Recently found superconductor (Ca_1-xSr_x)Pd₃P has an interesting structural phase transition [1]. It has an antiperovskite structure in all regions of Sr concentration x, but has inversion symmetry in the region where x is small and no inversion symmetry in the region where x is large. Its T_c is large in the former (~3.5 K at x=0.4) and small in the latter (~0.7 K at x=0.75). The lack of inversion symmetry in antiperovskite superconductor reminds us the pioneer work for CePt₃P by Bauer et al [2]. On the other hand, the lack of inversion symmetry may not so important in other anti-perovskite-related superconductors Li₂Pd₃P and Mg_1.9Rh₃P [3-5].

In this paper we report the results of the first-principles calculations for CaPd₃P, using the experimental crystal structure of (Ca_0.6Sr_0.4)Pd₃P and (Ca_0.25Sr_0.75)Pd₃P. We found that the density of states at the Fermi level per CaPd₃P (=D(E_F)) is large (=2.65 eV^-1) in the former and small (=1.35 eV^-1) in the latter.

Moreover, the electronic specific heat calculated from D(E_F) well agrees with the experimental result [1]. This result strongly suggests that the large change of T_c is mainly caused by the large change of D(E_F).

 [1] A. Iyo et al. in preparation.
[2] E. Bauer et al. Phys. Rev. Lett. 92 (2004) 027003.
[3] K. Togano et al. Phys. Rev. Lett. 93 (2004) 247004.
[4] A. Iyo et al. Phys. Rev. Mater. 3 (2019) 124802.
[5] I. Hase et al. J. Phys.: Conf. Ser. 1293 (2019) 012028.

Fig. Density of states curves of CaPd₃P with the crystal structure of (a) (Ca_0.6Sr_0.4)Pd₃P and (b) (Ca_0.25Sr_0.75)Pd₃P. Unit of the vertical axis is eV^-1 for the corresponding unit cell.

Keywords: (Ca,Sr)Pd3P, band structure calculation, superconductivity, structural phase transition