Thermal expansion of francisite, [Cu₃BiO₂](SeO₃)₂Cl, and its interpretation based on oxocentered copper-bismuth tetrahedra

Abstract

The thermal expansion of francisite, [Cu₃BiO₂](SeO₃)₂Cl, a rare copper(II)-bismuth(III) oxide chloride selenite, was studied by high-temperature X-ray powder diffraction over the temperature range 293–773 K. The mineral is stable up to 748 K at which temperature it decomposes to [Bi₂O₂]Se, CuO and, probably, [BiO][CuSe]. The thermal expansion has an anisotropic character (α_a = 9.0, α_b = 4.7, α_c = 17.0 · 10⁻⁶ K⁻¹), which is determined by the distribution of the bonds between additional oxygen atoms (^aO) and metal atoms (A). These bonds form two-dimensional systems in which each ^aO atom is tetrahedrally coordinated by one Bi and three Cu atoms. Thus the structure can be described in terms of oxocentered metal atom tetrahedra consisting of [^aO₂Cu₃Bi] layers of (^aOCu₃Bi) tetrahedra, (SeO₃) groups and Cl anions. This description gives the most reasonable viewpoint to explain the anisotropic thermal expansion character of francisite.