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

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 M2AX3 are found (Rb2AgCl3, Cs2AgCl3, Rb2AgBr3, Cs2AgBr3, K2AgI3, Rb2AgI3, Cs2AgI3, K2CuCl3, K2CuBr3, Rb2CuBr3 and Rb2CuCl3) which crystallize in one of two closely related crystal structures with space group Pnma. The x=0.4 composition is characterized by three compounds Cs3Cu2X5 (X=Cl, Br, I) which all adopt space group Pnma, whilst the x=0.5 case comprises the two compounds CsAgCl2 and CsAgBr2, 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 MCu2X3 have been identified (CsCu2Cl3, CsCu2Br3, CsCu2I3, RbCu2Br3, and RbCu2I3). Together with CsAg2I3, 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 CsCu9Br10 and CsCu9I10 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.