Proton Conduction and Incorporation into La_1−xBa_xYb_0.5In_0.5O_3−δ

Abstract

In order to clarify the effect of the dopant concentration and phase transition on the proton conduction and proton concentration, the electrical conductivity and proton concentration were determined for La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7). The phase transition from the orthorhombic system to the cubic system was over x = 0.3. The proton/deuteron isotope effect on the conductivity was observed for all samples at 673 K. The proton concentration was independent of the barium content above 673 K. The proton concentration increased with the barium content below 673 K, but the concentration ratio of the proton to dopant decreased following an increase in the barium concentration. It was determined that the dopant concentration and phase transition do not have an influence on the conduction and incorporation of protons into the La_1−xBa_xYb_0.5In_0.5O_3−δ.

1. Introduction

Some perovskite-type oxides undergo the proton conduction at the intermediate temperature. The oxide ion vacancies were formed in the crystal when a cation of the perovskite-type oxide was replaced by a cation having a low valence. Protons are incorporated into crystal by the flowing reaction¹⁾:   

\[ {\rm H}_{2} {\rm O} + V_{\rm O}^{\bullet\bullet} = 2{\rm H}_{\rm i}^{\bullet} + {\rm O}_{\rm O}^{\times} \](1)

The water uptake takes place at the oxide ion vacancy. According to eq. (1), the proton concentration is expected to depend on the number of oxide ion vacancies. However, the maximum proton concentration does not reach the dopant level or the oxide ion vacancy level. The proton solubility depends on the dopant species and matrix.

The ratio of the proton concentration to dopant concentration for the La-based perovskite oxide was in the order La_0.9Sr_0.1InO_3−δ > La_0.9Sr_0.1ScO_3−δ > La_0.9Sr_0.1YbO_3−δ > La_0.9Sr_0.1AlO_3−δ²⁾. It was found that the proton concentration depends on the B-site of the ABO₃ perovskite.

In a previous study^3–6), we suggested that the limitation on the degree of hydration in the oxide is attributed to the existence of the immobile oxide ion vacancy. Oikawa et al. reported that some oxygen vacancies in the MO₅ polyhedra are structurally stable; they do not affect the hydration⁷⁾. It is expected that the dissolution limit of the protons is related to the local structure of the oxygen octahedral.

Ba-doped LaInO₃ was reported to be the mixed conductor of the oxide ion and hole^8–10). Moreover, the phase transition occurred by increasing the Ba content. In a part of report¹¹⁾, proton conduction was observed for this system in a wet atmosphere. However, the evidence of proton conduction has not been thoroughly obtained and the effect on phase transition is not known.

In order to clarify the effect of the dopant concentration and phase transition on the proton conduction and proton concentration, the proton conductivity and proton concentration were determined for La_1−xBa_xYb_0.5In_0.5O_3−δ. In this study, the isotope effect on the conductivity was observed for La_1−xBa_xYb_0.5In_0.5O_3−δ and the proton concertation was measured by a thermogravimetric analysis.

2. Experimental

2.1 Sample preparation

La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7) were prepared by a solid state reaction method. The reagent-grade La₂O₃ (99.9%), Yb₂O₃ (99.9%), BaCO₃ (99.9%), and In₂O₃ (99.9%) powders were weighted to form the composition La_1−xBa_xYb_0.5In_0.5O_3−δ, then mixed in ethanol using a mortar. The powder mixtures were calcined in air for 10 h at 1573 K. The calcined powder was pressed into a pellet at 200 MPa by CIP. The pellets were next sintered at 1873 K for 10 h in air. The compact density was over 98%. The crystal structures were identified by an X-ray diffraction analysis using a MinFlex (Rigaku) with CuKα-radiation.

2.2 Thermogravimetric analysis

The proton concentrations were determined from the weight change of the sample due to water absorption based on a thermogravimetric analysis (TGA). The samples were first treated at 1473 K for 1 h in dry-N₂-Ar to remove any existing hydrogen. The weight changes of the samples were then measured in 1.9%H₂O-N₂ in the temperature range of 473–1473 K using a STA449F3 Jupiter (NETZSCH). The effect of buoyancy was corrected using the data from a blank test without any sample.

2.3 AC impedance measurement

The electrical conductivity was measured by a four-terminal AC technique. Porous platinum electrodes were attached to each end of the bar sample (10.0 mm × 3.0 mm × 2.0 mm) by a painting paste, then sintering at 1273 K in air. In order to verify the protonic conductivity, the electrical conductivity was measured under 1.9%H₂O–1%O₂ and 1.9%D₂O–1%O₂ in the temperature range of 673–1173 K. The samples were equilibrated with 1.9% H₂O–1%H₂ (or O₂)-Ar in the temperature range of 573 K–1173 K. The complex impedance was measured in the frequency range of 0.02 Hz–1 MHz using an LCR meter (NF corporation: ZM2375 or HIOKI corporation: IM3536).

2.4 Electromotive force measurement

The EMF of the gas concentration cells was measured for La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ at 1073 K. The samples were formed into pellets (φ13.5 mm diameter and 0.5 mm thickness). Porous platinum electrodes were prepared on both surfaces of the pellet samples. The samples were held between the alumina tubes by pyrex glass gaskets, thus separating the two electrode compartments. The gas concentration cell is represented by the following cell formula:   

\[ \begin{split} & (-), {\rm Pt} (p_{{\rm H}_{2}} ({\rm I})\ {\rm or}\ p_{{\rm O}_{2}} ({\rm I}),\ p_{\rm H_{2}O} ({\rm I}))\\ &\quad \left| {\rm La}_{0.5} {\rm Ba}_{0.5} {\rm Yb}_{0.5} {\rm In}_{0.5} {\rm O}_{3 - \delta} \right|\\ &\quad {\rm Pt}(p_{{\rm H}_{2}} ({\rm II})\ {\rm or}\ p_{{\rm O}_{2}} ({\rm II}),\ p_{{\rm H_{2}O}} ({\rm II})),\ (+) \end{split} \](2)

where p_i represents the activities of the gas species with reference to one bar of the respective gas in the pure state. A gas mixture of H₂ (or O₂) and Ar was humidified with water vapor saturated at a given temperature using a bubbler in a thermostatic bath. The EMF measurements were performed using a digital voltmeter (Keysight Technology 34461A).

3. Results and discussion

Figure 1 shows the X-ray diffraction pattern of La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7). As shown in Fig. 1, the phases of x = 0.1 and x = 0.3 were determined to be the orthorhombic perovskite structure. The phase of x = 0.5 was confirmed to be the cubic system and single phase. The diffraction pattern of x = 0.7 showed an unknown peak of the second phase.

Fig. 1

X-ray diffraction pattern of La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7).

Figure 2 shows the proton concentration of La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7). As shown in Fig. 2(a), the proton concentration decreased with an increase in the temperature. The substitution of barium ions on the lanthanum ion-site then lead to an enhancement of the proton concentration at a low temperature. Figure 2(b) shows the proton concentration normalized by the barium ion concentration. The normalized proton concentration decreased with an increase in the barium content. This suggested that the proton concentration is not proportional to that of the formed oxygen vacancy.

Fig. 2

Proton concentration of La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7). (a) Temperature dependence of the proton concertation. (b) Concentration ratio of proton to dopant as a function of the barium content.

Figure 3(a) shows the decay curve of the conductivity for evaluateing the proton/deuteron isotope effect at 400℃. As shown in the figure, the isotope effect on the conductivity was observed. the proton was found to be the predominant charge carrier. Figure 3(b) shows the ratio of the conductivity in 1.9%H₂O–1%O₂ to that in 1.9%D₂O–1%O₂. The conductivity ratio decreased with an increase in the temperature or Ba content. This suggested that the proton transport number decreased with an increase in the temperature or Ba content. At a low temperature, the conductivity ratio was over 1.34 which is the value expected for a local mode with a reduced mass approximation. This might be attributed to the difference in the zero-point motion between the proton and deuteron¹²⁾.

Fig. 3

Proton/deuteron isotope effect on conductivity. (a) Conductivity change with exchange of the proton for the deuteron. (b) Ratio of the conductivity under H₂O–O₂ and the conductivity under D₂O–O₂.

Figures 4(a) and (b) show the temperature dependence of the electrical conductivity. As shown in Fig. 4(a), the conductivity was independent of the Ba content at a low temperature. According to the isotope effect on the conductivity, the proton conduction is dominant at a low temperature. Therefore, the proton conduction is independent of the Ba content. On the other hand, the conductivity increases with the increasing in Ba content at a high temperature. In order to clarify the dominant charge carrier, the EMF of the gas concentration cells was measured for La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ at 1073 K. Figures 5(a) and (b) show the EMF of the hydrogen concentration and the water vapor concentration under a reducing atmosphere, respectively. The EMF of the hydrogen concentration was in agreement with the theoretical value according to the Nernst equation. This indicated that the ionic transport number is unity. The EMF of the water vapor concentration was observed, but was lower than the calculated theoretical value based on the Nernst equation when the transport number of the oxide ion is unity. Therefore, La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ was found to be a mixed ion conductor of the proton and oxide ion under the reducing atmosphere. The EMF of the mixed ion conductor can be represented as follows:²⁾   

\[ E = - \frac{RT}{F} \ln \frac{p_{\rm H_{2}O(II)}^{-1/2} + \sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ}/\sigma_{V_{\rm O}^{\bullet\bullet}}^{\circ}} {p_{\rm H_{2}O(I)}^{-1/2} + \sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ}/\sigma_{V_{\rm O}^{\bullet\bullet}}^{\circ}} \](3)

Fig. 4

Temperature dependence of conductivity. (a) Conductivity under a reducing atmosphere containing hydrogen. (b) Conductivity under an oxidizing atmosphere containing oxygen.

Fig. 5

EMF of gas concentration cell using La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ as the electrolyte. (a) Hydrogen concentration cell under H₂O–H₂. (b) Stem concentration cell under H₂O–H₂. (c) Oxygen concentration cell under H₂O–O₂.

where $ \sigma^{\circ}_{{\rm H}_{\rm i}^{\bullet}}$ and $\sigma^{\circ}_{V^{\bullet\bullet}_{\rm O}}$ are the unique constants only depending on the temperature. The proton conductivity and the oxide ion conductivity were assumed using $\sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ}$ and $\sigma_{V_{\rm O}^{\bullet\bullet}}^{\circ}$ as follows²⁾:   

\[ \sigma_{{\rm H}_{\rm i}^{\bullet}} = \sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ} p_{\rm H_{2}O}^{1/2} \](4)

  

\[ \sigma_{{\rm V}_{\rm O}^{\bullet\bullet}} = \sigma_{{\rm V}_{\rm O}^{\bullet\bullet}}^{\circ} \](5)

By setting $\sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ}/\sigma_{V_{\rm O}^{\bullet\bullet}}^{\circ}$ as the fitted parameter, the EMF was calculated from eq. (3). As shown in Fig. 5(b), the calculated value was in good agreement with the experimental value when the value of $\sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ}/\sigma_{V_{\rm O}^{\bullet\bullet}}^{\circ}$ was 12. The proton transport number can be determined from eqs. (4) and (5) as follows:   

\[ t_{{\rm H}_{\rm i}}^{\bullet} = \frac{ \sigma_{{\rm H}_{\rm i}^{\bullet}} } { \sigma_{{\rm H}_{\rm i}^{\bullet}} + \sigma_{{\rm V}_{\rm O}^{\bullet\bullet}}} = \frac{\sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ} p_{\rm H_{2}O}^{1/2}/ \sigma_{{\rm V}_{\rm O}^{\bullet\bullet}}^{\circ}} {\sigma_{{\rm H}_{\rm i}^{\bullet}}^{\circ} p_{\rm H_{2}O}^{1/2}/ \sigma_{{\rm V}_{\rm O}^{\bullet\bullet}}^{\circ} + 1} \](6)

The transport number of the proton and the oxide ion vacancy were 0.62 and 0.38 at pH₂O = 0.019 and 1073 K, respectively. In a previous study²⁾, La_0.9Sr_0.1InO_3−δ with the orthorhombic system was reported to be the mixed conduction of the proton and oxide ion. The transport numbers of the proton and oxide ion were same vales as the La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ with the cubic system. The phase transition of LaInO₃ system does not significantly affect the transport number of the proton and oxide ion.

Figure 4(c) shows the EMF of the oxygen concentration cell under an oxidizing atmosphere. The observed EMF was lower than the theoretical value according to the Nernst equation. Therefore, La_0.5Ba_0.5Yb_0.5In_0.5O_3−δ was found to be a mixed conductor of the hole and ions.

The phase of La_1−xBa_xYb_0.5In_0.5O_3−δ transforms from orthorhombic to cubic following an increase in the barium content. The proton conductivity did not change with the phase transition. The transform to cubic does not improve the proton conductivity and concentration. The crystal system does not affect on the proton conductivity and concentration for La_1−xBa_xYb_0.5In_0.5O_3−δ. Moreover, the proton concentration was independent of the barium content above 673 K. The concentration ratio of the proton to dopant decrease with an increase in the barium content as shown in Fig. 2(b). The proton concentration is not proportional to the number of oxide ion vacancy. This indicated that the proton concentration does not simply increase with an increase in the number of the oxide ion vacancies. The proton concentration might depend on the local structure around the oxide ion vacancy and the acceptor dopant.

4. Conclusion

The conduction and incorporation of a proton into La_1−xBa_xYb_0.5In_0.5O_3−δ (x = 0.1. 0.3. 0.5, 0.7) were examined in the temperature range of 573–1073 K. The effects of the phase transition and the dopant concentration on the proton conductivity were not observed. The proton concentration was independent of the barium content above 673 K. The concentration ratio of the proton to dopant decreases with an increase in the dopant content. This indicated that all the oxygen vacancies do not act during the hydration reaction. The change in the mean structure and the number of the oxygen vacancies might not be important regarding incorporation of a proton into La_1−xBa_xYb_0.5In_0.5O_3−δ.

Acknowledgement

This study was partially supported by JSPS KAKEN Grant number 16H06124 (Grant-in-Aid for Young Scientist(A)).

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