RESUMEN
Aliovalent substitution of Li in salts by Mg can generate Li vacancies and thus in principle improve the ionic conductivity. In fact the influence of substitution on ionic conductivity is far more complex. Here the impact of Mg substitution on Li ion mobility is studied in the example of Li4P2S6 by a combination of nuclear magnetic resonance experiments on 31P and 7Li at variable temperatures, impedance spectroscopy and X-ray powder diffraction to elucidate the relationship with structural changes and the effect on mobility on different length scales. It is found that substituting Li ions with Mg ions in Li4-2xMgxP2S6 with 0 ≤ x ≤ 0.2 increases the local Li ion mobility up to a certain concentration at which a phase transition induces a different structural realignment of the P2S64- units. The determined activation energies can be assigned to vacancy hopping processes by comparison with nudged elastic band calculations at the density functional level of theory, which shows not only the possibilities but also limitations of substituting Li with Mg.
RESUMEN
The structure of Li4P2S6 was solved, based on a combination of X-ray powder diffraction data, quantum chemical calculations and solid state nuclear magnetic resonance (NMR). Two-dimensional 31P single quantum/double quantum correlation spectra yielded important constraints regarding the space group symmetry allowing the crystal structure to be solved by the Rietveld method. Li4P2S6 crystallizes in a trigonal space group with a = 10.51452(6) Å; c = 6.59149(8) Å. The structure contains two distinct P2S64- ions in a 2 : 1 ratio: in the first one the two P atoms of the hexahypothiophosphate unit are crystallographically distinct, whereas in the second one they are crystallographically identical.