In the Bi2O3-SiO2-VO25+O5 system, single crystal solid solutions of the sillenite family of the general composition Bi-24(Bi,Si,V)(2)O-40 are obtained by a hydrothermal method and for the first time characterized by neutron and X-ray diffraction analysis. The tetrahedral position is found to contain vanadium ions with different formal charges (V4+ and V5+) responsible for green and orange colors, respectively, of the samples. For the first time, for some sillenites of this system dissymmetrization of the structure (a transition from the I23 space group into P23) is revealed, which is caused by the presence of several atoms in one crystallographic position and also by crystal growth conditions.

X-ray diffraction studies of sillenite Bi24V2O40 single crystals grown by the hydrothermal method are performed for a separate crystal and powdered crystals. It is found that the composition of the two specimens is described by the (Bi24 − x ▭ x )[Bi y 3+ V 1−y 5+ ]2 O40 general formula with completely populated oxygen sites but differs in the content of vacancies at the bismuth site (this was established for the first time) and the Bi: V ratio at the tetrahedral site. The structural models of all the vanadium-containing sillenites reported in the literature are considered, and the possibility that Bi atoms are located at the centers of BiO4 tetrahedra is established.

In the Bi 2 O 3 -SiO 2 -V 2 5+ +O 5 system, single crystal solid solutions of the sillenite family of the general composition Bi 24 (Bi,Si,V) 2 O 40 are obtained by a hydrothermal method and for the first time characterized by neutron and X-ray diffraction analysis. The tetrahedral position is found to contain vanadium ions with different formal charges (V 4+ and V 5+ ) responsible for green and orange colors, respectively, of the samples. For the first time, for some sillenites of this system dissymmetrization of the structure (a transition from the I 23 space group into P 23) is revealed, which is caused by the presence of several atoms in one crystallographic position and also by crystal growth conditions.

Выполнено рентгеноструктурное исследование монокристаллов силленита Bi24V2O40, выращенных гидротермальным методом, как в виде отдельного кристалла, так и измельченных в порошок кристаллов. Обнаружено, что их состав может быть описан общей формулой (Bi24 - x x)[Bi V ]2O40, но с разным содержанием вакансий в позиции висмута (выявлено впервые), отсутствием кислородных вакансий и разным соотношением Bi и V в тетраэдрической позиции. Рассмотрены все известные в литературе модели строения ванадийсодержащих силленитов и установлена принципиальная возможность нахождения атомов Bi в центре тетраэдра BiO4.

We report the optical and dielectric properties and microhardness of La 3 Ga 5.5 Ta 0.5 O 14 lanthanum gallium tantalate (langatate) crystals. Analysis of the optical transmission spectra of the crystals in relation to their refined compositions indicates that the bands at 34000–35000 and 27000–28000 cm −1 are due to lanthanum and oxygen vacancies, respectively, and that the band at 20000–21000 cm −1 is responsible for the yellow (orange) coloration of the crystals. Their resistivity and microhardness decrease with increasing oxygen vacancy concentration.

Langatate crystals with the nominal composition La3(Ga0.5Ta0.5)Ga5O14 grown by Czochralski pulling along 〈0001〉 have been studied by X-ray and neutron diffraction before and after postgrowth annealing in air and vacuum. Their compositions are determined, the relationship between the Ga/Ta ratio and unit-cell parameters is established, and their color (colorless, yellow, and orange crystals) is shown to depend on their oxygen content and the growth and annealing conditions. Vacuum annealing at sufficiently high temperatures leads to precipitation of a green phase, La(Ta,Ga)3+O3.

Synthesis parameters of multiwall carbon nanotubes were prepared by catalytic pyrolysis of toluene and isopropyl alcohol under isochome conditions in the presence of nickel oxalate as catalyst were investigated. The synthesis products were analyzed by electron microscopy and x-ray diffraction spectroscopy

Crystals of langasite La3Ga4(GaSi)O14 grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La3Ga5.14Si0.86O14 seed in an (Ar + O2) atmosphere (the 〈 0001 〉 growth direction) can be written as(La2.85(2)□0.15)(Ga0.95(2)□0.05) Ga3(Ga1.15 Si0.85(5))(O13.72□0.28(7)) and(La2.89(1)□0.11)·(Ga0.98(1)□0.02) Ga3(Ga1.06Si0.94(4))(O13.81□0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La 2.97(4)□0.03) Ga(1)(Ga2.94(9)□0.06)(Ga0.7(1) Si1.3)(O13.9□0.1(1)), which is grown from the La3 Ga5SiO14 seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ⌚growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.

Впервые проведено нейтроноструктурное изучение кристаллов лангасита La3Ga4(GaSi)O14, выращенных методом Чохральского. Найдено, что составы верха и низа оранжевого кристалла, полученного из шихты La3Ga5.14Si0.86O14 в атмосфере (Ar + O2) (направление роста 0001 ), могут быть описаны как (La2.85(2) 0.15)(Ga0.95(2) 0.05)Ga3(Ga1.15Si0.85(5))(O13.72 0.28(7)) и (La2.89(1) 0.11) · (Ga0.98(1) 0.02)Ga3(Ga1.06Si0.94(4))(O13.81 0.19(9)), причем содержание La и Ga соответственно меньше и больше в верхней части кристалла, чем в нижней, и Ga > Si в позиции (GaSi). Напротив, в бесцветном кристалле состава (La2.97(4) 0.03)Ga(1)(Ga2.94(9) 0.06)(Ga0.7(1)Si1.3)(O13.9 0.1(1)), выращенном из шихты La3Ga5SiO14 в Ar (направление роста 01 1 ), в позиции (GaSi) содержание Ga меньше, чем Si. Найдена связь между соотношением Ga : Si и межатомными расстояниями (GaSi)–3O.

The frequency coefficient, an important performance parameter of piezoelectric materials, which is used in determining vibrational frequencies of crystalline piezoelectric elements, is shown to vary along the length of a crystal. The dependences of the frequency coefficient and elastic stiffness coefficient on unit-cell parameters have the form of a parabolic function with a maximum, while the dependence of the frequency coefficient on \({1 \mathord{\left/ {\vphantom {1 {(\sqrt \rho )}}} \right. \kern-\nulldelimiterspace} {(\sqrt \rho )}}\) (ρ is the density of the crystal) is linear. Polarity changes within a sample are revealed, which may be due to the stress arising from variations in melt composition during crystal growth.

The problems of Langasite single crystals growth by Czochralski and Bridgman methods were analyzed. It was found that the deferent kinds of defects in langasite crystals influence on physical properties and thus on langasite crystals application. Vernel method was first proposed for obtaining of qualitative single crystal charge from initial components. A monophase single crystal charge obtaining was confirmed by X-ray diffraction method. A single crystal with uniform composition was obtained by Czochralski method from this charge

The temperature-dependent electrical conductivity and dielectric permittivity and room-temperature optical absorption spectra of La3Ga5SiO14 (langasite) crystals grown under different conditions are measured. The resistivity and peak-loss temperature t tanδ of the crystals are shown to be determined by the concentration of oxygen vacancies, which originate from changes in melt composition during crystal growth. The t tanδ of langasite is shown for the first time to be anisotropic (measurements on Z- and X-cuts). The properties of the crystals are suitable for the fabrication of stable piezoelectric elements capable of operating above 600°C.

The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czochralski technique from charges of nominal composition La3Ga5.5M0.5O14 = La3(Ga0.5M0.5)(1)Ga3(2)Ga2(3)O14 (M=Ta5+, Nb5+) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La3(Ga0.52)Ta0.48(2)5+Ga3(Ga0.94Ta0.06(1)3+)2O14 in LGT-I (Ta5+ in the octahedral site Ga(1) and Ta3+ in the trigonal-pyramidal site Ga(3)) and La3(Ga0.55Ta0.45(2)5+)Ga3Ga2O13.93(2)□0.07 in LGT-II (Ta5+ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La3(Ga0.47Nb0.53(1)5+)Ga5O14 and La3(Ga0.48Nb0.52(1)5+)Ga5O14, respectively, but differ in polarity.

A Czochralski-grown langasite, La3Ga5SiO14 (La3Ga4(GaSi)O14), crystal was studied by x-ray diffraction. The crystal was found to consist of two isostructural solid solutions, except in the center of the tail end. The solid solutions differ in the Ga and Si occupancies on the (Ga,Si) site: Ga > Si in solid solution I, and Si > Ga in solid solution II. The composition of I (center of the tail end) is La3Ga4(Ga1.14Si0.86(4))O13.84□0.16(11). The composition of II, which coexists with I in the peripheral part of the tail end, is (La2.95□0.05)Ga4(Ga0.84Si1.16(6))O14.

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