R. S. Bubnova

New layered polyanion in α-CsB5O8 high-temperature modification

Crystal structure of high-temperature polymorphic modification, α-CsB5O8, was determined from single-crystal X-ray diffraction data. The phase crystallizes in space group P21/c with a = 7.122(2), b = 9.640(3), c = 11.411(3) A, β = 116.64(2) ◦ and V = 700.3(3) A 3 , Z = 4. It contains new zigzag layer polyanions built up from rigid [B5O8] − pentaborate groups that consist of four triangles and a tetrahedron condensed to a double ring via common tetrahedron. The cesium cations are located in large cavities of the layers and they are coordinated by nine oxygen atoms with distances from 3.037 to 3.429 A. Five of the oxygen atoms are placed in the same layer as cesium atom; other four ones are distributed between two adjacent layers. Under heating this structure demonstrates a highly anisotropic thermal expansion (α11 = 27, α22 = 61, α33 =− 8 × 10 −6 K −1 ); the thermal expansion in the direction perpendicular to the layer is the intermediate one.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Thermal expansion and phase transitions in K 1 − x Rb x BSi 2 O 6 leucite borosilicate solid solutions

Continuous solid solutions and the reversible phase transition from the I-43d cubic phase to the Ia-3d cubic phase are revealed in the borosilicate series K1 − xRbxBSi2O6. Samples in the KBSi2O6-RbBSi2O6 system are prepared by solid-phase synthesis and crystallization of glasses and investigated using the annealing and quenching technique, high-temperature X-ray diffraction, and dilatometry. The above polymorphic phase transition is observed in all solid solutions at temperatures in the range from 330 to 430°C depending on the composition: an increase in the rubidium content in the solid solution leads to a gradual decrease in the phase transition temperature. The linear thermal expansion coefficients α are determined for solid solutions of different crystalline modifications and glasses. The linear thermal expansion coefficients α for the I-43d low-temperature phase are equal to (20–23) × 10−6 K−1 according to the X-ray diffraction data and (21–24) × 10−6 K−1 according to the dilatometric data. The values of α for the Ia-3d high-temperature phase lie in the range (4–9) × 10−6 K−1 according to the X-ray diffraction data and in the range (6–9) × 10−6 K−1 according to the dilatometric data. The linear thermal expansion coefficients for both modifications decrease with an increase in the rubidium content in the solid solutions. The linear thermal expansion coefficients for glasses α = (10–11) × 10−6 K−1 are close to those for the high-temperature modification and virtually independent of the sample composition. The I-43d (cubic) ai I41/a (tetragonal) o Ia-3d (cubic) polymorphic phase transitions in the KBSi2O6 compound are revealed by differential scanning calorimetry (DSC) and dilatometry. Their reversibility is confirmed by the DSC data.

Structure of bismuth oxoborate Bi4B2O9 at 20, 200, and 450°C

Single crystals of bismuth oxoborate Bi4B2O9 have been grown by slowly cooling the melt of a stoichiometric Bi2O3 + H3BO3 mixture. The structure of the borate (monoclinic space group P21/c, a = 11.107 Å, b = 6.629 Å, c = 11.044 Å, β = 91.04°, Z = 4) has been studied at 20, 200, and 450°C. The structure is described not only in terms of full BiO6− and BiO7 polyhedra but also in terms of truncated BiO3− and BiO4− polyhedra and BO3 triangles, as well as oxo-centered OBi3 triangles and OBi4 tetrahedra. It is shown that both the B-O and Bi-O bond lengths are practically unaffected by temperature. Only the angles between polyhedra change with temperature, being responsible for the strong anisotropy of Bi4B2O6 thermal expansion, which was measured by high-temperature powder X-ray diffraction: α11 = 20, α22 = 15, α33 = 6 × 10−6 °C−1, and μ = (c, α33) = −19°.

Crystal-structure refinement, thermal expansion, and chemical distortion of Bi2Ga4O9

Single crystals of Bi2Ga4O9 are grown from a solution in a bismuth oxide melt. The structure (orthorhombic, space group Pbam, a = 7.918(2) Å, b = 8.299(2) Å, c = 5.894(2) Å, 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 coordinations. The thermal expansion of Bi2Ga4O9 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 Bi2M4O9 compounds with M = Fe(III), Al, or Ga is analyzed and compared with the thermal expansion of Bi2Ga4O9.

Crystal structure of the low-temperature modification of α-RbB3O5

The crystal structure of α-RbB3O5 was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms to Rp = 2.93, Rwp = 3.80, RB = 2.53, RF = 2.84, and s = 1.54. The compound is isostructural to CsB3O5: it is orthorhombic, sp. gr. P212121, a = 8.209(1), b = 10.092(1), c = 5.382(1) Å, and V = 445.9 Å 3. The framework structure is formed by the boron-oxygen [B2IIIBIVO5] − rings consisting of two [BO3]-triangles and a [BO4]-tetrahedron. The rings are linked to form systems of helical chains running along the twofold screw axes parallel 21 to the a-and b-axes and infinite channels parallel to the a-and c-axes, which accommodate Rb atoms. The data were collected on an ADP-2 diffractometer [CuKα radiation, Ni-filter, 12.00° < 2θ < 110.00°, a step in 2θ equal to 0.02°, count time 8 s per step, and 711 reflections α1 + α2)]. All the calculations were performed using version 3.3 of the WYRIET program. The comparison of the structures of α-and β-RbB3O5 and CsB3O5 revealed that the type of deformations in the framework structures of alkali-metal borates due to the changes of the temperature or the substitution of cations is determined by the role played by metal atoms, and especially, by large and heavy ions.

Thermal expansion and polymorphism in a series of rubidium cesium boroleucites

The crystal structure of the boroleucite solid solution Rb0.40Cs0.54B0.94Si2.06O6 is refined in space group I-43d by the Rietveld method with the use of the X-ray powder diffraction data. The refinement data complement the available crystal chemical characteristics of Rb1−xCsxBSi2O6 solid solutions. The thermal expansion and phase transformations of Rb1−xCsxBSi2O6 borosilicates are investigated in parallel by high-temperature X-ray diffraction with conventional powdered samples and by the dilatometric method with samples in the form of pressed pellets. It is demonstrated that the thermal expansion coefficients, as well as the temperatures and sequence of polymorphic transitions, which are determined from the data obtained by two methods are in close agreement. The temperature curve of the I-43d ⇄ Ia3d phase transition for the Rb1−xCsxBSi2O6 solid solution system is constructed from the data obtained by both methods. It is shown with the use of the structural data obtained by the Rietveld method that, at temperatures above 800°C, rubidium-cesium boroleucites undergo decomposition due to the release of alkali cations.

Thermal expansion of β-BaB2O4 and BaB4O7 borates

The thermal behavior of β-BaB2O4 and BaB4O7 borates is investigated using high-temperature X-ray powder diffraction analysis. The components of the thermal expansion tensor and the tensor orientation with respect to the crystallographic axes are calculated. Thermal deformations are analyzed in relation to the crystal structure. The thermal expansion of the β-BaB2O4 borate is maximum along the c axis and close to zero in the ab plane. The thermal expansion anisotropy correlates with the layered structure, the orientation of the optical indicatrix, and the thermal ellipsoids of atoms. The thermal expansion of the BaB4O7 borate is studied in three high-temperature X-ray diffraction experiments (two experiments with heating and one experiment with cooling). The thermal expansion of this compound is strongly anisotropic: the expansion is maximum in the ac monoclinic plane, whereas the contraction is observed along the b axis.

Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB4O7

γ-NiB4O7 was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6522 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm3, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB4O7 was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB4O7.

Thermal expansion and phase transitions in K1 − xCsxBSi2O6 borosilicate solid solutions

AbstractSolid solutions K1 − xCsxBSi2O6 (x (atomic fraction) = 0.00, 0.20, 0.30, 0.35, 0.40, 0.80, 0.90,1.00) have been prepared by solid-phase synthesis and crystallization of glasses in the KBSi2O6-CsBSi2O6 borosilicate series. The thermal behavior of the solid solutions has been investigated using the annealing and quenching techniques, dilatometry, and high-temperature X-ray powder diffraction. It has been shown that solid solutions with x = 0.00–0.35 and 0.40–1.00 correspond to space groups I