Article pubs.acs.org/cm
Local Structure, Dynamics, and the Mechanisms of Oxide Ionic Conduction in Bi26Mo10O69 Chris D. Ling,†,* Wojciech Miiller,†,‡ Mark R. Johnson,§ Didier Richard,§ Stéphane Rols,§ Jim Madge,∥ and Ivana R. Evans∥ †
School of Chemistry, The University of Sydney, Sydney 2006, Australia The Bragg Institute, ANSTO, PMB 1, Menai 2234, Australia § Institute Max Von Laue Paul Langevin, F-38042 Grenoble, France ∥ Department of Chemistry, Durham University, Science Site, Durham DH1 3LE, United Kingdom ‡
S Supporting Information *
ABSTRACT: We report the results of a computational and experimental study into the stabilized fluorite-type δ-Bi2O3-related phase Bi26Mo10O69 aimed at clarifying the local and average structure, for which two distinct models have previously been proposed, and the oxide ionic diffusion mechanism, for which three distinct models have previously been proposed. Concerning the structure, we propose a new model in which some molybdenum atoms have higher coordination numbers than 4; that is, some MoO5 trigonal bipyramids coexist with MoO4 tetrahedra. This accounts for the additional oxygen required to achieve the nominal composition (a tetrahedrononly model gives Bi26Mo10O68) without invoking a previously proposed unbonded interstitial site, which we found to be energetically unfavorable. All these MoOx units are rotationally disordered above a first-order transition at 310 °C, corresponding to a first-order increase in conductivity. Concerning oxide ionic diffusion above that transition temperature, we found excellent agreement between the results of ab initio molecular dynamics simulations and quasielastic neutron scattering experiments. Our results indicate a mechanism related to that proposed by Holmes et al. (Chem. Mater. 2008, 20, 3638), with the role previously assigned to partially occupied interstitial oxygen sites played instead by transient but stable MoO5 trigonal bipyramids and with more relaxed requirements in terms of the orientation and timing of the diffusive jumps. KEYWORDS: Bi26Mo10O69, delta-Bi2O3, oxide ionic conduction, synchrotron X-ray diffraction, quasielastic neutron scattering, density functional theory, ab initio molecular dynamics, fluorite-type
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temperatures.1,4 Ionic conductivity research has largely focused on the RE-stabilized phases, which have disordered structures and are generally the better conductors, but suffer from instability issues. Work on the TM-stabilized phases has concentrated on their crystallography, where they are a rich source of novel modulated structures. However, some of these modulated structures appear to incorporate well-defined conduction pathways that may make them resistant to the aging phenomena that affect RE-doped phases. The conductivity of the recently reported Bi1−xVxO1.5+x (x = 0.087, 0.095) materials reaches 1.2 × 10−2 S/cm at 400 °C, the best observed in a stable δ-Bi2O3 superstructure.5 Among the bismuth molybdates, the highest conductivities are found in Bi26Mo10O69 and Bi38Mo7O78, up to 1 mS cm−1 at 500 °C.6,7 Vannier et al.6 identified a unique structure type in the Bi2O3−MoO3 pseudobinary system of nominal composition Bi26Mo10O69, with a small solid solution range (2.57 ≤ Bi/Mo
INTRODUCTION The high-temperature phase δ-Bi2O3 is the best oxide ion conductor known (as high as 1−1.5 S cm−1 around 800 °C1,2) due to the presence of 1/4 oxygen vacancies in its face-centered cubic fluorite-type structure. However, δ-Bi2O3 is only stable from 730 °C up to its melting point of 830 °C. The low temperature α phase and the intermediate β and γ phases eliminate these vacancies, resulting in drastically reduced conductivity; transformations to these phases creates strain that fatally compromises the structural integrity of the films or ceramics used in any working device. Consequently, yttriastabilized zirconia Zr1−xYxO2−x/2 (YSZ), which shows no such transformation, has remained the leading solid-state oxide ionic conductor for commercial (and most prototype) solid-oxide fuel cells since the middle of the last century, despite far lower conductivities in the order of 0.01 S cm−1 for optimally doped (x = 0.16) YSZ at 730 °C.3 The average fluorite-type structure of δ-Bi2O3 can be preserved to room temperature by doping with rare earth (RE) or transition metal (TM) cations such as Mo6+, in some cases retaining similar conductivity to pure δ-Bi2O3 at high © XXXX American Chemical Society
Received: October 3, 2012 Revised: November 13, 2012
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dx.doi.org/10.1021/cm303202r | Chem. Mater. XXXX, XXX, XXX−XXX
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≤ 2.77), exhibiting pure oxide ionic conduction. They reported that it undergoes a reversible monoclinic → triclinic phase transition on cooling through 310 °C, coinciding with a significant decrease in ionic conductivity and an increase in activation energy. They collected single-crystal X-ray diffraction data at room temperature, but could only solve and refine a model in a monoclinic unit cell a = 11.74, b = 5.80, c = 24.79 Å, β = 102.84° with P2/c space-group symmetry. This cell is related to the fluorite-type (subscript f) δ-Bi2O3 parent structure by a = 3/2af + 3/2cf, b = bf, c = −4af + 2cf. It consists of bismuth-rich fluorite-type (δ-Bi2O3-related) infinite columns along the [010] ≡ ⟨010⟩f direction, surrounded by MoO4 tetrahedra and a single isolated bismuth site (labeled Bi7). It is unclear whether this model represents a quenched version of the high-temperature monoclinic form or an approximate solution to the low-temperature triclinic form. This space-group symmetry and unit cell are not compatible with a fully ordered structural model at the nominal composition Bi26Mo10O69. The model described above gives Bi26Mo10O68, which is oxygen-deficient based on the presumed oxidation states of the cations, Bi3+ and Mo6+. (Mo can adopt lower oxidation states, but the fact that the compound is very pale yellow in color and electrically insulating suggests that this is not the explanation.) Buttrey et al.8 accounted for this discrepancy by identifying a 25% occupied interstitial oxygen site O19, between MoO4 tetrahedra but not directly bonded to any Mo atom. All these features are shown in Figure 1. The tetrahedra themselves were found have large anisotropic ADPs, suggesting significant thermal libration. Vannier et al.9,10 then proposed a model in which O19 and the adjacent MoO4
tetrahedra provide an oxide ion diffusion pathway along the [010] direction. Galy et al.11,12 reported a much wider solid-solution range (2.53 < Bi/Mo ≤ 3.50) and invoked vacancies on the isolated bismuth site, rather than an interstitial oxygen site, to explain the discrepancy between the nominal composition and the space-group symmetry. They then proposed a different oxide ionic diffusion mechanism involving continuous exchange of oxygen atoms between all MoO4 tetrahedra via a cooperative mechanism, principally along the [201] direction. Holmes et al. 13 (with Vannier) later investigated Bi26Mo10O69 in a variable-temperature 17O nuclear magnetic resonance (NMR) study. They observed a significant change in line width at the 310 °C phase transition. Their analysis led to the conclusion that while MoO4 tetrahedral rotations are the dominant motion in the triclinic low-temperature regime, the rate of exchange of oxide ions between tetrahedra is extremely slow. This is still the case in the monoclinic high-temperature regime; however, the rate of exchange of oxide ions bonded to Bi (i.e., within the δ-Bi2O3-like columns) was consistent with the oxide ionic conductivity experimentally observed by Vannier et al.6 On this basis, they proposed a diffusion mechanism involving exchange between the δ-Bi2O3-like columns and the partially occupied O19 site, via concerted bond formation and bond breaking on opposite sides of MoO4 tetrahedra, but not directly from one MoO4 tetrahedron to another or from one O19 site to another. Diffusion then occurs primarily along the [010] direction, along the edges of the δBi2O3-like columns, mediated by rapidly rotating MoO4 tetrahedra. Here, we report the results of a combined synchrotron X-ray diffraction, ab initio molecular dynamics, and experimental quasielastic neutron scattering study on Bi26Mo10O69 aimed at clarifying the nature of the long-range and local structure (especially the existence of the partially occupied O19 site) and of the oxide ionic diffusion mechanism.
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EXPERIMENTAL AND COMPUTATIONAL METHODS Pure polycrystalline samples of Bi26Mo10O69 were prepared by conventional solid-state reaction in air from commercial Bi2O3 and MoO2 (99.99% purity or greater). The stoichiometric mixture was ground in high-energy planetary ball-mill, pressed into pellets and calcined at 600 °C for 16 h. Sample purity was confirmed by X-ray powder diffraction (XRD) collected on a Panalytical X’Pert Pro diffractometer using nonmonochromated Cu Kα radiation. Single crystals of Bi26Mo10O69 were prepared by melting the polycrystalline material and slow-cooling it (3 °C h−1) to room temperature. Single-crystal synchrotron X-ray diffraction data were collected on beamline I19 at the Diamond Light Source, U.K. A yellow prism-shaped crystal, ∼40 × 10 × 10 μm in size, was mounted in paratone oil on a mitegen micromount and cooled to 100 K using an Oxford Cryosystems Cryostream device. A full sphere of data was collected, using a wavelength 0.6889 Å and an exposure time of 1 s/degree. Data reduction was performed using the Rigaku Crystal Clear software, and data analysis using Crystals.14 Variable temperature synchrotron XRD (S-XRD) data were collected at the Powder Diffraction beamline of the Australian Synchrotron between 200 and 380 °C at a wavelength of λ = 0.82458 Å (calibrated against a LaB6 standard). Samples were
Figure 1. Monoclinic structure of Bi26Mo10O69 according to Buttrey et al.8 Bismuth atoms are blue, molybdenum atoms are green, and oxygen atoms are red. An isolated bismuth (Bi7) and some of the 25% occupied interstitial oxygen (O19) sites are labeled. The shaded red area shows an oxide ionic diffusion pathway along [010] (into the page) proposed by Vannier et al.9,10 The shaded green area shows the diffusion pathway along [201] (in the plane of the page) proposed by Galy et al.11,12 The black arrows in the shaded orange area refer to the diffusion mechanism discussed in this work and related to that of Holmes et al.13 B
dx.doi.org/10.1021/cm303202r | Chem. Mater. XXXX, XXX, XXX−XXX
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simulation cell that forces the heavy atoms to vibrate about their equilibrium positions. This observation indicates that such small-scale NVT simulations cannot be expected to accurately reproduce the measured temperature dependence of oxygen ion dynamics. AIMD trajectories were analyzed using the nMoldyn code20 to calculate correlation functions from the last 15K steps of simulation: mean square displacement (MSD), vibrational density-of-states (DOS) from the velocity autocorrelation function, and the neutron scattering function S(Q, ω). AIMD trajectories were visualized using the LAMP Program (http://www.ill.eu/instruments-support/computing-forscience/cs-software/all-software/lamp).
placed in unsealed 0.2 mm diameter quartz capillaries, which were heated with a hot air blower. Rietveld-refinements against S-XRD data were carried out using the GSAS15 program with the EXPGUI16 front-end. Scale factors, zero-shifts, background functions, and a single Lorentzian broadening term on top of the standard pseudoVoigt peak shape function for the instrument were refined in addition to a global isotropic atomic displacement parameter (ADP) for each species of ion. Inelastic (INS) and quasi-elastic neutron scattering (QENS) data were collected on the time-of-flight spectrometer IN4 at the Institute Max von Laue Paul Langevin (ILL), France. Data were collected at room temperature and from 100−800 °C in 100 °C steps. The 9.31 g sample, in powder form, was loaded inside a Nb cylindrical can and placed inside an evacuated chamber inside the furnace. IN4 was operated in inelastic-timefocusing mode, with an incident neutron wavelength of 2.4 Å, optimizing the resolution on the anti-Stokes side of the spectrum while keeping a high neutron flux (ΔE = 0.66 meV at elastic scattering, incident flux = 105 n/cm2/s). Typical counting time was 3 h per temperature. The spectra were corrected from the Nb and furnace contributions by subtracting empty can runs obtained in the same conditions. A vanadium standard was also measured to correct from variable detector efficiency. It was also used as a resolution function in the analysis of the quasi-elastic scattering. In the following, the neutron results will be presented in the form of the generalized density of states (GDOS, G(ω)), angle-integrated scattering function S(ω), and scattering function S(Q, ω), depending on the most appropriate representation to discuss the data. The function S(ω) is obtained by summing the angle-dependent spectra S(2Θ, ω) over the total scattering range covered by the 300 detectors on IN4. The GDOS G(ω) is obtained from S(ω)using the equation G(ω) =
S(ω)ω Q (2⟨θ ⟩, ω)2 B(ω , T )
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RESULTS AND DISCUSSION One of the first observations in the AIMD calculations at all temperatures (including 20 °C) was the apparently extreme instability of the interstitial O19 site originally proposed by Buttrey et al.8 and incorporated into all subsequent models of oxide ionic diffusion. The O19 atoms immediately moved toward adjacent MoO4 tetrahedra, forming MoO5 units that rapidly relaxed to regular MoO5 trigonal bipyramids. This suggests that the stoichiometric discrepancy between Vannier et al.’s6 ordered monoclinic model of Bi26Mo10O68 and the charge-balanced form Bi26Mo10O69 should not be accounted for by an interstitial site, but instead, by some Mo sites having 5fold coordination. The implications for the previously unsolved low-temperature triclinic crystal structure are considered first, and those for the ionic diffusion mechanisms are considered second. Single-crystal synchrotron X-ray diffraction data were sufficiently precise to resolve the triclinic distortion, and they indexed to a unit cell with parameters a = 11.728(6) Å, b = 5.7953(13) Å, c = 24.755(6) Å, α = 89.999(3)°, β = 102.881(3)°, γ = 89.951(3)°, V = 1640.2(10) Å3. This unit cell is very similar to that reported by Buttrey et al.8 using synchrotron X-ray powder diffraction. The structure of the lowtemperature form of Bi26Mo10O69 was solved in triclinic space group P1̅, using the charge-flipping algorithm as implemented in Crystals. Charge-flipping gave the fractional coordinates of all cation sites (corresponding to the unit cell content of Bi52Mo20). Subsequent difference Fourier maps readily revealed oxygen atoms sites that led to the Bi52Mo20O136 stoichiometry (i.e. the Bi26Mo10O68 formula). The model at this stage was topologically equivalent to previous monoclinic models, with the triclinic distortion due to relatively small rotational modulations of MoO4 tetrahedra; at the same time, this is the stage at which the previous structure determination failed to provide a chemically plausible stoichiometric model. To get the best possible partial structure, the Bi26Mo10O68 model was refined, using soft restraints on the Mo−O distances (1.75(10) Å) before a final difference Fourier map reveled a peak at ∼0.52, −0.02, 0.46. Placing the final oxygen atom on this general position would give the Bi26Mo10O70 stoichiometry and a five-coordinate Mo atom with all five bond lengths shorter than 1.80 Å, which was not deemed plausible. Instead, the final oxygen atom was placed at (1/2, 0, 1/2), giving the correct Bi26Mo10O69 formula, and a 4 + 1 coordination environment around Mo (four bond lengths
