Crystal Structure and Oxide-Ion Conductivity of Highly Grain-Aligned

Article pubs.acs.org/crystal

Crystal Structure and Oxide-Ion Conductivity of Highly Grain-Aligned Polycrystalline Lanthanum Germanate Oxyapatite Grown by Reactive Diffusion between Solid La2GeO5 and Gases [GeO + 1/2O2] Koichiro Fukuda,*,† Toru Asaka,† Shinji Hara,† Abid Berghout,‡ Emilie Béchade,‡ Olivier Masson,‡ Jenny Jouin,‡ and Philippe Thomas‡ †

Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan Science des Procédés Céramiques et de Traitements de Surface (SPCTS), UMR 7315 CNRS, Université de Limoges, Centre Européen de la Céramique, 12 Rue Atlantis, 87068 Limoges Cedex, France

‡

S Supporting Information *

ABSTRACT: We prepared the highly c*-axis-aligned polycrystalline lanthanum germanate oxyapatite, La9.68Ge6O26.52, by the reactive diffusion that occurs between a randomly grainoriented La2GeO5 polycrystal and [GeO + 1/2O2] gases at 1723 K in air. The resulting La9.68Ge6O26.52 polycrystal was examined by optical microscopy, X-ray diffractometry, microRaman spectroscopy, and impedance spectroscopy. The crystal structure (space group P1) showed the appreciable positional disordering of 9 of the 24 O atoms, bonding to Ge atoms, in the unit cell. We have found an extra O site, with the occupation factor of 0.51, which was located close to one of the six Ge sites with the Ge−O distance of ∼0.198 nm. The Raman extra band at 649 cm−1 also suggested the existence of the five coordinate Ge atoms. The conductivity along the c* axis of oxide ions steadily increased from 6.3 × 10−7 to 1.04 × 10−2 S/cm as the temperature increased from 573 to 973 K. The activation energy of conduction was 1.2 eV.

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regions.6 The LSO crystals of the former La2SiO5 region are grown by the reaction of La2SiO5 with SiO2 component, the latter of which is released from La2Si2O7. The LSO crystals of the former La2Si2O7 region are generated by the loss of the SiO2 component from La2Si2O7; the contribution of in-coming La2O3 flux, which crosses the original contact boundary from the La2SiO5 region, is negligible. The chemical reaction processes of these unbalanced diffusion of SiO2 component are described by7,11

INTRODUCTION The sintered polycrystalline materials (e.g., ceramics) that are formed by the conventional sintering methods normally possess a randomly grain-aligned structure. Such materials generally show isotropic mechanical (e.g., fracture toughness and bending strength) and physical (e.g., magnetic, pyroelectric, piezoelectric, and thermoelectric) properties. On the other hand, the ceramics with grain-aligned microtextures can demonstrate anisotropic properties. One of the most common texturing methods would be of the templated grain growth process,1,2 and the others would comprise hot pressing, applying magnetic field, and utilizing centrifugal force.3−5 However, these manufacturing processes are quite complicated and also the texture fraction of the grain-aligned ceramics is not necessarily satisfactory. Recently, the highly c-axis-oriented polycrystalline lanthanum silicate oxyapatite (LSO) has been successfully prepared by the solid-state reactive diffusion between La2SiO5 and La2Si2O7.6−11 The LSO is thermodynamically stable, in the binary system La2O3−SiO2, between the intermediate compounds of La2SiO5 and La2Si2O7. The grain-aligned polycrystals of LSO have been readily formed as layers at the original contact boundaries of the La2SiO5/La2Si2O7 diffusion couples after the heat treatment between 1773 and 1873 K for 5−100 h. The product LSO layer was separated by the original boundary into two adjacent © 2015 American Chemical Society

(14 + 3x)La 2SiO5 + (4 − 3x)SiO2 → 3La 9.33 + 2xSi6O26 + 3x for La 2SiO5 side

(1)

and (14 + 3x)La 2Si 2O7 − (10 + 6x)SiO2 → 3La 9.33 + 2xSi6O26 + 3x for La 2Si 2O7 side

(2)

The general formula of the product LSO is applicable to describe the La2O3-excess chemical composition, the amount of which is expressed by the x value. Received: April 14, 2015 Revised: May 31, 2015 Published: June 17, 2015 3435

DOI: 10.1021/acs.cgd.5b00509 Cryst. Growth Des. 2015, 15, 3435−3441

Crystal Growth & Design

Article

from calculation to be 1.3 × 10−3 MPa at 1723 K (Figure S2, Supporting Information). The formation reaction of LGO substantially occurred between the polycrystalline La2GeO5(s) and [GeO(g) + 1/ 2O2(g)]. Optical Microscopy and Micro-Raman Spectroscopy. The outer shape of the disk-shaped specimens was, before and after the reactive diffusion, almost unchanged. One of the reacted specimens was cut perpendicular to its flat surface with a diamond saw to expose the cross section, which was polished with 1 μm diamond paste and subsequently made into a thin section. The microtexture was observed using a polarizing microscope under crossed polars. We collected the micro-Raman spectra, ranging from 600 to 900 cm−1, in the backscattering geometry (resolution = 1 cm−1) using a double grating monochromator (model NRS-2000, JASCO Co., Tokyo, Japan). The green line at 514.5 nm of an argon-ion laser was used for the excitation of Raman spectra on the thin-section surface. The laser beam had the spot size of ϕ5 μm with the objectivelens magnification of 20 times, and the incident power was 10 mW. Orientation Degree. We collected the X-ray profile intensity data from the flat surface of the disk-shaped specimen using a diffractometer (X’Pert PRO Alpha-1, PANalytical B.V., Almelo, The Netherlands) in the range of 20.0° ≤ 2θ (CuKα1) ≤ 62.0° (2514 total data points). A programmable divergence slit was employed to maintain the illuminated length to be ∼5 mm on the specimen surface. We estimated the texture fraction of {0 0 l}apatite planes from the Lotgering factor f 00l,28 which is defined by

Lanthanum germanate oxyapatite (LGO) materials have stirred up great interest for their potential electrolytes for SOFC applications since they show relatively high conductivity (σ) of oxide ions at intermediate temperatures.12−27 There are a number of structural studies for LGO, La9.33+2yGe6O26+3y, with different amounts of excess La2O3 component (= y). LeónReina et al.14 have reported the two phases with different space groups in LGO: one is P1̅ for 9.66 ≤ 9.33 + 2y ≤ 9.75 (0.165 ≤ y ≤ 0.21), and another one is P63/m for 9.52 ≤ 9.33 + 2y ≤ 9.60 (0.095 ≤ y ≤ 0.135). Actually, the crystal structures of LGO having the relatively high concentrations of La2O3 such as La10Ge6O27 (y = 0.33),20 La9.92Ge6O26.88 (y = 0.29),22 La9.75Ge6O26.62 (y = 0.21),14 and La9.64Ge6O26.46 (y = 0.15)22 have been determined with the space group P1.̅ On the other hand, the space group of the structures has been P63/m for La9.60Ge6O26.40 (y = 0.13)15 and La9.33Ge6O26 (y = 0),13,27 the La2O3 concentrations of which are relatively low. Nakayama and Sakamoto 27 have prepared the single crystal of La9.33Ge6O26. They have determined the σ values parallel to and perpendicular to the c axis at 573−1073 K to demonstrate the isotropy of conduction. The present study reports the successful preparation of a highly grain-aligned LGO polycrystal by the reactive diffusion between solid La2GeO5 and gases [GeO + 1/2O2]. One of the most remarkable features of this grain-alignment method is that the La2GeO5 substrate is composed of the randomly grainoriented polycrystal.

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f00l = (P00l − P0)/(1 − P0) where

EXPERIMENTAL SECTION

P00l = (ΣI00l)/(ΣIhkl)

Materials. A powder specimen with the chemical formula of La2GeO5 was prepared from reagent-grade chemicals of La2O3 (99.99%, Mitsuwa Chemicals Co. Ltd., Osaka, Japan) and GeO2 (99.999%, Mitsuwa Chemicals Co. Ltd., Osaka, Japan). We precalcined the hygroscopic La2O3 reagent at 1473 K for 10 min to remove La(OH)3 and/or La2O2(CO3) phases and obtained the proper amount of La2O3. The well-mixed chemical with the desired chemical composition was heated at 1873 K for 3 h, and then quenched in air. The reaction product was a slightly sintered polycrystalline material, which was ground to obtain the fine powder specimen. The sinterability of La2GeO5 powder was poor under ambient pressure; hence, we used the spark plasma sintering system (model SPS-2040, Sumitomo Coal Mining Co., Ltd., Tokyo, Japan) to obtain the densely sintered polycrystalline material. About 5 g of the powder specimen was charged into a graphite die of ϕ15 mm, compressed by a press mechanism, and heated by application of a pulsed current. The sample was heated at ∼150 K/min to 1773 K, annealed at that temperature for 10 min, and then cooled to ambient temperature. To avoid crack formation in the densely sintered specimen, the pressure was immediately released at the beginning of the cooling process. We eventually obtained the columnar-shaped sintered material (ϕ15 mm × 5 mm thickness), which was exclusively composed of the randomly grain-oriented La2GeO5 polycrystal with the crystal-grain size of ∼5 μm. The relative density (measured density over theoretical density) was ∼98%. We cut the columnar sample in round slices using a diamond saw to prepare several pieces of disk-shaped specimens (ϕ15 mm × ∼0.77 mm thickness). They were subsequently polished with 1 μm diamond paste to eventually obtain the circular thin plates with ϕ15 mm × ∼0.40 mm thickness. Reactive Diffusion between Solid and Gas. We covered the bottom of a Pt crucible with GeO2 powder and then suspended, using a Pt wire, one of the disk-shaped La2GeO5 specimens between the bottom and opening of the crucible (Figure S1, Supporting Information). We subsequently heated the whole assembly at 1723 K for 10 h in air. At that temperature, the GeO2 powder was readily melted to generate [GeO + 1/2O2] gases; the vapor pressure of GeO(g) under the atmosphere of P(O2) = 0.021 MPa was determined

and

P0 = (ΣI00l 0)/(ΣIhkl 0) where Ihkl and Ihkl0 are the integrated intensities of hkl reflections for the textured specimen and the randomly oriented one, respectively. We extracted the Ihkl values from the whole X-ray diffraction (XRD) pattern by the Le Bail method29 using the program RIETAN-FP.30 The Ihkl0 values for the simulated XRD pattern with random grain orientation were also generated using the same computer program. Impedance Spectroscopy. Electrodes were prepared by coating opposite sample faces with a platinum paste and subsequent heating at 1273 K to harden the Pt residue. We collected the impedance spectroscopy data in the range of 4−1 M Hz during the heating process in air at 573−973 K using an impedance analyzer (model 3532-80, HIOKI E. E. Co., Nagano, Japan). A nonlinear least-squares fitting method using ZView software was employed to analyze the equivalent circuits.31 Single-Crystal X-ray Diffractometry. A rectangular microcrystal (∼40 μm × ∼10 μm × ∼10 μm) was selected from the crushed specimen and attached on the end of a glass capillary. We collected the single-crystal XRD data, up to the maximum 2θ value of 73.94°, on a Bruker Smart Apex II Ultra diffractometer using MoKα radiation (50 kV and 50 mA). We determined the unit-cell parameters and obtained the observed structure factors using the program package Apex2W2K/NT.32 The initial structural model was derived by the charge flipping method.33 The structural parameters and the atomic displacement parameters (ADPs) were refined by the computer program SHELXL-97.34 The validity of the refined structural model was verified by the difference Fourier maps using the JANA2006 program.35 The 3D difference electron-density distributions and balland-stick models were drawn by the computer program VESTA.36 The crystal information and the parameters for data collection and refinement are summarized in Table S1 (see the Supporting Information). 3436



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RESULTS AND DISCUSSION Highly Grain-Oriented LGO Polycrystal. The optical microscopic observation revealed that the sample consisted of prismatic crystallites (Figure 1). We obtained the Raman

(50.03°) for randomly grain-oriented LGO crystallites (Figure 3a), the structural parameters of which were taken from those

Figure 1. Polarizing-microscope images of the crystal-oriented sample (thin section) after the reactive diffusion between La2GeO5(s) and [GeO(g) + 1/2O2(g)]. The white arrows P and A indicate the vibrational orientations of, respectively, polarizer and analyzer of the microscope. Most of the La9.68Ge6O26.52 crystallites are diagonal position in (a) and extinction position in (b).

Figure 3. X-ray diffraction patterns of La9.68Ge6O26.52 polycrystals. (a) Simulated profile for randomly oriented polycrystal. (b) Observed profile intensities (red symbol: +) collected on the sample surface, the fitted pattern (green solid line), and the difference curve (blue solid line in lower part of the diagram). The positions of possible Bragg reflections are indicated by vertical bars. The very weak extra reflections with 25° ≤ 2θ ≤ 30° are of the trace amount of La2Ge2O7 polycrystal, which is formed by the reaction between LGO and [GeO + 1/2O2].

spectra from the various regions on the sample to find that it was composed almost exclusively of the LGO crystallites (Figure 2). It is noteworthy that these LGO crystallites are

determined by the single-crystal XRD as mentioned below. The experimental XRD pattern, which was obtained from the sample surface, showed an intensity enhancement of 00l (l = 2 and 4) reflections (Figure 3b). The texture fraction of {0 0 l}apatite planes (i.e., orientation degree along the reciprocal c* axis) was determined to be f 00l = 0.70. Thus, we have confirmed that the resulting LGO polycrystal consists of highly c*-axisoriented crystallites, with the orientation directions being almost normal to the sample surface. The very weak extra reflections with 25° ≤ 2θ ≤ 30° are of the trace amount of La2Ge2O737 polycrystal, which must be formed by the reaction between LGO and [GeO + 1/2O2] on the sample surface. The micro-Raman spectrum showed the overlapping bands ranging from 690 to 800 cm−1, together with a weak band at 649 cm−1 (Figure 2). The spectral decomposition of the former bands revealed that they were made up of five components, probably corresponding to at least five different environments of the [GeO4]4− tetrahedra (Figure S3 and Table S2, Supporting Information).19 The extra band at 649 cm−1 is attributable to the existence of oxide ions at the interstitial sites, which lead to the formation of five coordinate Ge.24 The existence of the [GeO5]6− polyhedra as well as the different environments of the [GeO4]4− tetrahedra will be demonstrated by the structural model as determined below. We obtained the

Figure 2. Micro-Raman spectra from La9.68Ge6O26.52 formed by the reactive diffusion between La2GeO5(s) and [GeO(g) + 1/2O2(g)].

mostly at the extinction position under crossed polars (Figure 1b), which gives evidence that their elongation directions are nearly perpendicular to the flat surfaces of the disk-shaped specimen. The simulated XRD pattern shows the intense reflections with 002 (2θ ≈ 24.41°), 21̅1̅ (30.05°), 131 (30.02°), and 004 3437



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spectroscopy data from the various points on the polished LGO sample in Figure 1 and found that the constituent LGO crystal grains were homogeneous. Thus, each of the crystal grains was probably saturated with the GeO2 component at 1723 K. Structure Determination and Description. The LGO single crystal demonstrated the intensity distributions that are consistent with the Laue symmetry 1̅. Thus, there will be two possible space groups of P1̅ (centrosymmetric) and P1 (noncentrosymmetric). We first examined the possibility of the centrosymmetric model. However, this attempt was entirely unsuccessful, because the relevant structural model necessarily contained the split sites for all cations, with the significant remaining electron-density distributions found on the difference Fourier maps. Hence, we have adopted the noncentrosymmetric structural model. There are 42 independent sites (Wyckoff positions 1a) in the unit cell of the initial structural model; there are 10 La sites (labeled from La1 to La10), 6 Ge sites (Ge1−Ge6), and 26 O sites (O1−O26). We have fixed one of the O sites (O12) at the coordinate (0, 0, 1/4) for easy comparison among the structural models of apatite-type crystals (space group P63/ m) that are proposed in literature.13,15,27 At the early stages of the refinement process, the site occupancies (g) were separately refined without any constraints for La and Ge. The g(La1) and g(La2) values remained less than unity; hence, we necessarily refined these values in successive least-squares cycles. Since we found the relatively high residual electron densities nearby the nine Oi sites with i = 18−26, we split each of these sites into two, OiA and OiB. The distribution of O atoms between them was determined by imposing the linear constraints on occupancies; g(OiA) + g(OiB) = 1. After the refinement based on the anisotropic ADPs for all cations, the Gram− Charlier expansion up to the third-rank tensor was applied to those of the La1−La4 sites in order to assess the anharmonicity.38 The similar anharmonic refinements were also performed for the La sites of LSO in our previous studies.10,11 The resulting difference Fourier maps still showed a significant residual electron density peak nearby the Ge1 site. Thus, we have added an extra O site (O27) and continued the refinement based on the modified split-atom model, which finally converged to the reliability (R) indices of R = 0.0298 and wR = 0.0333. The O27 site was partially occupied with g(O27) = 0.513(7). This implies that ∼51% of Ge atoms occupying at the Ge1 site is five coordinated with the Ge1−O27 distance being ∼0.198 nm, which is in accord with the finding by microRaman spectroscopy. Figure 4 shows the refined crystal structure represented by the ball-and-stick model. The crystallographic data are summarized in Table 1. The refinement resulted in the chemical formula of La9.675(6)Ge6O26.513(7). The crystal structure is characterized by the substantial disordering of atomic positions for O18−O26 that are bonded to Ge atoms. The structural parameters and isotropic and anisotropic ADPs are given in Table S3 (Supporting Information). The anharmonic ADPs of La1−La4 are given in Table S4 (Supporting Information). The distances between the selected atoms are listed in Table S5 (Supporting Information) with their standard deviations. The average Ge−O bond lengths of [GeO4]4− tetrahedra ranged from 0.172 to 0.179 nm. The Ge1 atom that is 5-fold coordinated by O2, O17, O18, O25, and O27 atoms forms a distorted trigonal bipyramid with the mean bond length of 0.180 nm. These distances are in accord with those predicted by the bond valence sum (0.1748 nm for the former

Figure 4. Crystal structure of La9.68Ge6O26.52. The occupancies of parts of La and O sites are less than unity; hence, these atoms are depicted as circle graphs for occupancies. Atom numbering corresponds to that in Table S3 (Supporting Information). The atomic coordinate of O12 is fixed at (0, 0, 1/4).

Table 1. Crystallographic Data for La9.68Ge6O26.52 La9.675(6)Ge6O26.513(7) space group a/nm b/nm c/nm α/deg β/deg γ/deg V/nm3 Z Dx/Mg m−3

P1 0.98949(7) 0.99056(7) 0.72865(5) 89.46(1) 89.944(1) 60.09(1) 0.61903(10) 1 5.912

environment and 0.1831 nm for the latter).39,40 The cation sites of La1, La5, La7, La8, and La10 are 8-fold coordinated, and those of La2, La3, and La4 are 9-fold coordinated, although La2 and La5 sites are, respectively, 8-fold and 7-fold coordinated when excluding the O27 atom from the ligands. The two remaining La sites (La6 and La9) are 7-fold coordinated. Since the O18−O26 atoms are positionally disordered, and also the La1 and La2 sites are to some extent deficient, we have evaluated the bond lengths by the charge distribution method.41,42 In this method, the ratio of the formal oxidation number (q) to the computed charge (Q) of cation (= (q/ Q)cation) indicates the correctness of the structure determination. In the present LGO, these ratios of the La sites ranged from 0.929 (La1) to 1.135 (La10) and those of Ge varied from 0.958 (Ge5) to 1.093 (Ge3) (Table S6, Supporting Information). Since all of these ratios are close to unity, we have concluded that the final structural model is satisfactory. The present split-atom model (space group P1) with the interstitial oxide-ion site at around (0.472, 0.569, 0.524) has been, as far as the authors know, used for the first time for LGO. Oxide-Ion Conductivity. We constructed the equivalent circuit, which was modeled in parallel with the grain-alignment direction. The circuit consisted of three elements of bulk (b), grain boundary (gb), and electrode (e) that are connected in series as (RbQb)(RgbQgb)(ReQe), where the resistance R is 3438



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temperature examined, the linearity of the Arrhenius plot of σb (Figure 6), and hence determined the activation energy of

parallel to the constant phase element Q. The capacitance (C) values were estimated by

C = Q 0(ωmax )n − 1 where Q0 is numerically equal to the admittance (1/|Z|) at ω = 1 rad/s and the imaginary part of the impedance reaches a maximum at the frequency of ωmax.43 The C values are generally 10−11−10−8 F/cm for Cgb and 10−7−10−5 F/cm for Ce.44 Each of the impedance diagrams at 573−673 K (Figure 5a and Figure S4, Supporting Information) exhibits two semi-

Figure 6. Comparison of oxide-ion conductivity of lanthanum germanate oxyapatite materials. The conductivity of the present specimen La9.68Ge6O26.52 and those of La9.33Ge6O26 determined by Nakayama and Sakamoto.27

conduction (= Ea) to be 1.20(3) eV. With T > 773 K, both σb and Ea values of the present La9.68Ge6O26.52 were slightly higher than those parallel to and perpendicular to the c axis of La9.33Ge6O26 (space group P63/m)27 and vice versa with T < 773 K. Because the unit cell of La9.68Ge6O26.52 is nearly hexagonal (α ≈ 89.46°, β ≈ 89.94°, and γ ≈ 60.09°), its c axis is almost parallel to the reciprocal c* axis, which corresponds to the grain-alignment direction. Although these two materials have distinct crystal structures regarding space groups and excess amounts of La2O3 component, they do not show a distinctive difference of σ values along their c axes. This implies that the interstitial oxide ions would not play an important role for the conduction along the c axis of LGO. The conduction mechanism of LGO is still uncertain, and hence, it should be investigated further. Formation Mechanism of Grain-Oriented LGO Polycrystal. For the solid-state reactive diffusion between La2SiO5 and La2Si2O7, the highly c-axis-oriented LSO polycrystal in the former La2SiO5 region has been formed by the reaction of La2SiO5 with the SiO2 component, as shown in eq 1. The present reactive diffusion between La2GeO5(s) and [GeO(g) + 1/2O2(g)] would proceed basically in the same mechanism, the chemical reaction of which is described by

Figure 5. Nyquist plots of La9.68Ge6O26.52 electrolyte. Data collected at (a) 623 K and (b) 973 K. The equivalent circuits composed of the three elements corresponding to bulk, grain boundary (gb), and electrode.

circles. The observed arc at the higher frequency can be assigned to the bulk response, and another one at the lower frequency corresponds to the contribution of RgbQgb element with Cgb = 5−8 × 10−8 F/cm. The deconvolution of different contributions and the final fitting results are displayed in red dashed-line semicircles and blue solid lines, respectively. With increasing temperature to 973 K, the grain-boundary semicircle progressively became smaller and eventually disappeared at 973 K (Figure S4, Supporting Information). Thus, we adopted the Qb-free equivalent circuit of (Rb)(RgbQgb)(ReQe) with 823 K ≤ T ≤ 923 K. The impedance response at 973 K (Figure 5b) consists solely of a semicircle due to the electrode/electrolyte interfaces (Ce = 1.0 × 10−5 F/cm). The present Cgb and Ce values agreed well with those of the LSO polycrystals and ceramics.6,7,9−11,44 The Rb values were determined from the impedance spectroscopy data. The bulk conductivity (σb) increased steadily from 6.3 × 10−7 to 1.04 × 10−2 S/cm by the increase of temperature from 573 to 973 K. We found, over the

(14 + 3y)La 2GeO5(s) + (4 − 3y)[GeO(g) + 1/2O2 (g)] → 3La 9.33 + 2yGe6O26 + 3y (s)

(3)

where the y value represents the excess amount of the La2O3 component in the resulting LGO. At the initial stage of the reaction, the LGO crystals would start to grow on the surfaces of La2GeO5 specimen. The reaction would proceed by the diffusion of the GeO2 component toward the central part of the specimen. The relevant material flows along the perpendicular directions with respect to the specimen surfaces would be closely related to the oriented growth of the prismatic LGO crystallites. In the present study, the La2GeO5 polycrystal was 3439



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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research (No. 25630281) from the Japan Society for the Promotion of Science. We thank Mr. H. Oka, Nagoya Institute of Technology, for technical assistance.

completely converted to the LGO polycrystal during heating at 1723 K for 10 h in air due to the sufficient supplementation of the GeO2 component. The resulting LGO contained the excess La2O3 component, the amount of which was determined by the single-crystal XRD to be y ≈ 0.17 in eq 3. We determined by calculation the unit-volume change on conversion from La2GeO545 to LGO, based on their unit-cell volumes and eq 3 with y = 0.17. Because the unit volume increases by 5.2%, the densification would proceed on forming the grain-aligned LGO polycrystal. The interaction between the LGO and the [GeO + 1/2O2] gas occurred on further heating to form the trace amount of La2Ge2O7 polycrystal on the sample surface (Figure 3b). In our previous studies, we reported the formation of a highly c-axis-aligned polycrystalline LSO by the solid-state reactive diffusion. The present study reports, for the first time, the successful preparation of the grain-aligned polycrystalline material, using the same technique, between solid and gases. The authors think that one of the most significant implications of this research is that we successfully extended the reactive diffusion system from solid−solid to those including solid− gases in the preparation of grain-aligned ceramics. The present solid−gas interaction technique could be widely applicable to the other grain-aligned materials.

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ASSOCIATED CONTENT

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REFERENCES

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We succeeded in the synthesis of a highly c*-axis-oriented polycrystal of LGO by the reactive diffusion between La2GeO5(s) and [GeO(g) + 1/2O2(g)] at 1723 K in air. The formation reaction was described by (14 + 3y)La2GeO5 + (4 − 3y)[GeO + 1/2O2] → 3La9.33+2yGe6O26+3y, where y represents the excess amount of the La2O3 component in the resulting LGO. The polycrystal with y ≈ 0.17 (La9.68Ge6O26.52) showed the conductivity of oxide ions ranging from 6.3 × 10−7 S/cm at 573 K to 1.04 × 10−2 S/cm at 973 K, with the empirical activation energy of 1.2 eV. The crystal structure (space group P1) demonstrated the disordering of atomic positions of oxygen that are bonded to Ge atoms, together with an extra O site in the unit cell. The formation of five coordinate Ge was also confirmed by the Raman extra band at 649 cm−1. The material flow of the GeO2 component along the perpendicular direction to the former La2GeO5 specimen surface would be closely related to the oriented growth of the prismatic LGO crystallites.

S Supporting Information *

CIF, tables (data collection and refinement parameters for single-crystal XRD, fitted parameters for Raman spectroscopy, structural parameters, atomic displacement parameters, and selected interatomic distances) and figures (schematic drawing of the sample assembly, vapor pressure of GeO gas, Raman spectra, and a series of Nyquist plots).The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00509.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3440



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Crystal Structure and Oxide-Ion Conductivity of Highly Grain-Aligned

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