Synthesis of the perovskite-type oxyfluoride AgTiO2F: an approach adopting the HSAB principle

Abstract

A synthetic approach for oxyfluorides involving the hard and soft acids and bases (HSAB) principle was examined. Using AgF composed of the Ag⁺ ion as a soft acid and F⁻ as a hard base, we attempted to synthesize an oxyfluoride, AgTiO₂F, that included the O²⁻ ion as a softer base than the F⁻ ion and the Ti⁴⁺ ion as a harder acid than the Ag⁺ ion. Consequently, a perovskite (Pv)-type oxyfluoride AgTiO₂F was synthesized by a solid-state reaction under ambient pressure and under high pressure, and a heat treatment at 1000 °C for 30 min under a pressure of 4 GPa produced the Pv-type phase with minimal impurity phases. The lack of an optical second harmonic generation (SHG) response and the result of Rietveld refinement indicated that the compound is centro-symmetric and shows the anti-phase tilt of Ti(O, F)₆ octahedra along the c-axis on the basis of the space group I4/mcm. O/F ordering on the anion site was not clarified. The phase stability of a Pv-type AgTiO₂F is discussed in terms of the HSAB principle and the thermodynamics in the formation of the Pv-type phase by comparing KTiO₂F and NaTiO₂F. AgTiO₂F has a light yellow color and a band gap energy of 2.8 eV in the visible region, which originates from the covalent bonding between the soft cation Ag⁺ and O²⁻ as a softer anion than F⁻. We propose that the HSAB principle is useful for selecting a cation for the synthesis of oxyfluorides.