K3Mo2O5.6F3.4 and K3V2O3.3F5.7 – exploring transition metal cation valence and anion distribution in oxyfluorides

Abstract

Oxyfluorides come in many different structures and are highly adaptable in composition, not least because of their mixed-anionic nature. Slight changes, unless specifically looked for, can easily go unnoticed. In this paper, we present two oxyfluorides, K₃Mo₂O_5.6F_3.4 and K₃V₂O_3.3F_5.7, synthesized under high-pressure/high-temperature conditions, and demonstrate the importance of careful analysis of composition, oxidation state and O/F anion distribution for an accurate description of oxyfluorides. Their crystal structures were determined by single-crystal X-ray diffraction and the transition metal cation valences analyzed by X-ray photoelectron spectroscopy (XPS). The O/F anion ratio was calculated using the principle of charge neutrality and the local distribution within the crystallographic framework was studied using bond valence (BV) and charge distribution (CHARDI) calculations. Madelung Part of Lattice Energy (MAPLE) calculations and magnetic measurements provide insight into phase stability and corroborate the mixed-valent nature of the compounds.