Thermal evolution and crystal structure features of Cs<sub>2</sub>SO<sub>4</sub> and Cs<sub>2</sub>Ca<sub>3</sub>(SO<sub>4&a

For the first time, the thermal expansion of two modifications of α- and β-Cs2SO4, as well as the compound Cs2Ca3(SO4)4, was studied by the high-temperature powder X-ray diffraction method in the temperature ranges of 25–960 and 25–540°C, respectively. β-Cs2SO4 transforms into the high-temperature α-Cs2(SO4) modification through a two-phase region – in the range of 600–750°C. The thermal expansion of all the studied phases is sharply anisotropic: αa = 37.3(10), αb = 36.2(4), αc = 12(5), αV = 85.1(5) at 30°C for β-Cs2SO4; αa = 55(5), αc = 115(9), αV = 224(12) ∙ 10–6 °С–1 at 750°С for α-Cs2SO4. The thermal expansion coefficients for Cs2Ca3(SO4)4 are: α11 = 18.8(5), αb = 18.2(5), α33 = –7.5(2), αβ = –10.6(2), αV = 29.6(9) ∙ 10–6 °С–1 at 25°С. The inheritance of the polymorphic transformation of Cs2SO4 is shown, consisting in the fact that with an increase in temperature, the corrugated columns or rods elongated along the c axis in both modifications, consisting of Cs(SO4)6 microblocks, straighten due to the rotation of SO4 tetrahedra. The interpretation of the anisotropy of the thermal expansion of Cs2Ca3(SO4)4 is based on the mechanism of rocking polyhedra, a hinge deformation at the level of Ca(SO4)6 microblocks is revealed, leading to a large negative thermal expansion in the α33 direction.