Subtle symmetry breaking in averievite-related compounds, (MX)Cu5O2(VO4)2 (MX = CsBr, CsI, RbBr, RbI and KI)

Abstract

In this work, we report a previously unrecognized type of phase transition within the averievite family of compounds, (MX)Cu₅O₂(VO₄)₂ (M = Cs, Rb, K; X = I, Br). Using temperature-dependent single-crystal X-ray diffraction analysis we identified a structural transition from the high-symmetry P3 @#x0305;m1 aristotype to a superstructure with a = b ≈ √3a₀, and c = c₀. The superlattice reflections were observed in the five studied (MX)Cu₅O₂(VO₄)₂ compounds: the Cs-bearing phases undergo the transition at approximately 125 K, the RbI compound shows the presence of superlattice reflections at temperatures above 375 K, whereas the KI and RbBr compounds exhibit persistent superlattice reflections across the entire temperature interval. The transition is accompanied by a pronounced pseudotranslational symmetry, which hinders the choice between nonpolar P3 @#x0305;and polar P31m space groups on the basis of X-ray diffraction alone. Due to the high absorption of the pure-phase RbBr sample and instrumental limitations, no second-harmonic generation signal was detected under the present experimental conditions. However, detailed intensity-statistics analysis of the observed reflections, symmetry and crystal-chemical arguments suggest that the polar P31m space group is the most plausible one.