Rb2Ca3(SO4)4: Crystal Structure, Thermal Expansion, Phase Transformations and Comparison with Cs2Ca3(SO4)4 and Langbeinite Structure Type

The Rb2Ca3(SO4)4 compound was obtained by rapid cooling of the stoichiometric melt. The crystal structure was solved and refined using single crystal X-ray diffraction analysis (P21/c, a = 9.2847(9), b = 9.4094(6), c = 9.2917(8) Å, β = 114.646(1)°, V = 737.80(12) Å3, R1 = 0.051). The thermal behavior of Rb2Ca3(SO4)4 was investigated by high-temperature powder X-ray diffraction in the range 25–1000 °C. Thermal decomposition of the Rb2Ca3(SO4)4 phase occurs at 300 °C, forming Rb2Ca2(SO4)3 and CaSO4. The decomposition is complete at 450 °C, and the mixture of Rb2Ca2(SO4)3 + CaSO4 persists up to 890 °C. Homogenization of the phases occurs at 900 °C, resulting in the formation of the Rb2Ca3(SO4)4 compound again at 970 °C. A structural interpretation of this thermal phase transformation is presented, and the relationship between the crystal structures of Rb2Ca3(SO4)4 and Rb2Ca2(SO4)3 of the langbeinite structure type is demonstrated. Thermal expansion of Rb2Ca3(SO4)4 is highly anisotropic: α11 = 23.9(4), αb = 19.2(3), α33 = 7.7(1), αβ = −1.9(7), αV = 50.8(9) × 10−6 °C−1 at 25 °C and α11 = −7(2), αb = 17(5), α33 = 25(7), αβ = −1.1(1), αV = 35(9) × 10−6 °C−1 at 1000 °C. The anisotropy of the thermal expansion is described in comparison with the Rb2Ca3(SO4)4 crystal structure. The optical band gap for the Rb2Ca3(SO4)4 compound was determined to be 3.7 eV from absorption spectroscopy data.