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Analysis of phase transition behavior of BaCeO3 with thermal analyses and high temperature X-ray diffraction
Solid State Ionics 180 (2009) 1034–1039
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Analysis of phase transition behavior of BaCeO3 with thermal analyses and high temperature X-ray diffraction Takeshi Ohzeki, Shinya Hasegawa, Misa Shimizu, Takuya Hashimoto ⁎ Department of Integrated Sciences in Physics and Biology, College of Humanities and Sciences, Nihon University, 3-8-1 Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan
a r t i c l e
i n f o
Article history: Received 14 January 2009 Received in revised form 19 May 2009 Accepted 23 May 2009 Keywords: BaCeO3 Crystal structure Phase transition High temperature X-ray diffraction Differential scanning calorimetry Thermal expansion
a b s t r a c t Structural phase transitions in BaCeO3 have been investigated with combination of differential scanning calorimetry (DSC), dilatometry and high temperature X-ray diffraction with high sensitivity and resolution. In DSC curve at heating procedures, baseline shift, endothermic peak and another baseline shift were observed at 260 °C, 385 °C and 895 °C, respectively. From DSC curve at cooling procedure, it was revealed that all the baseline shifts and peak were reversible. No hysteresis was observed in the both baseline shifts indicating second order phase transition at 260 °C and 895 °C with variation of specific heat capacity, ΔCp, of 10 J/mol K and 7 J/mol K, respectively; whereas the order of the phase transition at 385 °C was revealed to be the first since hysteresis was detected around 370–385 °C. Variation of enthalpy, ΔH, at the phase transition was 45 J/mol. High temperature X-ray diffraction measurements have revealed that the crystal structure of BaCeO3 changes from primitive orthorhombic perovskite through body-centered one, rhombohedral distorted one to cubic one around 280 °C, 400 °C and 900 °C, showing correspondence with DSC curves. Dependence of molar volume on temperature estimated from high temperature X-ray diffraction showed agreement with thermal expansion behavior observed with dilatometry. © 2009 Elsevier B.V. All rights reserved.
1. Introduction BaCe1 − xMxO3 − δ (M: trivalent ion such as Y and rare earth) has been studied as a high proton conducting oxide operated at 400– 600 °C, which is expected as an electrolyte of solid oxide fuel cells, H2 gas sensor and so on [1]. It has been reported that one of the problems for application of BaCe1 − xMxO3 − δ is structural phase transitions between room temperature and high temperature. However, there have been contradictions in literatures on the structural phase transition behavior even in mother phase, BaCeO3. By using neutron diffraction, Knight reported that crystal structure of BaCeO3 at room temperature was orthorhombic distorted perovskite with space group of Pnma (No. 62). He also reported three types of structural phase transitions from primitive orthorhombic perovskite to body-centered one with space group of Imma (No. 74), from the Imma to rhombohedral distorted one with space group of R3̄c (No. 167) and from the R3̄c to cubic one with space group of Pm3̄m (No. 221) at 290 °C, 400 °C and 900 °C, respectively [2,3]. Genet et al. also performed high temperature neutron diffraction measurements of BaCeO3. They reported almost the same phase transition behavior with Knight's with slight different phase transition temperatures of 300 °C, 400 °C and more than 927 °C [4]. However, neutron diffraction measurement involves several problems as a method for evaluation of phase transition. One is a large quantity of the specimens required for ⁎ Corresponding author. Tel.: +81 3 3329 1151x5516; fax: +81 3 5317 9432. E-mail address: [email protected] (T. Hashimoto). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.05.019
the measurement. It makes accurate and systematic measurements of phase transition temperature difficult. Another problem is that thermodynamic character, such as the order of the phase transition, cannot be investigated directly. In addition, the results reported by Knight cannot be assessed easily since the institute, in which high temperature neutron diffraction measurements can be carried out is limited. Therefore, other general measurements which require small amount of specimens should be employed to investigate the phase transition behavior of BaCe1 − xMxO3 − δ. Thermal analyses such as differential scanning calorimetry (DSC) and dilatometry are regarded as suitable for systematic investigation of the phase transition behavior since they can be generally carried out with far lower cost and smaller amount of specimens. However, we regard that no reliable thermal analyses have been reported even on BaCeO3 system. Yamanaka and coworkers reported the phase transitions of BaCeO3 by using DSC and dilatometry [5,6]. They reported anomalies in DSC curve at 267 °C, 327 °C and 907 °C ascribed to the phase transition. However, their result was not convincing enough since the observed signals were so broad, resulting in failure on characterization of the phase transition behavior such as order, variation of enthalpy (ΔH) and variation of heat capacity (ΔCp). They also reported no anomaly in their measured thermal expansion behavior, which showed contradiction with the result of DSC. Melekh et al. reported two kinds of endothermic peaks at 260 °C and 357 °C in DSC curve of BaCeO3 and attributed them to the structural phase transition [7]. However, it is probable that composition of their sample was not accurate since their preparation method involved melting of
T. Ohzeki et al. / Solid State Ionics 180 (2009) 1034–1039
the specimen by using inductively coupled plasma. Egorov et al. reported DSC curve of ceramic BaCeO3 prepared by ordinary solid state reaction method [8]. Although reversibility nor existence of hysteresis were not confirmed since the DSC curve was measured only at heating process, they concluded that the phase transition at 262 °C was second order with ΔCp of 97.6 J/mol K and the first order phase transition at 394 °C with ΔH of 58.6 J/mol; however, ΔCp were extraordinary large compared to those for other structural phase transition of perovskite-type oxide [9]. Kuzmin et al. investigated phase transition in BaCeO3 by using dilatometry [10]. They concluded that six kinds of phase transitions existed at 207 °C, 257 °C, 392 °C, 627 °C, 757 °C and 897 °C, some of which could not be detected by diffraction analysis. Yamaguchi and Yamada also reported dilatometric curve of BaCeO3 [11]. Besides the anomalies at 263 °C, 394 °C and 891 °C, which can be attributed to the structural phase transition reported earlier, they observed anomalies at 474 °C, whose origin was not clarified; however, we regard that both results of dilatometry are not convincing because their observed anomalies on thermal expansion were so small except one around 392–394 °C. It can be expected that information of the phase transition of BaCeO3 can be obtained by using high temperature X-ray diffraction measurements; however, it has not been reported so far. Only reported one is high temperature X-ray diffraction measurements of BaCe1 − xYbxO3 − δ (x = 0.05, 0.12 and 0.2) reported by Yamaguchi and Yamada [11]. However, we evaluate their investigation was insufficient for analysis of the structural phase transition since their measurements were limited to variation of particular diffraction peaks on temperature between 350 °C and 900 °C. In this study, phase transition behavior of BaCeO3 has been investigated by combination of DSC, dilatometry and high temperature X-ray diffraction. Employing measurement method with enough sensitivity and resolution, accurate information on the structural phase transition behavior has been obtained. 2. Experimental BaCeO3 was prepared by solid state reaction method. Nominal amount of BaCO3 (99.9%, Furuuchi Chemistry Corp.) and CeO2 (99.9%, Furuuchi Chemistry Corp.) were mixed in ethanol with alumina mortar. The obtained powder was uniaxially pressed into cylindrical shape at 30 MPa. The pressed specimens were sintered at 1300 °C for 10 h in air. In order to confirm the crystal structure of obtained
Fig. 1. X-ray diffraction pattern of BaCeO3 at room temperature in 2θ range of 15–90° obtained using CuKα radiation. Diffraction pattern can be indexed as orthorhombic distorted perovskite structure.
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Fig. 2. DSC curves of BaCeO3 in static air at heating and cooling procedure. Heating and cooling rate is 10 °C/min. The weight of the sample and reference are about 70 mg and about 40 mg, respectively.
specimens, the powder prepared by crushing the sintered specimen was subjected to X-ray diffraction at room temperature by using Rigaku RINT-2500VPC (CuKα: 50 kV, 250 mA). Phase transition behavior of BaCeO3 was analyzed with DSC, dilatometry and high temperature X-ray diffraction measurements. DSC of powder specimen in temperature range from room temperature to 1200 °C was performed in static air with heating and cooling rate of 10 °C/min by using DSC8270 (Rigaku Co., Ltd.). The weight of sample and Al2O3 powder employed as a reference were optimized in order to obtain DSC curve with low noise and flat base line. Pt was used as a material of pan for sample and reference. Thermal expansion of sintered specimen with about 5.0 mm diameter and 10–15 mm height from room temperature to 1200 °C was measured in air with heating and cooling rate of 10 °C/min by using TMA8310 (Rigaku Co., Ltd.). Al2O3 was employed as materials for reference, push rod and sample stage. In order to analyze crystal structure of BaCeO3 at high temperature, X-ray diffraction patterns of powder specimen were measured in air at
Fig. 3. DSC curves of BaCeO3 around 260 °C. The baseline shift with no hysteresis was observed at 260 °C.
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Fig. 4. X-ray diffraction patterns of BaCeO3 at 30–280 °C in 2θ range of 15–45°. The ▼ peaks with h + k + l = odd number disappear at 280 °C.
various temperatures between 30 °C and 1200 °C by using RINT2500VPC (CuKα: 50 kV, 250 mA) with heating attachment. Pt was used as a material for sample holder. 3. Results and discussion 3.1. DSC curves of BaCeO3 between room temperature and 1200 °C
Fig. 6. X-ray diffraction patterns of BaCeO3 at 280–400 °C in 2θ range of 15–90°. Apparently, no variation on temperature was observed; however, the diffraction pattern at 360 °C is indexed as body-centered orthorhombic, while that at 400 °C as rhombohedral symmetry.
lowest noise level and the highest flatness of base line could be obtained in this study. On heating procedure, thermal anomalies were observed at 260 °C, 385 °C and 895 °C more clearly than so far reported DSC measurements possibly due to higher sensitivity and temperature resolution. By comparison with DSC curves at heating and cooling, it has been revealed that all the observed thermal anomalies are reversible. To clarify the origin of the thermal anomalies, DSC signal and variation of X-ray diffraction patterns around each temperature were minutely investigated.
Fig. 1 shows X-ray diffraction pattern in 2θ range of 15–90° of obtained specimen at room temperature. All the diffraction peaks could be indexed assuming orthorhombic symmetry with a = 8.784 Å, b = 6.220 Å and c = 6.239 Å. No peak assigned as impurity was detected, showing that single phase of BaCeO3 was successfully obtained. The peaks with h + k + l = odd as 111 and those with mixed h, k, l, such as 112, were observed, indicating primitive lattice which agreed with the space group, Pmnb (No. 62), proposed by Knight [2,3], Genet et al. [4] and Takeuchi et al. [12]. Fig. 2 shows DSC curves of BaCeO3 powder in static air with the sample and reference weight of about 70 mg and 40 mg, respectively. By using these amounts of the sample and Al2O3, DSC curve with the
Fig. 3 shows a close up of DSC curves around 260 °C. Observed at 260 °C was the baseline shift with no hysteresis, indicating the second order phase transition. ΔCp estimated from the baseline shift was about 10 J/mol K, which is one order smaller than that reported by Egorov et al. [8].
Fig. 5. DSC curves of BaCeO3 round 380 °C. The peaks originating from latent heat and hysteresis were observed around 373–383 °C.
Fig. 7. X-ray diffraction patterns of BaCeO3 at 280–400 °C in 2θ range of 87.5–89.5°. Two peaks assigned as 044 and 800 of orthorhombic symmetry were observed below 360 °C, whereas only one peak indexed as 440 of rhombohedral symmetry was detected at 400 °C.
3.2. Structural phase transition around 260 °C
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the more accurate phase transition temperature can be measured in this study than those obtained by using neutron diffraction due to small quantity of the specimens. 3.3. Structural phase transition around 380 °C
Fig. 8. DSC curves of BaCeO3 around 900 °C. The baseline shift with no hysteresis was observed at 895 °C.
Since existence of structural phase transition from orthorhombic distorted perovskite with space group of Pmnb (No. 62) to orthorhombic one with Imma (No. 74) around 290 °C has been reported by neutron diffraction [2–4], the second order phase transition at 260 °C can be assigned as this structural phase transition. For confirmation, X-ray diffraction patterns of BaCeO3 at various temperatures between 30 and 280 °C were measured. Fig. 4 shows observed X-ray diffraction patterns. All the peaks could be indexed assuming orthorhombic symmetry; however, the peaks with h + k + l = odd represented by ▼ disappeared in diffraction pattern at 280 °C, indicating that crystal system of BaCeO3 at 280 °C is body-centered orthorhombic. Peaks with h + k + l = odd remain at 250 °C showing agreement with the result of DSC depicted in Fig. 3. This showed agreement with so far reported structural phase transition behavior proposed by using neutron diffraction except for transition temperature. We regard that
Fig. 5 shows a close up of DSC curves around 380 °C. The endothermic and exothermic peaks were observed in heating and cooling procedure, respectively. The temperature difference of both peaks, i.e. hysteresis indicated that phase transition around 380 °C was the first order. ΔH calculated from peak area was about 45 J/mol. Since Knight reported that crystal system of BaCeO3 changed from Imma (No. 74) to R3̄c (No. 167) at 400 °C by neutron diffraction [2,3], it was highly possible that the first order phase transition originated from this structural phase transition. The deviation of temperature could be ascribed to large amount of specimens for neutron diffraction measurements. Fig. 6 shows X-ray diffraction patterns of BaCeO3 in temperature range of 280–400 °C. Apparently, no variation was observed except peak shift due to thermal expansion; however, variation of peak shape was observed at high 2θ angle as shown in Fig. 7. Below 360 °C, two peaks identified as 044 and 800 of bodycentered orthorhombic symmetry were observed in 2θ range of 87.5– 89.5°, whereas only one peak was detected at 400 °C. This indicates that crystal symmetry of BaCeO3 at 400 °C is higher than those below 360 °C, i.e., Imma (No. 74). The diffraction pattern at 400 °C depicted in Fig. 6 could be indexed assuming rhombohedral symmetry with a = 6.234 Å and α = 60.260°, showing agreement with DSC curves depicted in Fig. 5 and the results of neutron diffraction measurements. The first order structural phase transition from orthorhombic distorted perovskite to rhombohedral distorted perovskite has been reported also in LaCrO3 and LaGaO3 [13,14]. However, the phase transition for LaCrO3 and LaGaO3 is from orthorhombic Pnmb (No. 62) to rhombohedral R3̄c (No. 167), which is different from the first order phase transition of BaCeO3. ΔH for phase transition of LaCrO3 and LaGaO3 are 350 J/mol and 250 J/mol, respectively, which is larger than ΔH, 45 J/mol, for phase transition of BaCeO3. It is suspected that smaller ΔH of BaCeO3 can be attributed to higher crystal symmetry below the transition temperature than those of LaCrO3 and LaGaO3.
Fig. 9. (a) X-ray diffraction patterns of BaCeO3 at 400–900 °C in 2θ range of 15–90°. The diffraction pattern at 400 °C can be indexed as rhombohedral. The diffraction peak represented by ▼ disappear at 900 °C, resulting in successful identification as cubic symmetry. (b) Enlarged X-ray diffraction patterns of (a).
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Fig. 10. X-ray diffraction patterns of BaCeO3 at 900–1200 °C in 2θ range of 15–90°. All the diffraction patterns can be indexed as cubic symmetry.
Fig. 11. Temperature dependence of rhombohedral lattice angle of BaCeO3. In cubic symmetry, rhombohedral lattice angle is equal to 60°.
3.4. Structural phase transition around 895 °C Fig. 8 shows an enlargement of DSC curves around 900 °C. Reversible baseline shift with no hysteresis was observed at 895 °C, indicating second order phase transition. From baseline shift, ΔCp was evaluated to be about 7 J/mol K. Fig. 9(a) shows X-ray diffraction patterns of BaCeO3 at temperature range of 400–900 °C. Apparently, no variation on temperature was observed; however, peak represented by ▼ was not observed in the diffraction pattern at 900 °C as also shown in Fig. 9(b), which is enlargement of Fig. 9(a). Below 800 °C, the ▼ peak, this cannot be indexed as cubic symmetry but rhombohedral symmetry remains, showing agreement with second order phase transition at 895 °C observed by DSC. Due to lack of the peak, X-ray diffraction pattern at 900 °C could be indexed assuming cubic symmetry with Pm3̄m (No. 221). Fig. 10 shows X-ray diffraction patterns of BaCeO3 at 900– 1200 °C. All the peaks could be indexed assuming ideal cubic symmetry, which showed agreement with the results of neutron diffraction measurement. The phase transition can also be investigated from variation of lattice constants on temperature. Fig. 11 shows variation of rhombohedral angle, α, calculated from Bragg angles of 400 and 440 peaks of rhombohedral lattice,1 on temperature. In the cubic structure, α is equal to 60° [15]. The α approaches 60° not suddenly but gradually with increase of temperature and variation of α was not observed above 900 °C, showing correspondence with second order phase transition at 895 °C observed by DSC.
895 °C, which can be attributed to the second order phase transition from R3̄c (No. 167) to Pm3̄m (No. 221). Mean linear thermal expansion coefficients were 1.10 × 10− 5 K− 1 and 1.04 × 10− 5 K− 1 below and above 895 °C, respectively. At about 260 °C and 380 °C where phase transitions exist, influence of phase transition on thermal expansion was scarcely detected; however, slight anomalies were observed in thermal expansion coefficient at the temperatures. It is concluded that variation of thermal expansion coefficient derived from the second order phase transition at 260 °C and ΔV from the first order phase transition at 380 °C are so small that their influence on thermal expansion could not be observed. Fig. 13 shows the temperature dependence of molar volume of BaCeO3 calculated from high temperature X-ray diffraction patterns. It has been revealed that the phase transitions from Pmnb (No. 62) to Imma (No. 74) and from Imma to R3̄c (No. 167) scarcely affect the thermal expansion behavior, whereas clear variation of inclination due to structural phase transition from R3̄c (No. 167) to Pm3̄m (No. 221) was observed around 900 °C. These results showed
3.5. Effect of phase transition on thermal expansion behavior of BaCeO3 Thermal expansion behavior is important information as for application to high temperature devices such as H2 gas sensor, electrolyte of solid oxide fuel cells and so on. It has been frequently reported that the discrete variation of volume, ΔV, at first order structural phase transition and variation of thermal expansion coefficient at second order structural phase transition influence property of materials. Fig. 12 shows thermal expansion and expansion coefficient of sintered BaCeO3 ceramics in air measured with dilatometry. In thermal expansion curve, variation of slope was detected at about 1
As for diffraction patterns more than 900 °C, 222 and 400 peaks of cubic symmetry were regarded as 400 and 440 peaks of rhombohedral lattice, respectively, to calculate α.
Fig. 12. Thermal expansion behavior and temperature dependence of expansion coefficient of BaCeO3 ceramics measured with dilatometry. The variation in the thermal expansion coefficient was observed at 260 °C, 380 °C and 895 °C.
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detected in BaCeO3 and their thermodynamic character has been established with high reliability. The second order structural phase transition from primitive orthorhombic perovskite with space group of Pmnb (No. 62) to body-centered one with space group of Imma (No. 74) was detected at 260 °C. ΔCp of the phase transition was 10 J/mol K. At 385 °C, observed was the first order structural phase transition with ΔH of 45 J/mol from Imma (No. 74) to rhombohedral distorted perovskite with space group of R3c̄ (No. 167). Another second order structural phase transition with ΔCp of 7 J/mol K from R3c̄ to ideal cubic ̄ (No. 221) was observed at 895 °C. perovskite with space group of Pm3m Due to small variation of thermal expansion coefficient on the phase transition at 260 °C and small ΔV on the first order structural phase transition at 385 °C, anomalous variation was not observed at the temperatures in thermal expansion curve of BaCeO3, measured with dilatometry. Only at 895 °C, variation of thermal expansion coefficient due to the second order phase transition was detected, showing agreement with temperature dependence of molar volume evaluated by high temperature X-ray diffraction. References Fig. 13. Temperature dependence of molar volume of BaCeO3 evaluated from X-ray diffraction at high temperatures. The symbols of ○, Δ, ◊ and □ represent primitive orthorhombic perovskite, body-centered orthorhombic one, rhombohedral one and cubic one, respectively.
[1] [2] [3] [4] [5] [6]
quantitative agreement with thermal expansion observed with dilatometry. It can also be concluded that variation of thermal expansion coefficient derived from the second order phase transition at 260 °C and ΔV from the first order phase transition at 380 °C are so small that they cannot be observed by our employed X-ray diffraction. Small ΔV compared to those for LaCrO3 and LaGaO3 can be attributed to higher crystal symmetry for BaCeO3 below the first order phase transition temperature, as also was observed for ΔH at the phase transition. Mean volume thermal expansion coefficients were 3.54 × 10− 5 K− 1 and 3.16 × 10− 5 K− 1 below and above 895 °C, respectively. Assuming isotropic thermal expansion, volume thermal expansion should be as three times as linear one, which shows agreement with this study. 4. Conclusion With combination of DSC, high temperature X-ray diffraction and dilatometry, three kinds of structural phase transitions have been
[7] [8] [9] [10] [11] [12] [13] [14] [15]
H. Iwahara, H. Uchida, K. Kondo, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. K.S. Knight, Solid State Ionics 74 (1994) 109. K.S. Knight, Solid State Ionics 145 (2001) 275. F. Genet, S. Loridant, C. Ritter, G. Lucazeau, J. Phys. Chem. Solids 60 (1999) 2009. S. Yamanaka, T. Hamaguchi, T. Oyama, T. Matsuda, S. Kobayashi, K. Kurosaki, J. Alloys Compd. 359 (2003) 1. S. Yamanaka, M. Fujikane, T. Hamaguchi, H. Muta, T. Oyama, T. Matsuda, S. Kobayashi, K. Kurosaki, J. Alloys Compd. 359 (2003) 109. B.T. Melekh, Y.u.M. Baikov, N.F. Kartenko, Y.u.N. Filin, M.E. Kompan, I.I. Novak, G.B. Kulik, Solid State Ionics 97 (1997) 465. V.M. Egorov, Y.u.M. Baikov, N.F. Kartenko, B.T. Melekh, Y.u.N. Filin, Phys. Solid State 40 (1998) 1911. T. Hashimoto, Y. Ueda, M. Yoshinaga, K. Komazaki, K. Asaoka, S. Wang, J. Electrochem. Soc. 149 (2002) A1381. A.V. Kuzmin, V.P. Gorelov, B.T. Melekh, M. Glerup, F.W. Poulsen, Solid State Ionics 162 (2003) 13. S. Yamaguchi, N. Yamada, Solid State Ionics 162–163 (2003) 23. K. Takeuchi, C.K. Loong, J.W. Richardson Jr., J. Guan, S.E. Dorris, U. Balachandran, Solid State Ionics 138 (2000) 63. F. Nakamura, Y. Matsunaga, N. Ohba, K. Arai, H. Matsubara, H. Takahashi, T. Hashimoto, Thermochim. Acta 435 (2005) 222. T. Shibasaki, T. Furuya, J. Kawahara, Y. Takahashi, H. Takahashi, T. Hashimoto, J. Therm. Anal. Calorim. 81 (2005) 575. N. Ohba, E. Oikawa, T. Hashimoto, Defect Diffus. Forum 242–244 (2005) 9.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Analysis of phase transition behavior of BaCeO3 with thermal analyses and high temperature X-ray diffraction Takeshi Ohzeki, Shinya Hasegawa, Misa Shimizu, Takuya Hashimoto ⁎ Department of Integrated Sciences in Physics and Biology, College of Humanities and Sciences, Nihon University, 3-8-1 Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan
a r t i c l e
i n f o
Article history: Received 14 January 2009 Received in revised form 19 May 2009 Accepted 23 May 2009 Keywords: BaCeO3 Crystal structure Phase transition High temperature X-ray diffraction Differential scanning calorimetry Thermal expansion
a b s t r a c t Structural phase transitions in BaCeO3 have been investigated with combination of differential scanning calorimetry (DSC), dilatometry and high temperature X-ray diffraction with high sensitivity and resolution. In DSC curve at heating procedures, baseline shift, endothermic peak and another baseline shift were observed at 260 °C, 385 °C and 895 °C, respectively. From DSC curve at cooling procedure, it was revealed that all the baseline shifts and peak were reversible. No hysteresis was observed in the both baseline shifts indicating second order phase transition at 260 °C and 895 °C with variation of specific heat capacity, ΔCp, of 10 J/mol K and 7 J/mol K, respectively; whereas the order of the phase transition at 385 °C was revealed to be the first since hysteresis was detected around 370–385 °C. Variation of enthalpy, ΔH, at the phase transition was 45 J/mol. High temperature X-ray diffraction measurements have revealed that the crystal structure of BaCeO3 changes from primitive orthorhombic perovskite through body-centered one, rhombohedral distorted one to cubic one around 280 °C, 400 °C and 900 °C, showing correspondence with DSC curves. Dependence of molar volume on temperature estimated from high temperature X-ray diffraction showed agreement with thermal expansion behavior observed with dilatometry. © 2009 Elsevier B.V. All rights reserved.
1. Introduction BaCe1 − xMxO3 − δ (M: trivalent ion such as Y and rare earth) has been studied as a high proton conducting oxide operated at 400– 600 °C, which is expected as an electrolyte of solid oxide fuel cells, H2 gas sensor and so on [1]. It has been reported that one of the problems for application of BaCe1 − xMxO3 − δ is structural phase transitions between room temperature and high temperature. However, there have been contradictions in literatures on the structural phase transition behavior even in mother phase, BaCeO3. By using neutron diffraction, Knight reported that crystal structure of BaCeO3 at room temperature was orthorhombic distorted perovskite with space group of Pnma (No. 62). He also reported three types of structural phase transitions from primitive orthorhombic perovskite to body-centered one with space group of Imma (No. 74), from the Imma to rhombohedral distorted one with space group of R3̄c (No. 167) and from the R3̄c to cubic one with space group of Pm3̄m (No. 221) at 290 °C, 400 °C and 900 °C, respectively [2,3]. Genet et al. also performed high temperature neutron diffraction measurements of BaCeO3. They reported almost the same phase transition behavior with Knight's with slight different phase transition temperatures of 300 °C, 400 °C and more than 927 °C [4]. However, neutron diffraction measurement involves several problems as a method for evaluation of phase transition. One is a large quantity of the specimens required for ⁎ Corresponding author. Tel.: +81 3 3329 1151x5516; fax: +81 3 5317 9432. E-mail address: [email protected] (T. Hashimoto). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.05.019
the measurement. It makes accurate and systematic measurements of phase transition temperature difficult. Another problem is that thermodynamic character, such as the order of the phase transition, cannot be investigated directly. In addition, the results reported by Knight cannot be assessed easily since the institute, in which high temperature neutron diffraction measurements can be carried out is limited. Therefore, other general measurements which require small amount of specimens should be employed to investigate the phase transition behavior of BaCe1 − xMxO3 − δ. Thermal analyses such as differential scanning calorimetry (DSC) and dilatometry are regarded as suitable for systematic investigation of the phase transition behavior since they can be generally carried out with far lower cost and smaller amount of specimens. However, we regard that no reliable thermal analyses have been reported even on BaCeO3 system. Yamanaka and coworkers reported the phase transitions of BaCeO3 by using DSC and dilatometry [5,6]. They reported anomalies in DSC curve at 267 °C, 327 °C and 907 °C ascribed to the phase transition. However, their result was not convincing enough since the observed signals were so broad, resulting in failure on characterization of the phase transition behavior such as order, variation of enthalpy (ΔH) and variation of heat capacity (ΔCp). They also reported no anomaly in their measured thermal expansion behavior, which showed contradiction with the result of DSC. Melekh et al. reported two kinds of endothermic peaks at 260 °C and 357 °C in DSC curve of BaCeO3 and attributed them to the structural phase transition [7]. However, it is probable that composition of their sample was not accurate since their preparation method involved melting of
T. Ohzeki et al. / Solid State Ionics 180 (2009) 1034–1039
the specimen by using inductively coupled plasma. Egorov et al. reported DSC curve of ceramic BaCeO3 prepared by ordinary solid state reaction method [8]. Although reversibility nor existence of hysteresis were not confirmed since the DSC curve was measured only at heating process, they concluded that the phase transition at 262 °C was second order with ΔCp of 97.6 J/mol K and the first order phase transition at 394 °C with ΔH of 58.6 J/mol; however, ΔCp were extraordinary large compared to those for other structural phase transition of perovskite-type oxide [9]. Kuzmin et al. investigated phase transition in BaCeO3 by using dilatometry [10]. They concluded that six kinds of phase transitions existed at 207 °C, 257 °C, 392 °C, 627 °C, 757 °C and 897 °C, some of which could not be detected by diffraction analysis. Yamaguchi and Yamada also reported dilatometric curve of BaCeO3 [11]. Besides the anomalies at 263 °C, 394 °C and 891 °C, which can be attributed to the structural phase transition reported earlier, they observed anomalies at 474 °C, whose origin was not clarified; however, we regard that both results of dilatometry are not convincing because their observed anomalies on thermal expansion were so small except one around 392–394 °C. It can be expected that information of the phase transition of BaCeO3 can be obtained by using high temperature X-ray diffraction measurements; however, it has not been reported so far. Only reported one is high temperature X-ray diffraction measurements of BaCe1 − xYbxO3 − δ (x = 0.05, 0.12 and 0.2) reported by Yamaguchi and Yamada [11]. However, we evaluate their investigation was insufficient for analysis of the structural phase transition since their measurements were limited to variation of particular diffraction peaks on temperature between 350 °C and 900 °C. In this study, phase transition behavior of BaCeO3 has been investigated by combination of DSC, dilatometry and high temperature X-ray diffraction. Employing measurement method with enough sensitivity and resolution, accurate information on the structural phase transition behavior has been obtained. 2. Experimental BaCeO3 was prepared by solid state reaction method. Nominal amount of BaCO3 (99.9%, Furuuchi Chemistry Corp.) and CeO2 (99.9%, Furuuchi Chemistry Corp.) were mixed in ethanol with alumina mortar. The obtained powder was uniaxially pressed into cylindrical shape at 30 MPa. The pressed specimens were sintered at 1300 °C for 10 h in air. In order to confirm the crystal structure of obtained
Fig. 1. X-ray diffraction pattern of BaCeO3 at room temperature in 2θ range of 15–90° obtained using CuKα radiation. Diffraction pattern can be indexed as orthorhombic distorted perovskite structure.
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Fig. 2. DSC curves of BaCeO3 in static air at heating and cooling procedure. Heating and cooling rate is 10 °C/min. The weight of the sample and reference are about 70 mg and about 40 mg, respectively.
specimens, the powder prepared by crushing the sintered specimen was subjected to X-ray diffraction at room temperature by using Rigaku RINT-2500VPC (CuKα: 50 kV, 250 mA). Phase transition behavior of BaCeO3 was analyzed with DSC, dilatometry and high temperature X-ray diffraction measurements. DSC of powder specimen in temperature range from room temperature to 1200 °C was performed in static air with heating and cooling rate of 10 °C/min by using DSC8270 (Rigaku Co., Ltd.). The weight of sample and Al2O3 powder employed as a reference were optimized in order to obtain DSC curve with low noise and flat base line. Pt was used as a material of pan for sample and reference. Thermal expansion of sintered specimen with about 5.0 mm diameter and 10–15 mm height from room temperature to 1200 °C was measured in air with heating and cooling rate of 10 °C/min by using TMA8310 (Rigaku Co., Ltd.). Al2O3 was employed as materials for reference, push rod and sample stage. In order to analyze crystal structure of BaCeO3 at high temperature, X-ray diffraction patterns of powder specimen were measured in air at
Fig. 3. DSC curves of BaCeO3 around 260 °C. The baseline shift with no hysteresis was observed at 260 °C.
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Fig. 4. X-ray diffraction patterns of BaCeO3 at 30–280 °C in 2θ range of 15–45°. The ▼ peaks with h + k + l = odd number disappear at 280 °C.
various temperatures between 30 °C and 1200 °C by using RINT2500VPC (CuKα: 50 kV, 250 mA) with heating attachment. Pt was used as a material for sample holder. 3. Results and discussion 3.1. DSC curves of BaCeO3 between room temperature and 1200 °C
Fig. 6. X-ray diffraction patterns of BaCeO3 at 280–400 °C in 2θ range of 15–90°. Apparently, no variation on temperature was observed; however, the diffraction pattern at 360 °C is indexed as body-centered orthorhombic, while that at 400 °C as rhombohedral symmetry.
lowest noise level and the highest flatness of base line could be obtained in this study. On heating procedure, thermal anomalies were observed at 260 °C, 385 °C and 895 °C more clearly than so far reported DSC measurements possibly due to higher sensitivity and temperature resolution. By comparison with DSC curves at heating and cooling, it has been revealed that all the observed thermal anomalies are reversible. To clarify the origin of the thermal anomalies, DSC signal and variation of X-ray diffraction patterns around each temperature were minutely investigated.
Fig. 1 shows X-ray diffraction pattern in 2θ range of 15–90° of obtained specimen at room temperature. All the diffraction peaks could be indexed assuming orthorhombic symmetry with a = 8.784 Å, b = 6.220 Å and c = 6.239 Å. No peak assigned as impurity was detected, showing that single phase of BaCeO3 was successfully obtained. The peaks with h + k + l = odd as 111 and those with mixed h, k, l, such as 112, were observed, indicating primitive lattice which agreed with the space group, Pmnb (No. 62), proposed by Knight [2,3], Genet et al. [4] and Takeuchi et al. [12]. Fig. 2 shows DSC curves of BaCeO3 powder in static air with the sample and reference weight of about 70 mg and 40 mg, respectively. By using these amounts of the sample and Al2O3, DSC curve with the
Fig. 3 shows a close up of DSC curves around 260 °C. Observed at 260 °C was the baseline shift with no hysteresis, indicating the second order phase transition. ΔCp estimated from the baseline shift was about 10 J/mol K, which is one order smaller than that reported by Egorov et al. [8].
Fig. 5. DSC curves of BaCeO3 round 380 °C. The peaks originating from latent heat and hysteresis were observed around 373–383 °C.
Fig. 7. X-ray diffraction patterns of BaCeO3 at 280–400 °C in 2θ range of 87.5–89.5°. Two peaks assigned as 044 and 800 of orthorhombic symmetry were observed below 360 °C, whereas only one peak indexed as 440 of rhombohedral symmetry was detected at 400 °C.
3.2. Structural phase transition around 260 °C
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the more accurate phase transition temperature can be measured in this study than those obtained by using neutron diffraction due to small quantity of the specimens. 3.3. Structural phase transition around 380 °C
Fig. 8. DSC curves of BaCeO3 around 900 °C. The baseline shift with no hysteresis was observed at 895 °C.
Since existence of structural phase transition from orthorhombic distorted perovskite with space group of Pmnb (No. 62) to orthorhombic one with Imma (No. 74) around 290 °C has been reported by neutron diffraction [2–4], the second order phase transition at 260 °C can be assigned as this structural phase transition. For confirmation, X-ray diffraction patterns of BaCeO3 at various temperatures between 30 and 280 °C were measured. Fig. 4 shows observed X-ray diffraction patterns. All the peaks could be indexed assuming orthorhombic symmetry; however, the peaks with h + k + l = odd represented by ▼ disappeared in diffraction pattern at 280 °C, indicating that crystal system of BaCeO3 at 280 °C is body-centered orthorhombic. Peaks with h + k + l = odd remain at 250 °C showing agreement with the result of DSC depicted in Fig. 3. This showed agreement with so far reported structural phase transition behavior proposed by using neutron diffraction except for transition temperature. We regard that
Fig. 5 shows a close up of DSC curves around 380 °C. The endothermic and exothermic peaks were observed in heating and cooling procedure, respectively. The temperature difference of both peaks, i.e. hysteresis indicated that phase transition around 380 °C was the first order. ΔH calculated from peak area was about 45 J/mol. Since Knight reported that crystal system of BaCeO3 changed from Imma (No. 74) to R3̄c (No. 167) at 400 °C by neutron diffraction [2,3], it was highly possible that the first order phase transition originated from this structural phase transition. The deviation of temperature could be ascribed to large amount of specimens for neutron diffraction measurements. Fig. 6 shows X-ray diffraction patterns of BaCeO3 in temperature range of 280–400 °C. Apparently, no variation was observed except peak shift due to thermal expansion; however, variation of peak shape was observed at high 2θ angle as shown in Fig. 7. Below 360 °C, two peaks identified as 044 and 800 of bodycentered orthorhombic symmetry were observed in 2θ range of 87.5– 89.5°, whereas only one peak was detected at 400 °C. This indicates that crystal symmetry of BaCeO3 at 400 °C is higher than those below 360 °C, i.e., Imma (No. 74). The diffraction pattern at 400 °C depicted in Fig. 6 could be indexed assuming rhombohedral symmetry with a = 6.234 Å and α = 60.260°, showing agreement with DSC curves depicted in Fig. 5 and the results of neutron diffraction measurements. The first order structural phase transition from orthorhombic distorted perovskite to rhombohedral distorted perovskite has been reported also in LaCrO3 and LaGaO3 [13,14]. However, the phase transition for LaCrO3 and LaGaO3 is from orthorhombic Pnmb (No. 62) to rhombohedral R3̄c (No. 167), which is different from the first order phase transition of BaCeO3. ΔH for phase transition of LaCrO3 and LaGaO3 are 350 J/mol and 250 J/mol, respectively, which is larger than ΔH, 45 J/mol, for phase transition of BaCeO3. It is suspected that smaller ΔH of BaCeO3 can be attributed to higher crystal symmetry below the transition temperature than those of LaCrO3 and LaGaO3.
Fig. 9. (a) X-ray diffraction patterns of BaCeO3 at 400–900 °C in 2θ range of 15–90°. The diffraction pattern at 400 °C can be indexed as rhombohedral. The diffraction peak represented by ▼ disappear at 900 °C, resulting in successful identification as cubic symmetry. (b) Enlarged X-ray diffraction patterns of (a).
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Fig. 10. X-ray diffraction patterns of BaCeO3 at 900–1200 °C in 2θ range of 15–90°. All the diffraction patterns can be indexed as cubic symmetry.
Fig. 11. Temperature dependence of rhombohedral lattice angle of BaCeO3. In cubic symmetry, rhombohedral lattice angle is equal to 60°.
3.4. Structural phase transition around 895 °C Fig. 8 shows an enlargement of DSC curves around 900 °C. Reversible baseline shift with no hysteresis was observed at 895 °C, indicating second order phase transition. From baseline shift, ΔCp was evaluated to be about 7 J/mol K. Fig. 9(a) shows X-ray diffraction patterns of BaCeO3 at temperature range of 400–900 °C. Apparently, no variation on temperature was observed; however, peak represented by ▼ was not observed in the diffraction pattern at 900 °C as also shown in Fig. 9(b), which is enlargement of Fig. 9(a). Below 800 °C, the ▼ peak, this cannot be indexed as cubic symmetry but rhombohedral symmetry remains, showing agreement with second order phase transition at 895 °C observed by DSC. Due to lack of the peak, X-ray diffraction pattern at 900 °C could be indexed assuming cubic symmetry with Pm3̄m (No. 221). Fig. 10 shows X-ray diffraction patterns of BaCeO3 at 900– 1200 °C. All the peaks could be indexed assuming ideal cubic symmetry, which showed agreement with the results of neutron diffraction measurement. The phase transition can also be investigated from variation of lattice constants on temperature. Fig. 11 shows variation of rhombohedral angle, α, calculated from Bragg angles of 400 and 440 peaks of rhombohedral lattice,1 on temperature. In the cubic structure, α is equal to 60° [15]. The α approaches 60° not suddenly but gradually with increase of temperature and variation of α was not observed above 900 °C, showing correspondence with second order phase transition at 895 °C observed by DSC.
895 °C, which can be attributed to the second order phase transition from R3̄c (No. 167) to Pm3̄m (No. 221). Mean linear thermal expansion coefficients were 1.10 × 10− 5 K− 1 and 1.04 × 10− 5 K− 1 below and above 895 °C, respectively. At about 260 °C and 380 °C where phase transitions exist, influence of phase transition on thermal expansion was scarcely detected; however, slight anomalies were observed in thermal expansion coefficient at the temperatures. It is concluded that variation of thermal expansion coefficient derived from the second order phase transition at 260 °C and ΔV from the first order phase transition at 380 °C are so small that their influence on thermal expansion could not be observed. Fig. 13 shows the temperature dependence of molar volume of BaCeO3 calculated from high temperature X-ray diffraction patterns. It has been revealed that the phase transitions from Pmnb (No. 62) to Imma (No. 74) and from Imma to R3̄c (No. 167) scarcely affect the thermal expansion behavior, whereas clear variation of inclination due to structural phase transition from R3̄c (No. 167) to Pm3̄m (No. 221) was observed around 900 °C. These results showed
3.5. Effect of phase transition on thermal expansion behavior of BaCeO3 Thermal expansion behavior is important information as for application to high temperature devices such as H2 gas sensor, electrolyte of solid oxide fuel cells and so on. It has been frequently reported that the discrete variation of volume, ΔV, at first order structural phase transition and variation of thermal expansion coefficient at second order structural phase transition influence property of materials. Fig. 12 shows thermal expansion and expansion coefficient of sintered BaCeO3 ceramics in air measured with dilatometry. In thermal expansion curve, variation of slope was detected at about 1
As for diffraction patterns more than 900 °C, 222 and 400 peaks of cubic symmetry were regarded as 400 and 440 peaks of rhombohedral lattice, respectively, to calculate α.
Fig. 12. Thermal expansion behavior and temperature dependence of expansion coefficient of BaCeO3 ceramics measured with dilatometry. The variation in the thermal expansion coefficient was observed at 260 °C, 380 °C and 895 °C.
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detected in BaCeO3 and their thermodynamic character has been established with high reliability. The second order structural phase transition from primitive orthorhombic perovskite with space group of Pmnb (No. 62) to body-centered one with space group of Imma (No. 74) was detected at 260 °C. ΔCp of the phase transition was 10 J/mol K. At 385 °C, observed was the first order structural phase transition with ΔH of 45 J/mol from Imma (No. 74) to rhombohedral distorted perovskite with space group of R3c̄ (No. 167). Another second order structural phase transition with ΔCp of 7 J/mol K from R3c̄ to ideal cubic ̄ (No. 221) was observed at 895 °C. perovskite with space group of Pm3m Due to small variation of thermal expansion coefficient on the phase transition at 260 °C and small ΔV on the first order structural phase transition at 385 °C, anomalous variation was not observed at the temperatures in thermal expansion curve of BaCeO3, measured with dilatometry. Only at 895 °C, variation of thermal expansion coefficient due to the second order phase transition was detected, showing agreement with temperature dependence of molar volume evaluated by high temperature X-ray diffraction. References Fig. 13. Temperature dependence of molar volume of BaCeO3 evaluated from X-ray diffraction at high temperatures. The symbols of ○, Δ, ◊ and □ represent primitive orthorhombic perovskite, body-centered orthorhombic one, rhombohedral one and cubic one, respectively.
[1] [2] [3] [4] [5] [6]
quantitative agreement with thermal expansion observed with dilatometry. It can also be concluded that variation of thermal expansion coefficient derived from the second order phase transition at 260 °C and ΔV from the first order phase transition at 380 °C are so small that they cannot be observed by our employed X-ray diffraction. Small ΔV compared to those for LaCrO3 and LaGaO3 can be attributed to higher crystal symmetry for BaCeO3 below the first order phase transition temperature, as also was observed for ΔH at the phase transition. Mean volume thermal expansion coefficients were 3.54 × 10− 5 K− 1 and 3.16 × 10− 5 K− 1 below and above 895 °C, respectively. Assuming isotropic thermal expansion, volume thermal expansion should be as three times as linear one, which shows agreement with this study. 4. Conclusion With combination of DSC, high temperature X-ray diffraction and dilatometry, three kinds of structural phase transitions have been
[7] [8] [9] [10] [11] [12] [13] [14] [15]
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