In Situ Determination of the Hfo2‐Ta2o5‐Temperature Phase

Home , Tantalum pentoxide

This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. 10.1111/jace.16271doi: email:[email protected] lead article this todifferences as version between this andthe ofPleasecite Version Record. been and throughtypesetting, proofreadingwhich may thecopyediting, process, pagination This article undergone has for and buthas accepted been not review publication fullpeer +1 Fax: 217 333 2736 +1Phone: 217 333 5258 *CorrespondingAuthor: Waltraud M. Kriven diagram hasbeen constructed temperatureof for equilibrium phases. From th testing reversibilityof composition from thermal arrest experiments determination of: (i) liquidus, solidus and invariant CO up to 30 The Abstract Articletype : Article PROFESSOR WALTRAUD M. KRIVEN (Orcid ID : 0000-0002-2230-1301) ALEXANDRADR NAVROTSKY (Orcid ID : 0000-0002-3260-0364) SCOTTMR. J. MCCORMACK (Orcid ID : 0000-0002-0715-3451) Scot 2 Accepted Article1

Department ofMaterials Science and Engineering, University Illinois of at Urbana-Champaign, 3 laser previously Pet t J.McCormack In SituIn er A. 00 ˚C usingquadrupole a lamp furnace and conical nozzle levitator system equipped

in conjunction with sy Rock Thermochemistry Laboratory NEATand ORU

Determination of the HfO unknown experimental HfO 2 Materials Development, Inc. Evanston, Illinois, 60202, USA via 1 , Kuo-Pin Tseng Alexandra Navrotsky

in - situ X

which is nchrotron X - ray diffraction, Illinois, Urbana,Illinois, 61801 California, 95616, USA consistentwith 1 , Richard Weber 2 2 - - Ta - Ta ray diffraction. These in ,

2 2 (ii) ese O 3 O and Waltraud M. Kriven 5 5 and - determination of -

t , an experiment transformation Temperature PhaseDiagram emperature phase

(iii the Gibbs ) molar volume , USA 2 , Denys Kapush , U P

al hase temperatures as equilibrium phases niversity California of Davis,

- HfO situ techniquesallowed the diagram

R measurements 2 ule - Ta 1

. 3 2

, O Sergey V.Ushakov

5 has been elucidated - t emperature up

to to 3000 ˚C function of

throu

as a function

gh with

pha

se a 3 ,

The HfO The ( polymorphsthree a as function temperatureof This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. thermophysical data for CALPHAD modelling. Thesearethe two main goals theof present study. temperature phasespace is (i) to: identify the equilibrium phases andto (ii) start collecting not mention any invariant transformations. The next step for developingHfO the addition of HfO reversibility. show Turcotteal. et polymorphs are considered beto metastablethey as canbe only formed cooling.on do They not around 320˚C as orthorhombic proposedwith a symmetry Immaof 1.1 The HfO Introduction I. rates >50 temperature (1360˚C)transforms the accepted inthe community.Sufficiently slow cooling and/or annealing belowthe transition tetragonal structure. They believed that this structure was more correct, butit has not been widely and Roth transformation then measured the thermal expansion as well as identified and characterized the peritectic and determined thehomologous seriesregime to be the Hf superstructures could also besynthesised via oxidation.McCormack and Kriven molten structure of Ta material systems proposedmonoclinic symmetry I2of metastable polymorphs forms.The first metastablepolymorph forms at around 940 ( reconstructive,reversible transition into thehigh temperature tetragonal polymorph having temperature orthorhombic polymorph with P resultsalso in a closely spaced compositional variation.Yang structures inwhich thedifference between successive memberssimplea is structural unit, which part of a homologous seriesTurcotte by and high temperaturedielectrics oxidation ofother high temperaturesystems such as Hf- SG AcceptedSG Article ) 14) ) when 141)

6 Ta Turcotte The equilibria ofthe Ta Hf , cubic, 27 29 2 ˚C/s, the symmetry (T- O 6 2 presented a monoclinic, spacegroup ( I2 Ta O 2 17 - 5 18 Ta superstructure (8-subcells stacked in thea-direction) spacewith group Ima2 ( 2 hastwo equilibrium polymorphspossible and two metastable polymorphs.low The O . The moltenThe . structure of pureHfO

15 2

2 17 . No information. No regarding thechange in structure rangeor was provided. , with , with 2 O 1 - et alet . was firstidentifiedwas Spiridonov by . Compositions thissystemwithin arecurrently usedthermal as barrier coatings Ta

5 -Temperature system T-

2 Ta O

2 5 O 2 system hasdistinct three compounds HfO collected preliminary liquidus datafrom thermal arrestexperiments butdid Ta O

5 has beenexamined by Alderman

2 becannot retained at room temperature.On cooling from T- O

, tetragonal, HfO 5 symmetry ( ) at approximatelyat ) 1360

2 - 6 22

Ta . Understanding ofthe HfO

suggested that Ta 2 O 27

T-

5 ( system are of interest applicationsfor inhigh temperature 17

SG Ta et alet . , w SG

at 2250at ˚C. 5) 5) (M 2 O ith P4 225) when 5 backinto O- 22 mm A homologous seriesis defined asgroup a of 15,16 ’- hesecond metastablepolymorph forms 2 2 Ta 2 has been examinedGallington by /nmcsymmetry ( : monoclinic, with P2 26 etal. SG 2 ( O 2

O SG Ta ˚C 5 5) alternative 5) structurethat is related to the ). T 5

formed ahomologous series with the 7,8 28 25) symmetry25) (O- 21 . It is worthIt is . mentioningStephenson that , and thensoon after determined be to Ta Hf 2 etal.

- 2 Ta et al.et - O

5 2

. However, even with fast cooling O

-C 20 5

2 23

system is also ofinterest forthe 9 , Hf – ( SG showed thatHf 13 SG and Hf-

6 137) when Ta 74) (O74)

1 /c symmetry /c (space group 2 O . McCormack al. et Ta 17 2 Ta and Ta O ’ - Ta -N 5 18 24 ) undergoesslow, a 2 and liquid at 14 2 were able were solve to -

O Ta alloys. 6 5 Ta etal. 2 ˚C ˚C with a ) 2 O 27 O 2 5 . These These . 5 O . Ta

- HfO 17 19 2

whilethe O SG I4 5 24,25 2 , two has 46) 1 /amd

2 – 5 , method energy, G from the thermodynamic parameters using the CALPHAD (CALculation of PHAse Diagrams) This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. conjunction withquadrupole the lamp furnace system equipped with experimentsperformed are: (i) Thermal arrest measurements temperature (3000 HfO ˚C) equilibrium phasesdue to fasttransformations cooling.on processing can inducethe formation of non-equilibrium phases or even highmiss temperature to kinetically entrap the equilibrium phasefrom the isothermal hold. However, insome systems, this quenched (cooled rapidlyto ambient conditions). The largecooling ratefrom quenchingassumed is whichinvolve heating samplestemperature to a of interestwhere held they are isothermally, then reversibility for any perturbation inthermodynamic variable. move to theglobal minimum.As such, thesekinetically stabilisedphases will not exhibit phase inlocal a minimum, which require additional kineticenergy to achieveequilibriumthe phase,i.e. kinetically stabiliz distinguish equilibrium phasesfrom non-equilibriumas phases, non-equilibrium phasescan become equilibrium phaseis one whichnot does follow either theseof key features. It candifficult be to anddirection reverse for any perturbationin thermodynamic variables. Conversely, a non- when aphase transformationat occurs its equilibrium conditions,reaction will occur intheforward systemnot is changing underconstant T, andP Thermodynamic parametersconstant are when thermodynamic variablesi.e. are constant the equilibrium phaseidentification. be not ableto predictits existence. highlightsThis the importance of the firststep experimental of equilibrium phasehas been missed andnot is included intheCALPHAD calculation, CALPHAD will applied in cases wherethe correct equilibrium phases beenhave identified and characterized. If an equilibrium phases will be those with thelowest free energyunder the specified conditions (T, andP j withinsystem, a using the measured calculatedor thermodynamic parameters (S, H, V thermodynamicof parameters Equilibrium phaseidentification as function of thermodynamic variables (T, P, materials.There are three main stagesin thedevelopment of accuratephase diagrams: (i) microstructure-property relations. These features are integral inthe design of all classes of ceramic generated from phase equilibria,accompanying the phasetransformations, as well as Building 1.2 accurate phase diagrams transformation temperatureswhile the systems

Accepted Article). Whilethis method is extremely powerful for building accurate phasediagrams, can it be only 30 Due to Due these factors, factorsThese haveled to mis-identification of equilibrium phasesfrom Experimentally, two key featurescan be used to identifyan equilibrium phase: (i) CALPHADThe method involves modelling the free energies, Mostapplications ceramicsof rely onfundamental a knowledge of phasediagrams 32 . . . The thermal arrest experiments determine will the liquidus, solidus and invariant ed . With respectthe to free energy, these kinetically stabilized phasesare trapped a 400 W CO 2 - Ta in 2

situ O

5 -Temperature phasespace accurately and efficiently. twoThe key 2 laser

at temperatureat experiments will be used elucidateto high

i n 32

and (ii) situ

XRPD will allow for equilibrium phaseidentification, 17,33

. (ii) Equilibrium(ii) . phase reversibility ( situin –

37 (QLF) and(QLF) CNL equipped with W 400 CO

X-ray powder diffraction (XRPD)in

using conical a nozzle levitator (CNL)

and (iii) modelling free of

of a of series of phases, ex

), (ii)), measurement situ

experiments

). The ) i.e.) 2 laser ( (XRD)with aBruker D5000 diffractometer (Bruker AXS Inc., Madison, WI, USA),using µm.<45 heatingh a at and cooling rate of 10 ˚C/min. annealedThe powders were thenground and sieved to USA) then pressed into pelletsin Carver a press (standard bench top press 3850, Carver,I 1050at for˚C hinzirconia3 a crucible Ta situ This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. phasefractions for each sample synthesized are summarized in Table 1. elemental hafnium and tantalum.The room temperaturecomposition measured from XRF and Shimadzu EDX-7000 (Shimazdu Inc., America Chicago, IL, USA)by collecting characteristicX-rays for Newton Square, accessedPA) Jade through software9.4.1 (Materials Data Inc., Livermore,CA, USA). forCentre Diffraction Data database PDF-4+ (ICDD 2015,v. International Centrefor Diffraction Data, and size step of 0.02 The˚. crystalline phase identifiedwas reference with toInternational the dried overnightat 100 ˚C to producea dry, porous m enough water and isopropanol were evaporated,forming a viscous gel. gelThe subsequentlywas glycol), the solution stirredwas for h 1 roomat temperature,followed by heating 300 at ˚C, until chargeto monomer chargeratio of four.After theaddition theof steric entrapment(ethylene agent having a molecular weight 67.07of amu, addedwas inproportion the to maintaincation a valence mixed and stirred forhour. one Ethyleneglycol (Aldrich Chemical Company, Milwaukee, WI, USA) determinedbased on cation stoichiometry theof oxidebeing fabricated.Thetwo solutions were dissolvedwas in isopropanol. The mass deionized water. Tantalum chloride,(V) 99.99 (metal% basis) Aesar, (Alfa Inc., Ward Hill, MA, USA 41 2.1 Powder synthesis and preliminary c II. ExperimentalProcedures parameter verification of transformation reversibility and measurements theof molar volume(thermodynamic

. Hafniumchloride, (IV) 99.9 (metal% basis) (Alfa Aesar, Inc., Ward Hill, MA, USA)dissolvedwas in

Accepted Article2 O volume measurements canused be for futureCALPHAD calculations to furtherrefine HfO the 5 at aat load of ~60 MPa. pelletsThese werethen annealed at 1300 ˚C inplatinum a crucible for 10 phase diagram.

Cr Elemental composition was measured by X-ray fluorescence (XRF) spectroscopy in a porousThe mass was then ground Hafniumtantalate powders weresynthesized by the organic steric entrapment of cations ystalline phasecomposition of the samples examinedwas bydiffraction powder X-ray

, 40 kV, 30 mA). XRD patterns wereacquired aover ) as a) function of temperatureand composition by Rietveld refinement , har a heating a and rate cooling of 10 ˚C/min.powders The were es at of hafnium of (IV)chloride andtantalum (V)chloride were acterization in a zirconia mortar and pestle, calcined and crystallized, ass.

range of

10 ˚ to 65 at˚ 1 ˚/min nc

., Wabash, IN,

. These . These radiation 39– 2 ) in -

approximately 5 min, or untilbeads the had sufficient green strength. This methoddescribed is in was vibrated ata frequency of 70 Hz ina cubed-walled container having 30 mm dimensions for vol% Darvin having200 a µmparticle size, 5 vol% methyl cellulose binder(Sigma-Aldrich, St Louis, MO, USA), 1 means of a vibrating table method 2.3.2 Conical nozzle levitator(CNL) equipped with aCO found to be 1040 mm and 0.589957 standard (SRM 660a; National Institute Standardsof and Technology, Gaithersburg,and MD) were detector. sampleThe to detector distanceand wavelength were determined by means of a diffractionpowder (XRPD) patterns were collected at each temperature with the Pilatus 70K ArgonneNational Lab,Advanced Photon Source(APS) Argonneat National Laboratory.The X-ray holdtimeat each temperature. The experiments were conductedBeamline at 33- (QLF) Alfamm; Aesar, Inc., Ward Hill, MA). Thesample was heated in air inaquadrupole lamp furnace 0.6 mm; Crytur, Turnov, Czech Republic) and mountedinlonger a alumina tube (OD = 2 mm, ID = 1.2 annealed The Hf standard 325-mesh (45 loosely µm), packed into a sapphire capillary(SapphiT OD 1.00 = mm, ID = St Louis, USA) MO, inan agate mortar and pestle. The mixed powderwas then sieved using a 2. 2. uncertaintyavoid from unknown effective sample emissivity. spectropyrometer (900 nm, 10 ms response - time, 700 were simultaneously monitoredusing two pyrometers. Intofast, addition a single-band pyrometer Ushakov.Temperatures on laser melting and cooling tracesduring quenching from theliquid state being heated laser.a with This methodhas been described in more detailby McMurray al et analyz tension of the oxide melt tendedtoform spheroidal beads. laser controller(Synrad UC-2000) which allowedincrementation the powerof by%. The 0.5 surface power ofthe beam was adjusteda by LabVIEW(National Instruments, Austin, TX, USA)controlled a 5 mm diameterat a 1 m fromaway theSynrad laser, FSi401SB, Mukilteo, WA, USA).laser The mm indiameterin a copperin air, hearth, with 400 a sealedW CO This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. 2. 3.1 Quadrupole lamp furnace (QLF) High3. temperature X-ray diffraction 2

Accepted Article . Thermal arrestfrom cooling traces 33 ed

from room temperature to a maximum of 1600 in ˚C, approximately 50 ˚C steps with a 3 min The HfThe Cooling profilesrecalescence on hafniumThe tantalate(HfO . Selected beads werelevitated and rotated inconicalnozzle a levitator ® dispersant (Vanderbilt Company, Inc., Norwalk,CT, USA)and 7 water.vol% This slurry 6 Ta 2 42 O (500 (500 17 was processedwas into polycrystallinesintered spheroids, mm in 2-3 diameter,by 6 Ta – nm,1000 -1400 2 O 17 powder waspowder mixed with 10 wt%Ptpowder (99.99%; Sigma-Aldrich, 53 2 . Ceramic slurries were prepared from 87 vol%Hf • Å, 17,33 3500 IR ˚C, Ta respectively. 2 31 – O 37 of the of hafnium tantalatebeads inair were recorded and 5

) powders ) were melted into polycrystallinespheroids 2-3 4000 4000 ˚C, FAR -CAS8CS;Chino Co., Tokyo, Japan), 2 laser Associates, Macedonia, was OH) used to

19,20,50 2 – laser (where 10.6 a beam µm had 52,32,43 – 49

32 (CNL) in(CNL) air,while BM a -C atthe 6 Ta 2 O 17

31 powder powder and

temperatures are required. pyrometers mayuseful be for temperature control butnot are so useful internal when and/or exact observed by X-ray diffraction. External temperature measuring devices,such as thermocouples and calculate averagethe temperature of the diffracted i.e.,volume, the volume of material being monitor the temperatureat these higher temperatures. Internal standardscan be toused accurately 2.3 errors of approximately apparatus. For the CNL system,sample the temperature was recorded from thepyrometer with variance inthermal expansion of the standard and (ii) thed-spacing resolution of theX-ray This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. the powder diffraction patterns to anaccuracy of characterized thermal expansion of platinum positions and profilefunctions were refined for each temperature.For theQLF system,well- the Analysis System Two (GSAS-II)program 2. and 0.123589Å, respectively. Institute Standardsof and Technology, Gaithersburg, MD, USA)and were found beto 1027.4 mm distanceand wavelength weredetermined with referenceto a the laser and CHINOpyrometer before each XRPD pattern measured. Thedetector sample to detector. The beam X-ray had dimensions 100 x 200 µm and soaligned aswas to incidentbe with XRD pattern heldwas atthe desired temperaturefor approximatelybefore 3 minutes taking measurements pyrometer wavelength. TheCNL system setupdescribed is in detailby Weber measured radiometric temperature correctedwas usingspectral a emissivity of 0.92 at the Japan)monitored the temperature of the bead surface incident with the laser and X-ray beam. The A CHINOpyrometer (900 nm, 10 ms response time, 700- melting point. Thesample temperature was controlledadjusting by incident laser the beam power. arrangementenabled the sample toheated be to approximately 3000 ˚C, FSi401SB, Mukilteo, WA, USA)beamthat partiallywas focused on the surfacetop of the sample. This axis simulateair)in a conical nozzle levitator (CNL)system Hafniumtantalate sintered beadswerelevitated in stream a argon of mixed with % 21 oxygen (to beads at the Advanced SourcePhoton (APS), Argonne National Laboratory,at Beamline 6- heating rate of 2 ˚C/min and cooling rate of 8 ˚C/min. moredetail by Santos et al. 3.3 Rietveld refinement.

Accepted.4 Article wh CNL temperaturecorrections ile beingheated usingbeam the from a 400 W sealed tube CO Unlike the QLF system, aninternal standard has yetto developedfor be the CNL systemto The resultingThe patternsXRD were refined via the R In situ s werecollected ~100 at temperature˚C interval , hightemperature, synchrotron XRD experimentswereperformed on the Hf

53 . The resulting. The spherical beads wereheat treated at 1300 for˚C hat6 a

˚C ˚C dueto temperature gradientsin 54 . The background,. The lattice constants, scalefactors, atomic 33 was used was toaccurately calculatethe temperature of

19,20,50

33,55 3500 3500 ˚C, IR ie tveldusing method the General Structure – . This error had. two sources:(i) the 52,32,43 by a-Si Perkin Elmer XRD1621 area

– the sample. 49 -CAS8CS;Chino Co., Tokyo,

. The levitated. The rotates sample on standard (SRM National660a; 2 laser (10.6 µm,Synrad wh ich wasaboveits et al . 43 . The sampleThe ID 6 Ta -D. 2 O . The 17

This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. while being cooled atbottom the these ceramic beadshadrelatively high temperaturegradients the sample position by as as much 100 um, aboutheighthalf the of the X-ray beam. Furthermore, its position with respect to the incident beam.In addition, changes inaerodynamiceffects can alter heating, the shape theof beadcould changedue to thermal expansionsintering and altered which steady control of the bead heightwith respect to theincident beamdifficult. X-ray was Upon for order the pyrometer to measurethe temperature theof diffracting volume. During levitation, by pyrometry. The incident X-ray beam neededto be located onsame the top surface theof beadin firstinvariant transformation is marked in green (monotectic), the second inred (eutectic)and th section 3.2.Toprevent confusion, the type invariantof transformations has been listedhere.The directly from thermal arrestexperiments but becould identified using tabulated inTable 3 average The liquidus temperature and standard error as functionsof composition are transformations due to their consistency intemperaturea functionas of compositionhave and been tabulated inTable 2. solidusThe temperatures have been defined as threeseparate invariant statistics onliquidus the andpoints. solidus temperatures are identified. Eachcomposition was heated and quenched fivetimesin orderto build is shown insupplementary informationclearly S1, demonstrating howliquidus and solidus changein temperature in thecooling trace, signifying thermalrepresentative arrest. A cooling trace Ta 3. ResultsDiscussion and III. temperature measurement the with volume measurement. sample temperaturefell within correct the temperature bounds. However, it coupledthe parameters polynomialallowingfit, for calculation of temperature the based theon observed lattice “melting pointlattice parameters”. Thesetwo data sets range. Thus, lattice parameters just priorcould to melting bemeasured andcould beused as the temperatures measured from thermal arrest experiments theover entiretemperature composition measuring thelattice parametersat well a characterizedtransition temperaturesuchthe as liquidus thermal expansion using theQLF system, where the temperature accuracy was to estimatethe temperatureof diffracted the volume.becould done This by first measuring the beused. resolved if the temperaturegradient in solid samplesreduced, was ifan internalor standardcould aligned be to incident with CO 1 1 Liquidus, solid Accepted2 Article O 5 composition. The liquidus and solidus temperaturesare defined by plateau a sudden or For example,For in theCNL system the surface temperature oflevitated the wasmeasured bead The thermalThe arrest datain theform coolingof are tracesdisplayed in Fig. for 1 each HfO Thistemperature correction greatlyuncertainty reduced and ensuredthat the observ Sincean internal standard not was available, the sample itselfbecould calibrated and used 25 . us and invarianttransformation temperatures . However, the exacttype invariantof transformation cannot bedetermined 2 laser beam theon surface the of bead.These issuescould be 50,51 . This is why theThis iswhy . beam X-ray small is (200 x 100 µm) and is couldbe then interpolated using a 56

due to beingdue to heated from the top, in situXRD anddiscussed is in

, as, well by as ed 2

- e For For changed. Determining whichsatellite peaks appear and disappearfunctionas a of compositionis This isindicatedbecause satellite lowintensity peaks appear and disappear as the composition is the Hf/Taratio is changed inthe compound closely spaced phases ratherthan truesolida solution.as Thatis, HfO constant,as expected for aregime two-phase in equilibrium. homologous series orsolid solution thisover composition range.O- Ta When O- of present within the range of showslattice the parameters of O- monoclinic symmetry ( symmetry( present:O- fraction. Each triangle corresponds to the sample composition,and temperature equilibrium phase composition.(iii in equilibrium.This is also supported bythe molar beingvolume constant as function of potential of each componentmust be equal in allphasespresent) to the that ensure two phases are observed, they mustsatisfy equi-parti the not does changewhen the temperaturebeing is held (ii)constant. When multiple phasesare thermodynamic variables (T,P, Couplingwith XRD the conical nozzle levitator (CNL)system equipped with CO equilibrium.The (i) thermodynamic parameters H, (S, V, able to effectively map out phases 3. the Hf This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Ta liquidus The datacollected here temperatures.We know no ofother studies that mention invarianttransformations intheHfO presentdata [HfO third inpurple (peritectic). liquidusThe and invariant transformationtemperatures as functions of 2 diagramPhase from in-situpowder X-ray diffractionthefor HfO 2 Accepted Article2 O O

5 5 Ta ] mol fraction] mol and O- -Temperature phase equilibria. McCormack

, the lattice, the parameters of O- 6

cubic symmetry ( Ta 2

The The constancyThe theof chemical potential in two phase regions can be observed in Fig Fig.showsthe 3 phases observed in equilibrium asfunction a temperatureof and [HfO O 2 5

steadily increase, signifyingthe existence a homologous of series solidor solution Ta SG

17 “solid solution”

ha peritectic transformation. 2

141)

O 6 ve Ta ) 5

with P Thereversibility the observed of transformations.

higher resolution incomposition andinvariant identify transformation 2

27 O are , O-

; O- ; 17 are constantbecause these two phasesin are equilibrium each with other plotted inFig. 2. SG Hf mm Hf SG the lattice parameters O- of 14); 14); T-HfO 6 Ta 6 225). Ta 2

like ranges for O- 2 26 O 2 orthorhombic symmetry( O 17 Ta

forms. forms. Between

) with Ima2with orthorhombic symmetry ( 2 Ta O are constant.This is achieved by ensuringthat the molar volume in Hf 57

5 2 situ . When 2 is no longer observedwhen with P4 6

O Ta

, O- 2 are as a functionas a temperature.of There are three key testsfor O ti 17 on Hf similar to thosecollect Hf vary as a functionas a vary of composition, signifying another

2 Ta ing theof chemical potential (i.e. thechemical . Within this composition rangethe lattice parameters /nmctetragonal symmetry ( 6 6

Ta Ta 2 O 2 2 et alet

O 5 O and O-

17 17 Hf and M-HfO ,

. 25

a seriesrelated of ordered structures form. 6 Ta recently publishedrecently a morein-depth study of

Hf 2 ) be must constant when the

O 25); T-

6 17 , O-

Ta and M-HfO

2 , the lattice ,the parameters of O- Hf O 2 at roomat temperature. O-

17 Ta 6 ed 2 Ta probably - Ta 2 by Turcotte al. et O 2 2 is added to O- O SG 2 5 O with

46); M-HfO 5 and M-HfO 2 SG -temperaturesystem are observed to be

137);and C-HfO

I4 a

. For For . homologous series of 1 /amd tetragonal /amd

Ta 2 2 with 2

laser, is one are 22 2 O but the 5 observed.

or or when Ta

P2 4. 57 2 2

1 . with O which 2 2 /c /c - ] mol ] mol

s This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. that ofO difficult for X-ray powder diffraction due to thescattering large factors Hfandcomparedof Ta to These include:These verified andtested for reversibility to thatensure they in are factthe equilibrium transformations with their associated changesin molar volumeat temperature. Several transformations need to be andisthere appearance no satellite of peaks. solution. Thisisbecause thereis an observed changein lattice parameter asHfO room temperatureexhibits homologous a series, whentransforms it to T- composition via movements of andO changes in superstructure multiplicity.Although the O- proposed howthe Hf Ta function high peaks intensity (predominantlyscattering from Hf/Ta) changed-spacing continuously changeswhich may be caused by oxygen ordering.It is clear that the The second The invarianttransformation can be identified eutectic as a by examining the liquid- transformationtemperature for ( solid two-phase regions. The confirm theimmiscible liquid region that anin features,either becausethey were notkinetically trapped because or it Itdoes notexpected is exist. withinproposed the two-phaseliquid regimedid reveal not immiscible liquidmicrostructural observation of the two-phaseliquid inregion equilibrium. Unfortunately, quenchingsample the to Due the rarity of monotectic transformations, whatis required for confirmationis a clear below the monotectictransformation temperatureas depicted in supplementary information (S3). likelymost monotectic. This monotectictransformation was observed beto reversible aboveand monotectic and notsyntectic a invariant transformation, the first so that invarianttransformation i melting pointhigher than thefirstinvariant transformation temperature. Thisis characteristic a of transformations: monotectic or syntectic. (ii) Itconfirmed is that thepure end member (Ta transformationtemperature spanning composition its andrange signifying possible two invariant two key observations:It(i) is confirmed that no solid phaseexists above the first invariant T-

(iv) (iii) (ii) (i)

2 Accepted Article O

, liquidexisted in equilibrium with O-

homologous series as well as the

The phaseThe transformations observed in the phase diagram (Fig. are 4) summarized in Table 4 The firstThe invarianttransformation can be identified asmonotectic a transformationbased on . However, when abovethe second invarianttransformation temperature, for the thirdthe invarianttransformation (peritectic: secondthe invarianttransformation (eutectic: firstthe invariant transformation(monotectic: the O- the Ta of 58,59 -situ

composition, suggesting that cation sublattice is essentiallythe same for boththe O- 2

5 The small small The contribution of oxygen to the total scatteringobscur scattering techniquesmall-anglesuch as X-ray scattering (SAXS) wouldberequired to

. + O Ta This eutectictransformation is reversible aboveand below the eutectic 2 O - Hf . 5

It is estimatedIt from extrapolation that the eutecticcomposition occurs at

6 6 Ta Ta T- 2 2 O O Ta 17 17 2 structurecan accommodatea

O in 5 situ transformation and the eutectoid

XRD was able to confirm that,above the second invariant .

Hf 6 Ta 6

Ta 2

2 ), liquid), was observed to 17 O homologous series. McCormackal. et 17 . Thissuggeststhat the eutectic temperaturei

T- T-

Ta Ta 2 2 O O al

5 5 transformation: T-

+ + O se

structures related, are the as

co ) )

-existin equilibrium with

2 es

slight symmetry 5 ),

it becomesa solid

2 is added to T-

change in ), ), Ta

2 have have

O 2 as 5

Ta 5 ) has) a

O- 2 Ta

O s . 5 2 at at O s 5

T- Ta experiment. Ittook hours40 annealing inaboxfurnace at 12 form. Thesephases did notback revert to O- cooling, a - M’ clearIt is thatroom the temperatureO- This stemmed from thedifficulty in verifying reversibilityO- the of here. This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. cooling of T- from the high temperatureT- in depth andpublished is elsewhereby McCormack peritecticwas observed to bereversible temperature at decomposition of the O- (S transformation temperaturehypo-eutecticfor a compositionandhyper-eutectic (S4) a composition temperatures at which themetastable Ta must must exist inthe HfO Although therelittle is evidencefor theeutectoid latent heatfrom the eutectoid in the microstructurewhen quenching from the liquid phaseinhyper-eutectoid the eutectoid transformationT- composition rangewhere T- metastable transformations inTa homologous seriesare lines drawn for theTa regime matchthose predicted by the work of McCormackand Kriven CALPHADcomputational methodin the future.The homologous serieslines drawn intheHf also one needs theGibbs free energyfor each equilibrium which phase, should be attainable bythe While the identification of equilibrium phases partitioning chemicalof potential and transformation energy. It has been constructed based on stressed that this preliminaryphasediagram has not been builtbased minimizing on Gibbs free the HfO beshown to reversible and thus not will be discussed indetail more here. It was observed, that C- Phase Rule 5

Accepted Article ). O 2

existed inexisted equilibrium with liquid when 5 phases to revertback to the equilibrium O-

The equilibriumThe structure of room temperature O- The thirdThe invarianttransformation can be identified asperitectica the by observed This transformation The M-HfOThe The phasediagramThe can be seen moreclearly without data points inFig. It 5. mustbe transformation reversible, is containing metastable forward transformations on cooling

al 60 which are similar to what has beenpreviouslyreported inlietrature the transformationto be needs verified. transformationThe hasnotbeen directly observed

is satisfied. Ta

O Ta

5 follow: follow: T- 2 (monoclinic - to T-HfO 2 O 2 - 5 Ta

2 O O-

5 al

Ta phase diagram atlow [HfO Ta

reversibility transformation al 2

2 O

transformation was not observed

5 I2 2

O wa

+ O 27 phase into T-HfO 5 phase. is proposed It thatthe metastable phases formed on

need to bestudied more rigorously. ( s stable,thusconfirming the reversibility theof eutectoid SG

- M’ Hf Ta

5) 6 2 - in 2 Ta O Ta of O ) situ 5 T- and anO’ Ta 5 2 2

2 begin to form: M’ phase transforms to T- the O O 17

O is Ta 2 17 5 and the T-HfO 5 O of utmostimportance for accurate phase equilibria, equilibrium phaseidentification (according to equi-

regime . The exact composition exact The . and temperature of the Ta 2 5 al

O on on the time scale of the O’

transformation based theon data collected, it 2 5 et al. O

- 2

al Ta and liquid.O- The 5 - phase Ta O- reversibility) and the GibbsPhaseRule 2 2 ] mol fractions to ensurethat theGibbs are (S O

25 2 O Ta 6).This transformation hasbeenstudied 5 Ta and discussed be thus not will further

. simply schematic a not have andbeen 5 2 (orthorhombic Imma- 2 00 O- O O 2 (S to to C-HfO - 5 5

Ta 7). 7). This shows that the O- was consistentwas acro was thewas difficultmost to determine.

˚C for the T 2

O in

5 cooling traceexperiments

24 at

and Roth . Thisisbased on the

Ta transformation

2 in - T- O

Ta situ

phase. However, on

2 O

et al.et 5 diffraction

, M’

27 al 27

ss and O’ transformation ( . The regime. The - 61 the entire T- SG Ta The 18

2 74)

O have beenhave -

5 Ta 6

and O’

Ta )

2 phase 60 + al O

2 . . O

- . to cubicto discussed in moredetail by McCormack et al. thermalThe expansion and transformation mechanisms thisof peritectictransformation are from the peritectic transformation is observed to undergo complete melting volume change of peritectictemperature transformation mechanismswill be discussed in more detail inafuturepublication. function temperatureof up until melting at approximately measurements. isThere small a change inmolar volume of co a unitTherefore, cell. N volume of the unit cell, This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. transformation mechanismsdiscussed are in detailmore Haggerty by H symmetry Fig.shows the 9 molar volumefunctiona as temperatureof for the HfO throughfirst a ordertransformation from monoclinic molar volume of M-H Fig.shows the 8 molar volume as function temperatureof for Hf the molar volume of O- associated I4 Fig. displays 7 the molar functionasa volume temperatureof for the Ta 1360 ˚C, there molar volume of O- Hf Fig. depicts 6 the molar asvolume function of [HfO plottedbelow and tabulated inthesupplementary information (S8, S10, S9, S11). powder X-ray diffraction (XRPD)can beused to calculate the per molar volume cation of phasesusing Rietveld refinementaccording to 3. homologous series need still be to determined. determined experimentally theoretically or in this work.lineThe compositions of the Ta structures room at temperature.There is a large changein molar volume of nd fO 1 3 Molar volumesof equilibrium phases 6 mparing O- /amd /amd (

Accepted Ta Article composition for the equilibrium phases thewithin HfO 2 increases further, itundergoes a first order transformation from tetragonal 2 O 17

and M-HfO at 1715 ˚C

141

is a phaseis a transition from the orthorhombic P (225) symmetry(225) withvolume a change of 2 ) symmetry,) along a with first order transformationwhich is accompanied byan O 5

and O- Hf Ta

2 with an associated changein volume fO 6 C 2

. Ta O Z isZ thecation formula unit.The molar volumes

2 5 increases accordingsecond to a polynomial. order On heating, M-HfO 2 increases, following second-order a polynomial as expected. Ataround .

O The molar volume of the T- Hf is the number of cations intheformula unitand 17 6

increases inasigmoidal manneras function of temperature up to the

Ta between O- the . At the transformation. Atthe temperaturethere is crystallographic a molar 2 O 17 , which, hasbeen fromverified pycnometer bulk density 25 Hf elsewhere.

Ta P2

17 , where 2 and the T-HfO 1 O /c (

2 2

- phasethen increasesalmost linearly as ] mol fraction for thethree equilibrium

Ta mm2

2 to14) tetragonal

is the molarvolume, 5 - ( Temperaturephase space are

The thermalThe expansion and The thermal expansion and

etal. 25 as 2 phase.The T-HfO when comparing O- a function temperature of ) symmetry) to tetragonal . Asthetemperature T- of 17

6 and by Tobaseand by

at is the formulais the unit per Ta

around 2

2 2 O compound. The O P4 P4 17 5 compounds. The 2 compound. The 2

/nmc ( /nmc (

2 formed 5 SG SG is the

et al.et when 137) 137) 2

goes . 18 . a This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Conclusions IV. experiment molar volume measurementsfunctionas a of temperaturefor equilibriumFromdata, this phases. an equilibrium phases through testing theof reversibility condition in-situ X-rayvia diffraction, (iii) temperaturesa as function composition of from thermal arrest experiments,(ii) determination of techniquesallowed for the determination(i) of: liquidus, solidus and invarianttransformation equipped with a elucidated up to 3400 ˚C usingquadrupole a lamp furnaceand conical nozzle levitator system Hf trace data Supplementary Information is that expectedbased on theGibbsPhase Rule using directly Small Angle X-rayScattering (SAXS), (ii) a eutectic, (iii)a peritectic and a (iv) eutectoid, identification of invariant four transformations: (i) a monotectic, that needs still be to verified diffraction. diffraction. 6

AcceptedTa Article 2 O Tabulated room temperature lattice parameters as function of [HfO of function as parameters lattice temperature room S2:Tabulated cooling from determined are solidus and liquidus the how traceshowing cooling S1:Representative article: this of version the online on found be can information supplementary Additional Reversibility of the orthorhombic Ta orthorhombic the of S7:Reversibility on based invariant peritectic the of S6:Reversibility on based invariant hyper-eutectic the of S5:Reversibility on based invariant hypo-eutectic the of S4:Reversibility on based invariant monotectic the of S3:Reversibility 17 Tabulated molar volume of Ta of volume molar S9:Tabulated HfO of function asa volume molar S8:Tabulated S10: S11: The previouslyThe unknown experimental HfO The thermalThe arrest experimentsand in-situ diffraction X-ray experiments led to the and M-HfO and Tabulated molar volume of Hf volume molar Tabulated Tabulated molar volume of HfO volume molar Tabulated al HfO CO 2 - 2 Ta 2

laser, in conjunction with synchrotronpowder X-ray diffraction. Thesein-situ 2 O 5 phase diagram was been constructedbased onGibbs the PhaseRule 2 O 6 Ta 5 2 as a function of temperature. temperature. of function as a as a function of temperature. of function a as 2 O 2 17 O as a function of temperature. temperature. of function as a 5 to tetragonal Ta tetragonal to 60 . This. eutectoid still needs to be directly.verified 2 2 - composition for O- for composition Ta in 2 O -situ X-ray diffraction. diffraction. -situ X-ray in 5 -situ X-ray diffraction. diffraction. X-ray -situ -temperature phasediagram has been in in -situ X-ray diffraction. diffraction. X-ray -situ -situ X-ray diffraction. diffraction. -situ X-ray 2 O 5 transformation based on on based transformation

Ta 2 2 O ] composition for O- for ] composition 5 , O - Hf 6 Ta 2 O 17 and M-HfO and ex -situ X-ray X-ray -situ Ta 2 O 60 5 . , O 2 . . - . CourtrightEL, PraterJT, Holcomb GR, PierreGR St., Rapp RA.Oxidation of hafnium carbide 9. Marnoch High-temperature K. oxidation-resistant hafnium-tantalum alloys. JOM [Internet]. 8. Mardare AI, Ludwig A, Savan A, WieckAD, Hassel AW. Combinatorial investigation Hf of 7. Makovec ZuoD, JM, Twesten R, PayneDA. Ahigh-temperature structure for Ta 6. M, Li Xu WangQ, High-TemperatureL. Chemical Stability Hf6Ta2O17of Ceramic Thermalfor 5. M, Li Xu WangQ, L. Preparation and Thermal Conductivity of Hf6Ta2O17 Ceramic. TransTech 4. Rana H. AReview Paper on Thermal BarrierCoatings (TBC)to Improve the Efficiency Gas of 3. This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Accepted M, Li Qiang X, Zhu S, Wang WangL, F. Preparation and Thermal Expansion Hf of 2. Perepezko J. New Oxide Materials for an Ultra High Temperature Environment DE- 1. References Article preparation. sample with assistance their for Truskowski Mathew Tso and Whitney Sherman, Nicholas Ribero and 33 beamlines at was completed and 06CH11357 of Ener Department DMR1835848. NSF under funded of Materials Laboratory Research Materials Seitz Frederick the at part in out carried was research Acknowledgements at - 6 Rodriguez http://link.springer.com/10.1007/BF03378394 modulations by TiO 1965 Nov1965 12 [cited Nov2017 13];17(11):1225 Barrier Coatings. Key Eng Mater. 2012 Jun;512 Pubclications, Switz. 2010;434 Turbine.-International IJSRD J SciRes Dev. 2016;4(03):2321 https://www.sciencedirect.com/science/article/pii/S0013468610004688 31];55(27):7884 thin and films their anodic oxides. Electrochim Acta[Internet]. 2010 Nov 30 [cited 2018 Jul S S Ceramic. Rare Met MaterEng. 2011;40(6):612 SC0010477. 2017. - Use of the Advanced Photon Source at Argonne National Laboratory was supported by theS. U. by was supported Laboratory National Argonne at Source Photon Advanced Use the of UC and Davis at performed traces was ofcooling recording melting and laser by Sample preparation the National by funded work was This ID pecial thanks pecial thanks pecial thanks - D with t D with

at the University o University the at

and Ben Hulbert Ben and he assistance of of he assistance gy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE No. Contract under Sciences, Energy Basic Officeof of Science, Office gy, also go also go go – to members of the Kriven Research Group Research Kriven the of members to 91. Available from: 2 substitution. J Solid StateChem. 2006;179(6):17 to the Illinois S Illinois the to f Illinois at

who who Dr. Chris Benmore. Chris helped with helped – Urbana 435:459 cholar Undergraduate Research Undergraduate cholar Science - BM - Champaign. –

the beamline experiments. beamline the - 61. 61. C with the assistance of of assistance the C with

Foundation (NSF) under grant grant under (NSF) Foundation – – – 31. Available from: 4. 515:635

: Andrew Steveson, Kevin Seymour Kevin : Andrew Steveson, – 8. – 613.

Dr. (ISUR)

Jenia (Evguenia) Karapetrova Karapetrova Jenia(Evguenia)

82 students ,

DMR Centre – 91. 91.

. 1411032

: Nicole Crosby, Crosby, : Nicole for Microanalysis forMicroanalysis 2 6 - O Ta AC02 5 with 2 O

This 17 -

, Daniel Daniel ,

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signifieswhite two-phase a regime. Thisdatais tabulated in supplementary informationS2. HfO2 mol fraction [HfO2] atroom temperature. Grey signifiesa homologous seriesregime while Fig. Lattice 4. parameters ofO-Ta2O5, the O-Hf6Ta2O17 and M-HfO2 compoundsa function as of levitator system (CNL). liquid present, (black: white: notpresent). Thesedata points werecollected usingconical the nozzle HfO2 (black: monoclinic, red: tetragonal, blue: cubic, white: not present)and thecenter refers to the top refersto Hf6Ta2O17 (black: orthorhombic, white: notpresent) the rightcorner torefers corners. leftThe corner refersto Ta2O5 (black: orthorhombic, red: tetragonal, white: notpresent), equilibrium phases. The obs diffraction. Each triangle corresponds to the sample composition, temperature and the observed Fig. Construction 3. of the HfO2-Ta2O5-Temperature phasespace from in-situ X-raypowder Eutectic (L Fig. Liquidus 2. temperaturesand invariantreaction temperatures: (i) Monotectic (L_1 determineto the recalescencetemperature (liquidus)and invariantreaction temperatures. Fig. Cooling 1. tracecurves collected HfO2-Ta2O5.on five samples of each composition tested were Figures GalyJ,Roth RS. The crystal structure Nb of 61. Daub Gibbs EE. phaserule: A centenary retrospect. JChem Educ[Internet]. 1976 [cited Dec 60. ChantlerCT. Detailed tabulation atomicof form factors, photoelectric absorption and 59. ChantlerCT. Theoretical form factor, attenuation, and scattering tabulation for Z =1 58. This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Accepted Article from: http://aip.scitation.org/doi/abs/10.1063/1.1321055 Phys Chem Ref Data[Internet]. 2001 Mar [cited5 2017 Nov 1];29(4):597 http://www.sciencedirect.com/science/article/pii/0022459673901345?via%3Dihub [cited 2017 Oct 27];7(3):277 2018 May 24];53(12):747. Available from: http://pubs.acs.org/doi/abs/10.1021/ed053p747 insoft the X-ray (Z=30 scattering cross section, and mass attenuation coefficientsin thevicinity edges absorption of 1];24(1):71 E =1 E http://rsta.royalsocietypublishing.org/cgi/doi/10.1098/rsta.1973.0078 from: Philos Trans R Soc A Math Phys Eng Sci[Internet]. 1973 Aug 9;274(1245):627

– α+β) and (iii) Peritectic(α+L 10 eV to E =0.4 – 643. Available from: http://aip.scitation.org/doi/10.1063/1.555974 – erved equilibrium phases correspond to thecolor of the triangle’s 1.0 1.0 MeV. J Phys Chem Ref Data [Internet]. 1995 Jan 15 [citedNov 2017 – 36, Z=60 – 85. Available from: –

89, E=0.1 keV β), plottedβ), as 2 Zr 6 O – 17 10 10 keV), addressing convergence issues.J function [HfO2]of (mol fraction) inTa2O5. . J Chem Solid State [Internet]. 1973 Jul 1 – 1048. Available – 61. Available

α+L_2),(ii) – 92 from This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Fig. theoretically.or drawn for the Ta2O5 regimeare simplyschematic a and not have been determined experimentally predicted by the work of McCormack and Kriven24 and Roth al.61et homologous The serieslines homologous series. homologousThe serieslines drawn intheHf6Ta2O17 regime matches that peritectictie lines would have toadjusted be based onGibbs the Phase Ruleto accommodate the Hf6Ta2O17 regime signify thepotential homologous series compounds.of Theeutectoid and compounds which exhibit a homologous series. The dashedin the lines O-Ta2O5 andO- the in Fig. HfO2-Ta2O5-Temperature 5. phase spacebuilt based on the observed equilibrium phases from is discussed in depthmore by Haggerty et al.17 AtomicS11. mechanisms for the anisotropic thermal expansion related to this molar volume change signifiesdata collected usingQLF the system. This datatabulated is insupplementaryinformation change of (∆ )/ second order transforma on from T M-HfO2 to T- Fig. Molar 9. volume of as HfO2 function of temperature. Notethe first order transformation from transformationdiscussed is depth inmore by McCormacket al.25 anisotropic thermal expansion related to this molar volumechange and the peritectic system. This data is tabulated in supplementary information S10. Atomic mechanisms for the signifiesdata collected usingCNL the system whilewhite signifiesdata collected usingQLF the =1.16% Thischange involumedoes notinclu transforma onat T 2244˚C withan associated crystallographic molar volume change(∆ of )/ Fig. will bediscussed in moredetail infuture a paper. S9. Atomic mechanisms foranisotropic the thermal expansion related to this molar volumechange signifiesdata collected usingQLF the system. This datatabulated is insupplementaryinformation melting was observed Tat 1870 Grey˚C. signifiesdata collected using transforma onat T 1350˚C withan associated molar volume change(∆of )/ Fig. 7. tabulated in supplementary information S8. Grey signifiesa homologous series regime while white signifiesa two-phaseregime. This data is volume changebetween theequilibrium O Ta2O5 and O-Hf6Ta2O17 has been verified from pycnometer density measuremen [HfO2

Accepted Article powder-situ X-ray diffraction and theGibbs Phase Rule.H Thesubscriptis referring the to 6 8 . . Molar volume of O ] Molar volume of the O Molar volume of O at room temperature HfO2 at T with anassociated molar volume change(∆of )/ T 0. 1700 ˚C. Grey signifies data collected using theCNL syste - - Ta Hf 2O 6 . - The largeThe change in molar volume Ta Ta 5 2O17 and 2O17 T 2O5, O 2O5, and T - HfO2 - - Ta Hf 2O 6 to C - - Ta Hf HfO2 5 dethe volume theof liquid phase. T 2450 ˚C .Grey 2O17 and 2O17 M 6 as func ofon temperature - Ta HfO2 at2600 T ˚C with anassociated molar volume 2O17and M as func onof temperature - HfO2 - HfO2 (( as func ofon HfO ∆ )/ ∆ structures(∆ is )/ the CNL system while white

=-25.24%) between O- . Note therst order

. Notethe peritec c -

2. 78 =-2.32% .=-2.32% Complete m while white 2 ts . Also . note the mol frac on . The molar

=3.07%.

0.95HfO 0.15HfO 0.05HfO 0.9HfO 0.8HfO 0.7HfO 0.6HfO 0.5HfO 0.4HfO 0.3HfO 0.2HfO 0.1HfO Hf This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. resolvephase peaks and intensities. characteristic X-rays. Phase fraction error (synchrotron diffraction) X-ray comes from the ability to Composition error (XRF)comes from deconvolutethe ability to and quantify the Hf and similar Ta Table1 Tables Sample Ta 6 HfO Ta 2 2 2 2 2 2 2 2 2 Accepted2 Article2 2

2 0.05Ta 0.1 0.2Ta 0.3Ta 0.4Ta 0.5Ta 0.6Ta 0.7Ta 0.8Ta 0.85Ta 0.9Ta 0.95Ta O 2 O 2 5

Ta 17

–

Summary HfOof 2 2 2 2 2 2 2 2 2 O O O O O O O O O 2 2 2 O O O 5 5 5 5 5 5 5 5 5

5 5 5

[ HfO 1 0.95 0.90 0.86 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.15 0.10 0.05 0.00 .00 (mol fraction) (mol 2 C

] omposition 2

- Ta 2 O 5 binary samples fabricated using thesteric entrapment method. [ Ta 0 0.05 0.10 0.14 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.85 0.90 0.95 1.00

.00 2 O

5 ]

XRF (mol fraction) (mol XRF [ HfO 0.31 0.23 0.15 0.12 0.0 0 1 0.9 0.9 0.86 0. 0.7 0.60 0. 0. .00 .00 77 50 40 5 6 0 2 2

]

[ Ta

0.6 0.7 0.8 0.8 0. 1 0 0.04 0. 0.1 0. 0.28 0.4 0.5 0 .00 .00 .60 2 9 10 23 O 9 7 5 8 5 4 0 0

5 ]

Phases (mass fracti (mass Phases HfO 0.00 0.00 0.00 0 0.00 0 1.00 0.48 0.0 0.00 0 0.00 0.00 0.00 0.00 .00 .00 .00 2 2

on at room t room at on Ta

0.5 1.00 1 1.00 1.00 0.00 0.00 0.00 0.00 0.03 0.09 0.1 0.2 0.3 .00 2

2 3 3 5

emperature) Hf 6 0.00 0.52 0.98 1.00 0.97 0.9 0.87 0.77 0.65 0.48 0.18 0.00 0 0.00 0.00 Ta .00 2 1 O

[HfO 1.00 0.96 0.86 0.77 0.72 0.31 0.23 0.15 0.12 0.05 0.00 0.9 0.6 0.5 0.4 2

] (mol] fraction)

Liquidus

Temperature

(˚C)

referenceto Figs. 6-9 This article is protected by copyright. All rights reserved. This article isprotected rights by All copyright. reserved. Change** in molar volume for crystallographic phases (does notinclude volume liquid of phase). Acceptedtemperature. Referto Fig 6. These* are reactions comparingthe changein molar volumeas function [HfO of 4Table ArticleGibbs Phase Rule.Thepresented composition and temperaturesare estimates. *The eutectoidpoint notdirectly was determined. Itis included hereasis it required to satisfy the 3Table

( T O O -

- Fourth invariant: - Ta First invariant: Hf

Second invariant: Ta T – – Third Third invariant: Transformation - Change in molar volume for transformations intheHfO Invariant Transformation/Reactions. (O O (O 2

HfO 6 2 O Ta

- O - -

Hf Hf 5

T 5 O

2 M

Invariantpoint 6 -

O T O 2 +M Ta 6 - Ta

Ta 5 - Ta

- 17

HfO 2 O compared to O HfO O

2 - 2

- 17 O T O HfO O Ta

compared to - 5

Ta 2 17 5

2 M

+ O

2 2 -

- O Monotectic HfO 5

Hf T

T - Peritectic + T 5 Eutectoid - Hf /Reaction - -

HfO - Ta

O 6 HfO Eutectic O HfO Ta - 2 6 Hf -

- Ta 2 + O Hf M

Hf O 6

2 2 Ta 2 - O

2 2 HfO 5 6

6 O

2 Ta 17 - + L Ta O Ta 17 17

2 2 )

2 )

O 17 17 5 * *

Change Molar in Volume Composition

[HfO

2 ] (mol] fraction)

(%)

-Temperaturesystem with

Temperature

2 ] room at ~2600 ~1700 ~2244 ~1350 Temperature ~26 ~26

* *

~1300*

(˚C)