Negative thermal expansion of β-Rb2SO4

Abstract

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 (α_a = 65.4(3)∙10^–6°C^–1, α_b = 59.7(2)∙10^–6°C^–1, α_c = 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 (α_a = –10.3(3)∙10^–6°C^–1, α_b = –8.6(2)∙10^–6°C^–1, α_c = –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.