For the isotropic values, the conventional approach to convert from σ iso to δ iso is to use a linear transformation function δ iso = aσ iso +b, where the parameters a and b are obtained by linear regression between computed σ iso and experimental δ iso data. Several different linear transformation functions for 19 F have been published for a wide range of cations bonded to fluorine (see Table ). One approach for the quantitative prediction of 19 F δ iso values for a disordered system containing one metal cation is to derive an appropriate linear transformation function from data for a reference ordered (oxy)fluoride, containing the same cation as the system of interest and with fluoride ions occupying multiple crystallographic sites. This approach has previously been used to simulate the 19 F NMR spectra of the disordered oxyfluorides MO 2 F and MOF 3 (M = Nb, Ta), with transformation functions derived by fitting to data for the corresponding MF 5 fluorides .