Superconducting and calorimetric properties of the metastable palladium borocarbide ThPd₂B₂C

A broad superconducting transition with bulk T_c of 14 K and a diamagnetic onset as high as 20 K was observed for the metastable, as-melted ThPd₂B₂C sample from magnetic studies. Low temperature specific heat data yield a bulk T_c of 14 K, an electronic-term coefficient 7 of 12 mJ/mol K² and a Debye temperature θ_D of 320 K. The systematic variation of T_c for the nonmagnetic quaternary borocarbides RT₂B₂C (R = Sc, Y, Lu, La, Th; T = Ni, Pd, Pt) is correlated with the electronic-term coefficient γ and the transition metal T-T nearest-neighbor in-plane distance. The variation of the transitional metal 3d/4d/5d-dominated conduction band and thus the Fermi-level density of states N(E_F) is believed to be the determining factors for T_c variation of these borocarbides.