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

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.