Role of different negatively charged layers in Ca10(FeAs)10(Pt4As8) and superconductivity at 30 K in electron-doped (Ca0.8La0.2)10(FeAs)10(Pt3As8)

The recently discovered compounds Ca-10(Fe1-xPtxAs)(10)(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of calcium atoms. We show that T-c > 15 K only emerges by electron doping of pure FeAs layers, and not by platinum substitution in (Fe1-xPtx) As layers, as anticipated so far. Indeed, two different negatively charged layers [(FeAs)(10)](n-) and (Pt3+yAs8)(m-) compete for the electrons provided by the Ca2+ ions. The charge between the layers is formally balanced to (FeAs)(1-) in the parent compound Ca-10(FeAs)(10)(Pt3As8), and superconductivity emerges by electron doping if this balance is shifted. The latter is achieved either by adding electrons as in (Ca0.8La0.2)(10)(FeAs)(10)(Pt3As8) (T-c = 30 K), or intrinsically in Ca-10(FeAs)(10)(Pt4As8) (T-c approximate to 38 K), where the Pt4As8 layer itself provides extra electrons.