Assessment of thermodynamic functions in the ZrO2–La2O3–Al2O3 system

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Journal of Alloys and Compounds 453 (2008) 271–281

Assessment of thermodynamic functions in theZrO2–La2O3–Al2O3 system

O. Fabrichnaya a,∗, S. Lakiza a,b, Ch. Wang a, M. Zinkevich a, F. Aldinger a

a Max-Planck-Institut fur Metallforschung, Stuttgart and Institut fur Nichtmetallische Anorganische Materialien,Universitat Stuttgart, Heisenbergstr. 3, 70569 Stuttgart, Germany

b I.M. Frantsevich Institute for Materials Problems, Krzhyzhanovskiy Str. 3, 03680 Kiev, Ukraine

Received 24 July 2006; accepted 14 November 2006Available online 19 December 2006

bstract

The thermodynamic database for the ZrO2–La2O3–Al2O3 system is derived using the Calphad approach. The compound energy formalism andwo-sublattice ionic liquid model are applied for phase description in this system. Thermodynamic parameters for the ZrO2–La2O3 system aressessed in this study with special attention to triangulation in the ternary system of ZrO2–La2O3–Al2O3. It is demonstrated that the phase equilibriaetween solid phases in the ternary ZrO2–La2O3–Al2O3 system constrain thermodynamic parameters in the binary ZrO2–La2O3 system. The ternary
nteraction parameter is assessed for the liquid phase using experimental data on liquidus surface. Isothermal sections at 1523, 1923 K, liquidusurface and several vertical sections are calculated and compared with available experimental data. The revealed several inconsistencies betweenalculations and experimental data are due to calculations indicated wider filed of primary crystallisation of pyrochlore. The SEM investigation ofelted samples performed in this work confirmed the calculated primary crystallisation field of pyrochlore.2006 Elsevier B.V. All rights reserved.
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eywords: Oxide materials; Scanning electron microscopy; Phase diagrams; T

. Introduction

The Yttria-stabilised zirconia (YSZ) has various industrialpplications, such as thermal barrier coatings (TBS) or solidtate electrolytes and oxygen sensors. It was shown that La2O3dditives improve the thermal conductivity and resistance tointering of the YSZ that makes it promising material forBC [1]. However, too high concentration of La2O3 leads

o decrease of the tetragonal (t’) phase stability and dura-ility of coating in thermal cycling [2,3]. It is important tond the concentration of the La2O3, which improves thermalonductivity and resistance to sintering without a remarkableoss of durability of coating. The compound La2Zr2O7 withyrochlore structure stable up to ∼2300 ◦C is also promisingandidate for TBC due to its low thermal conductivity [4–7].
he pyrochlore/YSZ double-layer system revealed an excellent
hermal cycling behaviour at temperatures up to 1450 ◦C [8].he important issue to make a judgment about the suitability

∗ Corresponding author. Tel.: +49 711 6893106; fax: +49 711 6893131.E-mail address: [email protected] (O. Fabrichnaya).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.11.102

dynamic modeling

f matter as TBC is its interaction with thermally grown oxideTGO) layer consisting of Al2O3. The thermodynamic data forhe ZrO2–La2O3–Y2O3–Al2O3 system should help to eluci-ate the stability issues of TBC and its interaction with TGO.he ternary system ZrO2–La2O3–Al2O3 is an important partf the above mentioned quaternary system. The knowledge ofhe phase interaction in the ZrO2–La2O3–Al2O3 system shouldlarify if the pyrochlore phase without an intermediate layer ofSZ could be used as TBC.There were several assessments available for the

rO2–La2O3 system [9,10] where the substitutional modelas used for the description of the liquid and solid solutionhases. This model is rather formal and does not reflect thetructure of phases. Furthermore, the substitutional model isot compatible with the ionic liquid model and the compoundnergy formalism. In the assessment for the La2O3–Al2O3ystem [11] quasichemical model was used for the liquid phase,hich is also not compatible with the ionic liquid model.
ecently, new measurements of thermodynamic properties of
he binary systems of La2O3–Al2O3 [12–14] and ZrO2–La2O315–19] have become available. The assessment of thermody-amic parameters in the La2O3–Al2O3 system based on phase

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dx.doi.org/10.1016/j.jallcom.2006.11.102

272 O. Fabrichnaya et al. / Journal of Alloys and Compounds 453 (2008) 271–281

Table 1Thermodynamic parameters in the ZrO2–La2O3–Al2O3 system

Phase Model, parameter (J mol−1)

Fluorite (F) (La3+, Al3+, Zr4+)2(O2−, Va)4

G(FLUORITE,Zr4+:O2−) = 2·GZRO2CG(FLUORITE,Zr4+:Va) = 2·GZRO2C-4·GHSEROOG(FLUORITE,La3+:O2−) = GLA2O3F + GHSEROO + 18.702165·TG(FLUORITE,La3+:Va) = GLA2O3F-3·GHSEROO + 18.702165·TG(FLUORITE,Al3+:O2−) = GCORUND + GHSEROO + 18.702165·T + 100000G(FLUORITE,Al3+:Va) = GCORUND-3·GHSEROO + 18.702165·T + 1000000L(FLUORITE,Al3+,Zr4+: O2−) = 185000L(FLUORITE,Al3+,Zr4+: Va) = 185000L(FLUORITE,La3+,Zr4+:O2−) = −1700100L(FLUORITE,La3+,Zr4+:Va) = −1700101L(FLUORITE,La3+,Zr4+:O2−) = −1346211L(FLUORITE,La3+,Zr4+:Va) = −134621

Tetragonal (T) (La3+, Al3+, Zr4+)2(O2−, Va)4

G(TETR,Zr4+:O2−) = 2·GZRO2TG(TETR,Zr4+:Va) = 2·GZRO2T-4·GHSEROOG(TETR,La3+:O2−) = GLA2O3T + GHSEROO + 18.702165·TG(TETR,La3+:Va) = GLA2O3T-3·GHSEROO + 18.702165·TG(TETR,Al3+:O2−) = GCORUND + GHSEROO + 18.702165·T + 100000G(TETR,Al3+:Va) = GCORUND-3·GHSEROO + 18.702165·T + 1000000L(TETR,Al3+,Zr4+:O2−) = 200000L(TETR,Al3+,Zr4+:Va) = 200000L(TETR,La3+,Zr4+:O2−) = 200000L(TETR,La3+,Zr4+:Va) = 20000

Monoclinic (M) (La3+, Al3+, Zr4+)2(O2−, Va)4

G(MONO,Zr4+:O2−) = 2·GZRO2MG(MONO,Zr4+:Va) = 2·GZRO2M-4·GHSEROOG(MONO,La3+:O2−) = GLA2O3A + GHSEROO + 100000 + 18.702165·TG(MONO,La3+:Va) = GLA2O3A-3·GHSEROO + 100000 + 18.702165·TG(MONO,Al3+:O2−) = GCORUND + GHSEROO + 200000 + 18.702165·TG(MONO,Al3+:Va) = GCORUND-3·GHSEROO + 200000 + 18.702165·T

LA2O3 A (A) (La3+, Zr4+)2(O2−, Va)1(O2−)3

G(LA2O3 A,Zr4+:O2−:O2−) = 2·GZRO2C + 50000G(LA2O3 A,Zr4+:O2−:Va) = 2·GZRO2C-GHSEROO + 50000G(LA2O3 A,La3+:O2−:O2−) = GLA2O3A + GHSEROOG(LA2O3 A,La3+:O2−:Va) = GLA2O3A0L(LA2O3 A,La3+,Zr4+:O2−:O2−) = 200000L(LA2O3 A,La3+,Zr4+:O2−:Va) = 20000

LA2O3 H (H) (La3+, Zr4+)2(O2−, Va)1(O2−)3

G(LA2O3 H,Zr4+:O2−:O2−) = 2·GZRO2C + 20000G(LA2O3 H,Zr4+:O2−:Va) = 2·GZRO2C-GHSEROO + 20000G(LA2O3 H,La3+:O2−:O2−) = GLA2O3H + GHSEROOG(LA2O3 H,La3+:O2−:Va) = GLA2O3H0L(LA2O3 H,La3+,Zr4+:O2−:O2−) = −318000L(LA2O3 H,La3+,Zr4+:O2−:Va) = −31800

LA2O3 X (X) (La3+, Zr4+)2(O2−, Va)1(O2−)3

G(LA2O3 X,Zr4+:O2−:O2−) = 2·GZRO2C + 10000G(LA2O3 X,Zr4+:O2−:Va) = 2·GZRO2C-GHSEROO + 10000G(LA2O3 X,La3+:O2−:O2−) = GLA2O3X + GHSEROOG(LA2O3 X,La3+:O−2:Va) = GLA2O3X0L(LA2O3 X,La3+,Zr4+:O2−:O2−) = −280000L(LA2O3 X,La3+,Zr4+:O2−:Va) = −28000

Pyrochlore (Pyr) (La3+, Zr4+)2(Zr4+, La3+)2(O2−, Va)6(O2−)1(Va, O2−)1

G(PYROCHLORE,Zr4+:Zr4+:O2−:O2−:Va) = GPYROZR-GHSEROOG(PYROCHLORE,La3+:Zr4+:O2−:O2−:Va) = GPYROG(PYROCHLORE,Zr4+:La3+:O2−:O2−:Va) = GPYRO + GANCAG(PYROCHLORE,La3+:La3+:O2−:O2−:Va) = 2·GPYRO-GPYROZR + GHSEROO+ GANCA + GREC1G(PYROCHLORE,Zr4+:Zr4+:O2−:O2−:O2−) = GPYROZRG(PYROCHLORE,La3+:Zr4+:O2−:O2−:O2−) = + GPYRO + GHSEROO + GREC2G(PYROCHLORE,Zr4+:La3+:O2−:O2−:O2−) = GPYRO + GHSEROO + GANCA

O. Fabrichnaya et al. / Journal of Alloys and Compounds 453 (2008) 271–281 273

Table 1 (Continued )


−GREC1 + GREC2 + GREC3 + GREC4G(PYROCHLORE,La3+:La3+:O2−:O2−:O2−) = 2·GPYRO + 2·GHSEROO−GPYROZR + GANCA + GREC1 + GREC2 + GREC3G(PYROCHLORE,Zr4+:Zr4+:Va:O2−:Va) = 6·GPYROLA-12·GPYRO + 7·GPYROZR−7·GHSEROO−6·GANCA + 134.8548·T−6·GREC1 + GREC5 + GREC6G(PYROCHLORE,La3+:Zr4+:Va:O2−:Va) = 6·GPYROLA−11·GPYRO + 6·GPYROZR−6·GHSEROO−6·GANCA + 134.8548·T−6·GREC1 + GREC5G(PYROCHLORE,Zr4+:La3+:Va:O2−:Va) = 6·GPYROLA−11·GPYRO + 6·GPYROZR−6·GHSEROO−5·GANCA + 134.8548·T−6·GREC1 + GREC7G(PYROCHLORE,La3+:La3+:Va:O2−:Va) = 6·GPYROLA−10·GPYRO + 5·GPYROZR−5·GHSEROO−5·GANCA + 134.8548·T−5·GREC1G(PYROCHLORE,Zr4+:Zr4+:Va:O2−:O2−) = 7·GPYROZR−12·GPYRO + 6·GPYROLA−6·GHSEROO + 134.8548·T−6·GANCA−6·GREC1 + GREC5 + GREC8 + GREC10G(PYROCHLORE,La3+:Zr4+:Va:O2−:O2−) = 6·GPYROZR−11·GPYRO + 6·GPYROLA−5·GHSEROO + 134.8548·T−6·GANCA−6·GREC1 + GREC2 + GREC5 + GREC8G(PYROCHLORE,Zr4+:La3+:Va:O2−:O2−) = 6·GPYROZR−11·GPYRO+6·GPYROLA−5·GHSEROO + 134.8548·T−5·GANCA−7·GREC1+GREC2 + GREC3 + GREC4 + GREC5 + GREC8 + GREC10 + GREC11G(PYROCHLORE,La3+:La3+:Va:O2−:O2−) = 5·GPYROZR−10·GPYRO+6·GPYROLA−4·GHSEROO + 134.8548·T−5·GANCA−5·GREC1 + GREC2+GREC3 + GREC5 + GREC8 + GREC9

Perovskite (LaAP) (La3+)1(Al3+)1(O2−)3

G(LaAP,La3+:Al3+:O2−) = 0.5·GCORUND + 0.5·GLA2O3A−65380−3.861607·T�-alumina (�) (La3+)1(Al3+)11(O2−)18

G(LaAP,La3+:Al3+:O2−) = 0.5·GCORUND + 0.5·GLA2O3A−65380−3.861607·TCorundum Al2O3 (Al3+)2(O2−)3

G(CORUNDUM,Al3+:O2−) = GCORUND

Liquid (L) (La3+, Zr4+)P(O2−, AlO3/2)Q

G(IONIC LIQ,Zr4+:O2−) = 2·GZRO2LG(IONIC LIQ,La3+:O2−) = GLA2O3LG(IONIC LIQ,AlO3/2) = 0.5·GAL2O3 L0L(IONIC LIQ,La3+,Zr4+:O2−) = −1683701L(IONIC LIQ,La3+,Zr4+:O2−) = −274260L(IONIC LIQ,La3+:O2−,AlO3/2) = −2026651L(IONIC LIQ,La3+:O2−,AlO3/2) = −798100L(IONIC LIQ,Zr4+:O2−,AlO3/2) = 500001L(IONIC LIQ,Zr4+:O2−,AlO3/2) = −40000L(IONIC LIQ,La3+,Zr4+:O2−,AlO3/2) = 175000

FunctionsGHSEROO = −3480.87−25.503038·T−11.136·T·ln(T) −0.005098888·T2 + 6.61846·10−7·T3−38365/T T:298.15−1000−6568.763 + 12.65988·T−16.8138·T·ln(T)−5.95798·10−4·T2 +6.781·10−9·T3 + 262905/T T:1000-3300−13986.728 + 31.259625·T−18.9536·T·ln(T)−4.25243·10−4·T2 +1.0721·10−8·T3 + 4383200/T T:3300-6000GZRO2M = −1126164 + 424.8908·T−69.38751·T·ln(T) −0.0037588·T2 + 683000/TGZRO2T = 5468.0−4.0·T + GZRO2MGZRO2C = 10336−4.0·T + GZRO2TGZRO2L = 87027−29.17432·T + GZRO2CGLA2O3A = −1833257 + 692.9664·T−120.629·T·ln(T)−0.006854·T2 + 808000/T−1.0·107/T2

GLA2O3H = GLA2O3A + 32350−13.986·TGLA2O3X = GLA2O3A + 43192−18.555·TGLA2O3F = GLA2O3A + 50000GLA2O3T = GLA2O3F + 10000GLA2O3L = 141329−56.622·T + GLA2O3AGCORUND = −1707351.3 + 448.021092·T−67.4804·T·ln(T)−0.06747·T2 + 1.4205433·10−5·T3 + 938780/T T:298.15-600−1724886.06 + 754.856573·T-116.258·T·ln(T)−0.0072257·T2

+2.78532·10−7·T3 + 2120700/T T:600-1500−1772163.19 + 1053.4548·T−156.058·T·ln(T) + 0.00709105·T2

−6.29402·10−7·T3 + 12366650/T T:1500-3000GAL2O3 L = −1607850.8 + 405.559491·T−67.4804·T·ln(T)−0.06747·T2

+1.4205433·10−5·T3 + 938780/T T:298.15-600−1625385.57 + 712.394972·T−116.258·T·ln(T)−0.0072257·T2

+2.78532·10−7·T3 + 2120700/T T:600-1500


Table 1 (Continued )


−1672662.69 + 1010.9932·T−156.058·T·ln(T) + 0.00709105·T2

−6.29402·10−7·T3 + 12366650/T T:1500-1912+29178041.6−168360.926·T + 21987.1791·T·ln(T)−6.99552951·T2

+4.10226192·10−4·T3−7.98843618·109/T T:1912-2327−1757702.05 + 1344.84833·T−192.464·T·ln(T) T:2327-4000GPYRO = −4194070 + 1531.07053·T−260.811·T·ln(T) + 1898400/T−0.00891455·T2

GANCA = 400000GPYROZR = 4·GZRO2C + 10000 + 16·TGPYROLA = 2·GLA2O3A + 160000

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GREC1 = 0GREC2 = 120000GREC3 = GREC4 = GREC5 = GREC6 = GREC7 = GREC8 = GREC9 = GREC

quilibrium data and recent calorimetric measurements waserformed by [20].

The phase relations in the ZrO2–La2O3–Al2O3 system werextensively studied experimentally by Lakiza and Lopato [21]nd isothermal sections, vertical sections and liquidus surface forernary system were constructed. The thermodynamic assess-

ent of the ternary system was not available so far. The aimf this study is to derive thermodynamic parameters of therO2–La2O3–Al2O3 system using experimental data [21], toalculate phase diagrams and to compare them with experimen-al ones.

.1. Evaluation of experimental data

The experimental investigation of the ZrO2–La2O3–Al2O3ystem reveals the formation of the ZrO2 based solid solu-ions with relatively small solubility of La2O3 and Al2O3.he La2O3 based solid solutions dissolves some amountf the ZrO2 (∼20 mol.%), while the solubility of Al2O3n these solutions was not indicated in experiments. Theolubility of ZrO2 and La2O3 in the corundum phase,l2O3, is not indicated in experiments. Ternary compounds

re not found in the ZrO2–La2O3–Al2O3 system similaro other ZrO2–RE2O3–Al2O3 systems [22–24]. Binary stoi-hiometric compounds, LaAlO3 (perovskite) and LaAl11O18�-alumina), are indicated in the La2O3–Al2O3 system. Thea2Zr2O7 phase with pyrochlore structure, having a rela-

ively narrow homogeneity range, is stable in the ZrO2–La2O3ystem.

The subsolidus phase relations in the ZrO2–La2O3–Al2O3ystem were experimentally studied by [25] for the first time.ater Lakiza and Lopato extensively studied phase relations in

he ZrO2–La2O3–Al2O3 system [21,26–29]. They constructedsothermal sections based on XRD data and optical method.ertical sections and the liquidus surface for ternary systemere derived from DTA measurements up to 2300 ◦C, XRD,etrographic and microstructure phase analyses. The isothermalections obtained by Lakiza and Lopato [21] as well as Leckie
nd Levi [30] revealed a contradiction with the results of [25].he reason for this inconsistency could be that equilibrium wasot attained in the experiments of [25] since the �-alumina phaseas not found among the reaction products.
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GREC11 = 0

Experimental thermochemical data for the ternaryrO2–La2O3–Al2O3 system are not available so far.

.2. Thermodynamic modelling and optimisation

The Calphad method [31] based on computer coupling ofhermochemistry and phase diagrams is used in this study tossess thermodynamic parameters in binary and ternary systems.he phases stable in the system ZrO2–La2O3–Al2O3, and ther-odynamic models used to describe them are listed in Table 1.ost of the solid phases are described by the compound energy

ormalism [32]; the remaining solid phases (LaAlO3-perovskiter LaAP, LaAl11O18-�-alumina and Al2O3-corundum) arereated as stoichiometric compounds. The liquid phase isescribed by the two-sublattice ionic liquid model [32]. Thebsence of any ternary compounds makes possible an extrapo-ation from binary systems to the ternary system.

The structure of the fluorite-type phase can be describedy two sublattices. One is filled by Zr4+ and La3+ cations.nother sublattice contains disordered oxygen anions andacant positions, i.e. (Zr4+,La3+)2(O2−,Va)4. The other ZrO2-ased solid solutions are described by the same formulae. Thea2O3 based solid solutions (A, H and X) are described as

La3+,Zr4+)2(O2−)3(O2−,Va) the same way as the Gd2O3-basedhases in the ZrO2–Gd2O3 system [23]. The structure of theyrochlore phase is well known [33]. It contains five differentrystallographic sublattices with strong preference of cationsnd anions to each sublattice, i.e.,

La3+, Zr4+)2(Zr4+, La3+)2(O2−, Va)6(O2−)(Va, O2−).

The Gibbs energy of a solution phase with mixing in twoublattices (i.e. fluorite, tetragonal, monoclinic phase, A, H and) is expressed as:

G =∑

i

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, αs is the number of sites on sublattice s per mole of formulanit of phase and �Gex is the excess Gibbs energy of mixing

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xpressed as:

Gex =∑

s

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si,j + �Gex,h−o

here:

si,j =

∑

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j )nLi,j,

re the binary interaction parameters between i and j species inhe sublattice s, �Gex,h-o is high-order contribution to the excessibbs energy. In the solid solutions only binary interactions are

aken into account.In the case of more sublattices (i.e. pyrochlore) the Gibbs

nergy is expressed by:

G =∑

Gend

∏ysj + RT

∑ ∑αsy

sj + �Gex

here Gend is the Gibbs energy of the compound representinghe end member. The excess energy for pyrochlore is assumedo be zero.

The liquid phase is described by the partially ionic sublatticeodel [32](La3+,Zr4+)P(O2−,AlO3/2)Q, where P and Q are the

umber of sites on the cation and anion sublattices, respectively.he stoichiometric factors P and Q vary with the composition inrder to maintain electroneutrality. The ternary mixing parame-er L(La3+,Zr4+:O2−,AlO3/2) was introduced in the liquid phaseo fit experimental liquidus surface data.

The thermodynamic functions derived for the phases in therO2–La2O3–Al2O3 system are presented in Table 1.

. Results and discussion

.1. Binary systems

The thermodynamic description of the La2O3–Al2O3 sys-em was accepted from [20]. The thermodynamic descriptionf the ZrO2–Al2O3 systems was adopted from [34] except for
he mixing parameters of tetragonal and fluorite phases whichere adjusted in the present study because of changes in the
hermodynamic description of the ZrO2 polymorphs [35]. TherO2–La2O3 system was assessed in the present work taking

2

p

able 2alculated and experimental thermodynamic parameters for pyrochlore La2Zr2O7

(K) Parameter Calculated

98.15 �◦f,oxH, J mol−1 −110063

74 �◦f,oxH, J mol−1 −112824

98.15 ◦S, J mol−1 K−1 242.498.15 CP, J mol−1 K−1 223.440

�◦f,oxG, J mol−1

234.080 241.200 −123483000 −124691100 −125810

200 −126837

36]: Combustion calorimetry; [15]: lead borate solution calorimetry; [16]: ◦S and C17]: EMF 870–1240 K; [18]: EMF 1073–1273 K; [19]: HT-H298 at 888–1567 K, dro

nd Compounds 453 (2008) 271–281 275

nto account phase relations in the ternary ZrO2–La2O3–Al2O3ystem. Bolech et al. [16] published thermodynamic functionsor pyrochlore being in a good agreement both with experi-ental calorimetric [15,16] and EMF [17,18] data. However,

t was demonstrated in present work, that if these functionsf pyrochlore were used in the ternary system the tie-linesetween tetragonal, �-phase, perovskite and pyrochlore phasesere not consistent with experimental tie-lines [21,30]. The

tability of pyrochlore phase was then reduced by makingts Gibbs energy more positive to get tie-lines in agreementith experimental data. Both enthalpy and entropy of forma-

ion from oxides were optimised. It should be mentioned that36] obtained enthalpy of formation of pyrochlore by com-ustion calorimetry ∼10 kJ less negative than was obtainedy [16] using solution calorimetry. Navrotsky [37] presentedalue for enthalpy of formation of pyrochlore from elements−100 kJ mol−1, which is even less negative than data of [36].owever, the source of data presented in Fig. 8 of [37] wasot indicated and neither the details of experiment nor dis-ussion were given. Furthermore, there is an indication fromb initio calculations [38] that the enthalpy of formation ofhe La2Zr2O7, pyrochlore phase, should be almost twice lessegative than obtained by [16]. Obviously new experimentaleasurement of pyrochlore phase is necessary. The heat capac-

ty function for the pyrochlore, La2Zr2O7, was accepted from19], where the data of [16] along with the new measurements ofnthalpy increment up to 1550 K had been used. The calculatedhermodynamic functions for pyrochlore phase are presented inable 2.

The phase diagram for the ZrO2–La2O3 system is presentedn Fig. 1. The calculated data on invariant reactions are presentedn Table 3. The calculations agree with experimental phase equi-ibria [21,39–41]. The detailed comparison of the calculatedhase diagrams with the experimental data is given elsewhere42].

.2. Ternary system

The calculated isothermal sections at 1523 and 1923 K areresented in Fig. 2a,b. The same phase fields are indicated in

Experimental Reference

−126000 [36]−135800 [15]

08 238.53 [16]15 223.05 [16]29 234.03 [16]92 241.55 [16]

−143088 [17]−144120 [17]−145152 [17]−145894 [18]−146184 [17]−146948 [18]

P at 298 K, adiabatic calorimetry; HT-H298 at 500–900 K – drop calorimetry;p calorimetry.

276 O. Fabrichnaya et al. / Journal of Alloys a

tL

t

litdii[usiTrpipicss

TI

R

L

L

L

X

F

H

T

Fig. 1. Calculated phase diagram of the ZrO2–La2O3 system.

he present calculations and in the experiments of Lakiza andopato [21].

The calculations performed in the ZrO2–La2O3–Al2O3 sys-em demonstrated that if the thermodynamic description of the

odAp

able 3nvariant equilibria in the ZrO2–La2O3 system

eaction T(K), x(La2O3)

This work calculated [39]

= Pyr (congr) 2567 2553L:0.3252 L:0.3333

3

= F + Pyr (eut) 2540 2493L:0.2468 L:0.25F:0.1732Pyr:0.3117

= Pyr + X (eut) 2285 2303L:0.6357 L:0.625Pyr:0.3478X: 0.7443

= Pyr + H (eut) 2181 2173X:0.7804 X:0.699Pyr:0.3479 2H: 0.8021 H:0.850

1

= T + Pyr (eut) 2148 2223F:0.0646 F:0.070T:0.0081Pyr:0.3120

= Pyr + A (eut) 2039 1987a

H:0.8412Pyr:0.3471A:0.9619

= M + Pyr (eut) 1364 –T:0.0008Pyr:0.3303M:0.0005

a – Tentative

nd Compounds 453 (2008) 271–281

iquid phase was combined from binaries without introduc-ng of ternary interaction parameter the liquidus surface andemperatures of invariant reactions were not very well repro-uced by the calculations. Therefore a ternary mixing parameters necessary to fit the liquidus surface and temperatures ofnvariant reactions. Experimental data of Lakiza and Lopato21] were used to assess ternary interaction parameter in liq-id. The calculated liquidus surface of the ZrO2–La2O3–Al2O3ystem is presented in comparison with the calculated onen Fig. 3a,b. Data on invariant reactions are presented inable 4. The temperatures of the invariant reactions wereeproduced in calculations reasonably well. The phase com-ositions for several reactions (i.e. E2, U4, e1(max)) are alson a good agreement with experimental data. It should beointed out that the general character of liquidus surfaces reproduced in calculations. However, several inconsisten-ies between calculations and experimental data on liquidusurface still exist. The calculated pyrochlore primary crystalli-ation field is larger than the one obtained in experiments
f Lakiza and Lopato [21]. This results in the followingiscrepancies between the calculated and experimental data.ccording to calculations the character of reaction betweenyrochlore, perovskite, tetragonal and liquid phases is tran-
[21] [41] [40]

2613 –L:0.33333

2588 –L:0.25

2253 –L:0.625

– –

– –

– –

– 1383 1373T:0.0076 T:0.0076


sAtsnbrceoq

tbgirpsis

Fig. 3. Liquidus surface of the ZrO2–La2O3–Al2O3 system: (a) calculated; opencircles are selected points for the present SEM investigations; (b) experimentala

2

iZtocF

gaaon

Fig. 2. Isothermal sections at 1523 K (a) and 1923 K (b).

itional, while according to experimental data it is eutectic.lso according to experimental data there is a maximum at

he L = LaAP + T univariant line and thus the ZrO2–LaAlO3ection is quasibinary, while according to calculation there iso maximum on this univariant line where it represents a sta-le equilibrium. In the La2O3-rich compositions the invarianteactions presented in [21] are different from the present cal-ulations. However, these reactions were not established inxperiments [21] and considered as tentative ones, becausef the high-temperature modifications of the La2O3 are notuenchable.

Fig. 4a–e presents calculated isopleths along with experimen-al points determined by DTA-method by [21]. The agreementetween the calculations and experimental data is reasonablyood. The indicated differences between calculations and exper-mental isopleths are caused by different character of invarianteaction between perovskite, pyrochlore, tetragonal and liquidhases. Also according to the calculations the LaAlO3–ZrO2
ection is not a quasibinary and pyrochlore phase participatesn several equilibria, which were not indicated in experimentaltudy [21].
ati

s published by Lakiza and Lopato [05Lak].

.3. Experimental study of solidification microstructures

In order to resolve the inconsistencies between exper-mental results and calculations of interaction in therO2–La2O3–Al2O3 system synthesis and SEM microstruc-

ure analysis of several samples were carried out. The selectionf compositions of these samples was guided by the cal-ulations and these compositions are shown in Table 5 andig. 3a.

Samples were prepared from pure oxide powders by carefulrinding in agate mortar in alcohol medium. Mixtures were driedt ambient temperature, pressed into pellets 5 mm in diameternd 5 mm in height and fired at 900 ◦C to react La2O3 with otherxides preventing hidration. Samples were melted in molybde-um crucibles in a differential thermal analyze (DTA) facility
t total pressure of H2 about 1.2 atm, annealed below solidusemperature for 1 hour and their microstructures were analyzedn SEM JSM 6400.


Table 4Temperature and liquid composition for invariant reactions in the ZrO2–La2O3–Al2O3 system

Reaction Temperature, K x(La2O3) x(Al2O3) Source

e1(max): L = LaAP + Pyr 2219 0.4277 0.2917 Calculated, this work2258 0.42 0.26 [21]

U1: L + F = T + Pyr 2167 0.2203 0.2981 Calculated, this work2223 0.30 0.18 [21]

U2: L + X = H + Pyr 2181 0.6582 0.0889 Calculated, this workU3: L + H = A + Pyr 2039 0.6680 0.1873 Calculated, this work

E1: L = LaAP + A + Pyr 2034 0.6675 0.1909 Calculated, this work2023 0.53 0.18 [21]

U4: L + Al2O3 = � + T 2019 0.1128 0.5862 Calculated, this work2003 0.13 0.56 [21]

U5: L + Pyr = T + LaAP 2009 0.2192 0.4244 Calculated, this workE2: L = Pyr + T + LaAP 1988 0.36 0.26 [21]

E2: L = T + � + LaAP 1934 0.1883 0.5301 Calculated, this work1938 0.2 0.53 [21]

L + X = A + Pyr 2213 0.62 0.03 [21]2072, metastable 0.6711 0.1617 Calculated, this work

L + H = A + X 2248 0.68 0.05 [21]1114, metastable 0.4318 0.5355 Calculated, this work

e2(max):L = T + LaAP 2088 0.32 [21]

msZcm[

mgv

dAitt

TS

S

IIIIVVV

vwtZbpLtatc

(

2062, metastable

The sample II (Fig. 5, a) presents fine ternary eutecticicrostructure E2, consisting of LaAlO3 (light triangular-

haped phase) + �-LaAl11O18 (black needle-shaped matrix) + T-rO2 (very fine light-grey dots in �-LaAl11O18 matrix). Theomposition of this sample is very close to the experi-entally determined phase composition of ternary eutectic

21].The sample III (Fig. 5, b) presents a binary eutectic

icrostructure, in which the light phase is La2Zr2O7 and therey phase is LaAlO3. The composition of this point agreesery well with the calculated saddle point e1(max).

The sample IV (Fig. 5, c) contains white T-ZrO2 primaryendrites, surrounded by grey grains of monovariant eutecticl O + T-ZrO and areas of ternary eutectic � + T + LaAP. It
2 3 2
ndicates that sample IV lies in the T-ZrO2 primary crystalliza-ion field and monovariant curves e4–U4 and U4–E2 lies beneathhis composition.

able 5elected compositions for the SEM investigation

ample number Composition, mol.%

Al2O3 La2O3 ZrO2

22 70 8I 50 19 31II 29 43 28V 56 9 35

20 65 15I 39 24 37II 26 59 15

(etotgtfih

gatctc

0.2737 0.2737 Calculated, this work

The microstructure of the sample VI (Fig. 5, d) consists ofery few La2Zr2O7 primary crystals in the middle (light), edgedith fine two-phase (T + LaAP) zone, being the results of transi-

ional reaction L + Pyr�T + LaAP(U5). Light-grey phase is T-rO2, grey phase is LaAP. The fine two-phase zone is surroundedy coarse zone, consisting of the same phases, T and LaAP, thatresents a monovariant simultaneous crystallization of T andaAP. The very last crystallization process of ternary eutec-

ic �-LaAl11O18 + T + LaAP is presented by coarse three-phasessemblages in the peripheral areas of Fig. 5d. The microstruc-ure of the samples II and VI reveal that the La2Zr2O7 primaryrystallization field stretches just beyond the composition VI.

The analysis of the microstructures of the samples VII and VFig. 5e, f) reveal that the phase LaAlO3 crystallizes primarilylarge dark faceted areas). Apart from these crystals the ternaryutectic LaAP + La2Zr2O7 + A-La2O3 (E1) can be seen, wherehe dark phase is LaAP, the grey phase is La2Zr2O7 and the whitene is A-La2O3. It should be stressed, that the constituents ofhis ternary eutectic (E1) do not demonstrate the cooperativerowth and it looks like simple mixture. The microstructures ofhe samples (VII, V) reveal that the LaAP primary crystallizationeld stretches beyond these two compositions to the areas withigher La2O3 contents.

The sample I (Fig. 5g) contains areas with the relatively largerains of LaAP (dark) and La2O3 (white) and areas of fine LaAPnd larger La2Zr2O7 crystals (grey) in the La2O3 matrix. This
estifies that this sample belongs to the monovariant curve, sorystallization in this sample starts with simultaneous precipita-ion of LaAP and La2O3 phases followed by the ternary eutecticomposition consisting of LAP + La2Zr2O7 + A-La2O3 (E1).


Fig. 4. Isoplethal sections along with expreimental points from [05Lak]: (a) LaAlO3-La2Zr2O7; (b) LaAlO3-(1.5La2O3-98.5ZrO2); (c) 20 mol.% ZrO2; (d) 60 mol.%ZrO2; (e) 50Al2O3-50ZrO2-La2O3.


Fig. 5. Solidification microstructures of selected compositions in the ZrO2–La2O3–Al2O3 system: (a) II: black phase is �-LaAl11O18, LaAP is grey, light dots inthe black phase are T-ZrO2; (b) III: light grey phase is pyrochlore La2Zr2O7, dark grey phase is LaAP. Saddle point e1(max); (c) IV: white phase is T-ZrO2, greyare two-phase grains of monovariant eutectic Al2O3 + T-ZrO2, ternary eutectic is � + T + LaAP; (d) VI: white phase is pyrochlore La2Zr2O7, �-LaAl11O18 is black,LaAP is dark grey, light grey phase is T-ZrO2; (e) VII: black phase is LaAP, La2Zr2O7 is grey, La2O3 is white; (f) V: black phase is LaAP, La2Zr2O7 is grey, La2O3

is white; (g) I: black phase is LaAP, La2Zr2O7 is grey, La2O3 is white; LaAP phase crystallised together with La2O3.

oys a

3

atst

u

si

w

po

smm

A

R

R

[

[[[[[

[

[

[[

[

[

[[

[[

[

[

[

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O. Fabrichnaya et al. / Journal of All

. Conclusions

Thermodynamic parameters of the ZrO2–La2O3 system aressessed in this study with special attention to triangulation inernary system ZrO2–La2O3–Al2O3. Phase equilibria betweenolid phases in the ternary ZrO2–La2O3–Al2O3 system constrainhermodynamic parameters in the binary system ZrO2–La2O3.

Ternary interaction parameter is assessed for the liquid phasesing experimental data on liquidus surface of [21].

Isothermal sections at 1523, 1923 K, liquidus surface andeveral vertical sections are calculated and compared with exper-mental data [21].

The calculated primary crystallisation field of pyrochlore isider than previously reported [21].Isoplethal section of LaAP– ZrO2-0.015La2O3 must be inter-

reted as regular isoplethal section, not as a partially quasibinaryne.

Experimental investigation of selected samples in the ternaryystem confirmed the calculations and it made possible to deter-ine the phase relations in the system ZrO2–La2O3–Al2O3ore accurately.

cknowledgments

Support for this investigation was provided by the Germanesearch Society DFG (project AL384/34-1).

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Assessment of thermodynamic functions in the ZrO2–La2O3–Al2O3 system

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