We discuss the impact of concomitant substitution of Fe by Mn and La by Y in optimally F-doped ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$. Mn has a known poisoning effect on superconductivity which is particularly strong in the La1111 system, where 0.2% of Mn were reported to completely suppress superconductivity. Through isovalent substitution of La by the much smaller Y we are able to inflict chemical pressure on the structure, which we show is stabilizing the superconducting state, resulting in a drastically larger amount of Mn needed to completely quench superconductivity. Interestingly, we find that the lattice parameter $\mathit{c}$ changes significantly even for small amounts of Mn substitution within a series, which is unexpected taking only the differences between ionic radii into account. We discuss our findings in the light of electron localization caused by small amounts of paramagnetic Mn impurities in ${\mathrm{La}}_{1-y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ also indicated by resistivity and Mößbauer measurements.

• Received 3 November 2017

DOI:https://doi.org/10.1103/PhysRevB.97.054522