NaCu<sub>6</sub>Se<sub>4</sub>: A Layered Compound with Mixed Valency and Metallic Properties

A new ternary compound NaCu<sub>6</sub>Se<sub>4</sub> was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group <i>R</i>3̅<i>m</i> with <i>a</i> = 4.0465(3) Å and <i>c</i> = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of <sup>1</sup>/<sub>∞</sub>[Cu<sub>6</sub>Se<sub>4</sub>] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 10<sup>3</sup> S cm<sup>–1</sup> at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4–11 μV K<sup>–1</sup> from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 10<sup>21</sup> cm<sup>–3</sup> and a hole mobility of ∼8.75 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> at 300 K. NaCu<sub>6</sub>Se<sub>4</sub> exhibits temperature-independent Pauli paramagnetism.