Structural Characterisation of the Li Argyrodites Li7PS6 and Li7PSe6 and their Solid Solutions: Quantification of Site Preferences by MAS-NMR Spectroscopy

Li7PS6 and Li7PSe6 belong to a class of new solids that exhibit high Li+ mobility. A series of quaternary solid solutions Li7PS6-xSex (0 <= x <= 6) were characterised by X-ray crystallography and magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The high-temperature (HT) modifications were studied by single-crystal investigations (both F43m, Z=4, Li7PS6: a=9.993(1) A, Li7PSe6: a= 10.475(1) A) and show the typical argyrodite structures with strongly disordered Li atoms. HTLi7PS6 and HT-Li7PSe6 transform reversibly into low-temperature (LT) modifications with ordered Li atoms. X-ray powder diagrams show the structures of LT-Li7PS6 and LT-Li7PSe6 to be closely related to orthorhombic LTa-Cu7PSe6. Single crystals of the LT modifications are not available due to multiple twinning and formation of antiphase domains. The gradual substitution of S by Se shows characteristic site preferences closely connected to the functionalities of the different types of chalcogen atoms (S, Se). High-resolution solid-state 3113 NMR is a powerful method to differentiate quantitatively between the distinct (P54_ Se)3- local environments. Their population distri- bution differs significantly from a statistical scenario, revealing a pronounced preference for P S over P Se bonding. This preference, shown for the series of LT samples, can be quantified in terms of an equilibrium constant specifying the melt reaction Sep+ S2-,=Sp+Se2-, prior to crystallisation. The 775e MAS-NMR spectra reveal that the chalcogen distributions in the second and third coordination sphere of the P atoms are essentially statistical. The number of crystallographically independent Li atoms in both LT modifications was analysed by means of 6Li(7Lil cross polarisation magic angle spinning (CPMAS).