  The recently discovered compounds Ca10(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 Ca atoms. We show that high
Tc's above 15 K only emerge if the iron-arsenide layers are at most free of
platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact
Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by
charge doping of pure FeAs layers. We point out, that two different negatively
charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the electrons provided
by the Ca2+ ions, which is unique in the field of iron-based superconductors.
In the parent compound Ca10(FeAs)10(Pt3As8), no excess charge dopes the
FeAs-layer, and superconductivity has to be induced by Pt-substitution, albeit
below 15 K. In contrast, the additional Pt-atom in the Pt4As8layer shifts the
charge balance between the layers equivalent to charge doping by 0.2 electrons
per FeAs. Only in this case Tc raises to 38 K, but decreases again if
additionally platinum is substituted for iron. This charge doping scenario is
supported by our discovery of superconductivity at 30 K in the electron-doped
La-1038 compound (Ca0.8La0.2)10(FeAs)10(Pt3As8) without significant
Pt-substitution.
