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 non-magnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare earth magnetism. 
E. A. Goremychkin, R. Osborn, C. H. Wang, M. D. Lumsden, M. A.   McGuire, A. S. Sefat, B. C. Sales, D. Mandrus, H. M. R{\o}n...
cond-mat.supr-con