Lithium Argyrodites with Phosphorus and Arsenic: Order and Disorder of Lithium Atoms,Crystal Chemistry,and Phase Transitions

Crystal chemical data of high‐ (HT) and low‐temperature (LT) modifications of lithium argyrodites with the compositions Li₇PCh₆ (Ch=S, Se), Li₆PCh₅X (X=Cl, Br, I), Li₆AsS₅Br, and Li₆AsCh₅I (Ch=S, Se) based on single‐crystal, powder X‐ray (113 K<T<503 K) and neutron measurements (5 K<T<293 K) are presented. In the HT modifications, the Li atoms are strongly disordered over a fraction of the available tetrahedral holes, whereas in the LT modifications they occupy ordered crystallographic positions with a pronounced site preference that is analysed on a crystal chemical basis. The Ch/X partial structures remain nearly unchanged upon the reversible phase transitions. Crystal chemical and crystallographic relations between HT and LT modifications based on the Frank–Kasper model of tetrahedral close packing are discussed. X‐ray single‐crystal data for HT‐Li₆PS₅I show the electron density of the disordered Li to be smeared out over an extended region preferably inside face‐sharing double tetrahedra. A series of temperature‐dependent powder neutron data for Li₆PS₅I gives clear evidence for an HT/LT phase transition at ≈175 K with an ordering of the Li atoms in different polyhedra with coordination numbers between three and four.