Antiperovskite Superconductor LaPd 3 P with Noncentrosymmetric Cubic Structure.

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd ₃ P, following our recent synthesis of A Pd ₃ P ( A = Ca, Sr, Ba). While A Pd ₃ P and (Ca,Sr)Pd ₃ P were found to be tetragonal or orthorhombic systems, LaPd ₃ P is a new prototype cubic system ( a = 9.0317(1) Å) with a noncentrosymmetric space group ( I 4̅3 m ). LaPd ₃ P exhibited superconductivity with a transition temperature ( T _c ) of 0.28 K. The upper critical field, Debye temperature, and Sommerfeld constant (γ) were determined to be 0.305(8) kOe, 267(1) K, and 6.06(4) mJ mol ^-1 K ^-2 f.u. ^-1 , respectively. We performed first-principles electronic band structure calculations for LaPd ₃ P and compared the theoretical and experimental results. The calculated Sommerfeld constant (2.24 mJ mol ^-1 K ^-2 f.u. ^-1 ) was much smaller than the experimental value of γ because the Fermi energy ( E _F ) was located slightly below the density of states (DOS) pseudogap. This difference was explained by the increase in the DOS at E _F due to the approximately 5 atom % La deficiency (hole doping) in the sample. The observed T _c value was much lower than that estimated using the Bardeen-Cooper-Schrieffer equation. To explain the discrepancy, we examined the possibility of an unconventional superconductivity in LaPd ₃ P arising from the lack of space inversion symmetry.