Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalides—II: ordered phases within the (Ag X) x–( MX) 1− x and (Cu X) x–( MX) 1− x ( M=K, Rb and Cs; X=Cl, Br and I) systems

Abstract

The crystal structures of 23 ternary phases present in the systems ( AX) x –( MX) 1− x ( A=Ag, Cu; M=K, Rb and Cs; X=Cl, Br and I) have been determined and/or refined using X-ray and neutron diffraction studies of powder samples. A total of 11 x=0.333 phases of stoichiometry M 2 AX 3 are found (Rb 2AgCl 3, Cs 2AgCl 3, Rb 2AgBr 3, Cs 2AgBr 3, K 2AgI 3, Rb 2AgI 3, Cs 2AgI 3, K 2CuCl 3, K 2CuBr 3, Rb 2CuBr 3 and Rb 2CuCl 3) which crystallize in one of two closely related crystal structures with space group Pnma. The x=0.4 composition is characterized by three compounds Cs 3Cu 2 X 5 ( X=Cl, Br, I) which all adopt space group Pnma, whilst the x=0.5 case comprises the two compounds CsAgCl 2 and CsAgBr 2, which possess space group Cmcm. The latter undergo phase transitions at 408(5) and 413(8) K, respectively, to higher symmetry structures in space group P4/ nmm. Five x=0.667 compounds of stoichiometry MCu 2 X 3 have been identified (CsCu 2Cl 3, CsCu 2Br 3, CsCu 2I 3, RbCu 2Br 3, and RbCu 2I 3). Together with CsAg 2I 3, these form a family of compounds, which crystallize in one of two closely related structures (space group Pbnm or Cmcm). Impedance spectroscopy measurements indicate that all the above-mentioned compounds have relatively low values of ionic conductivity ( σ<10 −4 Ω −1 cm −1 at room temperature). Contrary to previous reports, no evidence of high conductivity phases of composition CsCu 9Br 10 and CsCu 9I 10 was obtained during high-temperature powder neutron diffraction studies. Some general remarks concerning the structural properties of these systems are given, together with a discussion of the influence of the M + cations on the ionic conductivity. Finally, the structural information for all the ambient temperature phases has been used to derive improved bond valence parameters for Ag– X and Cu– X bonds.