Structural Phase Transitions and Superconductivity Induced in Antiperovskite Phosphide CaPd3P

Hidden Structural and Superconducting Phase Induced in Antiperovskite Arsenide SrPd3As

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPd₃As and revealed a hidden structural and superconducting phase in Sr(Pd_1-xPt_x)₃As. The Pd-rich samples (0 ≤ x ≤ 0.2) had the same noncentrosymmetric (NCS) tetragonal structure (a space group of I4₁md) as SrPd₃P. For the samples with 0.3 ≤ x ≤ 0.7, a centrosymmetric (CS) tetragonal structure (P4/nmm) identical to that of SrPt₃P was found to appear, accompanied by superconductivity at a transition temperature (T_c) up to 3.7 K. In the samples synthesized with Pt-rich nominal compositions (0.8 ≤ x ≤ 1.0), Sr₂(Pd,Pt)_8-yAs_1+y with an intergrowth structure (CS-orthorhombic with Cmcm) was crystallized. The phase diagram obtained for Sr(Pd,Pt)₃As was analogous to that of (Ca,Sr)Pd₃P in which superconductivity (T_c ≥ 2 K) occurred in the CS phases induced by substitutions to the NCS phases. This study indicates the potential for further material variation expansion and the importance of elemental substitutions to reveal hidden phases in related antiperovskites.

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.