figure_9_description.txt

#(a) Perspective view of [NbO12O22F1F2]5- octahedron, in which, atoms labeled X and X’ (X = O1 and O2) are equivalent by symmetry operations. Temperature dependence of (b) [MO4F2]5– octahedron volume, (c) bond lengths and (d) distortion index of NaNbO2F2 (red dots) and NaTaO2F2 (blue dots). Linear regressions (blue lines) are plotted in the [25–220°C] temperature range guiding the evolution of each quantity. The amplitude of significant evolutions is highlighted in blue.