  Electron Spin Resonance (ESR) measurements performed on the filled
skutterudite system Ce1-x$YbxFe4P12 (x< 0.003) unequivocally reveal the
coexistence of two Yb3+ resonances, associated with sites of considerably
different occupations and temperature behaviors. Detailed analysis of the ESR
data suggests a scenario where the fraction of oversized (Fe2P3)4 cages that
host Yb ions are filled with a low occupation of on-center Yb3+ sites and a
highly occupied T-dependent distribution of off-center Yb3+ sites. Analysis of
the 171Yb3+ (I=1/2) isotope hyperfine splittings reveal that these two sites
are associated with a low (~ 1 GHz) and a high (> 15 GHz) rattling frequency,
respectively. Our findings introduce Yb3+ in Th symmetry systems and uses the
Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions
dynamics.
