Two-dimensional metal NaCu6.3Sb3 and solid-state transformations of sodium copper antimonides

Abstract

A novel layered compound, NaCu_6.3Sb₃, has been successfully synthesized from elements. This unique structure crystallizes in the hexagonal space group P6₃/mmc and has lattice constants of a = 4.2166(2) Å and c = 24.041(1) Å. The structure consists of complex Cu/Sb 2D blocks separated by Na ions. These blocks contain graphene-like hexagonal layers of either Cu or CuSb. The solid-state transformations in the Na–Cu–Sb system between Cu₂Sb–NaCu_6.3Sb₃–NaCu₄Sb₂ were explored using annealing at different partial vapor pressures of Na, differential scanning calorimetry, and in situ synchrotron powder X-ray diffraction. Full reversibility of the transformation was observed, indicating that the bulk 3D Cu₂Sb phase is capable of reversible intercalation of Na ions, forming layered intercalated phases similar to intercalated graphite. The characterization of the transport properties shows that the metallic nature of the electrical conductivity is preserved, even for the Na-rich phase of NaCu₄Sb₂. Electronic structure calculations support metallic properties in the Cu–Sb layers and predict that no bands cross the Fermi level across the layers, supporting NaCu_6.3Sb₃ as a two-dimensional metal.