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

Abstract

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-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 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.