The solid electrolyte system RbCu₄Cl₃(I₂−_xCl_x), 0≤x≤0.40 I. Stability

Abstract

The stoichiometric solid electrolyte, RbCu₃Cl₃I₂, can be converted, by cold‐work, to a new low symmetry phase with the probable composition RbCu₄Cl₃(I_1.96Cl_0.04) (and very small amounts of CuI and probably, Rb₂Cu₃Cl₅ ). The new phase has a crystal structure which appears to be related to that of the cubic phase. It transforms rapidly to the cubic phase with formula RbCu₄Cl₃(I_1.96Cl_0.04) at temperatures above 75°C. The new phase is stable in a very narrow temperature range, almost surely not wider than 0°–35°C. It has a conductivity of 1.5×10^-4 Ω ^-1cm^-1 at 298 K. Stoichiometric RbCu₄Cl₃I₂ is thermodynamically stable in the narrow temperature range 200° ± 15° C. The solid solution range RbCu₄Cl₃(I_2-xCl_x)0.00 ≤ x ≤0.40 ± 0.01 has been confirmed. For 0.02 ≤ x ≤ 0.40, the materials appear to be stable from at least 78 K to their preparation temperatures which depend on x . None of these can be made to convert to the new phase; that is to say, the only established route to the new phase is through cold‐working of the stoichiometric phase.