Synthesis, structure, optical properties, and electronic structure of NaLiCdS2

The new compound NaLiCdS2 has been synthesized by the reaction of Cd and a Li2S/S/Na2S flux at 773 K. This compound, which has the Ce2O2S structure type, crystallizes with one formula unit in space group P3¯m1 of the trigonal system in a cell at T=153 K with a=4.1320(3) Å and c=6.8666(11) Å. The structure consists of two-dimensional image layers stacked perpendicular to the [001] direction. The two-dimensional layers are formed by corner-sharing LiS4 or CdS4 tetrahedra. The Na atoms are between these layers. Li incorporation in the compound is confirmed by an SIMS chemical composition map and by ICP measurements. The Li and Cd atoms are disordered in the crystal structure. First-principles calculations show that the optical excitations arise primarily from S→Cd charge-transfer transitions at 1.0 eV (very weak) and 2.4 eV (strong). Calculations also indicate that Na contributions around the Fermi level are significant. Polarized single-crystal optical measurements indicate an indirect optical band gap of 2.37 eV for light perpendicular to the (001) crystal face, in good agreement with theory. The compound NaLiZnS2 has also been synthesized and is found to be isostructural with NaLiCdS2.