Basicity and hydroxyl capacity of proton-conducting perovskites

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Solid State Ionics 16 17 (000) 11 15 www.elsevier.

and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan b Faculty of Science, Science University of Tokyo, Kagurasaka 1-, Shinjuku, Tokyo 16-085, Japan c Institute of

1 Solid State Ionics (000) locate/ ssi Basicity and hydroxyl capacity of proton-conducting perovskites Shu Yamaguchi *, Kenji Nakamura, Toru Higuchi, Shik Shin, Yoshiaki Iguchi a, a b c a a Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya , Japan b Faculty of Science, Science University of Tokyo, Kagurasaka 1-, Shinjuku, Tokyo , Japan c Institute of Solid State Physics, University of Tokyo, Tanashi , Japan Abstract In order to understand the large solubility of protons in perovskite proton conductors, a new approach employing the??? 1 1/ hydroxyl capacity, termed ; [, is proposed and examined for 5 mol% Yb1.5-doped Bae. Prior to the solubility experiments, the phase relation in an isothermal section of the phase diagram for Ba e Yb1.5 was determined and activity measurements for Ba were carried out. The values of were evaluated by measuring the solubility of water using the thermogravimetric method under a controlled Ba activity by employing the reaction 1 (g) 5 Ba. did not exhibit a strong dependency on the Ba activity, indicating that the basicity of Bae, defined as the activity of ions, is almost constant across the homogeneity region. It has been estimated that the A-site vacancy in AB mainly contributes to the steep change in Ba activity. 000 Elsevier Science B.V. All rights reserved. Keywords: Proton; Perovskite; Basicity; apacity; Solubility; Activity; Thermodynamics 1. Introduction to the basicity of the oxide at a given composition, a strong correlation between the solubility of water or ne of the most important characteristics of protons and basicity is expected. No discussion, proton-conducting oxides is the extent of solubility however, has been given on the relation between of the protons. The dissolution reaction of protons basicity and the solubility of protons, since the has been well described by defect chemical reactions basicity of complex oxides has never been taken into in earlier investigations. n the other hand, the account in the defect chemical treatment. It is also activity of the basic oxide (A) in an AB perov- known that the solubility of protons varies with the skite oxide is expected to show a steep change across concentration of dopant cations in perovskite-type its narrow homogeneous range. Because of the oxides. However, the relation between the dopant strong affinity of A for H, one can expect concentration and the solubility has never been enhanced solubility in the A-rich composition. As discussed in terms of the basicity. In the present the thermochemical activity of A is closely related study, a new approach employing the hydroxyl capacity, proposed by Wagner [1], is applied to the *orresponding author. Fax: defect chemistry in order to discuss the solubility of address: yamagchi@mse.nitech.ac.jp (S. Yamaguchi). protons in perovskite-type oxides / 00/ $ see front matter 000 Elsevier Science B.V. All rights reserved. PII: S (00)0005-

2 1 S. Yamaguchi et al. / Solid State Ionics (000) Theoretical background (a) acidic dissolution: 5 H 1 (A) The basicity of a complex oxide system is defined KA5 [H ]? a /p H ; (1) as the chemical potential of the oxide ion (m )or the thermochemical activity of the oxide ion (a ) (b) basic dissolution: 15() (B) in an ionic solution, since is the common anion K 5 [ ] /a in oxide systems. However, it is not measurable B? p. () without approximation because complex oxides are Both reactions are considered as buffering reactions concentrated ionic solutions, in contrast to ionic to neutralize the basicity or acidity. The hydroxyl aqueous solutions which may be regarded as ideal capacity ( ) is defined for liquid oxide systems dilute (Henrian) solutions. Many kinds of measurable such as oxide slags as measures of basicity using thermochemical and 1/ 1 1/ physical methods have been proposed. Among them, 5 [%H 0.5]? p ( 5 [H, ]? p ) the thermochemical activity of basic oxide and the 1/ 1 1/ solubility of gaseous components, termed capacity, 5 (K B? a ), or (K A? a ). () have been widely accepted as a thermochemical For the sake of application of the hydroxyl capacity measure of the basicity in liquid oxides [1]. to solid complex oxides, it is necessary to make a In liquid oxides, two types of dissolution mechasmall modification on the available site for the nisms for water have been proposed, indicating that dissolution of the anionic species as follows: acts as either an acid or a base depending on??? (a) acidic dissolution: H1V5Hi1 () the basicity of the solution. Typical results for liquid silicates are shown in Fig. 1:??? K 5 [H ]? [ ]/[V ]? p ; (4) 1 A i H??? (b) basic dissolution: 1V 15??? B H (D) K 5 [ ] /[ ]? [V ]? p. (5) The Kroger and Vink notation is used for the description of defect species in solids. Since H is believed to dissolve by the basic dissolution mechanism in AB perovskite [], the apparent hydroxyl capacity of a solid complex oxide can be written by the following equation:??? 1/ 1/ H ; [ ]? [V ]? p 1/ 5 (K? a ). (6) B The activity of is further related to a in equilibrium with the crystal through the following?? relation: 1V 5 (E):?? K 5 [ ]/a? [V ]. (7) Then, the overall reaction for the water dissolution??? may be written as 1V 15 (F)??? KKB5 [ ] /a? [V H. (8) Fig. 1. Relation between hydroxyl capacity and composition of alkaline silicates []. Therefore, the hydroxyl capacity,, for solid

3 S. Yamaguchi et al. / Solid State Ionics (000) oxides in the basic dissolution domain is defined by of Yb1.5-doped Bae and Ba were prepared the equation by a solid state reaction method. Powdery raw??? 1 1/ materials weighed to a proportion of 5 mol% ; [ H excess Ba were mixed and fired initially at 1/ 5 (K K? a ). () for 10 h followed by a sintering process at B 1500 or for 10 h. The samples were annealed Finally, a is related to the activity of a basic at 008 for about 1 month under a dry atmosphere 1 oxide (A), a A, by the equilibrium A5A 1 at P and the same P for subsequent (G): solubility measurements. Then, a sample piece was K 5 a? a /a, (10) installed into a TGA system (Shimazu TGA-40) and A A1 A equilibrated under a dry 11Ar atmosphere 1 0 until the weight of the sample stabilized. Subwhere A 1V 5A (H): A A sequently, a water-saturated gas mixture of 1 1 1Ar was introduced and the equilibrium weight 0 A A A K 5 [A ]/a? [V ]. (11) change was recorded after the weight increase The above discussion suggests a difference between?? reached a constant value. The time required for solid and liquid systems: vacant sites (V ) are equilibration was several hours to 0 h in most cases. explicitly necessary for the dissolution of to Prior to the solubility measurements, activity proceed in solid systems, while it is implicitly measurements of Ba in the Ba e Yb1.5 included in the case of liquids. Since aa changes system were made using the formation reaction of markedly across the AB perovskite phase from the Ba (reaction (I)) by a TGA method, in addition A-rich to the B-rich side, one may expect a to the determination of the isothermal phase relation change in across the AB phase. in the Ba e Yb1.5 system at Details ne of the disadvantage of the capacity is that all of the thermochemical stability will be published the activity coefficients of solutes and defects are elsewhere [4]. assumed to be constant, since the Henrian standard state is used for the activity unit. As the activity coefficient changes with variation of the composition even in a system of the same components, as well as in different component systems, it is not possible to make a direct comparison among the capacities of different perovskite systems. 4. Results and discussion 4.1. Thermochemical stability Fig. shows the isothermal section of the ternary phase diagram for Ba e Yb1.5 at 1008 determined in the present study. The phase relation. Experimental examined was almost identical to that reported for Ba e Nd 1.5 [5]. The three-phase regions of For the solubility measurements, 5 mol% Yb1.5 - [A] Bae1e1Yb1.5 and [B] Bae1 doped Bae (Bae0.5Yb ) was chosen, BaYb41Yb1.5 were identified. In addition, [] because of its appropriate range for controlling Bae1BaYb41Ba was estimated as being Ba activity. The solubility of water at 008 asa present, in which the activity of Ba is almost unity. function of Ba activity (a Ba) was examined by Fig. shows the equilibrium pressure as a thermogravimetric analysis (TGA) in order to estifunction of the absolute reciprocal temperature for mate. The value of aba was controlled by the two-phase coexistence of Bae1e in the equilibrating samples with in the atmosphere by binary system and the three-phase coexistence (TP) the reaction regions of [A] and [B] in the ternary system. The 1 (g) 5 Ba. (I) knee at about 008 observed for the binary system may correspond to the transformation between the Polycrystalline samples composed of two phases rhombohedral and cubic phase, while no indication

4 14 S. Yamaguchi et al. / Solid State Ionics (000) that extrapolation of the nose of the Bae homogeneity domain deviates towards the e side in Nd1.5-doped Bae [], indicating an increase in the extent of the A-site vacancy by Nd1.5 substitution. A similar tendency is expected for the Yb1.5- doped system. Fig. 4 shows the homogeneous domain of Bae as functions of log aba and dopant concentration. The thin solid curves indicate the variation of log aba under a fixed [A-site]/[B-site] ratio ([Ba]/ [e1yb] ratio in this case), estimated from the iso-activity lines of Ba. The present results suggest that Yb substitution increases both aba and the vacancy concentration. This may possibly be the reason for the strong dopant concentration dependency of the equilibrium constant of the H dissolution Fig.. Isothermal section of the phase diagram for the Ba reaction. The activity jump across the Bae phase e Yb1.5 quasi-ternary system at in 5 mol% Yb1.5-doped samples was estimated to. be from 1.0 on the Ba-rich side to 10 on the e-rich side. 4.. Hydroxyl capacity??? Fig. 5 shows the relation between [ ]/[V ] 1/ and P H. From the slope of the plot, the values of H are calculated, and are shown in Fig. 6, in which the variation of log a Ba, log, and log aba1 with log P are also plotted. As the expressions for aba1 and a (Eqs. (8) (10)) include the equilibrium constants in which the activity coefficients of the components are involved, the values of aba1 shown in Fig. 4 are relative ones, while those of a Ba Fig.. Relation between log(p / atm) and 1/T for the reaction 15Bae in the two-phase region of Bae1e and the three-phase regions of [A] Bae1 e1yb1.5 and [B] Bae1BaYb41Yb 1.5. of the transformation was observed for Yb1.5-doped ternary systems. The present results show excellent agreement with those of Levitskii et al. [6], but differ from those of Scholten and Schoonman [7] and Gopalan and Virkar [8]. The results of [A] and [B] Fig. 4. Variations of Ba activity in the homogeneity domain of TPs indicate an increase in the Ba activity with the Bae phase estimated at 008 and the concentration of the increase of Yb1.5 content. It has been reported oxygen vacancies with dopant concentration.

5 S. Yamaguchi et al. / Solid State Ionics (000) Results plotted in Fig. 6 show a steep change versus 0 log P. As log[v Ba] is inversely proportional to log aba through reaction (H), the large variation of 1 log aba is estimated to be caused by the extent of 1 the A-site deficiency. As mentioned earlier, an increase in the A-site vacancy in AB perovskite has been reported for lanthanide oxide substitution. Therefore, the relatively large extent of A-site vacancy dominantly contributes to the change in log aba across the Bae homogeneous domain.?? 1/ Fig. 5. Relation between []/[V ] and (P / atm) at onclusion The slope of each line corresponds to the hydroxyl capacity. A new approach to the understanding of the large solubility of water in AB perovskite oxides is proposed using the hydroxyl capacity defined as??? 1 1/ ; [ H, and examined for 5 mol% Yb1.5-doped Bae. Values of determined under controlled Ba activity indicate that the basicity, defined as the activity of ions, does not show a steep change across the homogeneity domain of Bae. It is estimated that the presence of the A-site vacancy in a AB crystal mainly contributes to the large variation in the Ba activity. References [1]. Wagner, Metall. Trans. 6B (175) 405. Fig. 6. Variation of log, log a Ba, and log aba 1 as a [] S. Ban-ya, M. Hinho (Eds.), hemical Properties of Molten function of log P. Slags, The Iron and Steel Institute of Japan, 11, p [] T. Norby, Y. Larring, urr. pin. Solid State Mater. Sci. are based on the Raoultian standard state. The values (17) 6. of log a were simply calculated from the equa- [4] S. Yamaguchi, K. Nakamura, unpublished. Ba 1 tion log a 5 log a log in the present [5] D. Makovec, Z. Samardzija, D. Kolar, J. Am. eram. Soc. 80 Ba1 Ba (17) 145. study. [6] V.A. Levitskii, S.L. Solokina, Y.Y. Skolis, M.L. Kovba, An extremely small dependence of on Inorg. Mater. 1 (186) 186. log P is observed in contrast to the large activity [7] M.J. Scholten, J. Schoonman, Solid State Ionics 61 (1) jump in a across the Bae phase. ne can 18. Ba carry out further calculations, by combining Eqs. () [8] S. Gopalan, A.V. Virkar, J. Electrochem. Soc. 140 (1) and (10), in order to estimate the variation of a Ba. 1

Homework. Computer Calculation of Equilibria and Phase Diagrams. Selection of parameters to optimize

Homework. Computer Calculation of Equilibria and Phase Diagrams. Selection of parameters to optimize Computer Calculation of Equilibria and Phase Diagrams Bo Sundman 4th lecture Homework Prepare a floppy (or send E-mail) with the SETUP and POP file for the system you selected to the lecture next week!