external stimulation-controllable heat-storage ceramics_ti3o5 phases

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ARTICLE Received 31 Oct 2014 | Accepted 25 Mar 2015 | Published 12 May 2015 External stimulation-controllable heat-storage ceramics Hiroko Tokoro 1,2,3 , Marie Yoshikiyo 1 , Kenta Imoto 1 , Asuka Namai 1 , Tomomichi Nasu 1 , Kosuke Nakagawa 1 , Noriaki Ozaki 1 , Fumiyoshi Hakoe 1 , Kenji Tanaka 1 , Kouji Chiba 4 , Rie Makiura 5,6 , Kosmas Prassides 7 & Shin-ichi Ohkoshi 1,2 Commonly available heat-storage materials cannot usually store the energy for a prolonged period. If a solid material could conserve the accumulated thermal energy, then its heat- storage application potential is considerably widened. Here we report a phase transition material that can conserve the latent heat energy in a wide temperature range, To530 K and release the heat energy on the application of pressure. This material is stripe-type lambda-trititanium pentoxide, l-Ti 3 O 5 , which exhibits a solid–solid phase transition to beta-trititanium pentoxide, b-Ti 3 O 5 . The pressure for conversion is extremely small, only 600 bar (60 MPa) at ambient temperature, and the accumulated heat energy is surprisingly large (230 kJ L 1 ). Conversely, the pressure-produced beta-trititanium pentoxide transforms to lambda-trititanium pentoxide by heat, light or electric current. That is, the present system exhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phase transitions. The material may be useful for heat storage, as well as in sensor and switching memory device applications. DOI: 10.1038/ncomms8037 OPEN 1 Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 2 CREST, JST, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan. 3 Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8577, Japan. 4 Science and Technology System Div., Ryoka Systems Inc., Tokyo SkytreeEast Tower, 1-1-2 Oshiage, Sumida-ku, Tokyo 131-0045, Japan. 5 Nanoscience and Nanotechnology Research Center, Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Osaka 599-8570, Japan. 6 PRESTO, JST, 4-8-1 Honcho, Kawaguchi 332-0012, Japan. 7 WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan. Correspondence and requests for materials should be addressed to S.O. (email: [email protected]). NATURE COMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications 1 & 2015 Macmillan Publishers Limited. All rights reserved.

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ARTICLEReceived31Oct2014|Accepted25Mar2015|Published12May2015Externalstimulation-controllableheat-storageceramicsHirokoTokoro1,2,3, MarieYoshikiyo1, KentaImoto1, AsukaNamai1, TomomichiNasu1,KosukeNakagawa1, NoriakiOzaki1, Fumiyoshi Hakoe1, KenjiTanaka1, KoujiChiba4, RieMakiura5,6,KosmasPrassides7&Shin-ichiOhkoshi1,2Commonlyavailableheat-storagematerialscannotusuallystoretheenergyforaprolongedperiod. If asolidmaterial couldconservetheaccumulatedthermal energy, thenitsheat-storageapplicationpotential is considerablywidened. Herewereport aphasetransitionmaterial that canconservethelatent heat energyinawidetemperaturerange, To530Kandreleasetheheat energyontheapplicationof pressure. This material is stripe-typelambda-trititaniumpentoxide, l-Ti3O5, which exhibits a solidsolid phase transition tobeta-trititaniumpentoxide, b-Ti3O5. Thepressurefor conversionis extremelysmall, only600bar (60MPa) atambienttemperature, and the accumulatedheat energy is surprisinglylarge (230kJ L1). Conversely, the pressure-produced beta-trititanium pentoxide transformsto lambda-trititanium pentoxide by heat, light or electric current. That is, the present systemexhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phasetransitions. Thematerial maybeusefulforheatstorage,aswell asinsensorandswitchingmemorydeviceapplications.DOI:10.1038/ncomms8037 OPEN1DepartmentofChemistry, School ofScience,TheUniversityofTokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo113-0033, Japan.2CREST, JST, KsGobancho, 7Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan.3Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,Tsukuba, 305-8577, Japan.4Science and Technology System Div., Ryoka Systems Inc., Tokyo Skytree East Tower, 1-1-2 Oshiage, Sumida-ku, Tokyo 131-0045,Japan.5Nanoscience and Nanotechnology Research Center, Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Osaka 599-8570, Japan.6PRESTO, JST,4-8-1Honcho, Kawaguchi332-0012, Japan.7WPI-AdvancedInstituteforMaterialsResearch, TohokuUniversity, 2-1-1Katahira, Sendai 980-8577, Japan.CorrespondenceandrequestsformaterialsshouldbeaddressedtoS.O. (email:[email protected]).NATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications 1&2015MacmillanPublishersLimited.Allrightsreserved.Phase transition phenomena, such as metal-insulator,ferroelectric ferromagnetic, and spin transitions, areattractive issues in the elds of physics, chemistry andmaterials science. Phasetransitions arecontrollednot onlybytemperaturechangebut alsobyother external stimuli suchaspressure, light-irradiationorelectriccurrentow. Forexample,for pressure-induced phase transitions, pressure-inducedmetal-semiconductor transitionina molybdenumdisulphide1,pressure-inducedsuperconductor transitioninafulleride2andpressure-induced ferroelectricantiferroelectric transition in aperovskitesystem3havebeenreported. Forlight-inducedphasetransitions, light-induced crystalline-amorphous transitions inchalcogenides4,5, light-induced metal-semiconductor transitionin a trititaniumpentoxide6and insulator-metal transition inperovskite manganites7,8, light-induced spin-crossover transitionsin metal complexes912and light-induced charge-transfertransitionin organicmolecules13,14and metalcomplexes15havebeen reported. Furthermore, for current-induced phasetransitions1618, current-induced insulator-metal transition inorganic compoundandcurrent-inducedmagnetic-domain-wallswitching in gallium manganese arsenide have been reported.Inrecent years, heat-storage materials have beenattractingattentionfromtheviewpointofenergysaving. Developmentofhigh-performance heat-storage materials is important for theeffective use of waste heat from blast furnaces in factories. Phasetransition materials are considered to be useful as latentheat-storagematerials. Thesearedividedintosolidliquidandsolidsolid phase transition types. In the former, the phasetransition at the melting point (m.p.) is used for the heat storage.For example, water (320 kJ L1at m.p. 0 C), parafn(140 kJ L1at m.p. 64 C)19and polyethylene glycol(165 kJ L1at m.p. 20 C)20areknown. Inthesecases, thereare concerns of liquidspill fromthe systemandmixing (orreaction)withthesurroundingmedia. Fromthisangle, asolidsolid phase transition material is stiff and its form is maintainedwithout support, while at the same time it has chemical stabilityagainst the surrounding media. Well-knownsolidsolidphasetransitionmaterials for heat-storage usage include copolymers(for example, hyperbranched polyurethane: 150 kJ L1at67 C)21, organic compounds (for example, neopentylglycol:165 kJ L1at 48 C and pentaerythritol: 360 kJ L1at188 C)22,23and organometallic compounds (for example,bis(n-hexadecylammonium) tetrachlorozincate: 120 kJ L1at103 C and bis(n-decylammonium) tetrachlorocuprate:60 kJ L1at 34 C)19,24,25. Ingeneral, suchphasechangeheat-storage materials cannot store the energy for a prolonged periodbelow the phase transition temperature. If a solid material couldconservetheaccumulatedthermalenergyandreleaseitonlyondemand, then its heat-storage application potential isconsiderablywidened. Fromthisangle, ourworkfocusedonaphase transition where the latent heat of thermal phase transitioncould be stored.Inthispaper, wereportaheat-storagematerial composedoflambda-trititanium pentoxide. The solidsolid phase transition ofthis material canbe controlled by heat, pressure application,light-irradiation and current ow. This heat-storage material canconserve ahighaccumulationof energyandrelease it bytheapplication of a remarkably small external pressure.ResultsMaterialandmorphology. Thesampleofthetitaniumoxide, anewseriesoflambda-trititaniumpentoxide(l-Ti3O5), waspro-duced by sintering rutile-TiO2 particles in a hydrogen atmosphere(see Methods). Elemental analysis using inductively coupledplasma mass spectrometry conrms that the formula of thematerial is Ti3O5. Scanning electron microscopy (SEM) andtransmission electron microscopy (TEM) images of the obtainedsample show a coral-like morphology with particle size ofB4 1 mm(SupplementaryFig. 1), composedof aggregatesofrectangular-shaped nanorods, of which the majority areB200 30 nm dimensions (hereafter called stripe-type-l-Ti3O5, Fig. 1a). Thehigh-resolutionTEM(HRTEM) imageisshown in Fig. 1b. The Fourier transform analysis of the HRTEMimage showed that the growth direction of the nanorods is alongthe crystallographic b axis. The atomic level image from HRTEMcorrespondstothevisualizedelectrondensitydistributionmaponthe bc plane calculatedby the maximumentropy method(MEM; Fig. 1c), described later.Pressure-induced phase transition. X-ray powder diffraction(XRPD) measurements were performed to investigate thepressure(P) dependenceof thecrystal structureof thestripe-type-l-Ti3O5. The XRPDpatternat 300 Kunder atmosphericpressure(P 0.1 MPa)isshowninFig. 1dandSupplementaryFig. 2. Rietveldanalysisindicatesthat thissampleiscomposedof 80.0(2)% l-Ti3O5and 20.0(2)% b-Ti3O5. l-Ti3O5adoptsamonocliniccrystal structure(spacegroupC2/m)withlattice parameters of a 9.83119(19) , b 3.78798(7) ,c 9.97039(19) andb 91.2909(7), anda unit cell volume,V371.207(12) 3. l-Ti3O5 has three symmetry-inequivalent Tisites, Ti(1), Ti(2) andTi(3), andve-symmetry-inequivalent Osites, O(1) to O(5). All the Ti sites forma six-coordinatestructure. Inthepreviousinvestigation6ofthesamepolymorphprepared fromanatase-TiO2nanoparticles, we observedsomeindications of a pressure effect. In the present research, thesamplewas pressedat various external pressures withapelletpress, andXRPDpatterns were measuredfor thepellets afterpressurerelease. WithincreasingP, theintensityof theXRPDpeaks of l-Ti3O5decreased and those of b-Ti3O5increased(Fig. 1d and Supplementary Fig. 3). The pressure wherethe fraction of l-Ti3O5becomes 50%(P1/2) is B60 MPa asshown in Fig. 1e. The crystal structure of b-Ti3O5 is monoclinic(space group C2/m; a 9.75252(18) , b 3.80034(6) ,c 9.44413(19) , b 91.5322(10)and V349.902(11) 3)(Supplementary Fig. 4). After pressurizing the sample andreleasingthepressureatroomtemperature, heatingthesamplecauses b-Ti3O5 to revert back to l-Ti3O5 at 470 K (Fig. 1d,f andSupplementary Fig. 5a). Above 530 K, l-Ti3O5 further transformsto a-Ti3O5. On the other hand, in the cooling process from 620 to300 K, a-Ti3O5 returns to l-Ti3O5 (Supplementary Fig. 5b). Thisl-Ti3O5is very stable in the wide temperature range of0oTo530 K. Furthermore, whenexternal pressurewasappliedtothis recoveredl-Ti3O5sample, l-Ti3O5exhibitedagainthephase transition to b-Ti3O5 (Supplementary Figs 6 and 7).Thevisualizedelectrondensitydistributions of l-Ti3O5andb-Ti3O5obtained using MEMfromthe XRPDpatterns, areshowninFig. 2a. TheMEMimageof l-Ti3O5showsthat theelectron density is spread between both Ti and O atoms, while inb-Ti3O5, the electron density is localized around each atom. Thisresult indicates the electron delocalized character of l-Ti3O5 andlocalized character of b-Ti3O5, which are consistent with the factthat l-Ti3O5is a metallic conductor and b-Ti3O5is asemiconductor. In addition, the visualized electron densitydistribution of l-Ti3O5in the bc plane well reproduces theHRTEM image, as mentioned in Figs 1b and c.First-principlescalculationofphononmode. Toelucidatethepressure-inducedphasetransition, rst-principlesphononmodecalculations were conducted. Figure 2b shows the phonon densityof states (DOS) based on the lattice vibrations for l-Ti3O5 and b-ARTICLE NATURECOMMUNICATIONS|DOI: 10.1038/ncomms80372 NATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications&2015MacmillanPublishersLimited.Allrightsreserved.Ti3O5. Thephonondispersionandphononfrequencies at theBrillouin zone centre, G point, for each of the phonon dispersionsare listed in Supplementary Fig. 8. Comparison of the two crystalstructuresshowsthatthecoordinationgeometryofTi(3)isdif-ferent between l-Ti3O5 and b-Ti3O5; Ti(3) is connected to O(5)in l-Ti3O5, while it bonds to O(4) in b-Ti3O5. Therefore, in thepressure-inducedphasetransitionfrom l-Ti3O5to b-Ti3O5, theTi(3) O(5)bondisconsideredtobreak, andtheTi(3) O(4)bond to form. The corresponding phonon modes of l-Ti3O5 lie at248.6, 318.5and445.8 cm1. Forexample, fortheBuphononmode at 445.8 cm1, Ti(3) vibrates signicantly towardO(4)and moves further away fromO(5) (Fig. 2c (upper) andSupplementaryMovie1). Onthecontrary, inthecourseofthethermal phase transition (that is, heat-storage process) fromb-Ti3O5tol-Ti3O5, theTi(3) O(4) bondis brokenandtheTi(3) O(5) bond is generated. The corresponding phononmodesnowlieat 226.7and339.3 cm1. Forexample, visuali-zationoftheBuphononmodeat226.7 cm1showsthatTi(3)signicantly vibrates towards O(5) (Fig. 2c (lower) andSupplementary Movie 2).Accumulated heat energy and pressure-released energy. Toinvestigate the heat-storage process frompressure-producedb-Ti3O5tol-Ti3O5andthe amount of accumulatedthermal 1516171819202122232 ()Intensity (a.u.)0.1 MPa230 MPa15 MPa45 MPa60 MPa75 MPa530 MPa, 300 K400 K450 K470 K480 K490 K500 K510 KPressure (MPa)Temperature (K)Phase fractions (%)Phase fractions (%)PTTP100806040200600 500 400 300 200 100 0100806040200500 450 400 350 300Low High0.8e 319.0e 3Ti3 O1 O4 Ti1cba-Ti3O5-Ti3O5-Ti3O5-Ti3O5-Ti3O5-Ti3O5Figure 1|Morphology of stripe-type-k-Ti3O5 and pressure-and-heat-induced phase transition between k-Ti3O5 and b-Ti3O5. (a) TEM image of stripe-type-l-Ti3O5. ThescalebarbelowtheTEMimageindicates50nm.(b)HRTEMimageofthesurfaceofstripe-type-l-Ti3O5showingtheatomicarrangement onthebc plane.The scale barbelowtheTEM imageindicates 1 nm. (c) Visualizedelectrondensity mapsonthebc plane ofstripe-type-l-Ti3O5 obtained by the MEM (isosurface 0.8e 3). The scale bar below the electron density map (left) indicates 1 nm. (d) Pressure (P) and temperature(T)dependenceoftheXRPDpatterns(l 1.5418). The ambient-temperatureXRPDpatternoftheas-preparedsampleatatmosphericpressure(P 0.1 MPa) is shown in the front,followed by XRPD patterns ofthepellet samples pressurizedby P 15 530MPa,measured after pressure release.Theseare followedbytheXRPDpatternsofpressure-produced b-Ti3O5withincreasingtemperaturefrom300K to510K. (e)Pressureevolutionofthephase fractions of l-Ti3O5 (blue) and b-Ti3O5 (red). The pressure where the fraction of l-Ti3O5 becomes 50% (P1/2) is an extremely small value of B60MPa. (f)Temperatureevolutionofthephasefractionsof l-Ti3O5(blue)and b-Ti3O5(red)intheheatingprocess.NATURECOMMUNICATIONS|DOI:10.1038/ncomms8037 ARTICLENATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications 3&2015MacmillanPublishersLimited.Allrightsreserved.energy in the system, heat capacity measurements wereperformed. First, we investigated the heat capacity of thepressure-produced b-Ti3O5. In the temperature region from 5 to300 K, specicheat was measuredbytherelaxationtechniqueusing the physical properties measurement system (Fig. 3a), andabove 300 K, specic heat accompanying the thermal phasetransition frompressure-produced b-Ti3O5to l-Ti3O5wasmeasured by differential scanning calorimetry (DSC; Fig. 3b). Bycombining the results from the physical properties measurementsystem and DSC measurements and integrating with temperature,theexperimental enthalpy(H) curves of l-Ti3O5andb-Ti3O5versus temperature were obtained up to 600 K(Fig. 3c; seeMethods). The transition enthalpy (DH) associated with the rst-orderphasetransitionfrom b-Ti3O5to l-Ti3O5was23020 kJL1(121 kJ mol1). In the temperature decreasing process oftheDSCmeasurement, therewas nopeak, indicatingthat theaccumulated heat energy of the phase transition from b-Ti3O5 tol-Ti3O5 was conserved in the system.Next the released energy of the pressure-induced phasetransitionfromstripe-type-l-Ti3O5tob-Ti3O5was measuredusingahigh-pressuremicro-DSCmeasurementsystematroomtemperature. After applying pressure, heat energy of24040 kJ L1wasreleased, whichalmost correspondstotheheat accumulated energy (Fig. 3d). Therefore, this materialconserves the heat energy of the phase transition from pressure-produced b-Ti3O5to l-Ti3O5and releases the accumulatedheat energy by applying lowpressure through the pressure-induced phase transition from l-Ti3O5to b-Ti3O5(Supplementary Movie 3).Thermal conductivityandsensibleheat-storageperformance.Bricks and concrete are useful as sensible heat-storagematerials20,2628since they release thermal energy slowly.Thermal conductivity measurements were performed for thestripe-type-l-Ti3O5 and pressure-produced b-Ti3O5. The thermalconducti-vities were 0.200.02 Wm1K1and 0.410.02 Wm1K1forl-Ti3O5andb-Ti3O5, respectively, whicharesimilartothevalues of bricks (for example, 0.16 Wm1K1)26and concrete(for example, 0.57 Wm1K1)28.Current-inducedandlight-inducedphasetransitions. Electriccurrentwasowedtothepressure-producedb-Ti3O5sampleat298 K. By owing a current of 0.4 Amm2, the colourof thesamplechangedfrombrowntodarkblue(Fig. 4a). TheXRPDpatterns before and after owing the current indicatethat b-Ti3O5istransformedintol-Ti3O5(Fig. 4bandSupple-mentaryMovie4). Theelectriccurrentdependenceonthecon-versionfromthepressure-producedb-Ti3O5tol-Ti3O5showsthat the thresholdcurrent value of the current-inducedphasetransitionis0.2 Amm2(SupplementaryFig. 9). Theoriginofthiscurrent-inducedphasetransitionisregardedasbreakingofcharge ordering or (and) Joule heat1618. The mechanismbybreaking of charge ordering is considered as follows: b-Ti3O5 is acharge-localized state whose charge is localized on Ti3 (3) withempty orbital on Ti4 (2). In contrast, l-Ti3O5is a charge-delocalized state whose charge is delocalized on Ti(2) and Ti(3).Byowingelectriccurrent tob-Ti3O5, thelocalizedchargeonTi(3) is forcedly moved to the empty orbital on of Ti(2), resultingin a transition to metallic l-Ti3O5.Light irradiation experiment was also conducted on a pressure-produced b-Ti3O5. The reverse phase transition from b-Ti3O5 tol-Ti3O5was observed by irradiation of 410-nmlaser light(Supplementary Fig. 10 and Supplementary Movie 5).Ti1Ti2Ti3O2O1O4O5O3 Ti2Ti1Ti3O1O2O4O3O5Ti1Ti2Ti3O2O1 O4O5O3O5Ti3O3Ti1O4 O1Ti2O2Ti3O4O5Ti3O4O5Phonon mode at 445.8 cm1Phonon density of statesPhonon density of statesb acb acElectron densityLow High60040020006004002000OTiTotalOTiTotalb acb ac Electron densityLow High0.45e 30.45e 37.5e 37.5e 3-Ti3O5-Ti3O5Frequency (cm1)Frequency (cm1)Phonon mode at 226.7 cm1Figure 2|Electron density maps and the phonon modes of k-Ti3O5 and b-Ti3O5. (a) Visualized electron density maps (isosurface 0.45e 3) of l-Ti3O5(upper) and b-Ti3O5 (lower) obtained using MEM from the XRPD patterns. (b) Phonon density of state (DOS) for l-Ti3O5 (upper) and b-Ti3O5 (lower).Blue,lightblueandgreyareasindicatethecontributionsfromphononsduetoTi, O,andthetotalphononDOS,respectivelyfor l-Ti3O5(upper). Red,orangeandgrey areasindicatethecontributionsfrom phononsduetoTi,OandthetotalphononDOS,respectively,for b-Ti3O5(lower).(c)SchematicillustrationoftheBuphononmodeat445.8cm1for l-Ti3O5(upper)andtheBuphononmodeat226.7cm1for b-Ti3O5(lower). Arrowsandtheirlengthsindicate thedirectionofthemovementoftheatoms andtherelative amplitudeofoscillation,respectively(see SupplementaryMovies 1and 2).ARTICLE NATURECOMMUNICATIONS|DOI: 10.1038/ncomms80374 NATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications&2015MacmillanPublishersLimited.Allrightsreserved.DiscussionThe generation of stripe-type-l-Ti3O5 originates from the changeintheGibbsfreeenergy(G)ofthematerial comparedwiththebulk or single crystal Ti3O5. This change in the Gvalue isconsideredtobeduetotheinterface(and/orsurface)energyofthe nanoscale domain. It is noted that there is no oxygen vacancy,which was conrmed by electron spin resonance. To understandwhy the stripe-type-l-Ti3O5 undergoes a pressure-induced phasetransition to b-Ti3O5, we considered the thermodynamics of thepresent phase transition phenomena using the mean-eld model,developed by Slichter and Drickamer29. In this model, Gisdescribed by DH, the transition entropy (DS) and the interactionparameter between l-Ti3O5 and b-Ti3O5 phases. The calculationshows that at atmospheric pressure (P 0.1 MPa), the sampleexists as l-Ti3O5(Supplementary Movie 6). This is becausel-Ti3O5 is synthesized by sintering at a high temperature, and itremains as l-Ti3O5with decreasing temperature due to theenergy barrier between l-Ti3O5and b-Ti3O5as shown intheGversusfraction(x)of l-Ti3O5curves(Fig. 5a(i)). Onthecontrary, onapplyingexternal pressure, theGversus xcurveschange; for example, the energy barrier disappears o400 K whenPis 60 MPa, andhence, l-Ti3O5transforms intob-Ti3O5onapplying pressure (Fig. 5a (ii)). The x versus temperature curvesof P 0.1 MPa and P 60 MPa are shown in Fig. 5b. As showninFig. 5c, xversuspressureplotsindicatethethresholdof thepressure-inducedphase transition. The originof the pressure-induced phase transition is the PDV term of DH( DUPDV),where DU and DV are the changes of internal energy and volume,respectively. At such a low pressure, the pressure-induced changeonDUisverysmall andnegligible. Infact, thephononmodecalculation under external pressure shows that the pressure-induced change of DU is B1 103kJ mol1at 60 MPa, whichistwoorderssmallercomparedwithPDV0.19 kJ mol1. Thepressure-inducedchangeonDSis alsoverysmall andcannotcontribute to the pressure-induced phase transition in the presentsystem (see Methods, Supplementary Fig. 11 and SupplementaryTables 1, 2). It is notedthat theobservedxversus Pplots ofFig. 1e is somewhat gradual. This is explained by the presence of adistribution in the transition pressure of the Slichter andDrickamer model, which may be due to the crystal sizedistribution. We have simulatedthis gradual pressure-inducedphase transition with a distribution of transition pressures(Supplementary Fig. 12).In summary, we report the rst metal oxide capable ofconservingtheaccumulatedheat energyof aphasetransition.Stripe-type-l-Ti3O5canstorealargeheatenergyof230 kJ L1,and this energy can be released by applying external pressure onlywhen demanded. The magnitude of the required pressure isextremely small, B60 MPa. This value is remarkably smaller thanthe typical pressures observed in the pressure-induced phasePICurrent22 20 18 16 22 20 18 16 2 () 2 ()Figure 4|Current-induced phase transition from b-Ti3O5 to k-Ti3O5. Anelectriccurrentof0.4Amm2owedthroughthepressure-producedb-Ti3O5 at 298K. (a) Photographs of the pressure-produced b-Ti3O5 before(left) and after the application of an electric current of 0.4Amm2(right).(b)XRPDpatterninthe2yrangeof16.022.5ofthepressure-producedb-Ti3O5(left)andaftertheapplicationoftheelectriccurrent(right).Blueandredareasmarkthepeaksof l-Ti3O5and b-Ti3O5, respectively.-Ti3O5PHeat energy release2,2402,2202,2002,1802,160500 400 300 200 100 0Temperature (K)H (kJ mol1)-Ti3O5-Ti3O5-Ti3O5Time (s)50004002000200500 450 400 350Heat flow (mW g1)-Ti3O5-Ti3O5Temperature (K)Heat flow (mW g1)0 50-Ti3O5200150100500350 300 250 200 150 100 50 0-Ti3O5-Ti3O5Cp (J K1 mol1)Temperature (K)Figure3|Thermodynamicpropertiesofstripe-type-k-Ti3O5andpressure-produced b-Ti3O5. (a) Molar heat capacity of l-Ti3O5 (blue) andb-Ti3O5(red)asafunctionoftemperature. ExperimentaldatawerettedwithaDebyemodel(seeMethods). (b)DSC chartsofthepressure-produced b-Ti3O5 with increasing temperature and l-Ti3O5 with decreasingtemperature. Apeak duetothelatentheatoftherst-orderphasetransitionfrom b-Ti3O5to l-Ti3O5(230kJ L1)wasobservedintheheating process, whereas no peak was observed in the cooling process. (c)Temperaturedependenceoftheenthalpy(H)for l-Ti3O5(blue)and b-Ti3O5(red). Whenpressureisappliedto l-Ti3O5,theaccumulatedheatenergyisreleasedasshowninthelowerenlargedgure(seeSupplementary Movie 3). (d) Pressure-released heat energy accompanyingthe pressure-induced phase transition from stripe-type-l-Ti3O5 to b-Ti3O5.Pressurewasappliedatt 0.NATURECOMMUNICATIONS|DOI:10.1038/ncomms8037 ARTICLENATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications 5&2015MacmillanPublishersLimited.Allrightsreserved.transitions in metal oxide materials3035and metalliccompounds3641, for example, the pressure-induced phasetransitionfromrutile-TiO2tobaddeleyite-typeTiO2at 1,043 Koccursat 20,000 MPa( 20 GPa)30. Fromtheviewpointof theenergybalanceofthethermodynamiccycle, pressureof60 MPacorresponds to B10 kJ L1, which is o5%of the pressure-releasing heat energy. Pressure of B60 MPa can be realized evenbythewaterpressureofahigh-pressurewashingmachine, andhence, l-Ti3O5has the potential tobe employedas pressure-sensitivesheets or reusableportableheatingpads. Inaddition,sincel-Ti3O5is ametallic conductor andb-Ti3O5is asemi-conductor, it has possibilities as a pressure-sensitive conductivitysensor or pressure-sensitive optical sensor. Furthermore, becausel-Ti3O5is composed of common elements (titanium andoxygen), it issafeandenvironmentallyfriendly. l-Ti3O5couldbeusefulforheat-retainingsystemsforresidentialuseandmayrealize more efcient uses of industrial waste heat generated fromfurnaces (Supplementary Fig. 13)42,43. In addition, light-inducedandcurrent-inducedphasetransitions frompressure-producedb-Ti3O5 to l-Ti3O5 are also observed, that is, stripe-type-l-Ti3O5shows reversible pressure-and-light-induced phase transition andreversible pressure-and-current-inducedphasetransition. Theseeffects are also attractive phenomena fromthe viewpoint ofadvanced electronic devices.MethodsMaterial. A new series of l-Ti3O5 nanocrystalliteswas produced by sinteringrutile-TiO2 particles in a hydrogen atmosphere (ow rates of 0.7 dm3min1) at1,117 C for 2 h, followed by a slow cooling process of B9 h from the sinteringtemperature to room temperature (SupplementaryFig. 14). Elemental analysisusing inductively coupled plasma mass spectrometry conrms that the formula isTi3.00(1)O5.00(6); Calc.: Ti, 64.2%. Found: Ti, 64.2(1)%. The experimentally obtaineddensity is 4.0000.048 g cm3, which is consistent with the theoretical valueof 4.00 g cm3from the crystal structure of l-Ti3O5 as determined by XRPDmeasurements.SEM and TEM images of the obtained sample show a coral-likemorphology with particle size of B4 1 mm, composed of rectangular-shapednanorods, of which the majority are B200 30 nm dimensions (SupplementaryFig. 1a). The Fourier transform analysis of the HRTEM image showed that thegrowth direction of the nanorods is along the crystallographic b axis. This newseries of l-Ti3O5 have larger crystal size than the previous series, which wereprepared from anatase-TiO2 (ref. 6; Supplementary Fig. 1b).XRPDmeasurements. XRPD measurements were performed with a RigakuUltima IV diffractometer with Cu Ka radiation (l 1.5418 ). The temperature-dependent XRPD measurements were undertakenusing a high-temperaturechamber with atmosphere control (RIGAKU-OAT003S) under N2 ow. TheRIETAN-FP computer programme was used for the Rietveld analyses,whileDysnomia was used for the MEM analyses.The rened crystal structures andcharge densities were visualized by the computer programme VESTA. Althoughboth l-Ti3O56and its high-temperature phase44,45can be considered as candidatesof the present material with C2/m crystal structure, we assigned the presentmaterial to l-Ti3O5 because it is obtained by a very slow cooling process taking ofca. 9 h from the sintering temperature to room temperature,and it is thermallystable.Heat capacity measurements. To investigate the temperature dependence of thelattice specic heat, C(T), in the temperature range of 5300 K, we carried out curvetting of the observed plots with the equation based on the two-Debye model46expressedby CT P2i1 9RciT=yi 3Ryi =T0x4ex

ex1 2dx, where R is gasconstant, ci is coefcient, yi is Debye temperature, x is o=kBT, : is thereduced Planck constant, o is phonon frequencyand kB is Boltzmann constant,with the t parameters of c13.2(1), c25.6(1), y14.1(1) 102K andy29.3(1) 102K for l-Ti3O5, and c12.7(1), c25.8(1), y14.3(1) 102Kand y29.3(2) 102K for b-Ti3O5. We then developed the temperaturedependencecurve of the specic heat in the temperature range of 5600 K usingboth the tted curve and the anomalous specic heat associated with the rst-orderphase transition from b-Ti3O5 to l-Ti3O5 obtained from the DSC measurement.Released heat energy on pressure application. Released heat energy on pressureapplication was measured with a high-pressure DSC measurement system (mDSCVII, SETARAMInstrumentation) at 300 K. Pressure application of 40 MPa wasachieved by instant injection of N2 gas into the sample cell.Thermal conductivity measurements. The specic heat and thermal diffusivity ofl-Ti3O5 and b-Ti3O5 pellet samples were measuredwith a DSC measurementsystem (DSC200F3 Maia (NETZSCH), NSST Co., Ltd.) and Light Flash Apparatus(LFA447NanoFlash, NSST Co., Ltd.), respectively.First-principles phonon mode calculations. First-principles calculations based onthe density functional theory were carried out for l-Ti3O5 and b-Ti3O5 using theVASP (Vienna ab initio simulation package) code. The wavefunctions based onplane waves and potentials of the core orbitals were represented by the projector-augmented wave of Blochl, and the exchange-correlation term was evaluated by thegeneralized gradient approximation by Perdew, Burke, and Ernzerhof. The crystalstructures of l-Ti3O5 and b-Ti3O5 obtained from the XRPD measurements wereused for computed models as the initial structures. The lattice parameters andatomic positions were optimized under no pressure and 1000 MPa with an energy10100 50 0Pressure (MPa)Fraction (x)-Ti3O5-Ti3O510500 400 300Temperature (K)Fraction (x)-Ti3O5-Ti3O5PP = 60 MPaP = 0.1 MPa-Ti3O5-Ti3O5-Ti3O5P250 K250 K500 K500 KFraction (x)0211012345G (kJ mol1)Fraction (x)0211012345G (kJ mol1)i iiP = 60 MPa P = 0.1 MPa0.9 0.7 0.9 0.7Figure 5|Mechanism of the pressure-induced phase transition based on a thermodynamic model. (a) Gibbs free energy (G) versus l-Ti3O5 fraction (x)for every 10K between 250K to 500K calculated using the SlichterDrickamer mean-eld model at P 0.1 MPa (i) and 60MPa (ii). Blue and red circlesindicate l-Ti3O5 and b-Ti3O5, respectively. l-Ti3O5 undergoes a pressure-induced phase transition to b-Ti3O5 because the energy barrier (shown by brownshadows) disappears by the application of external pressure above B60MPa as shown in the insets (see Supplementary Movie 6). (b) Calculated x versustemperature curves at P 0.1 MPa (blue) and 60MPa (red). (c) Calculated x versus pressure curve at 300K indicating a threshold pressure of B60MPa.ARTICLE NATURECOMMUNICATIONS|DOI: 10.1038/ncomms80376 NATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications&2015MacmillanPublishersLimited.Allrightsreserved.cutoff of 500 eV and 3 7 3 k-mesh until satisfying 105eVpm1forcetolerance. Supercells (1 3 1) of the optimized structures were used to calculatethe phonon modes and thermodynamic functions of l-Ti3O5 and b-Ti3O5, whichwere calculatedby the direct method implemented in Phonon code with 2 pmdisplacementsusing the optimized structures.Thermodynamicanalysis. In the Slichter and Drickamer mean-eld model, theGibbs free energy of the system is described as Gx(DH) gx(1 x) T{R[xlnx (1 x)ln(1 x)] x(DS)} Gb, where x is the ratio of the charge-delocalizedunit of Ti(1)3.3 Ti(2)3.3 Ti(3)3.3 corresponding to l-Ti3O5, g is theinteraction parameter between l-Ti3O5 and b-Ti3O5 phases, Gb is Gibbs freeenergy of b-Ti3O5 set as the origin of the energies, and R is the gas constant. Theobserved phase transition was considered to be a metal-semiconductor phasetransition between charge-delocalized Ti(1)3.3 Ti(2)3.3 Ti(3)3.3 andcharge-localized Ti(1)3.0 Ti(2)3.7 Ti(3)3.3 systems, which were regardedas l-Ti3O5 and b-Ti3O5, respectively. The values of DH11.5 kJ mol1andDS 25.2 J K1mol1, and a suitable value of g gagb f(T ), where ga14kJ mol1and gb1.08 102J K1mol1to be consistent with the observa-tion results, were used. When the external pressure is applied to the sample, DH isperturbed by the pressure-induced change on the DU and PDV terms. 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Okada (NIKKISO Co., Ltd.) for the knowledge of the pressureexperiment. The present research was supported in part by the CREST project of JST, theGrant-in-Aid for Young Scientists (A), (B), Research Activity Start-up and the NEXTprogramme from JSPS, the WPI Research Center Initiative for Atoms, Molecules andMaterials, SCF for Promoting Science and Technology and APSA from MEXT, IzumiFoundation and Kurata Foundation. M.Y. and N.O. are grateful to JSPS Research Fellowshipfor Young Scientists. M.Y. is grateful to ALPS and T.N. is grateful to MERIT from MEXT.AuthorcontributionsS.O. designed and coordinated this study, contributed to all the measurements andcalculations and wrote the paper. H.T. conducted the temperature-dependentXRPDNATURECOMMUNICATIONS|DOI:10.1038/ncomms8037 ARTICLENATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications 7&2015MacmillanPublishersLimited.Allrightsreserved.measurements, heat capacity measurements and thermodynamic calculations. M.Y.analysed the rst-principles phonon mode calculation results and partially wrote thepaper. K.I. carried out the Rietveld analyses of the XRPD patterns and heat capacity dataanalyses. A.N. conducted the Rietveld analyses and prepared the gures. T.N. carried outthe pressure- and temperature-dependent XRPD measurements. K.N. carried out theelemental analyses and background research. N.O. carried out the DSC measurementsand MEM analyses. F.H. obtained the TEM and SEM images. K.T. contributed to thesynthesis and Rietveld analyses. K.C. carried out the rst-principles phonon mode cal-culations. R.M. and K.P. contributed to the interpretation of the results and to the writingof the paper. All the authors commented on the manuscript.AdditionalinformationSupplementary Information accompanies this paper at http://www.nature.com/naturecommunicationsCompeting nancial interests: The authors declare no competing nancial interests.Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/How to cite this article: H Tokoro et al. External stimulation-controllable heat storageceramics.Nat. Commun. 6:7037 doi: 10.1038/ncomms8037 (2015).This work is licensed under a Creative Commons Attribution 4.0International License. The images or other third party material in thisarticle are included in the articles Creative Commons license, unless indicated otherwiseinthecreditline; ifthematerialisnotincludedundertheCreativeCommonslicense,userswill need to obtain permissionfrom the license holder to reproducethe material.To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ARTICLE NATURECOMMUNICATIONS|DOI: 10.1038/ncomms80378 NATURECOMMUNICATIONS | 6:7037 | DOI: 10.1038/ncomms8037 | www.nature.com/naturecommunications&2015MacmillanPublishersLimited.Allrightsreserved.