Thermal Expansion and Phase Transitions in K1-xCsxBSi2O6 Borosilicate Solid Solutions

Solid 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(4) over bar 3d and Ia(3) over bar d, respectively. The cubic-cubic phase transition I (4) over bar 3d reversible arrow Ia (3) over bard occurs in the composition range x = 0.35-0.40 at room temperature. In the series of solid solutions with x = 0-0.30, the reversible cubic-cubic polymorphic transition I (4) over bar 3d reversible arrow Ia (3) over bard with an increase in the temperature has been revealed, the temperature of the polymorphic transition has been determined, and the thermal expansion in both polymorphic modifications has been studied using high-temperature X-ray diffraction and dilatometry. The solid solutions belonging to the space group Ia (3) over bard are characterized by a lower thermal expansion than the solid solution belonging to the space group I (4) over bar 3d. According to the calculations, the equivalents a/gamma for the space groups I (4) over bar 3d and Ia (3) over bard are 0.03 and 0.02 ( in cesium atomic fractions per degree Celsius), respectively.