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 × 10(3) 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) × 10(21) cm(-3) and a hole mobility of ∼8.75 cm(2) V(-1) s(-1) at 300 K. NaCu6Se4 exhibits temperature-independent Pauli paramagnetism.