dybaco2o5

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Journal of Solid State Chemistry 177 (2004) 33393345

Metamagnetism in DyBaCo 2O5 x ; x E 0.5H.D. Zhou* and J.B. Goodenough

Texas Materials Institute, ETC 9.102, 1 University Station, C2201, University of Texas at Austin, Austin, TX 78712-1063, USA

Received 14 April 2004; received in revised form 20 May 2004; accepted 23 May 2004

Available online 20 July 2004

Abstract

The temperature dependence of the paramagnetic susceptibility wm

T taken in 2500Oe, the resistivity r T ; and thethermoelectric power aT of DyBaCo 2O 5 x ; which has Ba and Dy ordered into alternate (001) planes of an oxygen-decientperovskite, have revealed a phase segregation in the compositional range 0 :3p xo 0:5: Orthorhombic DyBaCo 2O5:51 has, inaddition, oxygen vacancies ordered into alternate rows of the DyO 0.51 (001) planes; a cold-pressed polycrystalline sample exhibits arst-order insulatormetal transition at T IM 320 K, a Curie temperature T C 300 K, and a broadened metamagnetic transitiontemperature T M E 265 K in 2500Oe. A ferromagnetic M 2 H hysteresis curve fails to saturate at 5 T, and a minority ferromagneticphase below T M has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in theDyBaCo 2O5:5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power aT 4 0 of DyBaCo 2O5:51changing continuously across T IM is interpreted to manifest a metallic minority phase crossing a percolation threshold; aT alsoprovides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above T IM : Aprevious model of the RBaCo 2O 5:5 phase is extended to account for the Ising spin conguration below T C ; the magnetic order in thepresence of higher-spin octahedral-site Co(III), and the aT data.r 2004 Elsevier Inc. All rights reserved.

Keywords: Cobaltites; Spin-state transitions; Orbital ordering; Oxygen-decient perovskites

1. Introduction

The RBaCo 2O5:5 family has the larger Ba2+ and

smaller rare-earth R3 ions ordered into alternate (001)planes of a tetragonal perovskite structure; the oxygenvacancies are conned to the RO 0.5 (001) planes. With aprimitive cubic perovskite lattice parameter ap ; thestructure is tetragonal (1 ap 1ap 2ap ) where theoxygen vacancies are disordered in the RO 0.5 (001)planes; it is orthorhombic Pmmm (2ap 1ap 2ap )where the oxygen vacancies order into alternate [010]rows within the RO 0.5 (001) planes [1]. This orderingcreates alternating [010] rows of square-pyramidal andoctahedral Co(III) sites, Fig. 1 . The square-pyramidalsites form two-leg ladders aligned parallel to the b-axis;they are coupled along the a-axis across (010) planes of corner-shared octahedra and along the c-axis acrossoxygen vacancies.

The orthorhombic phase is metallic above a rst-order insulatormetal transition at a temperature T IMthat increases with the size of the rare-earth ion over therange 310K p T IM p 360K for R Dy ;y ,Eu [24]. In asmall temperature interval T M p T p T C ; whereT C o T IM ; the orthorhombic phases are ferromagnetic;but below a metamagnetic transition temperature T M ;they became antiferromagnetic in zero applied magneticeld. Below T

M; the ferromagnetic phase is restored

above a critical magnetic eld strength H cT thatincreases with decreasing temperature. Moreover, me-tallic behavior above a T IM is peculiar to the oxygenstoichiometry RBaCo 2O5:5d with dX 0; the high-tem-perature metallic phase can tolerate a little excessoxygen, but additional oxygen vacancies suppress thetransition at T IM to a metallic state.

It has been recognized that the key to understandingthese phenomena may be the variation with temperatureof the spin state of the Co(III) ions. In the RCoO 3perovskites, the octahedral-site Co(III) ions are in theirlow-spin (LS) state t6 e0 at low temperatures, but with

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*Corresponding author. Fax: +1-512-471-7681.E-mail address: [email protected] (H.D. Zhou).

0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jssc.2004.05.047

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increasing temperature they convert to the intermediate-spin (IS) state t5 e1 and/or to the high-spin (HS) state t4 e2

as a result of the thermal expansion of the site and thehigher entropy of the higher spin states. Moreover, asmooth insulator-metal transition near 600 K inLaCoO 3 reects the growth of an itinerant-electronphase to beyond percolation in a localized-electronmatrix; the itinerant electrons occupy a narrow s bandof e-orbital parentage in a phase having a higherconcentration of HS Co(III) ions. These observationshave suggested that the insulatormetal transition atT IM may be associated with a transition from the LS tothe HS state at the octahedral-site Co(III) in

RBaCo 2O5:5 compounds, and Frontera et al. [5] havereported an anomalous volume expansion on increasingthe temperature across T IM in GdBaCo 2O5:5: On theother hand, the ve-fold oxygen coordination at thesquare-pyramidal Co(III) sites can be predicted tostabilize the IS state at all temperatures, and a studyby Taskin et al. [6a] of the magnetic susceptibility of adetwinned GdBaCo 2O5:5 single crystal has providedclear evidence for the ferromagnetic moment comingfrom an IS Co(III) on the square-pyramidal sites havinga spin S 1 pinned along the b-axis. They arguedconvincingly that the spins within a square-pyramidaltwo-leg ladder are strongly coupled ferromagnetically togive a T C E 300 K and that the metamagnetic transitionis associated with a weaker interladder coupling. Theyalso pointed out that a changing population below T IMof higher-spin octahedral-site Co(III) could be respon-sible for a change in the sign of the a-axis couplingbetween ferromagnetic b2 c planes of square-pyramidalIS Co(III) across b2 c planes of octahedral-site Co(III).

In this paper, we report the temperature dependencesof the thermoelectric power aT ; the resistivity r T ;and the magnetic susceptibility wm T of cold-pressed,polycrystalline DyBaCo 2O5 x ; 0:005p xp 0:51; samples.We extend the model of Taskin et al. [6a] for x 0:5 by

considering the origin of the Ising-like behavior of thespins of the IS Co(III) on the square-pyramidal sites andthe magnetic coupling between any HS Co(III) ionswithin the octahedral-site b c planes. The inuence of added oxygen vacancies on the transport and magneticproperties of DyBaCo 2O5:5 is also discussed.

2. Experimental

Polycrystalline samples of DyBaCo 2O5 x (0:005pxp 0:51) were prepared by standard solid-state reaction.Stoichiometric mixtures of Dy 2 O 3 , BaCO 3 , and Co 3 O 4were ground and calcined in air at 1150 C for 24h. Theproduct was then reground, pelletized, and sintered at1150 C for another 24 h in air. The stoichiometry of theas-prepared sample was DyBaCo 2O5:212 as determinedby thermogravimetric analysis (TGA) under a owingmixture of 5% H 2 Ar at 1000 C. The as-prepared samplewas also single phase to powder X-ray diffraction. TheTGA curve of Fig. 2 taken in oxygen provided a guideas to how to obtain samples with different oxygencontent. The as-prepared sample was then cold-pressedand annealed in the TGA apparatus at differenttemperatures in either pure oxygen or argon to adjustthe oxygen content; weight change was used to calculatethe oxygen stoichiometries of the end products. Asprepared samples were annealed in pure oxygen for 10 hat 230 C, 210 C and 180 C leading to x values 0.51(1),0.46(2), and 0.31(1), respectively; the values x 0:1153 and 0.005(3) were obtained after annealing inargon for 8 h at 410 C and for 30min at 1000 C,respectively.

A room temperature powder X-ray diffractionpattern for DyBaCo 2O5:51 was also recorded with aPhilips 1729 diffractometer equipped with a pyrolytic-graphite monochromator and Cu K a radiation; Si was

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Fig. 2. Thermogravimetric (TGA) curve of DyBaCo 2O5:21 taken in anoxygen ow at a heating and cooling rate of 1 C/min; solid line is theheating process and dotted line is the cooling process.

Fig. 1. Schematic crystal structure of DyBaCo 2 O 5.5 .

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the internal standard. Data were collected in steps of 0.020 over the range 20 p 2yp 80 with a count time of 15 s per step. Peak proles were tted with the programJADE. DyBaCo 2O5:51 has the orthorhombic Pmmmstructure with lattice parameters a=7.725(1)A ,b=3.904(2)A , and c=7.509(3)A , which corresponds

to the 2 ap 1ap 2ap superstructure reported byAkahoshi and Ueda [1] for YBaCo 2O5:5:Magnetic-susceptibility measurements were made

with a Quantum Design DC SQUID magnetometerafter cooling in zero eld (ZFC) or after cooling in ameasuring eld (FC) of 2500 Oe. M H hysteresis loopsof magnetization M versus applied magnetic eld H were measured over the range 5 T p H p 5 T at 265 and180K.

The thermoelectric power aT was obtained with alaboratory-built apparatus as described elsewhere [7].The resistivity r T was measured by a four-probetechnique.

3. Results

Fig. 3 shows the temperature dependences of themolar magnetic susceptibility wm T and its inversew1m T ; the resistivity r T ; and the thermoelectricpower aT for orthorhombic ( Pmmm ) DyBaCo 2O5:51:Clearly evident are an insulatormetal transition atT IM E 320 K where the resistivity changes from 10

1 to

103 O cm, a ferromagnetic Curie temperature

T C E 300 K, and a metamagnetic transition temperatureT M E 265 K in 2500Oe. Lowering the applied eldstrength sharpens the transition at T M : We were notable to obtain reliable values for aT below 120K; inthe interval 120 K o T o 180 K, an Arrhenius plot of

aT gave a trapping energy for mobile Co(IV) polaronsof D H t =2 14 meV and an Arrhenius plot of r T overthe same temperature interval gave an activation energyE a D H m D H t=2 80meV corresponding to amotional enthalpy D H m 66 meV for the Co(IV) ions.

Fig. 4 shows M 2 H hysteresis loops taken atT 265K E T M and T 180 K o T M for DyBaCo 2O5:51 : Lack of saturation at 50KOe in our poly-crystalline sample is consistent with the Ising-likebehavior of the spins reported by Taskin et al. [6a].Although the remanence is strongly reduced atT 180K from that at 265 K, nevertheless a residualferromagnetic component is retained as is also evident inthe divergence of the ZFC versus FC wm T curves of Fig. 3 .

Fig. 5 shows that as oxygen is removed fromDyBaCo 2O5:5; the volume fraction of the ferromagneticphase decreases, vanishing by DyBaCo 2O5:21 : More-over, the r T curve for DyBaCo 2O5:46; not shown,exhibits no anomaly at a T IM ; the compound remainspolaronic at higher temperatures. The insulatormetal

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Fig. 4. Field dependences of magnetization of DyBaCo 2O5:51 mea-sured at (a) 265 K and (b) 180K.

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transition is completely suppressed with only 0.04oxygen vacancies per formula unit in the octahedral-site b c plane even though aT shows a sharperincrease on cooling below T C in Fig. 6 . This gure alsoshows that low-temperature aT becomes negative forx 0:21; returning to a positive value at x 0:005 inDyBaCo 2O5 x ; Taskin et al. [6b] also observed a changein the sign of aT in GdBaCo 2O5 x :

4. Discussion

DyBaCo 2 O 5.5 is a distinguishable phase that canaccept some excess oxygen; it is a p-type conductor inboth the polaronic and metallic states. CompositionsDyBaCo 2O5 x with 0 :3p xo 0:5 segregate into two

phases, one corresponding to p-type x 0:5 and theother to a phase that is n-type with 0 :21o xo 0:31: Bothof these segregated phases are semiconductive at alltemperatures where they coexist. In DyBaCo 2O5:5 d; themobile holes are progressively trapped out as thetemperature decreases below T IM : The square-pyramidalsite Co(III) ions have been shown [6a] to be in the ISstate at all temperatures and the octahedral-site Co(III)to be in their low-spin state at low temperatures, whichwe take to be T o 180 K. At temperatures T o 180K, theholes are small-polaron Co(IV) ions that are progres-sively trapped out by shallow trapping centers; thesetraps are probably octahedral sites neighboring aninterstitial oxygen that introduced the Co(IV) ions.The origin of an increase in E a D H m D H t =2below105 K is not identied; it could represent a condensationof the holes into ordered clusters. The paramagneticsusceptibility data of Frontera et al. [5] for T 4 T IM areconsistent with HS Co(III) in the octahedral sites and ISCo(III) in the square-pyramidal sites in the metallicphase. At T 4 T IM ; the spin degeneracy of the electronsof e-orbital parentage is removed by the intraatomicexchange with the localized spins of the p-bonding telectrons, and spin-disorder scattering can account forpoor-metal behavior with an a4 0 if the octahedral-site

Co(III) are in their HS state.The smooth non-Arrhenius increases in aT and

r T on cooling DyBaCo 2O5:51 through the interval180K o T o T IM must, it follows, be associated with thetransition from LS to HS octahedral-site Co(III) on theapproach to T IM : The fact that a positive aT isretained as the population of electrons in the e-orbitalsdecreases to zero appears to require the coexistence of two phases, one containing mostly HS Co(III) and theother mostly LS Co(III) on the octahedral sites. Eitherthe metallic, HSCo(III) phase becomes non-percolativebelow T IM or the conductivity of the HS Co(III) phase ismade polaronic by the perturbation of the periodicpotential by the second phase; the latter phenomenon isobserved in DyBaCo 2O5:46 where the metallic phaseabove T IM is transformed to a polaronic phase withaT 4 0 by the coexistence of a second phase. As thevolume fraction of the HS Co(III) phase decreases, themobile holes would be transferred progressively intolow-spin Co(IV) small polarons in the LS-Co(III) phase.The coexistence of two phases is consistent with the rst-order character of the transition at T IM : This HSLSphase segregation is to be distinguished from the phaseseparation occurring in the compositional range0:31p xo 0:5:

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Fig. 5. Molar magnetic susceptibility wm T for DyBaCo 2O5 x(0:005p xp 0:51), for x 0:005; 0.115 and 0.21 ZFC and FC curvesoverlap.

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Discussion of the magnetic data begins with theobservation of Taskin et al. [6a] that the spins of the ISCo(III) on the square-pyramidal sites are pinned to theb-axis to give an Ising magnetic lattice and with theirobservation that the square-pyramidal sites form two-leg ladders within which the spins are strongly coupled

ferromagnetically to give a T C E 300 K. They alsopointed out that coupling between ladders across aCo(III) octahedral-site b2 c plane could change sign asthe population of octahedral-site LS Co(III) increasedwith decreasing temperature. They retained a ferromag-netic coupling between ladders along the c-axis acrossan oxygen vacancy. In addition, they showed that an e-orbital ordering on the IS Co(III) ions like that shown inFig. 7 could account for the ferromagnetic coupling withthe GoodenoughKanamori rules for the sign of thespinspin exchange interactions. An e-orbital orderingthat alternates occupancy of ( x 2 y2 ) and (3 z2 r2 )orbitals on traversing a leg of a two-leg ladder without-of-phase ordering on the second leg requiresasymmetric Co ? OCo bonds in both the rungs andlegs of the ladder, and these displacements were notdetected by the diffraction experiments of eitherFrontera et al. [5] or Moritomo et al. [8]. However, alack of long-range order or a vibronic coupling withquasi-static long-range order would make it difcult todetect atomic displacements within the ladders. It is alsopossible to achieve ferromagnetic order within theladders and between ladders along the c-axis across anoxygen vacancy by an e-orbital ordering that alternatesoccupancy of (3 x 2 r2 ) and (3 z2 r2 ) orbitals on traver-

sing a leg of a ladder with out-of-phase orbital ordering

on the other leg. Since the e-orbital ordering responsiblefor the ferromagnetic interactions within the ladders isambiguous, we begin with a consideration of the originof the pinning of the Co(III) spins of the two-leg laddersalong the axis of the legs.

In an octahedral site, an IS Co(III): t5 e1 conguration

has a three-fold t-orbital degeneracy as well as a two-fold e-orbital degeneracy. In a square-pyramidal site,these degeneracies are removed, but the cation displace-ment toward the apical oxygen determines how theorbital degeneracies are removed. In addition, atomicdisplacements within the CoO 2 (001) sheets inuencehow the t-orbital degeneracy is removed. The observa-tion of an Ising spin lattice within the ladders signalsthat there is an unquenched orbital angular momentumthat is oriented along the ladder legs and that it is pinnedby a strong elastic force. This observation can onlymean that the t-orbital hole of an IS Co(III) occupies a( yz7 izx) orbital.

This conclusion would be compatible with a contrac-tion of the b-axis on cooling through T IM as reported byMoritomo et al. [8], but Frontera et al. [5] as well asAkahoshi and Ueda [9] have reported that the b-axisexpands on cooling through T IM : A transition from HSto LS on the octahedral-site Co(III) should contract thelattice parameters, and indeed two axes and the volumecontract on cooling through T IM ; but one axis expands,and it appears to be the ladder axis b. One of us [10] hasargued from the virial theorem that the equilibrium (M O) bond length associated with localized d electrons islonger than the equilibrium bond length where the d

electrons become itinerant, and this prediction has beenextensively validated by experiment. Therefore, anelongation of the ladder axis on cooling through T IMmay reect the transition from itinerant to localizedbehavior of the e electrons where no spin-state transitionoccurs. We are thus led to conclude that the t-orbitalordering is present above as well as below T IM and thatthe t-orbital hole occupies a ( yz7 izx ) orbital below T IMat the sites where the (3 z2 r2 ) orbital is occupied; at theother IS Co(III) sites of a ladder leg, the CoO a bondsshould be lengthened so as to introduce stabilization of a hybrid (3 x 2 r2 ) and ( x 2 y2 ) orbital that lifts the t-orbital degeneracy without quenching signicantly theorbital angular momentum oriented along the b-axis.The ferromagnetic exchange with the pinned spinswould then keep all the spins pinned to the b-axis. Notethat this model hybridizes the two possible solutionsdiscussed above for the e-orbital ordering. The magneticdata thus suggest the individual (CoO) bond lengthsare more complex than those obtained in previousstructural studies.

A signicant HS population on the octahedral-siteCo(III) at T C ; which is indicated by the aT data,would strongly couple the ladder legs ferromagneticallyacross the octahedral-site Co(III) b2 c planes since

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Fig. 7. Schematic e-orbital ordering of GdBaCo 2 O 5.5 suggested byTaskin et al. [6a].

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antiferromagnetic coupling within the octahedral-siteplanes would match parallel spins with (3 z2 r 2 ) spins inthe ladders and antiparallel spins with (3 x 2 r 2 ), (x 2 y2 )spins in the ladders. This magnetic order would giveantiferromagnetic coupling with octahedral-site b cplane and a ferromagnetic moment from the square-

pyramidal sites equal to the ferromagnetic momentreported by Taskin et al. [6a]. As the population of HSCo(III) in the octahedral sites decreases, the model callsfor a segregation within octahedral-site Co(III) planes of islands of LS Co(III) that grow with decreasingtemperature. Across a LS Co(III), the (3 x 2 r 2 ),(x 2 y2 ) spins of neighboring ladders interact antiferro-magnetically. Above a critical fraction of LS Co(III), thea-axis coupling between ladders changes sign to give themetamagnetic behavior below T M : However, the two-phase character of the LSHS transition is manifest inthe retention of a ferromagnetic minority phase withinthe metamagnetic matrix to quite low temperature as ismanifest by the divergence of the FC and ZFC wm T curves in Fig. 3 .

Reduction of the volume fraction of the ferromagneticphase below T C in DyBaCo 2O5:46 ; Fig. 5 , is indicative of a two-phase character in the compositions 0 :3p xo 0:5of DyBaCo 2O5 x ; but this observation begs the questionof why the metallic phase above a T IM is suppressed inthe DyBaCo 2O5:5 phase within this composition. Estab-lishing a s band of itinerant-electron states of e-orbitalparentage in an octahedral-site Co(III) array dependsnot only on a critical population of HS Co(III) ions, butalso on an unperturbed periodic potential and an e O e

overlap integral that exceeds a critical value. Sincethe metallic phase is at the threshold of its formation,it would not take a large perturbation to suppress it.The limiting composition of the DyBa 2Co 2O5:5 phasecan be expected to contain some oxygen vacancies.Every oxygen vacancy introduces two HS Co(III) ionshaving localized t 5 e2 congurations; both specieswould perturb the periodic potential. The introductionof only a small concentration of oxygen vacancies intothe DyBaCo 2O5:5 phase appears to be sufcient tosuppress the transition to the metallic phase.

5. Conclusions

Orthorhombic DyBaCo 2O5:5 contains an ordering of Ba and Dy ions into alternate (001) planes of theperovskite structure and an ordering of oxygen vacan-cies into alternate [010] rows in the DyO 0.5 planes. Thisordering creates two-leg ladders parallel to the b-axis of IS Co(III) ions in square-pyramidal sites; the ladders of a b c plane are separated by an oxygen vacancy, but theladder spins of the b c planes remain ferromagneticallycoupled below T C and pinned to the b-axis. The ladderspins of neighboring b c planes are coupled across

octahedral-site Co(III) ions, and the sign of thiscoupling changes from antiferromagnetic to ferromag-netic at T M as the population of higher-spin Co(III) onthe octahedral sites increases with temperature. How-ever, a ferromagnetic volume fraction persists belowT M ; this fraction decreases with decreasing temperature,

disappearing below 50 K. Moreover, with spins pinnedto the b-axis and T M T M H ; a T M (2500 Oe) E 265Kis broadened.

A postulated model of orbital ordering that canaccount for the magnetic order and the pinning of thespins along the b-axis provides a net magnetization inthe ferromagnetic planes that is due to only the spins inthe square-pyramidal sites; the spins of the higher-spinoctahedral-site Co(III) would have a net antiferromag-netic order. The population of higher-spin Co(III) onthe octahedral sites increases progressively with tem-perature to T IM where a small step increase occurs.However, the thermoelectric power remains positive andchanges continuously across T IM ; which suggest thecoexistence of a metallic phase and a polaronic phaseacross T IM with the volume fraction of the metallicphase increasing discontinuously to beyond a percola-tion threshold at T IM :

The introduction of oxygen vacancies into the b cplanes of octahedral-site Co(III) suppresses the insula-tormetal transition, but not T C and T M ; and aprogressive decrease with x in the volume fraction of the ferromagnetic phase in the interval T M o T o T Cfound in the compositional range 0 :2o xo 0:5 of DyBaCo 2O5 x signals separation into two polaronic

phases of different x . At the phase limit DyBaCo 2O5:5dwith 0 o do 0:04; the metallic phase of DyBaCo 2O5:5 iscompletely suppressed.

Acknowledgments

The authors thank the National Science Foundationand the Robert A. Welch Foundation of Houston,Texas, for nancial support.

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R. Szymczak, M. Baran, Phys. Rev. Lett. 80 (1998) 3380;I.O. Troyanchuk, N.V. Kasper, D.D. Khalyavin, H. Szymczak,R. Szymczak, M. Baran, Phys. Rev. B 58 (1998) 2418.

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[6] (a) A.A. Taskin, A.N. Lavrov, Y. Ando, Phys. Rev. Lett. 90(2003) 227201;

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(b) A.A. Taskin, A.N. Lavrov, Y. Ando, Proceedings, Twenty-second International Conference on Thermoelectrics,P196200, IEEE Publisher, New York, 2003.

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