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.