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) angstrom and c = 31.438(8) angstrom. Its structure is built from two-dimensional slabs of 2/infinity [Cu4Se3] separated by Na+ cations. The compound is formally mixed-valent with Se2-/Se- atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of similar to 300 S cm(-1) at room temperature and a thermopower of similar to 10 mu V K-1. Hall effect measurements indicate holes as the dominant carrier with a concentration of similar to 6.12(1) X 10(21) cm(-3) at 300 K. Density functional theory electronic structure calculations indicate p -type metallic behavior for the 2/infinity [Cu4Se3] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism.