Antiperovskite Superconductor LaPd₃P with Noncentrosymmetric Cubic Structure

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd₃P, following our recent synthesis of APd₃P (A = Ca, Sr, Ba). While APd₃P and (Ca,Sr)Pd₃P were found to be tetragonal or orthorhombic systems, LaPd₃P is a new prototype cubic system (a = 9.0317(1) Å) with a noncentrosymmetric space group (I4̅3m). LaPd₃P 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 LaPd₃P 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 LaPd₃P arising from the lack of space inversion symmetry.