Metallicity of Ca₂Cu₆P₅ with single and double copper-pnictide layers

We report thermodynamic and transport properties, and also theoretical calculations, for Cu-based compound Ca₂Cu₆P₅ and compare with CaCu_2-δP₂. Both materials have layers of edge-sharing copper pnictide tetrahedral CuP₄, similar to Fe–As and Fe–Se layers (with FeAs₄, FeSe₄) in the iron-based superconductors. Despite the presence of this similar transition-metal pnictide layer, we find that both Ca₂Cu₆P₅ and CaCu_2-δP₂ have temperature-independent magnetic susceptibility and show metallic behavior with no evidence of either magnetic ordering or superconductivity down to 1.8 K CaCu_2-δP₂ is slightly off-stoichiometric, with δ = 0.14. Theoretical calculations suggest that unlike Fe 3d-based magnetic materials with a large density of states (DOS) at the Fermi surface, Cu have comparatively low DOS, with the majority of the 3d spectral weight located well below Fermi level. The room-temperature resistivity value of Ca₂Cu₆P₅ is only 9 μΩ-cm, due to a substantial plasma frequency and an inferred electron-phonon coupling λ of 0.073 (significantly smaller than that of metallic Cu). Also, microscopy result shows that Cu–Cu distance along the c-axis within the double layers can be very short (2.5 Å), even shorter than metallic elemental copper bond (2.56 Å). The value of dρ/dT for CaCu_2-δP₂ at 300 K is approximately three times larger than in Ca₂Cu₆P₅, which suggests the likelihood of stronger electron-phonon coupling. This study shows that the details of Cu–P layers and bonding are important for their transport characteristics. In addition, it emphasizes the remarkable character of the DOS of '122' iron-based materials, despite much structural similarities. - Highlights: • A comprehensive study on Cu-based compound Ca₂Cu₆P₅ and compare with CaCu_2-δP₂. • Both materials have layers of edge-sharing CuP₄ tetrahedral. • Ca₂Cu₆P₅ and CaCu_2-δP₂ have comparatively low Fermi-level density-of-states. • Our results indicate the details of structures dictate characteristics of materials.