Superposition model for 19F isotropic chemical shift in ionic fluorides:: from basic metal fluorides to transition metal fluoride glasses

Experimental measurements of F-19 isotropic chemical shifts on a large set of mainly ionic fluoride compounds (from simple metal fluorides to transition metal fluoride glasses) obtained by MAS NMR at 15 kHz spinning rate are simultaneously investigated. First, Ramsey's theory of die chemical shift with molecular orbitals obtained by Lodwin's orthogonalisation method is used to evaluate the isotropic part of the F-19 chemical shift in ionic fluorides for which the crystallographic structure and the atomic radial wavefunctions are known. Then, assuming that the paramagnetic part of the F-19 shielding in a given material is simply the summation of the paramagnetic contributions due to all the cations in the neighbourhood of the considered fluorine, a superposition model of the F-19 isotropic chemical shift is developed. Finally, this empirical approach is applied to complex fluoride compounds of unknown structure and it is demonstrated that it allows to obtain reliable structural informations. More generally, it can be applied to all the ionic fluorides. (C) 1999 Elsevier Science B.V. All rights reserved.