Crystal structure, properties and pressure-induced insulator-metal transition in layered kagome chalcogenides.

Layered materials with kagome lattice have attracted a lot of attention due to the presence of nontrivial topological bands and correlated electronic states with tunability. In this work, we investigate a unique van der Waals (vdW) material system, A 2 M 3 X 4 ( A = K, Rb, Cs; M = Ni, Pd; X = S, Se), where transition metal kagome lattices, chalcogen honeycomb lattices and alkali metal triangular lattices coexist simultaneously. A notable feature of this material is that each Ni/Pd atom is positioned in the center of four chalcogen atoms, forming a local square-planar environment. This crystal field environment results in a low spin state S = 0 of Ni 2+ /Pd 2+ . A systematic study of the crystal growth, crystal structure, magnetic and transport properties of two representative compounds, Rb 2 Ni 3 S 4 and Cs 2 Ni 3 Se 4 , has been carried out on powder and single crystal samples. Both compounds exhibit nonmagnetic p -type semiconducting behavior, closely related to the particular chemical environment of Ni 2+ ions and the alkali metal intercalated vdW structure. Additionally, Cs 2 Ni 3 Se 4 undergoes an insulator-metal transition (IMT) in transport measurements under pressure up to 87.1 GPa without any structural phase transition, while Rb 2 Ni 3 S 4 shows the tendency to be metalized.