Oxide-Ion Conductivity Enhancement of Polycrystalline Lanthanum

Jun 29, 2016 - The sandwich-type ternary diffusion couple, which was made up of La2SiO5/[BaO-doped La2Si2O7 + BaO-doped LSO]/La2SiO5, was heated ...
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Oxide-Ion Conductivity Enhancement of Polycrystalline Lanthanum Silicate Oxyapatite Induced by BaO Doping and Grain Alignment Koichiro Fukuda,*,† Ryoji Watanabe,† Masayuki Oyabu,† Ryo Hasegawa,† Toru Asaka,† and Hideto Yoshida‡ †

Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan

‡

S Supporting Information *

ABSTRACT: We prepared the c-axis-oriented polycrystal of BaO-doped lanthanum silicate oxyapatite (LSO) by a reactive diffusion technique. The sandwich-type ternary diffusion couple, which was made up of La2SiO5/[BaO-doped La2Si2O7 + BaOdoped LSO]/La2SiO5, was heated at 1873 K for 100 h. The thin-plate polycrystalline electrolyte was mechanically extracted from the inner part of the annealed couple and characterized by optical microscopy, transmission electron microscopy, electron probe microanalysis (EPMA), X-ray diffractometry (XRD), Raman spectroscopy, and impedance spectroscopy. On the basis of the numbers of cations determined by EPMA and structural data refined by single-crystal XRD, the chemical formula of the constituent crystallites was determined to be (La9.32Ba0.28)(Si5.87□0.13)O26, where □ denotes a vacancy in the Si site. The combined use of BaO-doping and grain-alignment techniques effectively improved the oxide-ion conductivity, which steadily increased from 2.17 × 10−2 to 1.42 × 10−1 S cm−1 as the temperature increased from 723 to 1023 K. The activation energy of conduction was 0.48 eV.

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conductivity (σ) at low temperatures (below 873 K).6,7 Because the oxide ion conducts much higher along the c-axis than perpendicular to this direction in the crystal structure (space group P63/m), the polycrystalline LSO is a material which requires further improvement of conductivity. There are two different methods that significantly enhance the oxide-ion conductivity: doping and grain alignment. The promising dopants which have effectively increased the oxide-ion conductivity include GeO2 and BaO.8−13 The σ-value at 1073 K of polycrystalline GeO2-doped LSO (La9.33Si5GeO26) with random grain orientation is 0.01 S cm−1, which is about 5 times larger than that of nondoped LSO.8 Recently, a facile method utilizing reactive diffusion has been developed for the preparation of c-axis-oriented polycrystalline LSO, which actually showed much higher σ-values than the randomly grain-oriented materials.14−16 The experimental

INTRODUCTION Electricity generation from solid oxide fuel cells (SOFCs) as an alternative to fossil fuels is one of the most promising technologies for solving the environmental and energy problems facing the world. A popular electrolyte material of SOFCs includes yttria-stabilized zirconia (YSZ). YSZ-based SOFCs typically operate at relatively high temperatures, ranging from 1023 to 1273 K, mainly due to the limited transport property of the electrolyte material at low temperatures.1,2 The lowering of operating temperature is beneficial for the improvement of stability, durability, and reliability of SOFCs.3 It also contributes to shorten the startup time and decrease energy loss from heat radiation. Expensive heat-resistant alloys used as peripheral materials can also be replaced with inexpensive common materials (e.g., stainless steel). To reduce the operating temperature, investigators have attempted to use Sr- and Mg-doped lanthanum gallate (LSGM) as an advanced alternative electrolyte.4,5 In recent years, the lanthanum silicate oxyapatite (LSO) has attracted our interest because of its relatively high oxide-ion © 2016 American Chemical Society

Received: April 26, 2016 Revised: June 6, 2016 Published: June 29, 2016 4519

DOI: 10.1021/acs.cgd.6b00638 Cryst. Growth Des. 2016, 16, 4519−4525

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NRS-2000, JASCO Co., Tokyo, Japan). The green line at 514.5 nm of an argon-ion laser (incident power of 20 mW) was used for the excitation. The spot size of the laser beam was 5 μm in diameter on the thin-section surface. A backscattered electron micrograph and concentration distribution maps for La2O3, BaO, and SiO2 were obtained for the thin-section sample using an electron probe microanalyzer (EPMA, model JXA8900L, JEOL Ltd., Tokyo, Japan). The analysis area was 1080 × 1080 μm with 540 × 540 pixels and the dwell time was 20 ms. A quantitative line analysis (2 μm step) was made within the same area. We defined the one-dimensional coordinate system that consists of a single axis x, which is parallel to the analysis line, to describe the locations of analysis points by the x-values in μm. The accelerating voltage, probe current, and electron probe diameter were, respectively, 15 kV, 0.010 μA, and about 2 μm for mapping, and 25 kV, 0.020 μA, and about 1 μm for line analysis. The corrections were made by ZAF routines. Transmission Electron Microscopy. We peeled the thin specimen off the glass slide and used an argon-ion beam (Model 695 PIPSII, Gatan Inc., CA, United States) for the preparation of the transmission electron microscope (TEM) specimen. We used a highresolution TEM (HR-TEM, JEM-2100F, JEOL Ltd.), and a scanning TEM (STEM, JEM-ARM200F, JEOL Ltd.) configured with a probeside aberration corrector and a low-angle annular detector, which provided annular dark-field (ADF) imaging. We investigated the crystal grains and their grain-boundary regions that were located close to the original interfacial contact boundary of Sample A/Sample B. The multislice simulation image was generated using software package xHREM (HREM Research Inc., Saitama, Japan) and compared with the ADF-STEM image to confirm that the investigated crystal is isostructural with apatite. X-ray Diffraction and Impedance Spectroscopy. We equally polished both surfaces of an annealed diffusion couple (about 1.2 mm in thickness) with 800-grid SiC paper and 1 μm diamond paste and obtained the thin-plate electrolyte (about 250 μm in thickness) consisting exclusively of lanthanum barium silicate oxyapatite (LBSO). The X-ray diffraction (XRD) patterns in the 2θ range from 10.0° to 90.0° (2394 total data points) were taken on the electrolyte surface using a diffractometer (model X’Pert PRO Alpha-1, PANalytical B.V., Almelo, The Netherlands) in the Bragg−Brentano geometry. The {00l} texture fraction ( f 00l) was determined from the XRD pattern using the Lotgering method.19 We extracted the integrated intensities from the entire XRD pattern by the Le Bail method20 using computer program RIETAN-FP.21 We also used this computer program to generate the simulated XRD pattern of the randomly grain-oriented LBSO polycrystal. The detailed procedure for the determination of f 00l based on the observed and calculated intensities was given in our previous studies.14−17 We coated both sides of the thin-plate electrolyte with a platinum paste and heated it at 1273 K to decompose the paste and harden the Pt residue. The impedance spectroscopy data were collected during heating from 723 to 1023 K using an impedance analyzer (model 3570, HIOKI E. E. Co., Nagano, Japan) over the frequency range from 4 to 5 MHz. We analyzed parts of the impedance spectra by a nonlinear least-squares fitting method using equivalent circuits with ZView software.22 We crushed the thin-plate LBSO electrolyte, picked up a single crystal, and mounted it onto the end of a glass capillary. The XRD intensities were measured on a Bruker Smart Apex II Ultra diffractometer (Mo Kα radiation, 50 kV and 50 mA). We refined the unit-cell parameters and extracted the observed structure factors using the program package Apex2-W2K/NT.23 The structural parameters were refined by the computer program SHELXL-97.24 The validity of the final structural model was confirmed by the different Fourier maps with the use of the JANA2006 program.25 The structural model was visualized with the computer program VESTA.26 We also used this program to calculate the Madelung energy per unit cell of the relevant crystal structure. The data collection and refinement parameters are summarized in Table S1.

procedure of this method simply required the isothermal heating of the diffusion couples. Thus, we combined the GeO2doping and reactive diffusion techniques to increase the σ-value of c-axis-oriented La9.33(Si0.87Ge0.13)6O26 polycrystal, which was prepared by the heat treatment of a sandwich-type binary diffusion couple of La2Si2O7/La2(Si0.833Ge0.167)O5/La2Si2O7.17 The σ-values at relatively high temperatures were actually improved (e.g., 6.92 × 10−2 S cm−1 at 973 K). On the other hand, the σ-values rapidly decreased as the temperature decreased (e.g., 3.35 × 10−2 S cm−1 at 873 K and 9.8 × 10−3 S cm−1 at 773 K) due to the relatively large activation energy of conduction (0.75 eV). Thus, the electrolyte of grain-aligned polycrystalline GeO2-doped LSO has been found to not be perfectly suited for those of low-temperature SOFCs. Vincent et al. successfully increased the σ-values of randomly grain-oriented LSO polycrystals by BaO doping; the σ-value of La9BaSi6O26.5 at 973 K was about 38 times larger than that of La9.33Si6O26.12 In the present study, we have, for the first time, combined the two independent techniques of BaO doping and grain alignment by reactive diffusion and greatly improved the oxide-ion conductivity of the resulting c-axis-oriented polycrystal at relatively low temperatures. To accomplish this purpose, we used the ternary diffusion couple, which was composed of a two-phase material for one end member and single-phase material for the other end member.

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EXPERIMENTAL SECTION

Materials. We prepared the La2SiO5 (theoretical density = 5.490 Mgm−3) powder specimen (Sample A) by a conventional solid state reaction using the stoichiometric amounts of La2O3 (99.99%, Mitsuwa Chemicals Co. Ltd., Osaka, Japan) and SiO2 (99.0%, Kishida Chemical Co. Ltd., Osaka, Japan). These raw materials were thoroughly mixed, pressed into pellets, heated at 1873 K for 3 h, and quenched in air. The sintered pellets were pulverized to obtain the fine powder specimen. The powder specimen with a starting composition of 1.95:0.05:2 La:Ba:Si in an atomic ratio (Sample B) was prepared from La2O3, BaCO3 (99.0%, Kishida Chemical Co. Ltd.), and SiO2. The mixture was pressed into pellets, heated at 1473 K for 0.5 h and then at 1873 K for 3 h, and finally cooled to ambient temperature. We finely ground the sintered pellets and obtained the powder specimen. The phase composition, which was determined by the Rietveld method18 as described in the Supporting Information, was found to be 98.92 mol % BaO-doped La2Si2O7 and 1.08 mol % BaO-doped LSO. The probable chemical composition and volume fraction were, respectively, 1.93:0.03:2 La:Ba:Si and 98.96 vol % for the former compound and 8.1:1.9:6 La:Ba:Si and 1.04 vol % for the latter compound. The average chemical composition of the two-phase material was 1.953:0.050:2 La:Ba:Si, which was in good agreement with the bulk chemical composition of Sample B. We prepared the sandwich-type ternary diffusion couples of Sample A/Sample B/Sample A, each of which was made up of one Sample B powder compact (0.200 g) with the size of φ 13 × 0.50 mm and two Sample A powder compacts (0.270 g each) with the size of φ 13 × 0.36 mm. They were heated at 1873 K for 100 h followed by slow cooling at 100 K/h to ambient temperature. The annealed couples were eventually made up of La2SiO5/apatite/La2SiO and were free from the unreacted regions of Sample B. Optical Microscopy, Raman Spectroscopy, and EPMA. We cut one of the annealed couples using a diamond saw along the diffusion direction to expose a section. The specimen was mounted on a glass slide and subsequently made into a thin transparent section. We polished the upper surface of the thin specimen with a 1 μm diamond paste to avoid irregular reflection of emergent light. The microtexture was observed using a polarizing microscope. We collected the micro-Raman spectra, ranging from 100 to 1000 cm−1, from the thin-section surface in the backscattering geometry (resolution = 1 cm−1) using a double grating monochromator (model 4520

DOI: 10.1021/acs.cgd.6b00638 Cryst. Growth Des. 2016, 16, 4519−4525

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RESULTS AND DISCUSSION Highly c-Axis-Oriented LBSO Polycrystal. Each of the annealed diffusion couples was composed of apatite and a small amount of unreacted La2SiO5, which remained on both surfaces (Figure 1). The red lines in Figure 1a indicate the locations of

Figure 2. La, Ba, and Si distributions. The points represent the numbers of cations on the basis of 26 oxygen atoms along the yellow line in Figure 1. Figure 1. Optical micrographs of the reaction product formed by the reactive diffusion taken in (a) plane polarized transmitted light and (b) under crossed polars. White arrows indicate the vibrational orientations of polarizer (P) and analyzer (A) of the microscope. Most of the crystallites are in the diagonal position in panel a and extinction position in panel b. The white open square depicts the EPMA mapping area, and the yellow line indicates the location of quantitative line analysis. The surface layers with thicknesses of 30 and 40 μm are of the unreacted La2SiO5 (Sample A).

points at the original interfacial contact boundaries, which will be detailed later. We extracted the thin-plate LBSO electrolyte with about a 250 μm thickness (Figure S3) from the central part of the innermost apatite layer. Hence, the relevant concentration profiles for La, Ba, and Si along the diffusion direction of this electrolyte should be almost identical to those of the region with 427 ≤ x/μm ≤ 677 (Δ = 250 μm) in Figure 2. Because the concentrations were almost uniform within the narrow region, we averaged the numbers of atoms over this region and determined the chemical formula of LBSO with electrolyte to be La9.32(5)Ba0.28(1)Si5.87(3)O26, where the figures in parentheses indicate standard deviations. When viewed under crossed polars, most of the LBSO crystallites of the innermost layer came to the extinction position at the same time (Figure 1b), which indicates that they are highly oriented along their crystallographic axes. The XRD pattern from the LBSO electrolyte surface (Figure 3) demonstrated the predominant intensities with the reflection indices of 00l (l = 2, 4, and 6). Hence, we confirmed that the caxis directions of the constituent crystal grains were highly aligned parallel to the diffusion direction (perpendicular to the electrolyte surface). The f 00l value was determined from the observed and calculated intensities (Figure S4) to be 0.58. Crystal Structure of LBSO. The crystal structure was wellrefined based on the Si-deficient structural model (space group P63/m), and hence the refinement process was very similar to

the original interfacial contact boundaries of Sample A/Sample B before annealing. The innermost apatite layer between the boundaries (about 492 μm in thickness) corresponds to the former Sample B region. The regions near the interfacial contact boundaries were necessarily dark under transmitted light due to the presence of minute voids in high concentrations. The white open square in Figure 1a depicts the area for EPMA mapping. The concentration distribution maps (Figure S2) showed that the innermost layer contained the BaO component in a relatively high concentration. During the annealing process, the BaO component was found to diffuse and intrude into the adjacent Sample A regions on both sides, as evidenced by the concentration line profile (Figure 2) along the yellow line in Figure 1a. Each of the concentration line profiles for La and Si demonstrated two projections of the data 4521

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0.01260(8) 0.01080(8) 0.0076(4) 0.0158(10) 0.0193(11) 0.0261(11) 0.083(5) 0 0 0 0 0 −0.0152(10) 0 0 0 0 0 0 −0.0086(9) 0 U11/2 0.00747(8) 0.0037(4) 0.0074(10) 0.0023(11) 0.0290(12) U11/2 a

4f 6h 6h 6h 6h 12i 2a M1 M2 Si O1 O2 O3 O4

Space group: P63/m. Unit-cell dimensions: a = 0.97158(1) nm, c = 0.72024(1) nm, and V = 0.58880(2) nm3.

0.01889(14) 0.01179(10) 0.0077(4) 0.023(2) 0.0195(14) 0.0161(11) 0.22(2) U11 0.01149(10) 0.0084(4) 0.0104(12) 0.0078(11) 0.046(2) U11 0.00945(9) 0.01144(10) 0.0066(4) 0154(13) 0.023(2) 0.0294(12) 0.016(2) 0.00102(5) 1/4 1/4 1/4 1/4 0.0693(3) 1/4 2/3 0.22714(3) 0.40249(12) 0.5950(3) 0.1614(3) 0.3467(3) 0

U33 U22 U11 z y x g

and all O sites were fully occupied. Thus, we fixed the siteoccupancy values (g) of M1 and M2 as g(M1) = 0.9 and g(M2) = 1. The chemical formula was expressed by (La9.32Ba0.28)(Si5.87□0.13)O26, where □ denotes a vacancy in the Si site, with the degree of deficiency being 2.2%. The refinement resulted in the satisfactory reliability (R) indices of R = 0.0169 and wR = 0.0197. The structural parameters and atomic displacement parameters are summarized in Table 1. Selected interatomic distances and their standard deviations are listed in (Table S2). The Ba atom could actually show the site preference between M1 and M2. Thus, we compared the Madelung energies for the unit cells of (La9.32Ba0.28)(Si5.87□0.13)O26 structures with different M2 site compositions. The Madelung energy steadily decreased as the Ba/(Ba + La) value increased in the M2 site from 0 to 0.0467 (Figure S5). We specified the crystal structure with the lowest energy in which all of the Ba atoms favorably occupied the M2 site. The structural formula was therefore expressed by (La 3.60 ) M1 (La 5.72 Ba 0.28 ) M2 (Si 5.87 □ 0.13 )O 26 . Although the Madelung energy principally contributes to the lattice energies of ionic crystals,28 this value has been

Wyckoff position

Figure 4. Structural model of (La9.32Ba0.28)(Si5.87□0.13)O26. The displacement ellipsoids are plotted at the 75% probability level. The numbering of atoms corresponds to that given in Table 1.

site

Table 1. Structural Parameters and Atomic Displacement Parameters (×10−2 nm2) for (La9.32Ba0.28)(Si5.87□0.13)O26a

U12

those given in our previous studies.14,27 The chemical composition of the crystal examined was fixed at 9.32:0.28:5.87:26 La:Ba:Si:O in molar ratios. Because it is impossible to distinguish between La and Ba using XRD, we used a virtual chemical species named M composed of 97.08 mol % La and 2.92 mol % Ba throughout the refinement process. The crystal structure consisted of two M sites of M1 (Wyckoff position 4f) and M2 (6h) one Si site and four O sites (Figure 4). The M1 site was partially occupied, while the M2

1/3 0.23921(3) 0.02984(11) 0.1221(3) 0.4858(4) 0.0903(3) 0

Figure 3. X-ray diffraction pattern from the surface of grain-aligned (La9.32Ba0.28)(Si5.87□0.13)O26 electrolyte.

0.9 1 0.9783 1 1 1 1

U13

U23

Ueq

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DOI: 10.1021/acs.cgd.6b00638 Cryst. Growth Des. 2016, 16, 4519−4525

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( f 00l = 0.79),17 the excellent performance of oxide-ion conductivity for the (La9.32Ba0.28)(Si5.87□0.13)O26 electrolyte must be substantially induced by the effect of BaO-doping into the M sites. Accordingly, it might be of great importance to clarify the site preference of Ba to more deeply understand the enhancement mechanism of conductivity in LSO. When the σ b -values along the c-axes between La9.50(Si5.87□0.13)O26 and La9.33Si6O26 at the same temperatures are compared (Figure 5), the σb-values were necessarily much higher for the former than for the latter. This implies that the Si deficiency would effectively increase the σb-values, although the detailed mechanism of conductivity enhancement is still uncertain. The crystal structure of LBSO prepared in the present study showed the Si deficiency, which would also contribute to the increment of the σb-value. LBSO Crystals with Relatively High La/Si Ratios. At the original interfacial contact boundaries of x/μm = 306 and x/μm = 798 in Figure 2, we found two upward projections of La data points and two downward projections of Si data points for the corresponding concentration line profiles. The local maximum La/Si-values were 1.73 at x/μm = 306 and 1.70 at x/μm = 798. There are two plausible explanations for this anomaly: parts of the starting material of La2SiO5 (La/Si = 2) remained unreacted and coexisted with LBSO, and/or the LBSO crystal grains with the relatively high La/Si ratios actually formed and locally distributed at the original interfacial contact boundaries. We collected a set of micro-Raman spectra (Figure S7) from the sampling points near the original interfacial contact boundary (Figure S8) and confirmed that they showed no Raman bands attributable to La2SiO5. We further investigated these regions, including the grain-boundary areas between the LBSO crystallites, using HR-TEM and tried to find out the secondary phases other than LBSO (Figure S9). However, there were no impurity phases in either the intracrystalline or intercrystalline regions (Figure S10), and hence we concluded that the crystallites near the contact boundaries were exclusively composed of LBSO. Using EPMA, we found the LBSO crystal with the highest La/Si value of 1.73, the probable chemical formula of which is (La9.75Ba0.11)Si5.63O26 ((La + Ba)/Si = 1.75). We selected, under STEM, one LBSO crystal grain that was located in nearly the same place at the original interfacial contact boundary and obtained the ADF-STEM image viewed along the a-axis (Figure 6). The inset is the simulated image, the crystallographic and structural data of which were taken from those of the present (La9.32Ba0.28)(Si5.87□0.13)O26 as the representative of the BaOdoped LSO structure. The distributions of black and white contrast of the simulated image were in good accordance with those of the observed one. Thus, the crystals of apatite structure with relatively high La/Si ratios must be actually formed at the contact boundaries. Because the σb-value of LSO showed a tendency to increase with the La/Si ratio,32 the LBSO with a relatively high La/Si ratio is of interest and should be investigated further concerning not only the formation mechanism but also the conductivity. Growth Process of Grain-Oriented Polycrystalline LBSO. The crystallites of LBSO located at the original interfacial contact boundaries between Sample A and Sample B must be formed at the initial stage of the reactive diffusion. After the formation of these crystallites, the prismatic crystals of LBSO, elongated along the c-axes, would subsequently grow toward the center of the Sample B region. This reaction would be accompanied by the one-dimensional diffusion of SiO2 and

successfully used as the stability indicator of covalent crystals such as oxynitrides.29,30 A more detailed discussion on the site preference of Ba should be made based on, for example, the density functional theory,31 which enables us to compute the ground state energy in various structural models. The reliable structural parameters that are indispensable for the calculation are now available for this compound. Oxide-Ion Conductivity. We determined the bulk oxideion conductivity (σb) by extracting the bulk contribution from the impedance spectra at 723−1023 K using a nonlinear leastsquares fitting method (Figure S6). The equivalent circuit used was constructed by the three elements of bulk, grain boundary (gb), and electrode, which are connected in series. As the temperature increased, the σb-value steadily increased from 2.17 × 10−2 S cm−1 at 723 K to 1.42 × 10−1 S cm−1 at 1023 K (Figure 5). The activation energy of conduction was 0.475(9)

Figure 5. Comparison in oxide-ion conductivity of c-axis-oriented polycrystalline apatite (ZrO 2 ) 0 . 9 1 (Y 2 O 3 ) 0 . 0 9 (YSZ) 2 and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM).5 The conductivity and crystal structure of (La9.32Ba0.28)(Si5.87□0.13)O26 was determined in the present study, and those of La9.50(Si5.87□0.13)O26,14 La9.50Si6O26.25,15 La9.33Si6O26,16 and La9.33(Si0.87Ge0.13)6O2617 were reported in previous studies. A red cross represents the conductivity of randomly grainoriented polycrystalline La9BaSi6O26.5 at 973 K.12

eV. The σb-values were 6.26 × 10−2 S cm−1 at 873 K and 3.36 × 10−2 S cm−1 at 773 K, both of which were much higher than those of (ZrO2)0.91(Y2O3)0.09 (YSZ)2 and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM).5 The total (bulk + gb) conductivity steadily increased from about 2.6 × 10−3 to about 1.2 × 10−1 S cm−1 as the temperature increased from 723 to 1023 K. The σb-values of the present polycrystal at 723−1023 K were, when compared at the same temperature, necessarily superior to those of the grain-aligned LSO-based polycrystals prepared by the reactive diffusion technique. Because the present orientation degree along the c-axis (f 00l = 0.58) was not significantly high compared to those of the previous specimens of La9.50(Si5.87□0.13)O26 ( f 00l = 0.90),14 La9.50Si6O26.25 (f 00l = 0.84),15 La9.33Si6O26 ( f 00l = 0.81),16 and La9.33(Si0.87Ge0.13)6O26 4523

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were relatively low in volume fraction (1.04 vol %), they could eventually cause the inferior orientation degree of the resulting polycrystal. It may be possible to further improve the conductivity if we successfully modified the reactive diffusion technique to eventually increase the degree of orientation of the polycrystalline electrolyte.

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CONCLUSION We successfully synthesized the c-axis-aligned polycrystal of (La9.32Ba0.28)(Si5.87□0.13)O26 by the reactive diffusion between La2SiO5 and BaO-doped La2Si2O7 + BaO-doped LSO at 1873 K for 100 h. Due to the combined effect of BaO doping and grain alignment, the resulting polycrystalline electrolyte showed conductivity superior to that of the other grain-aligned LSObased polycrystals; the present polycrystalline material showed an extremely enhanced oxide-ion conductivity along the grainalignment direction ranging from 2.17 × 10−2 S cm−1 at 723 K to 1.42 × 10−1 S cm−1 at 1023 K with the activation energy of conduction being 0.48 eV. Extraction of the SiO2 component from the BaO-doped La2Si2O7 crystals principally induced the oriented growth of the (La9.32Ba0.28)(Si5.87□0.13)O26 crystals. The coexisting BaO-doped LSO crystals played an important role as the main BaO-component supplier; however, they could cause the inferior degree of orientation of the polycrystalline material during the crystal growth process. We would be able to further improve the conductivity under the condition that the orientation degree is sufficiently increased.

Figure 6. ADF-STEM image taken along the [100]-zone axis of apatite structure. Inset is the simulated image of the (La9.32Ba0.28)(Si5.87□0.13)O26 crystal structure (convergence semiangle = 20 mrad, detector semiangle = 68−280 mrad, and specimen thickness = 9.7 nm).

BaO components, both of which were released from the Sample B region along the normal direction to the original Sample A/Sample B boundaries. Thus, there must be an incoming vacancy flux into the innermost LBSO layer (former Sample B region) to compensate for the unequal material flow. Actually, this layer showed a relatively high concentration of intracrystalline minute voids (probably Kirkendall voids33) compared with that of the former Sample A region (Figure 1a).15 In a previous study, the growth rate of the grain-oriented LSO polycrystal formed in the binary diffusion couple of La2Si2O7/La2SiO5 was found to be controlled by the volume diffusion of SiO2.14 Hence, the main determinant of the oriented growth rate of the present LBSO polycrystal would also be the one-dimensional diffusion of the SiO2 component. As a result, the smooth material flow of this component would be essential to the highly oriented growth processes of both polycrystals of LSO and LBSO. In previous studies on the preparation of LSO-based grainaligned electrolytes,14−16 we annealed the sandwich-type binary diffusion couples (e.g., La2SiO5/La2Si2O7/La2SiO5), the end members of which were necessarily composed of single-phase materials, and eventually obtained the polycrystals with the f 00l values higher than 0.79. On the other hand, the present end member of Sample B was made up of the two phases of BaOdoped La2Si2O7 (98.92 mol %) and BaO-doped LSO (1.08 mol %). The formation of grain-aligned LBSO polycrystal in the ternary diffusion couple of Sample A/Sample B/Sample A would be principally induced by the one-dimensional material flow of the SiO2 component from the BaO-doped La2Si2O7 crystals in Sample B into the La2SiO5 crystals in Sample A. The small amount of coexisting BaO-doped LSO crystals, which were oriented randomly and dispersed uniformly in [Sample B], would be valid only as the main BaO component supplier. Accordingly, the presence of these crystal grains would act as the heterogeneous nucleation site for the randomly oriented LBSO crystals and also obstruct the smooth material flow during the oriented growth process. Although the obstacles

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00638. CIF, phase composition of Sample B, and additional tables and figures (PDF) Accession Codes

CCDC 1461673 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research (15K14125) from the Japan Society for the Promotion of Science. Thanks are due to Ms. M. Okabe and Mr. H. Banno, Nagoya Institute of Technology, for their technical assistance in TEM.

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REFERENCES

(1) Ormerod, R. M. Chem. Soc. Rev. 2003, 32, 17−28. (2) Choudhary, C. B.; Maiti, H. S.; Subbarao, E. C. In Solid Electrolytes and Their Applications; Subbrao, E. C., Ed.; Plenum Press: New York, 1980; pp 1−80. (3) Brett, D. J. L.; Atkinson, A.; Brandon, N. P.; Skinner, S. J. Chem. Soc. Rev. 2008, 37, 1568−1578. 4524

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Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.6b00638 Cryst. Growth Des. 2016, 16, 4519−4525