An insulator–metal transition in a van der Waals kagome chalcogenide

Layered materials with kagome lattice have attracted a lot of attention due to the presense of nontrivial electronic bands and the resulting exotic quantum states. In this work, we examine a van der Waals system A₂Ni₃X₄ (A = K, Rb, Cs; X = S, Se) which contains the kagome lattice of the transition metal ions Ni²⁺. A noteble feature of this material is that the Ni atom sits in the center of four chalcogen atoms thus forms a square planar environment, leading to a low spin state S = 0. A systematic study of the 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 have been indentified as nonmagnetic band insulators. Unexpectedly, Cs₂Ni₃Se₄ shows an insulator-metal transition (IMT) in transport measurements under pressure up to 87.10 GPa without any structural phase transition, evidenced by high-pressure raman spectroscopy, while Rb₂Ni₃S₄ exhibits persistent insulating behavior. Our first-principles calculations suggest the density of states around the Fermi level are primarily dominated by Ni 3d orbitals and the hybridization of the nickel and chalcogen orbitals, thus the occurrence of IMT could be attributed to the bandgap closure by the reduction of Ni-Se bond lengths under pressure.