Phase evolution and temperature-dependent behavior of averievite, Cu5O2(VO4)2(CuCl) and yaroshevskite, Cu9O2(VO4)4Cl2

Abstract

The exhalation copper oxychloride vanadates attract increasing interest in the fields of both physics and chemistry. Based on the results of HT X-ray diffraction study of synthetic analogs of averievite (1) and yaroshevskite (2) and products of their thermal decomposition in air within the temperature range from 25 °C to 800 °C, it was found that 1 is stable up to 500 °C, and 2 is stable up to 480 °C. Both copper oxychloride vanadates expand anisotropically, but exhibit completely different thermal expansion patterns. 1 demonstrates an expansion in the direction perpendicular to the [O₂Cu₅]⁶⁺ layers, but inside the layer, the expansion is isotropic. The thermal expansion of 2 is much more anisotropic. The compression direction α₃₃ is close to the c axis, along which the structure tends to align the chains [O₂Cu₆]⁸⁺ into positions they would occupy in the layers [O₂Cu₅]⁶⁺ of the kagome type which exist in averievite. Meanwhile, the expansion direction α₁₁ is close to the a axis, along which the [O₂Cu₆]⁸⁺ chains shift tending to arrange as fragments of [O₂Cu₅]⁶⁺ layers. The thermal decomposition proceeds with loss of chlorine (most likely, both via hydrolysis/oxidation and evaporation of copper halides) and formation of copper vanadates.