Correlated Anion-Disorder in Heteroanionic Cubic TiOF2

The functional properties of heteroanionic materials often depend on the crystallographic arrangement of their constituent anion species. Understanding and controlling the properties of practical heteroanionic materials, therefore, requires resolving their anion configurations. In the case of anion-disordered oxyfluorides, conventional diffraction methods cannot fully resolve the anionic structure, necessitating the use of alternative structure-determination methods. We have investigated the anionic structure of the anion-disordered transition-metal oxyfluoride, cubic (ReO3-type) TiOF2, using X-ray PDF, 19F MAS NMR analysis, density functional theory, cluster expansion modelling, and genetic algorithm structure-prediction. Our computational data predict that cubic TiOF2 exhibits strong short-range anion ordering characterised by an absence of collinear O–Ti–O units, resulting in predominant cis-[O2F4] titanium coordination. This local coordination preference gives correlated anion disorder at longer ranges, consistent with the long-range O/F disorder observed in average structure diffraction data. To validate our computational predictions, we use genetic algorithm structure prediction to generate partially disordered supercells to compute simulated X-ray PDF data and 19F NMR spectra, which we directly compare to our experimental data. To construct our simulated 19F NMR spectra, we derive a new transformation function for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated X-ray PDF and 19F NMR spectra and the corresponding experimental data, which supports our computationally predicted structural model, and demonstrates how complementary experimental and computational techniques can be used to resolve anionic structure in anion-disordered oxyfluorides. Additionally, we performed DFT calculations to assess how the degree of anion disorder in cubic TiOF2 affects lithium intercalation behaviour. These calculations predict that increasing anion disorder makes lithium intercalation more favourable by, on average, up to 2 eV, highlighting the significant effect that variations in short-range order can have on the intercalation properties of anion-disordered materials.