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Light-induced charge transfer processes in Mn-doped Bi12GeO20 and Bi12SiO20 single crystals
0022-3697/M $3.M) f .OO 0 1985 pc%alMm mss Lid.
1. Phys. Chm. Solids Vol. 46, No. IO, pp. I 117-l 129, 1985 Pfinted in
aeatBlitin.
LIGHT-INDUCED CHARGE TItANSEER PROCESSES IN Mn-DOPED Bi12GeOz0 AND Bi12Si020 SINGLE CRYSTALS H. SZYMCZAK, M. T. BOROWIEC and K. PATAJ Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland
W. WARDZY&SIG,
and T. LUKA~IEWKZ and J. ~;MIJA Military Technical Academy, Warsaw, PoIand (Received 11September 1984; accepted 1 March 1985) Abstract-The optical absorption and ESR spectra of Bi,sGeGm and B1sSiOsOdoped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm-’ were performed. The observed ESR spectrum is a su~~~tion of six lines resulting from the hype&e intem~ion of manganese ions in tetrahedral positions. The g-factor and hyperhne constant are g = 1.999 r 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears alter illumination. The sharp line is interpreted as due to a transition inside the Mn+ center in tetrahedral coordination. Bands in the region lO,OOO-16,000cm-’ are due to Mn3+ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal held parameters for this center were found: B = 565 cm-‘, Dq = 1400 cm-‘, Dt = -330 cm-‘, Ds = 4170 cm-’ and C = 2260 cm-i. The illumination conditions which are needed for homogeneous coloration of tbe sample are also discussed.
1. INTRODUCIION
In a previous article [l] the photochromic effect observed in Bi&eOto @GO) and Bi&Ozs (BSO) doped with manganese was explained qualitatively in terms of light-induced charge transfer in the manganese ions. It was assumed that before illumination the absorption was connected with Mn4+ ions located on Ge sites and that illumination caused the charge transfer Mn4+ -) Mn’+. Since the spectrum of Mn-doped BGO and BSO is relatively simple, there was some hope that it would be possible to decompose the absorption bands into Gaussian components and then to assign them according to crystal field theory using reasonable crystal field parameters, and thus verify the previous gumption. It is also desirable to verify this explanation by other measurements such as ESR and lowtemperature absorption. In this study we present the results of ESR measurements, a uniaxial stress experiment and a detailed analysis of the absorption induced by light. These results and the analyses call for irn~~nt changes in the previous interpretation.
2. EXPERIMENTAL a.
RESULTS
ESR measurements
Crystals were oriented along the [ lOO], [ 1101 and [ 11 I] axes using X-rays. Investigations were carried out with an X-band (frequency 9.051 GHz) ESR spectrometer at liquid helium temperatures. It was
found that all the crystals showed in addition to the spectrum from manganese, a spectrum which was identified as connected with Fe3+ impurities [2]. The ESR spectrum of rnan~n~~o~ BGO is shown in Fig. I (the angle between the magnetic field H and the [lOO] direction being about 30”) and in Fig. 2 (where H is parallel to the [ 1001 direction). Dotted lines represent the spectrum before illumination and the broken lines represent the spectrum after illumination. The single central line in Fig. 1 as well as the three central lines in Fig. 2 are due to Fe3+. This unwanted Fe3+ spectrum has to be substracted from the measured spectrum. Crystals were illuminated for 15 min from both sides with the light of a lOOW xenon lamp through a filter consisting of a saturated water solution of C&O,. Similar results were found for Mn-doped BSO crystals. It is believed that observed resonance spectrum is a superposition of several lines resulting from hyperfine interactions which always exist in the case of manganese ions. In order to determine the spin Hamiltonian parameters from the observed spectrum one has to decompose the spectrum into six absorption lines, since the nuclear spin of a manganese atom is I = 4. To perform such a decomposition information about the shape of the resonance lines is needed. This has been calculated by Grant and Stranberg 131 for diluted paramagnets in the framework of the statistical method. They have shown that for diluted paramagnets the resonance line has a Lorentzian shape in the center and a Gaussian shape far from the center of the line. For simplicity in the subsequent analysis a Gaussian shape was assumed. In such a
1117
W. W~uDmksK1
1118
et al.
ganese spectrum and makes it impossible to perform a more detailed analysis of the hyperfine structure. Nevertheless, the agreement of the above-described fit with the experimental results indicates that the ESR spectrum related to manganese is isotropic and that its shape is the result of the superposition of six hyperfine structure components due to the nuciear spin of the Mn. This means that the spin Hamiltonian describing the spectrum has the following form:
where @ is the Bohr magneton, ,? is the spin of the manganese ion and f is the nuclear spin. If the spin S L 2, a crystal field term has to be added to (3) AT-=,= f
A
I
1 2.0
I
I
I
I
I
I
3.0
I
I I
3.2 HGs(i03)
r&s:+
I I
s; + s; - (1/3)S(S + 1)(3S2 + 3s - l)].
I
(4)
I
3.4
Fig 1. ESR spectrum for Mndoped BGO: before illumination, dotted line: after ilIumj~tion. broken line. The solid line is cakulated a&ording to formula (2). The single central line is due to a Fe3+ center [2]. The angle between the magnetic field H and the [NO] direction is about 30”.
Since the observed spectrum has an isotropic character it follows that u = 0. The analysis of the ESR spectrum leads to the foilowing values for the spin Hamilton~ parameters: A = 78 Gs,
g = 1.999 + 0.003.
b. Absorption measurements case an ESR signal connected with a single absorption line can be expressed in the following form:
The absorption coefficient was measured at 1.8 K and at room temperature in the range between 7500
+2vGiE So(ff,,x
W
- HI 2G-5
X exp -
7
I[
t (1) 11
V&n,, - H) 2
where the energy is expressed in units of magnetic field, w is the half-width of the absorption line, I&,,, is the position of the absorption line and SO is a constant. Since our FSR signal is a superposition of six equal and equahy spaced absorption lines we have S=
2@-G-? so ;: (Kw
W
X
-I-nA-H)
n=O
exp
{C
W
where A is the hypertine constant. The solid lines in Figs. I and 2 are derived from eqn (2) with the following values for the fitted parameters: H max= 3040 Gs;
A =
78 Gs;
w = 120 Gs.
The assumption that the lines are equally spaced is an approximation which can be justified only by the fact that the Fe3+ ESR spectrum overlaps the man-
-I
2.8
I
I
3.0
I
I
3.2 t-lGS(103)
I
I
I
I
3.4
Fig. 2. ESR spectrum for Mn-doped BGO: before iliumination, dotted line; after illumination, broken line. The solid line is calculated according to formula (2). The three central lines are due to a Fe’+ center (21. The magnetic field H is parallel to the [ 1001direction.
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Light-induced charge transfer processes and 23,000 cm-’ before and after illumination of Mndoped BGO and BSO samples. Since one may expect that the absorption coefficient measured after illumination will depend on the illumination time, the wavelength and the intensity of the light used for coloration, the conditions of the illumination are of great importance and should be carefully chosen. Furthermore, the illumination can cause different absorption coefficients for different distances from the illuminated surface, producing a gradient of the absorption coefficient along the thickness of the crystal. In such a case the absorption coefficient CI=:
1
ln(l
1
-Z?)‘-In1
Zo
(5)
room temperature for l/X = 16,000 cm-‘, A being the wavelength of the beam A. After switching on the mercury lamp, the illumination causes coloration of the crystal (i.e. increases its absorption coefficient) and the measured signal from the light beam A decreases with time. This allows us to determine the change of the absorption coefficient of the crystal, Acu. Results of such measurements for different illumination intensities are given in Fig. 4. The absorption coefficient induced by light, Aa (the difference between the absorption coefficient after and before illumination) increases exponentially, with a time constant which depends on the intensity of the light beam B (illumination light): ALY= Acu,( 1 - e-‘I’),
will not be the “true” absorption coefficient but some kind of “mean” absorption coefficient. Here t is the crystal thickness, R the reflection coefficient, IO the intensity of the incident beam and Z the intensity of the transmitted beam. To avoid this problem, the conditions of illumination should be such as to assure homogeneous coloration of the sample, and these were determined from measurements of the kinetics of the coloration process. The measurements of the kinetics of the coloration and decoloration processes are in themselves an interesting problem which will be published in a separate article. At present only those results important from the point of view of obtaining homogeneous samples will be given. The geometry used for these measurements is given schematically in Fig. 3. The light beam A used for measurements of the absorption coefficient comes from the monochromator and then passes through the crystal as near as possible to the illuminated surface. The light beam B from a 100-W mercury lamp used for illumination passes through a green Hg-line filter (546 nm) and neutral filters which allow us to change the intensity. This light is perpendicular to the light beam used for the measurement of the absorption coefficient. Measurements were made at
(6)
where Aa, is the light-induced absorption coefficient and T the time constant. T-I was found to be proportional to the light intensity, (see Fig. 5) T-’ = BZ, where Z is the intensity of the light used for coloration. For Mndoped BSO crystals used in these measurements it was found that B = 0.26 cm2 J-‘. Aar, does not depend within the limits of the experimental errors on the light intensity. To calculate the absorption coefficient induced by light as a function of the distance from the illuminated surface, let us divide the crystal into very thin layers, each of thickness Ax. Let us assume that the intensity of the light which causes the coloration in the first layer is lo and that the intensity of the light within each layer is constant, but changes from one layer to another. The absorption induced in the first layer by the light is described by the formula (6), A(Y, = Aa,( 1 - ewB’Or). The intensity II of the light causing coloration in the second layer is Z, = Z. e-A-l& and the absorption induced in the second layer:
Hg Light
lamp
Aa2 = a(~,[ I - exp(-Blot e-‘“I”)].
beam B
Hg 546 pm Filter
T
For the nth layer whose distance from the illuminated surface is given by
Neutral Filter
&2n-‘&
the light-induced follows: Aa, = Aa,[l for the measurements of the time dependence of light-induced absorption.
Fig. 3. The geometry used
’
2 absorption
may be expressed as
- expf-Blot
e
This formula enables us to calculate the lightinduced absorption as a function of a distance d
W. WARDzwkKt et al.
X 0900795 0 0.00398 +
Watt ,,2
0.00159
Fig. 4. The light-induced absorption Aor as a function of time for different ihumination intensities. The solid Lines are drawn according to formula (6).
from the illuminated surface for a given intensity of illumination light, and a known value of B. The results of such calculations provide useful information about the conditions of illumination which assure approximately homogeneous coloration of the sample. The results of the compu~tion are shown in Fig. 6 (curve A) for the following ~~rnete~: Ax = 5 X 10e4 cm, ACY, = 4 cm-‘, B = 0.26 cm2 J-‘, ia = 0.016 W cmT2, t = 5 min. These parameters
a2--
0.1
I
1
,
,
,
0.05
,
I
#
,
0.1
cm
mWatt /cm2
Fig. 5. The dependence of the time constant on the illumination intensity.
Fig. 6. The absorption induced by light calculated according to formula (7) as a function of the distance from the illuminated surface. A, illumination from one side of the crystal for 5 min; B, illumination from one side of the crystal for 5 min and subsequent illumination from the opposite surface of the crystal for 5 min.
Light-induced charge transfer processes characterize the conditions which can be used in our experiment. As one can see, the expected absorption coefficient induced by light varies through a thickness of 1 mm from 2.9 to 2.5 cm-‘, and for such conditions of illumination there would be a pronounced gradient in the absorption coefficient. Therefore, we calculate the absorption coefficient as a function of distance from the surface for the case when the crystal which was illuminated as before is subsequently illuminated from the opposite surface of the crystal. The subsequent illumination from the opposite surface of the crystal during 5 min causes an increase of the absorption and a distinct homogenization of the absorption coefficient, as shown in Fig. 6 (curve B). The expected absorption coefficient for such illumination varies from about 3.40 to 3.46 cm-‘, i.e. no more than 0.1 cm-‘. Similar calculations show that 15 min illumination from one side and subsequent illumination for 15 min from the opposite side of the crystal should produce an absorption coefficient which varies by about 0.03% throu~out a l-mmthick crystal, and that the value of this coefficient differs from the saturation value Aha, by no more than 0.2%. Althou~ these results are valid for the spectral range around 16,000 cm-’ (the value of B used in the calculation was measured in this region) we will assume that the 15 min illumination time from one side of the crystaI and subsequent illumina~on for another 15 min from the opposite side of the crystal, will assure the homogeneity of the absorption coefficient in the entire volume of the crystal, over the whole spectral region. These conditions of illumination (100-W mercury light through the green Hg-line filter for 15 min first from one side then from the other) were used in the,experiment. The absorption spectra of the Mn-doped BSO crystal at room temperature before and after illumination are given in Figs. 7(a), 7(b) and 8(a) and 8(b), respectively, and the absorption spectra of Mn-doped BGO at liquid helium temperature before (solid line) and after (broken Iine) illumination are given in Fig. 9.
1121
SO-
40-
30T E Y0 20-
10 -
m
16000
Fig. 7(b). Absorption spectrum of Mn-doped BSO in the
range 16,000-23.000 cm-’ at room temperature before illumination. Solid line, see text.
The absorption coefficient was calculated using eqn (5). The reflection coefficient was calculated from refractive index data [4] using the following eqn [5]: n2 = a2
xi-+b,-&+ /
with the following parameters: a2 = 78.5, X, = 0.2421
Fig. 7(a). Absorption spectrum of Mn-doped BSO in the range 700040,000 cm-’ at. room temperature before illummatron.
1,
2
x -G
b2 = 0.00035, and
X,= 0.4015.
The absorption spectra for both Mn-doped BGO and BSO are very similar. At room temperature before illumination they show a broad band around 13,500 cm-‘, a band which overlaps the fundamental absorption edge above 23,000 cm-‘, and a weak narrow band at 8000 cm-‘. The band at 13,500 cm-’ exhibits some structure which become sharper at liquid helium temperature. The weak narrow band at 8000 cm-’ becomes very sharp and much stronger at liquid helium temperature. The positions of this line are a littie different in Mn-doped BGG and BSG, and are similar to the 9620 cm-’ line in Crdoped BGO and BSO crystals (6). After illumination the 13,500 cm-’ band and the 8000 cm-’ line disappear
1122
W. WARDZY~SKIet al.
5
l
l
i’ so
l . . . .
.
.
.
4
.
40
. . t .
l .
3
l
30
~
-7 E 0
. l
7 E 0 e
2
l l l l l :
20
. : :
2’ _* 1 IO
t
10000
I
A(cm-$1
15000
Fig. 8(a). Absorption spectrum of Mn-doped BSO in the
range 7500-16,000 cm-’ at room temperature after illumination. The solid line shows the bound polaron absorption calculated according to formula (IO).
and a quite intense and very broad band appears {see Figs. 8(a) and 8(b)]. This band, which has a maximum around 20,~-22,~ cm-‘, covers the whole visible region of the spectrum and overlaps the fundamen~1 absorption edge of the crystal.
2 t
16000
I
I
t
t
2 0000 A (cm+
I
I
f
f
Fig. 8(b). Absorption spectrum of Mn-doped BSO in the range 16,000-23,000 cm-’ at room temperature aRcr illumination. The solid line shows the bound polaron absorption calculated according to formula (IO).
[2]. The behavior of the ESR signal at different temperatures indicates that the ground state of the center connected with a rnan~n~ ion which appears
c. Uniaxial stress measurements A uniaxial stress experiment was carried out on the BGO crystals at 1.8 K using the sharp line at 8000 cm-‘. Samples used for these measurements were oriented using X-rays, and stress was applied parallel to the [ 1001, [ 1 lo] and [ 11 I] directions. No splitting and broadening of the line under the influence of the uniaxial stress was observed, only a small shift of the line toward lower energy. The results of the rn~umrnen~ are shown in Fig. 10(a) (stress paraBe to the [ 1001 and [ 1lo] directions) and in Fig. 10(b) (stress parallel to the [ 11 l] direction).
3. Dl!SCUSSlON As one can see from Figs. l(a) and l(b), the ESR signal from manganese increases considerably after illumination. The isotropic character of the resonance line in BGO indicates that the coordination of the center is tetrahedral, similar to the case of Fe centers
A (cmmt)
Fig. 9. Absorption spectrum of Mn-doped BGO at 1.8 K. Solid Iine, be&! illumination; broken tine, after illumination.
Light-induced charge transfer processes
*
PIIE
o
pl.E
b001
1123
degeneracy and the piezospectroscopic tensor has the form
The change of the energy under uniaxial stress for stress p applied in the [ 1001, [ 1 IO] or [ Ill] directions will be 8026
A = Cp.
\
6025 -i .
I
200
’
I
’
I
6bO
,
1
1000
kG /cm* Fig. 10(a). Shift of 8026-cm-’ line by uniaxial stress applied parallel to the [MO] and [ 1lo] directions.
after illumination is an orbital singlet. Therefore, we have two possibilities for the ground state of the manganese ion after illumination: a dS configuration with a 6Alground state which leads to Mn2+, or a d2 configuration with a 3A2 ground state which leads to Mn5+. The near free-electron value of the g-factor and the value of the hyperhne constant A suggest Mn2+ rather than Mn’+ ions. ESR measurements indicate therefore that the manganese related center which appears after illumination is Mn2+-d’. These also indicate that Mn ions. which are oresent in the crystal before illumination and are transformed by light into Mn2+ ions, should be Mn3+-d4 or Mn+d’s, (d6) in tetrahedral coordination. For the d5 configuration all transitions are spin forbidden and the absorption connected with such a center should be weak and its observation would be rather unlikely. If such an absorption is observed, it should be present in the high-energy region, presumably above 20.000 cm-‘. Although we have Mn2+-d5 centers in the crystal and the concentration of these centers increases considerably with illumination as indicated by ESR measurements, there would be no optical absorption from these. The results of the uniaxial stress experiment may be understood by assuming that the center responsible for the 8000 cm-’ line has tetrahedral symmetry. For such a high symmetry center there is no orientational
The solid lines in Figs. 10(a) and 10(b) are obtained from this formula with C = -9 X 10e4 cm-‘/kG X cmm2. The uniaxial stress experiment indicates that the center connected with the 8000-cm-’ line indeed has tetrahedral symmetry like the center observed in the ESR measurements. It is reasonable to assume that the center connected with the KKlO-cm- line observed before illumination, and the center observed after illumination have the same origin. Therefore, because after illumination the center is Mn2+ in tetrahedral coordination, the center before illumination which is responsible for the 8000-cm-’ line, will be Mn3+ or Mn+, also in tetrahedral coordination. Illumination will cause the charge transfer Mn3+ - Mn2+ or Mn+ - Mn2+, assuming that the most probable change of the charge state by light is -Cl.
1 +PllE
o-
pl.E
8026
-7 5i ;
+
kG/cm*
Fig. IO(b).Shift of 8026-cm-’ line by uniaxial stress applied parallel to the [ 11 I] direction.
W. WAuozvkstu et al.
1124
Let us assume that the center connected with the 8000-cm-’ line before illumination is Mn’+-d4 in tetrahedral coordination. The observed line in this case would be the no-phonon line of the ‘T2 - 5E transition with Dq - 800 cm-‘. The ‘T, - ‘E transition was observed in the region of 5500 cm-’ (Dq - 550 cm-‘) in the case of C?+-d4 in ZnSe [7]. Although a value of Dq - 800 cm-’ Seems to be too high in comparison with 550 cm-‘, such an assignment cannot be excluded. For Mn+ we should take into account two electronic configurations: d5s’, or d6 if the s electron were to fall into the d shell. For a d6 configuration in tetrahedral coordination we should expect an absorption band in the region of 3000-6000 cm-’ due to the ‘E - ‘T2 transition (many Fez’ derivatives which form tetrahedral complexes possess an absorption band in this region, i.e. Dq - 300-600 cm-’ [8]). It seems rather unlikely that the 8000-cm-’ line is due to a ‘E - ‘T, transition in a d6 configuration; however such an assignment cannot be definitely excluded. The d5s’ configuration in tetrahedral coordination could also explain the narrow line at 8000 cm-‘. Such a center would be similar to the neutral chromium center reported previously in Crdoped BGO [6]. The spectrum of Mn+ in the gaseous phase consists of the line ‘.S,[3d5(%)4s] -
5S2[3d5(6S)4s]
at
9472.86 cm-‘,
and the lines 7S3[3d5(6S)4s] ,- ‘D4 5D3
‘D2 [3d6] 5DI :I 5D’_l
at
14,325.64 14,593.62 14,781.03 14,90 1.06 14,959.68
cm-’ cm-’ cm-’ . cm-’ cm-’
Other lines lie above 27,000 cm-‘. Because of the relatively large energy difference between the 5S2 and the ‘0 states, only a weak mixing between these states can be expected, and it is possible to assume that the 8000-cm-’ line is due to the ‘S3 - ‘S2 transition. The 5D[3d6] levels will form (neglecting spin-orbit interactions) in a crystal field of tetrahedral symmetry two bands, ‘E and ‘T2, and one should expect, as discussed above, that the energy difference between these bands may be as large as 3000-6000 cm-‘. However, it is difficult to make any assumption about the positions of these bands compared with the energy of the ‘D levels of the free ion. Moreover, the transitions from the ground state to these excited states should be strongly forbidden and should form a broad band. It is very doubtful whether such transitions can be observed. Therefore, one should expect only one sharp line for a Mn+ [3d54s’] center in tetrahedral coordination. We believe that the most probable explanation of the 8OOO-cm-’ line is that it is due to a Mn* [d5s1] center.
The light will change this center according to the scheme Mn+ [d5s’] 5 Mn*+ [d’]. The remaining absorption observed in the experiment [Fig. 7(a)] cannot be explained by means of a center with tetrahedral symmetry. Therefore, one should try to explain the absorption in the region lO,OOO-16,000 cm-’ in terms of centers of lower symmetry, probably Mn in an interstitial position (orthorhombic symmetry). Spectra of light-induced absorption (the difference between absorption after and before illumination) are very useful for the interpretation of absorption in photochromic materials. Such spectra eliminate the absorption connected with centers which do not change during illumination and enhance more clearly all those which do change with illumination. The light-induced absorption Aa of Mn-doped BSO at room temperature in the range 7500-16,000 cm-’ is shown in Fig. I l(a), and in the range 15,00023,000 cm-’ in Fig. 1l(b). Between 9000 and 14,000 cm-’ the light-induced absorption is negative, which means that the concentration of the centers responsible for this absorption decreases with illumination. Even if there is any absorption in this region which increases with illumination, the main role is due to the absorption connected with centers which vanishes with illumination. In the range between 14,000 and 15,000 cm-‘, Acuchanges rapidly, and at about 14,400 cm-’ the light-induced absorption changes sign. This is the region where centers which decrease and increase with illumination play an equal role. In the range between 15,000 and 23,000 cm-’ the light-induced absorption is positive, which means that in this region centers whose concentration increases with illumination are dominant. If one attempts to decompose the observed spectrum into various components the results will be as follows. In the region between 9000 and 17,000 cm-’ there will be some bands with halfwidths of about 2000-3000 cm-’ which decrease alter illumination, and there will be a very broad band ranging from the absorption edge to the near infrared region (halfwidth 8000- 10,000 cm-‘) whose intensity strongly increases after illumination. It is very difficult to assume that this broad band is due to a transition in a Mn center with a d” configuration. Schirmer et al. [9-121 showed that such a broad and quite intense band with a maximum close to the absorption edge which is observed in MnO, BeO:Li and in quartz can be explained as due to a small hole polaron bound to a defect or impurity. According to this concept the hole trapping is accompanied by a lattice distortion around the trapping site. The bonding orbitals between the impurity and ligand ions can capture a hole forming a small polaron. In tetrahedral coordination there are four equivalent orbitals. The optical absorption is explained by a light-induced transfer of the hole from one to another equivalent 02- site near the impurity.
1125
Light-induced charge transfer processes
(9)
Replacing w by A = l/X we have
a=-
A max
amax
e-t(2~/H*Wa-nmu~l~
9
A
(10)
where A
=mm
‘=JT
3uhc
and Hi = 8AoAmal In 2 coth
m
r
1oobo
’ *;cm_,) -15&xl
’
Fig. 11(a). Spectrum of tight induced absorption Aa for Mndoped BSO at room temperature in the range 7000-16400 cm-‘.
(
$T
1
.
Amax,HA and a,,, are fitting parameters. in the light-induced absorption spectrum [Fig. 1 l(b)] the band connected with such a polaron does not overlap with the bands from some other origin in the region between about 17,000 and 2 1,000 cm-‘, and fitting to the experimental data should be performed in this region. Such a fit gives the following values of the fitted parameters: A,,,ax= 22,700 cm-‘, Hn = 9500 cm-‘, a,,,= = 13.3 cm-‘, and A,J = 658 cm-‘. The solid line in Fig. I l(b) is drawn according to eqn (10) using the above given parameters.
According to Schirmer [ 121 such a process will produce an absorption which in general will form three quasi-Gaussian bands:
16 w-’ = 3 E&two coth
(8)
where EJT is the stabilization energy gained by the hole capture, J is the resonance integral, tro, is the energy of a representative phonon mode describing the distortion, and RF and ECF are connected with the crystal field splitting. Let us assume that in BGO and BSO crystals with transition metal impurities similar centers may exist. We will denote these centers as X”+ 0. This means that in an X04- complex, where X is a transition metal element, a hole is localized on the bonding orbital forming a small radius polaron. If in the first approximation we neglect the crystal field effect and the influence of J, then the absorption band will be given by
15000
A(cm-0
2oboo
Fig. 1l(b). Spectrum of tight-induced absorption Aa for Mn-doped Bso at room temperature in the range 15,00023,000 cm-‘. [Solid line shows the fitting of the bound polaron absotption according to formula (IO). The fit agrees with the experimental points in the region between 17,000 and 21,000 cm-‘.]
W. WARDZYtiSKIet al.
1126
Valence band
Ml?+@ Smal I
radius
polaron
It,
-
la,
‘-
Fig. 12. The schematic presentation of the energy levels of the molecular orbitals for (GeO.$ and (Mn04)‘-, and its illustrative connection with the valence band of BGO and a bound polaron.
The polaron absorption band found this way agrees well with the absorption below the absorption edge measured for the illuminated BSO crystal [(Figs. S(a) and 8(b)]. The solid lines in these figures were calculated according to formula (10) with the fitting parameters given above. The small differences between the calculated curve and the experimental points in Fig. 8(a) in the range 10,000-15,000 cm-’ can be explained by the fact that the illumination does not destroy completely the absorption observed at 13,500 cm-’ before illumination. The discrepancies between the polaron curve and the experimental points in the region 18,000-23,000 cm-’ are due to the band-toband absorption (absorption edge). The difference between the polaron band (solid curve) and absorption measured in the experiment represents the absorption edge of the crystal. This absorption is given by the solid line in Fig. 7(b). The difference between this curve and the experimental points indicates that there is a band in this region which disappears after illumination. This band is also seen in Fig. 1l(b). If one subtracts the calculated polaron absorption from the experimental light-induced absorption [Figs. I l(a) and 1 l(b)], one obtains the absorption spectrum due to centers which disappear after illumination, as shown in Figs. 13(a) and 13(b). We attempt to decompose this spectrum into Gaussian bands of the form
(y =
oL,.,
e-t(2vminxa-am,)l* (11)
As one can see such a decomposition gives us a band at 10,700 cm-’ [solid line in Fig. 13(a)] with the following parameters: A,, = 10,700 cm-‘, HA = 1700 cm-‘, fxmax= 0.65 cm-‘; a band at 14,000 cm-’ [solid line in Fig. 13(a)] with the following fitting parameters: A,,, = 14,000 cm-‘, HA = 2900 cm-‘, shown ffmax = 2.8 cm-‘; a small residual absorption in Fig. 14 consisting of a weak line at 8000 cm-’ and probably its phonon structure in the region of 9000 cm-‘, and three rather narrow bands in the region of 12,000 cm-‘. We treat the weak absorption which is apparently present in the region 14,000-19,000 cm-’ as being within the limits of experimental error, since this absorption results from the separation process for the Gaussian components. This also applies for the region of 9000 cm-‘, although the accuracy in this region is better. There are also the bands above 23,000 cm-’ [see Fig. 13(b)]. The line at 8000 cm-’ is the same line as observed at helium temperature and already discussed. As pointed out earlier, the bands found V at 10,700, at 14,000, W at’ - 12,000 and V at -23,000 cm-‘, cannot be explained using the center connected with manganese in tetrahedral coordination, and we should take into account orthorhombic symmetry.
an interstitial
position
Since this absorption
with
dis-
1127
Light-induced charge transfer processes
10000
h:cm-‘t
15Jix
Fig. 13(a). The difference between the light-induced absorp tion and the bound pokon auction as a function of wavenumber in the range 7000-20,000 cm-‘. Solid line, possible Gaussian components.
appears with illumination and we do not find any other absorption of this kind which increases with illumination, we will assume that this interstitial Mn center after illumination has the charge state 8. In such such a case after illumination we will have a Mn2+ center. Absorption by such centers will be spinforbidden and should not be observed. The center before illumination should be Mn’+ [d4]. In Fig. 15 the energy level schemes for octahedral, tetragonal and orthorhombic symmetry for the d4 configuration
Fig. 14. The residual absorption found from the decomposition of the observed light-induced absorption into a polaron absorption band and two Gaussian bands from Fig. 13(a).
are shown. The actual orthorhombic symmetry of the interstitial in BilZGe020 can be treated in first approximation as tetragonal [6]. As one can see from Fig. 15, a tetragonal distortion will split the 5E level into 5B, and ‘A, levels, and the ‘T2 level into ‘B2 and ‘E levels. Reduction of the symmetry to orthorhombic leads to additional splitting of the highest SE level into two levels. The transition to these highest levels gives rise to the absorption above 23,000 cm-‘, and cannot be analyzed in detail. Therefore, we limit ourselves to tetragonal symmetry. The band found at 10,700 cm-” is interpreted as due to the 5B, - 5.4, transition, that at 14,000 cm-’ as due to the “B, - ‘B2 transition, and that at 24,~ cm-‘+23,000 cm-‘) as due to the ‘8, - ‘E, transition. Using the Ligand Field Energy Diagrams [ 131 we find the following crystal field parameters: B = 565
w l
. l . .
. * .. .
-4
l l
17000
20600
23000
,
Ab(cm+ Fig. 13(b). The difference between the light-induced absorp tion and the bound polaron absorption as a function of wave number in the range 16,~-23,~ cm-‘. PCS4b:lO-0
Orthorhombic
Octahedral Tetragonot
Fig. IS. Schematic diagram of the energy levels for a center with d’ electronic configuration in the environment of octahedral, tetragonat and o~horhombic symmetries.
1128
W. WAuozvAsut et al.
cm-‘, Dq = 1400 cm-‘, Dt = -330 cm-‘, Ds = 4170 cm-’ and C = 2260 cm-‘. From the above-cited Energy Diagrams we find the following spin forbidden transitions which depend weakly on the crystal field: 5B, - 3BI and ‘B, - 3&. Using the above-mentioned parameters one can find E 1~,-5~, = 10,500 cm-‘,
Ed,+.+ = 14,000 cm-‘,
E s~,__s~= 24,000 cm-‘,
EQ+_Q, = 12,000 cm-‘,
E Q,_)~~ = 12,830 cm-‘, in good agreement with experiment. It is clear that the crucial point in the interpretation presented above is the assumption about the small polaron. It is important therefore to have some idea, even in an approximate and qualitative manner, how such a polaron could be formed and consequently about the energy band structure of BGO type crystals. Only very little is known about the energy band structure of these crystals. Futro [14], using the results of reflection measurements in the region above the fundamental absorption edge and some analogy between BGO and &By’ compounds, suggested that the energy band scheme of BGO is determined mainly by the bismuth and oxygen net structure. He paid attention to the layer structure of rhombohedral B&X3 type compounds and cubic BGO. According to such a concept, the layer structure BizX3 type compounds is modified in BGO by inserting Ge02 molecules. We propose a different approach to this problem. If one analyzes the position of the atoms in the crystallographic structure of BGO [ 151, a possible description of this structure is as follows. The BiizGeOZO molecules are placed at the center and all corners of a cubic elementary cell. Such a Bii2GeOZ0 molecule consists of a (Ge0,)4- tetrahedron and four (Bi,04)+ groups attached to the four faces of the Ge04 tetrahedron. The oxygen ions in the B&O4 group form a flat equilateral triangle with the fourth @- at its center. Three Bi ions are attached to three 02- ions and are placed almost in the same plane as the @- ions, forming a similar equilateral triangle. The GeO, and four Bis04 form together a compact and slightly distorted spherical BilzGeOzo complex. Let us imagine the energy levels of the molecular orbitals of BilzGeOzo or for simplicity those of Ge04 and Bis04. The energy band structure will be formed from these levels in a similar way as from the atomic levels. Let us take into account the energy levels of the molecular orbitals connected with (GeO,r-. Such orbitals were calculated in [ 161. One can see that the highest molecular orbitals which are completely filled with electrons are: 2a,, le, 2t2, 3t2 and It,. It is reasonable to assume that the energy levels of these orbitals, due to interactions in the crystal lattice, will form a valence band of the crystal. Of course, the molecular orbitals from (Bis04)+ will also contribute
to the band structure, but unfortunately there are no data connected with the molecular orbitals of such a molecule. Therefore, the discussion has only a qualitative and illustrative significance. If Mn replaces Ge in the crystal lattice, a (MnO,rmolecule will be formed. The energy levels of the molecular orbitals for MnO, have also been calculated [ 171. The energy levels for Ge04 and MnO, according to [ 161 and [ 171 are given schematically in Fig. 12. One can note that the molecular orbitals of Mn04 are very similar to those of Ge04, and the It,, 3t2, 2a,, 2t, and le levels could overlap the valence band. If a hole appears in the valence band, it could be self-trapped on the molecular orbital of the MnO, complex, forming a small radius polaron (see Fig. 12). This orbital will probably be of the 2a,, 3t, types. Such orbitals will be populated about 30-40% by s electrons from the metal atom, SO-60% by p electrons from the oxygen atom and 10% by s electrons from an oxygen atom for the 2~ orbital, and 80% by p electrons from an oxygen atom, 2% by s electrons from an oxygen atom and the rest (18%) by p electrons from the metal atom for the 3t, orbital [16]. The 2ai level is sigma bonding between the metal s orbital and the oxygen 2p orbitals. The 3t, orbital is mainly oxygen 2~. It is possible that the position of such an orbital in relation to the valence band edge (near the top of the valence band, or just above the valence band edge) is decisive from the point of view of the formation of the bound polaron state and the formation of a Mn+ state with d5s’ or d” configuration. CONCLUSIONS (1) In Mn-doped BGO and BSO, the Mn ions are located in tetrahedral positions on a Ge or Si site with the charge state Mn+ [3dS4s’], and in interstitial positions in the charge state Mn’+ [3d4]. Mn3+ centers may be compensated by Mn+ centers. Charge compensation could also be obtained through oxygen and bismuth vacancies. (2) Illumination can remove the 4s electron from a Mn+ tetrahedral center producing a Mn’+ [3d5] center. This leads to the disappearance of the 8000cm-’ line. Illumination can also change the charge state of the Mn3+ interstitial center, giving Mn2+ and a hole in the valence band. This leads to the disap pearance of the absorption bands in the region 1O,OOO15,000 cm-‘, and above 23,000 cm-‘. The hole is bound to the Mn2+ tetrahedral center, giving a bound polaron center and a broad absorption band which appears after illumination. Therefore, f
tetrahedral center-Mn+ orthorhombic
center-Mn3+
bound polaron center-Mn2+
Mn2+ + e f
Mn2+ +‘h,
(tetrahedral) + h-
Mn2+ 0.
(3) Although the separation of some overlapping bands into simple components may often create some
Light-induced charge transfer pmcesses problems, the results presented above indicate that the presence of a very broad band is rather likely. Moreover such a broad band was also observed in
Cu- and Co-doped BGO, where in the case of Cu, the absorption spectrum does not overlap with the absorption due to other centers. This supports the interpretation of the photochromic effect in terms of a polaron model. Measurements on Co- and Cudoped BGO are in progress and will be published in separate
articles.
REFERENCES 1. Wardzynski W., Vukasiewicz T. and imija J., Opt. Commun. 30, 203 (1979). 2. Wardzynski W., Szymczak H. and Baran M., Physica 111B. 47 (1981).
3. Grant W. J. C. and Stranherg M. W. P.. Phys. Rev. A
135, 715, 727 (1964). 4. Aldrich R. E., Hou S. I. and Harvill M. L., J. Appl. I’hys. 42,493 (197 1).
1129
5. Vedam K. and Hennessey P., J. Opt. Sot. Am. 65,442
(1975).
H., J. Phys. Chem. Solids. 45, 887 (1984). 7. Wray E. M. and Allen J. W., J. Phys. C: Solid St. Phys. 4, 512 (1971). 8. Lever A. B. P., Inorganic Electronic Spectroscopy, p. 300. Elsevier, Amsterdam (1968). 9. Schirmer 0. F., Koidl P. and R&k H. G., Phys. Status Solidi b62, 385 (1974). IO. Schirmer 0. F. and Schnadt R., Solid State Commun. 18, 1345 (1976). 11. Schirmer 0. F., Solid State Commun. 18, 1349 (1976). 12. Schirmer 0. F.. J. Phvs. C: Solid St. Phvs. 11. L65 (1978): J. Phys. CollotI.C6 suppl. No. 7;41, C6-479 (1980): 2. Phys. B24, 235 (1976). 13. Kijnig E. and Kmmer S., Ligand Field Energy Diagrams, Plenum, New York (1977). 14. Futro A. T., J. Phys. Chem. Solids 40,201 ( 1979). 15. Abrahams S. C., Jam&on P. B. and Bernstein J. L., J. Chem. Phys. 47,4034 (1967). 16. Hijjer C., Menza-Hajer S. and Hemandez de Pedro G., Chem. Phys. Lett. 37, 30 I (1976). 17. di Bartolo B., OpticalInteractionsin Solids, Wiley, New York (1968). 6. Wardzyirski W. and Szymczak
1. Phys. Chm. Solids Vol. 46, No. IO, pp. I 117-l 129, 1985 Pfinted in
aeatBlitin.
LIGHT-INDUCED CHARGE TItANSEER PROCESSES IN Mn-DOPED Bi12GeOz0 AND Bi12Si020 SINGLE CRYSTALS H. SZYMCZAK, M. T. BOROWIEC and K. PATAJ Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland
W. WARDZY&SIG,
and T. LUKA~IEWKZ and J. ~;MIJA Military Technical Academy, Warsaw, PoIand (Received 11September 1984; accepted 1 March 1985) Abstract-The optical absorption and ESR spectra of Bi,sGeGm and B1sSiOsOdoped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm-’ were performed. The observed ESR spectrum is a su~~~tion of six lines resulting from the hype&e intem~ion of manganese ions in tetrahedral positions. The g-factor and hyperhne constant are g = 1.999 r 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears alter illumination. The sharp line is interpreted as due to a transition inside the Mn+ center in tetrahedral coordination. Bands in the region lO,OOO-16,000cm-’ are due to Mn3+ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal held parameters for this center were found: B = 565 cm-‘, Dq = 1400 cm-‘, Dt = -330 cm-‘, Ds = 4170 cm-’ and C = 2260 cm-i. The illumination conditions which are needed for homogeneous coloration of tbe sample are also discussed.
1. INTRODUCIION
In a previous article [l] the photochromic effect observed in Bi&eOto @GO) and Bi&Ozs (BSO) doped with manganese was explained qualitatively in terms of light-induced charge transfer in the manganese ions. It was assumed that before illumination the absorption was connected with Mn4+ ions located on Ge sites and that illumination caused the charge transfer Mn4+ -) Mn’+. Since the spectrum of Mn-doped BGO and BSO is relatively simple, there was some hope that it would be possible to decompose the absorption bands into Gaussian components and then to assign them according to crystal field theory using reasonable crystal field parameters, and thus verify the previous gumption. It is also desirable to verify this explanation by other measurements such as ESR and lowtemperature absorption. In this study we present the results of ESR measurements, a uniaxial stress experiment and a detailed analysis of the absorption induced by light. These results and the analyses call for irn~~nt changes in the previous interpretation.
2. EXPERIMENTAL a.
RESULTS
ESR measurements
Crystals were oriented along the [ lOO], [ 1101 and [ 11 I] axes using X-rays. Investigations were carried out with an X-band (frequency 9.051 GHz) ESR spectrometer at liquid helium temperatures. It was
found that all the crystals showed in addition to the spectrum from manganese, a spectrum which was identified as connected with Fe3+ impurities [2]. The ESR spectrum of rnan~n~~o~ BGO is shown in Fig. I (the angle between the magnetic field H and the [lOO] direction being about 30”) and in Fig. 2 (where H is parallel to the [ 1001 direction). Dotted lines represent the spectrum before illumination and the broken lines represent the spectrum after illumination. The single central line in Fig. 1 as well as the three central lines in Fig. 2 are due to Fe3+. This unwanted Fe3+ spectrum has to be substracted from the measured spectrum. Crystals were illuminated for 15 min from both sides with the light of a lOOW xenon lamp through a filter consisting of a saturated water solution of C&O,. Similar results were found for Mn-doped BSO crystals. It is believed that observed resonance spectrum is a superposition of several lines resulting from hyperfine interactions which always exist in the case of manganese ions. In order to determine the spin Hamiltonian parameters from the observed spectrum one has to decompose the spectrum into six absorption lines, since the nuclear spin of a manganese atom is I = 4. To perform such a decomposition information about the shape of the resonance lines is needed. This has been calculated by Grant and Stranberg 131 for diluted paramagnets in the framework of the statistical method. They have shown that for diluted paramagnets the resonance line has a Lorentzian shape in the center and a Gaussian shape far from the center of the line. For simplicity in the subsequent analysis a Gaussian shape was assumed. In such a
1117
W. W~uDmksK1
1118
et al.
ganese spectrum and makes it impossible to perform a more detailed analysis of the hyperfine structure. Nevertheless, the agreement of the above-described fit with the experimental results indicates that the ESR spectrum related to manganese is isotropic and that its shape is the result of the superposition of six hyperfine structure components due to the nuciear spin of the Mn. This means that the spin Hamiltonian describing the spectrum has the following form:
where @ is the Bohr magneton, ,? is the spin of the manganese ion and f is the nuclear spin. If the spin S L 2, a crystal field term has to be added to (3) AT-=,= f
A
I
1 2.0
I
I
I
I
I
I
3.0
I
I I
3.2 HGs(i03)
r&s:+
I I
s; + s; - (1/3)S(S + 1)(3S2 + 3s - l)].
I
(4)
I
3.4
Fig 1. ESR spectrum for Mndoped BGO: before illumination, dotted line: after ilIumj~tion. broken line. The solid line is cakulated a&ording to formula (2). The single central line is due to a Fe3+ center [2]. The angle between the magnetic field H and the [NO] direction is about 30”.
Since the observed spectrum has an isotropic character it follows that u = 0. The analysis of the ESR spectrum leads to the foilowing values for the spin Hamilton~ parameters: A = 78 Gs,
g = 1.999 + 0.003.
b. Absorption measurements case an ESR signal connected with a single absorption line can be expressed in the following form:
The absorption coefficient was measured at 1.8 K and at room temperature in the range between 7500
+2vGiE So(ff,,x
W
- HI 2G-5
X exp -
7
I[
t (1) 11
V&n,, - H) 2
where the energy is expressed in units of magnetic field, w is the half-width of the absorption line, I&,,, is the position of the absorption line and SO is a constant. Since our FSR signal is a superposition of six equal and equahy spaced absorption lines we have S=
2@-G-? so ;: (Kw
W
X
-I-nA-H)
n=O
exp
{C
W
where A is the hypertine constant. The solid lines in Figs. I and 2 are derived from eqn (2) with the following values for the fitted parameters: H max= 3040 Gs;
A =
78 Gs;
w = 120 Gs.
The assumption that the lines are equally spaced is an approximation which can be justified only by the fact that the Fe3+ ESR spectrum overlaps the man-
-I
2.8
I
I
3.0
I
I
3.2 t-lGS(103)
I
I
I
I
3.4
Fig. 2. ESR spectrum for Mn-doped BGO: before iliumination, dotted line; after illumination, broken line. The solid line is calculated according to formula (2). The three central lines are due to a Fe’+ center (21. The magnetic field H is parallel to the [ 1001direction.
1119
Light-induced charge transfer processes and 23,000 cm-’ before and after illumination of Mndoped BGO and BSO samples. Since one may expect that the absorption coefficient measured after illumination will depend on the illumination time, the wavelength and the intensity of the light used for coloration, the conditions of the illumination are of great importance and should be carefully chosen. Furthermore, the illumination can cause different absorption coefficients for different distances from the illuminated surface, producing a gradient of the absorption coefficient along the thickness of the crystal. In such a case the absorption coefficient CI=:
1
ln(l
1
-Z?)‘-In1
Zo
(5)
room temperature for l/X = 16,000 cm-‘, A being the wavelength of the beam A. After switching on the mercury lamp, the illumination causes coloration of the crystal (i.e. increases its absorption coefficient) and the measured signal from the light beam A decreases with time. This allows us to determine the change of the absorption coefficient of the crystal, Acu. Results of such measurements for different illumination intensities are given in Fig. 4. The absorption coefficient induced by light, Aa (the difference between the absorption coefficient after and before illumination) increases exponentially, with a time constant which depends on the intensity of the light beam B (illumination light): ALY= Acu,( 1 - e-‘I’),
will not be the “true” absorption coefficient but some kind of “mean” absorption coefficient. Here t is the crystal thickness, R the reflection coefficient, IO the intensity of the incident beam and Z the intensity of the transmitted beam. To avoid this problem, the conditions of illumination should be such as to assure homogeneous coloration of the sample, and these were determined from measurements of the kinetics of the coloration process. The measurements of the kinetics of the coloration and decoloration processes are in themselves an interesting problem which will be published in a separate article. At present only those results important from the point of view of obtaining homogeneous samples will be given. The geometry used for these measurements is given schematically in Fig. 3. The light beam A used for measurements of the absorption coefficient comes from the monochromator and then passes through the crystal as near as possible to the illuminated surface. The light beam B from a 100-W mercury lamp used for illumination passes through a green Hg-line filter (546 nm) and neutral filters which allow us to change the intensity. This light is perpendicular to the light beam used for the measurement of the absorption coefficient. Measurements were made at
(6)
where Aa, is the light-induced absorption coefficient and T the time constant. T-I was found to be proportional to the light intensity, (see Fig. 5) T-’ = BZ, where Z is the intensity of the light used for coloration. For Mndoped BSO crystals used in these measurements it was found that B = 0.26 cm2 J-‘. Aar, does not depend within the limits of the experimental errors on the light intensity. To calculate the absorption coefficient induced by light as a function of the distance from the illuminated surface, let us divide the crystal into very thin layers, each of thickness Ax. Let us assume that the intensity of the light which causes the coloration in the first layer is lo and that the intensity of the light within each layer is constant, but changes from one layer to another. The absorption induced in the first layer by the light is described by the formula (6), A(Y, = Aa,( 1 - ewB’Or). The intensity II of the light causing coloration in the second layer is Z, = Z. e-A-l& and the absorption induced in the second layer:
Hg Light
lamp
Aa2 = a(~,[ I - exp(-Blot e-‘“I”)].
beam B
Hg 546 pm Filter
T
For the nth layer whose distance from the illuminated surface is given by
Neutral Filter
&2n-‘&
the light-induced follows: Aa, = Aa,[l for the measurements of the time dependence of light-induced absorption.
Fig. 3. The geometry used
’
2 absorption
may be expressed as
- expf-Blot
e
This formula enables us to calculate the lightinduced absorption as a function of a distance d
W. WARDzwkKt et al.
X 0900795 0 0.00398 +
Watt ,,2
0.00159
Fig. 4. The light-induced absorption Aor as a function of time for different ihumination intensities. The solid Lines are drawn according to formula (6).
from the illuminated surface for a given intensity of illumination light, and a known value of B. The results of such calculations provide useful information about the conditions of illumination which assure approximately homogeneous coloration of the sample. The results of the compu~tion are shown in Fig. 6 (curve A) for the following ~~rnete~: Ax = 5 X 10e4 cm, ACY, = 4 cm-‘, B = 0.26 cm2 J-‘, ia = 0.016 W cmT2, t = 5 min. These parameters
a2--
0.1
I
1
,
,
,
0.05
,
I
#
,
0.1
cm
mWatt /cm2
Fig. 5. The dependence of the time constant on the illumination intensity.
Fig. 6. The absorption induced by light calculated according to formula (7) as a function of the distance from the illuminated surface. A, illumination from one side of the crystal for 5 min; B, illumination from one side of the crystal for 5 min and subsequent illumination from the opposite surface of the crystal for 5 min.
Light-induced charge transfer processes characterize the conditions which can be used in our experiment. As one can see, the expected absorption coefficient induced by light varies through a thickness of 1 mm from 2.9 to 2.5 cm-‘, and for such conditions of illumination there would be a pronounced gradient in the absorption coefficient. Therefore, we calculate the absorption coefficient as a function of distance from the surface for the case when the crystal which was illuminated as before is subsequently illuminated from the opposite surface of the crystal. The subsequent illumination from the opposite surface of the crystal during 5 min causes an increase of the absorption and a distinct homogenization of the absorption coefficient, as shown in Fig. 6 (curve B). The expected absorption coefficient for such illumination varies from about 3.40 to 3.46 cm-‘, i.e. no more than 0.1 cm-‘. Similar calculations show that 15 min illumination from one side and subsequent illumination for 15 min from the opposite side of the crystal should produce an absorption coefficient which varies by about 0.03% throu~out a l-mmthick crystal, and that the value of this coefficient differs from the saturation value Aha, by no more than 0.2%. Althou~ these results are valid for the spectral range around 16,000 cm-’ (the value of B used in the calculation was measured in this region) we will assume that the 15 min illumination time from one side of the crystaI and subsequent illumina~on for another 15 min from the opposite side of the crystal, will assure the homogeneity of the absorption coefficient in the entire volume of the crystal, over the whole spectral region. These conditions of illumination (100-W mercury light through the green Hg-line filter for 15 min first from one side then from the other) were used in the,experiment. The absorption spectra of the Mn-doped BSO crystal at room temperature before and after illumination are given in Figs. 7(a), 7(b) and 8(a) and 8(b), respectively, and the absorption spectra of Mn-doped BGO at liquid helium temperature before (solid line) and after (broken Iine) illumination are given in Fig. 9.
1121
SO-
40-
30T E Y0 20-
10 -
m
16000
Fig. 7(b). Absorption spectrum of Mn-doped BSO in the
range 16,000-23.000 cm-’ at room temperature before illumination. Solid line, see text.
The absorption coefficient was calculated using eqn (5). The reflection coefficient was calculated from refractive index data [4] using the following eqn [5]: n2 = a2
xi-+b,-&+ /
with the following parameters: a2 = 78.5, X, = 0.2421
Fig. 7(a). Absorption spectrum of Mn-doped BSO in the range 700040,000 cm-’ at. room temperature before illummatron.
1,
2
x -G
b2 = 0.00035, and
X,= 0.4015.
The absorption spectra for both Mn-doped BGO and BSO are very similar. At room temperature before illumination they show a broad band around 13,500 cm-‘, a band which overlaps the fundamental absorption edge above 23,000 cm-‘, and a weak narrow band at 8000 cm-‘. The band at 13,500 cm-’ exhibits some structure which become sharper at liquid helium temperature. The weak narrow band at 8000 cm-’ becomes very sharp and much stronger at liquid helium temperature. The positions of this line are a littie different in Mn-doped BGG and BSG, and are similar to the 9620 cm-’ line in Crdoped BGO and BSO crystals (6). After illumination the 13,500 cm-’ band and the 8000 cm-’ line disappear
1122
W. WARDZY~SKIet al.
5
l
l
i’ so
l . . . .
.
.
.
4
.
40
. . t .
l .
3
l
30
~
-7 E 0
. l
7 E 0 e
2
l l l l l :
20
. : :
2’ _* 1 IO
t
10000
I
A(cm-$1
15000
Fig. 8(a). Absorption spectrum of Mn-doped BSO in the
range 7500-16,000 cm-’ at room temperature after illumination. The solid line shows the bound polaron absorption calculated according to formula (IO).
and a quite intense and very broad band appears {see Figs. 8(a) and 8(b)]. This band, which has a maximum around 20,~-22,~ cm-‘, covers the whole visible region of the spectrum and overlaps the fundamen~1 absorption edge of the crystal.
2 t
16000
I
I
t
t
2 0000 A (cm+
I
I
f
f
Fig. 8(b). Absorption spectrum of Mn-doped BSO in the range 16,000-23,000 cm-’ at room temperature aRcr illumination. The solid line shows the bound polaron absorption calculated according to formula (IO).
[2]. The behavior of the ESR signal at different temperatures indicates that the ground state of the center connected with a rnan~n~ ion which appears
c. Uniaxial stress measurements A uniaxial stress experiment was carried out on the BGO crystals at 1.8 K using the sharp line at 8000 cm-‘. Samples used for these measurements were oriented using X-rays, and stress was applied parallel to the [ 1001, [ 1 lo] and [ 11 I] directions. No splitting and broadening of the line under the influence of the uniaxial stress was observed, only a small shift of the line toward lower energy. The results of the rn~umrnen~ are shown in Fig. 10(a) (stress paraBe to the [ 1001 and [ 1lo] directions) and in Fig. 10(b) (stress parallel to the [ 11 l] direction).
3. Dl!SCUSSlON As one can see from Figs. l(a) and l(b), the ESR signal from manganese increases considerably after illumination. The isotropic character of the resonance line in BGO indicates that the coordination of the center is tetrahedral, similar to the case of Fe centers
A (cmmt)
Fig. 9. Absorption spectrum of Mn-doped BGO at 1.8 K. Solid Iine, be&! illumination; broken tine, after illumination.
Light-induced charge transfer processes
*
PIIE
o
pl.E
b001
1123
degeneracy and the piezospectroscopic tensor has the form
The change of the energy under uniaxial stress for stress p applied in the [ 1001, [ 1 IO] or [ Ill] directions will be 8026
A = Cp.
\
6025 -i .
I
200
’
I
’
I
6bO
,
1
1000
kG /cm* Fig. 10(a). Shift of 8026-cm-’ line by uniaxial stress applied parallel to the [MO] and [ 1lo] directions.
after illumination is an orbital singlet. Therefore, we have two possibilities for the ground state of the manganese ion after illumination: a dS configuration with a 6Alground state which leads to Mn2+, or a d2 configuration with a 3A2 ground state which leads to Mn5+. The near free-electron value of the g-factor and the value of the hyperhne constant A suggest Mn2+ rather than Mn’+ ions. ESR measurements indicate therefore that the manganese related center which appears after illumination is Mn2+-d’. These also indicate that Mn ions. which are oresent in the crystal before illumination and are transformed by light into Mn2+ ions, should be Mn3+-d4 or Mn+d’s, (d6) in tetrahedral coordination. For the d5 configuration all transitions are spin forbidden and the absorption connected with such a center should be weak and its observation would be rather unlikely. If such an absorption is observed, it should be present in the high-energy region, presumably above 20.000 cm-‘. Although we have Mn2+-d5 centers in the crystal and the concentration of these centers increases considerably with illumination as indicated by ESR measurements, there would be no optical absorption from these. The results of the uniaxial stress experiment may be understood by assuming that the center responsible for the 8000 cm-’ line has tetrahedral symmetry. For such a high symmetry center there is no orientational
The solid lines in Figs. 10(a) and 10(b) are obtained from this formula with C = -9 X 10e4 cm-‘/kG X cmm2. The uniaxial stress experiment indicates that the center connected with the 8000-cm-’ line indeed has tetrahedral symmetry like the center observed in the ESR measurements. It is reasonable to assume that the center connected with the KKlO-cm- line observed before illumination, and the center observed after illumination have the same origin. Therefore, because after illumination the center is Mn2+ in tetrahedral coordination, the center before illumination which is responsible for the 8000-cm-’ line, will be Mn3+ or Mn+, also in tetrahedral coordination. Illumination will cause the charge transfer Mn3+ - Mn2+ or Mn+ - Mn2+, assuming that the most probable change of the charge state by light is -Cl.
1 +PllE
o-
pl.E
8026
-7 5i ;
+
kG/cm*
Fig. IO(b).Shift of 8026-cm-’ line by uniaxial stress applied parallel to the [ 11 I] direction.
W. WAuozvkstu et al.
1124
Let us assume that the center connected with the 8000-cm-’ line before illumination is Mn’+-d4 in tetrahedral coordination. The observed line in this case would be the no-phonon line of the ‘T2 - 5E transition with Dq - 800 cm-‘. The ‘T, - ‘E transition was observed in the region of 5500 cm-’ (Dq - 550 cm-‘) in the case of C?+-d4 in ZnSe [7]. Although a value of Dq - 800 cm-’ Seems to be too high in comparison with 550 cm-‘, such an assignment cannot be excluded. For Mn+ we should take into account two electronic configurations: d5s’, or d6 if the s electron were to fall into the d shell. For a d6 configuration in tetrahedral coordination we should expect an absorption band in the region of 3000-6000 cm-’ due to the ‘E - ‘T2 transition (many Fez’ derivatives which form tetrahedral complexes possess an absorption band in this region, i.e. Dq - 300-600 cm-’ [8]). It seems rather unlikely that the 8000-cm-’ line is due to a ‘E - ‘T, transition in a d6 configuration; however such an assignment cannot be definitely excluded. The d5s’ configuration in tetrahedral coordination could also explain the narrow line at 8000 cm-‘. Such a center would be similar to the neutral chromium center reported previously in Crdoped BGO [6]. The spectrum of Mn+ in the gaseous phase consists of the line ‘.S,[3d5(%)4s] -
5S2[3d5(6S)4s]
at
9472.86 cm-‘,
and the lines 7S3[3d5(6S)4s] ,- ‘D4 5D3
‘D2 [3d6] 5DI :I 5D’_l
at
14,325.64 14,593.62 14,781.03 14,90 1.06 14,959.68
cm-’ cm-’ cm-’ . cm-’ cm-’
Other lines lie above 27,000 cm-‘. Because of the relatively large energy difference between the 5S2 and the ‘0 states, only a weak mixing between these states can be expected, and it is possible to assume that the 8000-cm-’ line is due to the ‘S3 - ‘S2 transition. The 5D[3d6] levels will form (neglecting spin-orbit interactions) in a crystal field of tetrahedral symmetry two bands, ‘E and ‘T2, and one should expect, as discussed above, that the energy difference between these bands may be as large as 3000-6000 cm-‘. However, it is difficult to make any assumption about the positions of these bands compared with the energy of the ‘D levels of the free ion. Moreover, the transitions from the ground state to these excited states should be strongly forbidden and should form a broad band. It is very doubtful whether such transitions can be observed. Therefore, one should expect only one sharp line for a Mn+ [3d54s’] center in tetrahedral coordination. We believe that the most probable explanation of the 8OOO-cm-’ line is that it is due to a Mn* [d5s1] center.
The light will change this center according to the scheme Mn+ [d5s’] 5 Mn*+ [d’]. The remaining absorption observed in the experiment [Fig. 7(a)] cannot be explained by means of a center with tetrahedral symmetry. Therefore, one should try to explain the absorption in the region lO,OOO-16,000 cm-’ in terms of centers of lower symmetry, probably Mn in an interstitial position (orthorhombic symmetry). Spectra of light-induced absorption (the difference between absorption after and before illumination) are very useful for the interpretation of absorption in photochromic materials. Such spectra eliminate the absorption connected with centers which do not change during illumination and enhance more clearly all those which do change with illumination. The light-induced absorption Aa of Mn-doped BSO at room temperature in the range 7500-16,000 cm-’ is shown in Fig. I l(a), and in the range 15,00023,000 cm-’ in Fig. 1l(b). Between 9000 and 14,000 cm-’ the light-induced absorption is negative, which means that the concentration of the centers responsible for this absorption decreases with illumination. Even if there is any absorption in this region which increases with illumination, the main role is due to the absorption connected with centers which vanishes with illumination. In the range between 14,000 and 15,000 cm-‘, Acuchanges rapidly, and at about 14,400 cm-’ the light-induced absorption changes sign. This is the region where centers which decrease and increase with illumination play an equal role. In the range between 15,000 and 23,000 cm-’ the light-induced absorption is positive, which means that in this region centers whose concentration increases with illumination are dominant. If one attempts to decompose the observed spectrum into various components the results will be as follows. In the region between 9000 and 17,000 cm-’ there will be some bands with halfwidths of about 2000-3000 cm-’ which decrease alter illumination, and there will be a very broad band ranging from the absorption edge to the near infrared region (halfwidth 8000- 10,000 cm-‘) whose intensity strongly increases after illumination. It is very difficult to assume that this broad band is due to a transition in a Mn center with a d” configuration. Schirmer et al. [9-121 showed that such a broad and quite intense band with a maximum close to the absorption edge which is observed in MnO, BeO:Li and in quartz can be explained as due to a small hole polaron bound to a defect or impurity. According to this concept the hole trapping is accompanied by a lattice distortion around the trapping site. The bonding orbitals between the impurity and ligand ions can capture a hole forming a small polaron. In tetrahedral coordination there are four equivalent orbitals. The optical absorption is explained by a light-induced transfer of the hole from one to another equivalent 02- site near the impurity.
1125
Light-induced charge transfer processes
(9)
Replacing w by A = l/X we have
a=-
A max
amax
e-t(2~/H*Wa-nmu~l~
9
A
(10)
where A
=mm
‘=JT
3uhc
and Hi = 8AoAmal In 2 coth
m
r
1oobo
’ *;cm_,) -15&xl
’
Fig. 11(a). Spectrum of tight induced absorption Aa for Mndoped BSO at room temperature in the range 7000-16400 cm-‘.
(
$T
1
.
Amax,HA and a,,, are fitting parameters. in the light-induced absorption spectrum [Fig. 1 l(b)] the band connected with such a polaron does not overlap with the bands from some other origin in the region between about 17,000 and 2 1,000 cm-‘, and fitting to the experimental data should be performed in this region. Such a fit gives the following values of the fitted parameters: A,,,ax= 22,700 cm-‘, Hn = 9500 cm-‘, a,,,= = 13.3 cm-‘, and A,J = 658 cm-‘. The solid line in Fig. I l(b) is drawn according to eqn (10) using the above given parameters.
According to Schirmer [ 121 such a process will produce an absorption which in general will form three quasi-Gaussian bands:
16 w-’ = 3 E&two coth
(8)
where EJT is the stabilization energy gained by the hole capture, J is the resonance integral, tro, is the energy of a representative phonon mode describing the distortion, and RF and ECF are connected with the crystal field splitting. Let us assume that in BGO and BSO crystals with transition metal impurities similar centers may exist. We will denote these centers as X”+ 0. This means that in an X04- complex, where X is a transition metal element, a hole is localized on the bonding orbital forming a small radius polaron. If in the first approximation we neglect the crystal field effect and the influence of J, then the absorption band will be given by
15000
A(cm-0
2oboo
Fig. 1l(b). Spectrum of tight-induced absorption Aa for Mn-doped Bso at room temperature in the range 15,00023,000 cm-‘. [Solid line shows the fitting of the bound polaron absotption according to formula (IO). The fit agrees with the experimental points in the region between 17,000 and 21,000 cm-‘.]
W. WARDZYtiSKIet al.
1126
Valence band
Ml?+@ Smal I
radius
polaron
It,
-
la,
‘-
Fig. 12. The schematic presentation of the energy levels of the molecular orbitals for (GeO.$ and (Mn04)‘-, and its illustrative connection with the valence band of BGO and a bound polaron.
The polaron absorption band found this way agrees well with the absorption below the absorption edge measured for the illuminated BSO crystal [(Figs. S(a) and 8(b)]. The solid lines in these figures were calculated according to formula (10) with the fitting parameters given above. The small differences between the calculated curve and the experimental points in Fig. 8(a) in the range 10,000-15,000 cm-’ can be explained by the fact that the illumination does not destroy completely the absorption observed at 13,500 cm-’ before illumination. The discrepancies between the polaron curve and the experimental points in the region 18,000-23,000 cm-’ are due to the band-toband absorption (absorption edge). The difference between the polaron band (solid curve) and absorption measured in the experiment represents the absorption edge of the crystal. This absorption is given by the solid line in Fig. 7(b). The difference between this curve and the experimental points indicates that there is a band in this region which disappears after illumination. This band is also seen in Fig. 1l(b). If one subtracts the calculated polaron absorption from the experimental light-induced absorption [Figs. I l(a) and 1 l(b)], one obtains the absorption spectrum due to centers which disappear after illumination, as shown in Figs. 13(a) and 13(b). We attempt to decompose this spectrum into Gaussian bands of the form
(y =
oL,.,
e-t(2vminxa-am,)l* (11)
As one can see such a decomposition gives us a band at 10,700 cm-’ [solid line in Fig. 13(a)] with the following parameters: A,, = 10,700 cm-‘, HA = 1700 cm-‘, fxmax= 0.65 cm-‘; a band at 14,000 cm-’ [solid line in Fig. 13(a)] with the following fitting parameters: A,,, = 14,000 cm-‘, HA = 2900 cm-‘, shown ffmax = 2.8 cm-‘; a small residual absorption in Fig. 14 consisting of a weak line at 8000 cm-’ and probably its phonon structure in the region of 9000 cm-‘, and three rather narrow bands in the region of 12,000 cm-‘. We treat the weak absorption which is apparently present in the region 14,000-19,000 cm-’ as being within the limits of experimental error, since this absorption results from the separation process for the Gaussian components. This also applies for the region of 9000 cm-‘, although the accuracy in this region is better. There are also the bands above 23,000 cm-’ [see Fig. 13(b)]. The line at 8000 cm-’ is the same line as observed at helium temperature and already discussed. As pointed out earlier, the bands found V at 10,700, at 14,000, W at’ - 12,000 and V at -23,000 cm-‘, cannot be explained using the center connected with manganese in tetrahedral coordination, and we should take into account orthorhombic symmetry.
an interstitial
position
Since this absorption
with
dis-
1127
Light-induced charge transfer processes
10000
h:cm-‘t
15Jix
Fig. 13(a). The difference between the light-induced absorp tion and the bound pokon auction as a function of wavenumber in the range 7000-20,000 cm-‘. Solid line, possible Gaussian components.
appears with illumination and we do not find any other absorption of this kind which increases with illumination, we will assume that this interstitial Mn center after illumination has the charge state 8. In such such a case after illumination we will have a Mn2+ center. Absorption by such centers will be spinforbidden and should not be observed. The center before illumination should be Mn’+ [d4]. In Fig. 15 the energy level schemes for octahedral, tetragonal and orthorhombic symmetry for the d4 configuration
Fig. 14. The residual absorption found from the decomposition of the observed light-induced absorption into a polaron absorption band and two Gaussian bands from Fig. 13(a).
are shown. The actual orthorhombic symmetry of the interstitial in BilZGe020 can be treated in first approximation as tetragonal [6]. As one can see from Fig. 15, a tetragonal distortion will split the 5E level into 5B, and ‘A, levels, and the ‘T2 level into ‘B2 and ‘E levels. Reduction of the symmetry to orthorhombic leads to additional splitting of the highest SE level into two levels. The transition to these highest levels gives rise to the absorption above 23,000 cm-‘, and cannot be analyzed in detail. Therefore, we limit ourselves to tetragonal symmetry. The band found at 10,700 cm-” is interpreted as due to the 5B, - 5.4, transition, that at 14,000 cm-’ as due to the “B, - ‘B2 transition, and that at 24,~ cm-‘+23,000 cm-‘) as due to the ‘8, - ‘E, transition. Using the Ligand Field Energy Diagrams [ 131 we find the following crystal field parameters: B = 565
w l
. l . .
. * .. .
-4
l l
17000
20600
23000
,
Ab(cm+ Fig. 13(b). The difference between the light-induced absorp tion and the bound polaron absorption as a function of wave number in the range 16,~-23,~ cm-‘. PCS4b:lO-0
Orthorhombic
Octahedral Tetragonot
Fig. IS. Schematic diagram of the energy levels for a center with d’ electronic configuration in the environment of octahedral, tetragonat and o~horhombic symmetries.
1128
W. WAuozvAsut et al.
cm-‘, Dq = 1400 cm-‘, Dt = -330 cm-‘, Ds = 4170 cm-’ and C = 2260 cm-‘. From the above-cited Energy Diagrams we find the following spin forbidden transitions which depend weakly on the crystal field: 5B, - 3BI and ‘B, - 3&. Using the above-mentioned parameters one can find E 1~,-5~, = 10,500 cm-‘,
Ed,+.+ = 14,000 cm-‘,
E s~,__s~= 24,000 cm-‘,
EQ+_Q, = 12,000 cm-‘,
E Q,_)~~ = 12,830 cm-‘, in good agreement with experiment. It is clear that the crucial point in the interpretation presented above is the assumption about the small polaron. It is important therefore to have some idea, even in an approximate and qualitative manner, how such a polaron could be formed and consequently about the energy band structure of BGO type crystals. Only very little is known about the energy band structure of these crystals. Futro [14], using the results of reflection measurements in the region above the fundamental absorption edge and some analogy between BGO and &By’ compounds, suggested that the energy band scheme of BGO is determined mainly by the bismuth and oxygen net structure. He paid attention to the layer structure of rhombohedral B&X3 type compounds and cubic BGO. According to such a concept, the layer structure BizX3 type compounds is modified in BGO by inserting Ge02 molecules. We propose a different approach to this problem. If one analyzes the position of the atoms in the crystallographic structure of BGO [ 151, a possible description of this structure is as follows. The BiizGeOZO molecules are placed at the center and all corners of a cubic elementary cell. Such a Bii2GeOZ0 molecule consists of a (Ge0,)4- tetrahedron and four (Bi,04)+ groups attached to the four faces of the Ge04 tetrahedron. The oxygen ions in the B&O4 group form a flat equilateral triangle with the fourth @- at its center. Three Bi ions are attached to three 02- ions and are placed almost in the same plane as the @- ions, forming a similar equilateral triangle. The GeO, and four Bis04 form together a compact and slightly distorted spherical BilzGeOzo complex. Let us imagine the energy levels of the molecular orbitals of BilzGeOzo or for simplicity those of Ge04 and Bis04. The energy band structure will be formed from these levels in a similar way as from the atomic levels. Let us take into account the energy levels of the molecular orbitals connected with (GeO,r-. Such orbitals were calculated in [ 161. One can see that the highest molecular orbitals which are completely filled with electrons are: 2a,, le, 2t2, 3t2 and It,. It is reasonable to assume that the energy levels of these orbitals, due to interactions in the crystal lattice, will form a valence band of the crystal. Of course, the molecular orbitals from (Bis04)+ will also contribute
to the band structure, but unfortunately there are no data connected with the molecular orbitals of such a molecule. Therefore, the discussion has only a qualitative and illustrative significance. If Mn replaces Ge in the crystal lattice, a (MnO,rmolecule will be formed. The energy levels of the molecular orbitals for MnO, have also been calculated [ 171. The energy levels for Ge04 and MnO, according to [ 161 and [ 171 are given schematically in Fig. 12. One can note that the molecular orbitals of Mn04 are very similar to those of Ge04, and the It,, 3t2, 2a,, 2t, and le levels could overlap the valence band. If a hole appears in the valence band, it could be self-trapped on the molecular orbital of the MnO, complex, forming a small radius polaron (see Fig. 12). This orbital will probably be of the 2a,, 3t, types. Such orbitals will be populated about 30-40% by s electrons from the metal atom, SO-60% by p electrons from the oxygen atom and 10% by s electrons from an oxygen atom for the 2~ orbital, and 80% by p electrons from an oxygen atom, 2% by s electrons from an oxygen atom and the rest (18%) by p electrons from the metal atom for the 3t, orbital [16]. The 2ai level is sigma bonding between the metal s orbital and the oxygen 2p orbitals. The 3t, orbital is mainly oxygen 2~. It is possible that the position of such an orbital in relation to the valence band edge (near the top of the valence band, or just above the valence band edge) is decisive from the point of view of the formation of the bound polaron state and the formation of a Mn+ state with d5s’ or d” configuration. CONCLUSIONS (1) In Mn-doped BGO and BSO, the Mn ions are located in tetrahedral positions on a Ge or Si site with the charge state Mn+ [3dS4s’], and in interstitial positions in the charge state Mn’+ [3d4]. Mn3+ centers may be compensated by Mn+ centers. Charge compensation could also be obtained through oxygen and bismuth vacancies. (2) Illumination can remove the 4s electron from a Mn+ tetrahedral center producing a Mn’+ [3d5] center. This leads to the disappearance of the 8000cm-’ line. Illumination can also change the charge state of the Mn3+ interstitial center, giving Mn2+ and a hole in the valence band. This leads to the disap pearance of the absorption bands in the region 1O,OOO15,000 cm-‘, and above 23,000 cm-‘. The hole is bound to the Mn2+ tetrahedral center, giving a bound polaron center and a broad absorption band which appears after illumination. Therefore, f
tetrahedral center-Mn+ orthorhombic
center-Mn3+
bound polaron center-Mn2+
Mn2+ + e f
Mn2+ +‘h,
(tetrahedral) + h-
Mn2+ 0.
(3) Although the separation of some overlapping bands into simple components may often create some
Light-induced charge transfer pmcesses problems, the results presented above indicate that the presence of a very broad band is rather likely. Moreover such a broad band was also observed in
Cu- and Co-doped BGO, where in the case of Cu, the absorption spectrum does not overlap with the absorption due to other centers. This supports the interpretation of the photochromic effect in terms of a polaron model. Measurements on Co- and Cudoped BGO are in progress and will be published in separate
articles.
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135, 715, 727 (1964). 4. Aldrich R. E., Hou S. I. and Harvill M. L., J. Appl. I’hys. 42,493 (197 1).
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5. Vedam K. and Hennessey P., J. Opt. Sot. Am. 65,442
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