New superconductor SrPt₂Ge₂ with T_c = 10.2 K

Contrast to superconducting compounds SrNi₂Ge₂ (T_c = 1.1 K) and SrPd₂Ge₂ (T_c= 3.2 K) with the BaFe₂-As₂-type body-center-tetragonal (122-bct) layer structure (space group I4/mmm), SrPt₂Ge₂ crystallized in a pseudo-tetragonal, monoclinic structure with space group P21 and lattice parameters a = 0.4401(4) nm< b = 0.4435(4) nm, c = 0.9764(10) nm and β = 90.5° >90°. Superconductivity with T_c=10.2K was observed in this new compound. However, a weak diamagnetic shielding signal of -3.2 × 10 ² cm³/mol at 2K indicates that superconductivity is not from the bulk monoclinic phase but from a minor metastable 122-bct high-temperature phase with a ∼ 0.44 nm, c ∼ 0.98 nm, β = 90°. This is the highest T_c reported outside the iron-based layer compounds with the 122-bct structure. High-T_c superconductivity is due to strong quasi-2D Pt-Ge-Pt 5d _{xz, yz}-4p-5d_{xz, yz} hybridization in the Pt-Ge layer with squeezed PtGe4 tetrahedral crystal field and Pt-5d⁸ spin-orbital interaction in the metastable 122-bct phase and is another example of multi-band (d_xz/dy_z/d_xy) non-conventional s-wave superconductor.