Spatial inhomogeneity in RFeAsO1-xFx (R = Pr, Nd) determined from rare-earth crystal-field excitations

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of T-c compared to LaFeAsO1-xFx is not due to rare-earth magnetism.