Suppression of the superconducting transition of R FeAsO1-x Fx (R=Tb, Dy, and Ho)

A suppression of superconductivity in the late rare-earth R FeAsO1-x Fx materials is reported. The maximum critical temperature (Tc) decreases from 51 K for R=Tb to 36 K for HoFeAsO0.9 F0.1, 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 (V) across the R FeAsO1-x Fx series. A crossover in electronic structure around this optimum geometry is evidenced by a change in sign of the compositional d Tc /dV, from negative values for previously reported large R materials to positive for HoFeAsO0.9 F0.1. © 2009 The American Physical Society.