NaBa2Cu3S5: A Doped p‑Type Degenerate Semiconductor

Mixed S2–/S1– oxidation states have been discovered in the new quaternary compound NaBa2Cu3S5. Synthesized from the reaction of Cu in a molten alkali metal/polysulfide flux, the compound crystallizes in monoclinic space group C2/m with a = 16.5363(7) Å, b = 5.5374(5) Å, c = 10.3717(10) Å, β = 98.535(8)°. The Na+ Ba2+2 [Cu+3S3]3–S22– crystal structure contains layers of edge sharing CuS4 tetrahedra and sheets of S22– dimers. These layers are separated by mixed Ba/Na cation layers. The conductivity of the single crystals of NaBa2Cu3S5 is ∼450 S cm–1 at room temperature, and increasing conductivity with decreasing temperature is observed, indicating metallic behavior despite the optical band gap of 0.45 eV. A small positive thermopower (45–55 μV K–1 from 300 K to 500 K) and Hall effect measurements also confirm p-type conductivity with carrier concentration at 200 K of ∼1.6 × 1021 cm–3 and a hole mobility of ∼2 cm2 V–1 s–1. NaBa2Cu3S5 exhibits temperature-independent Pauli paramagnetism.