La12Sb9S38: a new semiconducting lanthanum antimony polysulfide with a mixed La/Sb site

Abstract

With the discovery of La_12+xSb_9−yS_38−z, for simplicity called La₁₂Sb₉S₃₈, we characterized the first lanthanum antimony polysulfide. Despite La/Sb mixed occupancies and S atom deficiencies, this material exhibits only a small phase width, as evident from our two single crystal structure studies. The refined formulae of La_12.17(1)Sb_8.59(2)S_37.86(3) and La_12.10(1)Sb_8.60(2)S_37.79(6) are almost equivalent, despite significantly different starting compositions. La_12.17(1)Sb_8.59(2)S_37.86(3) crystallizes in a new structure type, space group Pm, with Z = 1 and a = 11.1455(2) Å (La_12.10(1)Sb_8.60(2)S_37.79(6): a = 11.151(1) Å). This new structure type is composed of a complex three-dimensional network of different Sb–S polyhedra and disordered S₂²⁻ dumbbells. The material is a red semiconductor with an optical band gap of 1.75 eV. Despite its large sulfur content of 65 atomic-% and a therefore relatively low average molar mass of 67.4 g mol⁻¹, the material exhibits low thermal conductivity values between 0.80 W m⁻¹ K⁻¹ and 1.05 W m⁻¹ K⁻¹ as a consequence of its complex crystal structure and various disorders.