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 Li(7)PCh(6) (Ch=S, Se), Li(6)PCh(5)X (X=Cl, Br, I), Li(6)AsS(5)Br, and Li(6)AsCh(5)I (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-Li(6)PS(5)I 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 Li(6)PS(5)I gives clear evidence for an HT/LT phase transition at approximately 175 K with an ordering of the Li atoms in different polyhedra with coordination numbers between three and four.