Polymorphism and Oxide Ion Migration Pathways in Fluorite-Type

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Polymorphism and Oxide Ion Migration Pathways in Fluorite-Type Bismuth Vanadate, Bi46V8O89 Xiaojun Kuang,† Julia L. Payne,† James D. Farrell,† Mark R. Johnson,‡ and Ivana Radosavljevic Evans*,† †

Department of Chemistry, Durham University, Science Site, Durham DH1 3LE, United Kingdom Institute Laue Langevin, Grenoble, France

‡

S Supporting Information *

ABSTRACT: We report the synthesis, structural characterization, and ionic conductivity measurements for a new polymorph of bismuth vanadate Bi46V8O89, and an ab initio molecular dynamics study of this oxide ion conductor. Structure determination was carried out using synchrotron powder X-ray and neutron diffraction data; it was found that β-Bi46V8O89 crystallizes in space group C2/m and that the key differences between this and the previously reported α-form are the distribution of Bi and V cations and the arrangement of the VO4 coordination polyhedra in structure. β-Bi46V8O89 exhibits good oxide ion conductivity, with σ = 0.01−0.1 S/cm between 600 and 850 °C, which is about an order of magnitude higher than yttria stabilized zirconia. The ab initio molecular dynamics simulations suggest that the ion migration pathways include vacancy diffusion through the Bi−O sublattice, as well as the O2− exchanges between the Bi−O and the V−O sublattices, facilitated by the variability of the vanadium coordination environment and the rotational freedom of the VOx coordination polyhedra. KEYWORDS: Oxide ion conductors, bismuth vanadates, X-ray and neutron diffraction, AIMD simulations

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cell parameters proposed by Kashida et al.,7 except in space group P21/c. They described the Bi46V8O89 structure as built up by stacking of slabs made up of one (Bi18O27) layer and two consecutive (Bi14V4O31) layers along the c-axis; all VO4 tetrahedra in these layers were found to be isolated, contrary to the previously proposed V4O10 clusters.2 Substitution of Bi with Pb, Sr, Ca, and other metals can stabilize different fluoriterelated phases with general formula (BiM)46V8Oy,9 including Pb10Bi36V8O86 (I2/m)10,11 and Sr10Bi36V8O84 (Immm),9 which both have a fluorite supercell: a ∼ 3/2[1, −1, 0] aF, b ∼ 3/2[1, 1, 0] aF, and c ∼ [0, 0, 3] aF. Crystallographic information on the (BiM)46V8Oy vanadates and the analogous phosphate Bi46P8O89 is summarized in Table S1 in the Supporting Information. Among this group of compounds, composition Pb6Bi40V8O86 has been found to have two polymorphs; one (labeled Pb6Bi40V8O86-1 in Table S1, Supporting Information) has a unit cell similar to Bi46V8O89 reported by Darriet et al.,8 but crystallizes in C-centered monoclinic space group C2/m.9 The second form of Pb6Bi40V8O86 (labeled Pb6Bi40V8O86-2 in Table S1, Supporting Information) has a larger unit cell, but the same C2/m symmetry. The difference between the two polymorphs is in the stacking sequence of the Bi/Pb/V/O slabs: in Pb6Bi40V8O86-1, the repeat unit along the c-axis is formed by two slabs, while in Pb6Bi40V8O86-2 it consists of three such building blocks. Interestingly, both forms of Pb6Bi40V8O86 were identified from single crystals grown by slow-cooling of the

INTRODUCTION There is considerable interest in bismuth vanadates because of their structural diversity1,2 and attractive physical properties such as high oxide ion mobility3,4 and catalytic activity.5 In the Bi-rich part of the Bi2O3−V2O5 binary system, several phases with the general formula Bi2nV2O3n+5 (n = 2, 3.5, 4, 6, 8, and 9) have been reported. The n = 2 member Bi2VO5.5 is a layered Aurivillius phase; its Cu-doped derivative Bi2Cu0.1V0.9O5.35 has the highest, albeit two-dimensional, oxide ion conductivity at low temperatures.6 Phases with n = 3.5−9 possess fluoriterelated structures, classified by Zhou1 as type I (n = 9, pseudocubic 3 × 3 × 3 fluorite cell) and type II (triclinic for n = 3.5−6), with the n = 8 member being an intermediate phase between type I and type II. The structures of most type II Bi2nV2O3n+5 materials remain unknown. Using high resolution transmission electron microscopy (HRTEM) simulations, Zhou initially proposed the existence of V4O10 clusters in the n = 6 member.1,2 Cation distribution was subsequently determined by Kashida et al.7 from single crystal X-ray diffraction, using a pseudomonoclinic (P1̅) fluorite supercell with a ∼ 3/2[1, 1, 2] aF, b ∼ 3/2[−1, 1, 0] aF, and c ∼ 1/2[−5, −5, 2] aF; this metal ordering model led to a cation content of Bi92V16 per unit cell. Watanabe3 prepared polycrystalline Bi46V8O89 by heating the starting oxides at 850 °C and quenching the reaction mixture to room temperature and grew single crystals from the melt. He proposed a different triclinic fluorite-related cell: a ∼ 3/2[1, 1, 0] aF, b ∼ 3/2[−1, 0, −1] aF, and c ∼ 1/2[−5, 5, 2] aF; however, the structure solution was unsuccessful due to twinning of the crystals. The full crystal structure of Bi46V8O89 remained unknown until Darriet et al.8 solved it by single X-ray diffraction, using the unit © 2012 American Chemical Society

Received: March 14, 2012 Revised: May 3, 2012 Published: May 3, 2012 2162

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density) for electrical characterization. The ac impedance spectroscopy measurements in air were carried out with a Solartron 1260 frequency response analyzer over a frequency range from 10−1 to 106 Hz. The pellet was coated with platinum paste on each face, mounted in a ProboStat measurement cell, and fired at 800 °C for 30 min to burn out the organic components in the paste to form platinum electrodes. The impedance data were collected on heating to 800 °C and cooling back to room temperature. Zview3.0a software was used to analyze the complex impedance data. Computational studies were performed by the ab initio molecular dynamics method (AIMD), which allows accurate modeling of bond breaking and formation necessary for ion diffusion. DFT-based first principles calculations were performed using the projector-augmented wave (PAW) formalism16 of the Kohn−Sham DFT17,18 at the generalized gradient approximation (GGA) level, implemented in the Vienna ab initio simulation package (VASP).19,20 The GGA was formulated by the Perdew−Burke−Ernzerhof (PBE) density functional.21,22 The Gaussian broadening technique was adopted, and all results converged well with respect to k-mesh and energy cutoff for the plane wave expansion. AIMD simulations were performed, following geometry optimization to reduce residual forces on atoms, in the NVT ensemble, with temperature being controlled with a Nosé thermostat. The MD time step was 2 fs. Simulations of 10 000 steps (20 ps) were performed at 1373 K. The MD simulations were analyzed using LAMP (large array manipulation program)23 while mean square displacements (MSD), and coordination numbers were calculated with the nMoldyn code.24

melt from the same starting composition, suggesting that the differences in stability are very subtle. Watanabe3 performed dc conductivity measurements and determination of the oxide ion transport number and reported high oxide ion conductivity in his Bi46V8O89 sample (6 × 10−3 to 6 × 10−2 S/cm between 600 and 850 °C, with oxide ion transport number τ = 0.9−0.95 above 600 °C). Darriet et al.8 postulated that three partially occupied oxygen sites bonded to Bi provided the pathway for the ionic conductivity in Bi46V8O89. We have previously suggested that variable coordination numbers of smaller cation sites are important for ionic migration. For example, in lanthanum molybdate, La2Mo2O9, we identified the variable (4-, 5-, and 6-fold) Mo coordination as a key structural feature facilitating fast ion conductivity.12 We also showed that the high temperature cubic β-structure could be viewed as a time-average of the room temperature monoclinic α-La2Mo2O9, with an order−disorder phase transition between the two leading to high oxide ion conductivity.12 The role of variable central atom coordination in oxide ion mobility was also demonstrated in the melilitebased interstitial oxide conductor La1+xSr1−xGa3O7+0.5x.13 The ability of the V5+ cation to support flexible coordination environments has led us to undertake experimental and computational studies of structures and oxide ion dynamics in a number of Bi2O3−V2O5 systems, with the aim of elucidating structures, properties, and ion migration pathways. We recently reported the remarkably high low-temperature ionic conductivity in Bi1−xVxO1.5+x (x = 0.087, 0.095; σ ∼ 3.9 × 10−2 S/cm at 500 °C) and the roles that the different structural building blocks play in the conductivity.14 In this paper, we report the preparation and the structure of a new polymorph of Bi46V8O89, its conductivity from impedance measurements, and ab initio molecular dynamics simulations that elucidate the ionic migration pathways. Our results reveal that, contrary to the previous proposal, both Bi−O and V−O sublattices play important roles in oxide ion conduction, via O2− migration mechanisms facilitated by the variable coordination of V5+ and the rotational freedom of the VOx groups.

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RESULTS AND DISCUSSION Structure. Figure 1 shows the observed laboratory XRD pattern of our Bi46V8O89 sample (curve 1) in comparison with

EXPERIMENTAL SECTION

Polycrystalline Bi46V8O89 samples were synthesized by a hightemperature solid state reaction using Bi2O3 (99.9%, Sigma-Aldrich) and V2O5 (99.99%, Sigma-Aldrich) as starting materials, which were weighed in a stoichiometric ratio and ground under isopropanol. Gradual heating at 700, 750, 800, and 825 °C, for 12 h at each temperature with intermediate grinding, resulted in a 6 g sample, which was used for X-ray and neutron diffraction studies, chemical analysis, and ac impedance measurements. The phase content of the samples (powder and pellets) at ambient temperature was monitored by laboratory X-ray diffraction, performed on a Bruker AXS D8 Advance diffractometer with a Vantec detector, using Cu Kα radiation. The ambient temperature synchrotron X-ray diffraction (λ = 0.826430 Å) data were collected at Diamond Light Source beamline I11, on a sample loaded in a 0.3 mm glass capillary. The ambient temperature time-of-flight neutron diffraction data were collected at high resolution powder diffraction (HRPD) at ISIS, Rutherford Appleton Laboratory. Rietveld analysis of the diffraction data was carried out using Topas Academic.15 The sample composition was analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) on a Perkin-Elmer-Sciex Elan6000. The sample was dissolved in nitric acid. The composition from ICP analysis is Bi46.2(2)V7.8(2), in good agreement with the nominal composition. The powdered sample was pressed into a pellet and fired at 900 °C for 12 h, which resulted in a dense pellet (∼93% of X-ray theoretical

Figure 1. Observed laboratory XRD pattern of β-Bi46V8O89 (1) in comparison with the patterns calculated using structural models in space groups C2/m (curve 2, Pb6Bi40V8O86-1), P21/c (curve 3, primitive α-Bi46V8O89 model), and the triclinic P1̅ model (curve 4).

the patterns calculated using three existing models: the Ccentered monoclinic structure of Pb6Bi40V8O86-19 (curve 2), the primitive monoclinic Bi46V8O89 structure8 (curve 3), and the approximate (cations-only) triclinic model proposed by Kashida et al.7 (curve 4). The comparison reveals that our powder pattern does not contain weak reflections, which violate the C-centering, present in the pattern of Bi46V8O89 in space group P21/c; we hereinafter refer to this new material as β-Bi46V8O89 structure, and to the primitive structure as the α-form. In addition to one of the polymorphs of Pb6Bi40V8O86,9 Bi46P8O89 also adopts space group C2/m.8 The phosphate structure was determined from single crystal data,8 and the model obtained contained one split Bi site (in a 0.67:0.33 fractional occupancy ratio), four split O sites (50:50), and one O site with a fractional occupancy of 2163

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Figure 2. Rietveld plots obtained for β-Bi46V8O89: (a) synchrotron and (b) neutron diffraction data. The ticks mark Bragg reflection positions.

Figure 3. Structure of the two forms of Bi46V8O89: the ac view illustrating the layer description of the two polymorphs; the bc view in polyhedral representation illustrating different organization of structural building blocks. Yellow spheres , Bi atoms; red spheres, O atoms; orange tetrahedra, VO4 groups.

disordered model for the refinement against combined X-ray and neutron data was constructed. This model contained two rigid tetrahedra centered on vanadium V2 and allowed the central atom position to split into two sites. The model thus allowed for both orientational and translational disorder of this structural segment. This model was used for refinement against combined powder synchrotron X-ray diffraction and neutron diffraction data sets, both covering a range down to d = 0.9 Å. Structural parameters refined included unit cell parameters, three isotropic temperature factors, the positions of all freely defined atoms, the orientations of the VO4 rigid bodies, and the fractional occupancies of the two tetrahedra centered on vanadium V2. Two runs of 10 000 cycles of simulated annealing were performed from this starting model, and the same minimum was found several times. The obtained distance between the centers of the two tetrahedra disordered around the V2 site was 0.23(4) Å, and the ratio of their fractional occupancies was

0.75, to satisfy the nominal stoichiometry. Due to the complexity of the structure and the fact that only powder diffraction data were available, we simplified the phosphate structure by removing the splitting of the Bi and O sites and used this as the initial model for the Rietveld refinement of βBi46V8O89. The model obtained in this way consisted of 48 crystallographically unique atoms: 14 Bi sites, 4 V sites, and 30 O sites (12 of which are occupied by atoms bonding to V only (OV), and the remaining 18 sites by O atoms belonging to the fluorite-like Bi−O sublattice (OF)). The initial Rietveld analysis was performed on the powder synchrotron data to obtain good coordinates for the metal sites; the powder neutron diffraction data were then used to probe the structural features of the oxygen sublattice. When fractional occupancies of the OV oxygen sites were allowed to vary, they all refined to values between 0.9 and 1.0, except for oxygen atoms bonded to vanadium V2, suggesting disorder at this site. Based on the individual analyses of the two data sets, a 2164

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0.57(2):0.43(2). It is worth noting that similar orientational disorder was found around the equivalent P2 site in Bi46P8O89 structure, determined by single crystal X-ray diffraction; in this case, the refinement of the relative occupancies was not attempted, and the two PO4 tetrahedra were treated with occupancies of 0.50 each.8 The final agreement factors obtained were as follows: overall Rwp = 3.484%; Rwp = 3.381%, and RB = 1.505% for X-ray data; Rwp = 4.868% and RB = 3.705% for neutron data. The unit cell parameters obtained for β-Bi46V8O89 are a = 20.02938(5) Å, b = 11.59621(3) Å, c = 21.12860(6) Å, β = 111.3366(2)°, V = 4571.07(2) Å3. The atomic coordinates are supplied as Supporting Information, and the Rietveld plots for βBi46V8O89 are shown in Figure 2. Figure 3 shows the structure of β-Bi46V8O89, in comparison with the previously reported α form; disorder is omitted from these figures for clarity. Both polymorphs of Bi46V8O89 can be described as built up by stacking of slabs formed by a single (Bi18O27) layer and two consecutive (Bi14V4O31) layers, as shown in the ac view; the formula can be expressed as [(Bi14V4O31)2(Bi18O27)] to reflect this description. As a result of different cation distributions, the two forms exhibit different arrangements of the basic structural building blocks, illustrated in the right-hand pair of pictures in Figure 3, the bc projections. In both polymorphs, the Bi−O sublattice contains tetrahedral OBi4 groups; in addition, some oxygen atoms are surrounded by three Bi atoms, forming trigonal OBi3 groups. The OBi4 and OBi3 coordination polyhedra are connected by corner- and edge-sharing, providing pathways for oxide ion migration through the structure (vide inf ra). AC Impedance Measurements and Ionic Conductivity. The ac impedance data for β-Bi46V8O89 comprised the bulk, grain boundary and electrode responses. A typical complex impedance plot at 205 °C (Figure 4a) shows a large semicircular arc for the bulk response and a small semicircular arc for the grain boundary response. The low frequency intercepts of the semicircular arcs were used to estimate bulk and total resistivity, respectively. The electrode exhibited a linear Warburg-type response, with large capacitance (10−7 to 10−6 F/cm) at low frequency (below 10 Hz) above 250 °C, consistent with ionic conduction. With increasing temperature, the bulk and grain boundary responses gradually disappeared and the electrode response dominated the impedance data above 300 °C; this collapsed gradually above 500 °C, leading to a single semicircular arc at high temperature above 600 °C (inset in Figure 4a). The high frequency intercept of the semicircular arc was estimated as the total resistivity at high temperature. Figure 4b shows the Arrhenius plot of the total conductivity for βBi46V8O89, and exhibits a change of slope at around 400 °C. The activation energy of 0.98(2) eV in the low temperature region (150−300 °C) decreases to 0.729(2) eV in the high temperature region (400−850 °C). Between 600 and 850 °C, the total conductivity of β-Bi46V8O89 varies between 0.01 and 0.1 S/cm and is about an order of magnitude higher than yttria stabilized zirconia in the same temperature range.25 This is in good agreement with the values obtained from dc conductivity measurements by Watanabe,3 who reported an activation energy of 0.78 eV and oxygen transport number of 0.9−0.95 above 600 °C, indicating predominantly oxide ion conduction in Bi46V8O89.3 We note that Watanabe did not determine the structure of the material he characterized, but the indexed

Figure 4. Complex impedance plots at 205 °C (a) and Arrhenius plot of total conductivity (b) for β-Bi46V8O89 pellet. In (a), the inset shows the complex impedance plot at 679 °C and Rb, Rgb and Rt denote the bulk, grain boundary and total resistivity, respectively. The numbers denote the selected frequency logarithms at the points marked by filled squares in the plot. In (b), the activation energies in the high temperature region (400−850 °C) and the low temperature region (150−300 °C) are marked.

diffraction pattern he gave (Figure 1 in ref 3) suggests that this was β-Bi46V8O89, with the structure reported herein. Oxide Ion Migration Pathways in Bi46V8O89. We have previously reported a combined experimental and ab initio molecular dynamics (AIMD) study of fluorite-related oxide ion conductor Bi8La10O27, in which concerted O2− migration proceeds through oxygen vacancies, with the partially occupied oxygen crystallographic sites playing a key role in the process.26 During their structural analysis of α-Bi46V8O89, Darriet et al.8 similarly identified three partially occupied oxygen sites belonging to the coordination spheres of Bi atoms and suggested that these provided a pathway for oxide ion migration. To gain qualitative descriptive insight into the O2− migration mechanisms in Bi46V8O89, we performed AIMD simulations using the β-Bi46V8O89 structural model transformed into P1 symmetry. Because the structural models for DFT simulations cannot contain partial occupancies, all sites must be treated as fully occupied; 2 out of 180 oxygen atoms were therefore removed, at random, resulting in the simulation box contents of Bi92V16O178. We have also performed AIMD simulation on α-Bi46V8O89, for which the simulation box was 2165

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derived by removing 6 out of 184 oxygen atoms at random from the P1 symmetry structure. The fact that we obtained the same oxide ion exchange pathways (vide inf ra) for the two forms, which contain the same structural building blocks in different arrangements, indicates that the method of generating the starting configuration does not bias the qualitative results sought and obtained in the simulations on this time scale. Oxide ion motion can be expressed quantitatively by the mean square displacement (MSD) of different groups of atoms. The observation of a linearly increasing MSD curve indicates diffusional motion of the oxide ions in β-Bi46V8O89 (Figure 5).

Figure 7. Oxide ion migration mechanisms in Bi46V8O89, white clouds represent volume visited by oxide ions during simulations. (a) Exchange of an oxide ion between the Bi−O sublattice and a VO4 tetrahedron. (b, c) Diffusion within the Bi−O sublattice proceeds via O2− jumps between edge- and corner-sharing OBi4 and OBi3 groups. (d) O2− exchange between two VOx polyhedra through a sequence of jumps involving the OBi3 groups, the OBi4 tetrahedra, and octahedral □Bi6 vacancies. Red arrows are used as guide to the eye to accentuate the diffusion directions.

Figure 5. MSD of oxygen atoms in β-Bi46V8O89 in the 1373 K AIMD simulation.

sublattices are further illustrated by pathways shown in Figure 7d. This shows the O2− exchange between two VOx polyhedra through a sequence of jumps within the Bi−O sublattice, which proceed via vacancies in the OBi3 groups, the OBi4 tetrahedra, and octahedral □Bi6 vacancies, commonly found in δ-Bi2O3related materials. It is important to note that these oxide ion diffusion mechanisms are facilitated not only by the ability of vanadium to support variable coordination numbers, but also by the rotational flexibility of the VOx groups, illustrated in Figure 7 by large and nearly spherical white clouds. In the AIMD simulation on α-Bi46V8O89, we observed the same types of exchanges and the same O2− migration mechanisms as described above. The corresponding MSD curve lies slightly below that shown in Figure 5, suggesting a slightly lower O2− mobility in α-Bi46V8O89; however, this cannot be verified by the experimental conductivity data, since the α-form has only been prepared in single crystal form and its bulk physical properties have not been determined.8 In the absence of direct experimental comparison, drawing quantitative conclusions and comparisons between the two forms of Bi46V8O89 based on relatively short (20 ps) AIMD simulation does not seem justified. It should be noted, however, that the mechanisms we found in the AIMD simulations presented here are similar to the experimentally based conclusions of Holmes et al., who used 17O solid state NMR to provide detailed insight into the conductivity mechanism in the δ-Bi2O3‑related columnar phase Bi 26 Mo10O69.27 They observed oxygen exchange between the two sublattices, creating vacancies in the Bi−O sublattice and leading to oxide ion conductivity involving both the Bi−O slabs and the Mo−O coordination polyhedra, similar to the O2− migration pathways represented in Figure 7. Furthermore, similar O2− exchange mechanisms both within the Bi−O sublattice and between the Bi−O and V−O sublattices were recently found to be responsible for the exceptional low-temperature oxide ion conductivity in fluorite-

Figure 6 depicts the variation of the V average coordination number, calculated over 16 sites in the unit cell, over the course

Figure 6. Variation of the vanadium average coordination number in β-Bi46V8O89 over the course of the AIMD simulation at 1373 K.

of the MD simulations. The average coordination numbers rises during the simulation from its starting value of 4, reaching the maximum of 4.31 (corresponding to 5 out of 16 initially tetrahedral V sites being five-coordinate, or to a combination of 1 six-coordinate, 3 five-coordinate, and 12 four-coordinate V atoms). The key oxide ion migration pathways, observed many times in the 1373 K AIMD simulation of β-Bi46V8O89, are shown in Figure 7; white clouds in this figure represent volumes visited by oxide ions. The increase in the vanadium coordination number, shown in Figure 6, is associated with the diffusion of an oxide ion from the Bi−O sublattice onto a VO4 tetrahedron (Figure 7a). This creates a transient VO5 species and a vacancy in the Bi−O sublattice, promoting ionic diffusion in this part of the structure, which can occur by O2− migration between edgeand corner-sharing OBi4 and OBi3 groups (Figure 7b and c). Oxide ion exchange between the Bi−O and the V−O 2166

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related Bi1−xVxO1.5+x (x = 0.087, 0.095; σ ∼ 3.9 × 10−2 S/cm at 500 °C).14 It is therefore appropriate to address possible reasons for the differences in conductivity between good oxide ion conductors in the Bi/V/O system (such as the title compound) and the exceptional ones. We attributed the remarkable conductivity in Bi1−xVxO1.5+x (x = 0.087, 0.095) to the simultaneous presence of four advantageous structural features: the highly polarizable Bi−O sublattice; the variable coordination geometry for V; the favorable topology of V−O sublattice; and a pseudocubic ordered superstructure.14 The donor-doping of smaller cations into Bi2O3 reduces the oxygen vacancy content, and with increasing doping levels, vacancies tend to accumulate around dopants, eventually leading to formation of lower-symmetry superstructures with lower conductivities. At V-doping levels corresponding to the title compound, the key structural changes that have occurred have a detrimental effect on the oxide ion conductivity. Although the V cations, capable of supporting variable coordination environments, are present, the higher V content leads to shorter distances between adjacent VOx groups. The polarizable Bi−O sublattice, which exists in Bi46V8O89, is therefore significantly less “δ-Bi2O3-like”, as its three-dimensional connectivity has been largely disrupted. These factors lead to lower oxide ion conductivity relative to the compositions with low V content.

REFERENCES

(1) Zhou, W. J. Solid State Chem. 1988, 76, 290. (2) Zhou, W. Z. J. Solid State Chem. 1990, 87, 44. (3) Watanabe, A. Solid State Ionics 1997, 96, 75. (4) Boivin, J. C.; Mairesse, G. Chem. Mater. 1998, 10, 2870. (5) Boivin, J. C.; Pirovano, C.; Nowogrocki, G.; Mairesse, G.; Labrune, P.; Lagrange, G. Solid State Ionics 1998, 113, 639. (6) Abraham, F.; Boivin, J. C.; Mairesse, G.; Nowogrocki, G. Solid State Ionics 1990, 40−1, 934. (7) Kashida, S.; Hori, T. J. Solid State Chem. 1996, 122, 358. (8) Darriet, J.; Launay, J. C.; Zuniga, F. J. J. Solid State Chem. 2005, 178, 1753. (9) Labidi, O.; Drache, M.; Roussel, P.; Wignacourt, J. P. Solid State Sci. 2008, 10, 1074. (10) Giraud, S.; Wignacourt, J. P.; Swinnea, S.; Steinfink, H.; Harlow, R. J. Solid State Chem. 2000, 151, 181. (11) Giraud, S.; Obbade, S.; Suard, E.; Steinfink, H.; Wignacourt, J. P. Solid State Sci. 2003, 5, 335. (12) Evans, I. R.; Howard, J. A. K.; Evans, J. S. O. Chem. Mater. 2005, 17, 4074. (13) Kuang, X.; Green, M. A.; Niu, H.; Zajdel, P.; Dickinson, C.; Claridge, J. B.; Jantsky, L.; Rosseinsky, M. J. Nat. Mater. 2008, 7, 498. (14) Kuang, X. J.; Payne, J. L.; Johnson, M. R.; Evans, I. R. Angew. Chem., Int. Ed. 2012, 51, 690. (15) Coelho, A. A.; Evans, J. S. O.; Evans, I. R.; Kern, A.; Parsons, S. Powder Diffr. 2011, 26, S22. (16) Blochl, P. E. Phys. Rev. B 1994, 50, 17953. (17) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, B864. (18) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, 1133. (19) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (20) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396. (23) LAMP, the Large Array Manipulation Program. Available online: http://www.ill.eu/instruments-support/computing-forscience/cs-software/all-software/lamp. (24) Rog, T.; Murzyn, K.; Hinsen, K.; Kneller, G. R. J. Comput. Chem. 2003, 24, 657. (25) Kharton, V. V.; Marques, F. M. B.; Atkinson, A. Solid State Ionics 2004, 174, 135. (26) Li, Y. D.; Hutchinson, T. P.; Kuang, X. J.; Slater, P. R.; Johnson, M. R.; Evans, I. R. Chem. Mater. 2009, 21, 4661. (27) Holmes, L.; Peng, L. M.; Heinmaa, I.; O’Dell, L. A.; Smith, M. E.; Vannier, R. N.; Grey, C. P. Chem. Mater. 2008, 20, 3638.

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CONCLUSIONS We have prepared, structurally characterized, and measured the ionic conductivity of a new polymorph (β-form) of Bi46V8O89. Structure determination from powder synchrotron X-ray and neutron diffraction data demonstrates that the main difference between this and the previously reported α-form is the cation distribution (leading to C-centered and primitive space groups, respectively). Additional structural differences are manifested as variations in the arrangement of the OBi4/OBi3 coordination polyhedra in the Bi−O sublattice, and the VO4 tetrahedral groups. β-Bi46V8O89 is a good oxide ion conductor, with σ = 0.01−0.1 S/cm between 600 and 850 °C, which is about an order of magnitude higher than yttria stabilized zirconia. Oxide ion migration pathways include vacancy diffusion through the Bi−O sublattice, commonly found in δ-Bi2O3 based superstructures, but also O2− exchanges between the Bi−O and the V−O sublattices, facilitated by the flexibility of the vanadium coordination environment and the rotational freedom of the VOx groups.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the EPSRC for funding through Grant No. EP/F030371 and the ILL for the summer studentship for J.D.F. STFC is acknowledged for the facility beam time awards; Dr. C. C. Tang and Dr. A. Daoud-Aladine are thanked for the assistance with data collections at Diamond and ISIS, respectively. 2167

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