Coexisting on- and off-center Yb 3+ sites in Ce$_{1}$$_{−}$ x Yb x Fe$_{4}$P$_{12}$ skutterudites F. A. Garcia 1 , D. J. Garcia 2 , M. A. Avila 1 , J. M. Vargas 1 , P. G. Pagliuso 1 , C. Rettori 1 , M. C. G. Passeggi 3 , S. B. Oseroff 4 , P. Schlottmann 5 , B. Alascio 2 , and Z. Fisk 6 $^{1}$Instituto de Física “Gleb Wataghin", UNICAMP, Campinas-SP, 13083-970, Brazil. $^{2}$Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Centro Atómico Bariloche, S.C. de Bariloche, Río Negro, Argentina. $^{3}$INTEC (CONICET and UNL), S3000GLN, Santa Fe, Argentina. $^{4}$San Diego State University, San Diego, California 92182, USA. 5 Department of Physics, Florida State University, Tallahassee, Florida 32306, USA $^{6}$University of California, Irvine, CA, 92697-4573, USA. (Dated: October 31, 2018) Electron Spin Resonance (ESR) measurements performed on the filled skutterudite system Ce 1$_{−}$ x Yb x Fe 4 P 12 ( x ≲ 0 . 003 ) unequivocally reveal the coexistence of two Yb 3+ resonances, associ- ated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe 2 P 3 ) 4 cages that host Yb ions are filled with a low occupation of on-center Yb 3+ sites and a highly occupied T -dependent distribution of off-center Yb 3+ sites. Analysis of the $^{171}$Yb$^{3+}$(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 Yb 3+ in T h symmetry systems and uses the Yb 3+ ESR as a sensitive microscopic probe to investigate the Yb 3+ ions dynamics. The dynamics of guest or filler ions vibrating loosely inside oversized host cages has been a topic of current fo- cus in condensed matter physics. The anomalous behav- iors of these so-called rattler ions raise interest both from the fundamental understanding of the unusual potential wells they are subjected to (and consequent anharmonic- ities in their vibrational motions) as well as from the im- plications of such rattling on the dampening of thermal transport in the material, which invites application per- spectives in the field of thermoelectrics. 1 Thermoelectric materials, which can convert heat into electricity, are of great interest for energy sustainability and energy har- vesting (transformation of waste heat into useful elec- tricity). The main obstacle is the low thermoelectric ef- ficiency of materials for heat to electricity conversion, which is quantified by the thermoelectric figure of merit, $_{ZT}$. The high$_{ZT}$ value is the result of the high Seebeck coefficient and the low thermal conductivity. 2 Among the best-known cage systems displaying such characteristics are the filled skutterudite compounds RT 4 X 12 , where R is a rare earth or actinide, T is a transition metal (Fe, Ru, Os) and X is a pnictogen (P, As, Sb). Besides ex- hibiting a rich variety of ground states and promising thermoeletricity, 3 the question of whether the R ions in these compounds are sited on- and/or off-center in the oversized rigid (T 2 X 3 ) 4 -cages is a matter of intense debate. 4,5 There is also controversy over the extent to which the weakly bounded R ions can be regarded as in- dependent Einstein oscillators, and how effectively they contribute to a phonon-glass type of heat conduction. 6–8 In this work we take advantage of a uniquely favorable conjunction between the chemical and structural charac- teristics of skutterudites and their effect on the Electron Spin Resonance (ESR) of the$_{J}$ = 7 / 2 multiplet of Yb 3+ , to probe this ion’s dynamical behavior within oversized cages of the Ce 1 $_{$_{−}$x}$Yb$_{x}$ Fe 4 P 12 system ($_{x}$ ≲ 0 . $_{003}$). Skutterudites crystallize in the cubic LaFe 4 P 12 struc- ture with space group Im $_{3}$. 9 Each R ion is surrounded by eight transition metal ions forming a cube, and twelve pnictogen ions that form a slightly deformed icosahedron. Our work lies in the fact that the local point symmetry for the R ions is T$_{h}$, which lacks two symmetry operations ($_{C}$ 4 and$_{C}$ ′ 2 rotations) 10 when compared to common cubic structures. Thus, the electric crystal field (CF) Hamilto- nian ($_{H}$$_{CF}$ ) allows for an additional sixth order term with an extra crystal field parameter (CFP),$_{B}$ t $_{6}$. 11,12 This sys- tems present a complex magnetic behavior, and it is es- sential to know their CF level schemes for its complete description. 13,14 ESR is a powerful microscopic tool to provide in- formation about CF effects, site symmetries, valencies of the paramagnetic ions, $_{g}$-values, fine and hyperfine parameters. 12 The ESR of excited states may be also observable, then, by measuring a R-ion ESR at differ- ent frequencies and temperatures, one may obtain CF ground states and, in some cases, the full set of CFP’s that determine the overall splitting of a R-ion$_{J}$-multiplet ground state. 15 Previous works on Ce 1 $_{$_{−}$x}$R$_{x}$ Fe 4 P 12 for R = Nd, Dy, Er, Yb; ( x ≲ 0 . $_{005}$) succeeded in explaining the low-$_{T}$ ESR results using such an expanded$_{H}$$_{CF}$ , and the full set of CFP’s could be determined. 15 However, we now found that in the case of R = Yb, as T increases, a second Yb 3+ resonance emerges from the low-$_{T}$ spec- tra, corresponding to a distinct site, coexisting with the first one. The presence of the new term in the H$_{CF}$ has proven essential to explain the appearance of this second Yb 3+ resonance. The sensitivity of Yb 3+ 4 $_{f}$-electrons to this type of CF environment make it a rare and useful probing ion to help the understanding of the potential well responsible for its motion. I. INTRODUCTION arXiv:1001.3287v1 [cond-mat.str-el] 19 Jan 2010