First-Principles Selection of Solute Elements for Er-Stabilized Bi2O3

Mar 29, 2017 - *E-mail: [email protected] (K. Shitara)., *E-mail: [email protected] (I. Tanaka). Cite this:Chem. Mater. 2017, 29, 8, 376...
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First-Principles Selection of Solute Elements for Er-Stabilized Bi2O3 Oxide-Ion Conductor with Improved Long-Term Stability at Moderate Temperatures Kazuki Shitara,*,†,‡,§ Takafumi Moriasa,† Akifumi Sumitani,† Atsuto Seko,†,§,∥ Hiroyuki Hayashi,†,§ Yukinori Koyama,†,§ Rong Huang,⊥ Donglin Han,† Hiroki Moriwake,‡,§ and Isao Tanaka*,†,‡,§ †

Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan § Center for Materials Research by Information Integration, National Institute for Materials Science, Tsukuba, 305-0047, Japan ∥ JST PRESTO, Kawaguchi, Saitama 332-0012, Japan ⊥ School of Information Science & Technology, East China Normal University, Shanghai 200241, China ‡

S Supporting Information *

ABSTRACT: Quality oxide-ion conductors are essential for cleanenergy applications. Rare-earth-stabilized bismuth sesquioxide, δBi2O3, exhibits a much greater oxide-ion conductivity at high temperatures than commonly used ZrO2- or CeO2-based electrolytes, but it suffers from serious conductivity degradation while annealing at moderate temperatures of ∼773 K, which is the target temperature for many applications. Here, we demonstrate that a novel set of solute elements for δ-Bi2O3 can significantly enhance the long-term stability at 773 K. A pure oxide-ion conductivity of 0.035 S/cm at 773 K remains unchanged during annealing for 100 h, which is five times greater than the best known solid-state oxide materials after long-term annealing. For materials design, we explore a range of chemical spaces using theoretical methods based on first-principles calculations. The order−disorder transition temperature of the anion sublattice, oxygen-ion diffusivity, and solution free energy are used as descriptors. The design concept is verified experimentally.

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INTRODUCTION

suffers from a significant aging effect, namely, the conductivity degrades during long-term annealing.14,15 The aging effect causes a serious problem, because it occurs at the target service temperature for practical applications. This phenomenon was initially recognized a few decades ago. Since then, it has been the subject of intensive studies. Wachsman and co-workers16−19 reported that the origin of the aging effect is 2-fold, depending on the solute elements and the annealing temperature. One is the heterostructural phase transformation from a quenched high-temperature cubic phase to an equilibrium low-temperature (for example, rhombohedral) phase. The other is the order−disorder transition of the anion sublattice. Watanabe reported that most of “cubic stabilized Bi2O3” in the literature is indeed metastable below 973 K.20 As a result, a sluggish phase transformation occurs, and the aging effect is prominent during isothermal annealing below 973 K.16,20−22 The order−disorder transition within the

Oxide-ion conductors are widely used as oxygen sensors, solid oxide fuel cells, and oxygen separation membranes.1−3 Bismuth sesquioxide in the cubic form, δ-Bi2O3, shows a very high oxideion conductivity exceeding 1 S/cm at 1000 K, which is much higher than standard oxide-ion conducting electrolyte yttriastabilized zirconia (YSZ).2−4 The high ionic conductivity has been ascribed to the defective fluorite structure of δ-Bi2O3, in which the empty oxygen sites are distributed randomly over the anion sublattice of the fluorite structure.5−8 The disorder of the anion sublattice is related to its high ionic conductivity, which has been examined by first-principles molecular dynamics (FPMD) simulations.9,10 The δ-phase is stable only in the temperature range between 1002 K and its melting point of 1097 K, but the use of a solid solution of Bi2O3 and some rare-earth oxides can stabilize the δphase at lower temperatures.11−13 For instance, single-phase δBi2O3 can be obtained in the range of 17.5−45.5 cation% Er, which shows a high ionic conductivity close to undoped δBi2O3 at high temperatures.13 However, the stabilized phase © 2017 American Chemical Society

Received: February 28, 2017 Revised: March 28, 2017 Published: March 29, 2017 3763

DOI: 10.1021/acs.chemmater.7b00846 Chem. Mater. 2017, 29, 3763−3768

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Chemistry of Materials cubic phase occurs even when the heterostructural phase transformation is suppressed. Jung et al.18 proposed that the order−disorder transition in the absence of the phase transformation is the origin of the aging effect at 773 K. If this is the case, the order−disorder transition temperature of the oxide-ion sublattice (Tc) should be a key factor in determining the presence of the aging effect at 773 K. If doping of solute elements can lower Tc, the aging effect should be suppressed. Other important factors for designing solute elements are (1) the effects of the solutes on the oxide-ion conductivity and (2) the solution free energy. The desired solute elements should neither decrease the conductivity nor be detrimental to the solubility. In the present study, we focus on the design of solute elements for Er-stabilized δ-Bi2O3 free from aging at 773 K. The properties of the Er-stabilized system have been wellstudied in the literature.13−15,23 At 773 K, only the order− disorder transition occurs without a heterostructural phase transformation. The effects of the solute atoms on Tc, diffusivity, and solution free energy are systematically investigated by first-principles calculations. These quantities are then used as descriptors for the rational selection of solute elements from a given chemical space. The designed material is synthesized by experiments, confirming the improved longterm stability of the conductivity at 773 K.

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Figure 1. Three quantities estimated by first-principles calculations for Bi2O3 with 6.25 cation% of solute elements: (a) theoretical order− disorder transition temperature of the anion sublattice (Tc); (b) oxideion diffusivity at 1600 K by FPMD simulations; and (c) free energy of solution, relative to the end members of pseudo-binary systems (ΔGS) at 1173 and 773 K.

RESULTS AND DISCUSSION Theoretical Order−Disorder Transition Temperature. The order−disorder transition temperature (Tc) was estimated using a set of density functional theory (DFT) calculations and 80-atom cells. Two of the 32 cation sites were substituted by a solute element selected from eight isovalent (Y, La, Nd, Sm, Gd, Dy, Er, and Lu) and six aliovalent elements [group 4A (Zr and Hf), 5A (Nb and Ta), and 6A (Mo and W)]. When aliovalent cations of 4A, 5A, and 6A elements were used as solutes, one, two, and three extra oxide ions were respectively put into the cell, in order to maintain charge neutrality. Therefore, the cation concentration was fixed at 6.25 cation%. Assuming the cation arrangement is unchanged at the transition, Tc can be estimated by equating the free energy of the ordered and disordered arrangements of the anion sublattice. The estimated Tc is 931 K for undoped Bi2O3. Although the experimental data for Tc has not been reported, the experimental β-to-δ transition temperature3,24 of ∼900 K can be used to approximate the order−disorder transition temperature. The β-phase is considered as a distorted and ordered δ-phase where the oxide-ion vacancies in the anion sublattice are ordered along the [100] direction.5,13,15,24 Thus, the agreement between experimental and theoretical Tc is satisfactory for the undoped Bi2O3. Results of estimated Tc for alloy systems are shown in Figure 1a. Trivalent cations are shown in descending order of the ionic radius. The small amount of solute elements significantly impacts Tc, which decreases by ∼80 K when trivalent solutes are present. The estimated Tc is almost the same among trivalent solutes. On the other hand, the presence of aliovalent cations in groups 4A−6A changes Tc notably. The group 4A and 5A elements decrease Tc. The decrease is >300 K, relative to the undoped Bi2O3 for Nb5+. In contrast, group 6A elements increase Tc. Theoretical Oxide-Ion Diffusivity. The oxide-ion diffusivity was estimated by first-principles molecular dynamics (FPMD) simulations with the NVT ensemble using 80-atom

cells. A high simulation temperature of 1600 K was selected to mimic the disordered state and save computational time. Figure 1b summarizes the oxide-ion diffusivity at 1600 K. There is a weak tendency where the oxide-ion diffusivity decreases with the decrease in the ionic radius of trivalent cations. Such a weak tendency was anticipated based on the experimental data of a few systems.25 However, the dependence is weak in both the experiments and the present calculations. In addition, aliovalent cations of group 4A−6A elements do not change the oxygen diffusivity significantly. The oxide-ion conductivity is known to decrease almost linearly as the total amount of solute elements increases as long as the original cubic structure is maintained.17 Therefore, the oxygen diffusivity of the disordered solid solutions by itself is not an essential factor for the selection of solute element as long as the solute concentration is fixed. Theoretical Free Energy of Solution. The free energy of solution, relative to δ-Bi2O3 and the end members of solute elements (e.g., M2O3 for trivalent solutes), which is denoted by ΔGS, is a good measure of thermodynamical assessment. Figure 1c shows the values of both the ordered and the disordered structures at the sintering temperature (1173 K) and the annealing temperature (773 K). When multiple polymorph structures are known for the end-member oxide of M, the lowest energy structure was used as the reference. All solute elements show negative ΔGS values of the disordered structures at 1173 K. They are lower than the ΔGS values of the ordered structures, indicating a preference of solid-solution formation at the sintering temperature. On the other hand, the ΔGS value of the disordered structure is positive at 773 K for most of the trivalent elements. The ΔGS value of the ordered structure is lower than that of the disordered structure at 773 K for all of 3764

DOI: 10.1021/acs.chemmater.7b00846 Chem. Mater. 2017, 29, 3763−3768

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Chemistry of Materials the trivalent elements, since Tc is higher than 773 K. Group 4A elements exhibit similarly positive ΔGS values, whereas group 5A and 6A elements show negative values. These results indicate that only the disordered solid solutions with the trivalent elements are thermodynamically unstable at 773 K. They are formed temporarily by quenching from the sintering temperature and should suffer from either phase separation or anion-sublattice ordering during long-term annealing. In other words, the aging effects are unavoidable for solid solutions only with the trivalent lanthanide elements. Doping of Group 5A or 6A elements is expected to be a remedy to increase the thermodynamical stability of the solid solutions. A rational design of solute elements can be developed based on the theoretical results in Figures 1a−c. The Nb solute should be the best choice for reducing Tc with enhanced thermodynamical stability of the solid solution. Group 4A element solutes may be expelled due to the positive ΔGS value at 773 K. For group 6A elements, W should be the better choice than Mo, because of its lower Tc. Experimental Results for Designed Materials. To verify the alloy design given by the theoretical calculations, experiments were conducted. Three types of samples were examined: (1) Bi2O3−20 cation% Er (20Er), (2) Bi2O3−x cation% Er−y cation% Nb−z cation% W (x + y + z = 20) (xEryNbzW), and (3) Bi2O3−∑xi cation% Mi (∑xi = 20), where Mi is a trivalent cation [high-entropy alloy (HEA) series]. The total amount of solutes was fixed at 20 cation% for all samples, because the total amount of cation is crucial for the oxide-ion conductivity as long as the original cubic structure is maintained.17 A few studies have reported that codoping of W or Nb with rare-earth elements effectively enhances the long-term stability. Watanabe and Sekita reported that Bi0.705Er0.245W0.05O1.575 does not degrade by 873 K after aging for 1100 h.26 Jung et al. reported that Bi0.70Dy0.25 W0.05O 1.575 shows a negligible conductivity degradation at 773 K after 500 h.18 The effects of Nb on 923−973 K aging are also reported in the literature.27,28 Except for the study by Jung et al., these studies do not explicitly investigate the effects of W and/or Nb on the 773 K degradation (i.e., the order−disorder transition of the anion sublattice). The presence of a single cubic phase is identified by powder X-ray diffraction (XRD) in 20Er and 15Er2.5Nb2.5W (Figure S1 in the Supporting Information) before annealing. On the other hand, the formation of a secondary phase is evident when equal amounts of three elements are mixed (e.g., x = y = z = 6.7). Although the structure of the secondary phase is not carefully investigated in the present study, the XRD profile is similar to the orthorhombic Bi2WO6 reported in Dy+Wcodoped Bi2O3.19 In the present study, two samples were supplied for further examination: 20Er and 15Er2.5Nb2.5W. The Arrhenius plots of the conductivity of the two samples in Figure 2a indicate that the two samples have almost the same conductivities before the aging test. The aging effect is absent in both samples at 873 K, as shown in Figure 2b. However, the aging behaviors markedly differ at 773 K. The conductivity of the 20Er sample significantly degrades by more than an order of magnitude during the 773 K annealing, whereas no degradation is observed under the same conditions for the 15Er2.5Nb2.5W sample (Figure 2c). Although our XRD study cannot detect any changes in the profile after the 100 h of annealing at 773 K (Figure 3a), the superstructure electron-diffraction spots are frequently found

Figure 2. (a) Arrhenius plots of the conductivity of the two samples, 20Er and 15Er2.5Nb2.5W, compared to undoped Bi2O3,4 YSZ (yttriastabilized zirconia),2 and GDC (gadolinium-doped ceria).2 (b,c) Conductivity during isothermal annealing for the 20Er and the 15Er2.5Nb2.5W samples at 873 and 773 K.

Figure 3. (a) Powder XRD profiles of the 20Er and the 15Er2.5Nb2.5W samples after annealing at 773 K for 100 h in air. (b) Selected-area electron-diffraction patterns along the ⟨110⟩ zone axis for the two samples after annealing at 773 K for 100 h in air.

only in the 20Er sample after annealing (Figure 3b). This is consistent with the literature,15,16,21 which is well accepted as 3765

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Figure 4. (a) Arrhenius plots of the conductivity of the 20Er sample before/after 100 h of annealing at 773 K. (b−d) differential thermal analysis (DTA) during heating and cooling runs for the as-sintered 20Er (panel (b)), 20Er after 100 h of annealing at 773 K (panel (c)), and 15Er2.5Nb2.5W after 100 h of annealing at 773 K (panel (d)).

the fingerprint of the ordered structure with a cell parameter twice as large as that of the original fluorite structure. Such superstructure electron-diffraction spots are not found in the 15Er2.5Nb2.5W sample after annealing. Figure 4a shows the Arrhenius plots of the conductivity of the 20Er sample before/ after 100 h of annealing at 773 K. Because the aging effect occurs on the hour scale, the conductivity is not degraded before annealing in both the cooling and heating runs of the conductivity measurements. However, after annealing for 100 h, the slope of the Arrhenius plot clearly differs from that of the original sample before annealing. It is interesting that the degradation is rejuvenated after heating the annealed sample above 900 K. The rejuvenated sample shows the same Arrhenius plot for the conductivity as the original sample. The behavior is the same as that reported in the 20Er sample.21 Differential thermal analysis (DTA) of the original 20Er sample shows an exothermic peak at 670 K (Figure 4b). After the 100 h of annealing at 773 K, an extra endothermic peak appears at 910 K (Figure 4c), which should be related to the rejuvenation. In the 15Er2.5Nb2.5W sample, the 910 K peak does not appear, even after the 100 h of annealing at 773 K (Figure 4d). Since the origin of the aging effect at 773 K is attributed to the ordering of the anion sublattice, it is natural to ascribe the rejuvenation effect at ∼900 K to the order−disorder transition of the anion sublattice, which is consistent with the endothermic behavior. Because the concurrent addition of Nb and W effectively avoids the aging effect at 773 K, the 15Er2.5Nb2.5W sample does not show the rejuvenation behavior. Because of the absence of the aging effect in the 15Er2.5Nb2.5W sample, its conductivity at 773 K after the 100 h of annealing is the same as that of the initial value: 0.035 S/cm. This conductivity is five times greater than the best known Bi2O3-based material after the long-term annealing reported in the literature (i.e., Bi0.70Dy0.25W0.05O1.575 at 773 K after 100 h of annealing18). The conductivity is 1−2 orders of magnitude greater than commonly used ZrO2- or CeO2-based oxide-ion conductors.2,24,29,30 The oxide-ion transportation number was measured using an oxygen concentration cell. By a linear fit to the measured electromotive force (EMF), it is 0.98 at 773 K and 1.00 at 873 K (Figure 5a). The proton transportation is negligible when measured by a water vapor concentration cell. The high oxideion conductivity and the absence of the aging effect should be the great value for practical applications of the present material in the moderate temperature regime.

Figure 5. (a) EMF of an oxygen concentration cell at 773 and 873 K with the 15Er2.5Nb2.5W sample plotted against the oxygen partial pressure. (b) Photographs of the sample and the cell.

Experimental Results for HEA Samples. To observe the effects of aliovalent solutes, another set of samples only with trivalent solutes were examined. Five different types of chemical compositions using mixtures of 5−8 trivalent cations were chosen. The total amount of solutes was fixed at 20 cation%. The idea to mix many cations comes from the HEAs of metallic systems.31 The stability of the solid solution is expected to increase by the configurational entropy contribution of many elements. The experimental results are shown in the Supporting Information. All five of the HEA samples show a single cubic phase before annealing (Figure S2 in the Supporting Information). Their Arrhenius plots of the conductivity are almost the same as that of 20Er. After annealing at 773 K for 100 h, a rhombohedral phase is formed in two HEA samples (HE-1 and HE-2), whereas no change is found by XRD in three samples (HE-3, HE-4, and HE-5). All five HEA samples show a clear aging effect at 773 K. The present results clearly show that the addition of aliovalent solutes (i.e., Nb and W) is essential to avoid the aging effect. The configurational entropy contribution just by mixing up to eight trivalent solutes is insufficient to overcome the problem.

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CONCLUSION To realize a Bi2O3-based oxide-ion conductor free from the aging effect at moderate temperatures, a range of chemical 3766

DOI: 10.1021/acs.chemmater.7b00846 Chem. Mater. 2017, 29, 3763−3768

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the MSD and the diffusivity. Cation diffusion exceeding the nearestneighbor cation distance of MSD was not detected during the 100 ps simulation. Experimental Details. Samples were synthesized by the solid-state reaction process. The starting powders were commercial Bi2O3 (99.99%, Rare Metallic), Er2O3 (99.9%, Rare Metallic), Nb2O5 (99.9%, Kojundo Chemical), WO3 (99.99%, Kojundo Chemical), and others from similar suppliers with similar purities. They were mixed by a planetary ball mill with zirconia balls and ethanol followed by calcination in air at 1073 K for 10 h. They were then finely ground with an alumina mortar, uniaxially pressed, and sintered in air at 1173 K for 10 h. The density of the sintered bodies was ∼90%. All samples were ground and subjected to powder X-ray diffraction with Cu Kα radiation (Rigaku SmartLab). The electron diffraction patterns were observed in select pulverized samples on micromeshes using a transmission electron microscopy (TEM) system (JEOL, Model JEM-2100F). The electrical conductivity was measured in air using an impedance analyzer (Solartron, Model SI-1260) in a frequency range of 1 Hz to 10 MHz and an ac voltage of 100 mV. Sputtered gold electrodes were used. Heating and cooling measurements were conducted every 25 K with a heating/cooling rate of 3.3 K/min. The oxide-ion transportation number was measured using an oxygen concentration cell: O2(pO2)−Ar,Au(I) | sample | Au(II), O2 (Figure 5b), with varying pO2 values by mixing O2 with Ar. The water vapor concentration cell (Ar−20% O2−3.12% H2O, Au(I) | sample | Au(II),Ar−20% O2−0.86% H2O, was used to estimate the contribution of the proton conduction. The apparatuses were the same as those used in ref 41. The apparent proton transportation number was