figure_S21_description.txt

#(a) Perspective view of [NbO4F2]5– octahedron, in which, the equatorial plane (O2O2’F1F2) is highlighted. (b) Angles between the apical O1’ oxygen and the equatorial plane of the [MO4F2]5– octahedron from NaNbO2F2 (red dots) and NaTaO2F2 (blue dots) as a function of temperature. X and X’ (X = O1 and O2) are atoms equivalent by symmetry operations. Linear regressions (blue lines) are plotted in the [25–220°C] temperature range guiding the variation of each quantity. The amplitudes of significant variations are indicated and highlighted in blue.