Structural Characterisation of the Li Argyrodites Li_7PS _6 and Li_7PSe_6 and their Solid Solutions: Quantification of Site Preferences by MAS-NMR Spectroscopy

摘要

Li_7PS_6 and Li_7PSe_6 belong to a class of new solids that exhibit high Li~+ mobility. A series of quaternary solid solutions Li_7PS_(6-x_Se_x (0≤×≤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 F4?3m, Z = 4, Li_7PS_6: a = 9.993(1) ?, Li _7PSe_6: a = 10.475(1) ?) and show the typical argyrodite structures with strongly disordered Li atoms. HT-Li _7PS_6 and HT-Li_7PSe_6 transform reversibly into low-temperature (LT) modifications with ordered Li atoms. X-ray powder diagrams show the structures of LT-Li_7PS_6 and LT-Li_7PSe_6 to be closely related to orthorhombic LT-α-Cu_7PSe_6. 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 ~31P NMR is a powerful method to differentiate quantitatively between the distinct (PS _(4-n)Sen)~(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 Se_P+S~(2-)?S P+Se~(2-), prior to crystallisation. The ~(77)Se 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{~7Li} cross polarisation magic angle spinning (CPMAS).