Chloride-ion-stabilized strontium mayenite: Expansion of versatile material family

Sr-mayenite (S₁₂A₇) incorporating Cl^- ions in its crystallographic cages up to the theoretical maximum occupancy, Sr₁₂Al₁₄O₃₂Cl₂, is reported in our study. The addition of a stoichiometric amount of SrCl₂ to the starting mixture is effective for the formation of Sr₁₂Al₁₄O₃₂Cl₂ with high phase purity. Almost 100% densification is achieved using spark plasma sintering (SPS). Evaporation of SrCl₂ from Sr₁₂Al₁₄O₃₂Cl₂, which becomes significant at sintering temperatures above ∼1300°C, degrades the phase purity. However, SrCl₂ effusion is significantly suppressed in the samples fully densified by SPS, impeding the decomposition of Sr₁₂Al₁₄O₃₂Cl₂ up to temperature as high as ∼1400°C. The crystal structure of Sr₁₂Al₁₄O₃₂Cl₂ was investigated by Rietveld analysis of the X-ray diffraction pattern. It is found that the Cl^- ion is incorporated in the center of the inner cage with nearly theoretical maximum occupancy, which is responsible for the phase stability. Porous Sr₁₂Al₁₄O₃₂Cl₂ exhibits humidity-sensitive surface protonic conductivity. Dense Sr₁₂Al₁₄O₃₂Cl₂ prepared under reducing conditions such as SPS exhibits electronic conductivity. Sr-mayenite has various potential applications derived from its multifunctionalities.