Syntheses and characterization of two new layered ternary chalcogenides NaScQ2 (Q = Se and Te)

Abstract

Two new mixed metal ternary chalcogenides, NaScSe₂ and NaScTe₂, have been synthesized by the high-temperature reaction of elements using the sealed tube method. Single crystal X-ray diffraction studies show that both compounds are isostructural and adopt the trigonal crystal system with space group Rm with three formula units (Z = 3). The NaScQ₂ structure consists of three unique crystallographic sites: Na, Sc, and Q sites. The Sc atom in this structure sits at the center of an octahedron of six Q atoms. The six Sc–Q bonds are identical, but the geometry of ScQ₆ is slightly distorted compared with the ideal octahedron. The ScQ₆ octahedra are conjoined to adjacent units via edge-sharing to form two-dimensional layers of ²_∞[ScQ₂]¹⁻. Two such layers are separated by the filling of Na⁺ cations, each surrounded by six Q atoms forming distorted octahedral geometry. These two chalcogenides are valence precise [Na¹⁺Sc³⁺(Q²⁻)₂]. The optical absorption and thermal conductivity measurements were carried out on the polycrystalline samples of NaScQ₂. The optical bandgap value for NaScSe₂ (1.5(1) eV) and NaScTe₂ (1.2(1) eV) confirms their semiconducting nature. The high-temperature resistivity and Seebeck coefficient studies were also performed on a sintered polycrystalline sample of NaScTe₂. The emission spectroscopic analysis of the polycrystalline NaScSe₂ was also analyzed to understand the fluorescence properties. DFT calculations were performed to study the electronic structure, optical, and thermoelectric properties of NaScQ₂. The COHP analysis shows stronger chemical bonding between the Sc and Te atoms compared with that of Na atoms in the NaScQ₂ structure.