Charge states and optical transitions of vanadiumin Bi4Ge3O12 identified by MCD and ODMR

B. Briat a,*, A. Watterich b, F. Ramaz a, L. Kov�aacs b, B.C. Forget a,N. Romanov c

a Laboratoire d’Optique Physique, ESPCI, 10 rue Vauquelin, 75231 Paris C�eedex 05, Franceb Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Konkoly-Thege M. �uut 29-33,

1121 Budapest, Hungaryc Ioffe Physico-Technical Institute, Russian Academy of Sciences, Polytekhnicheskaia 26, 194021 St. Petersburg, Russia

Received 31 May 2002; accepted 4 June 2002

Abstract

The spectroscopic properties of V-doped Bi4Ge3O12 have been investigated in detail via several complementary

techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly

modified under UV-illumination and the initial state can be restored optically with visible light. Optical absorption and

magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UV-illumination

while a paramagnetic one is created. The latter shows a very characteristic S-shaped MCD pattern in the near-IR, which

is readily assigned to the 2E! 2T2 internal transition of tetragonal V4þ centers at the Ge sites. This assignment was

further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at

35 GHz. The g tensor of V4þ was found to be anisotropic with principal values gjj ¼ 1:81� 0:03 and g? ¼ 1:94� 0:02.Two additional MCD bands in the visible spectral region could be attributed to V4þ via ODMR measurements.

Ionization thresholds for holes (V5þ) and electrons (V4þ) were determined by optical absorption experiments and the

V4þ=5þ donor level was positioned 1.9 eV below the conduction band. The presence of the V4þ=3þ acceptor state in the

forbidden band is also suggested.

� 2002 Elsevier Science B.V. All rights reserved.

PACS: 7820.L; 7850; 7630.M; 4270.G

Keywords: Bismuth germanate; Eulytite; Magnetic circular dichroism; EPR; Photorefractive; Photochromic

1. Introduction

Scintillating crystals have been developed for a

long time as a basic component in detectors [1,2]

for high-energy physics and medical imaging. A

typical example is Bi4Ge3O12 which has becomethe main component of high-resolution positron

emission tomography (PET). Single crystals of

this material can be grown with high quality and

in large sizes. Their transparency range goes ap-

proximately from 300 to 6000 nm.

Another very interesting property of Bi4Ge3O12is the photorefraction. In this material, holographic

Optical Materials 20 (2002) 253–262

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*Corresponding author. Tel.: +33-40794583; fax: +33-

43362395.

E-mail address: briat@optique.espci.fr (B. Briat).

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PII: S0925-3467 (02 )00066-6

gratings based on the photorefractive effect have

first been induced at 442 nm in samples doped with

chromium [3]. It was later discovered [4] that

undoped samples present similar properties at

wavelengths shorter than 350 nm. Such short

wavelengths are in particular attractive for pho-tolithographic applications since they afford an

increased resolution. Doping Bi4Ge3O12 with Fe

and Mn leads to an enhanced photorefractive res-

ponse under UV-laser (351 nm) excitation [5].

Photorefraction in bulk crystals implies that at

least one deep level is present in the forbidden

band. Photoexcited charge carriers can be trapped

at the same or at a different level. When it occursat a different level, the absorption spectrum may

change and one refers to photochromism. This

effect has been investigated on Bi4Ge3O12 samples

doped with 3d elements such as iron [6,7], man-

ganese [6,8], cobalt [8,9] and chromium [7,10]. In

all samples the effect is optically and thermally [11]

reversible. In parallel, a number of electron para-

magnetic resonance (EPR) studies were carried out[10,12–18] on the same materials, in order to

specify the site and charge state of the doping ions.

In spite of these efforts, extremely little is known

about the charge transfer paths and the energy

levels implied in the photorefractive and photo-

chromic processes. Certainly, it is hard to make

correlations between broad optical absorption

bands in the ultraviolet–visible-near infrared (UV–VIS-near IR) regions and the EPR spectra of

paramagnetic transition metal defects. In the

present work we overcome this difficulty by

optically detected electron paramagnetic reso-

nance (ODMR) via a change of magnetic circular

dichroism (MCD) under resonance conditions

[19]. Basically, the magnetic field dependence of

MCD signals (saturation curve) is recorded inpresence of microwaves. A dip usually occurs in

this curve when the microwaves energy matches

some Zeeman splitting in the ground state.

ODMR measurements at different wavelengths

allow the tagging of MCD bands with the chemical

and structural information attached to EPR-active

defects while MCD-silent absorption bands con-

cern defects in their diamagnetic state.The ODMR technique is applied to vanadium

doped Bi4Ge3O12 which has not been investigated

before. Besides MCD and ODMR data, we shall

show results illustrating the change of the ab-

sorption spectrum under (or after) various sec-

ondary VIS-illuminations. A tentative model for

recharging processes will finally be presented. We

have already conducted a detailed optical, MCDand EPR study on chromium-doped Bi4Ge3O12[10] while preliminary MCD results have been

presented for samples doped with iron [7], man-

ganese and cobalt [8].

2. Experimental details

2.1. Structure and preparation of crystals

The crystal structure (eulytite) of Bi4Ge3O12belongs to the cubic space group I�443d, with fourchemical formulas in the unit cell. Each Ge4þ ion is

coordinated by four oxygen ions arranged in a

tetrahedron, which is slightly compressed along a

C4 axis of the crystal [17]. The local site symmetryat Ge4þ is D2d. Each Bi

3þ ion is coordinated by six

oxygen ions at a site with C3 symmetry, the

threefold axis being along a [1 1 1] direction of the

cubic cell. In this matrix, vanadium is expected to

occur as Vnþ, either with n ¼ 2� 5 at the Ge site orwith n ¼ 2� 3 at the Bi site. For both sites onemust consider different geometries of equivalent

centers in order to explain certain spectroscopicresults.

Single crystals of Bi4Ge3O12 were grown in

Budapest by the diameter controlled Czochralski

method using Bi2O3 and GeO2 powders. The

presence of impurities such as Al, Si, Te and Fe

was demonstrated earlier [20] in undoped Bi4Ge3-

O12 crystals, at the inclusions and cavities, by en-

ergy dispersive X-ray spectrometry (EDS). For thepresent studies 10�3 mol/mol V2O5 was added to

the melt and the V concentration in the crystal

was about 3:4� 10�4 mol/mol, as determined byatomic absorption spectroscopy besides some

other impurities (Fe, Cr, Mn and Zn) in the range

of 10�5–10�6 mol/mol. The crystal samples were

X-ray oriented, cut and polished.

Because of the photosensitivity of the materialwe had to standardize the initial or reference state:

the samples were annealed for 30 min in air at

254 B. Briat et al. / Optical Materials 20 (2002) 253–262

500 �C and then kept in the dark before furthertreatments. Even freshly grown samples should

have been annealed because they might have been

illuminated accidently. These standardized sam-

ples showed each time the same optical absorption:

hereafter the initial state will be designated as an-nealed.

Crystals were studied in the annealed state and

after/under various illuminations with broadband

and interference filters. Most absorption and

MCD experiments were conducted on a sample of

thickness 1.44 mm with the light propagating

along a [1 1 0] axis. Another thinner sample (1.02

mm) oriented perpendicular to [1 1 1] was also usedfor ODMR experiments.

2.2. Instrumentation

Following the practice of chemists [22,23] MCD

is defined as the differential absorbance DA (or

absorption coefficient Da) presented by a samplefor left (rþ) and right (r�) circularly polarizedlight (CPL) propagating along the direction of an

applied magnetic field B. Right-hand CPL means

that an observer receiving the radiation observes a

clockwise rotation of the electric vector tip in a

plane perpendicular to B. Then the evaluation of

the absorption cross-section of left CPL (right

CPL) for an electronic transition from a ground

state component jii to an excited one jji proceedsvia the evaluation of the quantity jhjjmþjiij2ðjhjjm�jiij2), where m� ¼ ð�i=

ffiffiffi

2p

Þðmx � imyÞ arecomponents of the electric dipole moment opera-

tor.

For MCD measurements the sample was im-

mersed in a liquid He cryostat, at the center of a

superconducting magnet providing B up to 3.2 T

at 1.4 K (pumping on the bath). The light origi-nating from a Triax Jobin Yvon monochromator

was passed through a linear polarizer (Glaze-

brook) and then through an infrasil photoelastic

quarter-wave modulator (PEM, home-made),

whose optical axes were placed at a 45� angle tothe axis of the polarizer, in order to create alter-

natively left and right circularly polarized com-

ponents with high-frequency (�50 kHz). Theintensity of the light beam is further chopped

mechanically at low-frequency (1 kHz). In the

present work we used a photomultiplier in UV–

VIS or a silicon photodetector in the near-IR

range. Two separate lock-in amplifiers then pro-

cess the signal arising from this detector and DA isobtained from the ratio of the high-frequency (Vhf )and low-frequency (Vlf ) outputs as DA ¼ CVhf=Vlf .The sign and absolute value of the constant C can

be determined by placing an optically active sam-

ple along the light path and comparing Vhf=Vlf tothe natural circular dichroism measured for the

same sample on a Jobin Yvon or Jasco commercial

instrument. We currently use an alcoholic solution

of camphorquinone or a crystal of a-nickel sul-phate hexahydrate grown in Paris.ODMR in the Q-bands (�35 GHz) was mea-

sured by monitoring MCD signals versus the

magnetic field strength under microwaves (Gunn

diode, 250 mW at maximum) without any cavity.

The sample was placed near an open end of the 35

GHz waveguide. Once a magnetic resonance was

detected, MCD spectra were eventually taken at

the appropriate magnetic field in the presence orabsence of microwaves, their difference providing

an MCD spectrum of a specific paramagnetic cen-

ter (MCD tagged by EPR). For absorption mea-

surements a Cary 05E Varian spectrometer was

used, from �300 K down to 5 K by means of a

cold finger Oxford helium cryostat. ODMR cali-

brations were made with Sn or Ge doped CdTe

since Sn3þCd and Ge3þCd show a sharp line which is

known from classical EPR [21] to correspond to

g ¼ 2:101 and g ¼ 2:145 respectively.

3. Results and discussion

3.1. Optical absorption and MCD

As illustrated in Fig. 1, the optical absorption

spectrum of annealed samples in the UV spectral

region shows a strong shoulder around 3.8 eV,

relatively close to the band gap (4.5 eV at 0 K).

UV-illumination above 3.2 eV, either at 300 K or

at 80 K, with a xenon lamp and a broad band filter

(UG11) covering also the gap region reduces

considerably the magnitude of this band and leadsto the creation of two new shoulders at lower en-

ergies (around 3.5 and 2.8 eV). At �300 K a

B. Briat et al. / Optical Materials 20 (2002) 253–262 255

qualitatively similar result was obtained by usingthe 365 nm (3.4 eV) line of a mercury lamp for 15

min. This demonstrates that band to band excita-

tion is not compulsory for the process to occur.

Another very spectacular effect of our illumina-

tions on Bi4Ge3O12:V is the appearance of a three-

component absorption band in the near-IR region

below 1.8 eV. None of these features occurs in

undoped crystals.As shown in Fig. 2, the initial spectrum can be

restored partially with VIS-illumination of suc-

cessively higher energies. Quite clearly the main

features of the difference spectra have signs op-

posite to those found after UV-illumination of the

initial (annealed) sample (Fig. 1). As indicated in

the inset of Fig. 2, the threshold for restoration is

found to be �1.9 eV at �300 K. The kinetics of thebleaching process was followed at 2.73 eV (455

nm) by recording (Fig. 3) the decrease of the ab-

sorbance under a secondary illumination at the

same energy (blue light). A good fit of the data

requires two exponential components, this being

the signature [24] for the participation of (at least)

two deep levels in the charge transfer processes.

Fig. 4 shows the MCD spectra in the visiblespectral region. Above 2.3 eV qualitatively similar

features are observed both in the annealed and

UV-illuminated states. The spectra are dominated

by two strong and broad components (gaussian

shape) peaking around 2.8 eV (Band 1) and 3.5 eV

Fig. 1. Optical absorbance of Bi4Ge3O12:V (thickness 1.44 mm)

at 80 K in the annealed state (a, dotted line) and after UV-

illumination (b, full line). The difference spectrum is shown in

c � a� b (dashed line).

Fig. 2. Optical restoring of the initial absorption at room

temperature. After taking one spectrum after UV-illumination

(a), the sample was illuminated for 20 min with a set of inter-

ference filters at successively higher energies ranging from 1.1 to

2.8 eV, and a new spectrum was taken. Curves b–d illustrate the

differences between each of these spectra and the UV-illumi-

nated one for red (650 nm, 1.8 eV), green (555 nm, 2.23 eV) and

blue (455 nm, 2.73 eV) illuminations respectively. The inset

shows the absorbance modification at 3.85 eV versus the pho-

ton energy of the secondary beam.

Fig. 3. Kinetics of the bleaching process. The absorbance of a

UV-illuminated sample is measured at 455 nm (2.73 eV) with a

weak probe beam of the spectrometer. Once a steady state is

reached (time 0), a more intense pump beam is directed towards

the sample and the decrease of the absorbance is recorded for

30 min (circles). The solid line shows the fit with two expo-

nential functions.

256 B. Briat et al. / Optical Materials 20 (2002) 253–262

(Band 2). They demonstrate the presence of one or

two paramagnetic defects since the MCD of broad

bands gives an insight into the spin polarization in

the ground state [10,23]. We emphasize that the

magnitude of the two components is considerably

smaller (19–25 times) in the annealed as compared

to the UV-illuminated state. No MCD band is

associated with the absorption component (Fig. 1)around 3.8 eV, this proving the diamagnetic

character of the associated defect. Below 2.2 eV we

observe additional weak MCD features in the

annealed state. These are certainly due to Cr4þ

impurities. Comparing the present dichroism

Da ¼ 2:3 DA=l (l is the thickness in cm) with theone measured in [10] for a chromium-doped sam-

ple grown in Madrid, we find that all featuresappear exactly at the same position. The ratio of

intensities is 3:2� 10�3 allowing to deduce chro-mium impurity concentration of 6 ppm (Cr atoms

per 106 host lattice molecules) in the V-doped

sample. In the annealed state we have also ob-

served a relatively sharp negative MCD feature at

2.91 eV (23,500 cm�1) with a vibrational compo-

nent at higher energy (inset of Fig. 4). A com-parison with [7] shows that these MCD signals are

due to a small amount of Fe3þ. In fact chemical

analysis with atomic absorption spectrometry has

also proven (see 2.1) the presence of Cr and Fe

impurities in Bi4Ge3O12:V.

In the near-IR (Fig. 5), an extremely weak

MCD was detected in the annealed state. In con-trast, very large (DA=A � 0:3) positive and nega-tive MCD signals are associated with the optically

created absorption features. At 1.4 K the MCD

and absorption spectra show a zero-phonon line at

1.14 eV (9231 cm�1) followed by a number of

components (separations <300 cm�1) due to lattice

vibrations [25].

3.2. Site and charges of vanadium

The spectroscopic properties of vanadium ions

in crystals are well documented. For example, yt-

trium aluminum garnet offers three different sites.

The absorption spectrum of V-doped samples

shows the presence of sharp (0.93 and 1.50 eV) and

broad (2.0 and 2.9 eV) features assigned to V3þ atthe tetrahedral site and at the octahedral site re-

spectively [26]. These features are absent from our

absorption and MCD spectra at 1.4 K. Actually,

the structure of the absorption spectrum shown in

Fig. 5 for the UV-illuminated state of Bi4Ge3O12:V

is reminiscent of that found in Mg2SiO4 and

Ca2GeO4 and assigned to the2E! 2T2 transition

Fig. 4. MCD spectum (visible spectral range) at 1.4 K of

Bi4Ge3O12:V (thickness 1.44 mm) in the annealed state (full

line) and in the UV-illuminated state (dots). The former spec-

trum has been magnified 19 times for comparison. In the an-

nealed state several features are assigned to Cr4þ and Fe3þ (see

inset). The open circles stand for the tagged MCD data (see

text).

Fig. 5. Near-IR absorption (thin solid line) and MCD (2.5 T,

thick solid line) spectra of Bi4Ge3O12:V (thickness 1.44 mm) at

1.4 K in the UV-illuminated state. The doted line shows the

MCD spectrum in the annealed state.

B. Briat et al. / Optical Materials 20 (2002) 253–262 257

of paramagnetic V4þ at a distorted tetrahedral site

[27]. We shall now provide additional evidence

that the presence of a V4þ=5þ level within the for-

bidden band is responsible for the gross features

observed in our absorption and MCD spectra.

3.3. Magnetic properties of the vanadium defects

At an early stage of this study EPR measure-

ments were carried out at a few temperatures down

to 20 K. Unfortunately, an EPR signal was found

neither in the annealed nor in the UV-illuminated

state. The lack of any kind of paramagnetic V

signal was puzzling. A possible cause was firstthought to be the usually very broad EPR line

width in doped Bi4Ge3O12 crystals. Additionally,51V (abundance 99.75%) has a nuclear spin I ¼7=2, therefore a splitting with eight lines was ex-pected due to the hyperfine interaction of the

electron with the V nucleus, this reducing further

the line intensities. The EPR signal could also be

quenched above a certain temperature, due to astrong vibronic coupling in the 2E ground state of

V4þ. Finally, it was also possible that the EPR

signal was fully saturated, even at 20 K.

Our ODMR measurements at 1.4 K demon-

strated that this was indeed the case. Our data at

two wavelengths in the near-IR region (colored

state of the crystal) for B along [1 1 0] are shown in

Fig. 6. The main feature corresponds to g ¼ 1:875.With the magnetic field set at 1.315 T (resonance

field), MCD spectra were taken at 1.4 K in the

absence (Fig. 7a) or presence (Fig. 7b) of micro-

waves. In the 1.1–1.8 eV region, they appear

identical, except for a 92% reduction when the

microwaves are on. This implies that MCD in the

selected spectral range is connected with the only

paramagnetic center under study.ODMR experiments were also conducted on

another UV-illuminated sample for B along [1 1 1].

The spectra recorded at maximum power of mi-

crowaves as well as at maximum attenuation (30

dB) with the monochromator set at 985 nm, are

presented in Fig. 8. Again, the MCD signal goes

close to zero at resonance (g ¼ 1:894). Quite re-markably this signal is solely reduced by roughly50% at maximum attenuation, demonstrating that

saturation is very easily achieved for the center.

This strongly suggests that the ground state of the

defect has a zero orbital angular momemtum,

which is consistent with the assignment to V4þ at

the Ge site with D2d symmetry.

Fig. 6. ODMR spectra (34.5 GHz) with the monochromator

set at 985 or 800 nm, for B oriented along [1 1 0]. The dotted line

shows the field dependence of the MCD at 800 nm in the ab-

sence of microwaves. The signal measured in the absence of

B has been substracted. The weak feature around 1.05 T (g ¼2:39) was not found in the course of experiments on a second

crystal with B along [1 1 1].

Fig. 7. MCD spectra (no smoothing) at 1.4 K for B set at the

resonance field, with microwaves off (a) and on (b). The second

spectrum has been magnified 12 times and translated for clarity.

The dip around 1.92 eV in spectrum b is possibly due to the

presence of Cr4þ impurities.

258 B. Briat et al. / Optical Materials 20 (2002) 253–262

Table 1 shows the EPR expectations for a te-

tragonal center (D2d site symmetry) when the

magnetic field is oriented along different symmetry

axes of a cubic crystal.

The EPR of V4þ at a fourfold coordinated D2d

site has been investigated in several zircon-type

crystals [26]. gjj and g? differ by less than 0.15 andthey are both smaller than 2. From the expressions

of g3–g5 given in Table 1, we expect that the mainODMR feature for B oriented along [1 1 1] and

[1 1 0] should be only slightly shifted, the latter

being broader than the former and more asym-

metric. Fig. 8 shows that this is indeed the case.

ODMR signals measured at B // [1 1 1] are broad

unresolved lines with the shape which can be fittedby a mixture of gaussian and lorentzian. The line

shape becomes closer to gaussian with a decrease

of the microwave power. The absence of the re-

solved hyperfine structure of vanadium may be

due to the broadening of the hyperfine compo-

nents because of strong saturation and exchange

interactions. The line shape from a record for[1 1 1] was used to fit ODMR spectra measured for

B along [1 1 0]. A good fit was obtained with two

components (1.325 and 1.282 T) with the intensity

ratio 2:1 which is expected (Table 1) for D2d site

symmetry.

Altogether it is unambiguous that UV-illumi-

nation formed V4þGe ions. Our analysis of the two

([1 1 0] and [1 1 1]) ODMR spectra lead to an esti-mate of the principal values of the g tensor:

gjj ¼ 1:81� 0:03 and g? ¼ 1:94� 0:02.

3.4. Spectroscopic model for V 4þ

According to Griffith [29], the 2E ground state

of a d1 ion in Td symmetry breaks up into two

doublets E0 � h and E00 � e under the combinedaction of the tetragonal field and spin-orbit cou-

pling. Under the approximation that orbitals have

a purely metallic character (ligand field theory),

the predictions concerning g-factors are the fol-lowing [29]:

e : gjj ¼ 2� 8x1 and g ?¼ 2� 2x2

h : gjj ¼ 2 and g? ¼ 2� 6x2

x1 ¼ f=E1 and x2 ¼ f=E2. E1 and E2 stand for theenergies of the two orbital components 2B1 and

2E(D2d notations) of the

2T2 manifold (�8500 cm�1

above 2E) and f is the spin-orbit coupling constantfor V4þ (250 cm�1 [30]). Since E1 � E2 � E1ðE2Þ,one predicts g? > gjj for an e ground state, whilethe converse holds for h. Similar results are ex-pected when the dynamic Jahn–Teller effect is ta-

ken into account [28]. Our experimental g-values

therefore demonstrate that je;�1=2i is the lowestground state component.

We return now to the absorption spectrum and

S-shaped MCD spectrum (UV-illuminated state)

shown in Fig. 5. The overall shape of the former

results from the combined effect of spin-orbit

coupling and the tetragonal distortion in the2T2 excited state, as well as the coupling of the

Fig. 8. The thick and thin solid lines show the ODMR signals

(34.5 GHz) observed at maximum microwave power at 985 nm

for B aligned along a [1 1 0] and [1 1 1] axis respectively. For the

sake of comparison, the former spectrum has been translated of

0.015 T towards higher fields. The [1 1 1] signal measured at

maximum attenuation is shown with a dotted line.

Table 1

Expression of the effective g factors for a tetragonal center in a

cubic matrix

B along Effective g value Intensity

[0 0 1] g1 ¼ gjj 1

g2 ¼ g? 2

[1 1 1] g3 ¼ 3�0:5½g2jj þ 2g2?�0:5

3

[1 1 0] g4 ¼ g? 1

g5 ¼ 2�0:5½g2jj þ g2?�0:5

2

B. Briat et al. / Optical Materials 20 (2002) 253–262 259

electronic states with lattice vibrations (Jahn–Teller effct). For a d1 ion, first order spin-orbit

coupling splits the 2T2 state into E00 at energy

f > O and U0 at energy �f=2. As demonstrated inearly MCD work, 1 selection rules for polarized

light can be evaluated between spinor subcompo-

nents of the ground state (2E) and of the excited

state (2T2) by using Griffith’s formalism [31]. These

are given in Table 2. Then, independently of thetetragonal distortion and of the electron-lattice

coupling, the expected MCD signs at low and high

energy in Fig. 5 depend solely upon the nature of

the ground level and the order of the spin-orbit

components. At very low temperatures, the sign

and magnitude of the expected MCD is imposed

by the selection rules from the lowest subcompo-

nent in the ground state since it carries most of thepopulation. Assuming that this state is j2E e�1=2i, then the balance between the intensities forrþ and r� lights is found (Table 2) to be positive

(+6) for the U0 state at lower energies and negative

()6) for the E00 state. These signs and relative

values are consistent with our experimental results

(Fig. 5). The reverse signs should be observed if

j2E h � 1=2i was lowest. MCD results are thusconsistent with our analysis of the ODMR data.

3.5. Charge transport properties

We return finally to our MCD data (Fig. 4) in

the VIS-near UV spectral region for the UV-illu-

minated sample. The spectrum is dominated by

two broad bands (1 and 2) peaking around 2.8 and3.5 eV. These bands are correlated to the presence

of vanadium since they are absent in the MCDspectrum of undoped samples. By measuring at

selected energies (from 1.9 to 3.5 eV) the magni-

tude of the dip in the MCD saturation curve at

resonance (tagged MCD), we could establish un-

ambiguously that bands 1 and 2 are both associ-

ated with V4þGe. Since this ion has only one internal

transition (2E! 2T2) in the near-IR, the bands are

necessarily associated to the ionization of electronsor holes from a localized state to the conduction

band or the valence band respectively. We assign

Band 1 at lower energies to the photoionization of

an electron from the V4þ=5þ level (semiconductor

notation) to the bottom of the conduction band:

V4þGe þ hm > 1:9 eV! V5þGe þ ecbThe level is located �1.9 eV below the conductionband. This follows from the fact that, after UV-

illumination, such an energy is required (Fig. 2) to

start erasing the near-IR band of V4þ while an

increase is simultaneously observed for the MCD-silent (V5þ is a diamagnetic d0 ion) absorption

band peaking at 3.9 eV. The latter is assigned to

the complementary direct process

V5þGe þ hm > 2:6 eV! V4þGe þ hvb

Upon illuminating into this band, V4þ is created at

the expense of V5þ and the characteristic and un-

ique internal band of the d1 ion is observed in the

near-IR. Band to band excitation may produce the

same effect with electrons being trapped by V5þ

and holes being trapped at some unknown accep-

tor level.

The existence of a second charge transfer bandcorrelated with V4þ is intriguing. The energy

threshold (most likely position of the non-

observed zero-phonon line) estimated from a

Table 2

Selection rules among the spinor subcomponents of 2E and 2T2 in tetrahedral symmetry. pc ¼ hJjjmcjIii (arbitrary unit), wherem� ¼ �ði=

ffiffiffi

2p

Þ ðmx � imyÞ and m0 ¼ imz

jh;þ1=2i jh;�1=2i je;þ1=2i je;�1=2iE00a00 p0 ¼ �2 pþ ¼ �p

2 p� ¼ �p6

E00b00 p� ¼ þp2 p0 ¼ þ2 pþ ¼ þp

6

U0k p� ¼ �1 p0 ¼ þ2p2 pþ ¼ �p3

U0k p� ¼ þp3 pþ ¼ þ3

U0l pþ ¼ þp3 p� ¼ þ3

U0m p0 ¼ þ2p2 pþ ¼ �1 p� ¼ �p3

1 Ref. [22] p. 177.

260 B. Briat et al. / Optical Materials 20 (2002) 253–262

gaussian analysis of the MCD curve (Fig. 4) in the

UV-illuminated state is �2.7 eV. Althought thebands structure in Bi4Ge3O12 does not seem to be

known, it is conceivable that this second MCD

band could concern the ionization of an electron

from the V4þ=5þ level to a level located above thebottom of the conduction band. Alternatively, one

can imagine that an acceptor state V4þ=3þ is lo-

cated slightly above the donor state. We note that

the two types of levels are thought to occur in

vanadium-doped silicon carbide [32]. Band 2 in

Fig. 4 would then be assigned to the ionization of

holes from this second level to the VB.

4. Conclusion

A detailed spectroscopic investigation has been

carried out on vanadium-doped Bi4Ge3O12 single

crystals. In annealed samples vanadium is only

present as V5þGe. Under near band gap illumination

holes are ionized to the valence band and V4þGe iscreated. Optical absorption demonstrates strong

photochromism, being reversed under illumination

with visible light, which ionizes electrons from V4þGeto the conduction band. Other valences of V seem

to be absent from our crystals.

As demonstrated by MCD measurements,

paramagnetic V4þGe shows a very characteristic2E! 2T2 internal band in the near-IR. Both thesign and magnitude of the observed features were

found to be consistent with this interpretation.

MCD was also used to detect EPR optically. Ac-

tually, we have a kind of textbook example for

ODMR since the MCD is unusually drastically

reduced (8% remains) at resonance. This means

that the two components of the lowest doublet in

the 2E ground state are almost equally populatedand this explains why we failed to detect conven-

tional EPR signal at 20 K. An analysis of our data

for two directions of the magnetic field leads to

gjj ¼ 1:81� 0:03 and g? ¼ 1:94� 0:02. From theseresults we conclude that e is the lowest tetragonalcomponent in the ground state.

ODMR was also used to tag two charge

transfer transitions in the visible spectral range ofthe UV-illuminated sample with the V4þGe EPR

signals. From a combination of these results and

optical absorption data, the V4þ=5þ donor state

was located �1.9 eV below the conduction band.The presence of the V4þ=3þ acceptor level �2.7 eVabove the valence band is also suggested by our

MCD/ODMR results. Finally, the presence of

minute amounts of Cr4þ and Fe3þ was demon-strated through the high sensitivity of MCD ex-

periments.

Acknowledgements

This research was supported by the National

Science Research Fund OTKA, Hungary (Grant

No. T22859), the French CNRS and the Center of

Excellence Program (Contract No ICA1-1999-75002). L. Kov�aacs and N. Romanov are gratefulto ESPCI and the City of Paris for the award of a

two-months fellowship. Thanks are also due to H.

Bou Rjeily and B. Farid for their participation to

certain experiments and to O. Szak�aacs for atomicabsorption measurements.

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