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We report thermodynamic and transport properties, and also theoreticalcalculations, for Cu-based compound Ca2Cu6P5 and compare with CaCu(2-x)P2. Bothmaterials have layers of edge-sharing copper pnictide tetrahedral CuP4, similarto Fe-As and Fe-Se layers (with FeAs4, FeSe4) in the iron-basedsuperconductors. Despite the presence of this similar transition-metal pnictidelayer, we find that both Ca2Cu6P5 and CaCu(2-x)P2 have temperature-independentmagnetic susceptibility and show metallic behavior with no evidence of eithermagnetic ordering or superconductivity down to 1.8 K. CaCu(2-x)P2 is slightlyoff-stoichiometric, with delta = 0.14. Theoretical calculations suggest thatunlike Fe 3d-based magnetic materials with a large density of states (DOS) atthe Fermi surface, Cu have comparatively low DOS, with the majority of the 3dspectral weight located well below Fermi level. The room-temperatureresistivity value of Ca2Cu6P5 is only 9 micro ohm-cm, due to a substantialplasma frequency and an inferred electron-phonon coupling lambda of 0.073(significantly smaller than that of metallic Cu). Also, microscopy result showsthat Cu-Cu distance along the c-axis within the double layers can be very short(2.5 A), even shorter than metallic elemental copper bond (2.56 A). The valueof dp over dT for CaCu(2-x)P2 at 300 K is approximately three times larger thanin Ca2Cu6P5, which suggests the likelihood of stronger electron-phononcoupling. This study shows that the details of Cu-P layers and bonding areimportant for their transport characteristics. In addition, it emphasizes theremarkable character of the DOS of '122' iron-based materials, despite muchstructural similarities.

Metallicity of Ca2Cu6P5 with Single and Double Copper-Pnictide Layers