Mixed-Valent NaCu₄Se₃: A Two-Dimensional Metal

The new ternary copper selenide NaCu₄Se₃ crystallizes in the RbCd₄As₃ structure type with the trigonal space group R3m and lattice constants a = 4.0316(4) Å and c = 31.438(8) Å. Its structure is built from two-dimensional slabs of ²/_∞[Cu₄Se₃] separated by Na⁺ cations. The compound is formally mixed-valent with Se^2-/Se^- atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm^-1 at room temperature and a thermopower of ∼10 μV K^-1. Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10²¹ cm^-3 at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the ²/_∞[Cu₄Se₃] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism.