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The layered oxide BiGeSbO6 has been synthesized for the first time via solid state reaction. Rietveld refinement showed that BiGeSbO6 belongs to PbSb2O6 structural type (P-31m space group) with unit cell parameters a = 5.1314(9), c = 4.9974(1) A, α = 90°, γ = 120°, Z = 1. Using the EXAFS and XANES methods, it was shown that a structural feature of this compound is a strong disordering of the Bi–O octahedron caused by the stereochemical activity of the 6s2 lone pair of Bi3+ electrons. A model according to which one oxygen vertex in the oxygen coordinated Bi3+ octahedron is absent, and the lone pair of electrons builds this polyhedron to the octahedron, was proposed. A particularity of the structure of BiGeSbO6 is that the existing oxygen vacancy does not have a specific position. It was established that BiGeSbO6 does not exhibit any polymorphic transition up until its incongruent melting point Тm = 985°С. Isobaric heat capacity was measured and thermodynamic functions (entropy, enthalpy increment and Gibbs free energy) were calculated for BiGeSbO6 in the 10–1200 K temperature range. We found that the band gap of BiGeSbO6 is due to the allowed direct interband transition with the energy of 3.16 eV. It was shown that BiGeSbO6 has a wide transmission region in the visible and near-IR ranges. BiGeSbO6 is a self-activated phosphor, but its luminescence spectrum is atypical for Bi3+-containing oxides. The luminescent properties of Bi1-xEuxGeSbO6 solid solutions (x = 0.3, 0.5) were also studied.

The existence of a wide range of pyrochlore-like (PL) Ln2FeTaO7 compounds (Ln = Pr–Yb, including Y) with a hexagonal structure (space group R$$\overline{3}$$) was shown. Two new low-temperature methods for the synthesis of Ln2FeTaO7 were developed to reduce the production temperature and time, namely, coprecipitation followed by annealing and synthesis in molten salts NaCl/KCl. The synthesis of PL phases was found to involve the formation of a nanocrystalline metastable fluorite phase, the fluorite irreversibly transforming to a PL phase as the crystallization time or temperature increases. No other transitions in Ln2FeTaO7 were recorded by DSC up to the melting point of 1320–1450°C (depending on the composition of the Ln2FeTaO7 compound). The isobaric heat capacity Cp(T) was measured and the thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) were calculated in the temperature range 5–1300 K for Y2FeTaO7 by the way of example. In the low-temperature region of 2–25 K, an abnormal behavior of Cp(T), apparently related to the Y2FeTaO7 magnetic transition, was detected.

A series of LnIII $${\text{Ga}}_{{0.{\text{5}}}}^{{{\text{III}}}}$$ $${\text{Sb}}_{{{\text{1}}.{\text{5}}}}^{{\text{V}}}$$ O6 (Ln = La–Tb) complex oxides were prepared by coprecipitation followed by annealing. The phases where Ln = Ce, Pr, Eu, and Tb were prepared for the first time. The 900°С isothermal section of the Ce2Ox–Ga2O3–Sb2Oy phase diagram was plotted. Structures were calculated by the Rietveld method to show that all of the LnGa0.5Sb1.5O6 (Ln = La–Tb) compounds have rosiaite (PbSb2O6) type structures. By the way of example, for LaGa0.5Sb1.5O6 the isobaric heat capacity was determined and thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) were calculated at temperatures in the range from 15 to 1300 K.

The detailed study of magnetic properties, including dc and ac measurements, has been carried out on the Bi-Cr-Sb-O pyrochlore. The region of pyrochlore phase existence was determined and photocatalytic properties have been also studied. We revealed that the Bi-Cr-Sb-O pyrochlore phase occurs along Bi2−xCr1+xSbO7 section within the region x = 0.1–0.4. The Cr3+ ions oxidation state in Bi-Cr-Sb-O pyrochlore was confirmed by XANES spectra. The best Rietveld refinement results for Bi1.8Cr1.2SbO7 composition were achieved for the disordered structure model (space group Fd-3m). According to the magnetic data Bi1.8Cr1.2SbO7 is a highly frustrated pyrochlore with short-range antiferromagnetic exchange interactions between the nearest Cr3+ ions. At Tg = 4 К this compound undergoes spin-glass transition.

The region of pyrochlore existence in the Bi-Mn-Sb-O system, Mn valence state and magnetic properties of pyrochlore compounds have been studied. It is shown that a pyrochlore phase exists in the vide range х = 0.2–0.8 along the Bi(2−x)Mn(1+x/2)Sb(1+x/2)O7 section. Rietveld structure refinement, XANES and EXAFS data show that Bi-Mn-Sb-O pyrochlores are structurally disordered and manganese is in Mn2+/Mn3+mixed-valence state. Mn2+ ions occupy a part of eight coordinated Bi sites while Mn3+ ions are in strongly distorted octahedra. Magnetic measurements confirm the structural data and reveal strong dependence of magnetic properties on Mn concentration. According to dc and ac susceptibility data all studied pyrochlores demonstrate a transition to the conventional spin glass state at Tf ≤ 7 K. The transition temperature decreases with the decrease of Mn3+ amount in the pyrochlore octahedral sublattice. It was also revealed that there exists a threshold Mn concentration above which we detected an antiferromagnetic transition at TN = 42 K along with an SG transition.

A technique has been developed for fluorinating the pyrochlore oxide Bi1.8Fe0.2FeSbO7, and a compound with the composition Bi1.8Fe1.2SbO7–x/2Fx has been obtained. The synthesized oxyfluoride also has the pyrochlore structure (sp. gr. Fd3m), with a lattice parameter a = 10.4443(1) A (R wp = 5.2). It has been shown that the charge balance upon fluorine substitution for oxygen is maintained not through partial reduction of Fe3+ to Fe2+ but through the incorporation of fluorine into oxygen vacancies. The magnetic behavior of the fluorinated pyrochlore phase is determined by the persisting frustration of the octahedral sublattice, which is responsible for the development of a spin glass state below T f = 12 K. The fluorination-induced changes in the anion sublattice led to an increase in the antiferromagnetic exchange interaction between neighboring Fe3+ ions and changes in the dynamic properties of the spin glass phase.

A series of Bi1.8Fe1.2(1−x)M1.2xSbO7 (M = Ga, Al) compositions with substitution of Fe3+ for diamagnetic Ga3+ and Al3+ has been synthesized and characterized by XRD, Mossbauer spectroscopy and magnetic measurements. X-ray diffraction analysis indicate that the Bi1.8Fe1.2(1−x)Ga1.2xSbO7 (x = 0–1) at the whole x range and Bi1.8Fe0.84Al0.36SbO7 (x = 0.3) are solid solutions of pyrochlore structure. With an increase of Ga3+ content the substitution of Fe3+ for Ga3+ occurs disproportionally over A- and B-sites while Al3+ ion incorporates entirely on B-site. Magnetic measurements showed that all studied compositions fall into the category of disordered, magnetically frustrated materials and at T ≤ 13 K undergo spin-glass transition. The spin-glass phases have a striking sensitivity to substitution of Fe3+ for Ga3+ or Al3+. The comparison of the magnetic behavior of Ga- and Al-containing pyrochlores revealed that, most likely, freezing temperature is governed by Fe3+ ions residing especially on octahedral B-sites.

New complex Cr-containing pyrochlores Ln2CrTaO7, Ln = Y, Sm, Gd, were synthesized. The Rietveld refinement of the Ln2CrTaO7 composition confirmed the pyrochlore type of the structure. The Cr3+ oxidation state is corroborated by XANES spectra. The detailed study of DC and AC magnetization of Ln2CrTaO7, Ln = Y, Sm, Gd, was carried out in the temperature range 2–300 K and magnetic field up to 5 T. FM phase transitions at 143, 192, and 163 K were observed in pyrochlores with Y, Sm, and Gd, respectively. It is shown that these transitions are due to canted antiferromagnetism or weak-ferromagnetism in the Ln2CrTaO7 pyrochlores. AFM interactions between Ln- and Cr-sublattices in Sm2CrTaO7 and Gd2CrTaO7 causes AFM transition at T = 42 K, after which magnetization continues increasing. Unusual magnetic properties were observed for Gd2CrTaO7 at low temperatures. Competitive FM and AFM interactions in Cr-and Gd-sublattice give rise to the occurrence of reentrant spin glass at T

The Bi–La–Fe–Sb–O, Pr–Fe–Sb–O, and Bi–Pr–Fe–Sb–O systems have been shown to contain pyrochlore solid solutions. We have determined the limits of the Pr2–xFe1 + xSbO7 and Bi2–xLnxFeSbO7 (Ln = La, Pr) solid solutions and shown that there is no compound corresponding to the “ideal” composition Pr2FeSbO7. Analysis of the magnetic properties of the synthesized compounds indicates that rare earth substitution for bismuth ions has no effect on the spin glass transition and that the spin glass state is only determined by the interaction between the Fe3+ spins on the octahedral site. The Pr3+ paramagnetic ion in Pr2–xFe1 + xSbO7 does not participate in the formation of the spin glass state but makes a paramagnetic contribution to the total magnetic susceptibility, which is especially significant at low temperatures.

Phase equilibria in the La 2 O 3 -Fe 2 O 3 –Sb 2 O 5 system have been studied. The isothermal section was constructed at T=900 °С. The existence of the ternary oxide LaFe 0.5 Sb 1.5 O 6 was confirmed. The structure of this compound was solved using Rietveld refinement of synchrotron radiation-based powder XRD data. LaFe 0.5 Sb 1.5 O 6 crystallizes in a trigonal layered structure relating to PbSb 2 O 6 type (space group P-31m, a=b=5.2446(3) A, c=5.1930(3) A, Z=1). The fine powder of LaFe 0.5 Sb 1.5 O 6 was prepared by molten salt synthesis. The compound was characterized by diffuse reflection and Mossbauer spectroscopy, magnetic measurements, scanning electron microscopy and photocatalytic tests. The magnetic behavior of LaFe 0.5 Sb 1.5 O 6 in the applied magnetic field H=5000 Oe is entirely paramagnetic. By contrast, in the small magnetic field H=100 Oe the magnetic data of LaFe 0.5 Sb 1.5 O 6 indicates an unusual critical behavior near phase transition at T

Phase equilibria in the Bi2O3–NiO–Sb2O5 system have been investigated. The isothermal section of the system (650°) has been constructed. For the first time the existence of the pyrochlore structure solid solution, (Bi2−xNix)Ni2/3−ySb4/3+yO7±δ, x=0.1−0.35, y=0−0.1, and a new compound Bi3Ni2/3Sb7/3O11 of KSbO3 structure type have been discovered. The structures and properties of these compounds were studied by XRD, XAFS, diffuse reflection spectroscopy and magnetic methods. Rietveld refinement of synchrotron radiation-based powder XRD data for pyrochlore sample of 38.43Bi2O3–33.0NiO–28.57Sb2O5 composition and Bi3Ni2/3Sb7/3O11 compound was performed. The best possible refinement of the positional parameters for both pyrochlore and Bi3Ni2/3Sb7/3O11 structures was achieved for disordered models. Magnetic, diffuse reflection and an X-ray absorption spectroscopy study confirmed that in both compounds nickel ions are mainly in a 2+ oxidation state. According to magnetic data, Bi1.84Ni0.16(Ni0.63Sb1.37)O7 pyrochlore and Bi3Ni2/3Sb7/3O11 are overall paramagnetic in nature.

The refinement of the Bi2O3–Fe2O3–Sb2O5 system phase diagram has been performed and the existence of the two ternary compounds has been confirmed. The first one with a pyrochlore-type structure (sp. gr. Fd 3m) exists in the wide solid solution region, (Bi2−xFex)Fe1+ySb1−yO7±δ, where x=0.1–0.4 and y=−0.13–0.11. The second one, Bi3FeSb2O11, corresponds to the cubic KSbO3-type structure (sp. gr. Pn 3) with unit cell parameter a=9.51521(2) A. The Rietveld structure refinement showed that this compound is characterized by disordered structure. The Bi3FeSb2O11 factor group analysis has been carried out and a Raman spectrum has been investigated. According to magnetization measurements performed at the temperature range 2–300 K it may be concluded that the Bi3FeSb2O11 magnetic properties can be substantially described as a superposition of strong short-range antiferromagnetic exchange interactions realizing inside the [(FeSb2)O9] 3D-framework via different pathways.

The subsolidus phase equilibria of the Bi 2 O 3 –Fe 2 O 3 –Sb 2 O x system have been investigated by X-ray powder diffraction. The isothermal section of the system at 775–800 °C has been constructed. It can be represented by 9 phase assemblage triangles. Ternary compound formation is limited to pyrochlore which forms a substantial solid solution region of Bi-deficient compositions relative to Bi 2 FeSbO 7 . As follows from the phase diagram, approximately 7–25% of Bi positions in pyrochlore may be occupied with Fe 3+ ions that was confirmed by Mossbauer data. Magnetic measurements were carried out on the pyrochlore specimen of quasi-chemical formula (Bi 1.8 Fe 0.2 )(FeSb)O 7 . The existence of the strong short-range antiferromagnetic superexchange interactions and spin-glass magnetic transition at around 9 K were revealed.

The kinetics of BiFeO3 formation from xerogel prepared by reverse coprecipitation were studied at different synthesis temperatures. We examined the influence of annealing temperature on the morphology of the reaction products and optimized synthesis conditions, which allowed us to obtain 99.7 wt % pure BiFeO3 consisting of crystalline grains less than 50 nm in size. Its magnetic properties were investigated.

Crystals of BSO doped with Se in two different concentrations (BSO:Se(I) and BSO:Se(II)) were grown by the Czochralski method. It is established that doping with Se is accompanied with preferential absorption of Fe from the melt. According to the in-depth chemical analysis, it is assumed that the doping Se ions enter the tetrahedral positions by means of the substitution of 3Si4+ by (Se6+ + 2Fe3+) ions. The measured absorption spectrum of the low-concentration Se-doped crystal (BSO:Se(I)) confirms such a conclusion as the absorption coefficient is increased in a broad spectral interval (1.4–3.1 eV) – an effect typical for all Fe-doped Bi12SiO20 crystals. The polarized Raman spectra of BSO:SeI show that the doping-induced lattice distortions are small. The IR spectrum of the BSO:SeI crystal yields indications for local lowering of the symmetry of the Fe-occupied tetrahedral positions. Doping with Se at high concentration (BSO:Se(II)) leads to occasional second phase inclusions and to a downshift of practically all modes in the Raman spectrum by 2–5.5 cm−1. It is concluded that the doping with Se at high concentrations follows the same mechanism as that at low concentrations but the introduced lattice distortions are more significant, and lead to an enlargement of the unit cell while preserving the overall cubic symmetry.

Single crystals of α- and β-polymorphs of Bi2B8O15 were grown by Czochralski method from a charge of the stoichiometric composition. The crystal structure of β-Bi2B8O15 was solved by direct methods from a twinned crystal and refined to R1=0.081 (wR=0.198) on the basis of 1584 unique observed reflections (I>2σ(I)). The compound is triclinic, space group P 1 ¯ , a=4.3159(8), b=6. 4604(12), c=22.485(4) A, α=87.094(15)°, β=86.538(15)°, γ=74.420(14)°, V=602.40(19) A3, Z=2. The B–O layered anion of β-Bi2B8O15 is topologically identical to the anion of α-Bi2B8O15 but the orientation of neighboring layers is different. Thermal expansion of α-Bi2B8O15 has been investigated by X-ray powder diffraction in air in temperature range from 20 to 700 °C. It is strongly anisotropic, which can be explained by the hinge mechanism applied to chains of Bi–O polyhedra. While the anisotropy of thermal expansion is rather high, the volume thermal expansion coefficient αV=40×106 °C−1 for α-Bi2B8O15 is close to those of other bismuth borates.

It is shown using Bi12TiO20 as an example that doping of a sillenite phase leads to the formation of a solid solution between it and the Bi12M x O20 ± δ phase corresponding to the dopant. This gives rise to structural features characteristic of the second phase and, accordingly, results in the formation of the corresponding optical centers. Dopant atoms occupy tetrahedral (M) sites. In addition, in the case of heterovalent substitutions the M site is partially occupied by the Bi atoms of [BiO3] groups. The structural features of sillenites are governed by the oxidation state of the M cation. Accordingly, doping with ions in the same oxidation state leads to the formation of similar optical centers and, as a consequence, produces similar changes in the properties of the material. The results obtained are supported by experimental studies of the absorption, circular dichroism, photoinduced absorption, and photoconductivity spectra of doped sillenite crystals in a wide spectral range.

A new compound, SrBi 2 B 4 O 10 , has been grown by cooling a melt with the stoichiometric composition. It is triclinic, P −1, a =6.819(1), b =6.856(1), c =9.812(2) A, α= 96.09(1), β =109.11(1), γ =101.94(1)°, V =416.5(1) A 3 , Z =2. The crystal structure of the compound has been solved by direct methods and refined to R 1 =0.050 (w R 2 =0.128). The structure contains Bi–O pseudolayers build up from Bi–O chains involving oxocentred OBi 3 triangles. Sr atoms and [B 4 O 9 ] 6− isolated anions (4B:3 Δ □: Δ □> Δ ) are located between the Bi–O packages. The thermal treatment as well as DSC experiment showed that the compound melts above 800 °C presumably according to the peritectic reaction: SrBi 2 B 4 O 10 ↔ SrB 2 O 4 +SrB 4 O 7 + Liquid. According to high-temperature X-ray powder diffraction study thermal expansion of SrBi 2 B 4 O 10 structure is anisotropic ( α 11 =13, α 22 =9, α 33 =2, α V =24×10 −6 °C −1 ).

The crystal structure of a new bismuth aluminoborate Bi0.96Al2.37(B4O10)O is studied by single-crystal X-ray diffraction. The Bi0.96Al2.37(B4O10)O single crystals are hexagonal (space group $$P\bar 6$$ 2m). The unit cell parameters are as follows: a = b = 4.587(4) A, c = 2.253(9) A, α = β = 90°, γ = 120°, V = 168.60 A3, Z = 1.

Phase equilibria in the BaO-Bi2O3-B2O3 system have been investigated by X-ray powder diffraction analysis and DTA. Quasi-binary sections have been determined, and an isothermal section of the system in the subsolidus region has been constructed. The BaO-Bi2O3-B2O3 ternary system has been divided into 22 triangles of coexisting phases. It has been found that four bismuth barium borates exist, namely, Ba3BiB3O9, BaBi2B4O10, BaBiB11O19, and BaBiBO4. Ba3BiB3O9 undergoes a phase transition at 850°C and exists up to 885°C, where it decomposes in the solid state. BaBiB11O19 and BaBi2B4O10 melt congruently at 807 and 730°C, respectively. BaBiBO4 melts incongruently at 780°C. X-ray powder diffraction data for the low-temperature polymorph of Ba3BiB3O9 are presented.

Single crystals of Bi 2 Ga 4 O 9 are grown from a solution in a bismuth oxide melt. The structure (orthorhombic, space group Pbam , a = 7.918(2) A, b = 8.299(2) A, c = 5.894(2) A, Z = 2) is refined to R = 0.052 in the anisotropic approximation based on single-crystal X-ray diffraction data. The structure is a framework. The bismuth(III) atoms are sixfold coordinated; gallium(III) exists in both tetrahedral and octahedral coordina- tions. The thermal expansion of Bi 2 Ga 4 O 9 is studied by high-temperature X-ray powder diffraction method and is found to be sharply anisotropic. A structural interpretation of the anisotropy is proposed. Chemical distortion in the Bi 2 M 4O9 compounds with M = Fe(III), Al, or Ga is analyzed and compared with the thermal expansion of Bi2Ga4O9.

The axial circular-dichroism spectrum of a synthetic amethyst crystal has been studied for the first time. Bands both of the positive and negative signs are revealed. New information on the color centers of amethyst and their electronic structure is obtained.

The vibrational spectra of crystals of bismuth borates Bi24B2O39, Bi4B2O9, BiBO3, and Bi2B8O15 were obtained for the first time, and the spectra of Bi3B5O12 and BiB3O6 crystals measured in the range 30–1600 cm−1 at room temperature were refined. The lines observed were assigned to the corresponding vibrational transitions on the basis of the theoretical group analysis and comparison of the obtained results with the vibrational spectra of borates of different composition. The complication of the structure of bismuth borates with increasing content of B2O3 was traced by the example of vibrational spectra.

The paper presents a new data on the crystal structure, thermal expansion and IR spectra of Bi 3 B 5 O 12 . The Bi 3 B 5 O 12 single crystals were grown from the melt of the same stoichiometry by Czochralski technique. The crystal structure of Bi 3 B 5 O 12 was refined in anisotropic approximation using single-crystal X-ray diffraction data. It is orthorhombic, Pnma , a =6.530(4), b =7.726(5), c =18.578(5) A, V =937.2(5) A 3 , Z =4, R =3.45%. Bi 3+ atoms have irregular coordination polyhedra, Bi(1)O 6 ( d (B–O)=2.09–2.75 A) and Bi(2)O 7 ( d (B–O)=2.108–2.804 A). Taking into account the shortest bonds only, these polyhedra are considered here as trigonal Bi(1)O 3 (2.09–2.20 A) and tetragonal Bi(2)O 4 (2.108–2.331 A) irregular pyramids with Bi atoms in the tops of both pyramids. The BiO 4 polyhedra form zigzag chains along b -axis. These chains alternate with isolated anions [B 2 IV B 3 III O 11 ] 7− through the common oxygen atoms to form thick layers extended in ab plane. A perfect cleavage of the compound corresponds to these layers and an imperfect one is parallel to the Bi–O chains. The Bi 3 B 5 O 12 thermal expansion is sharply anisotropic ( α 11 ≈ α 22 =12, α 33 =3×10 −6 °C −1 ) likely due to a straightening of the flexible zigzag chains along b -axis and decreasing of their zigzag along c -axis. Thus the properties like cleavage and thermal expansion correlate to these chains.

The experimental data on the optical and chiro-optical properties of crystals with the sillenite structure Bi12MxO20 ± δ (M= elements of II–VIII groups) have been reviewed. The relations between the changes in the chemical composition, the crystal structure of Bi12MxO20 ± δ compounds, absorption, circular dichroism, and optical rotation spectra of sillenites have been established. A model taking into account the contribution of the electronic transitions of optically active tetrahedral [MO4]n− complexes to the total optical rotation in sillenites is suggested. The data on the state (oxidation degree) and the recharging processes of 3d-elements in sillenites are systematized. The models of optical centers responsible for the photochromic effect in sillenites are critically analyzed.

The absorption and circular dichroism spectra of Bi12SiO20and Bi12TiO20crystals codoped with Mn or Cr and a penta- (P5+), tri- (Ga3+), or divalent (Cu2+) element were studied. The results demonstrate that the sillenite crystals contain Mn and Cr in two oxidation states, 5+ and 4+. The introduction of P5+increases the fractions of Mn4+and Cr4+, while Ga3+and Cu2+have the opposite effect. The bands observed in the spectra are assigned to electronic transitions of [MnO4]4–and [MnO4]3–.

Bi12SiO20 crystals doped with Cu were grown by the Czochralski method along 〈 001 〉 from a melt. The concentration of the impurities was determined by electro–thermal and flame atomic absorption spectroscopy. The lattice parameter a o = 10.165 A was determined by single-crystal diffractometry. Results from Raman measurements in different scattering configurations lead to the conclusion that distinguishable structural distortions of the bismuth–oxygen framework of the sillenite lattice take place due to Cu doping.

Single crystals of α- and β-polymorphs of Bi2B8O15 were grown by Czochralski method from a charge of the stoichiometric composition. The crystal structure of β-Bi2B8O15 was solved by direct methods from a twinned crystal and refined to R1=0.081 (wR=0.198) on the basis of 1584 unique observed reflections (I>2σ(I)). The compound is triclinic, space group P 1 ¯ , a=4.3159(8), b=6. 4604(12), c=22.485(4) A, α=87.094(15)°, β=86.538(15)°, γ=74.420(14)°, V=602.40(19) A3, Z=2. The B–O layered anion of β-Bi2B8O15 is topologically identical to the anion of α-Bi2B8O15 but the orientation of neighboring layers is different. Thermal expansion of α-Bi2B8O15 has been investigated by X-ray powder diffraction in air in temperature range from 20 to 700 °C. It is strongly anisotropic, which can be explained by the hinge mechanism applied to chains of Bi–O polyhedra. While the anisotropy of thermal expansion is rather high, the volume thermal expansion coefficient αV=40×106 °C−1 for α-Bi2B8O15 is close to those of other bismuth borates.

A new compound, SrBi 2 B 4 O 10 , has been grown by cooling a melt with the stoichiometric composition. It is triclinic, P −1, a =6.819(1), b =6.856(1), c =9.812(2) A, α= 96.09(1), β =109.11(1), γ =101.94(1)°, V =416.5(1) A 3 , Z =2. The crystal structure of the compound has been solved by direct methods and refined to R 1 =0.050 (w R 2 =0.128). The structure contains Bi–O pseudolayers build up from Bi–O chains involving oxocentred OBi 3 triangles. Sr atoms and [B 4 O 9 ] 6− isolated anions (4B:3 Δ □: Δ □> Δ ) are located between the Bi–O packages. The thermal treatment as well as DSC experiment showed that the compound melts above 800 °C presumably according to the peritectic reaction: SrBi 2 B 4 O 10 ↔ SrB 2 O 4 +SrB 4 O 7 + Liquid. According to high-temperature X-ray powder diffraction study thermal expansion of SrBi 2 B 4 O 10 structure is anisotropic ( α 11 =13, α 22 =9, α 33 =2, α V =24×10 −6 °C −1 ).

The results of experimental investigation of light induced absorption in Bi 12 TiO 20 single crystals with different stoichiometric composition are presented. We study the dynamic of the changes in absorption coefficient of Bi 12 TiO 20 single crystals grown from different melt composition for the incoherent light with the wavelength of λ ≈ 660 nm . It was shown that the change in light absorption reaches the values of ~ 0.4 cm -1 in the crystal containing 7.5 mol.% TiO 2 in the start melt composition.

The absorption, circular dichroism and photoinduced absorption spectra of Mn, P-doped Bi12SiO20 single crystal are studied. It is shown that dominant oxidation degree of manganese in this crystal is +4. The assignment of bands observed in studied spectra to electronic and vibronic transitions is proposed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical absorption, power rotary and circular dichroism of crystals of the sillenite family Bi12MxI20(M- elements I-VIII of groups or their combination) were measured. The t1-2e charge transfer transition of the [VO4]3- tetrahedral complexes is found to make a negative contribution to the optical rotation of the vanadium sillenite. We present a model, which takes into account a negative contribution from the charge-transfer transitions of [MO4]n- complexes to the optical rotation in Bi12MxO20 crystals. It was found that M vacancies and crystal field transitions of 3d- elements contribute significantly to the circular dichroism of sillenite crystals.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Bi12TiO20 single crystals were grown from the various melt compositions (5.0-10.0 mol.% TiO2). The stoichiometric dependence of optical and photoconductive properties of Bi12TiO20 crystals is studied. It was observed that the lattice constant value depends on TiO2-concentration in the melt. The melt composition dependencies of absorption coefficient and circular dichroism value have linear character in spectral range 410- 500 nm. The bismuth-richer melt-grown crystals performed the higher values of absorption coefficient and intensities of the band in the circular dichroism spectrum. It is shown that photoconductivity is more sensitive characteristic to melt composition. The crystal grown from the bismuth- poorest melt exhibits the photoconductivity which is two order of magnitude higher, than in crystal grown from the bismuth-richer melt, over studied spectral range. The crystal grown from 9 mol.% TiO2 melt composition performs the maximal values of photocurrent and photoinduced absorption. Based on these optical and electrical results, the nature of the local centers in Bi12TiO20 crystals is discussed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Bi12TiO20 crystals doped with Nb atoms ranging 0.05-0.2 wt.% are grown from non-stoichiometric melts bymodified Czochralski pulling technique. Doping studies showed that Nb5-ions occupy tetrahedral sites in sillenite lattice.Experimental results on spectral dependent characteristics, such as, absorption, photochromic effect, photoconductivity andelectrooptic efficiency are presented. Comparison with similar properties of Bi12TiO20 crystals doped with anotherpentavalent atoms (V, P) is discussed. A relationship ofthese characteristics with local structural defects in doped Bi12TiO20lattice is proposed.Keywords: Sillenite, Bi12TiO20, Crystal Growth, Nb-Doping, Optical Absorption, Photochromism, Photoconductivity,electrooptical efficiency, structural defects. 1. INTRODUCTION Photorefractive crystals of sillenite-type structure, such as Bi12GeO20, Bi12SiO20 and Bi12TiO20, are effectivephotoconductors in the visible. Taking into account the electrooptic properties they might be used as active elements for