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Hydration behavior of Ba2Sc2O5 with an oxygen-deficient perovskite structure
Solid State Ionics 177 (2006) 2447 – 2451 www.elsevier.com/locate/ssi
Hydration behavior of Ba2Sc2O5 with an oxygen-deficient perovskite structure Takahisa Omata ⁎, Tomonao Fuke, Shinya Otsuka-Yao-Matsuo Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan Received 25 June 2005; received in revised form 22 May 2006; accepted 29 May 2006
Abstract The hydration behavior of the barium scandates with oxygen-deficient perovskite-related structures of tetragonal Ba2Sc2O5 and cubic BaScO3−δ (δ ∼ 0.5) was studied. Both the tetragonal Ba2Sc2O5 and cubic BaScO3−δ absorbed water and formed Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O, respectively, during cooling in a humidified atmosphere. The hydration and the dehydration temperatures for the barium scandates were ∼ 533 K and ∼ 553 K, respectively; these temperatures were 30–70 K lower than that for Ba2In2O5. A distinct difference in the hydration behavior of the structurally different tetragonal Ba2Sc2O5 and cubic BaScO3−δ was not observed. The Ba2Sc2O5 shows protonic conduction below 673 K and p-type conduction above 773 K in a humidified atmosphere. © 2006 Elsevier B.V. All rights reserved. Keywords: Proton conductor; Barium scandate; Hydration; Thermogravimetry; Electrical conductivity
1. Introduction The discovery of protonic conduction below ∼ 673 K by Zang and Smyth [1] in 1995 stimulated the investigation of the protonic conduction of brownmillerite-type Ba2In2O5. The protonic conduction of Ba2In2O5 originates from the formation of the hydrate Ba2In2O5·H2O in a humidified atmosphere, and protons bound to oxygen in the hydrate crosses between oxygen [2–4]. The proton conductivity of the hydrate Ba2In2O5·H2O was less than 10− 4 S cm− 1 at ∼ 573 K, and it decomposed into dry Ba2In2O5 and H2O above 573 K [1]. For practical applications, such as fuel cell electrolytes, a proton conductivity higher than ∼10− 2 S cm− 1 is required, therefore, the hydrate Ba2In2O5·H2O cannot be utilized due to its low proton conductivity. Recently, we attempted to improve the proton conductivity of Ba2In2O5·H2O by stabilizing the hydrate compound at high temperature [5]. The phase transformation temperature between the dry Ba2In2O5 and the hydrate Ba2In2O5·H2O was success-
⁎ Corresponding author. E-mail address: [email protected] (T. Omata). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.05.044
fully increased by ∼ 50 K due to the substitution of In by Sc, Lu and Y. However, the increase in thermal stability of ∼ 50 K is not sufficient for improving the conductivity beyond 10− 2 S cm− 1. The substitution of In by MIII was not possible for x > 0.3 in Ba2(In1−xMxIII)2O5·H2O, therefore, a further increase in the thermal stability of the hydrate compound by substitution of In by Sc, Lu and Y cannot be expected. The compound corresponding to the terminal composition with x = 1 in Ba2 (In1−xScx)2O5, i.e., Ba2Sc2O5 was reported by Kwestroo et al. [6]. The structure is reported to be an oxygen-deficient perovskite-related type, such as the brownmillerite-type Ba2In2O5. In the present study, we have investigated the hydration temperature and the amount of water absorbed for two types of barium scandates, i.e., tetragonal Ba2Sc2O5 and cubic BaScO3−δ. The electrical conductivity of the sintered tetragonal Ba2Sc2O5 was studied. 2. Experimental Polycrystalline samples with a nominal composition of Ba2Sc2O5 were prepared via the following two synthesis routes. One was a solid-state reaction of the powders of the raw materials. The other was the thermal decomposition of the
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complex hydroxide of Ba3Sc2(OH)12 [7], which was prepared from Ba(OH)2·8H2O and Sc2O3 under hydrothermal conditions. For the former case, powdered barium and scandium oxalates, i.e., BaC2O4 and Sc2(C2O4)3, were weighed. Because the barium compound evaporates during sintering, excess barium oxalate was added to the starting powder, i.e., the molar ratio of Ba to Sc, XBa/XSc, was 1.3. The precursors were mixed using a mortar made of partially-stabilized zirconia. In order to ensure a thorough mixing of the powders, ethanol was added as a mixing medium. The powders were remixed using a planetary ball-mill; the pot and balls were made of partially stabilized zirconia. The dried powder was pressed into 17.2-mm diameter disks at 256 MPa. The disks were subsequently sintered at 1248 K for 20 h in air. For the thermal decomposition of the complex hydroxide, the Ba3Sc2(OH)12 was prepared using the following procedure. The raw materials of Ba (OH)2·8H2O (4.12 g) and Sc2O3 (0.30 g) were weighed in the indicated amount. The materials were sealed in a Teflon-lined stainless steel autoclave (50 mL) together with 25 mL of a 3 M Na(OH) aqueous solution and heated to 413 K for 20 h under autogenous pressure. The resulting solid product, Ba3Sc2 (OH)12, was recovered by filtration under an argon atmosphere in order to avoid formation of BaCO3 from the remaining Ba (OH)2 in the solution and CO2 in the ambient atmosphere. The obtained Ba3Sc2(OH)12 powder was pressed into 17.2-mm diameter disks at 256 MPa. The disks were subsequently sintered at 1073 K for 20 h in air. The obtained crystalline phases were identified using powder X-ray diffraction (XRD) (RIGAKU, RINT2500HF, Cu-Kα radiation using a curved graphite receiving monochromator). The chemical compositions of the products were determined by ICP emission spectroscopy. The two barium scandates were subjected to thermogravimetry (TG) (Seiko Instruments TG/DTA320) under dry and wet O2 gas flows in order to study their hydration behavior. A TG analysis was conducted as follows. First, the sample powder was heated to 1223 K and held there for 9 h under a dry O2 atmosphere in order to degas the powder. Subsequently, the powder was slowly cooled (1 K min− 1) to room temperature under a wet O2 atmosphere (run 1). After run 1, the sample was heated (5 K min− 1) to 1223 K under a dry O2 atmosphere, and the powder was then slowly cooled again to room temperature under a wet O2 atmosphere (run 2). The final run 3 consisted of heating to 1223 K and then slowly cooling to room temperature under a wet O2 atmosphere. The wet O2 gas was prepared by humidifying with water at room temperature; the partial pressure of water, P(H2O), in the wet O2 gas was 1.6– 2.6 × 103 Pa in the present study [8]. The crystalline phases after the TG experiments were characterized by powder XRD. The sintered Ba2Sc2O5 samples for the electrical conductivity measurements were prepared by spark-plasma-sintering (SPS) at 1123 K for 10 min at a pressure of 40 MPa under a vacuum of 10− 1 Pa. The electrical conductivity was measured in the temperature range between 523 and 1073 K under a dry or wet O2 atmosphere using an HP4192A impedance analyzer. The wet O2 with a partial pressure of 3 × 103 Pa water, P(H2O), was prepared by saturating the water vapor at room temperature.
3. Results 3.1. Sample preparation Fig. 1(a) shows the XRD profiles of samples prepared by the solid-state reaction between barium and scandium oxalate. The diffraction profile showed that the obtained sample was the tetragonal perovskite-related phase containing a trace of Sc2O3. One can see the small, but clear diffractions at 2θ ∼ 7.5° and ∼15° in the figure. All the diffractions, including the two small diffractions, were indexed as those for the tetragonal crystal with lattice parameters a = 0.415 nm and c = 2.405 nm. Although Kwestroo et al. reported that the unit cell volume of tetragonal Ba2Sc2O5 was approximately the same as that of the cubic perovskite structure [6], i.e., both the lattice parameters a and c were approximately the same as ap (ap denotes the lattice parameter for the cubic perovskite structure) for the well known tetragonal BaTiO3 structure [9,10]. However, the tetragonal Ba2Sc2O5 obtained in the present study possesses a six times larger unit cell volume than the cubic perovskite structure, and its lattice parameters a and c are approximately the same as ap and 6ap, respectively. Fig. 1(b) shows the XRD profiles of the sample obtained by the decomposition of Ba3Sc2(OH)12 at 1073 K for 20 h in air. All the diffraction lines were indexed as those for a cubic perovskite structure with a lattice parameter of a = 0.413 nm. Therefore, it was concluded that the cubic perovskite-type BaScO3−δ (δ ∼ 0.5) was formed by the decomposition of Ba3Sc2 (OH)12 at 1073 K. According to the ICP analysis, the atomic
Fig. 1. X-ray diffraction profiles of (a) the tetragonal Ba2Sc2O5 and (b) the cubic BaScO3−δ samples. The tetragonal Ba2Sc2O5 sample was prepared by the solid state reaction of barium and scandium oxalates. The cubic BaScO3−δ sample was prepared by the annealing of the complex hydroxide of Ba3Sc2(OH)12.
T. Omata et al. / Solid State Ionics 177 (2006) 2447–2451
ratio of barium and scandium was XBa/XSc = 1.18 for both the precursor Ba3Sc2(OH)12 and the product BaScO3−δ. This analytical composition implied that the cubic BaScO3−δ contained not only oxide-ion vacancies, but also cationic defects such as scandium-ion vacancies, because XBa > XSc.
Table 1 DTG peak temperatures during cooling and heating corresponding to the hydration and dehydration temperatures, respectively, for Ba2Sc2O5 and BaScO3−δ (δ ∼ 0.5) Sample
DTG peak temperature (K) Cooling (hydration)
3.2. Hydration of Ba2Sc2O5 and BaScO3−δ Fig. 2 shows the TG results for the tetragonal Ba2Sc2O5 and cubic BaScO3−δ as starting samples. Fig. 2(a) shows the temperature–time pattern. Fig. 2(b) and (d) show the TG curves and (c) and (e) show their differential (DTG) curves for Ba2Sc2O5 and BaScO3−δ, respectively. In run 1 of the tetragonal Ba2Sc2O5 (Fig. 2(b)), the mass decreased during the first heating and holding at 1223 K under a dry atmosphere. When cooling under a wet atmosphere was started, the mass gradually increased, i.e., a small but abrupt mass increase occurred at ∼ 540 K. In run 2, a mass variation similar to run 1 was observed. In the final run 3 in a wet atmosphere, the mass decrease during heating was lower than that observed in runs 1 and 2 under a dry atmosphere. This mass variation was qualitatively similar to that of the hydration of Ba2In2O5 and the dehydration of Ba2In2O5·H2O as previously reported [2,5]. Consequently, the mass variation observed in Fig. 2(b) was attributed to the hydration and dehydration expressing the reaction Ba2Sc2O5 + nH2O⇔Ba2Sc2O5·nH2O. The observed
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Ba2Sc2O5 BaScO3−δ Ba2In2O5 Ba2(In0.8Sc0.2)2O5
Heating (dehydration)
Run 1
Run 2
Run 3
Run 2
Run 3
527 538 566 601
528 538 557 602
525 535 561 595
548 553 619 641
570 575 639 660
The temperatures for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5 were the DTA peak temperatures reported in Ref. [5].
total mass change was calculated to involve 1 mol of Ba2Sc2O5 that absorbed 0.60 mol of H2O. The chemical formula of the hydrate was Ba2Sc2O5·0.60H2O. The hydration and dehydration temperatures were evaluated from the DTG peak as observed in Fig. 2(c). These values are summarized in Table 1 together with those for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5 [5]. The hydration and dehydration temperatures were lower than those for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5. For the cubic BaScO3−δ (Fig. 2(d)), the hydration behavior, i.e., the mass variation was similar to the case for the tetragonal Ba2Sc2O5. The hydration and dehydration temperatures in Table 1 were slightly higher than those for the tetragonal Ba2Sc2O5. However, a distinct difference in the hydration behavior due to the structural difference was not observed. The chemical formula after hydration was calculated to be BaScO3−δ·0.37H2O. The amount of absorbed water was slightly higher than that for the tetragonal Ba2Sc2O5. 3.3. Protonic conduction of Ba2Sc2O5
Fig. 2. TG and differential-TG (DTG) curves for the tetragonal Ba2Sc2O5 and the cubic BaScO3−δ exposed to dry and wet O2 atmospheres. (a) Temperature– time pattern, (b) TG curve for the tetragonal Ba2Sc2O5, (c) the first derivative curve of (b) (DTG), (d) TG curve for the cubic BaScO3−δ and (e) DTG for (d).
Fig. 3 shows the Arrhenius plot of the electrical conductivity of the sintered tetragonal Ba2Sc2O5. The conductivity difference between being in a wet atmosphere and a dry atmosphere was very small, but a distinct difference was observed after careful observation. For instance, the conductivity below 673 K in a wet atmosphere, under which the dry Ba2Sc2O5 transforms into the hydrated Ba2Sc2O5·nH2O, was slightly higher than that in the dry atmosphere, e.g., the conductivity at 573 K in a wet atmosphere was 1.2 times higher than that in a dry atmosphere. This shows that the conductivity of the hydrated Ba2Sc2O5· nH2O is higher than that of the dry Ba2Sc2O5. The higher conductivity in a wet atmosphere than that in a dry atmosphere suggests a protonic conduction based on the analogy of the conducting behavior of Ba2In2O5. However, it is implied that the transference number of protons, which is evaluated by tH = σH/σwet = (σwet − σdry)/σwet, is as low as 0.16 at 573 K. Above 773 K, the conductivity in a wet atmosphere was lower than that in a dry atmosphere. Based on the TG results, the material is dry Ba2Sc2O5 under both the wet and dry atmosphere. The oxygen partial pressure , P(O2) = P(total) − P (H2O), for the wet atmosphere of 0.983 × 105 Pa is slightly lower than that for the dry atmosphere, i.e., 1.013 × 105 Pa. The
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However, for the present Ba2Sc2O5 and BaScO3−δ compositions, corresponding to the case of x = 1 in Ba2(In1−xScx)2O5, the hydration temperatures evaluated from the DTG-peaks were 30– 70 K lower than that for Ba2In2O5 as summarized in Table 1. The oxygen-deficient sites in the brownmillerite-type Ba2(In1−x Scx)2O5 represent an ordered arrangement, while they are disordered in Ba2Sc2O5 and BaScO3−δ. These structural differences may be the origin of the lower hydration temperature of Ba2Sc2O5 and BaScO3−δ compared to that of Ba2In2O5. 5. Conclusion The hydration behavior and the electrical conductivity were studied for two types of barium scandates with oxygen deficient perovskite-related structures, i.e., the tetragonal Ba2Sc2O5 with lattice parameters of a ∼ ap and c ∼ 6ap and the cubic BaScO3−δ (δ ∼ 0.5) with a ∼ ap samples. The obtained results are summarized as follows:
Fig. 3. Electrical conductivity of the tetragonal Ba2Sc2O5 under dry (○) and wet ( ) O2 atmospheres as a function of the reciprocal temperature.
◇
decrease in conductivity for the wet atmosphere must be induced by a decrease in P(O2). It is reported that p-type conduction appears at high temperatures, at which protons desorb from the crystal, for the high temperature proton conductors, such as Ca(Zr,In)O3 and (La,Ca)2Zr2O7 [11,12], and the p-type conductivity increases with the increasing P(O2) [13]. Therefore, the present results are consistent by assuming that the Ba2Sc2O5 shows a p-type conduction above 773 K. 4. Discussion It was shown that both tetragonal Ba2Sc2O5 and cubic BaScO3−δ (δ ∼ 0.5) absorb water and that the hydrated compounds of Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O are formed during cooling under a wet atmosphere. The mass increase and decrease during hydration and dehydration, respectively, were gradually in contrast to the cases of Ba2In2O5 [2,5] and Ba2Al2O5 [14]. The XRD patterns of the Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O hydrates were identical to those of the dry Ba2Sc2O5 and BaScO3−δ, respectively. No structural transformation was observed during hydration. According to previous papers [2,5,14], for the Ba2In2O5 and Ba2Al2O5, a distinct structural transformation accompanies the hydration. Abrupt mass variations for Ba2In2O5 and Ba2Al2O5 are reported around the transformation temperature. In the present cases for Ba2Sc2O5 and BaScO3−δ the mass variation might become gradual, because no distinct structural transformation accompanied the hydration. For the brownmillerite-type Ba2(In1−xScx)2O5, the hydration temperature increased with the increasing Sc-content, x [5].
(1) Tetragonal Ba2Sc2O5 was formed by a solid-state reaction, and cubic BaScO3−δ was formed by the decomposition of the complex hydroxide of Ba3Sc2 (OH)12. (2) Both tetragonal Ba2Sc2O5 and cubic BaScO3−δ absorbed water, and hydrated compounds of Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O were formed during cooling in a wet atmosphere. The mass increase during hydration and mass decrease during dehydration were gradual, because the hydration and the dehydration occurred without distinct structural transformations. (3) The hydration and the dehydration temperatures observed for Ba2Sc2O5 and BaScO3−δ were 30–70 K lower than that for Ba2In2O5 and 70–100 K lower for that of Ba2 (In0.8Sc0.2)2O5. (4) Ba2Sc2O5 exhibits a protonic conduction with a small protonic transference number below 673 K in a wet atmosphere. Above 773 K, a p-type conduction was suggested. Acknowledgements The authors would like to thank Dr. Masao Kita for his experimental assistance and Dr. Shigeru Katsuyama for assistance with the spark plasma sintering. References [1] [2] [3] [4] [5]
G.B. Zhang, D.M. Smyth, Solid State Ion. 82 (1995) 153. T. Schober, J. Friedrich, F. Krug, Solid State Ion. 99 (1997) 9. T. Schober, J. Friedrich, Solid State Ionics 113–115 (1998) 369. W. Fischer, G. Reckand, T. Schober, Solid State Ion. 116 (1999) 211. T. Omata, M. Kita, Y. Goto, T. Okura, S. Otsuka-Yao-Matsuo, J. Electrochem. Soc. 152 (2005) A1068. [6] W. Kwestroo, H.A.M. van Hal, C. Langereis, Mater. Res. Bull. 9 (1974) 1623. [7] W. Kwestroo, H.C.A. van Gerven, H.A.M. van Hal, Mater. Res. Bull. 12 (1977) 161. [8] O. Kubacshewski, C.B. Alcock, Metallurgical Thermochemistry, 5th ed., Pergamon, Oxford, 1979, p. 362.
T. Omata et al. / Solid State Ionics 177 (2006) 2447–2451 [9] R.L. Needs, M.T. Weller, J. Solid State Chem. 139 (1998) 422. [10] R.H. Buttner, E.N. Maslen, Acta Crystallogr., B 48 (1992) 764. [11] N. Kurita, N. Fukatsu, K. Ito, T. Ohashi, J. Electrochem. Soc. 142 (1995) 1552.
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[12] T. Omata, S. Otsuka-Yao-Matsuo, J. Electrochem. Soc. 148 (2001) E252. [13] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed. John Wiley & Sons, New York, 1976. [14] A.H. Ahmed, L.S.D. Glasser, J. Chem. Technol. Biotechnol. 42 (1988) 31.
Hydration behavior of Ba2Sc2O5 with an oxygen-deficient perovskite structure Takahisa Omata ⁎, Tomonao Fuke, Shinya Otsuka-Yao-Matsuo Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan Received 25 June 2005; received in revised form 22 May 2006; accepted 29 May 2006
Abstract The hydration behavior of the barium scandates with oxygen-deficient perovskite-related structures of tetragonal Ba2Sc2O5 and cubic BaScO3−δ (δ ∼ 0.5) was studied. Both the tetragonal Ba2Sc2O5 and cubic BaScO3−δ absorbed water and formed Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O, respectively, during cooling in a humidified atmosphere. The hydration and the dehydration temperatures for the barium scandates were ∼ 533 K and ∼ 553 K, respectively; these temperatures were 30–70 K lower than that for Ba2In2O5. A distinct difference in the hydration behavior of the structurally different tetragonal Ba2Sc2O5 and cubic BaScO3−δ was not observed. The Ba2Sc2O5 shows protonic conduction below 673 K and p-type conduction above 773 K in a humidified atmosphere. © 2006 Elsevier B.V. All rights reserved. Keywords: Proton conductor; Barium scandate; Hydration; Thermogravimetry; Electrical conductivity
1. Introduction The discovery of protonic conduction below ∼ 673 K by Zang and Smyth [1] in 1995 stimulated the investigation of the protonic conduction of brownmillerite-type Ba2In2O5. The protonic conduction of Ba2In2O5 originates from the formation of the hydrate Ba2In2O5·H2O in a humidified atmosphere, and protons bound to oxygen in the hydrate crosses between oxygen [2–4]. The proton conductivity of the hydrate Ba2In2O5·H2O was less than 10− 4 S cm− 1 at ∼ 573 K, and it decomposed into dry Ba2In2O5 and H2O above 573 K [1]. For practical applications, such as fuel cell electrolytes, a proton conductivity higher than ∼10− 2 S cm− 1 is required, therefore, the hydrate Ba2In2O5·H2O cannot be utilized due to its low proton conductivity. Recently, we attempted to improve the proton conductivity of Ba2In2O5·H2O by stabilizing the hydrate compound at high temperature [5]. The phase transformation temperature between the dry Ba2In2O5 and the hydrate Ba2In2O5·H2O was success-
⁎ Corresponding author. E-mail address: [email protected] (T. Omata). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.05.044
fully increased by ∼ 50 K due to the substitution of In by Sc, Lu and Y. However, the increase in thermal stability of ∼ 50 K is not sufficient for improving the conductivity beyond 10− 2 S cm− 1. The substitution of In by MIII was not possible for x > 0.3 in Ba2(In1−xMxIII)2O5·H2O, therefore, a further increase in the thermal stability of the hydrate compound by substitution of In by Sc, Lu and Y cannot be expected. The compound corresponding to the terminal composition with x = 1 in Ba2 (In1−xScx)2O5, i.e., Ba2Sc2O5 was reported by Kwestroo et al. [6]. The structure is reported to be an oxygen-deficient perovskite-related type, such as the brownmillerite-type Ba2In2O5. In the present study, we have investigated the hydration temperature and the amount of water absorbed for two types of barium scandates, i.e., tetragonal Ba2Sc2O5 and cubic BaScO3−δ. The electrical conductivity of the sintered tetragonal Ba2Sc2O5 was studied. 2. Experimental Polycrystalline samples with a nominal composition of Ba2Sc2O5 were prepared via the following two synthesis routes. One was a solid-state reaction of the powders of the raw materials. The other was the thermal decomposition of the
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complex hydroxide of Ba3Sc2(OH)12 [7], which was prepared from Ba(OH)2·8H2O and Sc2O3 under hydrothermal conditions. For the former case, powdered barium and scandium oxalates, i.e., BaC2O4 and Sc2(C2O4)3, were weighed. Because the barium compound evaporates during sintering, excess barium oxalate was added to the starting powder, i.e., the molar ratio of Ba to Sc, XBa/XSc, was 1.3. The precursors were mixed using a mortar made of partially-stabilized zirconia. In order to ensure a thorough mixing of the powders, ethanol was added as a mixing medium. The powders were remixed using a planetary ball-mill; the pot and balls were made of partially stabilized zirconia. The dried powder was pressed into 17.2-mm diameter disks at 256 MPa. The disks were subsequently sintered at 1248 K for 20 h in air. For the thermal decomposition of the complex hydroxide, the Ba3Sc2(OH)12 was prepared using the following procedure. The raw materials of Ba (OH)2·8H2O (4.12 g) and Sc2O3 (0.30 g) were weighed in the indicated amount. The materials were sealed in a Teflon-lined stainless steel autoclave (50 mL) together with 25 mL of a 3 M Na(OH) aqueous solution and heated to 413 K for 20 h under autogenous pressure. The resulting solid product, Ba3Sc2 (OH)12, was recovered by filtration under an argon atmosphere in order to avoid formation of BaCO3 from the remaining Ba (OH)2 in the solution and CO2 in the ambient atmosphere. The obtained Ba3Sc2(OH)12 powder was pressed into 17.2-mm diameter disks at 256 MPa. The disks were subsequently sintered at 1073 K for 20 h in air. The obtained crystalline phases were identified using powder X-ray diffraction (XRD) (RIGAKU, RINT2500HF, Cu-Kα radiation using a curved graphite receiving monochromator). The chemical compositions of the products were determined by ICP emission spectroscopy. The two barium scandates were subjected to thermogravimetry (TG) (Seiko Instruments TG/DTA320) under dry and wet O2 gas flows in order to study their hydration behavior. A TG analysis was conducted as follows. First, the sample powder was heated to 1223 K and held there for 9 h under a dry O2 atmosphere in order to degas the powder. Subsequently, the powder was slowly cooled (1 K min− 1) to room temperature under a wet O2 atmosphere (run 1). After run 1, the sample was heated (5 K min− 1) to 1223 K under a dry O2 atmosphere, and the powder was then slowly cooled again to room temperature under a wet O2 atmosphere (run 2). The final run 3 consisted of heating to 1223 K and then slowly cooling to room temperature under a wet O2 atmosphere. The wet O2 gas was prepared by humidifying with water at room temperature; the partial pressure of water, P(H2O), in the wet O2 gas was 1.6– 2.6 × 103 Pa in the present study [8]. The crystalline phases after the TG experiments were characterized by powder XRD. The sintered Ba2Sc2O5 samples for the electrical conductivity measurements were prepared by spark-plasma-sintering (SPS) at 1123 K for 10 min at a pressure of 40 MPa under a vacuum of 10− 1 Pa. The electrical conductivity was measured in the temperature range between 523 and 1073 K under a dry or wet O2 atmosphere using an HP4192A impedance analyzer. The wet O2 with a partial pressure of 3 × 103 Pa water, P(H2O), was prepared by saturating the water vapor at room temperature.
3. Results 3.1. Sample preparation Fig. 1(a) shows the XRD profiles of samples prepared by the solid-state reaction between barium and scandium oxalate. The diffraction profile showed that the obtained sample was the tetragonal perovskite-related phase containing a trace of Sc2O3. One can see the small, but clear diffractions at 2θ ∼ 7.5° and ∼15° in the figure. All the diffractions, including the two small diffractions, were indexed as those for the tetragonal crystal with lattice parameters a = 0.415 nm and c = 2.405 nm. Although Kwestroo et al. reported that the unit cell volume of tetragonal Ba2Sc2O5 was approximately the same as that of the cubic perovskite structure [6], i.e., both the lattice parameters a and c were approximately the same as ap (ap denotes the lattice parameter for the cubic perovskite structure) for the well known tetragonal BaTiO3 structure [9,10]. However, the tetragonal Ba2Sc2O5 obtained in the present study possesses a six times larger unit cell volume than the cubic perovskite structure, and its lattice parameters a and c are approximately the same as ap and 6ap, respectively. Fig. 1(b) shows the XRD profiles of the sample obtained by the decomposition of Ba3Sc2(OH)12 at 1073 K for 20 h in air. All the diffraction lines were indexed as those for a cubic perovskite structure with a lattice parameter of a = 0.413 nm. Therefore, it was concluded that the cubic perovskite-type BaScO3−δ (δ ∼ 0.5) was formed by the decomposition of Ba3Sc2 (OH)12 at 1073 K. According to the ICP analysis, the atomic
Fig. 1. X-ray diffraction profiles of (a) the tetragonal Ba2Sc2O5 and (b) the cubic BaScO3−δ samples. The tetragonal Ba2Sc2O5 sample was prepared by the solid state reaction of barium and scandium oxalates. The cubic BaScO3−δ sample was prepared by the annealing of the complex hydroxide of Ba3Sc2(OH)12.
T. Omata et al. / Solid State Ionics 177 (2006) 2447–2451
ratio of barium and scandium was XBa/XSc = 1.18 for both the precursor Ba3Sc2(OH)12 and the product BaScO3−δ. This analytical composition implied that the cubic BaScO3−δ contained not only oxide-ion vacancies, but also cationic defects such as scandium-ion vacancies, because XBa > XSc.
Table 1 DTG peak temperatures during cooling and heating corresponding to the hydration and dehydration temperatures, respectively, for Ba2Sc2O5 and BaScO3−δ (δ ∼ 0.5) Sample
DTG peak temperature (K) Cooling (hydration)
3.2. Hydration of Ba2Sc2O5 and BaScO3−δ Fig. 2 shows the TG results for the tetragonal Ba2Sc2O5 and cubic BaScO3−δ as starting samples. Fig. 2(a) shows the temperature–time pattern. Fig. 2(b) and (d) show the TG curves and (c) and (e) show their differential (DTG) curves for Ba2Sc2O5 and BaScO3−δ, respectively. In run 1 of the tetragonal Ba2Sc2O5 (Fig. 2(b)), the mass decreased during the first heating and holding at 1223 K under a dry atmosphere. When cooling under a wet atmosphere was started, the mass gradually increased, i.e., a small but abrupt mass increase occurred at ∼ 540 K. In run 2, a mass variation similar to run 1 was observed. In the final run 3 in a wet atmosphere, the mass decrease during heating was lower than that observed in runs 1 and 2 under a dry atmosphere. This mass variation was qualitatively similar to that of the hydration of Ba2In2O5 and the dehydration of Ba2In2O5·H2O as previously reported [2,5]. Consequently, the mass variation observed in Fig. 2(b) was attributed to the hydration and dehydration expressing the reaction Ba2Sc2O5 + nH2O⇔Ba2Sc2O5·nH2O. The observed
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Ba2Sc2O5 BaScO3−δ Ba2In2O5 Ba2(In0.8Sc0.2)2O5
Heating (dehydration)
Run 1
Run 2
Run 3
Run 2
Run 3
527 538 566 601
528 538 557 602
525 535 561 595
548 553 619 641
570 575 639 660
The temperatures for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5 were the DTA peak temperatures reported in Ref. [5].
total mass change was calculated to involve 1 mol of Ba2Sc2O5 that absorbed 0.60 mol of H2O. The chemical formula of the hydrate was Ba2Sc2O5·0.60H2O. The hydration and dehydration temperatures were evaluated from the DTG peak as observed in Fig. 2(c). These values are summarized in Table 1 together with those for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5 [5]. The hydration and dehydration temperatures were lower than those for Ba2In2O5 and Ba2(In0.8Sc0.2)2O5. For the cubic BaScO3−δ (Fig. 2(d)), the hydration behavior, i.e., the mass variation was similar to the case for the tetragonal Ba2Sc2O5. The hydration and dehydration temperatures in Table 1 were slightly higher than those for the tetragonal Ba2Sc2O5. However, a distinct difference in the hydration behavior due to the structural difference was not observed. The chemical formula after hydration was calculated to be BaScO3−δ·0.37H2O. The amount of absorbed water was slightly higher than that for the tetragonal Ba2Sc2O5. 3.3. Protonic conduction of Ba2Sc2O5
Fig. 2. TG and differential-TG (DTG) curves for the tetragonal Ba2Sc2O5 and the cubic BaScO3−δ exposed to dry and wet O2 atmospheres. (a) Temperature– time pattern, (b) TG curve for the tetragonal Ba2Sc2O5, (c) the first derivative curve of (b) (DTG), (d) TG curve for the cubic BaScO3−δ and (e) DTG for (d).
Fig. 3 shows the Arrhenius plot of the electrical conductivity of the sintered tetragonal Ba2Sc2O5. The conductivity difference between being in a wet atmosphere and a dry atmosphere was very small, but a distinct difference was observed after careful observation. For instance, the conductivity below 673 K in a wet atmosphere, under which the dry Ba2Sc2O5 transforms into the hydrated Ba2Sc2O5·nH2O, was slightly higher than that in the dry atmosphere, e.g., the conductivity at 573 K in a wet atmosphere was 1.2 times higher than that in a dry atmosphere. This shows that the conductivity of the hydrated Ba2Sc2O5· nH2O is higher than that of the dry Ba2Sc2O5. The higher conductivity in a wet atmosphere than that in a dry atmosphere suggests a protonic conduction based on the analogy of the conducting behavior of Ba2In2O5. However, it is implied that the transference number of protons, which is evaluated by tH = σH/σwet = (σwet − σdry)/σwet, is as low as 0.16 at 573 K. Above 773 K, the conductivity in a wet atmosphere was lower than that in a dry atmosphere. Based on the TG results, the material is dry Ba2Sc2O5 under both the wet and dry atmosphere. The oxygen partial pressure , P(O2) = P(total) − P (H2O), for the wet atmosphere of 0.983 × 105 Pa is slightly lower than that for the dry atmosphere, i.e., 1.013 × 105 Pa. The
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However, for the present Ba2Sc2O5 and BaScO3−δ compositions, corresponding to the case of x = 1 in Ba2(In1−xScx)2O5, the hydration temperatures evaluated from the DTG-peaks were 30– 70 K lower than that for Ba2In2O5 as summarized in Table 1. The oxygen-deficient sites in the brownmillerite-type Ba2(In1−x Scx)2O5 represent an ordered arrangement, while they are disordered in Ba2Sc2O5 and BaScO3−δ. These structural differences may be the origin of the lower hydration temperature of Ba2Sc2O5 and BaScO3−δ compared to that of Ba2In2O5. 5. Conclusion The hydration behavior and the electrical conductivity were studied for two types of barium scandates with oxygen deficient perovskite-related structures, i.e., the tetragonal Ba2Sc2O5 with lattice parameters of a ∼ ap and c ∼ 6ap and the cubic BaScO3−δ (δ ∼ 0.5) with a ∼ ap samples. The obtained results are summarized as follows:
Fig. 3. Electrical conductivity of the tetragonal Ba2Sc2O5 under dry (○) and wet ( ) O2 atmospheres as a function of the reciprocal temperature.
◇
decrease in conductivity for the wet atmosphere must be induced by a decrease in P(O2). It is reported that p-type conduction appears at high temperatures, at which protons desorb from the crystal, for the high temperature proton conductors, such as Ca(Zr,In)O3 and (La,Ca)2Zr2O7 [11,12], and the p-type conductivity increases with the increasing P(O2) [13]. Therefore, the present results are consistent by assuming that the Ba2Sc2O5 shows a p-type conduction above 773 K. 4. Discussion It was shown that both tetragonal Ba2Sc2O5 and cubic BaScO3−δ (δ ∼ 0.5) absorb water and that the hydrated compounds of Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O are formed during cooling under a wet atmosphere. The mass increase and decrease during hydration and dehydration, respectively, were gradually in contrast to the cases of Ba2In2O5 [2,5] and Ba2Al2O5 [14]. The XRD patterns of the Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O hydrates were identical to those of the dry Ba2Sc2O5 and BaScO3−δ, respectively. No structural transformation was observed during hydration. According to previous papers [2,5,14], for the Ba2In2O5 and Ba2Al2O5, a distinct structural transformation accompanies the hydration. Abrupt mass variations for Ba2In2O5 and Ba2Al2O5 are reported around the transformation temperature. In the present cases for Ba2Sc2O5 and BaScO3−δ the mass variation might become gradual, because no distinct structural transformation accompanied the hydration. For the brownmillerite-type Ba2(In1−xScx)2O5, the hydration temperature increased with the increasing Sc-content, x [5].
(1) Tetragonal Ba2Sc2O5 was formed by a solid-state reaction, and cubic BaScO3−δ was formed by the decomposition of the complex hydroxide of Ba3Sc2 (OH)12. (2) Both tetragonal Ba2Sc2O5 and cubic BaScO3−δ absorbed water, and hydrated compounds of Ba2Sc2O5·0.60H2O and BaScO3−δ·0.37H2O were formed during cooling in a wet atmosphere. The mass increase during hydration and mass decrease during dehydration were gradual, because the hydration and the dehydration occurred without distinct structural transformations. (3) The hydration and the dehydration temperatures observed for Ba2Sc2O5 and BaScO3−δ were 30–70 K lower than that for Ba2In2O5 and 70–100 K lower for that of Ba2 (In0.8Sc0.2)2O5. (4) Ba2Sc2O5 exhibits a protonic conduction with a small protonic transference number below 673 K in a wet atmosphere. Above 773 K, a p-type conduction was suggested. Acknowledgements The authors would like to thank Dr. Masao Kita for his experimental assistance and Dr. Shigeru Katsuyama for assistance with the spark plasma sintering. References [1] [2] [3] [4] [5]
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[12] T. Omata, S. Otsuka-Yao-Matsuo, J. Electrochem. Soc. 148 (2001) E252. [13] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed. John Wiley & Sons, New York, 1976. [14] A.H. Ahmed, L.S.D. Glasser, J. Chem. Technol. Biotechnol. 42 (1988) 31.