NaCu6Se4: A Layered Compound with Mixed Valency and Metallic Properties

A new ternary compound NaCu6Se4 was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group R3̅m with a = 4.0465(3) Å and c = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of 1/∞[Cu6Se4] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 103 S cm–1 at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4–11 μV K–1 from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 1021 cm–3 and a hole mobility of ∼8.75 cm2 V–1 s–1 at 300 K. NaCu6Se4 exhibits temperature-independent Pauli paramagnetism.