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

We refined crystal structures of newly found members of the Fe-1111 superconductor family, CaFe1-xCoxAsF and SrFe1-xCoxAsF (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 are likely to originate from mismatches in atomic radii among the constituent elements. In this system, the enlarged FeAs4 tetrahedron resulting from the fact that the radius of Sr is larger than that of Ca, is flattened along a-b plane, whereas the smaller radius of Ca makes the tetrahedron closer to a regular one, their characteristic shapes are further enhanced by Co substitution. These results suggest that the CaFeAsF compound is a promising candidate for a higher- Tc superconductor.