Systematic study of ionic conduction in silver iodide/mesoporous alumina composites 3: effects of binary silver halide doping.

In our previous report (Y. Fukui et al., Phys. Chem. Chem. Phys., 2024, 26, 13675), we reported the effect of AgBr-doping in a AgI-loaded mesoporous alumina (MPA) composite on the phase stability and Ag+-ion conductivity. This study revealed that the AgI-AgBr/MPA composites showed the highest room-temperature conductivity (σ25°C) of 1.6 × 10-3 S cm-1 when the AgBr content was 10 mol%, which is more than twice as high as that of the parent AgI/MPA composite (7.2 × 10-4 S cm-1). In the present study, we investigated for the first time the effect of binary silver halide doping (i.e., AgBr/AgCl) in the AgI/MPA composite on Ag+-ion conducting behaviour using variable-temperature powder X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy measurements. The AgI-AgBr-AgCl/MPA composites formed a β/γ-AgI-structured ternary solid solution (β/γ-AgIss) phase when the AgX (X = Br and Cl) content was 10 mol% (Br5Cl5), and underwent phase separation into β/γ-AgIss and face-centred cubic structured ternary solid solution (AgBrClss) when the AgX content was 20 mol% (Br10Cl10). Owing to the absence of phase separation, Br5Cl5 showed a higher σ25°C value (1.1 × 10-3 S cm-1) than that of Br10Cl10 (9.0 × 10-4 S cm-1), though both surpassed the value of the parent AgI/MPA. However, the conductivity of the AgI-AgBr-AgCl/MPA composites was lower than that of AgI-AgBr/MPA composites with the same doping levels. This result must reflect the importance of Ag+⋯halide Coulomb interactions over Frenkel defects, which arise from the lattice distortion induced by the partial substitution of I- ions with smaller X- (X = Br and Cl) ions. In addition, it could be described that this study marks the first successful synthesis of ternary silver halide nanoparticles with the aid of porous space.