High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

O xide-ion conducting ceramic materials have attracted much attention due to their applications in solid-oxide fuel cells (SOFCs), oxygen separation membranes, gas sensors, and catalysts  . Yttria stabilized zirconia (YSZ) ceramics have widely been used, but the working temperature is high (700-1000°C). Thus, there remains a strong motivation to explore oxide-ion conductors with higher conductivities at temperatures below 600°C. High oxide-ion conductivities have been observed in specific structure families such as the fluorite-type, perovskite-type, melilite-type, and apatite-type structures  .
The perovskite and its related materials exhibit interesting physical and chemical properties 25 and can be classified into four structural groups of (i) AMX 3 perovskite-type, (ii) AMX 3 -related, (iii) hexagonal perovskite-related and (iv) modular structures 26 where A and M are larger and smaller cations, respectively, and X is an anion. A number of perovskite-type materials and perovskite related phases belonging to the groups of (i), (ii) and (iv) have been reported to be oxide-ion conductors. The hexagonal perovskite-related structures (iii) have hexagonal close packing of AX 3 layers or sequences of hexagonal (h) and cubic (c) AX 3 (and/ or anion-deficient AX 3-x (c′) where x is the anion vacancy content) close-packed layers. The hexagonal perovskite related oxides exhibit a variety of crystal structures [26][27][28][29][30][31] . However, oxide-ion conductors with hexagonal perovskite-related structures are quite rare. Several mixed ion (oxide-ion and/or proton) and electronic conductors with hexagonal perovskite related structures were reported in the literature [32][33][34][35] . The hexagonal perovskite related oxides Ba 3 MNbO 8.5-δ (M = Mo and W; δ is the oxygen deficiency) and their solid solutions exhibit significant oxide-ion conductivities 23,30,31,[36][37][38][39] , however, the conductivities are not high at temperatures below 600°C. The relatively low conductivities are ascribed to relatively high activation energy for conductivity (e.g., 1.2 eV for Ba 3 MoNbO 8.5-δ 23 ). Therefore, we have explored oxide-ion conductors with the hexagonal perovskite related structures. Ba 7 Nb 4 MoO 20 is a trigonal P 3m1 hexagonal perovskite polytype 7H 29,40 . Fop et al. found high oxide-ion and proton conductivities of Ba 7 Nb 4 MoO 20 40 . Herein, we report higher oxide-ion conductivities, crystal structure and oxide-ion diffusion pathways of the solid solution composition Ba 7 Nb 3.9 Mo 1.1 O 20.05 . Ba 7 Nb 3.9 Mo 1.1 O 20.05 is found to exhibit a bulk conductivity of 5.8 × 10 −4 S cm −1 at 310°C, which is higher than those of the "best" oxide-ion conductors. The present work also demonstrates the two-dimensional (2D) oxide-ion O1-O5 diffusion through the interstitial octahedral O5 and lattice tetrahedral O1 sites at a high temperature of 800°C.  Supplementary Fig. 4).

Results and discussion
Oxide-ion conduction in Ba 7 Nb 3.9 Mo 1.1 O 20.05 . Figure 1a, b shows the typical impedance spectra of Ba 7 Nb 3.9 Mo 1.1 O 20.05 , which indicates the bulk, grain boundary and electrode responses. Bulk conductivity (σ b ), grain-boundary conductivity (σ gb ), and grain-boundary capacitance were obtained by the equivalent circuit fitting (Red lines in Fig. 1a, b, Supplementary Figures 5-9). The σ b and σ gb were measured in dry O 2 , dry air and in dry N 2 at 295 and 598°C. They were independent of oxygen partial pressure at these temperatures, which indicates ionic conduction (Supplementary Figure 10). Figure 1c shows the temperature dependencies of the σ b and σ gb of Ba 7 Nb 3.9 Mo 1.1 O 20.05 on cooling in dry air. The activation energy for σ b was found to be lower than those for σ gb and the DC total electrical conductivity, σ tot . The σ b was higher than σ gb at temperatures below 550°C and higher than σ tot at temperatures below 850°C. The oxide-ion transport number (t ion ) was investigated using oxygen concentration cell measurements. The t ion values were 1.00 between 700 and 900°C and 0.99 at 600°C in air/O 2 , 0.99 at 800°C and 1.00 at 900°C in air/N 2 , and 0.98 at 600°C in air/5% H 2 in N 2 (Fig. 1d). Oxide-ion diffusion in dense Ba 7 Nb 3.9 Mo 1.1 O 20.05 was clearly confirmed by the isotope exchange depth profile method 41 , which calculated a high oxygen tracer diffusion coefficient D* value of 5.35 × 10 -9 cm 2 s -1 at 700°C and 7.25 × 10 -9 cm 2 s -1 at 800°C (Supplementary Figure 11). The diffusion lengths were about 150 μm and the grain sizes of the Ba 7 Nb 3.9 Mo 1.1 O 20.05 samples were 1-5 μm (Supplementary Figure 12), thus, the 18 O tracer species encountered a number of grains and grain boundaries. The total DC electrical conductivity (σ tot ) was independent of the oxygen partial pressure P(O 2 ) between P(O 2 ) = 7 × 10 −25~1 atm at 300°C , P(O 2 ) = 2 × 10 −26~1 atm at 600°C, and P(O 2 ) = 5 × 10 −181 atm at 900°C (Fig. 1e). Electronic conductivity was observed in the P(O 2 ) range of 6 × 10 −24~4 × 10 −26 atm at 900°C with the slope of −0.129 (19). Therefore, Ba 7 Nb 3.9 Mo 1.1 O 20.05 exhibits a wider electrolyte domain compared with Ba 7 Nb 4 MoO 20 as reported by Fop et al. 40 . To examine the possible proton conduction of this phase, the DC electrical conductivities, σ tot , of Ba 7 Nb 3.9 Mo 1.1 O 20.05 were measured in wet air (H 2 O partial pressure, P(H 2 O) = 2.3 × 10 −2 atm) and in dry air (P(H 2 O) < 1.8 × 10 −4 atm). The contribution of protons to σ tot of Ba 7 Nb 3.9 Mo 1.1 O 20.05 was small, even in wet air where the proton transport number was 0.03 − 0.12 ( Supplementary Fig. 13). Water incorporation behavior was also investigated and the results are shown in Supplementary Fig. 14  oxide-ion conductor. No change was observed in the X-ray powder diffraction patterns before and after the oxygen concentration cell measurements ( Supplementary Fig. 15), which demonstrates the high phase stability of Ba 7 Nb 3.9 Mo 1.1 O 20.05 at high temperatures and in the wide P(O 2 ) region between P(O 2 ) = 1.2 × 10 −27 and 1 atm. The σ b of Ba 7 Nb 3.9 Mo 1.1 O 20.05 was found to be higher than those of Ba 7 Nb 4 MoO 20 40 (Fig. 1c) and YSZ, and comparable to those of the best oxide-ion conductors (Fig. 1f). It should be noted that the σ b of Ba 7 Nb 3.9 Mo 1.1 O 20.05 was higher than the best oxide-ion conductors at temperatures of around 300°C, due to the low activation energy of Ba 7 Nb 3.9 Mo 1.1 O 20.05 (0.185-0.454 eV as shown in Fig. 1c). These results indicate the high potential of the hexagonal perovskite related oxide Ba 7 Nb 3.9 Mo 1.1 O 20.05 as a superior oxide-ion conductor.  La9.5Ge5.5Al0.5O26.5 La10Si6O26  Structural origin of the high oxide-ion conductivity in Ba 7 Nb 3.9 Mo 1.1 O 20.05 . Next, we discuss the structural origin of the high oxide-ion conductivity of Ba 7 Nb 3.9 Mo 1.1 O 20.05 , based on its refined crystal structure and neutron scattering length density (NSLD) at 800°C (Fig. 2). In the Rietveld refinements of the neutron-diffraction data, the Mo 6+ and Nb 5+ cations were assumed to be disordered, since they have quite similar neutron scattering lengths. By the trigonal P 3m1 hexagonal perovskite polytype 7H (c′hhcchh; Fig. 2a), the crystal structure of Ba 7 Nb 3.9 Mo 1.1 O 20.05 was successfully refined by Rietveld analyses of the neutron-diffraction data measured in situ at a temperature of 800°C in vacuum on the super-high-resolution diffractometer, SuperHRPD 42,43 at J-PARC, Japan ( Fig. 3 and Supplementary  Table 1). In order to examine the oxide-ion diffusion pathway and to validate the crystal structure of Ba 7 Nb 3.9 Mo 1.1 O 20.05 , the NSLD was analysed using the maximum-entropy method (MEM) and structure factors obtained through the Rietveld analysis. It is well known that the MEM is a powerful tool to study the structural disorder and ion-diffusion pathways in various ionic conductors 16,19,31 . Oxygen atoms were found to partially occupy the octahedral interstitial O5 site in the Ba1 (O1) 2−x (O5) 0.05+x layer (Fig. 2a), because (i) the Rietveld fit for the structural model with the O5 atom (weighted profile reliability factor R wp = 2.39%) was lower than those without the O5 atom (R wp = 2.47%) and (ii) the MEM NSLD distribution clearly shows the O5 site (Fig. 2b, d).
Here the x in Ba1(O1) 2−x (O5) 0.05+x is the vacancy content at the O1 site in the unit cell. We applied the split-atom model for the tetrahedral O1 site, because the atomic displacement parameter was quite high for the non-splitatom model and the Rietveld fit for the split-atom model (R wp = 2.39%) was better than that for the non-split atom model (R wp = 2.44%). The crystal structure of Ba 7 Nb 3.9 Mo 1.1 O 20.05 consists of an oxide-ion conducting Ba1(O1) 2−x (O5) 0.05+x layer (c′), two Ba2 (O2) 3 layers (h), two Ba4(O4) 3 layers (h), two Ba3(O3) 3 layers (c), and Nb and Mo cations at the Nb/Mo1, Nb/Mo2 and Nb/ Mo3 sites (Fig. 2a). A striking feature of the MEM NSLD distribution of Ba 7 Nb 3.9 Mo 1.1 O 20.05 at 800°C is the connected oxide-ion diffusional pathway between the tetrahedral O1 and interstitial octahedral O5 sites on the oxide-ion conducting Ba1 (O1) 2−x (O5) 0.05+x layer (c′) (Fig. 2b, d). The oxide ions twodimensionally migrate through both lattice O1 and interstitial O5 sites, which indicates the interstitialcy mechanism of oxideion diffusion. The bond-valence-based energy barriers for oxideion migration, E b , for the refined crystal structure of Ba 7 Nb 3.9 -Mo 1.1 O 20.05 at 800°C also supported this 2D feature, because the E b along the ab plane (0.19 eV) is much lower than E b along the c axis (1.54 eV). Ba 7 Nb 3.9 Mo 1.1 O 20.05 has an excess oxygen of x = 0.05 (O 20+x or O 0.05 in Ba 7 Nb 3.9 Mo 1.1 O 20.05 ) compared with the mother material Ba 7 Nb 4 MoO 20 , which leads to a larger amount of interstitial oxygen and the higher oxide-ion conductivity of Ba 7 Nb 3.9 Mo 1.1 O 20.05 (Fig. 1c).   In conclusion, we have discovered a structure family of rareearth-free oxide-ion conductors based on the hexagonal perovskite related oxide Ba 7 Nb 4 MoO 20 . Ba 7 Nb 3.9 Mo 1.1 O 20.05 shows a wide stability range and predominantly oxide-ion conduction in the oxygen partial pressure range from 2 × 10 -26 to 1 atm at 600°C. The bulk conductivity of Ba 7 Nb 3.9 Mo 1.1 O 20.05 is as high as 5.8 × 10 −4 S cm −1 at 310°C. This high conductivity is ascribed to the interstitial O5 oxygen, 2D oxide-ion O1 − O5 diffusion through the lattice tetrahedral O1 and interstitial O5 octahedral oxygen sites on the ab plane at z = 0 and to the low activation energy for oxide-ion conductivity. The (tetrahedral O1)-(octahedral O5) diffusion pathways in Ba 7 Nb 3.9 Mo 1.1 O 20.05 are along the [1 10], [120] and [ 2 10] directions (Arrows in Fig. 2c), which are the same as those for the (tetrahedral O3)-(octahedral O2) migration paths in the hexagonal perovskite related oxide Ba 3 MoNbO 8.5-δ 31 . This strongly suggests that the (tetrahedral)-(octahedral) oxide-ion migration pathway along the [1 10], [120] and [ 2 10] directions on the oxygen deficient c′ layer is a common feature of the oxideion conductors with hexagonal perovskite related structures. This feature would be a guide for design of oxide-ion conductors with the hexagonal perovskite-related structures. The present finding of high oxide-ion conductivities in rare-earth-free Ba 7 Nb 3.9 -Mo 1.1 O 20.05 suggests the ability of various hexagonal perovskite related oxides as superior oxide-ion conductors.

Methods
Synthesis and characterization. Ba 7 Nb 3.95 Mo 1.05 O 20.025 and Ba 7 Nb 3.9 Mo 1.1 O 20.05 were prepared by the solid-state reactions. High-purity (> 99.9%) BaCO 3 , Nb 2 O 5 , and MoO 3 were mixed and ground using an agate mortar and pestle as ethanol slurries and dry powders repeatedly for 0.5-2 h. The obtained mixtures were calcined at 900°C for 10-12 h in static air. The calcined samples were crushed and ground using an agate mortar and pestle as ethanol slurries and dry powders repeatedly for 0.5-2 h. The powders thus obtained were uniaxially pressed into pellets at 62-150 MPa and subsequently sintered in static air at 1100°C for 24 h.
Parts of the sintered pellets were crushed and ground into white powders to measure X-ray powder diffraction, atomic absorption spectroscopy (AAS, Hitachi Z-2300), inductively coupled plasma optical emission spectroscopy (ICP-OES, Hitachi PS3520UVDD), and thermogravimetric (TG) data. To identify the existing phases, X-ray powder diffraction patterns of Ba 7 Nb 3.95 Mo 1.05 O 20.025 and Ba 7 Nb 3.9 Mo 1.1 O 20.05 were measured at RT with an X-ray powder diffractometer (BRUKER D8 Advance, Cu Kα radiation, 2θ range: 5−90°). The chemical composition of Ba 7 Nb 3.9 Mo 1.1 O 20.05 was examined by energy dispersive XRF analyses (Rigaku, NEX DE). XPS spectra of Ba 7 Nb 3.9 Mo 1.1 O 20.05 were measured using an Xray photoelectron spectrometer (ULVAC PHI 5000 Versa Probe III). TG analysis was carried out in dry air using a Bruker-AXS 2020SA instrument at the heating and cooling rates of 10°C min −1 . The heating and cooling cycle was repeated three times to negate the influence of absorbed species, such as water and to confirm the reproducibility of the measurement.
Measurements of electrical conductivity, oxygen diffusion coefficient and transport properties. The electrical conductivities of Ba 7 Nb 3.9 Mo 1.1 O 20.05 were measured as a function of temperature by AC impedance spectroscopy in flowing dry air, N 2 , and O 2 gases (100 mL min −1 ) using a sintered pellet (20 mm in diameter, 2.7 mm in thickness, relative density of 100−98%) with Pt electrodes. Impedance spectra were recorded with a Solartron 1260 impedance analyser in the frequency range of 10 MHz−1 Hz at an applied alternating voltage of 100 mV. The activation energies, E a , for the conductivities were estimated using the Arrhenius equation: where A 0 , k, and T are the pre-exponential factor, Boltzmann constant, and absolute temperature, respectively. Oxygen concentration cell measurements were conducted to investigate the oxygen transport number t ion using a sintered pellet (20 mm in diameter, 4.5 mm in height, and relative density of 100−98%) attached to an alumina tube with a glass seal. One side of the pellet was exposed to flowing dry air and the other side to flowing dry O 2 (Air/O 2 ), N 2 (Air/N 2 ), or 5% H 2 in N 2 (Air/5% H 2 in N 2 ) gases at high temperatures. The electromotive forces of the concentration cell were recorded with a Keithley model 617 electrometer. The following Nernst equation was utilized to estimate the t ion : where F is the Faraday constant, R is the gas constant, T is the absolute temperature, p(O 2 ) is the oxygen partial pressure of the gas of O 2 , N 2 , 5% H 2 in N 2 , and p 0 (O 2 ) (= 0.21 atm) is the oxygen partial pressure of dry air. After the oxygen concentration cell measurements, the surface of the pellet was ground with sandpaper carefully to remove the Pt paste and then crushed and ground into powder. X-ray diffraction patterns of the resulting powders were measured to investigate the phase stability at high temperatures and different atmospheres. The total electrical conductivity σ tot of the Ba 7 Nb 3.9 Mo 1.1 O 20.05 pellet (relative density: 95%) was measured by a DC-4-probe method with Pt electrodes at various oxygen partial pressure p(O 2 ). The p(O 2 ) was controlled using a mixture of O 2 , N 2 , and 5% H 2 in N 2 and p(O 2 ) was monitored by an oxygen sensor. 18 O tracer diffusion measurements of dense Ba 7 Nb 3.9 Mo 1.1 O 20.05 pellets (relative density of 98-100%) were carried out using the line scan method by secondary ion mass spectrometry (SIMS) 41 . Each sample prepared was polished with diamond spray media down to a finish of 0.25 µm. Samples were pre-annealed in dry research grade oxygen (BOC 99.996%) of natural isotopic abundance for a duration of 10 times that of the isotopic exchange. The samples were subsequently annealed for 2 h in 18 O-enriched gas at a pressure of ≃200 mbar. After the exchange anneal, the samples were cut perpendicular to the original surface and the exposed cross-sections polished to 0.25 μm finish, as above. The oxygen diffusion profiles were measured by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) using a ToF-SIMS.5 instrument (IONTOF GmbH) using Bi + ions at 25 keV energy. Values of oxygen self-diffusion, D*, and surface exchange, k, coefficients were obtained by fitting the experimental data to Crank's solution of Fick's 2 nd law of diffusion 41,44 using the TraceX software 45 (8.7 mm in diameter, 43 mm in height) in a Ti-Zr alloy holder were carried out in vacuum using a super-high-resolution time-of-flight (TOF) neutron diffractometer (SuperHRPD) installed at the Materials and Life Science Experimental Facility of J-PARC, Japan 42,43 . The absorption correction was performed using the method given by Rouse and Cooper 46 . The diffraction data were analysed by the Rietveld method using the Z-Rietveld program 47 . The neutron scattering length density distribution was investigated using the MEM. The MEM analysis was carried out with computer program, Dysnomia 48 , using the structure factors obtained in the Rietveld refinement of the neutron diffraction data at 800°C. The MEM calculations were performed with the unit cell divided into 60 × 60 × 168 pixels. Computation of the bond-valence-based energy barrier for oxide-ion migration. The bond-valence-based energy landscapes for a test oxide ion were calculated using the SoftBV 49 software with a spatial resolution of 0.1 Å.
The refined crystal structure, MEM neutron scattering length density distributions, and bond-valence-based energy landscape were depicted using VESTA 50 .

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.