Electronic band structure, Fermi surface, and elastic properties of polymorphs of the 5.2 K iron-free superconductor SrPt_2As_2 from first-principles calculations

摘要

By means of first-principles calculations, we studied in detail the structural, elastic, and electronic properties of the tetragonal CaBe_2Ge_2-type 5.2 K superconductor SrPt_2As_2 in comparison with two hypothetical SrPt_2As_2 polymorphs with ThCr_2Si_2-type structures, which differ in the atomic configurations of the [Pt_2As_2] (or [As_2Pt_2]) blocks. We found that CaBe_2Ge_2-type SrPt_2As_2 is a unique system with near-Fermi bands of a complicated character and an "intermediate"-type Fermi surface, which consists of electronic pockets having a cylinderlike [two-dimensional (2D)] topology (typical of 122 FeAs phases) together with 3D-like electronic and hole pockets, which are characteristic of ThCr_2Si_2-like iron-free low-T_c superconductors. Our analysis revealed that, as distinct from ThCr_2Si_2-like 122 phases, other features of CaBe_2Ge_2-like SrPt_2As_2 are as follows: (1) There are essential differences in the contributions from [Pt_2As_2] and [As_2Pt_2] blocks to the near-Fermi region; conduction is anisotropic and occurs mainly in the [Pt_2 As_2] blocks. (2) A 3D system of strong covalent Pt-As bonds is formed (inside and between [Pt_2As_2] and [As_2Pt_2] blocks), which is responsible for enhanced stability of this polymorph. (3) There is essential charge anisotropy between adjacent [Pt_2As_2] and [As_2Pt_2] blocks. We also predict that CaBe_2Ge_2-like SrPt_2As_2 is a mechanically stable and relatively soft material with high compressibility, which will behave in a ductile manner. In contrast, the ThCr_2Si_2-type SrPt_2As_2 polymorphs, which contain only [Pt_2 As_2] or [As_2Pt_2] blocks, are less stable, have Fermi surfaces of a multisheet three-dimensional type like the ThCr_2Si_2-like iron-free 122 phases, and therefore will be ductile materials with high elastic anisotropy. Based on our data for the three simplest SrPt_2 As_2 polymorphs we assume that there may exist a family of higher-order polytypes, which can be formed as a result of various stackings of the two main types of building blocks ([Pt_2As_2] and [As_2Pt_2]) in various combinations along the z axis. This may provide an interesting platform for further theoretical and experimental search for other superconducting materials.