B. A. Maksimov

Zn Atoms in Lithium Niobate and Mechanism of Their Insertion into Crystals

Precision X-ray structural studies were carried out for LiNbO3:Znx single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic NbLi defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO3. A change in the intrinsic defect structure of the LiNbO3 crystals with different Zn concentrations is discussed.

Crystal structure and optical activity of La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals of the langasite family

Precision X-ray structural studies of the La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals were carried out. The space group P321 was confirmed. The anomalous X-ray scattering was taken into account to establish the absolute structures (chirality) of the crystals; they proved to be of different sign. The structural features responsible for the optical activity of crystals were revealed, and the dependence of the magnitude and sign of specific rotation on the structural parameters of these compounds were determined.

Phase transitions in compounds with the Ca3Ga2Ge4O14 structure

Possible structural changes described by the group-subgroup relationships in the Ca3Ga2Ge4O14-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La3SbZn3Ge2O14, whose crystal structure is refined as a derivative structure of the Ca3Ga2Ge4O14 structure type within the sp. gr. A2 (C2). At ∼250°C, La3SbZn3Ge2O14 undergoes a reversible phase transition accompanied by symmetry rise, A2 ⇄ P321. Similar phase transitions, P321 ⇄ A2, are also observed in La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.

Crystal structure of the new barium borate Ba5(BO3)2(B2O5)

The crystal structure of the new barium borate Ba5(BO3)2(B2O5) is established (R = 0.0436). Single crystals were grown by spontaneous crystallization in the BaO-B2O3-Na2O system using the flux method. This compound crystallizes in the orthorhombic system, sp. gr. P212121; the unit-cell parameters are a = 9.590(2) Å, b = 16.659(3) Å, c = 22.919(6) Å, and Z = 12. The structure consists of coordination polyhedra of barium cations and the anionic groups [BO3] (planar triangles) and [B2O5] (vertex-sharing double [BO3] triangles), which form a pseudohexagonal framework. Melting of barium borate occurs by a peritectic reaction at 1170 ± 10°C.

Growth and structure of barium sodium orthoborate NaBaBO3 crystals

Crystals of NaBaBO3 were grown by spontaneous crystallization on a platinum loop from the BaO-B2O3-Na2O system using the flux technique. The crystals have a highly disordered block structure. X-ray diffraction study λMoKα, 518 independent reflections, R = 0.0272) demonstrated that the structure of these crystals is identical with that established previously by other researchers for a sample prepared by cooling a stoichiometric melt.

Crystal structure and microtwinning of monoclinic La3SbZn3Ge2O14 crystals of the langasite family

The crystal structure of monoclinic La3SbZn3Ge2O14 crystals from the langasite family is determined by X-ray diffraction analysis [a = 5.202(1) Å, b = 8.312(1) Å, c = 14.394(2) Å, β = 90.02(1)°, sp. gr. A2, Z = 2, and R/Rw = (5.2/4.6)%]. The structure is a derivative of the Ca3Ga2Ge4O14-type structure (a = 8.069 Å, c = 4.967 Å, sp. gr. P321, Z = 1). The crystal studied is a polysynthetic twin with the twin index n = 2, whose monoclinic components are related by pseudomerohedry by a threefold rotation axis of the supergroup P321.

Growth and structure of La3Zr0.5Ga5Si0.5O14 crystals

The complete X-ray structure determination of Czochralski grown La3Zr0.5Ga5Si0.5O14 single crystals with the Ca3Ga2Ge4O14 structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo Kα1 radiation, 1920 crystallographically independent reflections, R = 0.0166, Rw = 0.0192). The absolute structure is determined. It is shown that possible transition of some of La atoms (∼1.2%) from the 3e to 6g position may give rise to the formation of structural defects.

X-ray diffraction study of cerium-and thulium-doped (Sr,Ba)Nb2 O6 single crystals

The structural models of cerium-and thulium-doped (Sr0.61Ba0.39)Nb2O6 solid solutions were established and refined on the basis of single-crystal X-ray diffraction data. The dopants were located, and their concentrations in the specimens were refined. The effect of the Sr/Ba ratio and doping with rare earth elements on the structural characteristics of (SrxBa1-x)Nb2O6 crystals was analyzed.

Crystal structure of La3Nb0.5Ga5.5O14 at 20 K

The atomic structure of La{sub 3}Nb{sub 0.5}Ga{sub 5.5}O{sub 14} single crystals (space group P321, R = 2.21%, R{sub w} = 2.31%) at 20 K was determined using X-ray diffraction. A comparative analysis of the structural characteristics determined at temperatures of 293 and 20 K did not reveal a noticeable redistribution of bond valences that could be considered precursors of a possible phase transition accompanied by the reduction of the symmetry of the crystal. The assumption was made that the anomalous behavior of the permittivity e{sub 33} of the La{sub 3}Nb{sub 0.5}Ga{sub 5.5}O{sub 14} single crystals is associated with the decrease in the possible relative displacements of the Ga(1) and Nb atoms occupying the cation position with point symmetry 32 in a 1: 1 ratio.The atomic structure of La3Nb0.5Ga5.5O14 single crystals (space group P321, R = 2.21%, Rw = 2.31%) at 20 K was determined using X-ray diffraction. A comparative analysis of the structural characteristics determined at temperatures of 293 and 20 K did not reveal a noticeable redistribution of bond valences that could be considered precursors of a possible phase transition accompanied by the reduction of the symmetry of the crystal. The assumption was made that the anomalous behavior of the permittivity e33 of the La3Nb0.5Ga5.5O14 single crystals is associated with the decrease in the possible relative displacements of the Ga(1) and Nb atoms occupying the cation position with point symmetry 32 in a 1: 1 ratio.

X-ray diffraction study of Ca0.88Gd0.12F2.12 single crystals with a modified fluorite structure

The characteristic structural features of a single crystal of the anion-excessive fluorite phase Ca0.88Gd0.12F2.12 have been studied by X-ray diffraction analysis (MoKα radiation), sp. gr. Fm3m, Z = 4, a = 5.5000(4) Å. The structure was refined in the anharmonic approximation of thermal atomic vibrations to Rw = 0.0047. A total of 78 independent structure amplitudes were measured; 14 parameters were refined; sinθ/λ ≤1.13 Å−1. In the structure under study, the replacement of the divalent Ca2+ cations by the trivalent Gd3+ cations in the position 4a (0, 0, 0) resulted in the statistical distribution of the fluorine atoms over three positions of the sp. gr. Fm3m. Thus, the fluorine atoms occupy the position 8c (1/4, 1/4, 1/4), the main position in the fluorite structure type, and the position 32f (v, v, v) with v = 0.417(1) on threefold symmetry axes. In addition, some fluorine atoms are shifted from the main 8c position along the threefold symmetry axis to the position 32f (w, w, w) with w = 0.313(2).