Advances in structural analysis of fluoroaluminates using DFT calculations of 27Al electric field gradients.

Based on the analysis of 23 aluminum sites from 16 fluoroaluminates, the present work demonstrates the strong potential of combining accurate NMR quadrupolar parameter measurements, density functional theory (DFT)-based calculations of electric field gradients (EFG), and structure optimizations as implemented in the WIEN2k package for the structural and electronic characterizations of crystalline inorganic materials. Structure optimizations are essential for compounds whose structure was refined from usually less accurate powder diffraction data and provide a reliable assignment of the 27Al quadrupolar parameters to the aluminum sites in the studied compounds. The correlation between experimental and calculated EFG tensor elements leads to the proposition of a new value of the 27Al nuclear quadrupole moment Q(27Al) = 1.616 (+/-0.024) x 10(-29) m2. The DFT calculations provide the orientation of the 27Al EFG tensors in the crystal frame. Electron density maps support that the magnitude and orientation of the 27Al EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the Al 3p orbital valence electrons. In most cases, the definition of relevant radial and angular distortion indices, relying on EFG orientation, allows correlations between these distortions and magnitude and sign of the Vii.

Electric field gradients in fluoride crystalline powders: correlation of NMR measurements with ab initio calculations.

Electric field gradients (EFG) of 23Na and 27Al in three model fluoride crystalline powders AlF3, Na3AlF6 and Na5Al3F14 were computed using the density functional based electronic structure code WIEN97 and compared to values derived from nuclear magnetic resonance (NMR). First, results of measurements of 23Na and 27Al quadrupolar parameters in AlF3, Na3AlF6 and Na5Al3F14 were revisited by using high-resolution solid-state NMR. To determine chemical shifts and quadrupolar parameters with a high precision, the experimental procedure involved magic angle spinning, satellite transition spectroscopy and multi-quanta techniques applied to the quadrupolar nuclei together with a computed reconstruction of the NMR spectra. The large discrepancies which appear between previously published results in some cases, justify the use of ab initio calculations of the corresponding EFG using the WIEN97 code based on the known structural data of the crystalline phases. The agreement obtained between these calculations and the experimental results which is better than 10% in almost all cases supports the reliability of the present NMR investigations and of the crystallographic data.