Control of the superconducting properties of Sr_2−xCa_xVO₃FeAs through isovalent substitution

The effect of the isovalent substitution of Sr²⁺ by Ca²⁺ on the structure and superconducting properties of Sr_2−xCa_xVO₃FeAs is described in the compositional range 0≤x≤0.5. SQUID magnetometry measurements reveal that after an initial increase in T_c, which is maximised at 29.5 K in Sr_1.95Ca_0.05VO₃FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr_2−xCa_xVO₃FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in T_c with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with T_c maximised in samples with regular FeAs₄-tetrahedra. - Graphical abstract: Superconducting transition temperature is controlled by structural parameters in Sr_2−xCa_xVO₃FeAs. - Highlights: • Substitution of Sr by Ca in the superconductor Sr₂VO₃FeAs is isovalent. • Relationship between superconducting T_c and structural parameters is demonstrated. • Linear dependence of T_c on structural parameters rather than composition