Origin of NMR shielding in fluorides

Abstract

In this work, we analyze in detail the relation between electronic structure and fluorine nuclear magnetic resonance (NMR) shielding in a series of solid state alkali fluorides (LiF, NaF, KF, RbF, and CsF). For that purpose, we use solid-state NMR calculations implemented in the density functional theory full potential wien2k code (APW+lo). Both measurements and calculations show that the NMR shielding varies across the series by approximately 200 ppm. We focus our discussion on an explanation of the origin of the observed trend, and we show that the variation is mainly determined by contributions from ``semicore'' metal-$p$ and valence F-$p$ bands. More specifically, the trend in fluorine shielding can be related to the small but significant change in the hybridization of these states. A second important ingredient determining the value of the shielding is the presence and position of metal-$d$ states in the unoccupied part of the Kohn-Sham bands. Although the present analysis has been demonstrated for the ${}^{19}$F nucleus in alkali fluorides, the main results are more general and can explain similar trends observed in other solids and for other nuclei.