Thermal Evolution and Crystal Structure Features of Cs2SO4 and Cs2Ca3(SO4)4 Sulfates

Abstract

The thermal expansion of two modifications α- and β-Cs₂SO₄, as well as the compound Cs₂Ca₃(SO₄)₄, was studied for the first time by 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 studied phases is sharply anisotropic: α_a = 37.3(10), α_b = 36.2(4), α_c = 12(5), α_V = 85.1(5) at 30°C for β-Cs₂SO₄; α_a = 55(5), α_c = 115(9), α_V = 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), α_b = 18.2(5), α₃₃ = –7.5(2), αβ = –10.6(2), α_V = 29.6(9) × 10^–6 °С^–1 at 25°С. The continuity of the polymorphic transformation of Cs₂SO₄ is shown. It consists 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, which leads to a large negative thermal expansion in the α₃₃ direction, is revealed.