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A comprehensive multinuclear ( 7 Li,  31 P,  75 As,  77 Se,  127 I) NMR study has been conducted to characterize local structural configurations and atomic distributions in the crystallographically ordered solid solutions of composition Li 6 PS 5- x   Se   x   I (0 ≤  x  ≤ 1) and in Li 6 AsS 5 I. Throughout the composition range, structural ordering between the atoms on the Wyckoff sites  4a  and  4c  is maintained, with the I −  ions exclusively occupying the  4a  sites.  31 P magic-angle spinning nuclear magnetic resonance (MAS NMR) can serve to differentiate between the various possible PS 4- n   Se   n3−  tetrahedral units in a quantitative fashion, indicating a preference of P-S relative to P-Se bonding. Each individual PS 4- n   Se   n3−  tetrahedron is represented by a peak cluster containing up to five resonances, representing the five different configurations in which the PCh 4   3−  units are surrounded by the four closest chalcogenide anions occupying the  4c  sites; the distribution of S 2−  and Se 2−  over these sites is close to statistical. Non-linear  7 Li chemical shift trends as a function of  x  are interpreted to indicate that the Coulombic traps created by sulfur-rich PS 4- n   Se   n3−  ions ( n  ≥ 2) within the energy landscape of the lithium ions are deeper than those of the other anionic species present (i.e. selenium-richer PCh 4   3−  tetrahedra, isolated chalcogenide or iodide ions), causing the Li +  ions to spend on average more time near them. Temperature dependent static  7 Li NMR linewidths measured on Li 6 PS 5 I and Li 6 AsS 5 I indicate a two-step motional narrowing process characterized by a clear dynamic distinction between a more rapid localized intra-cage process and a slower, long-range inter-cage process. In the solid solutions this differentiation gradually disappears, leading to an overall increase of lithium ionic mobility with increasing selenium content, which can be attributed to the influences of higher anionic polarizability and a widening of the lithium migration pathways caused by lattice expansion. Furthermore, the low-temperature phase transition in Li 6 PS 5 I, which tends to immobilize the lithium ions below 170 K, is suppressed in the solid solutions. The results offer interesting new insights into the -structure/ionic mobility correlations in this new class of compounds.

zeitschrift für physikalische chemie

Site preferences and ion dynamics in lithium chalcohalide solid solutions with argyrodite structure: I. A multinuclear solid state NMR study of the system Li 6 PS 5-x Se x I and of Li 6 AsS 5 I