Synthesis, structure and physical properties of layered quaternary sulfides NaLiMS2 (M = Mn, Fe, Co)

Three layered quaternary sulfides, NaLiMS2 (M = Mn, Fe, Co) were synthesized by solid-state reactions. They adopt a trigonal CaAl2Si2-type structure (S.G. P-3m1) with Li and M randomly distributed at the tetrahedral metal site. Magnetic measurements reveal spin-glass ground states for these compounds, in contrast to the long-range antiferromagnetic order in the ternary counterpart NaFe1.6S2. As evidenced by electrical resistivity and optical measurements, all these three compounds show semiconducting nature, consistent with the results of first-principles calculations. The measured band gaps are 3.4 eV and 1.25 eV for NaLiMnS2 and NaLiCoS2, respectively. Photoelectric measurements on NaLiCoS2 demonstrate its potential application in photovoltaic devices.