Negative thermal expansion of β-Rb₂SO₄

Research subject. The low-temperature modification of β-Rb2SO4 sulfate (Pmcn). Aim. Low-temperature study of the thermal expansion of β-Rb2SO4 by high-temperature powder X-ray diffraction in comparison with the crystal structure, as well as interpretation of the anisotropy of β-Rb2SO4 thermal expansion. Materials and Method. Powder X-ray diffraction and high-temperature powder X-ray diffraction. Results. The thermal expansion of β-Rb2SO4 sulfate was studied for the first time using low-temperature powder thermal X-ray diffraction in comparison with the crystal structure. The phase composition was confirmed by powder X-ray diffraction. The thermal expansion of β-Rb2SO4 is practically isotropic. Across the temperature range from –177 to –140°C, the sulfate experiences negative thermal expansion. A further increase in temperature leads to a change in its thermal expansion, which becomes positive. It is proposed to consider the crystal structure of β-Rb2SO4 sulfate as a mixed framework of [RbSO4]–1, which, in turn, consists of fundamental building units (microblocks) of Rb(SO4)6. Across the temperature range from room temperature to –100°C, the maximum expansion of β-Rb2SO4 sulfate occurs along the a axis. The minimum thermal expansion is observed along the c-axis, along the columns consisting of microblocks (αa = 65.4(3)∙10–6°C–1, αb = 59.7(2)∙10–6°C–1, αc = 58.6(2)∙10–6°C–1 at +25°C). In the temperature range from –177 to –140°C, thermal expansion is negative in all three directions (αa = –10.3(3)∙10–6°C–1, αb = –8.6(2)∙10–6°C–1, αc = –9.7(2)∙10–6°C–1 at –170°C). Conclusion. The thermal expansion of β-Rb2SO4 sulfate in the low-temperature range (from –177 to –25°C) was studied for the first time, its structural interpretation was performed. A comparison was given with the thermal expansion of isostructural β-K2SO4.