Superconductivity at 35 K by self doping in RbGd 2 Fe 4 As 4 O 2 .

We report synthesis, crystal structure and physical properties of a novel quinary compound RbGd2 Fe4 As4 O2 . The new iron oxyarsenide is isostructural to the fluo-arsenide KCa2 Fe4 As4 F2 , both of which contain separate double Fe2 As2 layers that are self hole-doped in the stoichiometric composition. Bulk superconductivity at [Formula: see text] K is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility and heat capacity. An exceptionally high value of the initial slope of the upper critical field ([Formula: see text]d[Formula: see text]/d[Formula: see text] [Formula: see text] T K-1 ) is measured for the polycrystalline sample.