ReO 3 -type TiOF 2 is chemically and structurally similar to ReO 3 -type NbO 2 F and TaO 2 F. In these materials, it has been proposed that collinear F-M -F units are disfavoured and that oxygen and fluorine anions preferentially adopt short-range orderings that give asymmetric F-M -O units . This coordination asymmetry allows the central cation to shift off-centre to form shorter Ti-O bonds, which has been suggested to increase the overall bonding strength of the M (O/F) 6 unit. By analogy, we might anticipate that in TiOF 2 , collinear O-Ti-O units are disfavoured compared to asymmetric collinear O-Ti-F units. Fig. shows the distribution of energies for all 2 × 2 × 2 supercells, grouped by the number of collinear O-Ti-O units in each structure, out of a maximum of 8 possible for this supercell size. In general, structures with a greater number of collinear O-Ti-O units have higher configurational energies, while the lowest energy structures have no collinear O-Ti-O subunits. This observed correlation supports the hypothesis that in ReO 3type TiOF 2 , oxygen and fluorine preferentially organise to give asymmetric collinear O-Ti-F units.