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Continuous solid solutions and the reversible phase transition from the I -43 d cubic phase to the Ia -3 d cubic phase are revealed in the borosilicate series K 1 -x Rb x BSi 2 O 6 . Samples in the KBSi 2 O 6 -RbBSi 2 O 6 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 -43 d 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 -3 d 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 -43 d (cubic)I 4 1 / a (tetragonal) Ia -3d (cubic) polymorphic phase transitions in the KBSi 2 O 6 compound are revealed by differential scanning calorimetry (DSC) and dilatometry. Their reversibility is confirmed by the DSC data.

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The phase relations in the NaB 3 O 5 -KB 3 O 5 pseudobinary system are investigated using high-temperature X-ray powder diffraction, differential scanning calorimetry, and the annealing and quenching techniques. Solid solutions Na 1x K x B 3 O 5 (0.5 < x ≤ 1.0) based on the KB 3 O 5 phase are found in the system at temperatures below 650-670°C. The replacement of potassium atoms by sodium atoms is accompanied by the contraction of the structure along the a and c axes. The parameter b (the direction of channels in the structure) is insensitive to the substitution of sodium atoms for potassium atoms. No isomorphic miscibility on the basis of NaB 3 O 5 is revealed in the system. In the region from 0 to 50 mol % KB 3 O 5 , there exist two phases, namely, Na 1x K x B 3 O 5 (x ≈ 0.5) and β-NaB 3 O 5 .

Structure D 2000 X-Ray Powder Diffraction Studies and Thermal Behavior of NaK 2 B 9 O 15 , Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15. -The title compounds are synthesized by glass crystallization from stoichiometric mixtures of H3BO3, Na2CO3, and K2CO3 (670°C, 20 h followed by 1000°C, 0.5-1 h). The compounds crystallize in the monoclinic space group P21/c with Z = 4 (powder XRD). Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The thermal behavior of the compounds is characterized by high-temperature XRD. -(BUBNOVA, R.; ALBERT, B.; GEORGIEVSKAYA, M.; KRZHIZHANOVSKAYA, M.; HOFMANN, K.; FILATOV*, S.; J. Solid State Chem.