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

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 (σ_25°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-AgI_ss) phase when the AgX (X = Br and Cl) content was 10 mol% (Br5Cl5), and underwent phase separation into β/γ-AgI_ss and face-centred cubic structured ternary solid solution (AgBrCl_ss) when the AgX content was 20 mol% (Br10Cl10). Owing to the absence of phase separation, Br5Cl5 showed a higher σ_25°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.