Layered kagome compound Na₂Ni₃S₄ with topological flat band

We report structural and electronic properties of Na₂Ni₃S₄, a quasi-two-dimensional compound composed of alternating layers of Ni₃S₄²⁻ and Na⁺. The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom. Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV. Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated d_xz-orbital with C₃ + T symmetry in the presence of crystal-field splitting.