Structural Characterisation of the Li Argyrodites Li 7 PS 6 and Li 7 PSe 6 and their Solid Solutions: Quantification of Site Preferences by MAS‐NMR Spectroscopy

Li 7 PS 6 and Li 7 PSe 6 belong to a class of new solids that exhibit high Li + mobility. A series of quaternary solid solutions Li 7 PS 6− x Se x (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 F $\bar 4$ 3 m , Z =4, Li 7 PS 6 : a =9.993(1) Å, Li 7 PSe 6 : a =10.475(1) Å) and show the typical argyrodite structures with strongly disordered Li atoms. HT‐Li 7 PS 6 and HT‐Li 7 PSe 6 transform reversibly into low‐temperature (LT) modifications with ordered Li atoms. X‐ray powder diagrams show the structures of LT‐Li 7 PS 6 and LT‐Li 7 PSe 6 to be closely related to orthorhombic LT‐α‐Cu 7 PSe 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 31 P NMR is a powerful method to differentiate quantitatively between the distinct (PS 4− n Se n ) 3− local environments. Their population distribution differs significantly from a statistical scenario, revealing a pronounced preference for PS over PSe 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 6 Li{ 7 Li} cross polarisation magic angle spinning (CPMAS).