Discovery of Oxide-Ion Conductors with a New Crystal Structure

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Discovery of Oxide-Ion Conductors with a New Crystal Structure, BaSc ASiO (A: Mg, Ca) by Screening Sc-Containing Oxides through the Bond-Valence Method and Experiments 2-x

x

3

10-x/2

Eiki Niwa, and Masatomo Yashima ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00701 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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ACS Applied Energy Materials

Discovery of Oxide-Ion Conductors with a New Crystal Structure, BaSc2-xAxSi3O10-x/2 (A: Mg, Ca) by Screening Sc-Containing Oxides through the Bond-Valence Method and Experiments Eiki Niwa and Masatomo Yashima* Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-ku, Tokyo, 1528551, Japan

Corresponding Author *E-mail [email protected]

(M. Y.)

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KEYWORDS : oxide-ion conductor, crystal structure, bond-valence method, electrical conductivity, barium scandium silicate, ionic conduction, X-ray diffraction, band gap

ABSTRACT We have discovered oxide-ion conductors with a new crystal structure by combining the bond-valence (BV) method and experiments. In the present work, the BV-based energy barrier Eb for oxide-ion migration was calculated for 123 kinds of Sc-containing oxides as a screening process. We found that the monoclinic BaGd2Si3O10-type barium scandium silicate BaSc2Si3O10 has a relatively low Eb, indicating that it is potentially a new oxide-ion conductor. BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were prepared by solid-state reactions. Rietveld analyses of X-ray powder diffraction data of these samples were successfully performed using the monoclinic P21/m BaGd2Si3O10-type structure. Lattice volume V of BaSc1.9Mg0.1Si3O9.95 was smaller than V of BaSc2Si3O10 due to the substation of smaller sized Mg2+ for Sc3+, while the V of BaSc1.9Ca0.1Si3O9.95 was larger than V of BaSc2Si3O10 due to the


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substation of larger sized Ca2+ for Sc3+, indicating the formation of solid solutions. Electrical conductivity and ultraviolet-visible (UVVis) diffuse reflectance measurements indicated that the dominant carrier of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) was oxide ion. Thus, BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were found to be new structure-type oxide-ion conductors. These materials are the first examples of pure oxide-ion conductors containing Sc as an essential element. The oxide-ion conductivity of BaSc2Si3O10 was enhanced by doping Mg or Ca owing to the increase in the carrier (oxygen vacancy) concentration. The oxide-ion conductivity of BaSc1.9Ca0.1Si3O9.95 was approximately 19 times higher than that of BaSc2Si3O10 at 1000 ºC. The BV-based energy landscape of BaSc2Si3O10 indicated two- or three-dimensional oxide-ion diffusion along the edges of Si3O10 groups and/or ScO6 octahedra. Introduction Ceramic oxide-ion conductors are utilized as high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs), batteries, catalysts and oxygen permeation membranes.1-3 The oxideion conductors have been studied by many research groups. The main


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materials exhibiting high oxide-ion mobility have a limited range of structure

types

such

as

fluorite-type,4-9

perovskite-type,10-17

pyrochlore-type,18-20 K2NiF4-type,21-27 and others.28-35 Therefore, the discovery of new structure-type oxide-ion conductors is important for the development of innovative devices based on oxide-ion conductors. The first aim of this study was to search for Sc-containing oxide-ion conductors with a new structure by the bond-valence (BV) method. We chose Sc-containing oxides because Sc3+ has the Ar electron configuration, which can lead to a high transport number of oxide ions in Sc3+-containing oxides. However, there have been no reports on pure oxide-ion conduction in materials containing Sc as an essential element where there is at least one crystallographic site fully occupied by a Sc atom. Sc species has been used as a dopant or a component where the site occupancy of Sc atom is less than unity (e. g., scandia-stabilized zirconia ((Sc2O3)x(ZrO2)1-x)), Sc-doped CaTiO3 and pyrochlore-type Ln2ScMO7 (Ln: Sm, Ho; M: Nb, Ta)).36-39 Perovskite-type LaScO3-based materials are not pure oxide-ion conductors but mixed conductors of protons, oxide ions and electrons.40,41


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There are various methods of estimating the energy barriers for oxide-ion migration such as density functional theory (DFT)-based calculations and molecular dynamics (MD) simulations.42-49 In this work, we utilized the BV method, because this method provides a quicker and more efficient computational approach to screening materials with potentially high oxide-ion conductivity than other methods owing to its simpler calculation procedure. As described in the Results and Discussion section, we discovered BaGd2Si3O10-type BaSc2Si3O10 to be a candidate oxide-ion conductor with a new crystal structure by the BV method. The electrical conduction of BaSc2Si3O10 has not been yet investigated, although its luminescence property has been reported.50-52 Furthermore, BaSc2Si3O10 is stable at high temperatures and its melting point is high (approximately 1300 ºC). The aims of this study were (i) to synthesize BaSc2Si3O10 and (ii) to investigate the crystal structure, oxide-ion conductivity and oxide-ion diffusion path of BaSc2Si3O10. In this work, we demonstrated that BaSc2Si3O10 is an oxide-ion conductor with a new structure. Furthermore, in the present work, we synthesized BaSc2-xAxSi3O10-x/2 samples (A: Mg, Ca; x = 0.1, 0.2) and investigated their oxide-ion


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conductivities. It was revealed that the oxide-ion conductivities of BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) are higher than that of BaSc2Si3O10 owing to the higher carrier (oxygen vacancy) concentration in BaSc1.9A0.1Si3O9.95. Methods BV-based energy landscapes for a test oxide ion in 123 kinds of Sccontaining oxides were examined to search for oxide-ion conductors with new crystal structures. The BV-based energy landscape for each Sc-containing oxide was calculated using its lattice parameters, atomic coordinates and occupancy factors from the inorganic crystal structure database ICSD Ver. 3.3.053 with the computer program 3DBVSMAPPER.46 The spatial resolution was set to 0.1 Å. The crystal structure and BV-based energy landscape were depicted with VESTA3.54 Using the BV-based energy landscapes, the BV-based energy barriers for oxide-ion migration were estimated for the 123 kinds of Sc-containing oxides. BaSc2-xAxSi3O10-x/2 (A: Mg, Ca; x = 0.0, 0.1, 0.2) samples were prepared by a solid-state reaction method. The starting materials,


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BaCO3 (99.95%), Sc2O3 (99.9%), SiO2 (99.9%), MgO (99.9%) and CaCO3 (99.99%) were mixed and ground as ethanol slurries and dried powders in an agate mortar for 30 minutes. The mixed powder was calcined in air at 1000 ºC for 10 hours. The calcined samples were crushed and ground by a planetary ball mill using YSZ balls at a rotation speed of 300 rpm for 30 minutes. The obtained samples were isostatically pressed into pellets at approximately 200 MPa and sintered in air at 1280 ºC for 10 hours. The relative densities of sintered BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were 90.9%, 89.9% and 88.9%, respectively. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) data were measured in order to determine the cation ratios of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca). Each sample was weighted in a platinum crucible and mixed with lithium borate, and the mixture was fused over a burner. The material thus obtained was crushed and ground into powders, and then dissolved in nitric acid solution. ICP-AES indicated that the cation molar

ratios

of

BaSc1.9Ca0.1Si3O9.95

BaSc2Si3O10, were

Ba/Sc/Si

BaSc1.9Mg0.1Si3O9.95 =

and

1.02(2)/1.98(3)/3.00(2),

Ba/Sc/Mg/Si = 1.01(2)/1.90(2)/0.100(2)/3.00(3) and Ba/Sc/Ca/Si =


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1.018(12)/1.900(13)/0.100(1)/3.002(2),

respectively,

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in

good

agreement with the average chemical compositions of the starting mixtures within two times of the standard deviation of the measured chemical composition and the number in the parenthesis is the last digit of the standard deviation. The existing phases in the obtained samples were investigated at 24

o

C by Cu Kα X-ray powder

diffraction (XRD) (RINT-2500, Rigaku Co. Ltd.). XRD data of the mixture of obtained samples of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) and silicon powders were also measured at 24 oC to obtain accurate lattice parameters, which were refined by the Rietveld method using the computer program Z-code.55,56 Scanning electron microscope (SEM) observation was performed using a JCM-5700 SEM (JEOL Co. Ltd.). The fractured cross sections of the sintered BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were observed to investigate the morphology. The electrical conductivities of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca; sample size: 4.3–4.5 mm in diameter, 11–12 mm in height) were measured at 800 ºC at various oxygen partial pressures P(O2) by a direct-current four-probe technique. P(O2) was controlled ACS Paragon Plus Environment

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by the flow ratio of an O2/N2 or dry H2/N2 gas mixture and monitored by a YSZ oxygen sensor set at downstream of the measurement apparatus.

The

electrical

conductivities

of

BaSc2Si3O10

and

BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were measured between 400 and 1000 ºC in air. Pt paste and wire were employed as an electrode and lead wire, respectively. To investigate the possibility of proton conduction, the electrical conductivities of BaSc2Si3O10 and BaSc1.9Ca0.1Si3O9.95 were measured also in wet air, and the humidified temperature was 20 ºC. Thermogravimetric (TG) measurements (Bruker-AXS TGDTA2020SA) of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were carried out in static air from 400 to 1000 ºC where the heating rate was 5 oC min–1. TG data of BaSc1.9Ca0.1Si3O9.95 were also measured at 800 ºC under air (P(O2) = 0.21 atm), Ar (P(O2) ≈ 10–5 atm) and 5% H2/N2 (P(O2) ≈ 10–24 atm) where the P(O2) was monitored using a YSZ oxygen sensor. DFT calculations were carried out to investigate the optimized structure, band structure and density of states of BaSc2Si3O10. The VASP code57 was used for a single cell of monoclinic (BaSc2Si3O10)2. The lattice parameters and atomic coordinates of the optimized ACS Paragon Plus Environment

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structure of BaSc2Si3O10 agreed well with those in the literature (Table S1 in Supporting Information (SI)). The calculated band structure of BaSc2Si3O10 indicated a direct band gap (Fig. S1 in SI). Details of the DFT calculations are shown in page S-2 of SI. The ultraviolet-visible (UV-vis) diffuse reflectance spectra of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were measured at room temperature between 200 and 700 nm at intervals of 1 nm using a JASCO V-670 UV-vis spectrometer. The optical direct band gap Eg was estimated using the Kubelka-Munk equation58 and a Tauc plot.59 Results and discussion The calculated BV-based energy barriers for oxide-ion migration Eb for 123 kinds of Sc-containing oxides ranged from 0.3 to 8.8 eV (Fig. 1a). Figures 1b and 1c show BV-based energy landscapes for an oxide ion in BaSc2Si3O10 and Ba3Cu3Sc4O10, respectively, with the isosurfaces at +1.5 eV, where the BV-based energy of the most stable position is set to 0 eV. The yellow isosurface of the BV-based energy of Ba3Cu3Sc4O12 did not connect across the unit cell (Fig. 1c) and the estimated Eb for Ba3Cu3Sc4O12 was high (Eb = 4.26 eV). In contrary,


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we found that BaSc2Si3O10 had a relatively low energy barrier for oxide-ion migration across the unit cell (Eb = 1.24 eV) as shown in Figs. 1a and 1b. Furthermore, BaSc2Si3O10 does not contain transition metal ions, which leads to less electronic conduction. Therefore, we chose BaSc2Si3O10 as a candidate oxide-ion conductor in the present study. Figures 2a and 2b show XRD patterns of BaSc2xMgxSi3O10-x/2

and BaSc2-xCaxSi3O10-x/2 (x = 0.0, 0.1, 0.2), respectively,

at 24 ºC. All the reflection peaks of BaSc1.9Ca0.1Si3O9.95 were indexed to a primitive monoclinic cell, indicating a single monoclinic phase with P21/m BaGd2Si3O10-type structure. Main phases of other compositions

were

also

monoclinic.

BaSc1.8Mg0.2Si3O9.9

and

BaSc1.8Ca0.2Si3O9.9 showed unknown peaks where the maximum peak intensities of unknown phase were 13% and 24% of those of the monoclinic phase, thus, we did not study these compositions further. Rietveld analyses of the XRD data of BaSc2-xMgxSi3O10-x/2 and BaSc2xCaxSi3O10-x/2

(x = 0.0, 0.1) were successfully performed using the

monoclinic P21/m BaGd2Si3O10-type structure51 (Table 1, Fig. S2 in SI). Fractions of the monoclinic and impurity BaCO3 phases were ACS Paragon Plus Environment

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98.8 wt% and 1.2 wt% in BaSc2Si3O10 sample, and 97.9 wt% and 2.1 wt% in BaSc1.9Mg0.1Si3O9.95. The refined lattice parameters a, b, c and the lattice volume V of BaSc1.9Mg0.1Si3O9.95 were smaller than those of BaSc2Si3O10 (Table 1). The smaller lattice volume indicated the substitution of smaller sized Mg2+ (0.72 Å in sixfold coordination) for Sc3+ (0.75 Å in sixfold coordination).60 a, b, c and V for BaSc1.9Ca0.1Si3O9.95 were larger than those for BaSc2Si3O10 (Table 1). The larger lattice volume of BaSc1.9Ca0.1Si3O9.95 showed the substitution of larger sized Ca2+ (1.00 Å in sixfold coordination) for Sc3+ (0.75 Å in sixfold coordination). On the other hand, the angle β for BaSc1.9Mg0.1Si3O10 was larger than that for BaSc2Si3O10 and β for BaSc1.9Ca0.1Si3O10 was smaller than that for BaSc2Si3O10. It can be seen that the distortion (β – 90o) decreases with increasing V. These results on the lattice parameters and lattice volume indicate the formation of BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) solid solutions. The electrical conductivities of BaSc2-xAxSi3O10-x/2 (A: Mg, Ca; x = 0.0, 0.1) were independent of the oxygen partial pressure P(O2) at 800 ºC from 10–24 to 1 atm (Fig. 3). TG measurements showed no weight change of BaSc1.9Ca0.1Si3O9.95 under flowing 3 kinds of gases (air, Ar, ACS Paragon Plus Environment

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5% H2/N2) at 800 ºC. These results indicate that the oxygen content and oxygen vacancy concentration in BaSc1.9Ca0.1Si3O9.95 were constant in the wide P(O2) range from 10–24 atm to 1 atm at 800 ºC. No

proton

conduction

was

observed

in

BaSc2Si3O10

and

BaSc1.9Ca0.1Si3O9.95 in wet air (Fig. 4). The optical band gaps of BaSc2Si3O10, BaSc1.9Mg0.1Si3O9.95 and BaSc1.9Ca0.1Si3O9.95 were 4.77, 3.76 and 3.75 eV, respectively (Fig. S3 in SI). The measured band gap of BaSc2Si3O10 (4.77 eV) was in good agreement with the value of 4.75 eV obtained by DFT calculations (Figs. S1 and S4 in SI). These results indicate that the dominant carrier is oxide ion and that BaSc2xAxSi3O10-x/2

(A: Mg, Ca; x = 0.0, 0.1) are pure oxide-ion conductors.

BaSc2-xAxSi3O10-x/2 (A: Mg, Ca; x = 0.0, 0.1) are the first examples of oxide-ion conductors with the P21/m BaGd2Si3O10-type structure. Therefore, we discovered new structure-type oxide-ion conductors BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca). Although there are Sc-doped oxide-ion conductors such as scandia-stabilized zirconia, Sc-doped CaTiO3 and pyrochlore-type oxides,37-39 Sc is not an essential element in these compounds. Perovskite-type La1-xAxScO3 (A: Sr, Ba) are not pure oxide-ion conductors but mixed conductors of


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protons, oxide ions and electrons.40,41 Therefore, BaSc2-xAxSi3O10-x/2 (A: Mg, Ca; x = 0.0, 0.1) are also the first examples of pure oxide-ion conductors containing Sc as an essential element. TG measurements of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) showed no significant weight change in static air from 400 to 1000 ºC (Fig. S5 in SI), indicating no change in oxygen content of these materials. Figure 4 shows Arrhenius plots of the oxide-ion conductivities of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) between 400 and 1000 oC in static air. The activation energies Ea for oxide-ion conduction were estimated from the Arrhenius equation

σ =

A0  E  exp  − a  , T  kT 

where A0, k and T are the pre-exponential factor, Boltzmann constant and absolute temperature, respectively. The value of Ea for BaSc1.9Mg0.1Si3O9.95 (1.24(2) eV) and BaSc1.9Ca0.1Si3O9.95 (1.29(2) eV) were higher than that for BaSc2Si3O10 (1.07(2) eV). The oxideion conductivities of BaSc1.9Mg0.1Si3O9.95 and BaSc1.9Ca0.1Si3O9.95 at 1000 ºC were approximately 19 times higher than that of BaSc2Si3O10, although the activation energies of BaSc1.9Mg0.1Si3O9.95 and ACS Paragon Plus Environment

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were

also

higher.

The

morphologies

in

BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were similar as shown by the SEM images in Fig. S6 of SI. Therefore, the improvement of ionic conductivity of BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) is not attributable to the microstructures such as porosity. The influence of the BaCO3 impurity on the oxide-ion conductivity was negligible, because the amounts of BaCO3 were quite small (1.2 wt% for BaSc2Si3O10, 0 wt% for BaSc1.9Ca0.1Si3O9.95, 2.1 wt% for BaSc1.9Mg0.1Si3O9.95; Fig. S2

in SI). The higher oxide-ion

conductivities of BaSc1.9Mg0.1Si3O9.95 and BaSc1.9Ca0.1Si3O9.95 than that of BaSc2Si3O10 can be ascribed to the higher carrier (oxygen vacancy) concentration (0.05) in BaSc. Mg . Si O. (V‥ ) . and BaSc. Ca . Si O. (V‥ ) . than that (0.00) in BaSc2Si3O10. An important question is why the oxide ions diffuse in BaSc2Si3O10. To answer this question, the oxide-ion diffusion pathway in BaSc2Si3O10 was investigated through the BV method (Fig. 5). Horseshoe-shaped trisilicate groups (Si3O10 groups) exist in the crystal structure of BaSc2Si3O10 (Fig. 5b). BV-based energy landscapes (Figs. 5a and 5c) indicate that an oxide ion can migrate ACS Paragon Plus Environment

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along the edges of Si3O10 groups (red, violet, blue and black lines with arrows in Figs. 5a and 5c). Possible oxide-ion diffusion pathways in the [010] direction are –O4–O3–O2–O3–O4–O6–O3–O1–O3–O6– O4– (Table S2 in SI) and –O4–O3–O2–O3–O4–O1–O3–O6–O4– (Table S3), where the atomic coordinates of each oxygen site Oi (i = 1, 2, …, 6) are shown in Table S1 in SI. Most of the paths in the [010] direction are curved and along the edges of Si3O10 groups. A possible oxide-ion diffusion path in the [101] direction is –O5–O6–O3–O4– O6–O5–O5– (Table S4 of SI). The other paths –O5–O6–O6–O4–O5– and –O5–O6–O6–O5–O4–O5– can be seen along the edges of both ScO6 octahedra and Si3O10 groups in the [101] direction (Tables S5 and S6 of SI, blue lines with arrows in Figs. 5c and 5d). The yellow isosurfaces in Fig. 5a did not exist around Ba cations but existed around Si3O10 groups and ScO6 octahedra. These results indicate that Si3O10 groups and ScO6 octahedra have key roles in the oxide-ion conduction of BaSc2Si3O10. The BV-based energy barriers Eb between Oi and Oj sites in BaSc2Si3O10 are listed in Tables S2, S3, S4, S5 and S6 in SI. The BV-based energy barriers for oxide-ion migration in the [010], [101] and [101] directions were estimated to be 1.24, 1.24 and


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1.63 eV, respectively, using these Tables. Therefore, BaSc2Si3O10 is expected to exhibit two- or three-dimensional oxide-ion diffusion.

Conclusions In the present work, we have discovered oxide-ion conductors with a new crystal structure, BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca), by combining BV-based energy calculations, sample synthesis, and X-ray diffraction, UV-Vis diffuse reflectance and electrical conductivity measurements. We demonstrated that the combined technique is a powerful means of searching for oxide-ion conductors with a new crystal structure. The BV-based energy barriers of 123 kinds of oxides containing Sc species were estimated as a screening process. BaSc2Si3O10 was chosen as a candidate because of its relatively low BV-based energy barrier (Figs. 1a and 1b). Monoclinic BaGd2Si3O10-type BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were synthesized. They were found to have wide optical band gaps and their electrical conductivities were independent of the oxygen partial pressure. BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) are


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the first examples of pure oxide-ion conductors including Sc as an essential element. The oxide-ion diffusion is an important step in catalysis

of

some

catalysts

such

as

ceria-zirconia.61

Thus,

BaSc2Si3O10-based materials can be used as catalysts and sensors. The BV-based energy barriers in the [010], [101] and [101] directions of BaSc2Si3O10 were estimated to be 1.24, 1.24 and 1.63 eV, respectively, which strongly suggested two- or three-dimensional oxide-ion diffusion. We also demonstrated that Si3O10 groups and ScO6 octahedra have key roles in the oxide-ion conduction in BaSc2Si3O10. Thus, materials including the B3O10 group (B: representative element such as Si or Ge) may potentially be oxide-ion conductors with a new structure. In this work, we showed that the oxide-ion conductivity of BaSc2Si3O10 is enhanced by the doping of Mg2+ or Ca2+ owing to the increase in carrier concentration. However, the oxide-ion conductivities of BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) were not very high, because their activation energies Ea for oxide-ion conduction were higher than that of BaSc2Si3O10. Therefore, the exploration of BaSc2Si3O10-based oxides with lower Ea is important to further improve oxide-ion conductivity.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ********. Information on DFT calculations, Rietveld patterns, TG curve, UVvis diffuse reflectance spectra, SEM images, crystal data, BV-based energy barriers and migration paths. *Corresponding Author Masatomo Yashima [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements We would like to express special thanks to Dr. K. Fujii and Mr. K. Hibino for helpful discussions. We thank Daiichi Kigenso Kagaku Kogyo Co. Ltd. for the ICP-AES analyses. We would like to also thank to Prof. T. Hashimoto, Dr. Kamioka, Mr. Okiba for SEM observations. This study was partly supported by Grants-in-Aid for ACS Paragon Plus Environment

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Figure 1. (a) Histogram of BV-based energy barrier for 123 kinds of Sccontaining oxides. BV-based energy landscapes for an oxide ion in (b) BaSc2Si3O10 and (c) Ba3Cu3Sc4O12 with the isosurfaces at 1.5 eV.


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X−ray Intensity / Arb. Unit

(a)

：P21/m BaSc 2−xMgxSi3O10−x/2 ：BaCO 3 ：Unknown

031 111 130 102 012 131 112 121 022 122 041 201 131 140 211 206 141 032 210 132 221 141 112 202 220 141

121

x = 0.1

101

120

101 110 111 021

001 020 011

x = 0.2

x = 0.0 10

20

30

40

Diffraction Angle, 2 θ / ° ：P21/m BaSc 2-xCa xSi3O10-x/2 ：BaCO 3 ：Unknown

031 111 130 102 012 131 112 121 022 122 041 201 131 140 211 206 141 032 210 132 221 141 112 202 220 141

x = 0.1

101

011

101 110 111 021 120 121

x = 0.2

001 020

(b)

X-ray Intensity / Arb. Unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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x = 0.0 10

20 30 Diffraction Angle, 2 θ / °

40

Figure 2. X-ray diffraction patterns of (a) BaSc2-xMgxSi3O10-x/2 and (b) BaSc2xCaxSi3O10-x/2 (x = 0.0, 0.1, 0.2) at 24 ºC. hkl denotes the reflection index of the monoclinic BaSc2-xAxSi3O10-x/2 phase (A: Mg, Ca).


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：BaSc 1.9Ca0.1Si3O9.95 ：BaSc 1.9Mg0.1Si3O9.95 ：BaSc 2Si3O10

−5

−6

−1

−6

−1

σion = 1.59(2)×10 (S cm )

−1

log(σ / S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−6

σion = 1.53(1)×10 (S cm )

−6

−1

σion = 0.36(3)×10 (S cm ) −7 ＠ 800℃ −20

−10 log(P(O2) / atm)

0

Figure 3. P(O2) dependence of electrical conductivities of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) at 800 °C.


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1000

Temperature / ℃ 700

400

：BaSc 1.9Ca0.1Si3O9.95 (dry) ：BaSc 1.9Ca0.1Si3O9.95 (wet)

−2

Ea = 1.29(2) eV

−1

log( σ ion T / S K cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ea = 1.24(2) eV

−4 Ea = 1.07(2) eV

−6

：BaSc 1.9Mg0.1Si3O9.95 ：BaSc 2Si3O10 (dry) ：BaSc 2Si3O10 (wet) 0.8

1

1.2

1000/T / K

1.4

−1

Figure 4. Arrhenius plots of σionT of BaSc2Si3O10 and BaSc1.9Ca0.1Si3O9.95 in dry and wet air. Arrhenius plot of σionT of BaSc1.9Mg0.1Si3O9.95 in static air. Here σion and T stand for the oxide-ion conductivity and absolute temperature, respectively.


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Figure 5. (a) BV-based energy landscape for an oxide ion in BaSc2Si3O10 with the isosurface at 1.5 eV viewed along the a axis (0.0 ≤ x ≤ 1.0, 0.0 ≤ y ≤ 1.0, 0.0 ≤ z ≤ 1.0) with blue SiO4 tetrahedra, purple ScO6 distorted octahedra and green Ba cations. The red and purple lines with arrows indicate the possible migration paths in the b direction. (b) Crystal structure of BaSc2Si3O10 viewed along the a axis (0.0 ≤ x ≤ 1.0, 0.0 ≤ y ≤ 1.0, 0.0 ≤ z ≤ 1.0). (c) BV-based energy landscape for an oxide ion in BaSc2Si3O10 with the isosurface at 1.67 eV viewed along the b axis (0.0 ≤ x ≤ 1.0, 0.38 ≤ y ≤ 0.61, 0.0 ≤ z ≤ 1.0). The blue and black lines with arrows denote the possible migration paths in the [10 1 ] and [101] directions, respectively. (d) Crystal structure of BaSc2Si3O10 viewed along the b axis (0.0 ≤ x ≤ 1.0, 0.38 ≤ y ≤ 0.61, 0.0 ≤ z ≤ 1.0).


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Table 1. Lattice parameters and volume refined by the Rietveld analyses of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A: Mg, Ca) samples including the internal Si standard taken at 24 ºC. Reliability factors in Rietveld analyses: Rwp = 0.0678, Rp = 0.0504, RB = 0.0326, RF = 0.0218 for BaSc2Si3O10, Rwp = 0.0628, Rp = 0.0485, RB = 0.0368, RF = 0.0192 for BaSc1.9Mg0.1Si3O9.95, and Rwp = 0.0595, Rp = 0.0462, RB = 0.0312, RF = 0.0143 for BaSc1.9Ca0.1Si3O9.95.

Lattice parameter

a/Å

b/Å

BaSc2Si3O10

5.28009(2)

11.93431(5) 6.59911(2)

107.0501(3)

397.56(4)

BaSc1.9Mg0.1Si3O9.95 5.27594(3)

11.92597(5) 6.59393(3)

107.0512(4)

396.64(5)


11.94032(8) 6.60286(3)

107.0450(4)

398.27(6)

5.28293(3)

c/Å

β/º

V / Å3

Composition


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Table of Contents / Abstract Graphics


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Discovery of Oxide-Ion Conductors with a New Crystal Structure

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