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-T_c superconductor

We refined crystal structures of newly found members of the Fe-1111 superconductor family, CaFe_1-xCo_xAsF and SrFe _1-xCo_xAsF (x = 0,0.06,0.12) by powder synchrotron x-ray diffraction analysis. The tetragonal to orthorhombic phase transitions were observed at ∼120K for unsubstituted CaFeAsF and at ∼180K 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 100K 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- T_c superconductor.