Ag₄B₇O₁₂X (X = Cl, Br, I) Heptaborate Family: Comprehensive Crystal Chemistry, Thermal Stability Trends, Topology, and Vibrational Anharmonicity

Abstract

Using glass crystallization and solid-state techniques, we were able to complete the family of salt-inclusion silver halide borates, Ag₄B₇O₁₂X, by the X = Cl and I members. The new compounds are characterized by differential scanning calorimetry, single-crystal and high-temperature powder X-ray diffraction, optical spectroscopy, and density functional theory calculations. In all structures, the silver atoms exhibit strong anharmonicity of thermal vibrations, which could be modeled using Gram-Charlier expansion, and its asymmetry was characterized by the skewness vector. The topology of the silver halide and borate sublattices has been analyzed separately for the first time. Along the I → Br → Cl series, we observe a decrease of the melting point and configuration entropy and an increase of thermal expansion and its anisotropy and thermal vibration anharmonicity, which indicates decreasing stability.