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 R3¯m 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 V 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.