Superconductivity up to 35 K in the iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 with layered structures

We report the synthesis and crystal structures of three new superconducting
iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 (x = 0-0.15, y = 0-0.4). The
structures are stacking variants of FeAs- and slightly puckered Pt4-yAs8-layers
with square coordinated platinum separated by calcium-layers, respectively.
Arsenic atoms in the Pt4-yAs8-layers form (As2)4- dumbbells according to
Zintl's concept, providing charge balance in (Ca2+Fe2+As3-)10(Pt2+)3[(As2)4-]4.
Superconductivity was observed at 13-35 K. We suggest that the highest Tc above
30 K occurs in the 1048 phase with clean FeAs-layers that are indirectly
electron-doped according to (Ca2+Fe2+As3-)10(Pt2+)4[(As2)4-]4*2e-. We also
suggest that the lower critical temperatures occur in the 1038- and a-1048
phases due to Pt-doping at the Fe-site. DFT band structure calculations
indicate that the contribution of the Pt4-yAs8-layers to the Fermi surface is
small and that the Fermi energy is slightly either below or above a quasi-gap
in the Pt-states. The new platinum-iron compounds represent the first
iron-based superconductors with so far unknown structure types and can serve as
a new platform for further studies that go beyond the known systems.