 1, 1.72 and 26.9, respectively. The inset shows the difference between the initial spectrum of sample SP and that after U.V. illumination.

rmo

Mooo

2mJo

24000

28ooo

-4 32m

Fig. 5. ComParison of the optical absorption spectra of sampies SP, RO1 and R02 at 1.4K. The ordin@ scale is only appropriate for sampk SP. As to RO1 and 802, we used the same sealing factors as those in Fig. 4.

Chromium impurities in Bi&,O,,

single crystals

813

tions for different vahres of S and a Iande factor g of 2. Our data follow the behavior expected for a spin 1 ground state. A closer examination of the slope of the experimental curves versus the field below 1 T shows that the g factor is slightly lower than 2, in good agreement with the EPR data [8]. Several saturation experiments have also been carried out in the range 22,000-3O,OOOcm-‘, with similar results. We therefore conclude that the broad MCD features in the visible and near U.V. have essentially the same origin as the sharper ones below 21,OOOcn-‘. 0

1

2

3

Fig. 6. Magnetic field dependence of the various MCD LX&S of Cr-doued BiLie,O,, single crvstals at 1.4 K. The &rvesare no&al&i ai ihe &xi&m magnetic field strength (3 T). The full lines represent the expected dependence for a spin-only ground state of l/2, 1 and 3/2 (we use an isotropic g factor of 2). For clarity, only three sets of experimental points (sample SP) are shown for 13,500 cm-’ (O), l4,449cm-’ (0) and 19,258cm-’ (0).

two extra weak MCD features at 24,000 and 28,OOOcm-‘, in fair agreement with the absorption results shown in Fig. 2. MCD and absorption spectra have been measured under a thickness of 3.91 mm for our least concentrated (colorless) crystal R03. These data have been shown previously in Fig. 4a of Ref. 9 (181 and compared to those for sample ROl in Ref. 10. Although a for R03 is very weak (0.6cm-’ at 14,000 cm-‘), the MCD shows again very clearly the features accompanying bands B and C on Fig. 4. However, they are now superimposed on two very broad negative MCD features with their maximum around 20,000 and 27,000 cm-‘. These are associated to at least one paramagnetic defect of unknown origin. In order to gain more information about the origin of MCD bands, we have determined the field variation (up to 3 T at 1.4 K) of the seven peaks encountered below 21,OOOcm-‘. In the case of a spin-only ground state S and if the spins are at thermal equilibrium, this procedure is expected [13b] to provide the spin value of the ground state associated to a particular optical transition, since Aa is then proportional to the corresponding magnetization. Two procedures were used with several crystals: (i) continuous rise of the field at a speed of 0.58 T per minute; (ii) measurements at a set of tixed field values after verifying that the MCD signal had reached its maximum. Our results did not depend upon the procedure (i.e. the spin temperature is the bath temperature) or the crystal. Representative data for three MCD peaks are shown in a normalized form (at 3 T) on Fig. 6, together with theoretical predic-

4. DISCUSSION

4.1. Chromium concentration

The MCD signals shown in Fig. 4 are huge since Au/a at 4.2 K is close to 0.1 for all the bands. Due to this very high sensitivity but also to the fortunate occurrence of alternating signs, the MCD spectrum is much better resolved than the absorption one. It appears in particular that band B is more complex than one could think since, e.g. the positive MCD peak around 16,080 cm-’ has no obvious counterpart in the absorption spectrum. Even for the weak band C in the 18,000-24,000 cm-’ range, the MCD signal to noise ratio is very large while the base line (in the absence of a magnetic field) is perfectly defined. Note that the three negative MCD peaks are equally spaced by 715cm-‘. This frequency is very close to one encountered in the Raman spectrum [19] and should be assigned to a breathing vibrational mode. On the basis of the data shown in Fig. 4, it is clear that we can safely use the MCD magnitude of the seven peaks encountered below 21,OOOcm-’ to determine the relative chromium concentration in the four crystals investigated. These results have been summarized in Table la. On the contrary, the relative intensities of the various absorption components (compare bands B and C on Fig. 5) vary from sample

Table 1. (a) Chromium concentration [Cr], determined in various samples from the MCD magnitude of the seven peaks up to 21,OOOcm-I. For sample SP, the concentration has been determined with a plasma emission spectrometer. (b) Product of the optical absorption wetlicient a at 14,500 cm-’ by the factor A = 50 obtained for sample SP through the formula [Cr], = 1800 ppm = k.am.

R02 R03

1800 1046 67 4

1800 1700 210 57

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E. MOYA et al.

Besides the relatively weak d-d band (2E+2T2) in the lO,OOO-14,000cm-’ region, two broad charge transfer bands have been observed with their maximum around 17,000 cm-’ and 27,OOOcm-’ (‘T, and 2T2 excited state, respectively). The relative strengths of these three bands is 1, 5 and 60. In view of our absorption spectra alone, the possible occurrence of Cr”+ at the Ge site would therefore seem quite reasonable. The saturation behavior of the MCD above 13,OOOcm-’ appears however to exclude the possibility of a spin doublet ground state. A similar 4.2. Spectroscopic assignments argument would hold in the case of CrS+ at the Bi site. Finally, if CrS+ is present or created upon irradiation, The question now arises as to the site occupied by chromium in the lattice and their oxidation state. it can only be in a very small amount. 4.2.3. Evidence for Cf’+. We are finally left with Bi,Ge,O,, belongs to the crystalline symmetry groups 43m. This structure, known as eulytite, is formed by the possibility of CIA+ at the Ge or/and at the Bi site. a cubic arrangement of strongly distorted oxygen Cr4+ in tetrahedral symmetry is thought to be responoctahedra surrounding each Bi3+ ion and slightly sible for the lasing properties of Cr-doped forsterite distorted oxygen tetrahedra surrounding each Ge4+ [17,30]. It occurs also in Cr- and Ca-codoped YAG ion [19-211. The point group symmetry of the center fibers [31] as well as in other Cr-doped garnets including only Bi’+ and the oxygens is C, while that [32,33]. For a d2 ion in Td symmetry, the ground state of [GeO,r- is D,. According to previous results is ‘A2 and three d-d transitions are spin-allowed to obtained by X-ray photoemission spectroscopy 181, the ‘T2, 3T,a and ‘T,,, excited states. Among these, plausible chromium centers in our crystals are CrJ+, 3A2--r3T2 is symmetry forbidden since the electric Cr4+ and CrS+. dipole moment operator transforms as T2. Cr’+ in 4.2.1. Absence ofCti+. Cti+ ions (d’) in oxides are approximately Oh sites has been found in garnets always octahedrally coordinated. Even when they [32,33]. The isoelectronic V3+ [23,34] and Ti2+ replace ions in lattices with another coordination, [23,35] ions have also been extensively investigated. such as in fluorites (eight F- ions are coordinated The presence of Cr“+ at the bismuth site is quite with divalent cations replaced by CIj+ ), Cr’+ induces unlikely in our crystals. The large trigonal distortion, a lattice distortion to remain coordinated octacoupled to spin-orbit coupling is expected [31] to hedrally [22]. The optical absorption of Cr3+ in Oh produce a ground state with a doublet and a singlet symmetry has been characterized extensively [23]. separated by several cm-‘. This situation cannot lead One observes two intense and well resolved bands to the saturation curves observed here. We advocate now that Cr“+ in Td symmetry is by corresponding to the 4A2,--t4T29, 4T,p spin-allowed transitions, the separation between them being typifar the dominant species in our crystals. First of all, cally 6000-75OOcm-‘. Additional weak and narrow it was the conclusion reached from our previous EPR lines associated to the 2E,,, 2T,, and 2T2r excited states data at 80 K [8]. The spin-Hamiltonian parameters are also present. Furthermore, luminescence emission used to fit them (spin 1 ground state) were g,, = 1.93 1, is usually observed. All these features lead to very gl = 1.919 and D = 0.054 cm-‘. Secondly, our satucharacteristic MCD spectra [9,24,25]. In Fig. 1 of [9] ration experiments for all the major MCD bands are one can see in particular that the MCD for sharp lines fully consistent with this model and exclude the is considerably enhanced at low temperatures, as presence of a spin l/2 or a spin 3/2 ground states. compared to that associated with the spin-allowed We therefore assign band A in the low energy bands. Actually, none of these features is observed region to the weak ‘A2-+‘T2 transition. The band is in the spectra presented here. No evidence has broadened by vibronic interactions and we deduce either been found from the analysis of our EPR and Dq = 8OOcm-’ from the most likely position of the fluorescence data or from our MCD saturation electric dipole origin. There is a considerable scope of experiments. We can therefore safely conclude that Racah B values in the literature according to Cr’+ does not substitute for Bi3+ in our crystals. the various authors [30-331. We have calculated 4.2.2. Plausible presence of CrS+. CrS+ (d’) is (Appendix) the energies of the lowest states expected another common oxidation state which has been for Cf’+ in T,, (or 0,) symmetry for extreme values found in several solid matrices [26-291. The spectroof B (see Table 2). For a B value of 6OOcm-‘, the scopic assignments given [28,29] for [Cr0413- in model predicts properly the observed positions of Ca,PO,Cl or in Li3P04 seem to be quite reliable. bands B and C. This low value of the B parameter to sample because of the contribution to the absorption of diamagnetic centres. This contribution leads to overestimate the chromium concentration if it is calculated from the optical absorption spectrum (see Table 1b). Knowing now that sample R03 contains 4 ppm of chromium and considering the MCD signal-to-noise ratio in the near i.r., we conclude that the minimum amount of dopant detectable by MCD (3 T, 1.4 K) is about 0.1 ppm.

Chromium impurities in Bi,Ge,O,, single crystals Table 2. Energies (cm-‘) Predicted for the lowest d2 states of Cr*+ in Td symmetry, as a function of the Racah E parameter (C/E = 4.5). The crystal field parameter Dq was chosen as 8OOcm-’ in tetrahedral symmetry (see text) Excited St&

‘T* ‘T,. ‘T,b ‘E,

Efor B=6OOcm-’ 1:z 20:295 9942

E for B = lOC@cm8000 13,480 25,520 16,320

815

lead us to the conclusion that at most 10% of the Cr*+ ions are transformed upon U.V. ilhnnination. No indication is found for the production of CP+ and we rather think that Cr’+ is formed, its absorption and MCD features being masked by those of the dominant Cr”+ species.

5. CONCLUSION

indicates a large influence of covalency. The question is finally whether the sign of the MCD is consistent with this interpretation. From this viewpoint a 3A,--r3T, transition in Td symmetry is expected to lead to similar results as a 3S-+3P transition for a free ion. First-order spin-orbit coupling lifts the degeneracy of the excited state and one obtains three components with J= 2, 1 and 0 at energies 1, -1 and -21. The relative intensities of the associated MCD peaks are predicted [36,371 to be - 1, 1 and 2, respectively. We have carried out the diagonalixation of the 20 x 20 matrix appropriate for d2 and found 1> 0 for ‘Tla and 1 < 0 for 3T,b. According to this model, MCD should have an S-shape with Au < 0 on the high energy side in the case of 3T,a,and Aa < 0 on the low energy side for 3Tlb. Of course, second-order spin-orbit coupling effects, as well as the tetragonal crystal field component may modify somewhat this picture. Considering the fact that our MCD spectra do show the predicted kind of structure, we finally assign band B to “Tlaand band C to 3T,b. The great difference of absorption strength for these two bands is presently unexplained. The strong positive MCD in the near U.V. is suggested to correspond to a charge transfer band. 4.3. Irradiation eflects As we have seen, U.V. irradiation of our crystals produces relatively small changes in the absorption, EPR and MCD spectra. This situation is drastically different from that found previously [6] for Fe-, Gd- and Mn-doped samples. The rise of a for band D in Fig. 2a is accompanied by a slight decrease of cc for band B. This however does not establish firmly that the valency of chromium is changed in the process since we have seen that one or several diamagnetic centers produced during the growth contribute to these bands. It might be that illumination simply changes their relative concentration. Band D is likely due to the creation of lattice defects, i.e. holes or electrons trapped by the lattice [161. Absorption, EPR and MCD data obtained for physically different samples cut from the same ingot

In summary we have presented a detailed spectroscopic investigation of four Cr-doped Bi,Ge3012 crystals with three complementary techniques: optical absorption, EPR and MCD. The latter was shown to be adequate for the in situ monitoring of the chromium concentration down to about 0.1 ppm. For such a low doping, the corresponding absorption bands would be completely masked by those of defect centers which are created during the crystal growth. In agreement with our previous EPR results, our MCD data demonstrate conclusively that Cr’+ at the Ge tetrahedral site is by far the dominant chromium ion in these crystals. This is mainly concluded from the saturation behaviour of the MCD versus the magnetic field at 1.4 K. Measurements have been performed on several crystals for a set of eight wave numbers corresponding to the major features in the spectra. In each case, the MCD follows the Brillouin function for a spin 1 system with a g factor slightly less than 2. The lack of observed luminescence for Cr4+ in our crystals is most likely due to the presence of a defect center with a relatively low-lying excited state. We have demonstrated the presence of a U.V. induced photochromic process in our crystals. This process has a low efficiency and it leads to the formation of lattice defects which increase the optical absorption in the visible region. The effects of an X-ray irradiation are much more spectacular on the absorption spectrum. The MCD and EPR features associated to CIA+ exhibit a small but reproducible reduction upon irradiation. We therefore believe that Ci+ is formed, although no firm spectroscopic evidence can be put forward at the moment, essentially because both Cr’+ and Cr5+ in tetrahedral symmetry present absorption bands and thus MCD features in the same spectral regions.

Acknowledgemrs-This collaboration has benefited from the CNRS-CSIC exchange program and is partially sup

ported by CAM (Spain) under grant C154/91. V. Topa gratefully acknowledaes the French Ministry of Research and Teclkology for &e award of a “Bourse de haut niveau” during his stay in Paris.

E. MOYAet al.

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+ 1 f {(lox + 9)2 + 144}“2]

‘E y + = 0.5[4x + 1 + 4.2 f {(20x - 1)2+ 48}“2] ‘T2 y f =0.51-6x

+ 1 i- 42 f ((10x - ly + 48}“2]

‘A, Y f = 0.5[4~ + 18 + 92 + ((20x - 2 - zy + 2q2 + 2~}“2] ‘A,

y=l2x-8

3T2 y=2x-8 ‘T,

y=2.x+4+2s.

x is chosen positive and negative for Oh and Td symmetries, respectively. Diagrams can be drawn by reference to the energy of the lowest state, i.e. ‘T, (y-,0,,) and ‘A, (T.,).