Thermal expansion and phase transitions in K1-xRbxBSi2O6 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 degrees 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 alpha are determined for solid solutions of different crystalline modifications and glasses. The linear thermal expansion coefficients alpha for the I-43d low-temperature phase are equal to (20-23) x 10(-6) K-1 according to the X-ray diffraction data and (21-24) x 10(-6) K-1 according to the dilatometric data. The values of alpha for the Ia-3d high-temperature phase lie in the range (4-9) x 10(-6) K-1 according to the X-ray diffraction data and in the range (6-9) x 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 alpha = (10-11) x 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 I4(1)/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.