Site Preference of Rare Earth Doping in Palladium‐Iron‐Arsenide Superconductors

Abstract

The solid solutions (Ca 1-y RE y Fe 1-x Pd x As) 10 Pd z As 8 with RE = La, Ce, and Pr were synthesized by solid state methods and characterized by X-ray powder diffraction with subsequent Rietveld refinements [(CaFeAs) 10 Pt 3 As 8 -type structure ("1038 type"), P1, Z = 1]. Substitution levels (Ca/RE, Fe/Pd, and Pd/ᮀ) obtained from Rietveld refinements coincide well with the nominal values according to EDS and the linear courses of the lattice parameters as expected from the ionic radii. The RE atoms favor the one out of five calcium sites, which is eightfold coordinated by arsenic. This leads to significant stabilization of the structure, and especially prevents palladium over-

Iron arsenide superconductors (CaFeAs)10MnAs8 with metallic interlayers (M = Pt, Pd; n = 3, 4)

“…Starting from this Pd‐1038 parent compound, we were able to synthesize the whole series of RE ‐doped compounds (Ca 1 −x RE x ) 10 (FeAs) 10 Pd 3 As 8 ( RE = La–Lu). We observe the preference of substitution at the eightfold coordinated Ca site similar to the platinum phases . Unfortunately, the RE solubility limit was around 10% only, which is not sufficient for highest T c .…”

Arsenic chemistry of iron-based superconductors and strategy for novel superconducting materials

“…Therefore, when the Fe favoring tetrahedral coordination is mixed with Pt favoring planar square coordination, it appears that, because they do not favor formation of a solid solution in nature, a new compound, 10-4-8-type, is produced. In the same way, compounds including the PdAs 4 planar squares [34][35][36] and IrAs 4 planar squares [37][38][39][40] were discovered. Since Ir 2+ favors tetrahedral coordination rather than planar square coordination, the crystal structure is unstable, and shows a structural phase transition at low temperatures [38,39].…”