High T _c electron doped Ca ₁₀(Pt ₃As ₈)(Fe ₂As ₂) ₅and Ca ₁₀(Pt ₄As ₈)(Fe ₂As ₂) ₅ superconductors with skutterudite intermediary layers

It has been argued that the very high transition temperatures of the highest T _c cuprate superconductors are facilitated by enhanced CuO ₂ plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T _c in the new high T _c iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca ₁₀(Pt ₃As ₈)(Fe ₂As ₂) ₅ (the "10-3-8 phase") and Ca ₁₀(Pt ₄As ₈)(Fe ₂As ₂) ₅ (the "10-4-8 phase"). Based on -Ca-(Pt _nAs ₈)-Ca- Fe ₂As ₂- layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T _c of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T _c to As-Fe-As bond angles does not explain the observed differences in T _c of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt ₄As ₈ layer. The interlayer coupling is thus revealed as important in enhancing T _c in the iron pnictide superconductors.