Comparison of crystal structures and effects of Co substitution in a new member of Fe-1111 superconductor family AeFeAsF(Ae = Ca and Sr): a possible candidate for higher Tc superconductor

TL;DR

This study refines the crystal structures of CaFeAsF and SrFeAsF with Co substitution, revealing structural changes and phase transition behaviors that suggest CaFeAsF as a promising candidate for higher-temperature superconductivity.

Contribution

The paper provides detailed structural analysis of Co-doped CaFeAsF and SrFeAsF, linking atomic radii mismatches to tetrahedral distortions and phase transition suppression, proposing CaFeAsF as a potential high-Tc superconductor.

Findings

Structural phase transitions occur at ~120 K for CaFeAsF and ~180 K for SrFeAsF.

Co substitution suppresses structural phase transitions below 100 K.

Larger Sr radius flattens FeAs4 tetrahedron, smaller Ca radius makes it more regular.

Abstract

We refined crystal structures of newly found members of the Fe-1111 superconductor family, CaFe\_{1-x}Co\_{x}AsF and SrFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) by powder synchrotron X-ray diffraction analysis. The tetragonal to orthorhombic phase transitions were observed at ~120 K for unsubstituted CaFeAsF and at ~180 K for unsubstituted SrFeAsF, the transition temperatures agreeing with kinks observed in temperature-dependent resistivity curves. Although the transition temperature decreases, the structural phase transitions were observed below 100 K in both samples of x = 0.06, and finally they were suppressed in the doping level of x = 0.12. The refined structures reveal that distortions of the FeAs4 tetrahedron from the regular tetrahedron likely originate from mismatches in atomic radii among the constituent elements. In this system, the enlarged FeAs4 tetrahedron resulting from larger radius of Sr than that of Ca is flattened along a-b plane, whereas the smaller radius of Ca makes the tetrahedron closer to regular one, and their characteristic shapes are further enhanced by Co substitution. These results suggest that the CaFeAsF compound is a promising candidate for higher-Tc superconductor.