Abstract
Metastable filamentary superconductivity with T <sub>c</sub> = 10.2 K was observed in SrPt <sub>2</sub> Ge <sub>2</sub> . This is the highest T <sub>c</sub> reported outside the iron-based 122-layer compounds. The stable phase for this compound is a non-superconducting monoclinic LaPt <sub>2</sub> Ge <sub>2</sub> -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 BaFe <sub>2</sub> As <sub>2</sub> -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 <sub>1-x</sub> Pt <sub>x</sub> ) <sub>2</sub> Ge <sub>2</sub> suggests that the stable bct to monoclinic structure phase transition occurs near x = 0.15. For x < 0.15 with bct phase, bulk T <sub>c</sub> increases from 3.2 K for SrPd <sub>2</sub> Ge <sub>2</sub> to 3.6 K for SrPd 1.8 Pt 0.2 Ge <sub>2</sub> . For x > 0.15 with metastable filamentary bct phase, T <sub>c</sub> increases from 7.5 K for x = 0.5 to 9.2 K for x = 0.7 and reaches a T <sub>c</sub> maximum of 10.2 K for SrPt <sub>2</sub> Ge <sub>2</sub> . High T <sub>c</sub> is due to a strong quasi-2D Pt-Ge-Pt 5d <sub>xz,yz</sub> -4p-5d <sub>xz,yz</sub> hybridization in the Pt-Ge layer with squeezed-along-c-axis PtGe <sub>4</sub> tetrahedral crystal field and Pt-5d <sup>8</sup> spin-orbital interaction in the metastable filamentary bct phase and is another example of multi-band (d <sub>xz</sub> /d <sub>yz</sub> /d <sub>xy</sub> ) non-conventional s-wave superconductor. © 2013 AIP Publishing LLC.