Atomic Structure of Doped Sillenites

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 10, pp. 1175–1182. © Pleiades Publishing, Ltd., 2009.Original Russian Text © A.V. Egorysheva, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 10, pp. 1253–1260.

1175

INTRODUCTION

Bi

12

GeO

20

(BGO), Bi

12

SiO

20

(BSO), and Bi

12

TiO

20

(BTO) sillenite crystals are known as efficient photore-fractive materials. One common way of varying theirfunctional properties is doping. There is, however, nouniversal model capable of accounting for the effects ofvarious dopants on the properties of these sillenites.Also, there is no general agreement as to which site inthe sillenite structure is occupied by a particular dopant[1–5]. Understanding the doping effect on the proper-ties of crystals requires knowledge of how dopantatoms are incorporated and what lattice distortions theyproduce, because lattice defects play a key role in deter-mining such properties of sillenites as photoconductiv-ity, photochromism, and absorption edge [6–9]. Thisproblem is difficult to resolve because of the low solu-bility of dopants in sillenites, which limits the densityof doping-induced defects to a level insufficient for X-ray structure analysis.

Cubic sillenites have space group

I

23

, with two for-mula units per unit cell. The M atoms reside in the cen-ter and corners of the cubic unit cell (position

2

‡

) andare in tetrahedral oxygen coordination [10]. The struc-tural basis is made up of [BiO

5

E] polyhedra (where E isthe Bi

6

s

2

lone pair), which share edges to form dimers(Fig. 1). Bi

12

GeO

20

has a nearly ideal sillenite structure.In Bi

12

TiO

20

, about 10% of the tetrahedra containdefects: one Ti vacancy and two O(3) vacancies. Thesillenite family is not limited to BGO, BSO, and BTO.At present, more than 60 sillenites are known (generalformula Bi

12

M

x

O

20

±δ

), with

å

n

+

cations in the oxidationstates from 2+ to 5+. Electroneutrality is maintainedthrough local changes in the bismuth–oxygen frame-

work. The degree of disordering is highest in com-pounds containing tri- and divalent cations (Bi

25

M

3+

O

39

and Bi

38

M

2+

O

58

), where half or two-thirds of the regular[MO

4

] tetrahedra are replaced by [BiO

3

] “umbrella-like” groups, with one O(3) vacancy per group. In theunit cell of Bi

24

O

41

, the excess charge on tetrahedral

M

5+

is compensated through incorporation of an extraoxygen atom into the spacious voids in every sixth

6

b

site [10].

The objective of this work is to consider, using ear-lier structural data [10], the general mechanisms behindthe formation of lattice defects in doped sillenites, illus-trated by the example of bismuth titanate. The conclu-sions drawn here apply as well to other sillenite com-pounds.

RESULTS AND DISCUSSION

The way in which dopant atoms are incorporatedmay be assessed from the density of doped crystals.Table 1 lists the X-ray densities of doped BTO crystalscalculated for two substitution schemes: dopant (A)atoms substituting for Ti

4+

on the tetrahedral site(

Ç

i

12

í

i

1 –

x

A

x

é

20

) and for

Ç

i

3+

– Ç

i

12 –

x

A

x

í

i

é

20

. The cal-culation results are compared with experimental data.For heterovalent substitution on the Ti

4+

site, two con-figurations are considered. One of them takes intoaccount the possibility of bismuth accommodation inposition

2

‡

in the proportion imposed by the crystal-chemical formulas of the corresponding individualphases.

M25+

Atomic Structure of Doped Sillenites

A. V. Egorysheva

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russiae-mail: [email protected]

Received February 25, 2009; in final form, April 14, 2009

Abstract

—It is shown using Bi

12

TiO

20

as an example that doping of a sillenite phase leads to the formation ofa solid solution between it and the Bi

12

M

x

O

20

±

δ

phase corresponding to the dopant. This gives rise to structuralfeatures characteristic of the second phase and, accordingly, results in the formation of the corresponding opti-cal centers. Dopant atoms occupy tetrahedral (M) sites. In addition, in the case of heterovalent substitutions theM site is partially occupied by the Bi atoms of [BiO

3

] groups. The structural features of sillenites are governedby the oxidation state of the M cation. Accordingly, doping with ions in the same oxidation state leads to theformation of similar optical centers and, as a consequence, produces similar changes in the properties of thematerial. The results obtained are supported by experimental studies of the absorption, circular dichroism, pho-toinduced absorption, and photoconductivity spectra of doped sillenite crystals in a wide spectral range.

DOI:

10.1134/S0020168509100173

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INORGANIC MATERIALS

Vol. 45

No. 10

2009

EGORYSHEVA

[Bi

5

E]

[BiO

3

E]

O(3) Bi

M

V

O(3)

O(1) O(2)

O(1)

[BiO

4

E]

Bi

6

b

O(1)

O(1)

O(2)

M

O(3)

O

O

V

M

V

O

Bi

V

O

(

‡

)

(

b

)

Fig. 1.

(a) Portion of the sillenite structure containing

å

2+

-, å

3+

cations [10]; (b) [

V

M

V

O

V

O

O

2

] “defective” tetrahedron, character-istic of BSO, BGO, and BTO crystals.

Comparison of the calculated and measured densi-ties indicates that the formation of substitutional solidsolutions containing the dopant on the

Ç

i

3+

site isunlikely: substitutions on the bismuth site lead to arapid reduction in density with increasing doping level.Substitutions on the tetrahedral site have little effect onthe density of the material because the dopants and tita-nium differ little in atomic weight. For crystals dopedwith zinc, cadmium, gallium, and iron (Table 1), thebest agreement (within experimental uncertainty)

between the calculated and measured densities isachieved when bismuth incorporation into the tetrahe-dral site is taken into account. Therefore, these crystalscontain [BiO

3

] umbrella-like groups, characteristic ofsillenites containing tri- and divalent cations.

The observed effect of doping with zinc and vana-dium on the unit-cell parameter of bismuth titanate pro-vides conclusive evidence for the formation of

(1

−

x

)Ç

i

12

í

i

é

20

·

x

Bi

38

ZnO

58

and

(1 –

x

)Ç

i

12

í

i

é

20

·

x

Bi

24

V

2

O

41

solid solutions. As seen in Fig. 2, the data

INORGANIC MATERIALS

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No. 10

2009

ATOMIC STRUCTURE OF DOPED SILLENITES 1177

points representing doped samples fall on straight linesconnecting BTO and Bi

38

ZnO

58

(BZnO) orBi

24

V

2

O

41

(

BVO

)

.From the intensity of the two-phonon absorption

bands of the [MO

4

] tetrahedral groups at 1537 and1528 cm–1 for vanadium and at 1378 and 1328 cm–1 fortitanium in doped and undoped samples, we evaluatedthe V5+ and Ti4+ ion occupancies on the tetrahedral site.The results, in conjunction with neutron diffractiondata for the vanadium and titanium sillenites [10], wereused to estimate the likely fraction of [BiO3] groups and�Ti and �V vacant tetrahedra, where �Ti and �V arevacancies on the tetrahedral site in the titanium andvanadium sillenites, respectively. The total occupancyof the tetrahedral site was varied in the range 99–100%at different vanadium concentrations (Table 2).

The results indicate that doping of bismuth titanateleads to the formation of a solid solution between bis-muth titanate and the sillenite phase corresponding tothe dopant. This correlates with the phase diagram datafor the multicomponent systems Bi2O3–TiO2–Si(Ge)O2,Bi2O3–MxOy–Si(Ge)O2, Bi2O3–MxOy–TiO2, andBi2O3--MxOy–P2O5 (M = Zn, Ga, Fe) [11–16], whichattest to the formation of limited solid solutionsbetween different sillenite phases. Therefore, dopantatoms occupy tetrahedral position 2‡. In addition, in thecase of heterovalent substitutions, position 2‡ is par-tially occupied by Bi atoms, which leads to the forma-

tion of [BiO3] umbrella-like groups. For this reason,doping of BTO is accompanied by the development ofstructural features characteristic of the second phaseand, accordingly, by the formation of the correspondingoptical centers. Their concentration is comparable toand in some instances (e.g., Zn) exceeds the dopant

Table 1. X-ray and measured densities of doped BTO crystals

Dopant A xX-ray density*, g/cm3

Measured density, g/cm3

Bi12Ti1 – xAxO20 Bi12 – xAxTiO20

Mn 0.057 9.05 9.02 9.07 ± 0.01

0.136 9.07 9.00 9.08 ± 0.01

0.190 9.07 8.97 9.08 ± 0.01

Zn

A = Zn1/3Bi2/3 A = Zn

0.022 9.08 9.07 9.05 9.08 ± 0.01

Cd 0.013 9.07 9.06 9.05 9.07 ± 0.01

Ca 0.086 9.10 9.05 9.01 9.08 ± 0.01

Cu 0.058 9.10 9.08 9.05 9.08 ± 0.01

A = Fe1/2Bi1/2 A = Fe

Fe 0.070 9.09 9.07 9.03 9.08 ± 0.01

0.092 9.10 9.07 9.02 9.09 ± 0.01

Ga 0.041 9.08 9.06 9.03 9.08 ± 0.01

A = V1/2Bi1/2 A = V

V 0.14 9.07 9.04 8.97 9.05 ± 0.01

0.28 9.08 9.02 8.87 9.02 ± 0.01

* Calculated from the unit-cell parameter of the crystals and the dopant concentration determined by laser ionization mass spectrometry.

10.18

0.2 0.4 0.6 0.8 1.0010.17

10.19

10.20

10.21

10.20

10.23

10.24

10.20a, Å

1

x

Bi24V2O41

Bi38ZnO58

2

Bi12TiO20

Fig. 2. Unit-cell parameter as a function of doping level for(1) (1 – x)Çi12íié20 · xBi36ZnO59 and (2) (1 – x)Çi12íié20 ·xBi24V2O41 crystals.

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INORGANIC MATERIALS Vol. 45 No. 10 2009

EGORYSHEVA

concentration. Since the structural features of sillenitesare governed by the oxidation state of the M cation,doping with ions in the same oxidation state leads to theformation of similar optical centers and, as a conse-quence, produces similar changes in the properties ofthe material.

Studies of different Bi12MxO20±δ sillenites and nons-toichiometric BGO, BSO, and BTO crystals [6–9] pro-vided insight into the relationship between the detailedatomic structure and properties of the crystals. Thespectroscopic and photoinduced properties of sillenitecrystals containing tetravalent cations (BGO, BSO, andBTO) are governed by a defect center consisting of anM vacancy and two O(3) vacancies. Oxygen vacanciesin such crystals act as traps for photoexcited carriers.This center is responsible for the strong band between2.4 and 3.1 eV in the absorption (absorption shoulder)and circular dichroism (CD) spectra of the sillenitesunder consideration, their high photoconductivity, andtheir strong photochromic effect upon excitation ofelectrons from the levels at Ö � 2.5–2.7 eV. The spectralfeatures of sillenites containing di- and trivalent cationsare related to the single O(3) vacancies due to the[BiO3] groups. The features indicated above are missingin the spectra of crystals containing å2+-, å3+ cations;

instead, there is a strong absorption for E < 2 eV. Withincreasing [BiO3] concentration, its strength increases,while the photoconductivity drops. In the excitationspectra of the photochromic effect in crystals contain-ing å2+-, å3+ cations, the band at Ö � 2.5–2.7 eV ismissing, which implies that the photochromic effect isonly due to interband transitions.

The formation of a solid solution upon doping, withstructural defects typical of both phases, accounts forthe observed changes in the spectroscopic and photoin-duced properties of the crystals. The spectra of dopedcrystals show features related to both phases, and thesechanges are concentration-dependent.

As shown previously [17–20], doping of BTO withany elements reduces the absorption in the shoulder at2.4–3.1 eV, shifts the absorption edge of BTO (Fig. 3),and reduces the intensity of the CD band at 2.4–3.1 eV(Fig. 4). This is attributable to the reduction in the con-centration of imperfect tetrahedra with decreasing BTOcontent. Note that doping with di- and trivalent impuri-ties has a substantially stronger effect in comparisonwith pentavalent impurities because, as follows fromthe structural formulas of Bi12MxO20±δ, there is (are) one(two) bismuth atom(s) in position 2a per å3+ (å2+) cat-

2.2 2.4 2.6 2.8 3.0 3.22.00

20

40

60

801 2 3 4 5 6 7

α, cm–1

Photon energy, eV.

Fig. 3. Absorption spectra of (1) undoped and (2–7) doped Çi12íié20 crystals: (2) 0.25 wt % P, (3) 0.35 wt % V, (4) 0.11 wt % Ca+ 0.12 wt % Ga, (5) 0.15 wt % Zn, (6) 0.18 wt % Cd, (7) 0.23 wt % Ga.



0 0.1 0.2 0.3 0.4 0.5 0.60

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16∆εC, cm–1

ë, wt %

Çíé ⟨Zn⟩BTO ⟨Ga⟩BTO ⟨Al⟩BTO ⟨P⟩BTO ⟨V⟩

BTO

Fig. 4. CD at the maximum of the defect-related band vs. doping level for BTO crystals.

–0.5

1.8 2.0 2.2 2.4 2.6 2.8 3.01.6

0

0.5

1.0

1.5

2.00.035 wt % Zn0.0140.009BTOBZnO

Photon energy, eV

logα [cm–1]

Fig. 5. Absorption spectra of BTO⟨Zn⟩ crystals.

Table 2. Occupancy of tetrahedral position 2a in BTO⟨V⟩ crystals

SampleOccupancy, %

Density, g/cm3 Lattice parameter, ÅV Ti Bi �V �Ti

BTO – 90 [10] – – 10 [10] 9.07 10.174

BTO⟨V⟩ (1) 8 82 0.3 0.7 9.0 9.07 10.176

BTO⟨V⟩ (2) 18 71 0.6 1.6 7.9 9.05 10.190

BTO⟨V⟩ (3) 23 66 0.8 2.1 7.3 9.04 10.191

BVO 89 [10] – 3 [10] 8 [10] – 8.89 [8] 10.247 [8]

ion. Accordingly, at a given å2+, å3+-, or M5+ concen-tration, the proportion of bismuth titanate in the solidsolution is higher in the case of M5+.

The example of BTO⟨Zn⟩ crystals demonstrates that,with increasing doping level, the absorption spectra ofdoped BTO crystals exhibit two trends (Fig. 5): a reduc-

1180


EGORYSHEVA

tion in the absorption on the shoulder (Ö > 2.3 eV) anda steady rise in the absorption coefficient for E < 2 eV.At a zinc content of 0.035 wt %, the absorption spec-trum of BTO⟨Zn⟩ is almost identical to that of BZnO.

As would be expected, the photoconductivity ofdoped BTO crystals in the range 2.2–3.0 eV decreaseswith a reduction in the concentration of imperfect tetra-hedra and an increase in the concentration of [BiO3]groups (Fig. 6). One exception is crystals doped withsmall amounts of pentavalent elements, which increasethe photocurrent by up to two orders of magnitude (forexample, 0.05 wt % P) because of the increase in thenumber of donor levels in the range 0.15–0.50 eV,readily ionizable at room temperature [21, 22]. Athigher dopant concentrations, the photoresponsedecreases over the entire spectral range studied.

The influence of the optical centers characteristic ofthe sillenite phase corresponding to the dopant isreflected in the photochromic properties of the crystals.Doping with elements in the oxidation state 4+ or abovehas no effect on the shape of the spectral dependence ofphotoinduced absorption (PIA). At the same time,because of the decrease in the concentration of imper-

fect tetrahedra, such doping reduces the magnitude ofPIA. Low concentrations of elements in the oxidationstate below 4+ also reduce the intensity of all the PIAbands. Increasing the doping level alters the shape of thespectrum: the PIA signal rises in a wide spectral range,from 1.4 to 2.8 eV, with a maximum at 2.0–2.2 eV. Thedoping effect on the PIA spectrum is well illustrated by theexample of BTO⟨Zn⟩ crystals (Fig. 7).

Based only on the fact that the dopant concentrationmay exceed the concentration of vacancies on the tetra-hedral site in undoped crystals, Nechitailov et al. [1]arrive at the conclusion that, at high doping levels,some of the dopant atoms always occupy bismuth orinterstitial sites. This conclusion is, however, refuted byour finding that, at high doping levels, the properties ofBTO are strictly monotonic functions of dopant con-centration. According to the above model, doping can-not fill all the vacancies on the tetrahedral site becausetheir concentration is governed by the proportion ofbismuth titanate in the solid solution formed. Table 2demonstrates that, even at the highest possible dopinglevel, corresponding to a 22% occupancy of vanadiumon the tetrahedral site, the fraction of imperfect tetrahe-

1.5 2.0 2.5 3.0 3.5 4.0 4.51.0100

109

108

107

106

105

104

103

102

101

1100

101

0.10.01

102

103

104

Phot

ocur

rent

, arb

. uni

ts

Çíé ⟨Zn⟩BTO ⟨Ga⟩BTO ⟨Ca⟩BTO ⟨P⟩

BTO ⟨Ca + Ga⟩

BTO 123456

(a) (b)

1

2

3

4

5

6

Photon energy, eV

E = 3.2 eV

E = 2.5 eV

P

V

Cd

Zn

Cu

Mn

Nb

C, wt %

Fig. 6. (a) Spectral dependences of the photocurrent through BTO crystals; (b) photocurrent through BTO as a function of dopinglevel under excitation at 2.5 and 3.2 eV.



dra in the crystal is �7%. For this reason, the spectra ofeven heavily doped crystals show all of the featuresrelated to this defect species and characteristic of BTO.

It is well documented [19, 23–25] that doping ofBSO and BGO crystals produces similar changes intheir spectra, which suggests that the above conclusionsapply as well to these sillenites.

CONCLUSIONS

Doping of bismuth titanate leads to the formation ofa solid solution between it and the Bi12MxO20±δ phasecorresponding to the dopant. Dopant atoms occupy tet-rahedral (M) sites. In addition, in the case of heterova-lent substitutions, the M site is partially occupied by theBi atoms of [BiO3] groups. For this reason, doping ofBi12MxO20±δ is accompanied by the development ofstructural features characteristic of the second phaseand, accordingly, by the formation of the correspondingoptical centers. Their concentration determines theproperties of doped crystals. These conclusions aresupported by experimental studies of the absorption,

circular dichroism, photoinduced absorption, and pho-toconductivity spectra of doped sillenite crystals in awide spectral range.

ACKNOWLEDGMENTS

This work was supported by the Presidium of theRussian Academy of Sciences through the programTargeted Synthesis of Substances with Tailored Proper-ties and Fabrication of Related Functional Materials.

REFERENCES1. Nechitailov, A.A., Krasinkova, M.V., Mokrushina, E.V.,

et al., Correlation between the Impurity Content, theAverage Charge State of Chromium Cation and OpticalAbsorption in Sillenite Crystals Doped by Chromium inWide Range of Concentrations, Cryst. Res. Technol.,2001, no. 2, pp. 147–156.

2. Marquet, H., Tapiero, M., Merle, J.C., et al., Determina-tion of the Factors Controlling the Optical BackgroundAbsorption in Nominally Undoped and Doped Silleni-tes, Opt. Mater., 1998, vol. 11, pp. 53–65.

3. Chevrier, V., Dance, J.M., Launay, J.C., and Berger, R.,Cu2+ in Bi12GeO20—A Tentative Electron-Spin-Reso-nance Study, J. Mater. Sci. Lett., 1996, vol. 15, pp. 363–365.

4. Wardzynsky, W. and Szymczak, H., The Center ofOrthorhombic Symmetry in ChromiumDopedBi12GeO20 andBi12GeO20 Single Crystals, J.Phys. Chem. Solids, 1984, vol. 45, nos. 8–9, pp. 887–896.

5. Borowiec, M.T., Photochromic Absorption of Bi12GeO20Doped with Copper, Phys. B (Amsterdam, Neth.), 1985,vol. 132, pp. 223–231.

6. Egorysheva, A.V., Absorption Edge of Bi12MxO20±δ (M =Zn, B, Ga, P, V, [Al,P], [Ga,P], [Fe,P], [Zn,V]) SilleniteCrystals, Zh. Neorg. Khim., 2005, vol. 50, no. 3, pp. 461–466.

7. Egorysheva, A.V., Burkov, V.I., Kargin, Yu.F., andMakhov, V.N., Reflection Spectra of Bi12MxO20±δ (M =Zn, Ga, Fe, [Ga,P], [Fe,P], [Zn,V]) Sillenite Crystals, Zh.Neorg. Khim., 2005, vol. 50, no. 7, pp. 1132–1137.

8. Kargin, Yu.F., Burkov, V I., Mar’in, A.A., and Egory-sheva, A.V., Kristally Bi12MxO20±δ so strukturoi sillen-ita. Sintez, stroenie, svoistva (Bi12MxO20±δ Sillenites:Synthesis, Structure, and Properties), Moscow: Azbuka,2005.

9. Egorysheva, A.V., Burkov, V.I., Kargin, Yu.F., and Sko-rikov, V.M., Stoichiometric Dependence of Optical andPhotoconductive Properties of Bi12TiO20 Single Crys-tals, Proc. SPIE–Int. Soc. Opt. Eng., 2001, vol. 4358,pp. 97–101.

10. Radaev, S.F. and Simonov, V.I., Sillenite Structure andAtomic Mechanisms of Isomorphic Substitutions in Sil-lenites, Kristallografiya, 1992, vol. 37, pp. 914–941.

11. Skorikov, V.M., Rza-Zade, P.F., Kargin, Yu.F., andDzhalaladdinov, F.F., Phase Equilibria between 24Bi2O3 ·Ga2O3 and Bi12AO20, Zh. Neorg. Khim., 1980, vol. 25,no. 10, pp. 2871–2872.

1.8 2.22.0 2.4 2.6 2.8 3.01.60

1

2

3

4

Photon energy, eV

∆α, cm–1

0.15 wt % Zn

0.035

0.014

0.009

Bi12TiO20

Bi38ZnO58

Fig. 7. Photoinduced absorption spectra of Çíé⟨Zn⟩ andBZnO crystals.

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EGORYSHEVA

12. Skorikov, V.M., Rza-Zade, P.F., Kargin, Yu.F., andDzhalaladdinov, F.F., Phase Equilibria in the Ga2O3-Bi2O3–AO2 (A = Si, Ge) Systems, Zh. Neorg. Khim.,1981, vol. 26, no. 4, pp. 1070–1074.

13. Skorikov, V.M., Rza-Zade, P.F., Kargin, Yu.F., andDzhalaladdinov, F.F., Phase Equilibria in ZnO–Bi2O3–AO2 Systems, Zh. Neorg. Khim., 1981, vol. 26, no. 7,pp. 1904–1908.

14. Skorikov, V.M., Kargin, Yu.F., and Nelyapina, N.I.,Phase Equilibria in the Ternary System of Bismuth, Tin,and Silicon Oxides, Zh. Neorg. Khim., 1988, vol. 33,no. 5, pp. 1354–1355.

15. Volkov, V.V., Kargin, Yu.F., Nelyapina, N.I., and Sko-rikov, V.M., Phase Equilibria between Bi12TiO20 andBi38ZnO60, Zh. Neorg. Khim., 1989, vol. 34, pp. 3131–3134.

16. Volkov, V.V., Endrzheevskaya, V.Yu., Kargin, Yu.F., andSkorikov, V.M., Solubility of Bi12SiO20 and Bi25FeO40 inBismuth Titanate, Zh. Neorg. Khim., 1989, vol. 34,no. 12, pp. 3128–3130.

17. Riehemann, S., Richermann, F., Volkov, V.V., et al., Opti-cal and Photorefractive Characterization of BTO Crys-tals Doped with Cd, Ca, Ga and V, J. Nonlin. Opt. Phys.Mater., 1997, vol. 6, no. 2, pp. 235–249.

18. Kargin, Yu.F., Egorysheva, A.V., and Volkov, V.V.,Growth and Characterization of Doped Bi12TiO20 SingleCrystals, J. Cryst. Growth, 2005, vol. 275, pp. 779–784.

19. Burkov, V.I., Egorysheva, A.V., and Kargin, Yu.F., Opti-cal and Chiroptical Properties of Sillenite-StructureCrystals, Kristallografiya, 2001, vol. 46, no. 2, pp. 356–380.

20. Coya, C., Zaldo, C., Volkov, V.V., et al., Gallium-Induced Inhibition of the Photo-Refractive Properties ofSillenite Crystals, J. Opt. Soc. Am. B, 1996, vol. 13, pp.908–915.

21. Dudkina, T.D., Egorysheva, A.V., and Volkov, V.V., Nb-Doped Bi12TiO20: Crystal Growth and Characterization,Proc. SPIE–Int. Soc. Opt. Eng., 1999, vol. 3734,pp. 408–414.

22. Egorysheva, A.V., Volkov, V.V., and Skorikov, V.M., TheEffect of Pentavalent Dopants on the Optical and Photo-conductive Properties of Bi12TiO20, Neorg. Mater., 1995,vol. 31, no. 3, pp. 377–383 [Inorg. Mater. (Engl. Transl.),vol. 31, no. 3, pp. 348–353].

23. Skorikov, V.M., Zakharov, I.S., Volkov, V.V., and Spirin,E.A., Transmission and Absorption Spectra ofBi12GeO20, Bi12SiO20, and Bi12TiO20 Single Crystals,Neorg. Mater., 2002, vol. 38, no. 2, pp. 226–232 [Inorg.Mater. (Engl. Transl.), vol. 38, no. 2, pp. 172–178].

24. Marquet, H., Tapiero, M., Merle, J.C., et al., Determina-tion of the Factors Controlling the Optical BackgroundAbsorption in Nominally Undoped and Doped Silleni-tes, Opt. Mater., 1998, vol. 11, pp. 53–65.

25. Grabmaier, B.C. and Oberschmid, R., Properties of Pureand Doped Bi12GeO20 and Bi12SiO20 Crystals, Phys.Status Solidi A, 1986, vol. 96, no. 1, pp. 199–210.

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Atomic Structure of Doped Sillenites