where 𝑦 is the anion fractional coordinate. The results of the geometry optimisations in terms of this octahedral rotation are shown for each composition and pressure in Fig. . For higher fluorine compositions, the octahedra also experience a trigonal distortion with equal length Nb-(O/F) bonds no longer oriented at 90 o to each other. This distortion can be quantified by fitting a minimum bounding ellipsoid to the octahedra and calculating the standard deviation of its principal radii, 𝜎(𝑅), the results of which are shown in Fig. . An attempt to determine the transition pressure using a common tangent approach (Fig. ) produced large errors at higher x compositions due to the low curvature of the rhombohedral energy against volume, therefore 𝜔 was used to identify the transition pressures shown in Fig. .