Competition between the Direct Exchange Interaction and Superexchange Interaction in Layered Compounds LiCrSe<sub>2</sub>, LiCrTe<sub>2</sub>, and NaCrTe<sub>2</sub> with a Triangular Lattice

Physical properties of new <i>S</i> = 3/2 triangular-lattice compounds LiCrSe<sub>2</sub>, LiCrTe<sub>2</sub>, and NaCrTe<sub>2</sub> have been investigated by X-ray diffraction and magnetic measurements. These compounds crystallize in the ordered NiAs-type structure, where alkali metal ions and Cr atoms stack alternately. Despite their isomorphic structures, magnetic properties of these three compounds are different; NaCrTe<sub>2</sub> has an A-type spin structure with ferromagnetic layers, LiCrTe<sub>2</sub> is likely to exhibit a helical spin structure, and LiCrSe<sub>2</sub> shows a first-order-like phase transition from the paramagnetic trigonal phase to the antiferromagnetic monoclinic phase. In these compounds and the other chromium chalcogenides with a triangular lattice, we found a general relationship between the Curie–Weiss temperature and magnetic structures. This relation indicates that the competition between the antiferromagnetic direct <i>d</i>-<i>d</i> exchange interaction and the ferromagnetic superexchange interaction plays an important role in determining the ground state of chromium chalcogenides.