Superconductivity up to 35 K in the iron platinum arsenides (CaFe(1-x)Pt(x)As)10Pt(4-y)As8 with layered structures.

The discovery of high-Tc superconductivity in iron arsenides in 2008 has arguably been the biggest breakthrough in this field since the appearance of the copper oxide superconductors in 1986. In iron arsenides, superconductivity up to 55 K originates in layers of edge-sharing [FeAs4/4]-tetrahedra. Meanwhile, a series of different structure types have been identified, but the family of superconducting iron arsenide compounds is still small in comparison with the cuprates. Its members are mainly derivatives of the relatively simple and long known anti-PbFCland ThCr2Si2-type structures. Thus extending the crystal chemistry of iron based superconductors is a foremost task of solid state chemistry. Compounds like Sr2VO3FeAs with thick perowskite-like oxideblocks between the FeAs-layers were derived from known copper sulphides and showed superconductivity up to 37 K. However, the combination of the metallic iron arsenide layers with transition metal oxides caused difficulties. [8, 9] Another approach is the combination of iron arsenide layers with other intermetallic building blocks. We considered the fact that a second transition metal should be one that can adopt coordination geometries other than tetrahedral. Platinum appeared promising, because Pt is known to be very flexible and forms arsenides with octahedral, tetrahedral, trigonal and square coordination in compounds like PtAs2, SrPt2As2, SrPtAs, and Cs2PtAs2, respectively. Recently, Nohara et al. mentioned superconductivity in the system Ca-Fe-Pt-As, but the structure and composition of the compound have not been reported. With these points in mind, we started explorative syntheses in the system Ca-Fe-Pt-As and found three new platinum-iron arsenides (CaFe1xPtxAs)10Pt4As8 (CaFe1xPtxAs)10Pt3As8 and (CaFeAs)10Pt4As8. These compounds crystallize in so far unknown structure types, where iron arsenide and platinum arsenide layers alternate. We have detected superconductivity up to 35 K, which is probably either induced by Ptdoping of the FeAs-layers in (CaFe1xPtx)10Pt3As8 or by indirect electron doping in (CaFeAs)10Pt4As8 due to additional Pt in the platinum arsenide layers. However, the concrete phase relationships are not yet completely resolved. In this communication, we report the synthesis, crystal structures, preliminary property measurements and DFT calculations of these new superconductors. The polycrystalline samples were mostly inhomogeneous and contained plate-shaped and well as needle-shaped crystals with metallic lustre. X-ray powder patterns could initially not be indexed, and the plate-like crystals easily exfoliated. Their diffraction patterns showed clean square motifs, but a disturbed periodicity of the third dimension which indicated stacking disorder. Only some crystals were of fairly sufficient quality for x-ray structure determinations. Finally, we found three different, but closely related crystal structures, whose compositions and lattice parameters are compiled in Table 1. The rather high R-values reflect the poor crystal quality and still not completely resolved twinning and/or intergrowth issues. For detailed crystallographic data we refer to the supplementary material.