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Article

Improved Oxide Ion Conductivity of Hexagonal Perovskite-Related Oxides Ba3W1+xV1−xO8.5+x/2

by
Yugo Kikuchi
1,†,
Yuta Yasui
1,†,
James R. Hester
2 and
Masatomo Yashima
1,*,†
1
Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 W4–17 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
2
Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, Sydney, NSW 2232, Australia
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Inorganics 2023, 11(6), 238; https://doi.org/10.3390/inorganics11060238
Submission received: 8 May 2023 / Revised: 25 May 2023 / Accepted: 26 May 2023 / Published: 29 May 2023
(This article belongs to the Special Issue Layered Perovskites: Synthesis, Properties and Structures)
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Abstract

:
Hexagonal perovskite-related oxides such as Ba3WVO8.5 have attracted much attention due to their unique crystal structures and significant oxide ion conduction. However, the oxide ion conductivity of Ba3WVO8.5 is not very high. Herein, we report new hexagonal perovskite-related oxides Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75). The bulk conductivity of Ba3W1.6V0.4O8.8 was found to be 21 times higher than that of the mother material Ba3WVO8.5 at 500 °C. Maximum entropy method (MEM) neutron scattering length density (NSLD) analyses of neutron diffraction data at 800 °C experimentally visualized the oxide ion diffusion pathways through the octahedral O2 and tetrahedral O3 sites in intrinsically oxygen-deficient layers. By increasing the excess W content x in Ba3W1+xV1−xO8.5+x/2, the excess oxygen content x/2 increases, which leads to more oxygen atoms at the O2 and O3 oxygen sites, a higher minimum NSLD on the O2–O3 path, and a higher level of conductivity. Another reason for the increased conductivity of Ba3W1.6V0.4O8.8 is the lower activation energy for oxide ion conduction, which can be ascribed to the longer (W/V)–O2 and (W/V)–O3 distances due to the substitution of V atoms with large-sized W species. The present findings open new avenues in the science and technology of oxide ion conductors.

Graphical Abstract

1. Introduction

Oxide ion and proton conductors are important materials for clean energy and environment [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. For example, oxide ion conductors can be used in electrochemical devices such as solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs). High oxide ion conductivity is needed for high-performance electrochemical devices and is attained in materials with specific crystal structures such as the fluorite-type and AMO3 perovskite-type structures [10,11,15,16,17]. Here, A and M are relatively larger and smaller cations. The conventional yttria-stabilized zirconia (YSZ) electrolytes exhibit low levels of oxide ion conductivity at intermediate temperatures, which restrict the widespread use of SOFCs with YSZ electrolytes. To solve this problem, it is of vital importance to search for new oxide ion conductors that exhibit higher conductivities.
There are four main groups of perovskite-type and perovskite-related structures: (i) the AMO3-perovskite-type structure, (ii) AMO3-perovskite-related structure, (iii) hexagonal perovskite-related structure, and (iv) modular structure [18,19]. Layered perovskites are emerging materials with layered structures that have perovskite or perovskite-like units [11]. The hexagonal perovskite-related oxides (iii) have a layered structure containing a hexagonal close-packed AO3 h layer or an oxygen-deficient, hexagonal close-packed AO3–δ h’ layer [9,11,12,18,19,20,21,22,23,24,25,26,27]. Compared with other perovskites, high levels of oxide ion and proton conduction were rarely found in hexagonal perovskite-related materials. Recently, high oxide ion conduction has been reported in various hexagonal perovskite-related oxides such as Ba3MoNbO8.5, Ba3WVO8.5, and Ba7Nb3.9Mo1.1O20.05 [18,20,21,24,25,26,27]. Ba3M2O8.5 oxides (e.g., M2 = MoNb, WV, and WNb) exhibit structural disorder in the oxygen-deficient, cubic close-packed c’ layer, leading to significant oxide ion conduction [19,20,21,22,24,25,26,27]. A high level of occupational disorder at the octahedral O2 and tetrahedral O3 sites yields a high probability density of oxygen atoms between the O2 and O3 sites, leading to a high level of oxide ion conductivity [27]. A possible means of increasing the probability density of oxygen atoms on the O2–O3 pathway is to have a larger amount of excess oxygen. Thus, the oxide ion conductivity of Ba3WVO8.5 can be expected to increase by increasing the amount of oxygen 8.5+x/2 via W/V substitution in Ba3W1+xV1−xO8.5+x/2. Herein, we report new hexagonal perovskite-related oxides Ba3W1+xV1−xO8.5+x/2 and their improved oxide ion conductivity compared with the mother material, Ba3WVO8.5. In the present work, we demonstrate that the bulk conductivity of Ba3W1.6V0.4O8.8 is 21 times higher than that of mother material Ba3WVO8.5 at 500 °C, which is ascribed to the greater amount of excess oxygen atoms O0.3 and the lower activation energy for oxide ion conduction in Ba3W1.6V0.4O8.8.

2. Results and Discussion

2.1. Formation of Ba3W1+xV1−xO8.5+x/2 Solid Solutions and Their Electrical Conductivities

Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) oxides were prepared via solid-state reactions. Most of their X-ray powder diffraction (XRD) peaks at room temperature (RT) were indexed to a rhombohedral lattice, indicating that the main phase of Ba3W1+xV1−xO8.5+x/2 is rhombohedral (space group: R 3 ¯ m and crystal symmetry: trigonal), which is consistent with the phase identification of Ba3WVO8.5 in the literature [21,22]. Figure S1a is a typical XRD pattern of Ba3W1.6V0.4O8.8, showing the main rhombohedral phase in addition to a small amount of BaWO4 phase. The lattice volume and lattice parameter c of Ba3W1+xV1−xO8.5+x/2 decrease with increasing excess W content x in the compositional range from x = 0.1 to 0.6, (Figure S1b,c), suggesting the formation of rhombohedral Ba3W1+xV1−xO8.5+x/2 solid solutions.
Figure 1a shows the Arrhenius plots of the direct current (DC) electrical conductivity σDC of Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) in static air. The σDC increases with increasing temperature. At any temperature between 600 and 1000 °C, the corrected full-density DC electrical conductivity σDC/corr increases with an increase in excess W content x in Ba3W1+xV1−xO8.5+x/2 from x = −0.1 to 0.6, which can be ascribed to the increase in carrier concentration due to the increase in excess oxygen atoms x/2 in Ba3W1+xV1−xO8.5+x/2 (Figure 1b). Meanwhile the σDC/corr for x = 0.75 is lower than that for x = 0.6, over the entire temperature range, which can be ascribed to overdoping due to defect association and/or a greater amount of BaWO4 impurity in the x = 0.75 sample. Similar overdoping has been observed in other hexagonal perovskite-related oxides, such as Ba7Ta4−xMo1+xO20+x/2 [26]. Ba3W1.6V0.4O8.8 exhibits the highest conductivity σDC among Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) over the entire temperature range (Figure 1a,b). Therefore, we focus on Ba3W1.6V0.4O8.8 for further studies. For example, at 600 °C, the σDC of Ba3W1.6V0.4O8.8 (3.0 × 10−4 S cm−1) is 86 times higher than that of the mother material, Ba3WVO8.5 (3.5 × 10−6 S cm−1). The σDC values of Ba3W1.6V0.4O8.8 at 800 and 1000 °C were as high as 3.7 × 10−3 and 0.021 S cm−1, respectively.

2.2. Oxide Ion Conduction of Ba3W1.6V0.4O8.8

X-ray photoelectron spectroscopy (XPS) spectra of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 showed that the valences of W and V at RT were +6 and +5, respectively, (Figure S2), indicating (Ba2+)3(W6+)1.6(V5+)0.4(O2−)8.8 and (Ba2+)3(W6+)(V5+)(O2−)8.5. The thermogravimetric analyses in dry air indicated no significant weight change in the second and third heating/cooling cycles (Figure S3), demonstrating no change in oxygen content. Therefore, the chemical compositions at a high temperature of 800 °C are the same as those at RT.
Figure 2a shows the oxygen partial pressure P(O2) dependence of the DC electrical conductivity σDC of Ba3W1.6V0.4O8.8 at 820 °C in dry conditions. The σDC increases with a decrease in P(O2) below 10−18 atm, suggesting electronic conduction due to the reduction in W and/or V cations. Meanwhile the σDC is almost independent of P(O2) between 10−17 and 0.21 atm at 820 °C, indicating that the influence of the p-type conducting BaWO4 impurity on the σDC is negligible (See the Supplementary Note in the Supporting Information). The UV-vis reflectance spectra of Ba3W1.6V0.4O8.8 indicated a wide optical band gap (Figure S4), suggesting an electronic insulator. To examine possible proton conduction, the σDC was also measured in wet and dry air. The σDC values in wet air almost agree with those in dry air (Figure 2b). These results suggest oxide ion conduction in Ba3W1.6V0.4O8.8.
Impedance measurements of Ba3W1.6V0.4O8.8 were carried out in dry air to obtain its bulk conductivity. The impedance data were analyzed using the equivalent circuits shown in Figure S5 to extract the bulk and grain boundary conductivities. Figure 2c shows typical Nyquist plots of Ba3W1.6V0.4O8.8, with the bulk and grain boundary components indicated by semicircles. The bulk conductivity in dry air σb of Ba3W1.6V0.4O8.8 increases with increasing temperature and is as high as 3.0 × 10−3 S cm−1 at 794 °C. The activation energy for σb is lower than the activation energy for grain boundary conductivity (Figure S6). The σb of Ba3W1.6V0.4O8.8 is higher than that of the mother material Ba3WVO8.5 (e.g., it is 21 times higher at 500 °C). The σb of Ba3W1.6V0.4O8.8 is higher than that of Ba3WNbO8.5 below 500 °C [20] and is comparable to that of Ba3MoNbO8.5 around 370 °C [24], indicating that Ba3W1.6V0.4O8.8 is a superior oxide ion conductor. The higher σb of Ba3W1.6V0.4O8.8 compared to Ba3WVO8.5 is attributable to both the lower activation energy for σb and the higher carrier concentration (a larger oxygen content of 8.8) of Ba3W1.6V0.4O8.8 compared to Ba3WVO8.5. The structural origins of the lower activation energy and higher carrier concentration will be described below.

2.3. Crystal Structure Analyses of Ba3W1.6V0.4O8.8 and Ba3WVO8.5

To investigate the origin of its high oxide ion conductivity, the crystal structure was analyzed using neutron diffraction data of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 taken in situ at 18 and 800 °C (Figure 3). Rietveld analyses of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 were successfully carried out as a hybrid structure (space group: R 3 ¯ m, Figure 4). Figure 3 shows the Rietveld patterns, indicating satisfactory fits. Preliminary structure analyses indicated no significant occupancy of W/V atoms at the Wyckoff position 3b and full occupancy of W/V atoms at the 6c position (W/V site), indicating the chemical order of W/V atoms and W/V vacancies. Anisotropic atomic displacement parameters (ADPs) provided lower reliability factors than isotropic ADPs for Ba1, Ba2, O1, and O2 atoms. A split-atom model for the apical oxygen O3 at the 36i Wyckoff position yielded lower reliability factors compared with a non-split atom model. Therefore, we used anisotropic ADPs for Ba1, Ba2, O1, and O2 atoms and a split-atom model for the O3 atom in the final refinements. The refined crystal parameters and reliability factors are listed in Tables S1–S4. The bond valence sums of the Ba1, Ba2, W, V, and O1 atoms agree with their valences of +2, +2, +6, +5, and −2, validating the refined crystal structures. The crystal parameters of Ba3WVO8.5 at room temperature and at 800 °C obtained in this work are consistent with those reported in the literature [21,22].
Figure 4c shows the crystal structure refined using Rietveld analysis and neutron diffraction data of Ba3W1.6V0.4O8.8 taken at 800 °C. The structure has hexagonal close- packed (h) BaO3 layers (Ba2–O1 layers) and oxygen-deficient, cubic close-packed (c’) BaO2.8 layers (Ba1–O2–O3 layers), forming a sequence of (hhc’)3. The (W/V)O5.408(7) polyhedron (Figure 4f) can be regarded as a hybrid of a (W/V)O6 octahedron (Figure 4d) and a (W/V)O4 tetrahedron (Figure 4e). Thus, the crystal structure of Ba3W1.6V0.4O8.8 (Figure 4c) is a hybrid of the structures of Figure 4a,b. The high oxide ion conductivity of Ba3W1.6V0.4O8.8 can be ascribed to the interexchange between the (W/V)O4 tetrahedron and (W/V)O6 octahedron, as discussed below.

2.4. Neutron Scattering Length Density Analyses of Ba3W1.6V0.4O8.8 and Ba3WVO8.5

To discuss the origin of the high oxide ion conductivity of Ba3W1.6V0.4O8.8, neutron scattering length densities (NSLDs) were analyzed via the maximum entropy method (MEM), using the structure factors estimated in the Rietveld analyses of the neutron diffraction data of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 taken in situ at 18 and 800 °C. It is known that MEM enables the visualization of the oxide ion diffusion pathways and structural disorder [18,19,22,27,29]. Figure 5 shows the NSLD distributions and corresponding refined structures of Ba3W1.6V0.4O8.8 at 800 °C. Connected NSLD distributions are clearly observed between the octahedral O2 and tetrahedral O3 atoms, which corresponds to the experimental visualization of the oxide ion O2–O3 diffusion pathways. Figure 5b–d show the two-dimensional network of the O2–O3 diffusion pathways. The oxide ion migrates along the O2–O3 edges of the (W/V)O5.408(7) polyhedron (Figure 4f). These features are similar to other Ba3M2O8.5 oxides (M2 = MoNb [19], WNb [27], and WV (this work, Figure 6a,c and [22])).
The NSLD distributions around the O2 and O3 sites are localized at 18 °C (Figure 6a,b), while they are connected between the O2 and O3 sites at 800 °C (Figure 6c,d), which is consistent with the higher conductivity at higher temperature (Figure 1a and Figure 2d). The minimum NSLD between the O2 and O3 sites (Figure 6e,f) can be a measure of oxide ion conductivity [27,29]. The minimum NSLD of Ba3WVO8.5 at 800 °C (0.73 fm Å−3) is higher than that at 18 °C (0.08 fm Å−3), which is consistent with the higher conductivity observed at 800 °C (Figure 1a and Figure 2d). Similarly, the minimum NSLD of Ba3W1.6V0.4O8.8 at 800 °C (1.62 fm Å−3) is higher than that at 18 °C (0.95 fm Å−3), which is also consistent with the higher conductivity observed at 800 °C. We have demonstrated that Ba3W1.6V0.4O8.8 exhibits higher oxide ion conductivity compared with the mother material Ba3WVO8.5. The minimum NSLD of Ba3W1.6V0.4O8.8 at 800 °C (1.62 fm Å−3) is higher than that of Ba3WVO8.5 at 800 °C (0.73 fm Å−3), which is consistent with the higher conductivity of Ba3W1.6V0.4O8.8.

2.5. Structural Origins of High Oxide Ion Conductivity in Ba3W1.6V0.4O8.8

Next, we discuss the origins of the higher conductivity of Ba3W1.6V0.4O8.8 compared with Ba3WVO8.5, based on their NSLD distributions at 800 °C (Figure 6). Figure 6e,f shows schematic NSLD distributions between the nearest-neighbor O2 and O3 sites of Ba3WVO8.5 (e) and Ba3W1.6V0.4O8.8 (f) at 800 °C. Ba3W1.6V0.4O8.8 has 0.3 excess oxygen atoms compared with the mother material Ba3WVO8.5. The excess oxygen atoms are incorporated at the O2 and O3 sites; therefore, the occupancy factors of the O2 and O3 atoms and oxygen content on the c’ layer of Ba3W1.6V0.4O8.8 are higher than those of Ba3WVO8.5. Thus, the NSLD and minimum NSLD between the O2 and O3 sites of Ba3W1.6V0.4O8.8 (e.g., 1.62 fm Å−3 at 800 °C) are higher than those of Ba3WVO8.5 (e.g., 0.73 fm Å−3 at 800 °C), leading to the higher oxide ion conductivity of Ba3W1.6V0.4O8.8. It is known that O2/O3 occupational disordering can enhance the oxide ion conductivity in Ba3WNbO8.5 [27]. The disorder parameter DP was calculated using the formulation of Yasui et al. [27] and the refined crystal parameters in Tables S1–S4. The DP of Ba3W1.6V0.4O8.8 (0.7660(15) at 800 °C) is slightly lower than the DP of Ba3WVO8.5 (0.860(4) at 800 °C), which cannot explain the conductivity enhancement observed in Ba3W1.6V0.4O8.8. These results indicate the enhancement of the oxide ion conductivity of Ba3W1.6V0.4O8.8 due to the excess oxygen.
We have indicated that the activation energy for the bulk conductivity of Ba3W1.6V0.4O8.8 (0.741(12) eV) is lower than that of the mother material Ba3WVO8.5 (1.13(1) eV) (Figure 2d). The occupancy factor of the larger W6+ cation at the W/V site in Ba3W1.6V0.4O8.8 (0.8) is higher than that in Ba3WVO8.5 (0.5). Therefore, the (W/V)–O2 bond length of Ba3W1.6V0.4O8.8 (2.1567(16) Å at 800 °C) is longer than that of Ba3WVO8.5 (2.123(4) Å at 800 °C). Similarly, the (W/V)–O3 bond length of Ba3W1.6V0.4O8.8 (1.622(5) Å at 800 °C) is longer than that of Ba3WVO8.5 (1.466(8) Å at 800 °C). These results suggest a size effect of the W/V cation on the activation energy for oxide ion conductivity Ea in which the lower Ea of Ba3W1.6V0.4O8.8 can be ascribed to the longer (W/V)–O2 and (W/V)–O3 distances.

3. Materials and Methods

3.1. Synthesis and Characterization of Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75)

Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, 0.75) samples were synthesized via high-temperature solid-state reactions. Stoichiometric amounts of BaCO3 (99.9%), WO3 (99.9%), and V2O5 (99.75%) were mixed as ethanol slurries and dry powders using an agate mortar for 1 h. The mixtures were calcined at 950 °C for 15 h in static air. The calcined samples were crushed into powders and mixed again as ethanol slurries and dry powders using the agate mortar for 1 h, followed by ball milling processes using 5, 3, and 1 mm diameter zirconia balls at 300 rpm for 30 min at each size with a FRITSCH PULVERISETTE 7. The powders thus obtained were isostatically pressed into pellets at 150 MPa and sintered at 950–1020 °C for 20 h. The phase purity of Ba3W1+xV1−xO8.5+x/2 were evaluated by XRD measurements using an X-ray diffractometer (Bruker D8, Cu Kα). The lattice parameters of Ba3W1+xV1−xO8.5+x/2 were refined via the Le Bail analyses of the XRD data of the mixture of Ba3W1+xV1−xO8.5+x/2 and an internal silicon standard using the software FullProf [30].
The chemical composition of Ba3W1.6V0.4O8.8 was examined via energy dispersive XRF (Rigaku, Tokyo, Japan, NEX DE) and ICP-OES (Agilent Technologies, Tokyo, Japan, 5100 VDV) analyses, which validated the nominal composition. The UV-vis diffuse reflectance spectra of Ba3W1.6V0.4O8.8 were measured using JASCO (Tokyo, Japan) V-650 at RT with spectral range of 850–200 nm. The optical band gaps for direct and indirect transitions of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 were estimated using the Kubelka–Munk function and Tauc plots. The weight changes in Ba3W1.6V0.4O8.8 and Ba3WVO8.5 were investigated using an NETZSCH (Yokohama, Japan) STA 449 F3 Jupiter. The temperature range of the measurement was 35–900 °C, with a 2 °C min−1 temperature ramp under 50 mL min−1 of dry air flow. The XPS spectra of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 were measured using a JPS–9010 X-ray photoelectron spectrometer (JEOL, Tokyo, Japan).

3.2. Electrical Properties of Ba3W1.6V0.4O8.8

The total DC electrical conductivities σDC of the sintered Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) were measured via a DC four-probe method in static air at 600–1000 °C. Pt electrodes with Pt wires were attached to each pellet. The σDC of Ba3W1.6V0.4O8.8 was measured in dry and wet (water vapor pressure: P(H2O) = 0.023 atm) air. The P(O2) dependence of Ba3W1.6V0.4O8.8 was measured at 820 °C in a P(O2) between 0.21 and 5.4 × 10−25 atm, and the P(O2) was controlled using a mixture of O2, N2, and 5% H2/N2 gases and monitored by a YSZ oxygen sensor downstream of the apparatus. The impedance spectra of the sintered Ba3W1.6V0.4O8.8 pellet with Pt electrodes were measured using a Solartron 1260 impedance analyzer in the frequency range of 1 MHz to 100 mHz with an AC voltage of 100 mV. The AC impedance measurements were carried out under a dry air flow from 794 to 382 °C. An equivalent circuit analysis was performed using ZView software (Scribner Associates, Inc., Southern Pines, USA) to exact the bulk and grain boundary conductivities.

3.3. Structural and MEM Neutron Scattering Length Density Analyses Using Neutron Diffraction Data of Ba3WVO8.5 and Ba3W1.6V0.4O8.8

Neutron powder diffraction (ND) patterns of Ba3WVO8.5 and Ba3W1.6V0.4O8.8 were obtained using a fixed-wavelength neutron diffractometer, Echidna [31], at the OPAL research reactor, ANSTO, Australia. The range of 2θ was 10° ≤ 2θ ≤ 160° at 18 and 800 °C. The ND data were analyzed via the Rietveld method using Z-Rietveld [32,33]. The neutron scattering length density distributions were investigated using the maximum entropy method (MEM). The MEM analyses were performed with the structure factors obtained from the Rietveld analyses of Ba3WVO8.5 and Ba3W1.6V0.4O8.8 using Z-Rietveld [32,33]. The refined crystal structures and NSLD distributions were visualized using VESTA [34].

4. Conclusions

We have synthesized new hexagonal perovskite-related materials: Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75). It was found that Ba3W1.6V0.4O8.8 exhibits the highest DC electrical conductivity in static air among the Ba3W1+xV1−xO8.5+x/2 (−0.1 ≤ x ≤ 0.75, Figure 1). Electrical properties and a wide optical band gap indicated the oxide ion conduction of Ba3W1.6V0.4O8.8 (Figure 2 and Figure S4). The bulk conductivity σb of Ba3W1.6V0.4O8.8 is higher than that of the mother material Ba3WVO8.5 over the entire temperature range (Figure 2d). The refined crystal structure and MEM NSLD distributions of Ba3W1.6V0.4O8.8 and Ba3WVO8.5 at 800 °C indicated that the oxide ions migrate via the octahedral O2 and tetrahedral O3 sites (Figure 3, Figure 4 and Figure 5, Tables S1–S4). The O2–O3 distance is short (e.g., 1.5880(6) Å in Ba3W1.6V0.4O8.8 at 800 °C), which suggests an interexchange between the octahedral and tetrahedral geometries and an interstitialcy cooperative diffusion mechanism of the oxide ions, as in Ba7Nb3.9Mo1.1O20.05 [18].
We have demonstrated that the excess amount of W species at the W/V site considerably improves the oxide ion conductivity. A larger excess of W species x produces a larger excess of oxygen x/2 in Ba3W1+xV1−xO8.5+x/2, which leads to more oxygen atoms at the O2 and O3 sites in the oxygen-deficient c’ layer. This creates a higher NSLD between the O2 and O3 sites, leading to the higher oxide ion conductivity (Figure 6). Thus, improvement in the oxide ion conductivity via excess oxygen can be a strategy to search for oxide-ion-conducting hexagonal perovskite-related oxides. Another reason for the higher oxide ion conductivity of Ba3W1.6V0.4O8.8 is the lower activation energy for oxide ion conduction (Figure 2d). We have suggested a size effect of the W/V cation on the activation energy for oxide ion conductivity Ea in which the lower Ea of Ba3W1.6V0.4O8.8 can be ascribed to the longer (W/V)–O2 and (W/V)–O3 distances. Therefore, the substitution of the M cation with a larger one in Ba3M2O8.5 oxides can be another strategy for improving oxide ion conductivity.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11060238/s1, Figure S1: X-ray powder diffraction pattern of Ba3W1.6V0.4O8, lattice parameters and lattice volume of Ba3W1+xV1−xO8.5+x/2 as functions of excess W content x; Figure S2: XPS spectra of Ba3W1.6V0.4O8.8 and Ba3WVO8.5; Figure S3: Thermogravimetric curves of Ba3W1.6V0.4O8.8; Figure S4: Diffuse reflectance spectra and Tauc plots of Ba3W1.6V0.4O8.8; Figure S5: Equivalent circuits and complex impedance plots of Ba3W1.6V0.4O8.8; Figure S6: Arrhenius plots of bulk and grain boundary conductivities of Ba3W1.6V0.4O8.8; Table S1: Refined crystallographic parameters and reliability factors of the neutron diffraction data of Ba3W1.6V0.4O8.8 at 18 °C; Table S2: Refined crystallographic parameters and reliability factors of the neutron diffraction data of Ba3WVO8.5 at 18 °C; Table S3: Refined crystallographic parameters and reliability factors of the neutron diffraction data of Ba3W1.6V0.4O8.8 at 800 °C; Table S4: Refined crystallographic parameters and reliability factors of the neutron diffraction data of Ba3WVO8.5 at 800 °C; Supplementary Note: Influence of BaWO4 impurity on the electrical conductivity of Ba3W1.6V0.4O8.8. References [35,36,37,38,39] are cited in the supplementary materials.

Author Contributions

Y.K. and M.Y. conceived and designed the experiments; Y.K. and Y.Y. performed the conductivity measurements; J.R.H. performed the neutron diffraction experiments; Y.K. and Y.Y. analyzed the neutron data; Y.K., Y.Y. and M.Y. wrote the original draft of the manuscript; M.Y. edited and revised the manuscript; M.Y. provided supervision, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Grants-in-Aid for Scientific Research (KAKENHI, JP19H00821, JP21K18182) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from the Japan Science and Technology Agency (JST), Grant Number JPMJTR22TC, and JSPS Core-to-Core Programs, A. Advanced Research Networks, Grant number: JPJSCCA20200004. Y.Y. acknowledges support in the form of a JSPS Fellowship for Young Scientists, DC1 (JP20J23124). The travel costs were partially supported by the Institute for Solid State Physics, The University of Tokyo (proposal No. 15616, 16604, and 16595).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, [M.Y.], upon reasonable request.

Acknowledgments

We express special thanks to T. Murakami, K. Fujii, W. Zhang, and H. Yaguchi for useful discussion and assistance in the experiments/analyses. We thank S. Iguchi for the assistance in the XPS measurements. Neutron diffraction measurements were carried out by the project approval (ANSTO: Proposal Nos. PPR6342, PP5198, and PPR5618; J-PARC: Proposal No. 2020L801, 2017L1300, 2017L1301 and 2017L1302). Synchrotron X-ray diffraction experiments were performed by the project approval (PF: 2014G508, 2015G047, 2016G644, 2017G168, and 2018G543; SPring-8: 2019A1052 and 2021A1599).

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) Arrhenius plots of DC electrical conductivities σDC of Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) in static air. (b) Variation of the corrected full-density DC electrical conductivity σDC/corr at constant temperature in static air with the excess W content x (oxygen content = 8.5 + x/2) in Ba3W1+xV1−xO8.5+x/2. σDC/corr = 2σDC/(2 − 3p), where the p is porosity [28].
Figure 1. (a) Arrhenius plots of DC electrical conductivities σDC of Ba3W1+xV1−xO8.5+x/2 (x = −0.1, −0.05, 0, 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, and 0.75) in static air. (b) Variation of the corrected full-density DC electrical conductivity σDC/corr at constant temperature in static air with the excess W content x (oxygen content = 8.5 + x/2) in Ba3W1+xV1−xO8.5+x/2. σDC/corr = 2σDC/(2 − 3p), where the p is porosity [28].
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Figure 2. Oxide ion conduction of Ba3W1.6V0.4O8.8. (a) Oxygen partial pressure P(O2) dependence of the DC electrical conductivity σDC of Ba3W1.6V0.4O8.8 at 820 °C in dry conditions. (b) Arrhenius plots of σDC in wet and dry air. (c) Complex impedance plots of Ba3W1.6V0.4O8.8 in dry air at 381 °C. (d) Arrhenius plots of the bulk conductivity σb of Ba3W1.6V0.4O8.8 (red open diamonds and line) and Ba3WVO8.5 (light green dotted line [21]) in dry air.
Figure 2. Oxide ion conduction of Ba3W1.6V0.4O8.8. (a) Oxygen partial pressure P(O2) dependence of the DC electrical conductivity σDC of Ba3W1.6V0.4O8.8 at 820 °C in dry conditions. (b) Arrhenius plots of σDC in wet and dry air. (c) Complex impedance plots of Ba3W1.6V0.4O8.8 in dry air at 381 °C. (d) Arrhenius plots of the bulk conductivity σb of Ba3W1.6V0.4O8.8 (red open diamonds and line) and Ba3WVO8.5 (light green dotted line [21]) in dry air.
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Figure 3. Rietveld patterns of neutron diffraction data of Ba3W1.6V0.4O8.8 measured at (a) 18 °C and (b) 800 °C. Rietveld patterns of neutron diffraction data of Ba3WVO8.5 at (c) 18 °C and (d) 800 °C. Red crosses and blue lines denote the observed and calculated intensities, respectively, and blue dots below show the difference patterns. Green and blue tick marks stand for the calculated peak positions of the rhombohedral R 3 ¯ m Ba3W1.6V0.4O8.8 phase and BaWO4, respectively.
Figure 3. Rietveld patterns of neutron diffraction data of Ba3W1.6V0.4O8.8 measured at (a) 18 °C and (b) 800 °C. Rietveld patterns of neutron diffraction data of Ba3WVO8.5 at (c) 18 °C and (d) 800 °C. Red crosses and blue lines denote the observed and calculated intensities, respectively, and blue dots below show the difference patterns. Green and blue tick marks stand for the calculated peak positions of the rhombohedral R 3 ¯ m Ba3W1.6V0.4O8.8 phase and BaWO4, respectively.
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Figure 4. (c) Refined crystal structure and (f) (W/V)O5.408(7) polyhedron of Ba3W1.6V0.4O8.8, which was obtained using Rietveld analysis and neutron diffraction data taken at 800 °C. The (W/V)O5.408(7) polyhedron in Ba3W1.6V0.4O8.8 (c,f) is regarded as a hybrid of (a,d) a (W/V)O6 octahedron and (b,e) a (W/V)O4 tetrahedron.
Figure 4. (c) Refined crystal structure and (f) (W/V)O5.408(7) polyhedron of Ba3W1.6V0.4O8.8, which was obtained using Rietveld analysis and neutron diffraction data taken at 800 °C. The (W/V)O5.408(7) polyhedron in Ba3W1.6V0.4O8.8 (c,f) is regarded as a hybrid of (a,d) a (W/V)O6 octahedron and (b,e) a (W/V)O4 tetrahedron.
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Figure 5. (a,c,d) Refined crystal structure and (b–d) corresponding isosurface of the neutron scattering length density (NSLD) at 1.0 fm Å−3 of Ba3W1.6V0.4O8.8 at 800 °C. NSLD distributions on the ab planes at (e) z = 0.01, (f) z = 0, and (g) z = −0.01 of Ba3W1.6V0.4O8.8 at 800 °C. Atomic displacement ellipsoids are drawn at an 80% probability level.
Figure 5. (a,c,d) Refined crystal structure and (b–d) corresponding isosurface of the neutron scattering length density (NSLD) at 1.0 fm Å−3 of Ba3W1.6V0.4O8.8 at 800 °C. NSLD distributions on the ab planes at (e) z = 0.01, (f) z = 0, and (g) z = −0.01 of Ba3W1.6V0.4O8.8 at 800 °C. Atomic displacement ellipsoids are drawn at an 80% probability level.
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Figure 6. Increased minimum neutron scattering length density (NSLD) between the O2 and O3 sites produced by excess oxygen (O0.3), which enhances the oxide ion conductivity. NSLD distributions on the ab plane at z = 0.01 of Ba3WVO8.5 (a,c) and Ba3W1.6V0.4O8.8 (b,d) at 18 °C (a,b) and 800 °C (c,d). Contour lines in the range of 0.5 to 9.5 fm Å−3 and 1 fm Å−3 per step. Schematic NSLD distributions between the nearest-neighbor O2 and O3 atoms of Ba3WVO8.5 (e) and Ba3W1.6V0.4O8.8 (f) at 800 °C.
Figure 6. Increased minimum neutron scattering length density (NSLD) between the O2 and O3 sites produced by excess oxygen (O0.3), which enhances the oxide ion conductivity. NSLD distributions on the ab plane at z = 0.01 of Ba3WVO8.5 (a,c) and Ba3W1.6V0.4O8.8 (b,d) at 18 °C (a,b) and 800 °C (c,d). Contour lines in the range of 0.5 to 9.5 fm Å−3 and 1 fm Å−3 per step. Schematic NSLD distributions between the nearest-neighbor O2 and O3 atoms of Ba3WVO8.5 (e) and Ba3W1.6V0.4O8.8 (f) at 800 °C.
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Kikuchi, Y.; Yasui, Y.; Hester, J.R.; Yashima, M. Improved Oxide Ion Conductivity of Hexagonal Perovskite-Related Oxides Ba3W1+xV1−xO8.5+x/2. Inorganics 2023, 11, 238. https://doi.org/10.3390/inorganics11060238

AMA Style

Kikuchi Y, Yasui Y, Hester JR, Yashima M. Improved Oxide Ion Conductivity of Hexagonal Perovskite-Related Oxides Ba3W1+xV1−xO8.5+x/2. Inorganics. 2023; 11(6):238. https://doi.org/10.3390/inorganics11060238

Chicago/Turabian Style

Kikuchi, Yugo, Yuta Yasui, James R. Hester, and Masatomo Yashima. 2023. "Improved Oxide Ion Conductivity of Hexagonal Perovskite-Related Oxides Ba3W1+xV1−xO8.5+x/2" Inorganics 11, no. 6: 238. https://doi.org/10.3390/inorganics11060238

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