# Gd 3+ rattling triggered by a “weak” M-I transition at 140-160 K in the Ce$_{1}$$_{−}$ x Gd x Fe$_{4}$P$_{12}$ x ≈ 0 . 001 skutterudite compounds: an ESR study. 

F. A. Garcia $^{1}$, J. G. S. Duque$^{2}$, P. G. Pagliuso$^{1}$, C. Rettori$^{1}$, Z. Fisk$^{3}$, and S. B. Oseroff$^{4}$. 

$^{1}$Instituto de Física “Gleb Wataghin”, C. P. 6165, UNICAMP, Campinas-SP, 13083-970, Brasil. 

$^{2}$Núcleo de Física, UFS, 49500-000, Itabaiana, SE, Brasil 

$^{3}$University of California, Irvine, CA, 92697-4573, USA. 

$^{4}$San Diego State University, San Diego, California 92182, USA. 

In this work we report electron spin resonance (ESR) measurements in the semiconducting Ce$_{1}$$_{−}$$_{x}$Gd$_{x}$Fe$_{4}$P$_{12}$ ( x ≈ 0 . 001 ) filled skutterudite compounds. Investigation of the temperature ( T ) dependence of the ESR spectra and relaxation process suggests, that in the T -interval of 140 − 160 K, the onset of a “weak” metal-insulator (M-I) transition takes place due to the increasing density of thermally activated carriers across the semiconducting gap of ≈ 1500 K. In addition, the observed low- T fine and hyperfine structures start to collapse at T ≈ 140 K and is completely absent for T ≳ 160 K. We claim that the increasing carrier density is able to trigger the rattling of the Gd 3+ ions which in turn is responsible, via a motional narrowing mechanism, for the collapse of the ESR spectra. 

The filled skutterudite RT 4 X 12 compounds, where R is a rare-earth or actinide, T is a transition metal (Fe, Ru, Os) and X is a pnictogen (P, As, Sb) have attracted great attention due to their broad range of physical prop- erties. They are of interest for those seeking more effi- cient thermoelectric materials 1,2 and also for those inves- tigating the basic issues of strongly correlated electron systems $^{3–5}$. 

These compounds crystallize in the LaFe 4 P 12 struc- ture with space group Im 3 and local point symmetry T$_{h}$ for the R ions. The R ions are guests in the oversized rigid cages constituted by the (T 2 X 3 ) 4 atoms $^{6}$. The dy- namics of these guests R ions is believed to be of great importance in dampening the thermal conductivity 7,8 as observed in the filled compounds of this family and it may also play an important role on the appearance of heavy fermion behavior and superconductivity $^{3,9}$. 

Electron spin resonance (ESR) is a sensitive and power- ful microscopic tool to provide information about crystal field (CF) effects, site symmetries, valencies of paramag- netic ions, g -values, fine and hyperfine parameters $^{10}$. In our recent work $^{11}$, ESR was found to be a useful tool to probe the dynamics of the R ions. The T -dependence of the guest R ions localization lead the ions to experi- ence slightly different symmetry environments causing a distribution of g -values that was detected by our ESR experiment. In addition, the remarkable reduction of the hyperfine parameters observed in the ESR spectra was attributed to a motional narrowing mechanism 12 caused by the rattling of the R ions. In our previous ESR exper- iments on Yb 3+ impurities in Ce 1$_{−}$ $_{x}$Yb$_{x}$ Fe 4 P 12 $^{11}$, these two effects were analyzed and the coexistence of two dis- tinct sites were determined. 

Ogita et al $^{13}$, performing Raman scattering exper- iments on several metallic skutterudite compounds of RT 4 X 12 (T = Fe,Ru,Os; X = P,Sb), found resonant 2nd order phonon modes associated with the vibration 

## I. INTRODUCTION 

that change the bond length of R-X (stretching mode). However, in the semiconducting CeFe 4 P 12 the 2nd order phonon modes were found to be non resonant. Based on these results the authors conclude that there should be a strong coupling between the R-X stretching modes and the conduction electrons. Thus, in CeFe 4 P 12 a weak stretching mode-conduction electron coupling should be expected. 

In this work, to further investigate these ideas, we have studied the T -evolution of the ESR spectra of the Gd $^{3+}$ion in the Ce 1$_{−}$ $_{x}$Gd$_{x}$ Fe 4 P 12 ( x ≈ 0 . 001 ) filled skut- terudite compounds. 

Single crystals of Ce 1$_{−}$ $_{x}$Gd$_{x}$ Fe 4 P 12 ( x ≲ 0 . 001 ) were grown in Sn-flux as described in Ref. $^{14}$. The cubic struc- ture ( Im 3 ) and phase purity were checked by x-ray pow- der diffraction. The Gd concentrations were determined from the H and T -dependence of the magnetization, M ( H, T ) , measured in a Quantum Design SQUID dc - magnetometer. The ESR experiments used crystals of ∼ 2 x 2 x 2 mm 3 of naturally grown crystallographic faces. The ESR spectra were taken in Bruker X ( 9 . 48 GHz)- band spectrometer using appropriated resonators cou- pled to a T -controller of a helium gas flux system for 4 . 2 K ≲ T ≲ 300 K. 

The low- T ESR spectra show the full fine structure of Gd $^{3+}$and its angular variation was studied at T = 4 . 2 K, T = 20 K and T = 300 K with the applied field, H , in the (110) plane. The experimental arrangement was set up in a way that θ = 0 corresponds to the [001] direction. The complete results of this work will be the subject of a more extended manuscript $^{15}$. In this work we will dis- cuss measurements in the T -interval 110 K ≲ T ≲ 160 K for various directions of H . In this interval, the fine structure collapses in one single broad line and its line- shape changes from Lorentzian (insulator) to Dysonian 

## II. EXPERIMENTAL 

