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, A₂M₃X₄ (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 Ni²⁺/Pd²⁺. A systematic study of the crystal growth, crystal structure, magnetic and transport properties of two representative compounds, Rb₂Ni₃S₄ and Cs₂Ni₃Se₄, 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 Ni²⁺ ions and the alkali metal intercalated vdW structure. Additionally, Cs₂Ni₃Se₄ undergoes an insulator-metal transition (IMT) in transport measurements under pressure up to 87.10 GPa without any structural phase transition, while Rb₂Ni₃S₄ persists in its semiconducting behavior.