High Oxide‐Ion Conductivity through the Interstitial Oxygen Site in Sillén Oxychlorides

Oxide‐ion conductors are gaining attention as future materials in energy applications, such as solid oxide fuel cells. Many Bi‐containing compounds exhibit high oxide‐ion conductivity via conventional vacancy mechanism. However, interstitial oxide‐ion conduction is rare in Bi‐containing materials. Herein, high oxide‐ion conductivity is reported through interstitial oxygen sites in Sillén oxychlorides, LaBi2−xTexO4+x/2Cl (Bi2LaO4Cl‐based oxychlorides). Oxide‐ion conductivity of LaBi1.9Te0.1O4.05Cl is 20 mS cm−1 at 702 °C, and higher than best oxide‐ion conductors as Bi2V0.9Cu0.1O5.35 below 201 °C. Despite of the presence of Bi and Te species, LaBi1.9Te0.1O4.05Cl shows extremely high chemical and electrical stability at 400 °C from oxygen partial pressure 10−25 to 0.2 atm and high chemical stability under CO2 flow, wet 5% H2 in N2 flow, and air with natural humidity. Neutron scattering length density analysis, DFT calculations, and ab initio molecular dynamics simulations indicate that the extremely high oxide‐ion conduction is attributed to cooperative diffusion through interstitial oxygen sites (interstitialcy diffusion mechanism) in triple fluorite‐like layers. The present findings demonstrate the ability of LaBi2−xTexO4+x/2Cl as superior oxide‐ion conductors, which can open new horizons for oxide‐ion conductors.

Generally, oxide ions in solids migrate through the oxygen vacancies (Figure 1a). [10][11][12][13][14] However, recently, there has been a growing interest in interstitialcy migration involving the knock-on motion of interstitial and lattice oxide ions (Figure 1d). Interstitialcy diffusion has been reported in Ruddlesden-Popper phases, melilite-type oxides, scheelite-type oxides, and hexagonal perovskite derivatives. [15][16][17][18][19][20][21][22][23][24][25] Bi-containing materials, such as Bi 2 O 3 , Na 0.5 Bi 0.5 TiO 3 , CsBi 2 Ti 2 NbO 10−δ , and Bi 3.9 Sr 0.1 NbO 8−δ Cl exhibit high oxide-ion conductivities, where δ is the amount of oxygen deficiency. [12,[26][27][28][29][30][31][32] In particular, the Aurivillius phases Bi 2 V 0.9 Cu 0.1 O 5. 35 (BICUVOX) and Bi 2 V 0.7 Sb 0. 3 O 5.5 show the highest oxide-ion conductivity at the low temperatures among known oxide-ion conductors. [33,34] The crystal structures of Bi 3.9 Sr 0.1 NbO 8−δ Cl, BICUVOX, and Bi 2 V 0.7 Sb 0.3 O 5.5 have a double fluorite-like Bi 2 O 2 layer ( Figure S1b,c, Supporting Information). Oxide ions in these known Bi-containing materials, such as fluorite-type Bi 1.4 Yb 0.6 O 3 migrate via the conventional vacancy mechanism (Figure 1a-c). [32] High oxide-ion conductivity can be attained by introducing interstitial oxygen atoms into Bi-containing materials. However, oxide-ion conduction via interstitialcy diffusion is rare in Bi-containing materials. Punn et al. reported that the defect fluorite-type oxide-ion conductors Bi 12.5 R 1.5 ReO 24.5 (= AO 1.63 ; A = Bi 0.83 R 0.1 Re 0.07 , R = La, Nd, Er, and Y) have interstitial oxygen sites of 32f Wyckoff position ( Figure S2e,f, Supporting Information). [35] However, the oxide ions in Bi 12.5 R 1.5 ReO 24.5 would migrate not by the interstitialcy mechanism, but by the vacancy mechanism, because the AO 1.63 oxides are highly defective and occupancy factor at the regular 8c site 0.615 is low (Table S1, Supporting Information). In the present study, we report high oxide-ion conduction and its origin in the Sillén phase, LaBi 1.9 Te 0.1 O 4.05 Cl (Bi 2 LaO 4 Cl-based oxychloride). A Sillén phase with a triple fluorite-like layer has been selected because of the presence of an interstitial site in the layer (Figure 1d-f), leading to high oxide-ion conduction. We have chosen Te 4+ as a higher-valent dopant than Bi 3+ to increase the amount of interstitial oxygen atoms, because Te 4+ has a similar ionic radius with Bi 3+ and is a lone-pair cation. Sillén phases have attracted much interest because of their applications in photocatalysis and luminescence. [36][37][38] However, oxide-ion conduction in Sillén phases has not yet been reported.  ,d and  Table S2, Supporting Information). Preliminary analysis suggested no interstitial atoms in LaBi 2 O 4 Cl. Refined crystal parameters of LaBi 2 O 4 Cl agreed with those in the literature. [36] The crystal structures of LaBi 2−x Te x O 4+x/2 Cl (x = 0.1, 0.2) at RT were successfully refined using the tetragonal P4/mmm Sillén phases (Figure S3b,c and Tables S3 and S4, Supporting Information). All the reflection spots in the selected-area electron diffraction patterns of LaBi 1.9 Te 0.1 O 4.05 Cl at room temperature were indexed to the primitive tetragonal lattice with a = 3.99 Å and c = 9.06 Å ( Figure S4, Supporting Information); this is consistent with the results of the Rietveld analyses based on the tetragonal P4/mmm Sillén phase. The absence of second-harmonic generation (SHG) signal of LaBi 1.9 Te 0.1 O 4.05 Cl indicated a center of symmetry; this is also consistent with the space group P4/mmm. The bulk conductivity, σ b and grain-boundary conductivity, σ gb of LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, 0.15, and 0.2) were investigated via impedance measurements in dry air (Figures 2  and 3a,b; Figures S5 and S6, Supporting Information). The values for σ b , σ gb , bulk capacitance, and grain-boundary capacitance were obtained by the equivalent circuit fitting ( Figure S7, Supporting Information). The bulk and grain-boundary capacitances were ≈8 × 10 −12 and ≈2 × 10 −10 F cm −1 , which are indicative of the bulk and grain boundary responses, respectively. [39] The σ b and σ gb values of LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, 0.15, and 0.2) increased with increasing temperature, and σ b was higher than σ gb at all the temperatures (  value in LaBi 1.9 Te 0.1 O 4.05 Cl than LaBi 2 O 4 Cl is attributable to the higher carrier concentration (that is, the amount of excess oxygen atoms x/2 in LaBi 2−x Te x O 4+x/2 Cl) and higher tetragonal symmetry. Meanwhile, the σ b of LaBi 2−x Te x O 4+x/2 Cl decreased with increasing of excess oxygen atoms x/2 from x =0.10 to 0.20. Such a decrease in the ion conductivity due to overdoping has been observed in several oxide ion conductors attributed to defect association or clustering among the dopants and oxygen defects. [40,41]  These results strongly suggest that oxide ions are the predominant conducting species. Ab initio molecular dynamics (AIMD) simulations show that the mean-square displacement (MSD) of oxide ions is much higher than that of cations and chloride ions ( Figure S8, Supporting Information), which further supports oxide-ion conduction in LaBi 2−x Te x O 4+x/2 Cl materials. The σ b of LaBi 1.9 Te 0.1 O 4.05 Cl (4.6 × 10 −5 S cm −1 at 121 °C) is five times higher than that of the best oxide-ion conductor in the literature, Bi 2 V 0.9 Cu 0.1 O 5.35 (9.5×10 −6 S cm −1 at 121 °C). The σ b of LaBi 1.9 Te 0.1 O 4.05 Cl (1.6 × 10 −3 S cm −1 at 285 °C) is 320 times higher than that of YSZ (5.0 × 10 −6 S cm −1 at 300 °C). It should be noted that the σ b of LaBi 1.9 Te 0.1 O 4.05 Cl is even higher than the best oxide-ion conductors from 96 to 201 °C (Figure 4). X-ray powder diffraction (XRD) patterns of LaBi 1.9 Te 0.1 O 4.05 Cl after annealing at 400 °C for 24 h in wet 5% H 2 in N 2 flow, after annealing under CO 2 gas flow at 400 °C for 24 h, and after annealing at 400 °C for 100 h in static air with natural humidity are in agreement with those before these annealing processes ( Figure S9, Supporting Information), indicating an extremely high chemical stability. Since the P(O 2 ) for Bi/Bi 2 O 3 equilibrium is of the order of 10 −13 atm, Bi oxides could be reduced into metallic Bi metal at low P(O 2 ) <10 −13 atm. [42] Compared with Bibased oxides, such as Bi 3.9 Sr 0.1 NbO 8−δ Cl and (Bi 0.725 Y 0.275 ) 2 O 3 , LaBi 1.9 Te 0.1 O 4.05 Cl shows higher chemical and electrical stability in wide P(O 2 ) regions down to 10 −25 atm (Figures S9 and S10, Supporting Information). [30,43] The high oxide-ion conduction, and chemical and electrical stability of LaBi 1.9 Te 0.1 O 4.05 Cl indicate its potential as a superior oxide-ion conductor.

Results and Discussion
To discuss the structural origin of the high oxide-ion conductivity of LaBi 1.9 Te 0.1 O 4.05 Cl, we investigated the crystal structure and neutron scattering length density (NSLD) distribution using neutron diffraction (ND) data measured in situ from 25 to 400 °C. Preliminary Rietveld and maximum-entropy method (MEM) analyses were performed using the ND data of LaBi 1.9 Te 0.1 O 4.05 Cl recorded at 25 °C, based on the "model without interstitial oxygen atoms" where the oxygen atom is placed only at the lattice O1 site ( Figure S11a, Supporting Information). The MEM analysis of the model without interstitial oxygen atoms indicated NSLD distributions around the hexahedral vacant sites (Wyckoff 1c site; atomic coordinates: 1/2, 1/2, 0) (dashed circle in Figure S11b, Supporting Information), suggesting the presence of interstitial oxygen atoms. Therefore, the Rietveld and MEM analyses were further carried out using the "interstitial model" wherein O atoms are placed at both the interstitial 1c O2 and lattice 4i O1 sites (red and orange ellipsoids in Figure 5a,c). The reliability (R) factors for the interstitial model (weighted profile R factor, R wp = 4.35%, R factor based on Bragg intensities, R B = 4.23%, and R factor based on structure factors, R F = 2.38%) were lower than those for the model without interstitial oxygen atoms (R wp = 4.86%, R B = 4.41%, and R F = 2.89%). The refined occupancy factors  of the O atom at the O1 and O2 sites are 0.9768(12) and 0.143(5), respectively. The obtained values are consistent with the numbers of oxygen atoms estimated using the NSLD distribution (O1:0.95 and O2:0.04). The presence of an energy minimum at the interstitial O2 site in the bond valence-based energy landscape for a test oxide ion also supports the presence of interstitial oxide ion at the O2 site ( Figure S11c, Supporting Information). Using DFT calculations, the structure of LaBi 1.9 Te 0.1 O 4.05 Cl was successfully optimized based on the interstitial model, and its formation energy was found to be negative, suggesting the formation of LaBi 1.9 Te 0.1 O 4.05 Cl (Note S1, Supporting Information). These results clearly indicate the presence of interstitial oxygen atoms at the hexahedral O2 sites in the triple fluorite-like layer of LaBi 1.9 Te 0.1 O 4.05 Cl. As described above, the bulk conductivity, σ b of LaBi 1.9 Te 0.1 O 4.05 Cl is higher than that of LaBi 2 O 4 Cl, which is attributable to the higher charge carrier concentration (higher occupancy factor of the interstitial oxygen atom and larger amount of excess oxygen atom O 0.05 ) compared to LaBi 2 O 4 Cl (occupancy factor of interstitial oxygen atom = zero and no excess oxygen atom, Tables S2 and S5, Supporting Information).
The calculated bond valence sums (BVSs) at 25 °C agreed with the average values of the formal charges, supporting the validity of the refined crystal structures (Table S5, Supporting Information). The refined lattice parameters and atomic coordinates using the ND data at 25 °C were in agreement with those obtained from the synchrotron X-ray diffraction data at 24 °C, as well as those from the DFT structural optimization (Tables S6-S9   occupancy factors are shown in Figures S13 and S14 (Supporting Information) and the details of the crystal structure are described in Note S3, Note S4, and Figure S16 (Figures 1d-f and 5). In contrast, known Bi-containing oxide-ion conductors, such as Aurivillius phase Bi 2 V 0.9 Cu 0.1 O 5.35 and Sillén-Aurivillius compound Bi 3.9 Sr 0.1 NbO 8−δ Cl, do not have a triple fluorite-like layer, but a double fluorite-like layer without interstitial oxygen sites ( Figure S1a-c, Supporting Information). [30,33] On the contrary to the known Bi-containing oxide-ion conductors via conventional vacancy mechanism, in the present work, we discovered high oxide-ion conduction through interstitial site and interstitialcy diffusion mechanism in LaBi 1.9 Te 0.1 O 4.05 Cl as discussed below.
To examine the oxide-ion diffusion pathway in LaBi 1.9 Te 0.1 O 4.05 Cl, the NSLD was analyzed using MEM and structure factors obtained through the Rietveld analysis. The NSLD distribution around an interstitial O2 site was localized at 25 °C (Figure 5d), whereas at higher temperatures, the O2 atom exhibited larger spatial distributions (Figure 5e,f). This observation is consistent with the higher oxide-ion conductivity at higher temperatures (Figure 3b). It should be noted that at 400 °C the NSLD distributions in the triple fluorite-like layer are connected between the lattice O1 and interstitial O2 sites in the <301> directions, which indicates the −O1−O2−O1− oxideion diffusion pathways in the triple fluorite-like layer, forming a two-dimensional (2D) network of diffusion paths (Figure 5b Figure 6; Video S1, Supporting Information), which is consistent with the experimental oxide-ion diffusion pathways observed in NSLD distributions (Figure 5b,f). An interstitial oxide ion O B (pink sphere) at the O2 site pushes (kicks) another oxide ion O A (red sphere) at the nearest-neighbor lattice O1 site toward an adjacent vacant interstitial O2 site, clearly showing interstitialcy migration (Figure 6a).
Oxide-ion migration in La 9 Bi 16 Te 2 O 37 Cl 9 was also investigated by static DFT calculations, where an oxide ion was moved step-by-step from an interstitial O2 site to its nearest-neighbor O1 site and structural optimization was carried out at each step fixing the reaction coordinate of the moved oxide ion. The estimated energy barrier for the oxide-ion migration via an interstitialcy mechanism 0.69 eV is consistent with the experimental activation energy for bulk conductivity 0.500 (17)
Many fluorite-type materials exhibit high fluoride-ion conductivity via the interstitialcy mechanism. [44,45] Meanwhile, the interstitialcy diffusion of oxide ions in the triple fluorite-like layer in LaBi 1.9 Te 0.1 O 4.05 Cl is a novel phenomenon because the interstitialcy diffusion of oxide ions is quite rare in fluorite-type materials and compounds with fluorite-like layers. Although fluorite-type UO 2+x also shows high conductivity via the interstitialcy mechanism, [46] it is a p-type semiconductor and is a radioactive material. [47] The high oxide-ion conduction via interstitialcy mechanism in the Bi-containing material LaBi 1.9 Te 0.1 O 4.05 Cl is unique. Oxide-ion conduction occurs via vacancy mechanism in most of the known Bi-containing materials, such as Na 0.5 Bi 0.5 TiO 3 , CsBi 2 TiNb 2 O 10−δ , Bi 2 V 0.7 Sb 0.3 O 5.5 , Bi 2 V 0.9 Cu 0.1 O 5.35 , and Bi 3.9 Sr 0.1 NbO 8−δ Cl, [12,[26][27][28][29][30][31][32][33][34]48,49] because of the absence of interstitial sites and presence of oxygen vacancies in these materials. The high ion conductivity due to the unique interstitialcy diffusion and high chemical and electrical stability of LaBi 1.9 Te 0.1 O 4.05 Cl open up new window of oxide-ion conductors, leading to high-performance SOFCs at low temperature.

Experimental Section
Synthesis and Characterization: LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, 0.15, and 0.2) samples were synthesized via a solid-state reaction route. Highpurity (99.9%) powders of La 2 O 3 , Bi 2 O 3 , TeO 2 , and BiOCl were used as starting materials. These materials were mixed and ground using an agate mortar for 30 min. The powders thus obtained were sintered at 800 °C for 24-60 h in Ar or vacuum. Sintered pellets of LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, 0.15, and 0.2) were used for electrical conductivity measurements. Parts of the sintered pellets of LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, and 0.2) were crushed and ground into powders for X-ray powder diffraction (XRD), UV-vis diffuse reflectance (UV-vis), and  [50] The XRD and ND data were analyzed by the Rietveld method using Z-code program. [51] To estimate the numbers of O atoms around the O1 and O2 sites, the local integration of the neutron scattering length density (NSLD) at each O site was performed based on the Voronoi tessellation using VESTA 3 software. [52] In order to investigate the oxide-ion diffusion pathway based on the NSLD distribution, MEM analyses were performed with the structure factors obtained via the Rietveld analysis of LaBi 1.9 Te 0.1 O 4.05 Cl and the computer program Z-MEM. [53] Bond valence-based energy (BVE) [54] for a test oxide ion O 2− in the crystal structure of LaBi 1.9 Te 0.1 O 4.05 Cl was obtained with SoftBV [55] software to investigate the oxide-ion migration paths and energy barriers for oxide-ion migration. The crystal parameters refined in the ND analyses at 400 °C were used for the BVE calculations. BVE landscapes were calculated with an approximate spatial resolution of 0.1 Å. The crystal structures, NSLD distributions and BVE landscapes were drawn with VESTA 3. [52] Selected-area electron diffraction (SAED) patterns were observed using an energy-filter transmission electron microscope of JEM-2010FEF operated at an accelerating voltage of 100 kV. Secondharmonic generation (SHG) measurements at 25 °C were performed with a YAG: Nd laser operating at a wavelength of 1064 nm (Note S5, Supporting Information).
Electrical Conductivity Measurements: The impedance spectra of LaBi 2−x Te x O 4+x/2 Cl (x = 0, 0.1, 0.15, and 0.2) were measured in dry air using sintered pellets (diameter 5 mm, thickness 5 mm) with Pt electrodes. Impedance spectra were recorded with Solartron 1260 impedance analyzer in the frequency range of 10 MHz-0.1 Hz at an alternating voltage of 100 mV. Equivalent-circuit analysis was performed to extract the bulk conductivity σ b and grain-boundary conductivity σ gb ( Figure S7, Supporting Information) using ZView software (Scribner Associates, Inc.). Electrical conductivities of LaBi 1.9 Te 0.1 O 4.05 Cl were also measured using DC 4-probe method in dry Ar with sintered pellets (diameter 5 mm, height 13 mm, and relative density 93%). The temperature dependence of the DC electrical conductivity σ DC of LaBi 1.9   The DC polarization measurements of LaBi 1.9 Te 0.1 O 4.05 Cl were carried out on a sintered pellet of 20 mm diameter and 2.4 mm height, by applying a constant current of 1 mA in an Ar atmosphere for 300 min at 700 °C. To investigate the transport number of the oxide ion, t ion , concentrationcell measurements of LaBi 1.9 Te 0.1 O 4.05 Cl were performed at 200 °C using a sintered pellet (18 mm diameter, 4.0 mm thickness, and 98% relative density) attached to an alumina tube with a sealing material. One side of the pellet was exposed to flowing dry air and the other side to flowing dry O 2 (Air/O 2 ) and 5%H 2 in N 2 (Air/5%H 2 in N 2 ) gases. The electromotive force of the concentration cell was measured using a Keithley 617 electrometer. t ion was estimated using the following equation, based on the Nernst equation for the oxygen concentration cell: where F is the Faraday constant, R is gas constant, T is absolute temperature, P(O 2 ) is oxygen partial pressure of O 2 and 5%H 2 in N 2 , and P 0 (O 2 ) (= 0.21 atm) refers to oxygen partial pressure of dry air. Density Functional Theory (DFT)-Based Calculations: The generalized gradient approximation (GGA) electronic calculations were performed using the Vienna Ab initio Simulation Package (VASP). [56] The oxide-ion diffusion was examined by both static DFT calculations (described in this section) and ab initio molecular dynamics (AIMD) simulations (described in the next section). Lattice parameters and atomic coordinates of La 9 Bi 16 Te 2 O 37 Cl 9 (3 × 3 × 1 supercell) and La 18 Bi 34 Te 2 O 73 Cl 18 (3 × 3 × 2 supercell) were optimized in the space group P1, with the convergence condition of 0.02 eV Å −1 . (La 9 Bi 16 Te 2 O 37 Cl 9 ) (3 × 3 × 1) and 3 × 3 × 2 (La 18 Bi 34 Te 2 O 73 Cl 18 ) supercells were used to study the interstitialcy oxide-ion migration and formation energy, respectively. For DFT calculations, projector augmented-wave (PAW) potentials for La, Bi, Te, O, and Cl atoms, plane-wave basis sets with a cutoff of 500 eV, and Perdew-Burke-Ernzerhof (PBE) GGA functionals were used. A 2 × 2 × 3 set of k-point meshes was used in the Monkhorst-Pack scheme. To investigate the interstitialcy oxide-ion migration, the relaxation of atom positions with the convergence criterion of 0.02 eV Å −1 was performed by moving an O anion step-by-step from one O2 interstitial site to a nearest-neighbor lattice O1 site knocking another O anion from the O1 to an adjacent O2 interstitial site. First, the initial and final structures were optimized, and then, a linear interpolation of 20 grid points between these was employed to define the migration process. Energy profiles were studied using a constrained energy minimization method. [57] Energy barriers were calculated using the total energy difference between the initial state and saddle point.
To investigate the mean-square displacements (MSDs), snapshots, and trajectories, AIMD simulations were performed at 900 °C using a 3 × 3 × 1 supercell of La 9 Bi 16 Te 2 O 37 Cl 9 . The simulations were performed with VASP at a constant temperature within the canonical (NVT) ensemble using a Nosé thermostat. The system was heated to the target temperature (900 °C) at a rate of 1 °C fs −1 , and further isothermal AIMD simulations were performed at 900 °C for 40 ps with a time step of 2 fs. Due to the large size of the supercells (≈12.0 × 12.0 × 9.23 Å 3 ), integration in the reciprocal space was performed only at the Γ-point. The AIMD snapshots and trajectories were visualized using the OVITO program. [58] The MSDs of all atoms were obtained using Pymatgen code. [59,60]

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.