Syntheses, crystal structures, and physical properties of three new chalcospinels: Ag2Sc2Sn2S8, Cu1.9(1)Sc1.9(1)Sn2.1(1)S8, and Cu1.4(1)Sc1.4(1)Sn2.6(1)Se8

Abstract

The present work describes the syntheses of three new chalcogenides, Ag₂Sc₂Sn₂S₈, Cu_1.9(1)Sc_1.9(1)Sn_2.1(1)S₈, and Cu_1.4(1)Sc_1.4(1)Sn_2.6(1)Se₈ with disordered spinel structures (space group: Fd$\bar{3}$m) as established from single crystal X-ray diffraction studies. As expected, the unit cell volume of the cubic Ag₂Sc₂Sn₂S₈ (a = 10.722(4) Å) structure is bigger than that of the Cu-containing sulfide Cu_1.9(1)Sc_1.9(1)Sn_2.1(1)S₈ (a = 10.4431(3) Å). Similarly, the refined a-lattice constant of 10.8961(10) Å for the Cu_1.4(1)Sc_1.4(1)Sn_2.6(1)Se₈ structure is longer than the sulfide Cu_1.9(1)Sc_1.9(1)Sn_2.1(1)S₈. These structures are based on cubic close packing of chalcogen atoms (S/Se) where the Sc and Sn atoms co-occupy the same octahedral sites, unlike the Cu/Ag atoms, which are distributed over the tetrahedral sites in the title structures. A phase pure polycrystalline selenide with the loaded composition of Cu_1.4Sc_1.4Sn_2.6Se₈ was prepared by a high-temperature reaction of elements at 1223 K. The polycrystalline samples with the loaded compositions of Cu_1.9Sc_1.9Sn_2.1S₈ and Ag₂Sc₂Sn₂S₈ were biphasic, consisting of the target spinel and SnS phases. The resistivity, Seebeck coefficient, and optical absorption studies show that the polycrystalline Cu_1.4Sc_1.4Sn_2.6Se₈ is an n-type semiconductor with a direct bandgap of 1.0(1) eV. The magnitude of the Seebeck coefficient value of the Se-sample varies from 122 μV/K (at 773 K) to 185 μV/K (at 573 K). The Se-sample shows a relatively high thermal conductivity (κ_tot) value of 1.75 Wm⁻¹K⁻¹ near room temperature, which drops to a low value of 0.77 Wm⁻¹K⁻¹ at 773 K. The biphasic samples, Cu_1.9Sc_1.9Sn_2.1S₈ and Ag₂Sc₂Sn₂S₈, also show semiconducting behavior from the resistivity and optical absorption studies.