Quaternary Chalcogenides La3Sn0.5InS7 and La3Sn0.5InSe7

Abstract

The quaternary chalcogenides La3Sn0.5InS7 and La3Sn0.5InSe7 were prepared by reactions of the elements at 1050 °C and 950 °C, respectively. They adopt noncentrosymmetric structures [hexagonal, space group P63, Z = 2; a = 10.2993(11) Å, c = 6.0921(6) Å for La3Sn0.5InS7; a = 10.6533(7) Å, c = 6.4245(4) Å for La3Sn0.5InSe7] in which the half‐occupancy of Sn atoms within octahedral sites classifies them as belonging to the La3Mn0.5SiS7‐type branch of the large family of quaternary rare‐earth chalcogenides RE3M1–xM′Ch7. The site distribution in La3Sn0.5InCh7, with higher‐valent Sn atoms occupying octahedral instead of tetrahedral sites, is reversed from the typical situation observed in other RE3M1–xM′Ch7 compounds. The ordered distribution of Sn atoms in octahedral sites and In atoms in tetrahedral sites was evaluated by bond valence sum analyses. Moreover, 119Sn solid‐state nuclear magnetic resonance (NMR) spectroscopy confirms the occupation of Sn4+ species exclusively within octahedral sites. An optical bandgap of 1.45 eV was found for La3Sn0.5InS7. Band structure calculations on an ordered superstructure model of La3Sn0.5InS7 reveal that avoidance of strongly Sn–S antibonding levels is an important driving force for the Sn deficiency.