Direct determination of the crystal field parameters of Dy, Er and Yb   impurities in the skutterudite compound CeFe$_{4}$P$_{12}$ by Electron Spin   Resonance
  Despite extensive research on the skutterudites for the last decade, their electric crystalline field ground state is still a matter of controversy. We show that Electron Spin Resonance (ESR) measurements can determine the full set of crystal field parameters (CFPs) for the Th cubic symmetry (Im3) of the Ce$_{1-x}$R$_{x}$Fe$_{4}$P$_{12}$ (R = Dy, Er, Yb, $x\lesssim 0.003$) skutterudite compounds. From the analysis of the ESR data the three CFPs, B4c, B6c and B6t were determined for each of these rare-earths at the Ce$^{3+}$ site. The field and temperature dependence of the measured magnetization for the doped crystals are in excellent agreement with the one predicted by the CFPs Bnm derived from ESR. 
D. J. Garcia, F. A. Garcia, J. G. S. Duque, P. G. Pagliuso, C.   Rettori, P. Schlottmann, M. S. Torikachvili, and S. B. Oseroff