Layered Kagome Compound Na_2Ni_3S_4 with Topological Flat Band

We report structural and electronic properties of Na_2Ni_3S_4, a quasi-two-dimensional compound composed of alternating layers of [Ni_3S_4]^2- 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 C_3 + T symmetry in the presence of crystal-field splitting.