Superconductivity up to 10 K in the Sr(Pd_1-xPt_x) ₂Ge₂ layer system

Metastable filamentary superconductivity with T_c = 10.2 K was observed in SrPt₂Ge₂. This is the highest T_c reported outside the iron-based 122-layer compounds. The stable phase for this compound is a non-superconducting monoclinic LaPt₂Ge₂-type structure with lattice parameters a = 0.4401(4) nm, b = 0.4435(4) nm, c = 0.9764(10) nm, β = 90.5°-90°. Weak diamagnetic shielding signal indicates that superconductivity is not from the major monoclinic phase, but from the minor BaFe₂As₂-type body-center-tetragonal (122-bct) phase with a = 0.44 nm, c = 0.98 nm, β = 90 °. Investigation on the pseudoternary system Sr(Pd_1-xPt_x) ₂Ge₂ suggests that the stable bct to monoclinic structure phase transition occurs near x = 0.15. For x < 0.15 with bct phase, bulk T_c increases from 3.2 K for SrPd₂Ge₂ to 3.6 K for SrPd 1.8 Pt 0.2 Ge₂. For x > 0.15 with metastable filamentary bct phase, T_c increases from 7.5 K for x = 0.5 to 9.2 K for x = 0.7 and reaches a T_c maximum of 10.2 K for SrPt₂Ge ₂. High T_c is due to a strong quasi-2D Pt-Ge-Pt 5d _xz,yz-4p-5d_xz,yz hybridization in the Pt-Ge layer with squeezed-along-c-axis PtGe₄ tetrahedral crystal field and Pt-5d ⁸ spin-orbital interaction in the metastable filamentary bct phase and is another example of multi-band (d_xz/d_yz/d _xy) non-conventional s-wave superconductor.