Rational Design of a Commensurate (3 + 3)-D Modulated Structure

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2017, 29, 9171-9181

Rational Design of a Commensurate (3 + 3)‑D Modulated Structure within the Fast-Ion Conducting Stabilized δ‑Bi2O3 Series Julia Wind,† Julia Polt,† Zhaoming Zhang,‡ Douglas A. Blom,§ Thomas Vogt,∥ Ray L. Withers,⊥ and Chris D. Ling*,† †

School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, New South Wales 2234, Australia § NanoCenter & Electron Microscopy Center, University of South Carolina, Columbia, South Carolina 29208, United States ∥ NanoCenter & Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States ⊥ Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 0200, Australia ‡

ABSTRACT: We report the systematic design, preparation, and characterization of the first commensurate member of the oxide-ionic conducting, (3 + 3)-D modulated, Type II phases of doped δ-Bi2O3. The incommensurate Type II modulation vector ε was previously described as continuously variable, but high-resolution synchrotron X-ray powder diffraction data show that close to the composition Bi23CrNb3O45, it “locks in” to ε = 1/3. The space group of the resulting 3 × 3 × 3 fluoritetype supercell was found to be F4̅3m by selected-area electron diffraction, and the structure was solved and Rietveld-refined against neutron powder diffraction data in conjunction with local structural information from X-ray absorption spectroscopy, high-resolution transmission electron microscopy, and ab initio geometry optimization calculations. The result unambiguously validates the crystal-chemical model of the Type II phases as being based on the local ordering of oxygen around transition metals M into tetrahedral clusters of MO6 octahedra and isolated MO4 tetrahedra, separating relatively disordered fluorite-type regions that facilitate the highest oxide-ionic conduction among transition metal-doped δ-Bi2O3 phases. conductors.9,10 Work on the TM-doped phases has concentrated on their crystallography, as they are a rich source of fascinating modulated structures based on the fluorite type cell. The most interesting among these from a crystallographic point of view are the (3 + 3)-dimensional incommensurately modulated “Type II”11 solid-solution phases Bi1−xNbx5+O1.5+x (0.06 < x < 0.23),12−14 Bi1−xTax5+O1.5+x (0.20 < x < 0.25),11,12,15−17 Bi1−xCrx6+O1.5+1.5x (0.05 < x < 0.15),18,19 Bi 1 − x Mo x 6 + O 1 . 5 + 1 . 5 x (0.05 < x < 0.16), 1 9 , 2 0 and Bi1−xWx6+O1.5+1.5x (0.15 < x < 0.19).21,22 The crystal structures of these (3 + 3)-dimensional incommensurately modulated materials have to be described in six dimensions using a superspace-symmetry formalism. They are characterized by a series of three coupled modulations of the fluorite-type subcell with lattice parameter a and a modulation parameter ε:

1. INTRODUCTION The cubic high-temperature polymorph of bismuth oxide, δBi2O3, is the best oxide-ionic conductor known. It shows conductivities of 1−2 S cm−1 between 729 and 817 °C,1,2 almost 2 orders of magnitude higher than yttria-stabilized zirconia (YSZ) at comparable temperatures.3 The exceptionally high conductivity of δ-Bi2O3 is attributed to the very high concentration (25%) of intrinsic vacancies on the oxygen sites of its disordered cubic fluorite-type (CaF2, space group Fm3̅m) lattice,4−6 combined with the presence of stereochemically active 6s2 electron lone-pairs on Bi3+ that help to stabilize those vacancies, reducing the energy barriers for oxygen diffusion.7 However, its narrow stability range between 729 °C and its melting point at 817 °C prevents its practical use. In the lower temperature polymorphs (β and γ phases1,8) these vacancies are eliminated, leading to an immense drop in conductivity of about 3 orders of magnitude. The average fluorite-type structure of δ-Bi2O3 can be preserved down to room temperature by doping with rare earth (RE) or transition metals (TM), while retaining conductivity comparable to that of pure δ-Bi2O3 at high temperatures. Most research has focused on the RE-doped phases, which have disordered structures and are better © 2017 American Chemical Society

(h + εm)2 + (k + εn)2 + (l + εp)2 1 = d2 a2 Received: July 19, 2017 Revised: September 28, 2017 Published: October 4, 2017 9171

DOI: 10.1021/acs.chemmater.7b03012 Chem. Mater. 2017, 29, 9171−9181

Article

Chemistry of Materials The most heavily studied Type II phase lies at the Birich end of the Bi2O3−Nb2O5 phase diagram. This is also the only one that has been fully refined14 in superspace group Fm3̅m(α,α,α) q00(α,α̅ ,α̅)q00(α̅ ,α,α̅ )00023 (no. 225.3.215.8; P:Fm3̅m:Fd3̅m in the notation of Yamamoto24,25). Its highly complex structure features corner-connected NbO6 chains along all the ⟨1100000⟩ directions, surrounded by three-dimensional δ-Bi2O3-like channels. The modulation vector was reported to vary over its solid-solution range, from ε = 0.354 (x = 0.1) to ε = 0.377 (x = 0.21).12,14 For other TM-doped Type II phases, detailed diffraction (neutron and X-ray) and high resolution transmission electron microscopy (HRTEM) as well as X-ray absorption spectroscopy (XAS) studies have been used to determine space group and lattice and modulation parameters as well as basic structural features. Type II Bi1−xTax5+O1.5+x was shown to crystallize in space group Fm3̅m(α,α,α)q00(α,α̅,α̅ )q00(α̅,α,α̅)000 with reported incommensurate modulation parameters in a similar range as observed in the niobium system (ε = 0.388 for x = 0.25;16 ε = 0.385 for x = 0.2017). XAS measurements confirmed exclusively octahedral coordination environments around the Ta dopants except at very low Ta concentrations.26 At these lower Ta concentrations, a larger pseudocubic Type I phase was reported between 0.05 < x < 0.07 (a ∼ 5.52 Å), and a mixture of Type I and Type II phases were found at intermediate compositions (0.07 < x < 0.20). The M6+-doped systems, on the other hand, all crystallize in superspace group Fm3̅m(α,α,α)000(α,α̅,α̅ )000(α̅ ,α,α̅ )000 (no. 225.3.215.7; P:Fm3̅m:Fm3̅m25); i.e., they do not have the dglide plane for the modulations. This symmetry suggests tetrahedral coordination around the dopant atoms, and the incommensurate modulation vectors are generally shorter (Cr6+: 0.285 < ε < 0.307;18,19 Mo6+: ε = 0.295 for x = 0.16,19,20 W6+: 0.317 < ε < 0.32121). With the modulation vector in the Bi1−xWx6+O1.5+1.5x system being very close to the commensurate value of 1/3, the crystal structure could be solved and refined within a 3 × 3 × 3 supercell in space group F4̅3m.27 However, there are still some crystal-chemical shortcomings to this solution, reflecting the fact that the real structure is actually slightly incommensurate over its entire solid solution range (0.317 < ε < 0.32121). Although the superspace description of the Type II phase is crystallographically correct and complete, in practice it is difficult to visualize and interpret the interplay among occupational, positional, and displacive atomic modulation functions (AMFs: linear combinations of sine and cosine functions that are related to each other by point-group symmetry). The most intuitive way to rationalize and verify the incommensurate model would be to find a truly commensurate version. This was the goal of the work reported here. Our approach was based on the fact that M5+-doped systems exhibit a modulation parameter slightly larger than 1/3, while modulation parameters in M6+-doped systems are lower than 1/3 (see Figure 1); that mixed doping has been shown to yield Type II phases in the W6+/Nb5+ system.28−30 We therefore hypothesized that mixed-valence transition metal doping could lead to a truly commensurate Type II phase. Because the modulation vector ε varies with both the type of M and the Bi:M ratio, we have two independent variables to chemically tune the composition in search for a commensurate ε = 1/3 structure. However, the proposed combination of M5+ and M6+ limits the accessible range of doping parameter space before the oxygen stoichiometry exceeds the crystal-chemical

Figure 1. Modulation parameter range for different dopants within the Type II solid solution ranges.

requirements of the Type II structure. For simplicity, in this study we have chosen to work with a fixed Bi:M ratio of 23:4, for two reasons. First, it lies comfortably within the Type II solid-solution field for M5+ and M6+ single dopants. Second, the integer stoichiometry 23 + 4 = 27 matches the number of fluorite-type subcells in a 3 × 3 × 3 supercell, maximizing the likelihood of the structure “locking in” to the desired commensurate value ε = 1/3.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Synthesis. Stoichiometric mixtures of MoO3 (99.96%), WO3 (99.999%), Cr2O3 (99.999%), Ta2O5 (99.99%), Nb2O5 (99.998%), and Bi2O3 (99.999%, preheated to 600 °C for 5 h) were thoroughly ground with acetone in an agate mortar and pestle and preheated to 850 °C (Mo-containing samples) or 850 °C (all other samples) for 5 h. The reground mixtures were then heated to 950 °C (Mo-containing samples) or 1000 °C (all other samples) for another 5 h and slowly cooled down to room temperature. When necessary, another heating step was performed after which samples were quenched to room temperature onto a steel plate. Larger sample quantities (>5 g) were ground together with ethanol in a planetary ball-mill with thorough regrinding in an agate mortar and pestle after the second heating step. 2.2. Data Collection and Analysis. To follow reaction progress and check for sample purity, room-temperature powder X-ray diffraction data were collected on a Panalytical X’pert Pro diffractometer in Bragg−Brentano geometry using nonmonochromated Cu Kα radiation. Synchrotron X-ray powder diffraction (SXRD) data were collected at the Powder Diffraction (PD) beamline of the Australian Synchrotron in a Debye−Scherrer geometry using a Mythen microstrip detector.31 Finely ground samples were packed into 0.1 mm boron glass capillaries for room temperature measurements. Depending on sample composition, data were recorded at 13 keV (λ = 0.952652(2) Å) or 21 keV (λ = 0.589263(3) Å, calibrated against a NIST LaB6 standard) over the range 2° ≤ 2θ ≤ 82°. Neutron powder diffraction (NPD) experiments were carried out at the high-resolution diffractometer Echidna at the OPAL research reactor at ANSTO.32 For room temperature measurements, 5−10 g of samples were packed into 9 mm vanadium cans. Structure refinements against these data were carried out using the Rietveld method as implemented in GSAS with the EXPGUI graphical interface.33 Le Bail profile fits were performed using Jana 2006.34 Low-energy XANES spectra at the O K-edge were collected at the soft-X-ray (SXR) beamline at the Australian Synchrotron.35 Finely ground powder samples were thinly dusted onto double-sided carbon tape and transferred into the high-vacuum analysis chamber (pressure less than 10−9 Torr). Spectra were collected in X-ray fluorescence yield (FY) mode. For the energy calibration purpose, all spectra were taken simultaneously with a total electron yield (TEY) signal measured from an oxidized Cr reference foil positioned upstream in the beamline. The reference foil removed approximately 10% of the beam intensity. This allowed for a precise energy alignment of the spectra obtained from different samples, with the maximum in the first derivative of the O Kedge from the oxidized foil set at 531.0 eV (i.e., the absolute energy scale was not calibrated). Intermediate-energy XANES spectra at the Nb and Mo L3-edges were collected at the Tender beamline 16A1 at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, 9172


Article

Chemistry of Materials Taiwan.36 Finely ground powder samples were dispersed onto Kapton tape and placed in front of the X-ray beam at a 45° angle. Spectra were collected in fluorescence yield mode using a Lytle detector. The energy scale for both the Nb and the Mo L3-edge spectra was calibrated using the L3-edge of a pure Mo foil (with the maximum in the first derivative set to 2520.2 eV). XANES spectra at the W L3-edge and Nb K-edge for selected samples were collected at the XAS beamline at the Australian Synchrotron.37 Powder samples were diluted with cellulose powder (concentrations ∼ 5000 ppm), sandwiched between Kapton tape and positioned in front of the X-ray beam. Measurements were performed at room temperature in fluorescence mode using a multielement solid state Ge fluorescence detector. The energy scale was calibrated using the L3-edge of a pure Pt foil (with the maximum in the first derivative set to 11562.8 eV) at the W L3-edge and the K-edge of a pure Nb foil (with the first derivative maximum set to 18986.0 eV) at the Nb Kedge. All XANES spectra were analyzed using the Athena software package.38 Selected area electron diffraction (SAED) patterns were collected using a JEOL 2100F transmission electron microscope (TEM) on crushed grains of the powder sample dispersed onto holey carboncoated copper grids. Aberration-corrected scanning TEM (AC-STEM) data in the Z-contrast high angle annular dark-field (HAADF) imaging modes were collected from the powdered samples. The dry powder was lightly ground in an agate mortar and pestle before loading onto holey carbon-coated copper grids. A JEOL JEM2100F with a CEOS aberration corrector for the electron probe was used at 200 kV to image the sample with a 24 mrad convergence angle. The images were acquired using a Fischione Model 3000 detector with a camera length such that the detector spanned between 75 and 178 mrad. The scanning acquisition was synchronized to the 60 Hz AC electrical power to minimize 60 Hz noise in the images, and a pixel dwell time of 15.8 μs was used. 2.3. Computational Details. Structure solution for commensurate Bi23CrNb3O45 was assisted by density functional theory (DFT) based methods as implemented in the Vienna ab initio simulation package (VASP). Calculations used projector-augmented wave (PAW)39 pseudopotentials within the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzerhof (PBE) exchangecorrelation functional.40 A cutoff energy of 400 eV (determined by oxygen) was used for the plane-wave basis set. The Brillouin zone was sampled using a Monkhorst−Pack k-point grid centered at the Γpoint, with one single k-point. Initially refined experimental structures were used as starting configurations and were optimized by allowing ions to relax to their minimum energy position using the conjugate gradient algorithm. The cell dimensions were fixed to the experimentally determined values. At each step, all forces were relaxed below 0.01 eV Å−1. Total energies were converged to within 10−5 eV.

Figure 2. Neutron powder diffraction patterns (λ = 2.4395 Å) across the Bi23(Cr,Nb)4Oy series. Bi23Cr3NbO46 is purple, Bi23Cr2Nb2O45.5 is green, and Bi23CrNb3O45 is blue. The patterns are staggered on the yaxis for clarity.

Table 1. Summary of Lattice Parameters and Modulation Parameters for Doped Type II Bi2O3:Nb2O5 Samples composition

a (Å)

ε

comment a

Bi23W4O46.5 Bi23Nb4O44.5 Bi23Mo4O46.5 Bi23W3NbO46

5.56888(2) 5.510201(5) 5.65400(2) 5.5530(1)

0.3208(2) 0.3632(1)b 0.2930(1)b -

Bi23W2Nb2O45.5 Bi22W2.5Nb2.5O46.75 Bi23WNb3O45 Bi23Cr3NbO46 Bi23Cr2Nb2O45.5 Bi23CrNb3O45 Bi23Mo3NbO46 Bi23Mo2Nb2O45.5 Bi23MoNb3O45

5.54018(2) 5.52221(2) 5.521381(8) 5.57876(2) 5.55755(2) 5.530375(5) 5.620840(8) 5.57947(4) 5.54269(6)

0.3288(1)a 0.3497(4)a 0.3563(1) 0.2964(2) 0.3130(2) 1/3 0.3054(1) 0.3211(1) 0.3456(2)a

very broad peaks; not Type II

commensurate

very weak superstructure reflections even in NPD data

a No superstructure reflections in SXRD patterns. bNo NPD data collectedmodulation parameter extracted from SXRD data only.

3. RESULTS AND DISCUSSION 3.1. Doping M6+ for Nb5+ in Type II Bi2O3:Nb2O5. For each dopant (M = Cr, Mo, W), we synthesized a range of samples of Bi23(M,Nb)4Oy with different M6+:Nb5+ ratios. Cubic fluorite-related structures were obtained over most of this parameter space. Figure 2 shows NPD patterns across the Bi23(Cr,Nb)4Oy series. Table 1 summarizes sample compositions and lattice and modulation parameters as extracted from combined Le Bail profile fits against SXRD and NPD data. All fits are based on a simple cubic δ-Bi2O3 model with lattice parameter aF in space group Fm3̅m with three coupled modulation vectors ⟨ε00⟩F. Lattice and modulation parameters follow Vegard’s law (Figure 3). Figure 4 shows the partial phase diagram for the Bi2O3− Nb2O5−W2O6 system, summarizing sample compositions and structure types. The cubic fluorite type structure was easily obtained for Bi23WNb3O45 and Bi23W2Nb2O45.5 by slow-cooling in the furnace ( 900 °C), further supports this explanation. The Type II Bi− Nb−O phase, on the other hand, incorporates a larger variety of NbO6 clusters (including corner connected NbO6 chains), 9173


Article

Chemistry of Materials

Figure 3. Lattice (top) and modulation (bottom) parameters vs dopant concentrations, based on a Type II cubic fluorite-related based unit cell with three coupled modulation vectors ⟨ε00⟩F.

As for the bismuth−tungsten niobates, slow-cooling of the Cr-rich sample Bi23Cr3NbO46 led to splitting of the (200), (220), and (311) reflections, i.e., tetragonal phases, while quenching retained cubic symmetry. All samples showed very weak superstructure reflections in SXRD patterns, which were much more intense in NPD data. The relationships between the modulation parameter and dopant concentration (Figure 3b) show that our approach was successful. In particular, Bi23CrNb3O45 appears to be truly commensurate at ε = 1/3. This sample will be discussed further in Section 3.4. Interpolation suggests two further possible commensurate compositions, Bi 23 W 1.75 Nb 2.25 O 45.375 and Bi23Mo1.5Nb2.5O45.25, which will be investigated in future studies. 3.2. Doping M6+ for Ta5+ in Type II Bi2O3:Ta2O5. For each dopant (M = Cr, Mo, W), we synthesized a range of samples of Bi23(M,Ta)4Oy with different M6+:Ta5+ ratios. The end-member sample Bi23Ta4O44.5 contained two cubic phases, in agreement with Struzik et al.:17 one with a = 5.5302(3) Å, referred to as Type I by Zhou11), and a smaller Type II phase with cell dimensions a = 5.4802(3) Å. Mixed doping also yielded two cubic phases (except for Bi23W3TaO46). As the superstructure peaks could not be unambiguously assigned to either of the two phases present in the samples, modulation parameters could not be determined. Extracted lattice parameters are summarized in Figure 5 and Table 2. On increasing the M6+ concentration (or decreasing the Ta content), the lattice parameter of the Type I phase, as

Figure 4. Partial phase diagram for the Bi2O3−Nb2O5−W2O6 system. Crosses indicate samples reported in literature29,30 and dots samples of this work. Red indicates samples with cubic fluorite-related structure.

resulting in samples with a higher Nb:W ratio being more stable. All Bi23(Mo,Nb)4Oy samples have Type II structures. The intensity of the superstructure reflections in SXRD data increases with increasing Mo content. No quenching was necessary to stabilize the cubic structure in samples that are richer in Nb, i.e., Bi23Cr2Nb2O45.5, Bi23CrNb3O45, Bi24Cr1.5Nb1.5O42.25, and Bi22Cr2.5Nb2.5O46.75.

Figure 5. (a) Lattice parameters vs dopant concentrations for doped Bi2O3:Ta2O5 samples, based on the presence of two cubic phases: Type I (filled markers) and Type II (hollow markers). (b) Weight fraction of Type I vs dopant concentrations. 9174


Article


look essentially identical to the spectrum for the Bi3Nb5+O7 standard. This indicates that Nb atoms adopt oxidation state 5+ and occupy octahedral sites in all samples. Figure 7b shows the Nb L3-edge XANES spectra, resulting primarily from the Nb 2p3/2 → 4d dipole transitions, for selected samples, i.e., Bi23CrNb3O45, Bi23Cr2Nb2O45.5, and Bi 23 Cr 3 NbO 46 as well as standards (Bi 3 Nb 5+ O 7 and LaNb5+O4). These spectra provide direct information on the occupancy and energy distribution of the final Nb(4d) states, which is determined mainly by the local crystal field exerted by the surrounding oxygen ligands. As shown in Figure 7b, the L3edge spectrum displays a bimodal feature due to crystal field splitting of the unoccupied Nb(4d) orbitals. The energy gap between the two peaks is directly related to the crystal field splitting (CFS). For an octahedral MO6 unit (where M is at the origin and O ligands are positioned on the Cartesian axes), the triply degenerate t2g orbitals are located at lower energy than the doubly degenerate eg orbitals, as the latter point toward the ligand orbitals and hence are subject to a stronger electrostatic repulsion from the ligands.44 The intensity of the t2g peak is higher than that of the eg peak due to higher degeneracy of the t2 orbitals (3) than that of the eg orbitals (2). In contrast, the three d-t2 orbitals lie higher in energy with respect to the two de orbitals for a tetrahedral MO4 unit (where M is at the center of a cube and O ligands are situated at the four alternate corners of the cube), as the t2 orbitals are now oriented with their lobes much closer to the ligands than the e orbitals. The peak intensity ratio is reversed from t2g/eg > 1 for an octahedral site to e/t2 < 1 for a tetrahedral site. In summary, the results shown in Figure 7b confirm that the Nb oxidation state is 5+ in all the samples and all Nb atoms are coordinated to 6 oxygen atoms. In addition, the CFS in all the samples is very similar to that in Bi3Nb5+O7, suggesting similar Nb−O bond distances. W Coordination Environments. W L3-edge XANES spectra originate predominantly from the dipole transition of the W 2p3/2 core electrons into the empty 5d states in the conduction band. This edge is sensitive to both the W oxidation state and its local coordination environment.45,46 Standards used were Aurivillius type Bi2W6+O6, in which W6+ is found to adopt purely octahedral coordination environments, 47,48 and Bi14W6+O24, which has exclusively tetrahedral W environments.49,50 Figure 8 shows W L3-edge spectra for selected (W,Nb) codoped samples, i.e., Bi23W3NbO46, Bi23W2Nb2O45.5, and Bi23WNb3O45. The W L3-edge position of all samples is essentially the same as that for the two standards, confirming the oxidation state of W6+ in all samples. The shape of the absorption peaks gives insights into the local coordination environments. Bi14WO24 looks like a single peak while Bi2WO6 exhibits two overlapping peaks (this difference is more apparent in the first derivative plots in the inset of Figure 8). This results from a smaller ligand field splitting of the W 5d orbitals in the tetrahedral environment than the octahedral environment.46 The XANES spectra shown in Figure 8 illustrate that W6+ is found predominantly in an octahedral coordination environment in all the codoped samples, with small tetrahedral contributions in Bi23W3NbO46 > Bi23W2Nb2O45.5 > Bi23WNb3O45. These tetrahedral contributions appear to be closely related to the stability of the cubic Type II phase vs the tetragonal Type Ib phase. Similar results were observed for other dopant combinations. Mo Coordination Environments. The Mo L3-edge XANES spectra arise from the transition from Mo 2p3/2 core level

Table 2. Summary of Samples Based on Ta6+-Doping and Rietveld Refinements Based on Cubic δ-Bi2O3 Models composition

a1 (Å)

a2 (Å)

weight % (phase 1)

Bi23W4O46.5 Bi23Mo4O46 Bi23W3TaO46 Bi23W2Ta2O45.5 Bi23WTa3O45 Bi23Cr3TaO46 Bi23Cr2Ta2O45.5 Bi23CrTa3O45 Bi23Mo4O46.5 Bi23Mo3TaO46 Bi23Mo2Ta2O45.5 Bi23MoTa3O45 Bi23Ta4O44.5

5.56888(2) 5.65400(2) 5.55888(9) 5.54264(8) 5.53847(4) 5.5894(7) 5.5863(7) 5.5708(5) 5.65400(2) 5.62714(5) 5.61804(8) 5.5928(1) 5.5302(3)

5.47449(8) 5.47081(4) 5.4683(7) 5.4728(7) 5.4753(5) 5.45848(6) 5.46456(8) 5.46845(9) 5.4802(3)

100 100 100 74 60 96 81 64 100 94 76 62 66

well as its weight fraction, generally increased (Figure 5). The Type II phase does not show any significant compositional dependency (empty symbols in Figure 5a). This suggests a constant Ta concentration in the Type II phase, with doping only occurring within the Type I phase. Figure 6 shows SXRD patterns across the Ta−W mixed doping range. Ta-rich samples contain two phases. Increasing

Figure 6. Excerpts of SXRD patterns for different compositions in the Bi−Ta−W−O system. Ta-rich samples contain two phases, while Bi 2 3 W 3 TaO 4 6 only contains one fluorite-related phase. Bi23WTa3NbO45 is purple, Bi23W2Ta2O45.5 is green, and Bi23W3TaO45 is blue. The patterns are staggered on the y-axis for clarity.

W content reduces the amount of Type I phase. For the W-rich sample Bi23W3TaO46, all peaks could be indexed using one simple cubic model. In the (Ta,Mo) and (Ta,Cr) systems, all samples exhibit two phases, and SXRD data could be fitted using two cubic fluorite-based models. 3.3. X-ray Absorption near-Edge Structure (XANES). X-ray absorption near-edge structure (XANES) spectroscopy was used to probe the short-range local chemical environments of the dopant species, including valence state, coordination, and site geometry. Nb Coordination Environments. Figure 7a shows selected Nb K-edge XANES spectra for mixed doped samples, Bi23Cr2Nb2O45.5, Bi23Mo2Nb2O45.5, and Bi23W2Nb2O45.5, along with two standards of Bi3Nb5+O7 (Nb in purely octahedral coordination environment) and LaNb5+O4 (Nb in purely tetrahedral coordination environment). The pre-edge feature of the Nb K-edge corresponds to a dipole-forbidden 1s to 4d transition42 and is very sensitive to local distortions and coordination number.43 The spectra of all mixed doped samples 9175


Article


Figure 7. Normalized XANES spectra: (a) at the Nb K-edge for Bi23Cr2Nb2O45.5, Bi23Mo2Nb2O45.5, and Bi23W2Nb2O45.5 and (b) at the Nb L3-edge for Bi23CrNb3O45 and Bi23Cr3NbO46, both including Bi3Nb5+O7 (Nb5+: octahedral) and LaNb5+O4 (Nb5+: tetrahedral) standards. (c) At the Mo L3edge for Bi23MoNb3O45, Bi23Mo2Nb2O45.5, Bi23Mo3NbO46, and Bi14Mo6+24 (tetrahedral) standard. (d) At the O K-edge for selected Cr-containing samples, Bi14Cr6+24 (Cr6+: tetrahedral) and Cr-free Bi14Mo6+24 standards (Mo6+: also tetrahedral).

position of all samples is the same as that of the Bi14Mo6+O24 standard, confirming the presence of Mo6+ in all Mo-doped samples. In addition, the same CFS and peak intensity ratio between the two peaks confirms that the Mo coordination environment is tetrahedral in all the compounds. Cr Coordination Environments. The Cr coordination environment was deduced from the O K-edge XANES spectra, as shown in Figure 7d for (Cr,Nb) codoped samples of Bi23CrNb3O45, Bi23Cr2Nb2O45.5, and Bi23Cr3NbO46, along with tetrahedral standards of Bi14Mo6+O2450 and Bi14Cr6+O24.50 These spectra correspond to transitions from the O(1s) core level to the unoccupied O(2p) states in the conduction band. If these compounds are purely ionic, no absorption peak would be observed as the O(2p) orbitals would be fully occupied. The large intensity seen at the O K-edge clearly demonstrates the covalent nature of M−O bonding in these materials, reflecting unoccupied density of states of the various cation orbitals hybridized with the O(2p) states. Hence the O K-edge line shape is also quite sensitive to the local structure.53 As shown in Figure 7d, the intensity of the lowest energy peak (∼529.4 eV) increases with increasing Cr content in the doped samples, showing a positive Cr concentration dependence. Furthermore, based on the similarity between the spectra from the (Cr,Nb) codoped samples and that of the Bi14Cr6+O24 tetrahedral standard, the presence of Cr6+ in the tetrahedral coordination environment is indicated in all the (Cr,Nb) codoped samples; i.e., the peak at ∼529.4 eV is attributed to Cr 3d-e states alone. 3.4. Bi23CrNb3O45A Truly Commensurate (3 + 3)-D Phase. For Bi23CrNb3O45, Le Bail fits against SXRD and NPD data yield a commensurate modulation parameter of ε = 1/3.

Figure 8. Normalized W L3-edge XANES spectra obtained from Bi23W 3NbO46, Bi23W2 Nb2 O45.5 , and Bi23WNb3O45 along with Bi14W6+O24 (tetrahedral) and Bi2W6+O6 (octahedral) standards.

electrons to the unoccupied 4d states.51,52 Two peaks, representing crystal field splitting (CFS), are observed in all spectra (Figure 7c). Bi14Mo6+O24, which has an exclusively tetrahedral Mo coordination environment,49,50 was included as a standard. As shown, the Mo L3-edge spectra for (Mo,Nb) codoped samples, Bi23MoNb3O45, Bi23Mo2Nb2O45.5, and Bi23Mo3NbO46, are essentially identical to that of the tetrahedral standard of Bi14Mo6+O24. The two peaks correspond to the Mo 4d-e and 4d-tg components, respectively. The intensity of the e peak is lower than that of the tg peak as explained above. The edge 9176


Article


Figure 9. (a) (100), (b) (112), and (c) (110) zone axis SAEDs of commensurate Type II Bi23CrNb3O45. The intense bright spots correspond to the simple fluorite-related unit cell, and less intense spots to the commensurate 3 × 3 × 3 superstructure. Bragg reflections are indexed to both the (3 + 3)-D modulated (yellow) and the 3 × 3 × 3 supercell (blue) of fluorite-type δ-Bi2O3.

The commensurate modulation was confirmed by selected area electron diffraction (SAED) patterns shown in Figure 9, in which all Bragg reflections can be indexed to both the (3 + 3)D modulated and the 3 × 3 × 3 supercell of fluorite-type δBi2O3, with perfect overlap. SAED patterns were collected from multiple crystallites, with no changes observed between them, demonstrating that the modulation parameter is genuinely “locked in” to this commensurate value. We carried out Rietveld refinements using a 3 × 3 × 3 supercell in space group F4̅3m against the NPD data, using the commensurate approximation for Bi22W5O4827 as a starting model. Careful initial refinement against NPD data pointed toward mixed Nb−Bi occupancy on the Bi1 site (within the tetrahedra of octahedra) and Cr on the fcc site, but at lower occupancy than the expected 100%. This suggests some underlying disorder in the structure, especially around the Cr site. To allow for local structural relaxation, we subsequently performed ab initio DFT simulations on a (in terms of composition) hypothetical perfectly ordered system with composition Bi22CrNb4O45 (100% Nb on the Bi1 site, 100% Cr on the fcc site). Geometry optimization relaxed the distorted average coordination environments obtained from the refinement to tetrahedra of highly symmetric NbO6 octahedra and CrO4 tetrahedra on the corner and face centered positions of the unit cell. This is in perfect agreement with XANES results for local coordination environments around Nb5+ (Figure 7a,b) and Cr6+ (Figure 7d). The local structural ordering suggested by these simulations was incorporated into our refinement model. Cr was found to be located on the tetrahedral site, and 75% Nb (+ 25% Bi) on the octahedral/cluster site. The O sites around the transition metals have large atomic displacement parameters (ADPs) and refine toward the fluorite-type positions. For our final refinement, these positions were constrained to their DFToptimized ones. Furthermore, to stabilize the Rietveld refinement with such a limited q-range, we assigned the same isotropic displacement parameter to all cation sites. This average disorder is unsurprising because the 25% Bi on the octahedral/cluster sites should have very different O coordination. On a local scale we suspect that either all four sites within a cluster are occupied by Nb atoms or none. This in turn would affect the stability of the O sites around the Cr. The final fit is shown in Figure 10, with results summarized in Table 3. Refined experimental and optimized experimental structures are shown in Figure 11. 3.5. HAADF-STEM Imaging. Figure 12 shows the experimental HAADF micrograph in the [110]F zone axis of

Figure 10. Rietveld refinement against NPD data collected on ECHIDNA (λ = 2.4395 Å) of a single phase Bi23CrNb3O45 sample. The blue line represents the difference between the observed (black crosses) and calculated (red line) profile, and the green line is background. The blue markers indicate peak positions for the 3 × 3 × 3 supercell in space group F4̅3m.

the fluorite lattices. The cation sites are all evident in the image, while the oxygen sites are not apparent. In this projection of the structure, each column of cations consists of two different atomic sites overlapped (cf. Figure 11d). Also shown in Figure 12 is a fast Fourier transform (FFT) of the real space image. Note that this FFT of the HAADF-STEM image is the same as that found in the SAED shown in Figure 9c. In order to obtain reliable dark-field intensities we needed to subtract a background that varied significantly due to sample thickness variations. After establishing regions of interest for the various atom columns shown in this projection we carefully chose regions where the electron probe did not interact with the projected location of atoms in the structure and subtracted these from the HAADF signal. In Figure 13 this procedure is shown for the Bi2Cr1 columns. Using this procedure, we obtained integrated HAADF signals for the six cation column projections shown in Figure 14. We analyzed a total of 781 atomic columns (85 Bi3BiNb1, 89 Bi2Cr1, 171 BiNb1Bi2, 85 Bi3Bi4, 180 Bi2/Bi3, and 171 Bi2Bi4 columns) to obtain good counting statistics for the HAADF-STEM intensities. Integrating the HAADF-STEM signal over many projected atom columns has shown to be insensitive to minute focus changes and drifts due to small environmental changes (temperature, vibration, electromagnetic interferences).54,55 9177


Article


Table 3. Refined Structural Parameters against NPD Data (ECHIDNA, λ = 2.4395 Å) for Bi23CrNb3O45 in Space Group F4̅3m, a = 16.5897(2) Åa

a

atom

x

y

z

Uiso

frac. occ.

site multiplicity

Bi1 Bi2 Bi3 Bi4 Nb1 Cr1 O1 O1 (bis) O2 O3 O4 O5 O6 O7

0.6647(7) 0.3363(4) 0.3272(10) 0.3279(5) 0.6647(7) 0 0.9392 0.0608 0.0802(8) 0.25 0.6274 0.0919 0.0736(17) 0.4269(13)

0.6647(7) 0.3363(4) 0 0.3279(5) 0.6647(7) 0 0.9392 0.0608 0.0802(8) 0.25 0.25 0.0919 0.25 0.4269(13)

0.6647(7) 0.0019(4) 0 0.3279(5) 0.6647(7) 0 0.9392 0.0608 0.2397(14) 0.25 0.25 0.7166 0.25 0.4269(13)

0.0867(7) 0.0867(7) 0.0867(7) 0.0867(7) 0.0867(7) 0.0867(7) 0.34(7) 0.34(7) 0.1435(31) 0.1435(31) 0.383(22) 0.383(22) 0.1435(31) 0.1435(31)

0.25 1 1 1 0.75 1 0.33(6) 0.67(6) 1 1 1 1 1 1

16 48 24 16 16 4 16 16 48 4 24 48 24 16

Overall powder R-factors: Rp = 0.0438, wRp = 0.0621, R(F2) = 0.0551.

Figure 11. (a, b) Experimental (refined) and (c, d) computational (geometry optimized) structures for Bi23CrNb3O45. Bi atoms are purple, CrO4 tetrahedra are blue, NbO6 octahedra are green, and O atoms are red.

Figure 12. Experimental HAADF micrograph in the [110]F zone axis of the fluorite lattices and fast Fourier transform (FFT) of the real space image. The area shown in the blue box was used for data analysis. Below the HAADF-STEM signal of the 6 different columns their atomic contents are shown.

As expected from the Z-contrast approximation, the column containing 50% Nb is the dimmest, followed by the Bi2Cr1 column containing 33.3% Cr. An initial model based on combined SXRD and NPD data had both Cr and Nb colocated on an octahedral site. As the XANES data suggested and the observed HAADF-STEM contrast variation proved unequivocally, Cr has a tetrahedral coordination and is located on (0,0,0). We note that the BiNb1Bi2 column is slightly brighter than expected from the model structure based on Z-contrast. Furthermore, the two columns Bi3Bi4 and Bi2Bi3 reveal less HAADF intensity when compared to the Bi2Bi4 column, even though they all contain only Bi. Our careful background evaluation and experience in comparing HAADF images with model structures55 suggests that ADPs of the cations, which were constrained to be the same in the Rietveld refinement of the NPD data, might vary due to the high degree of disorder in the oxygen sublattice. The HAADF signal is dominated by thermal diffuse scattering and highly sensitive to the ADPs. The

data suggest that the channeling of electrons in the Bi3Bi and Bi2Bi3 atomic columns are different from those in the Bi2Bi4 columns. To further investigate such a hypothesis one would require high-q NPD data permitting an atomic pair-distribution function analysis, which would then become the basis of extensive dark- and bright-field frozen phonon image calculations as outlined in Tate et al.56

4. CONCLUSIONS We have shown for the first time that (3 + 3)-D modulated “Type II” transition metal-stabilized δ-Bi2O3 phases can be rationally prepared in a truly commensurate form, with a 3 × 3 × 3 supercell at Bi23CrNb3O45, by appropriately varying the codopant concentration. The modulation vector does not smoothly cross over the commensurate value but “locks in” to ε = 1/3, as shown by the lack of any peak splitting in highresolution synchrotron X-ray powder diffraction and the consistency of selected-area electron diffraction patterns 9178


Article


Figure 13. HAADF-STEM signal is measured on a varying background shown on the left. To subtract this background we defined regions of interest (ROI). The ROIs for all Bi2Cr1 columns are shown (middle). The background was determined in the green rectangles to the left of the ROIs where no atoms are present in this projection as shown in Figure 11d.

reveals differences between projected atomic columns nominally containing only Bi. This should not be seen as a shortcoming, given that the underlying motivation for studying these stabilized δ-Bi2O3 phases is oxide ionic conduction, which requires local coordinative flexibility. Indeed, our failure to obtain single-phase Type II phases for (Ta,M) codoped phases may be a consequence of the lower coordinative flexibility of Ta compared to Nb. The promise of the transition metal-doped phases is that they provide better long-term stability than the (more conductive) rare earth-doped phases. If their conductivities could be tuned to the comparable levels, they would therefore be more attractive options. The improved crystal chemical understanding of the Type II phases delivered by the work presented here is an important step in that direction.

■

AUTHOR INFORMATION

ORCID Figure 14. Mean integrated HAADF-STEM signal of the 6 atomic columns shown in Figure 12. The error bars are the 95% confidence limit on the mean using Student’s t distribution.

Chris D. Ling: 0000-0003-2205-3106

obtained from multiple crystallites. Although analogous commensurate phases were not found for Bi23WNb3O45 or Bi23MoNb3O45, in the course of this work we mapped out the Type II phase space for Bi23(M,Nb)4Oy where M = Cr, Mo, and W. The reason for why ε locks in to 1/3 can be understood first in terms of the metal atom stoichiometry, whereby an integer number (4 × 4 = 16) of the 4 × 27 = 108 Bi sites in the targeted 3 × 3 × 3 supercell are replaced by a transition metal, and second in terms of the local ordering of oxygen coordination environments around those transition metals, based on tetrahedral clusters of MO6 octahedra and isolated MO 4 tetrahedra. Our ability to rationally design the commensurate phase is thus an intuitive “real space” validation of the crystal-chemistry of the Type II phases, developed by ourselves and others using the superspace formalism in reciprocal space. Nevertheless, there is still some residual disorder in the structure of Bi23CrNb3O45. This can be seen in the refined longrange average structure, which has mixed crystallographic site occupancies compared to an ideal model optimized in ab initio simulations; in the average local structure, where X-ray absorption spectroscopy shows a mixture of coordination environments for Nb5+ and Cr6+, and directly by highresolution scanning transmission electron microscopy, which

ACKNOWLEDGMENTS The authors thank Dr. Peter Kappen (XAS beamline), Dr. Lars Thomsen (Soft X-ray beamline), and Dr. Justin Kimpton (Powder Diffraction beamline) at the Australian Synchrotron and Dr. Maxim Avdeev (Echidna beamline) of the Australian Centre for Neutron Science, ANSTO, for their assistance with data collection. Computational resources were provided by National Computational Infrastructure Australia. J.W. received funding for this project from the Australian Institute for Nuclear Science and Engineering (Postgraduate Research Award) and C.D.L. from the Australian Research Council (DP150102863).

Notes

The authors declare no competing financial interest.

■

■

REFERENCES

(1) Harwig, H. A.; Gerards, A. G. Electrical properties of the α, β, γ, and δ phases of bismuth sesquioxide. J. Solid State Chem. 1978, 26, 265−274. (2) Takahashi, T.; Iwahara, H. Oxide Ion Conductors Based on Bismuthsesquioxide. Mater. Res. Bull. 1978, 13, 1447−1453. (3) Kharton, V.; Marques, F.; Atkinson, A. Transport Properties of Solid Oxide Electrolyte Ceramics: A Brief Review. Solid State Ionics 2004, 174, 135−149. (4) Gattow, G.; Schrö der, H. Ü ber Wismutoxide. III. Die Kristallstruktur der Hochtemperaturmodifikation von Wismut(III)oxid (δ-Bi2O3). Z. Anorg. Allg. Chem. 1962, 318, 176−189. 9179


Article

Chemistry of Materials (5) Battle, P. D.; Catlow, C. R. A.; Drennan, J.; Murray, A. D. The Structural Properties of the Oxygen Conducting Delta-phase of Bi2O3. J. Phys. C: Solid State Phys. 1983, 16, L561−L566. (6) Hull, S.; Norberg, S. T.; Tucker, M. G.; Eriksson, S. G.; Mohn, C. E.; Stolen, S. Neutron Total Scattering Study of the Delta and Beta Phases of Bi2O3. Dalton Trans. 2009, 8737−8745. (7) Laarif, A. The Lone Pair Concept and the Conductivity of Bismuth Oxides Bi2O3. Solid State Ionics 1986, 21, 183−193. (8) Harwig, H. A.; Gerards, A. G. The Polymorphism of Bismuth Sesquioxide. Thermochim. Acta 1979, 28, 121−131. (9) Sammes, N. M.; Tompsett, G. A.; Näfe, H.; Aldinger, F. Bismuth Based Oxide Electrolytes - Structure and Ionic Conductivity. J. Eur. Ceram. Soc. 1999, 19, 1801−1826. (10) Drache, M.; Roussel, P.; Wignacourt, J. P. Structures and Oxide Mobility in Bi-Ln-O Materials: Heritage of Bi2O3. Chem. Rev. 2007, 107, 80−96. (11) Zhou, W. Structural Chemistry and Physical Properties of Some Ternary Oxides in the Bi2O3‑Ta2O5 System. J. Solid State Chem. 1992, 101, 1−17. (12) Ling, C. D.; Withers, R. L.; Schmid, S.; Thompson, J. G. A Review of Bismuth-Rich Binary Oxides in the Systems Bi2O3−Nb2O5, Bi2O3−Ta2O5, Bi2O3−MoO3, and Bi2O3−WO3. J. Solid State Chem. 1998, 137, 42−61. (13) Withers, R. L.; Ling, C. D.; Schmid, S. Atomic Modulation Functions, Periodic Nodal Surfaces and the Three-dimensional Incommensurately Modulated (1-x)Bi2O3.xNb2O5, 0.06 < x < 0.23, Solid Solution. Z. Kristallogr. 1999, 214, 296−304. (14) Ling, C. D.; Schmid, S.; Blanchard, P. E.; Petricek, V.; McIntyre, G. J.; Sharma, N.; Maljuk, A.; Yaremchenko, A. A.; Kharton, V. V.; Gutmann, M.; Withers, R. L. A (3 + 3)-dimensional ″Hypercubic″ Oxide-ionic Conductor: Type II Bi2O3-Nb2O5. J. Am. Chem. Soc. 2013, 135, 6477−6484. (15) Saito, T.; Miida, R. Crystal Structure and Ionic Conductivity in Bi2O3-rich Bi2O3-Ta2O5 Sintered Oxides. Jpn. J. Appl. Phys. 1 1999, 38, 4838−4842. (16) Abrahams, I.; Krok, F.; Struzik, M.; Dygas, J. R. Defect Structure and Electrical Conductivity in Bi3TaO7. Solid State Ionics 2008, 179, 1013−1017. (17) Struzik, M.; Malys, M.; Wrobel, W.; Abrahams, I.; Krok, F.; Dygas, J. R. Ordered Fluorite Phases in the Bi2O3-Ta2O5 System: A Structural and Electrical Investigation. Solid State Ionics 2011, 202, 22−29. (18) Esmaeilzadeh, S.; Lundgren, S.; Hålenius, U.; Grins, J. Bi1−xCrxO1.5+1.5x, 0.05 ≤ x ≤ 0.15: A New High-Temperature Solid Solution With a Three-Dimensional Incommensurate Modulation. J. Solid State Chem. 2001, 156, 168−180. (19) Esmaeilzadeh, S. 3-dimensional Modulations in Bismuth Oxide Based Compounds With Average Fluorite Type Structures. Ferroelectrics 2001, 250, 63−66. (20) Valldor, M.; Esmaeilzadeh, S.; Pay-Gomez, C.; Grins, J. A New High-temperature Cubic Fluorite-type Phase Mo0.16Bi0.84O1.74 With a Rare Three-dimensional Incommensurate Modulation. J. Solid State Chem. 2000, 152, 573−576. (21) Wind, J.; Kayser, P.; Zhang, Z.; Radosavljevic Evans, I.; Ling, C. D. Stability and Range of the Type II Bi1‑xWxO1.5+1.5x Solid Solution. Solid State Ionics 2017, 308, 173−180. (22) Zhou, W. Defect Fluorite Superstructures in the Bi2O3-WO3 System. J. Solid State Chem. 1994, 108, 381−394. (23) Stokes, H. T.; Campbell, B. J.; van Smaalen, S. Generation of (3+d)-dimensional Superspace Groups for Describing the Symmetry of Modulated Crystalline Structures. Acta Crystallogr., Sect. A: Found. Crystallogr. 2011, 67, 45−55. (24) Yamamoto, A. Modulated Structure of Wustite (Fe1−xO) (Three-dimensional Modulation). Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, 38, 1451−1456. (25) van Smaalen, S.; Campbell, B. J.; Stokes, H. T. Equivalence of Superspace Groups. Acta Crystallogr., Sect. A: Found. Crystallogr. 2013, 69, 75−90.

(26) Gambino, M.; Giannici, F.; Longo, A.; Di Tommaso, S.; Labat, F.; Martorana, A. Dopant Clusterization and Oxygen Coordination in Ta-Doped Bismuth Oxide: A Structural and Computational Insight into the Mechanism of Anion Conduction. J. Phys. Chem. C 2015, 119, 26367−26373. (27) Wind, J.; Auckett, J. E.; Withers, R. L.; Piltz, R. O.; Maljuk, A.; Ling, C. D. Type II Bi1‑xWxO1.5+1.5x: A (3 + 3)-dimensional Commensurate Modulation that Stabilizes the Fast-ion Conducting Delta Phase of Bismuth Oxide. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, 71, 679−87. (28) Zhou, W. Microstructures of Some Bi−W−Nb−O Phases. J. Solid State Chem. 2002, 163, 479−483. (29) Krok, F. Effects of Low Levels of Tungsten Doping in Bismuth Niobates. Solid State Ionics 2008, 179, 172−177. (30) Borowska-Centkowska, A.; Kario, A.; Krok, F.; Abrahams, I.; Chan, S. C. M.; Liu, X.; Wrobel, W.; Malys, M.; Hull, S.; Dygas, J. R. D e f e c t S t r u c t u r e a n d E l e c t r i c a l C o n d u c t iv i t y i n t he Bi3+xNb0.8W0.2O7.1+3x/2 System. Solid State Ionics 2010, 181, 1750− 1756. (31) Wallwork, K. S.; Kennedy, B. J.; Wang, D. The High Resolution Powder Diffraction Beamline for the Australian Synchrotron. AIP Conf. Proc. 2006, 879, 879−882. (32) Liss, K.-D.; Hunter, B.; Hagen, M.; Noakes, T.; Kennedy, S. Echidna - The New High-resolution Powder Diffractometer Being Built at OPAL. Phys. B 2006, 385−386, 1010−1012. (33) Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (34) Petříček, V.; Dušek, M.; Palatinus, L. Crystallographic Computing System Jana2006: General Features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (35) Cowie, B. C. C.; Tadich, A.; Thomsen, L.; Garrett, R.; Gentle, I.; Nugent, K.; Wilkins, S. The Current Performance of the Wide Range (90−2500 eV) Soft X-ray Beamline at the Australian Synchrotron. AIP Conf. Proc. 2009, 307−310. (36) Dann, T. E.; Chung, S. C.; Huang, L. J.; Juang, J. M.; Chen, C. I.; Tsang, K. L. A High-performance Double-crystal Monochromator Soft X-ray Beamline. J. Synchrotron Radiat. 1998, 5, 664−6. (37) Glover, C.; McKinlay, J.; Clift, M.; Barg, B.; Boldeman, J.; Ridgway, M.; Foran, G.; Garret, R.; Lay, P.; Broadbent, A. Status of the X-Ray Absorption Spectroscopy (XAS) Beamline at the Australian Synchrotron. AIP Conf. Proc. 2006, 882, 884−886. (38) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−41. (39) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (40) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (41) Sharma, N.; Macquart, R. B.; Avdeev, M.; Christensen, M.; McIntyre, G. J.; Chen, Y. S.; Ling, C. D. Re-investigation of the Structure and Crystal Chemistry of the Bi2O3-W2O6 ’Type Ib’ Solid Solution Using Single-crystal Neutron and Synchrotron X-ray Diffraction. Acta Crystallogr., Sect. B: Struct. Sci. 2010, 66, 165−72. (42) Piilonen, P. C.; Farges, F.; Linnen, R. L.; Brown, G. E.; Pawlak, M.; Pratt, A. Structural Environment of Nb5+ in Dry and Fluid-rich (H2O, F) Silicate Glasses: A Combined XANES and EXAFS Study. Can. Mineral. 2006, 44, 775−794. (43) Iwasawa, Y. Applications of X-ray Absorption Fine Structure to Catalysts and Model Surfaces. J. Phys. IV 1997, 7, C2-67−C2-81. (44) Zhang, Z.; Middleburgh, S. C.; de los Reyes, M.; Lumpkin, G. R.; Kennedy, B. J.; Blanchard, P. E. R.; Reynolds, E.; Jang, L.-Y. Gradual Structural Evolution from Pyrochlore to Defect-Fluorite in Y2Sn2−xZrxO7: Average vs Local Structure. J. Phys. Chem. C 2013, 117, 26740−26749. (45) Kuzmin, A.; Purans, J.; Kalendarev, R. Local Structure and Vibrational Dynamics in NiWO4. Ferroelectrics 2001, 258, 21−30. (46) Yamazoe, S.; Hitomi, Y.; Shishido, T.; Tanaka, T. XAFS Study of Tungsten L1- and L3-Edges: Structural Analysis of WO3 Species 9180


Article

Chemistry of Materials Loaded on TiO2 as a Catalyst for Photo-oxidation of NH3. J. Phys. Chem. C 2008, 112, 6869−6879. (47) Rae, A.; Thompson, J.; Withers, R. Structure Refinement of Commensurately Modulated Bismuth Tungstate, Bi2WO6. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 870−881. (48) Knight, K. S. The Crystal Structure of Russellite; a Redetermination Using Neutron Powder Diffraction of Synthetic BiWO6. Mineral. Mag. 1992, 56, 399−409. (49) Ling, C. D.; Withers, R. L.; Thompson, J. G.; Schmid, S. Structures of Bi14WO24 and Bi14MoO24 from Neutron Powder Diffraction Data. Acta Crystallogr., Sect. B: Struct. Sci. 1999, 55 (3), 306−312. (50) Crumpton, T. E.; Francesconi, M. G.; Greaves, C. The Structural Chemistry of Bi14MO24 (M = Cr, Mo, W) Phases: Bismuth Oxides Containing Discrete MO4 Tetrahedra. J. Solid State Chem. 2003, 175, 197−206. (51) Hu, H.; Wachs, I. E.; Bare, S. R. Surface Structures of Supported Molybdenum Oxide Catalysts: Characterization by Raman and Mo L3Edge XANES. J. Phys. Chem. 1995, 99, 10897−10910. (52) Aritani, H.; Tanaka, T.; Funabiki, T.; Yoshida, S.; Eda, K.; Sotani, N.; Kudo, M.; Hasegawa, S. Study of the Local Structure of Molybdenum-Magnesium Binary Oxides by Means of Mo L3-Edge XANES and UV-Vis Spectroscopy. J. Phys. Chem. 1996, 100, 19495− 19501. (53) Zhang, Z.; Avdeev, M.; de los Reyes, M.; Lumpkin, G. R.; Kennedy, B. J.; Blanchard, P. E. R.; Liu, S.; Tadich, A.; Cowie, B. C. C. Probing Long- and Short-Range Disorder in Y2Ti2−xHfxO7 by Diffraction and Spectroscopy Techniques. J. Phys. Chem. C 2016, 120, 26465−26479. (54) E, H.; Pennycook, T. J.; Okunishi, E.; D’Alfonso, A. J.; Lugg, N. R.; Allen, L. J.; Nellist, P. D.; MacArthur, K. E. Probe Integrated Scattering Cross Sections in the Analysis of Atomic Resolution HAADF STEM Images. Ultramicroscopy 2013, 133, 109−119. (55) Rosenauer, A.; Mehrtens, T.; Mueller, K.; Gries, K.; Schowalter, M.; Satyam, P. V.; Bley, S.; Tessarek, C.; Hommel, D.; Sebald, K.; Seyfried, M.; Gutowski, J.; Avramescu, A.; Engl, K.; Lutgen, S. Composition Mapping in InGaN by Scanning Transmission Electron Microscopy. Ultramicroscopy 2011, 111, 1316−1327. (56) Tate, M. L.; Blom, D. A.; Avdeev, M.; Brand, H. E. A.; McIntyre, G. J.; Vogt, T.; Evans, I. R. New Apatite-Type Oxide Ion Conductor, Bi2La8[(GeO4)6]O3: Structure, Properties, and Direct Imaging of LowLevel Interstitial Oxygen Atoms Using Aberration-Corrected Scanning Transmission Electron Microscopy. Adv. Funct. Mater. 2017, 27, 1605625.

9181


Rational Design of a Commensurate (3 + 3)-D Modulated Structure

Recommend Documents