Nuclear magnetic resonance quadrupolar parameters and short range orderin disordered ionic fluorides

Abstract

Information on both radial and angular atomic coordinations is provided by thequadrupole interaction between the quadrupole moment of the atomic nuclei andthe electric field gradients (EFGs) originating from the distribution ofelectric charges around the nuclei. 69Ga and 71Ga arequadrupolar nuclei active in NMR. Their spectra are recorded in amorphousGaF3 which may be considered as a model compound for disorderedfluorides with fluorine corner sharing octahedra. An unique and continuousquadrupolar parameter Czjzek distribution allows us to simulate theexperimental spectra. The measured chemical shifts indicate that the Ga3+ ions are at the centre of (GaF6)3- octahedra in theamorphous GaF3 phase.A polarizable point charge model is used to explain NMR quadrupolar parameterswhich are related to the electric field gradients at the Ga site. Latticesummations are performed in the direct space over spherical volumes. Atomicposition sets generated by molecular dynamics are shown to give quadrupolarparameter distributions which look like Czjzek ones whatever the EFGcalculation approximations. Provided the polarizabilities are adjusted, theyallow us to reconstruct the experimental NMR spectra which correspond in anycase to slightly distorted (GaF6)3- octahedra. The presentapproach may be applied to any ionic disordered compound which containsquadrupolar nuclei and used to quantify short range order around such nuclei.