Ln3M(1-delta)TX7--quasi-isostructural compounds: stereochemistry and silver-ion motion in the Ln3Ag(1-delta)GeS7 (Ln = La-Nd, Sm, Gd-Er and Y; delta = 0.11-0.50) compounds

The crystal structures of the Ln3Ag(1-delta)GeS7 (Ln = La-Nd, Sm, Gd-Er, Y; delta = 0.11-0.50, space group P6(3)) compounds were determined by means of X-ray single-crystal diffraction and the similarities among the crystal structures of all Ln3M(1-delta)TX7 (space group P6(3); Ln--lanthanide element, M--monovalent element; T--tetravalent element and X--S, Se) compounds deposited in the Inorganic Crystal Structure Database (ICSD) are discussed. Substitutions of each element in Ln3M(1-delta)TX7 result in a different structural effect. On the basis of the data deposited in the ICSD the large family of the Ln3M(1-delta)TX7 compounds was divided into three groups depending on the position of the monovalent element in the lattice. This position determines what kind of stereoisomer is present in the structure, either the ++ enantiomer or the +- diastereoisomer. Since the silver ions can occupy a different position and the energy barriers between positions are low the ions can move through the channel. It was shown that this movement is not a stochastic process but a correlated one.