Dhalenne, G.

Isobe, M.

Khomskii, D. I.

Pryadun, V. V.

Revcolevschi, A.

Ueda, Y.

Vasilev, A. N.

English

Submitted to: Phys. Rev. Lett. Abstract: A huge increase of thermal conductivity k is observed at the phase transition in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04). This behavior is compared with that of pure and Zn-doped CuGeO3 where only modest kinks in the k(T) curves are observed at the spin-Peierls transition. The change of k at critical temperature Tc could be partially attributed to the opening of an energy gap in the magnetic excitation spectrum excluding the scattering of thermal phonons on spin fluctuations. However, the reason for such a strong anomaly in the k(T) may lie not only in the different energy scales of CuGeO3 and NaV2O5, but also in the different character of the phase transition in NaV2O5 which can have largely a structural origin, e.g. connected with the charge ordering

ARTS repository - University of Groningen

   University of GroningenAnomalous thermal conductivity of $NaV_{2}O_{5}$ as compared to conventional spin-Peierlssystem $CuGeO_3$Vasilev, A. N.; Pryadun, V. V.; Khomskii, D. I.; Dhalenne, G.; Revcolevschi, A.; Isobe, M.;Ueda, Y.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Vasilev, A. N., Pryadun, V. V., Khomskii, D. I., Dhalenne, G., Revcolevschi, A., Isobe, M., & Ueda, Y.(1998). Anomalous thermal conductivity of $NaV_{2}O_{5}$ as compared to conventional spin-Peierlssystem $CuGeO_3$. Physical Review Letters, 81(9).CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierlssystem CuGeO3A.N. Vasil'ev and V.V. PryadunLow Temperature Physics Department, Moscow State University, 119899Moscow,RussiaD.I. KhomskiiSolid State Physics Department, University of Groningen, 9747 AGGroningen,The NetherlandsG. Dhalenne and A. RevcolevschiLaboratoire de Chimie des Solides, Universite Paris-Sud, 91405 OrsayCedex,FranceM. Isobe and Y. UedaInstitute for Solid State Physics, The University of Tokyo, 7-22-1Roppongi,Minato-ku, Tokyo 106, JapanA huge increase of thermal conductivity  is observed at the phase transition in stoichiometricNaV2O5. This anomaly decreases and gradually disappears with deviation from stoichiometry inNa1 xV2O5 (x = 0.01, 0.02, 0.03, and 0.04). This behavior is compared with that of pure andZn-doped CuGeO3 where only modest kinks in the (T) curves are observed at the spin-Peierlstransition. The change of  at critical temperature Tc could be partially attributed to the openingof an energy gap  in the magnetic excitation spectrum excluding the scattering of thermal phononson spin uctuations. However, the reason for such a strong anomaly in the (T) may lie not only inthe dierent energy scales of CuGeO3 and NaV2O5, but also in the dierent character of the phasetransition in NaV2O5 which can have largely a structural origin, e.g. connected with the chargeordering.75.30.Kz, 66.70.+fJune 12, 1998Since the discovery of a spin-Peierls (SP) transition in CuGeO3 [1] and NaV2O5 [2] numerous experimental andtheoretical studies were continued to reveal the main features of this phenomenon in inorganic compounds. Whileconsidering that the study of CuGeO3 remains very active, the main physical properties of this compound are quitewell known by now. CuGeO3 belongs to the orthorhombic space group Pbmm. The structure contains magneticchains of Cu2+ (3d9, S = 1/2) ions parallel to the c axis. The SP transition observed at Tc ' 14 K is marked by anexponential drop of the magnetic susceptibility and a simultaneous lattice dimerization along the a and c axes. Dueto the magnetoelastic coupling the exchange constant between neighboring spins on the chain also alternates and thesinglet ground state in the dimerized phase is separated from the band of excited triplet states by an energy gap of about 25 K [3, 4].In comparison with CuGeO3, only few and somewhat contradictory data are presently available on the specicfeatures of the phase transition in the mixed (V4+/V5+) or intermediate (V4:5+) valence compound NaV2O5. A rapiddecrease of the magnetic susceptibility below Tc ' 35 K suggests the formation of a nonmagnetic spin-singlet groundstate. The simultaneous formation at this temperature of a superlattice structure characteristic of a SP transitionwas conrmed by X-ray measurements [5], while the opening of an energy gap ( ' 100 K) in the magnetic excitationspectrum was observed in inelastic neutron experiments [6]. These data, as well as the results of NMR [7] and ESR [8]studies, are strong evidence of the presence of S = 1/2 chains considered as a necessary condition for a SP transition.On the other hand, the two-step phase transition found in specic heat and thermal expansion measurements [9] makea simple SP scenario for this transition in NaV2O5 not so evident.The main controversy concerning the nature of the phase transition in NaV2O5 arises from the uncertainty of itscrystal structure identication at room temperature. The X-ray diraction pattern of single crystals ts well enoughwith both space groups P21mn and Pmmn [10-13]. In both cases the layers of edge/corner-sharing tetragonal VO5pyramids connected in the a   b plane are stacked along the c axis of the structure while the Na1+ ions are situated1between these layers. In the former non-centrosymmetric structure, there exists alternating chains of nonmagneticV5+ ions and magnetic V4+ ones (3d1, S = 1/2) aligned along the b axis (see e.g. Fig. 1 of Ref. [2]). In the latterstructure, all the V sites are crystallographically equivalent, which results formally in a random distribution of theV4+ and V5+ ions, or in a localization of electrons on V-O-V molecular orbitals on the rungs of the ladders runningalong the b axis (see e.g. Fig. 1 of Ref. [13]). In such a situation a one-dimensional magnetic behavior does not comeabout straightforwardly but can still be justied [13-15].In order to clarify a picture of the phase transition in NaV2O5, more experimental data are necessary. One ofthe few experimental techniques not yet applied to the study of SP transitions in metal oxides is the measurementof thermal conductivity which can provide useful information on the interaction of elementary excitations in thesecompounds.In this letter we report on the rst, to the best of our knowledge, such measurements, which show a surprisinglystrong eect in NaV2O5, in comparison with the rather weak anomaly observed in CuGeO3. A possible interpretationof this eect is considered below and it is concluded that the nature of the phase transition in NaV2O5 may be morecomplicated than previously assumed.Single crystals of CuGeO3 and Cu0:98Zn0:02GeO3 were grown in Paris University by a oating zone method [16].Single crystals of stoichiometric NaV2O5 were grown in Tokyo University by the self-ux method [17]. Single crys-tals of non-stoichiometric Na1 xV2O5 (x = 0.01, 0.02, 0.03, and 0.04) were prepared by heating a small crystal ofstoichiometric NaV2O5 embedded in a large quantity of Na1 xV2O5 powder in an evacuated silica tube at 650C forone week. The non-stoichiometric samples of Na1 xV2O5 contain vacancies on the Na sites. Typical dimensions ofthe crystals used in the thermal conductivity measurements were 0.20.84.0 mm3. The largest dimension coincidedwith the dimerization axis, i.e. the c axis in the case of CuGeO3, and the b axis in the case of NaV2O5. Thesmallest dimension coincided with the a axis in the layered structure of CuGeO3 and c axis in the layered structureof NaV2O5. In most cases, measurements were performed in the direction of dimerization, but in CuGeO3 thermalconductivity was measured also along the b axis. The measurements were performed by the longitudinal steady-statefour probe method [18] with a temperature gradient equal to 2% of the current temperature. The sample holder,surrounded by a special shell to suppress the thermal radiation, was placed into the vacuum chamber evacuated tobetter than 10 6 bar. The sample temperature and the temperature gradient were stabilized to better than 0.01% byan Oxford intelligent temperature controller with calibrated "Allen-Bradley" carbon resistors. Due to the relativelysmall dimensions of the samples the error in the determination of the absolute values of thermal conductivity was notbetter than 10%, while the precision on the thermal conductivity variations was about 1.5 %.The experimental results obtained in the c and b directions for pure CuGeO3 and for the Zn-doped sample areshown in Fig. 1. A broad phonon maximum of  is observed at T = 23.3 K in the c direction, while it is not wellpronounced in the b direction. The absolute value of  in the b direction is approximately three times lower thanthat in the c direction. The kinks on the (T) curves are evident at the temperature of the SP transition. The SPtransition temperature in the Zn-doped sample is reduced to Tc = 10.6 K to be compared to that of the pure sampleTc = 14.2 K. The absolute value of  at the SP transition in the doped sample is signicantly smaller than that inthe pure sample.The thermal conductivity of stoichiometric NaV2O5 exhibits a much more dramatic variation in the temperaturerange studied. As shown in Fig. 2, the thermal conductivity exhibits a broad maximum at T ' 70 K and decreasesupon approaching the SP transition. At Tc a sharp upturn of  occurs so that a ve times increase of thermalconductivity accompanied by a subsequent decrease of  upon cooling to liquid helium temperature takes place. Abehavior similar to the one described above can be seen in non-stoichiometric samples of Na1 xV2O5 (x =0.01, 0.02,0.03, and 0.04) with the progressively less pronounced maximumbelow Tc, as shown in Fig. 3. The position of the lowtemperature maximum remains unchanged in non-stoichiometric samples but absolute values of  gradually decreasewith x. Certain variation in the high temperature slopes and the vertical shift between the curves obtained for 0 and1% and the curves for 2, 3 and 4% Na deciency can be tentatively ascribed to a variation in the crystals morphology.These crystals can be easily cleaved perpendicular to the c axis, resulting in an uncontrolled extra scattering of thephonons on planar defects. The spin-Peierls transition temperatures in Na1 xV2O5 determined from the positionsof the minimum on the (T) curves are shown in Fig. 4, where the solid curve is drawn as a guide for an eye. Thesuppression of Tc by the Na-deciency was observed earlier in X-ray critical scattering measurements [19].Since no quantitative treatment of the transport phenomena for the spin-Peierls systems exists, we applied theBoltzmann equation for the analysis of the experimental data. In this approach the phonon thermal conductivity  vlC is determined by the sound velocity v, the mean free path of phonons l, and the specic heat C which dependson the number of elementary excitations. The interactions of the phonon and the spin subsystems can be taken intoaccount through the variation of the phonons mean free path. In the absence of these interactions, the mean freepath of the phonons increases upon cooling and eventually saturates while their number decreases. These oppositetendencies result in a phonon maximumTm on the (T) curves. The estimated phonon mean free path in CuGeO3 isstill much less than the sample dimensions; we believe that this is caused by the phonon scattering on planar defects2in the layered structure of this compound. The dierence in the absolute values of the thermal conductivity in theb and the c directions in CuGeO3 can naturally be attributed to a factor three dierence in ultrasonic velocities, i.e.2.7 103 m=sec in the b direction and 8.0103 m=sec in the c direction [20]. On the other hand, the small thermalconductivity value of in the Cu0:98Zn0:02GeO3 sample as compared with that of the pure CuGeO3 sample reects theincreased number of scattering centers in the doped sample.Anomalies observed in the temperature dependence of the thermal conductivity at the spin-Peierls transition canbe naturally connected with the change of the crystal lattice and with the formation of the energy gap in the magneticexcitation spectrum. The mutual positions of the temperatures of the phonon maximum Tm and of the spin-Peierlstransition Tc is of crucial importance for the analysis of these anomalies. The phonon maximumin CuGeO3 is reachedat Tm > Tc. In this case a possible decrease of the phonon scattering on spin uctuation at Tc cannot signicantlyinuence the thermal conductivity, because the phonon mean free path is already large, and only modest kinks on the(T) are observed. From the behavior of NaV2O5 we have to conclude that the situation here is exactly opposite. Ifthe maximum of  at 70 K would have the same origin, we would not be able to explain such a strong increase of below Tc. Therefore we have to assume that the decrease of  below 70 K in NaV2O5 is of a dierent nature, andmost probably can be explained by the enhancement of phonon scattering on approaching the phase transition. Theordering occurring below Tc apparently switches o this extra scattering and leads to a huge increase of the thermalconductivity. No variation of the sound velocity at Tc can explain the observed behavior of the thermal conductivityin NaV2O5. While no data for the absolute value of the sound velocity are available, a longitudinal velocity variationv=v 10 3 was observed at Tc preceded by a precursor eects observable below 70 K [21].Several factors may play a role in the much stronger enhancement of the thermal conductivity below Tc in NaV2O5as compared to CuGeO3. The spin gap is much larger in NaV2O5 (100 K vs 25 K in CuGeO3). Opening of this gapswitches o spin-phonon scattering for phonons with frequencies less that . A four times larger spin gap in NaV2O5implies that much more phonons will now strongly contribute to thermal conductivity below Tc. This picture could,in principle, have allowed also to explain the behavior of the thermal conductivity of NaV2O5 as a function of thedeviation from stoichiometry. Magnetic measurements show that, in contrast with the relatively small suppressionof Tc with x, the drop of magnetic susceptibility below Tc is strongly reduced and for x = 0.03 - 0.04 it nearlyvanishes. This shows that the spin gap is now, to a large extent, lled by magnetic excitations. Correspondingly, thephonon-spin uctuation scattering is being restored, which suppresses the increase of  below Tc in non-stoichiometricsamples.However, the comparison with CuGeO3 shows that pure spin-phonon scattering is hardly sucient to explain hugeincrease of  observed. Magnetoelastic coupling in NaV2O5 is not strong enough, which follows e.g. from the veryweak dependence of Tc on magnetic eld [23]. We believe that the huge increase of the thermal conductivity inNaV2O5 can be related to the specic features of the phase transition in this compound. If the crystal structureabove Tc is indeed that obtained in recent studies [11-13], all V ions are on the average equivalent, i.e. the V4:5+ions. Such a state can be visualized as the (dynamic) random mixture of V4+ and V5+. If slow enough, these chargeuctuations can strongly enhance phonon scattering and reduce thermal conductivity similar to a situation in glasses.The phase transition may then be accompanied, or even caused, by the charge ordering [22] at which this extrascattering mechanism is switched o. The opening of a spin gap may then be not so much a driving force but aconsequence of this ordering. The transition from the disordered glass-like phase to an ordered one could explain thestrong enhancement of the thermal conductivity below Tc. This picture can also explain the observation [23] that thetotal entropy of the transition in NaV2O5 is larger than the pure spin one.Although our measurements do not allow to draw a nal conclusion about the microscopic picture of the changesoccurring at the phase transition in NaV2O5, the qualitative dierence observed in the behavior of CuGeO3 andNaV2O5 can be taken as an evidence that the nature of the transition in NaV2O5 is more complicated than inCuGeO3 and most probably directly involves lattice and charge degrees of freedom, e.g. in a form of charge ordering.Further experiments, especially neutron scattering, should shed light on the microscopic picture of the spin-Peierlstransition in NaV2O5 and should clarify the nature of the strong anomaly in the thermal conductivity of NaV2O5reported in this letter.This work was supported by the Russian Foundation for the Basic Research (Grant-in-Aid No. 96-02-19474) and bythe Netherlands Foundation for the Fundamental Study of Matter (FOM). The authors are grateful to B. Buechner,D. van der Marel, A. Damascelli and M. Mostovoy for helpful discussions.[1] M. Hase, I. Terasaki, and K. Uchinokura, Phys. Rev. Lett. 70, 3651 (1993).3[2] M. Isobe and Y. Ueda, J. Phys. Soc. Jpn. 65, 1178 (1996).[3] M. Nishi, O. Fujita, and J. Akimitsu, Phys. Rev. B 50, 6508 (1994).[4] L.P. Regnault et al., Phys. Rev. B 53, 5579 (1996).[5] Y. Fujii et al., J. Phys. Soc. Jpn. 66, 326 (1997).[6] T. Yoshihama et al., Physica B 234-236, 539 (1997).[7] T. Ohama et al., J. Phys. Soc. Jpn. 66, 545 (1997).[8] A.N. Vasil'ev et al., Phys. Rev. B 56, 5065 (1997).[9] M. Koeppen et al., Cond-mat/9710183.[10] A. Carpy and J. Galy, Acta Cryst. B 31, 1481 (1975)[11] H.-G. von Schnering et al., to be published.[12] A. Meetsma et al., Submitted to Acta Cryst.[13] H. Smolinski et al., Cond-mat/9801276.[14] A. Damascelli et al., to be published.[15] P. Horsch and F. Mack, Cond-mat/9801316.[16] A. Revcolevschi and G. Dhalenne, Adv. Mater. 5, 657 (1993).[17] M. Isobe, C. Kagami, and Y. Ueda, J. Cryst. Growth 181, 314 (1997).[18] R. Berman, Thermal conductivity of solids. Clarendon Press, Oxford, 1976.[19] H. Nakao et al. in Proc. of Int. Conf. on Neutron Scattering, Toronto, 1997.[20] M. Poirier et al., Phys. Rev. B 51, 6147 (1995).[21] P. Fertey et al., submitted to Phys. Rev. B.[22] After submission of this paper the results of NMR studies of NaV2O5 became available (T.Ohama et al, Submitted toPhys. Rev. Lett.), which show that there appear inequivalent V ions at T< Tc, so that most probably indeed a chargeordering occurs at Tc in NaV2O5.[23] B. Buechner, private communication.FIG. 1. Temperature dependences of thermal conductivity in pure and Zn-doped CuGeO3.FIG. 2. Temperature dependences of thermal conductivity  and of magnetic susceptibility  in stoichiometricNaV2O5.FIG. 3. Temperature dependences of thermal conductivity in Na1 xV2O5 (x = 0, 0.01, 0.02, 0.03, and 0.04).FIG. 4. Critical temperatures of the spin-Peierls transition in Na1 xV2O5 as obtained from the thermal conductivitydata. The solid curve is drawn as a guide for an eye.40 5 10 15 20 25 3005101520253035Cu0.98Zn0.02GeO3, c axisCuGeO3, b axisCuGeO3, c axisκ (W / m K) T (K)0 20 40 60 80 100 120 14001020304050607025 30 35 40 45 501015202530Tcκ  (W / m K)T  (K)κNaV2O5κ (W / m K)T (K)02468χχ (10-4  emu / V4+  mol)0 20 40 60 80 100 120 140110100Na1-xV2O5x = 0%x = 1%x = 2%x = 3%x = 4% κ (W / m K)T (K)0 1 2 3 4152025303540DUNa1-xV2O5Tc (K)x (%)

Anomalous thermal conductivity of $NaV_{2}O_{5}$ as compared to conventional spin-Peierls system $CuGeO_3$

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Anomalous thermal conductivity of $NaV_{2}O_{5}$ as compared to conventional spin-Peierls system $CuGeO_3$

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Anomalous thermal conductivity of NaV2O5NaV_{2}O_{5}NaV2​O5​ as compared to conventional spin-Peierls system CuGeO3CuGeO_3CuGeO3​

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Anomalous thermal conductivity of $NaV_{2}O_{5}$ as compared to conventional spin-Peierls system $CuGeO_3$