Coexisting on- and off-center Yb3+ sites in Ce1-xYbxFe4P12 skutterudites

0.28 MB·English

by  F. A. Garcia

Coexisting on- and off-center Yb3+ sites in Ce Yb Fe P skutterudites
1−x x 4 12
F. A. Garcia1, D. J. Garcia2, M. A. Avila1, J. M. Vargas1, P. G. Pagliuso1, C. Rettori1,
M. C. G. Passeggi3, S. B. Oseroff4, P. Schlottmann5, B. Alascio2, and Z. Fisk6
1Instituto de Física “Gleb Wataghin", UNICAMP, Campinas-SP, 13083-970, Brazil.
2Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
and Centro Atómico Bariloche, S.C. de Bariloche, Río Negro, Argentina.
3INTEC (CONICET and UNL), S3000GLN, Santa Fe, Argentina.
4San Diego State University, San Diego, California 92182, USA.
5 Department of Physics, Florida State University, Tallahassee, Florida 32306, USA
6University of California, Irvine, CA, 92697-4573, USA.
0 (Dated: January 19, 2010)
1
0 Electron Spin Resonance (ESR) measurements performed on the filled skutterudite system
2 Ce1−xYbxFe4P12 (x.0.003) unequivocally reveal the coexistence of two Yb3+ resonances, associ-
ated with sites of considerably different occupations and temperature behaviors. Detailed analysis
n
a 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
J
distribution of off-center Yb3+ sites. Analysis of the 171Yb3+(I=1/2) isotope hyperfine splittings
9 reveal that these two sites are associated with a low (∼ 1 GHz) and a high (& 15 GHz) rattling
1
frequency, respectively. Our findings introduce Yb3+ in Th symmetry systems and uses the Yb3+
ESR as a sensitive microscopic probe to investigate theYb3+ ions dynamics.
]
l
e
-
r I. INTRODUCTION to probe this ion’s dynamical behavior within oversized
t
s cages of the Ce1−xYbxFe4P12 system (x.0.003).
.
at The dynamics of guest or filler ions vibrating loosely Skutterudites crystallize in the cubic LaFe4P12 struc-
m insideoversizedhostcageshasbeenatopicofcurrentfo- ture with space group Im3.9 Each R ion is surrounded
byeighttransitionmetalionsformingacube,andtwelve
- cus in condensed matter physics. The anomalousbehav-
d iorsoftheseso-calledrattler ionsraiseinterestbothfrom pnictogenionsthatformaslightlydeformedicosahedron.
n Our work lies in the fact that the local point symmetry
the fundamental understanding of the unusual potential
o fortheRionsisT ,whichlackstwosymmetryoperations
wellstheyaresubjectedto(andconsequentanharmonic- h
[c ities intheir vibrationalmotions)aswellasfromthe im- (C4andC2′ rotations)10whencomparedtocommoncubic
plications of such rattling on the dampening of thermal structures. Thus,the electriccrystalfield(CF) Hamilto-
1 nian(H )allowsforanadditionalsixthordertermwith
transport in the material, which invites application per- CF
v spectives in the field of thermoelectrics.1 Thermoelectric anextracrystalfieldparameter(CFP),B6t.11,12 Thissys-
7
tems present a complex magnetic behavior, and it is es-
8 materials, which can convert heat into electricity, are of
sential to know their CF level schemes for its complete
2 great interest for energy sustainability and energy har-
description.13,14
3 vesting (transformation of waste heat into useful elec-
. tricity). The main obstacle is the low thermoelectric ef- ESR is a powerful microscopic tool to provide in-
1
0 ficiency of materials for heat to electricity conversion, formation about CF effects, site symmetries, valencies
0 which is quantifiedby the thermoelectric figure ofmerit, of the paramagnetic ions, g-values, fine and hyperfine
1 ZT. The highZT value is the resultofthe highSeebeck parameters.12 The ESR of excited states may be also
: coefficientandthelowthermalconductivity.2 Amongthe observable, then, by measuring a R-ion ESR at differ-
v
i best-known cage systems displaying such characteristics ent frequencies and temperatures, one may obtain CF
X
are the filled skutterudite compounds RT4X12, where R ground states and, in some cases, the full set of CFP’s
r is a rare earth or actinide, T is a transition metal (Fe, thatdeterminetheoverallsplittingofaR-ionJ-multiplet
a
Ru, Os) and X is a pnictogen (P, As, Sb). Besides ex- groundstate.15 PreviousworksonCe1−xRxFe4P12 forR
hibiting a rich variety of ground states and promising = Nd, Dy, Er, Yb; (x . 0.005) succeeded in explaining
thermoeletricity,3 the question of whether the R ions thelow-T ESRresultsusingsuchanexpandedH ,and
CF
in these compounds are sited on- and/or off-center in thefullsetofCFP’scouldbedetermined.15 However,we
the oversized rigid (T2X3)4-cages is a matter of intense now found that in the case of R = Yb, as T increases,
debate.4,5 There is also controversy over the extent to a second Yb3+ resonance emerges from the low-T spec-
which the weakly bounded R ions can be regardedas in- tra, corresponding to a distinct site, coexisting with the
dependent Einstein oscillators, and how effectively they first one. The presence of the new term in the H has
CF
contribute to a phonon-glasstype of heat conduction.6–8 provenessentialtoexplainthe appearanceofthis second
In this work we take advantage of a uniquely favorable Yb3+ resonance. The sensitivityofYb3+ 4f-electronsto
conjunctionbetweenthe chemicalandstructuralcharac- this type of CF environment make it a rare and useful
teristics of skutterudites and their effect on the Electron probing ion to help the understanding of the potential
SpinResonance(ESR)oftheJ =7/2multipletofYb3+, well responsible for its motion.
2
II. EXPERIMENTAL

Single crystals of Ce1−xYbxFe4P12 (x . 0.003) were
grown in Sn-flux as described in Ref. 16. The cubic 2.60 Ce1-xYbxFe4P12 (x = 0.0023)
structure (Im3) and phase purity were checkedby x-ray
powder diffraction. The Yb concentrations were deter- e2.56
Ttmiohinnee,dEMSfrR(oHme,xTtph)e,ermiHmeeaansntusdreudTse-idndeapcreSynQsdtUeanlIscDeofdocf∼-tm2hxae2gmxn2eatgmonmmete3itzeaor-f. g-valu2.52 -2 H)/H x 10 123...000 (g H= )| g=( T)H -( Tg )( 4- .2H)|(4.2)
(
naturally grown crystallographic faces, as well as crys- 2.48 0.0 ( H)/H=1.3(3) g/g
tals crushed into fine powder. The ESR spectra were 0.00 0.g6/0g x1 1.200-2 1. 80 a)
tsapkeecntroinmeBtreurskeursiXng(9a.p4p8roGpHriza)tearnedsoQna(t3o4r.s4coGuHpzle)dbtaonda e)300 CPorywsdtaelr XX -- bbaanndd 2 mw)9 CX r-y bsatanld
TK-.cTonhteroElSleRrosfpeachtrealiuomfthgeas17fl0uYxbs3y+s(tIe=m0)foirso4t.o2p.esTho.w4ed5 dth (O200 Crystal Q - band I(P)/I(0.136 4.2 K 6.4 K BNraorarodw
the superposition of a narrow line and a slightly shifted wi 0.00 0.071/2 01./124 0.21
e P (W )
broad line. For single crystalsandpowderedsamples the n
spectra were, respectively, fitted by the superposition of Li100
twodysonian(metalliclineshape)andtwolorentzianres-
onanceswithadjustableresonancefields(H0),linewidths 0 b)
(∆H), A/B ratios and amplitudes.17 0 10 20 30 40 50
T (K)
III. EXPERIMENTAL RESULTS
Figure 2: (color online) X and Q-bands low T-evolution of:
a) g-valueand b) ∆H for the narrow and broad lines of Fig.
1andalso forapowderedcrystal. AtX-band,Inseta)shows
thecorrelationbetweenδ(∆H)/H andδg/g,andInsetb)the
(cid:13)
microwave power dependenceof the ESR intensity.
X - band (cid:13)Ce(cid:13) Yb(cid:13)Fe(cid:13)P(cid:13) (x = 0.0023)(cid:13) a(cid:13))(cid:13)
1-x(cid:13) x(cid:13) 4(cid:13) 12(cid:13)
s)(cid:13) x 10(cid:13) |(cid:13) 170(cid:13)Yb(cid:13) Best fit(cid:13)
unit 4.2 K(cid:13) 170Yb3+ isotope evolves into two lines, a narrow and
b. 173(cid:13)Yb(cid:13)|(cid:13) |(cid:13) |(cid:13) |(cid:13) |(cid:13) |(cid:13) x 4(cid:13) (cid:13) a broad one. At low-T the measured g-values for the
r 15.3 K(cid:13) (cid:13)
e (a 1x7 15(cid:13)Y(cid:13)b(cid:13) |(cid:13) |(cid:13) narrow and broad lines are essentially the same, g ≃
v x 20(cid:13) 2.57, different from the g-values of 2.666 and 3.428 ex-
vati 45 K(cid:13) (cid:13) pected for Γ6 and Γ7 doublets, respectively.12 At X-
eri 2.2(cid:13) 2.4(cid:13) 2.6(cid:13) (cid:13)2.8(cid:13) 3.0(cid:13) 3.2(cid:13) band the narrow line displays the full hyperfine spec-
n d Q - band (cid:13) (cid:13) b(cid:13))(cid:13) tra for the Yb isotopes 170Yb3+(I=0), 171Yb3+(I=1/2)
orptio x 3(cid:13) 5.5 K(cid:13) atrnada1r7e3Yabs3s+oc(Iia=t5ed/2t)o,cYonbfi3r+miinongst.haFtrtohme otbhseerhvyedpesrpfience-
bs 19 K(cid:13) splittings the corresponding hyperfine constants 171A =
(cid:13)A (cid:13) 440(10) Oe and 173A = 120(3) Oe were obtained. These
x 4(cid:13)
27 K(cid:13) values are ∼ 20% smaller than the hyperfine constants
of Yb3+ in a KDGS of any system with O cubic point
h
symmetry.12,18 The hyperfine lines corresponding to the
9.0(cid:13) 9.2(cid:13) 9.4(cid:13) 9.6(cid:13) 9.8(cid:13) 10.0(cid:13)
broad line of the 171Yb3+ isotope were not observed.
H (KOe)(cid:13)
Figures 2a and 2b show, respectively, the T-evolution
(4.2.T .40K)oftheg-valuesandlinewidths,∆H,for
the narrow and broad lines of Fig. 1 and also for a pow-
Figure 1: (color online) T-evolution (4.2 . T . 40 K)
of the normalized Yb3+ ESR spectra in a Ce1−xYbxFe4P12 deredsampleatX-band. Thefollowingfeaturesarenote-
(x ≃ 0.0023) single crystal: a) X-band and b) Q-band. The worthy: a) forthe sitescorrespondingtothe narrow line
enhancedlowfieldspectrashowstheabsenceofhyperfinelines the g-value and ∆H are frequency- and T-independent;
for the broad line. b) for the broad line sites the g-value and ∆H are T-
dependent, and only ∆H is frequency dependent; c) the
Figures 1a and 1b show, respectively, selected X- broad line of the powdered sample is broader than that
and Q-band ESR spectra for the Kramers doublet of the crystal, while the narrow line is about the same;
ground state (KDGS) of the 170Yb3+(I=0) isotope in a d) angular dependent ESR experiments found these res-
Ce1−xYbxFe4P12 (x ∼= 0.0023) single crystal. As T in- onances to be isotropic; e) for the various samples the
creases the nearly single line observed at low-T for the relative change of the broad line linewidth, δ(∆H)/H,
3
tive intensities the broad and narrow lines correspond,
(cid:13) respectively, to ∼ 95% and ∼ 5% of the Yb3+ ions fill-
nits)(cid:13) 9(cid:13) Ce(cid:13)1-x(cid:13)Yb(cid:13)x(cid:13)Fe(cid:13)4(cid:13)P(cid:13)12(cid:13) (x = 0.0023)(cid:13) a(cid:13))(cid:13) ing cages. Figure 3b presents the T-dependence of the
u (cid:13) relative population for the low occupied sites (narrow
rb. 1000(cid:13) line). The largeobserveddropstronglysuggeststhat, as
2(cid:13) (a 6(cid:13) (cid:13) 10100(cid:13)(cid:13) (cid:13) T-increases, the low populated Yb3+ sites migrate, in a
10(cid:13) 1(cid:13) (cid:13) reversible way, to the highly populated Yb3+ ones. The
y x 3(cid:13) 10(cid:13) T2 0(K(cid:13) )3(cid:13)0(cid:13) 40(cid:13) inset of Fig. 3b shows for the broad, narrow and hy-
sit perfine lines the T-dependence of their intensities (×T)
en 0(cid:13) (cid:13) normalized at T ≃ 4.2 K. This data reveals that the
Int Broad line(cid:13) b(cid:13))(cid:13) broad line practically follows a Curie-Weiss (C-W) law,
9(cid:13) Narrow line(cid:13) T (cid:13) 6(cid:13) (cid:13) while the narrow line surprisinglydropsfaster thana C-
/I(cid:13) %(cid:13)narrow(cid:13)total(cid:13) 6(cid:13) HCyuprieer-fWineei slisn(cid:13)e(cid:13) [I(T)/I(4.2)] x 0240(cid:13)(cid:13)(cid:13)(cid:13) 10(cid:13) T20 ((cid:13)K3)(cid:13)0(cid:13) 40(cid:13) (cid:13) (cid:13) Wi1tdh70eeYabr.ebeh3sTa+ovhniieaoonrnCc,se-gWsciavarerbrniyseefhulaofrrvctoaihomleirrzeaissduCapmFnpaooKgtrhntDeetrGtoiciStn.hmdeiocsmaitteeionsntmatinhgdarattthtiohanet
I(cid:13) 3(cid:13)
IV. ANALYSIS AND DISCUSSION
0(cid:13) 10(cid:13) 20(cid:13) 30(cid:13) 40(cid:13)
T(K)(cid:13)
In order to analyze our ESR data in Ref. 15 we used
the expanded Hamiltonian, H :
CFZ
Figure 3: a) T-dependence of the ESR intensity for the two
resonancesofthe170Yb3+(I=0)isotope. Theintensityofone
ofthe171Yb3+(I=1/2)hyperfineline,normalized bythenat- H = W (1−|y|) xO4c +(1−|x|)O6c +yO6t
ural abundance, is also shown. The inset presents the same CFZ (cid:26) (cid:20) F40 F60(cid:21) F62(cid:27)
datainlogscale. b)T-dependenceoftherelativeintensityfor +g µ H·J, (1)
thenarrow line. The inset displays thedata of Fig. 3a (×T) J B
normalizedat4.2K.ThegreenlinesshowtheC-Wbehavior. whereamagneticmomentJwithaLandég-factorg is
J
considered. The CF includes the usualcubic O 21 terms
h
parametrizedbythexvariablethatmeasurestherelative
scales at all-T with the relative change of its g-value, weightofthe4thand6thordertermsandalsotakesinto
δg/g (δ(∆H)/H ≃ 1.3(3) δg/g, see inset of Fig. 2a); f) consideration the new term Ot. The relative weight y
6
saturationESR intensity measurementsshowthatat4.2 linearlyinterpolatesbetweentheO cubictermsfory=0
h
K and ∼ 10 mW the broad and narrow lines present, re- and the Ot term for y= 1. This (x,y) parametrization
6
spectively, a ∼ 30% and ∼ 50% saturation (see inset of allows the entire range of the CFP’s to be accounted for
Fig. 2b). within the finite intervals −1≤x≤ 1 and |y|< 1 and the
All the experimental features given in Figs. 1 and 2 results do not depend on the sign of y. By diagonalizing
were confirmed in crystals from different batches with H oneobtains,asafunctionofxandy,theCFwave
CFZ
comparable Yb concentrations. They lead us to con- functions and energies for each of the R in units of W.
clude that: i) the narrow and the broad lines are, re- Fromthegroundstatewavefunctionthelowfieldg-value
spectively,homogeneousandinhomogeneousresonances; can be calculated15.
ii) the originof the inhomogeneity is a distribution of g- A combined analysis of the ESR data for Er3+, Dy3+
valuesoftheorderofthechangeintheg-value;iii)theT- and Yb3+ impurities in Ce1−xRxFe4P12 allowed us to
independent∆H forthenarrow line indicatesthatthere pinpointtheexact(x=0.523,y=0.082)valuescorrespond-
is no Yb3+ spin-lattice relaxation via exchange interac- ing to the Yb3+ narrow line observed at T ≃ 4.2 K and
tionwiththeconduction-electrons;19,20iv)thesaturation g ≃ 2.575.15 However, as T increases, a second Yb3+
of the spectra at low-T suggests slow spin-lattice relax- broad line emerges from the low-T narrow line (Fig. 1)
ationinvolvinglatticephononsviaspin-orbitcoupling;12 and its g-value decreases, reaching g = 2.54(1) at our
and v) at low-T the Yb3+ ions behave as an adiabatic highest-T (≃ 45 K). Therefore, these two resonances
spin system allowing the formation of Einstein oscilla- should be associated to two coexisting Yb3+ sites with
tors inside the (Fe2P3)4-cages. different peculiarities.
Figure 3a displays the T-dependence of the unsatu- In these compounds the R-ions are known to rattle
rated (∼2 mW) ESR intensity for the broad and narrow at frequencies of ∼ 103 GHz6–8 which are low compared
lines of the 170Yb3+(I=0) isotope of Fig. 1a. The inten- to the cage ion vibrations, but still much higher than
sityofoneofthe171Yb3+(I=1/2)isotopehyperfinelines, the ESR frequencies (∼10-30 GHz). Thus, we argue
normalizedbyits naturalabundance,isalsoshown. The thatthereducedhyperfineconstantforthehomogeneous
inset shows the data in a log scale. From their rela- narrow line spectra results from a motional narrowing
4
mechanism22 ofon-center Yb3+ ionsrattlinginthe rigid V. CONCLUSIONS
oversized(φ≃5 Å)(Fe2P3)4-cages.9 Inthe extreme mo-
tional narrowing regime23 a rattling frequency of ∼ 1
In summary, our ESR results have shown that the
GHz willreducein∼20%the hyperfine constant. More-
over,thehyperfinestructureintheinhomogeneousbroad origin for the large g-shift of the Yb3+ KDGS, rela-
tive to that in O symmetry (y = 0), may be associ-
line spectrawasnotobserved,suggestingthatadistribu- h
tion of Yb3+ ions are rattling at higher frequencies and ated to the B6t(O62 − O66) term in HCFZ. Coexisting
narrow and broad Yb3+ resonances were observed and
producing an even larger reduction of the hyperfine con-
associated, respectively, to a low occupation (∼5%) of
stant. Again,inthe extrememotionalnarrowingregime,
a rattling frequency &15 GHz will reduce in 90 to 95% on-center Yb3+ rattling ions (∼1 GHz) and to a highly
occupied (∼95%) T-dependent distribution of off-center
the hyperfine splitting, and the ESR spectra would look
rattling Yb3+ ions (&15 GHz). These assignments were
like the observed single broad line of ∆H ≃ 30-40 Oe.
basedon: i) themuchhigherexpectedYb3+ rattlingfre-
Weshouldmentionthatthereportedrattlingamplitudes
are . 0.1 Å.24 Hence, the broad line T-dependent shift quencies than the microwave frequency used in the ESR
experiments6–8 and; ii) on the observedreduction of the
andbroadeningismostlikelytheresultofaT-dependent
distribution of Yb3+ ions rattling at higher frequencies hyperfine constant for the on-center Yb3+ ions and the
absence of hyperfine structure in the spectra of the off-
inside the (Fe2P3)4-cages. Thus, we associate the ho-
center Yb3+ ionswhichwereattributedtomotionalnar-
mogeneous narrow line, corresponding to the low occu-
rowing effects.22,23 Although our findings relied on the
pied sites, to on-center (g = 2.575) of ≃1 GHz rattling
Yb3+ ionsat(x=0.523,y=0.082),whereasthe inhomoge- Yb3+ ESR results to witness the Yb3+ rattling mode,
they suggest that the R ions in other skutterudites and
neous broad line, corresponding to the highly occupied
sites with lower g-values, to a distribution of &15 GHz clathrate compounds may be also rattling in an analo-
rattling Yb3+ ions. Since this broad line is an inhomo- gous form as long as they are inside an oversized cage.
However,it maynot be alwaysobservablein anESR ex-
geneous resonance(distribution of g-values)andthe rat-
periment. We believe that the evidence for predominant
tling frequency is of the order of or higher than the ESR
frequency, the rattling Yb3+ ions responsible for these off-center rattling Yb3+ ions in these skuterudites is a
result that could justify the existence of Einstein oscilla-
spectrashouldbe spending moretime at off-center posi-
tors and help to understand the low thermal conductiv-
tions in the over-size cage.
ity and the strongly correlated phenomena exhibited by
Sincenoemergingsecondresonancewasobservedfrom
the low-T ESR spectra of Er3+ and Dy3+ ions diluted these type of materials.
in Ce1−xRxFe4P12,15 it is possible that for Yb3+, with
smaller ionic radius than that of Er3+ and Dy3+, larger
voided excursion space may be available for the Yb3+
ions inside the (Fe2P3)4-cages which may further favor
theYb3+ torattle. AT-dependentdistributionofCFPs, VI. ACKNOWLEDGMENTS
that inthis T symmetry allowsfor acontinuous change
h
on g 25, or even a distribution of new 2nd order CFPs
eff
in H associated to the off-center Yb3+ sites may be We thank FAPESP-SP and CNPq for financial sup-
CFZ
also a plausible reasonfor the observed T-dependence of port. PS is supported by DOE grant No. DE-FG02-
the inhomogeneous broad line. 98ER45707.
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