Structural Investigation of α- and β-Sodium Hexafluoroarsenate, NaAsF6, by Variable Temperature X-ray Powder Diffraction and Multinuclear Solid-State NMR, and DFT Calculations

Abstract

We report the phase transition between the α- and β-phases of NaAsF6 monitored by DTA, variable temperature 19F solid-state NMR and temperature controlled X-ray powder diffraction (XRPD) as well as their crystalline structures determined from XRPD data. The structural type of β-NaAsF6 has been determined thanks to 19F and 75As solid-state NMR experiments. 19F, 23Na, and 75As NMR parameters, including 19F–75As 1J coupling, have been measured for both phases. The 19F, 23Na, and 75As solid-state NMR investigations are in full agreement with both crystalline structures from a qualitative point of view. Chemical shielding tensors have been calculated from the gauge including projector augmented wave approach. The electric field gradient tensors of 23Na and 75As have been calculated in α-NaAsF6 from the all-electrons method and the projector augmented-wave approach. Two difficulties were encountered: the libration of the rigid and regular AsF6– anions in the β-phase, highlighted by the atomic anisotropic displacement parameters for F, which leads to erroneous shortened As–F bond length, and the overestimation of the As–F bond length with the PBE functional used in the density functional theory calculations. We show that both difficulties can be overcome by full optimization and rescaling of the cell parameters of the crystalline structures. Additionally, a linear correlation is observed between experimental 23Na δiso values and calculated 23Na σiso values from previously reported data and from our own measurements and calculations.