Calorimetric investigation of the La2Zr2O7, Nd2Zr2O7, Sm2Zr2O7 and LaYO3 compounds and CALPHAD assessment of the La2O3–Y2O3 system

Thermochimica Acta 526 (2011) 50–57

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Calorimetric investigation of the La2 Zr2 O7 , Nd2 Zr2 O7 , Sm2 Zr2 O7 and LaYO3 compounds and CALPHAD assessment of the La2 O3 –Y2 O3 system O. Fabrichnaya a,∗ , M.J. Kriegel a , J. Seidel a , G. Savinykh a , L.P. Ogorodova b , I.A. Kiseleva b , H.J. Seifert a,c a b c

Technical University of Freiberg, Freiberg, Germany M.V. Lomonosov Moscow State University, Geological department, Moscow, Russia Karlsruhe Institute of Technology, Karlsruhe, Germany

a r t i c l e

i n f o

Article history: Received 7 May 2011 Received in revised form 15 August 2011 Accepted 23 August 2011 Available online 30 August 2011 Keywords: Differential scanning calorimetry Drop-solution calorimetry CALPHAD Perovskite structure Pyrochlore structure

a b s t r a c t Temperature dependence of heat capacity of the La2 Zr2 O7 , Nd2 Zr2 O7 , Sm2 Zr2 O7 and LaYO3 is measured in the temperature range of 298.15–1373 K by differential scanning calorimetry (DSC). The enthalpy of formation from oxides for LaYO3 was measured by high-temperature lead borate solution calorimetry using transposed temperature drop method. Transformations in the LaYO3 were experimentally studied using differential thermal analysis (DTA) up to 2173 K. The obtained results for heat capacity and enthalpy of formation of the LaYO3 and temperatures of transformations were used to re-optimize thermodynamic parameters of the La2 O3 –Y2 O3 system. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The lanthanide zirconates (Ln2 Zr2 O7 ) attract interest as candidate materials for thermal barrier coatings (TBC), for nuclear waste matrix and other industrial applications. The Ln2 Zr2 O7 compounds for lanthanide with large atomic radius Ln = La–Gd form ¯ The LaYO3 compound pyrochlore structure (space group Fd3m). forms a distorted orthorhombic perovskite structure (Pnma). It presents interest due to proton conductivity especially when doped by low valent cations (i.e. Sr+2 ) or being in equilibrium with La2 Zr2 O7 pyrochlore [1,2]. Yttria stabilized zirconia (YSZ) co-doped with rare earth as well as pyrochlores Ln2 Zr2 O7 are new candidate materials for TBC due to their lower thermal conductivity them presently used YSZ [3,4]. The development of thermodynamic database for the ZrO2 –Ln2 O3 –Y2 O3 –Al2 O3 system is important to estimate stability of these materials and their interaction with thermally grown oxide (Al2 O3 ). The heat capacity measurement for the pyrochlore phases La2 Zr2 O7 , Nd2 Zr2 O7 , Sm2 Zr2 O7 and perovskite LaYO3 will provide more reliable data for modelling in the ZrO2 –Ln2 O3 –Y2 O3 –Al2 O3 systems (Ln = La, Nd, Sm). Beside thermodynamic database development heat capacity data are necessary for calculation of thermal

∗ Corresponding author. E-mail address: [email protected] (O. Fabrichnaya). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.08.021

conductivity based on experimental measurements of thermal diffusivity and density. The heat capacities of La2 Zr2 O7 and Nd2 Zr2 O7 were already measured by different techniques (by drop calorimetry, adiabatic calorimery and by differential scanning calorimetry) in several works [5–10]. Heat capacity data for the Sm2 Zr2 O7 are available at 573–1373 K from DSC measurements [11] and from enthalpy increment measurement by drop calorimetry at 573–1673 K [12]. The difference between measurements according to different authors reaches up to 15–20 J mol−1 K−1 in some cases. Additionally it should be mentioned that there is magnetic contribution into heat capacity of Nd and Sm pyrochlores which complicates estimation of systematic trend. The heat capacity data for the LaYO3 perovskite phase are not available so far. Therefore we are going to check the reliability of our heat capacity measurements first by comparing results with more established data for the La2 Zr2 O7 and Nd2 Zr2 O7 , then with less established data for the Sm2 Zr2 O7 and finally to obtain new data for LaYO3 . The thermodynamic assessment for the La2 O3 –Y2 O3 system derived by Fabrichnaya et al. [13] was based only on phase equilibrium data. Experimental thermodynamic data (heat capacity and enthalpy of formation) are important constraints to get reliable thermodynamic description of the system by CALPHAD approach. The aim of this study is therefore experimental determination of heat capacities of pyrochlore phases La2 Zr2 O7 , Nd2 Zr2 O7 , Sm2 Zr2 O7 as well as heat capacity and of enthalpy of formation of LaYO3 perovskite. The obtained data for LaYO3 perovskite will be

O. Fabrichnaya et al. / Thermochimica Acta 526 (2011) 50–57

used in this work to re-assess the thermodynamic parameters and calculate phase diagram of the La2 O3 –Y2 O3 system. 2. Experimental 2.1. Sample preparation Samples were prepared by a co-precipitation procedure, which was described in previous works of Fabrichnaya et al. [14,15]. The zirconium acetate solution in acetic acid, Zr(CH3 COO)4 (99.99%, Sigma–Aldrich), La(NO3 )3 ·6H2 O (99.99%, Alfa Aesar), Nd(NO3 )3 ·6H2 O (99.99%, Alfa Aesar), Sm(NO3 )3 ·6H2 O (99.99%, Alfa Aesar) and Y(NO3 )3 ·6H2 O (99.9%, Alfa Aesar) were used as the starting chemicals. In the first step, the Ln(NO3 )3 ·6H2 O (Ln = La, Nd, Sm) and Y(NO3 )3 ·6H2 O were dissolved in distilled water and the initial zirconium acetate solution was diluted. After the determination of the oxide yield of the prepared solutions, they were mixed according to the given ratios. Thus obtained precursor solution was dropped from a buret at a low speed (around 1 ml min−1 ) into a big beaker containing about 500 ml of distilled water. The pH value was maintained above 9.0 by adding ammonia aqueous solution. The precipitation occurred during dropping and stirring. The obtained suspension was heated up and held at 60 ◦ C for 1–2 h. The precipitate was filtered and dried at 80 ◦ C and finally, the white powder was obtained after pyrolysis of the dried precipitate at 800 ◦ C for 3 h in air. Initial solutions, filtrate and sample after drying were analyzed by Inductively Coupled Plasma (ICP-OES) spectrometry, with an experimental accuracy of ±2 at.%. The pyrolysed powder was pressed into cylindrical pellets and sintered in air atmosphere at a temperature of 1873 K in Pt-crucibles to obtain the equilibrium state. The duration of heat treatments was 100 h at 1873 K and 350 h at 1523 K. Synthesis of pyrochlore phases was performed at 1523 and 1873 K. The temperature of synthesis of Ln2 Zr2 O7 (Ln = La, Nd, Sm) pyrochlores was not important, because they do not show phase transformations in this range and for both temperatures single phase pyrochlore was obtained at both temperatures. The synthesis of LaYO3 was performed at 1523 K. According to [16,17] LaYO3 undergoes a phase transformation to monoclinic phase B at ∼1813 K. 2.2. Sample treatment and characterization The samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy combined with an energy dispersive X-ray spectrometry (SEM/EDX) and differential thermal analysis (DTA). The XRD measurements of powdered specimen were recorded using the URD63 diffractometer (Seifert, FPM, Freiberg, Germany). The goniometer working in the Bragg-Brentano geometry equipped with the graphite monochromator in the diffracted beam and the ˚ was used for the measurements. All CuK␣ radiation ( = 1.5418 A) measured diffraction patterns were refined using the Rietveld algorithm to obtain the volume fractions of present phases as well as lattice parameters and grain size. For the Rietveld refinement, the program BGMN [18] was used. The microstructures of sintered samples were examined by SEM (Leo1530, Carl Zeiss) equipped with EDX (Bruker AXS Mikroanalysis GmbH). The latter was employed to obtain the chemical compositions of sample (ratio of metal elements recalculated into the REO1.5 and ZrO2 content with an accuracy of ±4 mol.%). To determine temperature of transformations in the perovskite phase samples of LaYO3 heat treated at 1523 K for 336 h were investigated using two different SETARAM instruments: SETSYS EVOLUTION 1750 (TG-DTA) in PtRh10% crucible in Ar (or He) atmosphere in the temperature range 288–1973 K and SETSYS EVOLUTION 2400 (TG-DTA) in WRe5% crucibles in He atmosphere at

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temperatures up to 2173 K. The heating rate in both instruments was of 20 K min−1 up to 1473 K and then of 10 K min−1 ; cooling rate was of 30 K min−1 also the same in both instruments. Temperature calibration of SETSYS EVOLUTION 1750 was made using melting points of Al, Ag, Au, Cu and Ni. The determination of transition temperatures in the LaYO3 was necessary because of uncertainty between different sources of experimental data mentioned in Ref. [13]. 2.3. Calorimetric measurements The heat capacity of LaYO3 samples heat treated at 1523 K and pyrochlore samples heat treated at 1873 K were measured in Ar atmosphere in the temperature range from 573 to 1373 K by differential scanning calorimetry (DSC; NETZSCH Pegasus 404 C, Pt/Rh crucible). The classical three-step method (continuous method) with a constant heating rate of 10 K min−1 was used to measure specific heat. The system was calibrated using a certified sapphire standard material. The mass and radius of sample pellet was kept the same as for standard material 84.1 g and 5 mm. The measurements of two different samples were repeated three times with maximal uncertainty 2%. It should be mentioned that the CP measurements at temperature above 1200 K by described DSC equipment are becoming less reliable due to increase of heat radiation which decrease registered signal. The heat capacity measurements at temperature range of 293.15–313.15 were carried out using a Micro-DSC II calorimeter (SETARAM, France; Hastelloy cells, sample weight ca. 600–800 mg, N2 atmosphere). The instrument software assisted step-by-step method (steps of 1 K at the measuring temperature for sample, blank and reference material) was applied. Synthetic sapphire was also used as reference material (data from [19]). The mean values of CP measurements for two different samples at 293–313 K and at 573–1373 K are given in Supplement. Heat contents of La2 Zr2 O7 and LaYO3 at 973 K were determined on the Tian-Calvet heat-flux microcalorimeter (SETARAM, France) using the “drop” method. The samples were droped directly from room temperature into the calorimeter at T = 973 K and the enthalpy increments [H◦ (973 K)–H◦ (298.15 K)] were measured. The enthalpy of formation of LaYO3 was obtained by “transposed temperature drop solution calorimetry” on the same micro-calorimeter. In each run the samples at room temperature were dropped into molten 2PbO·B2 O3 in the calorimeter at T = 973 K. The measured heat contained two contributions, the enthalpy increment of the sample from room temperature to 973 K and the enthalpy of dissolution at T = 973 K [H◦ (973 K) − H◦ (298.15 K) + H◦ sol (973 K)]. Sample masses for the dissolution were 3–8 (±0.001) mg. When using 30–35 g of solvent for experiments, the ratio of dissolved substance to the melt can be attributed to an infinitely dilute solution. Calibration of the calorimeter was performed by dropping pieces of platinum wire (in the solution experiments) and standard corundum ␣Al2 O3 with known enthalpy increments [20] (by drop calorimetry measurements). The sensitivity of apparatus and the precision of calorimetric measurements have been tested using the reference materials – gold and quartz. 3. Results and discussions Investigations of pyrochlore samples by XRD indicated that only pyrochlore phase was present in samples heat treated at 1523 and 1873 K. Presence of only one phase was also indicated by SEM/EDX. Perovskite samples were also confirmed to be single phase by XRD and SEM/EDX investigation. The results of sample ICP-OES analysis compositions determined by EDX, lattice

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Table 1 Results of sample characterization. Sample N

Composition from ICP Zr/(RE + Zr) Y/(Y + La)

Composition from EDX Zr/(RE + Zr) Y/(Y + La)

Lattice parameters (nm)

Density (g cm−3 )

Pyr10 1250 Pyr10 1600 Pyr11 1600 NdPyr2mf 1600 SmPyr3mf 1600 Per 1250

0.5105 0.5105 0.5147 0.5066 0.4896

0.54 0.54 0.54 0.54 0.53

6.033 6.059 6.060 6.411 6.691 6.034

Per1 1250

0.5080

0.54

a = 1.08011 a = 1.07854 a = 1.07852 a = 1.06498 a = 1.05721 a = 0.60843 b = 0.58763 c = 0.8492 a = 0.60857 b = 0.58781 c = 0.84963

parameters determined by Rietveld refinement and calculated density are presented in Table 1. It should be mentioned that calculated lattice parameters and densities are in good agreement with other works [9,16,17,21,22]. 3.1. Heat capacity measurements for pyrochlores The heat capacity measurements for the La2 Zr2 O7 are compared with data from Refs. [5,8,21,22] and calculations based on Neumann–Kopp rule [23] in Fig. 1a. The Cp data were obtained by DSC from room temperature to 1573 K in the work of Vassen et al. [21] and up to 1673 K in the work of Lehmann et al. [22]. The heat capacity data at 500–900 K reported by Bolech et al. [5] and at 900–1600 K by Sedmidubsky et al. [8] were derived from enthalpy increment measurement by drop calorimetry and then they were combined with the CP data at 298.15–400 K obtained by adiabatic

Fig. 1. Heat capacity data for the La2 Zr2 O7 . (a) Experimental data measured in present work along with data from Refs. [5,8,21,22] and calculations using Neumann–Kopp rule. (b) Calculated curve using Eq. (1) along with experimental data used for coefficient fitting: results from present study obtained by DSC and micro DCS and from Ref. [5] by adiabatic calorimetry.

6.028

calorimetry [5] in work [8]. Results of our measurement are in a good agreement with data of Sedmidubsky et al. [8] at 700–1373 K and with data of Bolech et al. [5] at 600–800 K. The data of Vassen et al. [21] and Lehmann et al. [22] are lower than that obtained in the present work. In the temperature range of 1273–1373 K the results of Vassen et al. [21] are getting closer to present work data. It should be mentioned that calculations based on Neumann–Kopp rule are slightly above our results. To fit the temperature dependence of CP the polynomial of Maier–Kelly [24] was used. The temperature dependence of heat capacity derived by combination of DSC results in the range 573–1373 K, Micro-DSC data at 293–313 K and adiabatic calorimetry data in the range 298–400 K from Ref. [5] is expressed by Eq. (1):

CP (J mol−1 K−1 ) = 267.225 + 0.015994T −

4, 275, 180 T2

(1)

The calculated heat capacity using Eq (1) for the La2 Zr2 O7 along with experimental data from present work used for fitting of CP parameters are presented in Fig. 1b. The enthalpy increment of the La2 Zr2 O7 measured in present work from room temperature to 973 K by “drop” calorimetry to be 174. 9 ± 2.8 kJ mol−1 (Table 1) is in a good agreement with the value 177.25 kJ mol−1 calculated using Eq (1). The heat capacity data for Nd2 Zr2 O7 along with experimental data [7–10,22] and Neumann–Kopp calculations are presented in Fig. 2a. The data of Lehmann et al. [22] are much below than obtained in other studies [7–10] and in present work at 473–1473 K. It should be mentioned that in the range 600–1273 K our results agree very well with data of Wang [9] obtained by drop calorimetry. The measurements of Liu et al. [10] are slightly below our data. Our results are also in a good agreement with data of Lutique et al. [7] at 600–900 K, but at higher temperatures CP data of Litique et al. [7] are higher. Data of Ref. [7] are based on adiabatic calorimetry in the range 3–400 K [6] and DSC results between 400 and 1650 K [7]. Compared to the CP data of Sedmidubsky et al. [8] our results are higher in whole temperature range of investigation. The difference between our data and those by Sedmidubsky et al. [8] reached up to 18 J mol−1 K−1 . The reasons for this difference in the CP of Nd2 Zr2 O7 are not clear because our data for La2 Zr2 O7 agree very well with those from Ref. [8]. According to Lutique et al. [25] the CP of Nd2 Zr2 O7 should be systematically higher than for nonmagnetic La2 Zr2 O7 due to Schottky contribution (magnetic ordering). It should be mentioned the difference between heat capacity of La2 Zr2 O7 and Nd2 Zr2 O7 at high temperatures is very small according to data of Sedmidubsky et al. [8] and it is within uncertainty of measurement. The calculations based on the Neumann–Kopp rule are slightly below our results; the agreement is getting better in the range 1000–1200 K. The temperature dependence of heat capacity derived by fitting results of present work in the

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Fig. 2. Heat capacity data for the Nd2 Zr2 O7 . (a) Experimental data measured in present work along with data from Refs. [7–10] and calculations using Neumann–Kopp rule. (b) Calculated curve using Eq. (2) along with experimental data used for coefficient fitting: results from present study obtained by DSC and micro DSC and from Ref. [6] by adiabatic calorimetry.

temperature ranges 573–1373 K and 293–313 K and adiabatic calorimetry data [6] is expressed by Eq. (2): CP (J mol−1 K−1 ) = 274.1864 + 0.02736T −

4, 399, 651 T2

(2)

The calculated CP data for the Nd2 Zr2 O7 along with experimental data from present work and Ref. [6] used for fitting of the CP parameters are presented in Fig. 2b. The CP data for the Sm2 Zr2 O7 obtained in the present work are compared with data of Wang [12] obtained by drop calorimetry, data of Liu et al. [11] obtained by DSC and the Neumann–Kopp calculations in Fig. 3a. Values obtained in this work are higher than that of Wang et al. [12] and Liu et al. [11] at temperatures below 1000 K. At higher temperatures (1073–1273 K) the agreement between our measurements and data of Liu et al. [11] obtained by DSC is good. Also at a temperature of ∼1373 K our data agree with data of Wang et al. [12]. It should be mentioned that the CP measured in present work practically do not change in the range of 1073–1173 K and even start to decrease at temperatures above 1173 K. The observed decrease of heat capacity seems to be physically unlikely and therefore it should be checked at temperatures above 1173 K by other techniques. It should be mentioned that it is not quite clear why the decrease of CP starts at lower temperature for the Sm2 Zr2 O7 than for the other pyrochlores La2 Zr2 O7 and Nd2 Zr2 O7 , studied in present work. The CP values of the Sm2 Zr2 O7 obtained by adiabatic calorimetry at 298.15 K [26,27] are consistent with results obtained in present work at 293–313 K. The calculations based on the Neumann–Kopp rule are slightly below our measurement and they become closer to our measurements at higher temperature. The temperature dependence of heat capacity derived by

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Fig. 3. Heat capacity data for the Sm2 Zr2 O7 . (a) Experimental data measured in present work along with data from Refs. [11,12] and calculations using Neumann–Kopp rule. (b) Calculated curve using Eq. (3) along with experimental data used for coefficient fitting: results from present study obtained by DSC and micro DSC and from Ref. [26].

fitting of our experimental results in the ranges of 573–1373 K and 293–313 K is expressed by Eq. (3): CP (J mol−1 K−1 ) = 312.528 − 0.000238T −

7, 116, 731 T2

(3)

The calculated heat capacity of the Sm2 Zr2 O7 along with experimental data from present work and data of Kopan et al. [26] are presented in Fig. 3b. 3.2. Thermodynamic data for LaYO3 and phase diagram calculations The phase diagram of the La2 O3 –Y2 O3 system was calculated by CALPHAD method in work of Fabrichnaya et al. [13]. It should be mentioned that the assessment of thermodynamic parameters was based only on phase equilibrium data in the system. The LaYO3 perovskite phase was found to have small homogeneity range. It transforms into monoclinic phase B at a temperature of 1813 ± 10 K according to literature data [1,16,17] and according to DTA measurements performed in this study. Monoclinic phase B partially transforms back to perovskite on cooling. The DTA curves obtained on heating up to 1973 K and up to 2173 K are shown in Fig. 4a and b. High temperature DTA performed in present study indicates two transformations: LaYO3 perovskite into B phase at 1852 K and B into high-temperature hexagonal phase H at 2094 K, which transforms back into monoclinic phase on cooling. XRD results of samples after both DTA investigations indicate mainly B phase and small fraction of the LaYO3 phase. The onset point on heating (2096 K) is close to the onset point on cooling (2092 K). The transition temperature is 70 K above a value of 2023 K from the literature [16,17]. It should

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Fig. 5. Heat capacity data for the LaYO3 . (a) Experimental data measured in present work along calculations based on the Neumann–Kopp rule. (b) Calculated curve using Eq. (3) along with experimental data obtained by DSC and micro DSC in present study which were used for fitting.

Fig. 4. DTA data for LaYO3 sample heat treated at 1523 K. (a) Heating up to 1973 K. (b) Heating up to 2173 K.

be mentioned that there is systematic shift to temperatures ∼40 ◦ C at 1823 K between DTA running up to 1973 K and high temperature DTA (up to 2173 K). The temperatures for both transformations were corrected by taking into account melting data for Al, Au and Al2 O3 . The corrected temperature of transformation for LaYO3 to B is 1807 K and for B to H is 2041 K. The corrected temperatures are in agreement with literature data [16,17]. The temperature of transition of perovskite into B phase according to the work of Mizuno et al. [28] is substantially lower than that obtained in our work and [16,17], while the temperature of transformation of B phase into hexagonal phase H is substantially higher than that obtained in present study and [16,17]. The heat capacity was experimentally measured in this work in the range of 293–313 K and 573–1373 K. The results are compared with calculations based on the Neumann–Kopp rule in Fig. 5a. It is shown that the experimental results of the present work are slightly below Neumann–Kopp calculations, but the difference is within uncertainty limits. The temperature dependence of heat capacity derived by fitting of results in the temperature ranges of 573–1373 K and 293–313 K is expressed by Eq. (4): CP (J mol−1 K−1 ) = 123.3627 + 0.004427T −

1, 482, 838 T2

(4)

The calculated CP for LaYO3 along with experimental data from present work used for fitting are presented in Fig. 5b. The enthalpy increment calculated using Eq. (4) in the temperature range between 298.15 and 975 K is equal to 81.7 kJ mol−1 and

is close to value 78.4 ± 0.7 kJ mol−1 obtained by “drop” calorimetry in present work (Table 2). The enthalpy of formation of the LaYO3 from oxides at 298.15 K was calculated using experimental results of this study for two different samples of the same compositions (Table 3) and literature data for oxides [29–31] on the basis of thermochemical cycle shown in Table 4. The value of enthalpy of formation from oxides for LaYO3 f H◦ ox (298.15 K) equal to −50.8 ± 4.0 kJ mol determined in present work and the CP data according to Eq. (4) were used to reassess enthalpy and entropy values for the LaYO3 phase and mixing parameters for hexagonal La2 O3 based solid solution A and monoclinic B phase in the La2 O3 –Y2 O3 system. The parameters of cubic phase C (Y2 O3 based solid solution), high temperature hexagonal phase H and high temperature cubic phase X were kept the same as Table 2 Experimental data on enthalpy increment for La2 Zr2 O7 (M = 572.24 g mol−1 ) and LaYO3 (M = 275.81 g mol−1 ). Mass of sample, mg La2 Zr2 O7 41.741 41.681 23.076 Mean value LaYO3 19.987 12.967 19.982 6.528 40.725 31.427 Mean value a

H◦ (973 K)–H◦ (298.15 K) (kJ mol−1 ) 174.21 174.33 176.20 174.9 ± 2.8 (3)a 78.54 78.38 78.13 78.25 77.56 79.55 78.40 ± 0.70 (6)a

Uncertainty is expressed by 95% confidence interval.

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Table 3 Results of drop-solution experiments for LaYO3 (M = 275.8105 g mol−1 ). Ho (973 K)–H◦ (298.15 K) + H◦ sol (973 K)

H◦ (973 K)–H◦ (298.15 K) + H◦ sol (973 K) −1

Mass of sample, mg

kJ mol

Sample 1 (1523 K) 3.244 7.579 5.762 3.367 Mean

42.80 46.40 37.55 36.78 40.88 ± 7.28a

a

kJ mol−1

Mass of sample, mg Sample 2 (1523 K) 6.620 5.600 3.689 3.888 3.354 3.942 3.344 3.570 4.025 Mean Mean: 39.9 ± 2.4 (13)

38.38 45.79 33.27 40.05 33.56 40.07 42.96 39.32 41.13 39.39 ± 3.08

Uncertainty is expressed by 95% confidence interval.

Table 4 Thermochemical cycle used to calculate enthalpy of formation of LaYO3 at T = 298.15 K. N

Reaction

r H (kJ)

(1) (2) (3) (4) (5) (6)

La2 O3 (s, 298.15 K) → La2 O3 (s, 973 K) La2 O3 (s, 973 K) → dilute solution in 2PbO·B2 O3 (973 K) Y2 O3 (s, 298.15 K) → Y2 O3 (s, 973 K) Y2 O3 (s, 973 K) → dilute solution in 2PbO·B2 O3 (973 K) LaYO3 (s, 298.15 K) → dilute solution in 2PbO·B2 O3 (973 K) 0.5La2 O3 (s, 298.15 K) + 0.5 Y2 O3 (s, 298.15 K) = LaYO3 (s, 298.15 K) r(6) H = 0.5r(1) H + 0.5r(2) H + 0.5r(3) H + 0.5r(4) H − r(5) H

83·83 [31] −126.0 ± 4.4 [29] 81.94 [31] −61.7 ± 1.1 [30] 39.9 ± 2.4a −50.87 ± 4.0

a

Determined in this study.

Table 5 Standard entropy and entropy of formation from oxides for several perovskite phases. Phase

Structure

0 S298.15 calculated

0 S298.15 experimental

LaAlO3 NdAlO3 SmAlO3 GdAlO3 YAlO3 LaYO3

Rhombohedral Rhombohedral Orthorhombic Orthorhombic Orthorhombic Orthorhombic

93.02 [13] 107.3 [38] 106.2 [39] 97.22 [40] 81.81 [37] 98.66 this work

86.6 [41] 105.1 [42] – – – –

in works of Fabrichnaya et al. [13]. It should be mentioned that the optimization performed in present work was based on experimental data obtained by DTA in present study and phase equilibrium data and DTA results from literature [1,14,16,17,32–36] while data of Mizuno et al. [28] were not taken into account because they contradict other experimental data and our DTA results. The calculated phase diagram is presented in Fig. 6. The assessed value for the enthalpy of formation of LaYO3 perovskite phase equal to −42.44 kJ mol−1 at 298.15 K is in reasonable agreement with experimental value (−50.8 kJ mol−1 ). The calculated entropy value of 98.66 J mol−1 K−1 compared with calculated [13,37–40] and experimental [41,42] values for other perovskites seems to be reasonable (Table 5). However more experimental data on low-temperature CP is necessary for systematic estimates of perovskite entropies. The assessed mixing parameters for A and B phases in (J mol−1 ) described by sublattice model (La+3 ,Y+3 )2 (O−2 )3 are expressed as:

0f,ox S298.15 .15 calculated

3.86 2.88 5.23 −3.55 6.86 −15.51

system was discussed in Refs. [14,37]. It should be noted that is not possible to reach equilibrium experimentally at such low temperatures in the ceramic systems. However, the temperature of this reaction is too low to cause contradiction with results for the

A-phase: 0 LLa+3,Y+3:O−2 = 28,018; 1 LLa+3,Y+3:O−2 = 3073 0L B-phase: La+3,Y+3:O−2 = −56,419 + 36.2996T; 1L La+3,Y+3:O−2 = −108,890 + 49.7308T The calculated data on invariant reactions are compared with experimental data in Table 6. The new calculations are in good agreement with experimental phase equilibrium data. It should be mentioned that present calculation indicated additional eutectoid reaction B = A + Per at 974 K. Possibility of this reaction to proceed and it influence on phase equilibria in the ZrO2 –La2 O3 –Y2 O3

Fig. 6. Calculated phase diagram of the La2 O3 –Y2 O3 using the CP data and enthalpy of formation of LaYO3 from oxides determined in present work. Experimental data on phase equilibria from literature are presented. The phase name abbreviations: L – liquid, X – high temperature cubic phase, H – high temperature hexagonal phase, B – monoclinic phase, A – low temperature hexagonal phase, Per – LaYO3 perovskite.

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Table 6 Invariant equilibria in La2 O3 –Y2 O3 system. Reaction

T (K), x(Y2 O3 ) – calc.

[17]

[16]

[25]

[1]

L + H = X (p) L = X (c) X = H (c) B = H (c) H + C = B (p) H = B + C (e) H = B + A (e) B = Per(c) B = C + Per (e) B = A + Per (e)

2596 L:0.83 X:0.860 H:0.87 2476 0.367 2214 0.29 2009 0.4512 2083 H:0.617 C: 0.775 B:0.661

2583 L:0.825 X:0.890 H:0.874 2489 0.438 2231 0.339 2173 H:0.71 C:0.85 B:0.67 – 2003 H:0.45 B:0.5 A:0.4 1858 0.5 1843 B:0.55 C:0.85

2643 L:0.85 X:0.90 H:0.94 2483 0.35 2213 0.4 2033 H:0.60 C:0.80 B:0.50 – 2028 – 1823 0.5 1773 B:0.57 C:0.8

2623 L:0.82 X:0.85 H:0.93 2303 0.5 2253 0.37 2177 0.5 2133 C:0.85 B:0.6 H:0.7 2133 H:0.25 B:0.27 H:0.25 2177 0.5 1703 B:0.6 C:0.85

– – – – – – – 1778 0.5 –

2045 H:0454 B:0.496 A:0.394 1834 0.5 1820 B:0.584 C:0.800 974 B:0.149 A: 0.002

ZrO2 –La2 O3 –Y2 O3 system obtained in Ref. [14]. Low-temperature heat capacity measurements are highly desirable to constrain thermodynamic parameters of the La2 O3 –Y2 O3 system. 4. Conclusions Heat capacities of pyrochlore phases La2 Zr2 O7 , Nd2 Zr2 O7 , Sm2 Zr2 O7 and perovskite phase LaYO3 are measured in the temperature ranges 293.15–303.15 and 573–1373 K using micro-DSC and DSC. The obtained results for pyrochlores are in reasonable agreement with available literature data. This way the reliability of our measurements has been confirmed. The CP data fitted in temperature 293.15–1273 K can be used for further improvement of thermodynamic databases for the ZrO2 -based systems. The measured heat capacity data for the LaYO3 perovskite are also fitted to describe heat capacity in the range 293.15–1273 K. The heat capacity data of LaYO3 together with enthalpy of formation from oxides derived from high-temperature drop-solution calorimerty measurements are used for assessment of thermodynamic parameters of the La2 O3 –Y2 O3 system. The calculated phase diagram reproduces experimental data from literature. It should be mentioned that low-temperature heat capacity data would be desirable for LaYO3 perovskite to determine its standard entropy. Additionally, more systematic investigation of orthorhombic perovskites LnAlO3 is necessary because low-temperature heat capacity data for Ln = Sm, Gd are missing. Heat capacity data for the YAlO3 are not available at any temperatures so far. Therefore experimental data for heat capacities of orthorhombic perovskites are highly desirable for further development of thermodynamic databases for the ZrO2 –Ln2 O3 –Y2 O3 –Al2 O3 systems. Acknowledgement This study was financially supported by DFG Project SE 647/9-1. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tca.2011.08.021. References [1] V.P. Gorelov, Z.S. Martem’yanova, V.B. Balakireva, Unit cell, phase transition, and electric conductivity of LaYO3 , Inorg. Mater. 35 (1999) 153–157. [2] S.A. Speakman, R.D. Carneim, E.A. Payzant, T.R. Armstrong, Development of proton conductors using pyrochlore-perovskite phase boundaries, J. Mater. Eng. Perform. 13 (2004) 303–308. [3] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc. 24 (2004) 1–10. [4] C.G. Levi, Emerging materials and processes for thermal barrier system, Curr. Opin. Solid State Mater. Sci. 8 (2004) 77–91. [5] M. Bolech, E.H.P. Cordfunke, A.C.G. van Genderens, R.R. van der Laan, F.J.J.G. Janssen, J.C. van Miltenburg, The heat capacity and derived thermodynamic functions of La2 Zr2 O7 and Ce2 Zr2 O7 from 4 to 1000 K, J. Phys. Chem. Solids 58 (1997) 433–439.

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