Previous experimental work reported NbF3 as adopting a cubic ReO3-type structure, however the present results suggest a rhombohedral phase is more stable. One explanation may be that the synthesised NbF3 actually contained some quantity of oxygen. The method used for stoichiometric analysis of the final products is unspecified although a titration approach was likely used and could be inaccurate. It has also been suggested that NbF3 cannot be synthesised without some quantity of oxygen being present. From these results the transition pressure obtains a maximum at approximately x = 0.35, close to one additional electron per three niobium(V) ions. Similar cubic to rhombohedral transitions in other ReO3-type structures are typically governed by cationic repulsion; the reduction in Nb-Nb distance observed in the rhombohedral structure is unfavourable for highly charged cations. As such, the transition pressure is expected to decrease with decreasing cation charge, as is observed for x > 0.35. For x < 0.35 a different energetic contribution must therefore drive the increase in transition pressure with x. We attribute this to the greater covalency of Nb-O bonds cf. Nb-F bonds. The increased covalency drives hybridisation between O 2p and 2s orbitals, which in turn causes the Nb-O-Nb bond angle to shift away from 180° and oxygen lone pairs to occupy the vacant space within the ReO3 framework. The lone pairs can be seen from electron localisation function (ELF) calculations (Fig. ). This effect is strong enough to reduce the transition pressure for small x, in spite of the significant electrostatic repulsion between adjacent Nb(V) ions.