These minerals of the pearceite–polybasite group were characterized structurally. In general, all of the structures could be described as being a regular succession of 2 module layers stacked along the c-axis. The first module layer (A) had the general composition, [(Ag,Cu)6(As,Sb)2S7]2−, and the second module layer (B) had the general composition, [Ag9CuS4]2+. In the B layer of the pearceite structure, Ag cations were found in various sites which corresponded to the most pronounced probability density function locations of diffusion-like paths. It was shown for the first time that the structural disorder observed in the B-layer was strongly related to the fast-ion conduction exhibited by these minerals. An integrated XREF, differential scanning calorimetry, CIS and EPMA study was made of all members of the pearceite–polybasite group. Differential scanning calorimetry and conductivity measurements revealed that 222 members exhibited ionic transitions at 340K (arsenpolybasite-222) and 350K (polybasite-222), whereas 221 members exhibited transitions at 310 to 330K (arsenpolybasite-221) and 335K (polybasite-221). In 111 members (pearceite and antimonpearceite), the transition occurred at 273K. In situ single-crystal X-ray diffraction experiments showed that these minerals had the same high-temperature structure and were observed, at room temperature, in either the high-temperature fast-ion conductivity form or in one of the low-temperature fully-ordered (222), partially-ordered (221) or disordered (111) forms; with transition temperatures slightly above or below room temperature. The pearceite–polybasite group of minerals could be considered to be an homogeneous series having the same aristotype fast-ion conducting form at high temperatures. Ordering occurred at a transition temperature which was related to the Cu content: the lower the Cu content, the higher was the transition temperature from the fast-ion conducting form to the non ion-conducting form.

Fast Ion Conduction Character and Ionic Phase-Transitions in Disordered Crystals - the Complex Case of the Minerals of the Pearceite–Polybasite Group. L.Bindi, M.Evain, A.Pradel, S.Albert, M.Ribes, S.Menchetti: Physics and Chemistry of Minerals, 2006, 33[10], 677-90