A suppression of superconductivity in the late rare-earth $R{\text{FeAsO}}_{1-x}{\text{F}}_x$ materials is reported. The maximum critical temperature $\left(T_c\right)$ decreases from 51 K for $R=\text{Tb}$ to 36 K for ${\text{HoFeAsO}}_{0.9}{\text{F}}_{0.1}$, which has been synthesized under 10 GPa pressure. This suppression is driven by a decrease in the Fe-As-Fe angle below an optimum value of $110.6°$, as the angle decreases linearly with unit-cell volume $\left(V\right)$ across the $R{\text{FeAsO}}_{1-x}{\text{F}}_x$ series. A crossover in electronic structure around this optimum geometry is evidenced by a change in sign of the compositional $d{T}_c/dV$, from negative values for previously reported large $R$ materials to positive for ${\text{HoFeAsO}}_{0.9}{\text{F}}_{0.1}$.

• Received 31 July 2009

DOI:https://doi.org/10.1103/PhysRevB.80.052508