T. E.
Crumpton
a,
J. F. W.
Mosselmans
b and
C.
Greaves
*a
aSchool of Chemistry, University of Birmingham, Birmingham, UK B15 2TT. E-mail: c.greaves@bham.ac.uk
bCCLRC, Daresbury Laboratory, Warrington, WA4 4AD, UK, Cheshire
First published on 10th November 2004
A new Bi–Re–O phase, Bi28Re2O49, has been synthesized and characterized. Its structure, determined by neutron powder diffraction, is a superstructure of the cubic fluorite unit cell: tetragonal, I4/m, a = 8.7216(1) Å, c = 17.4177(2) Å. The structure comprises an ordered framework of linked BiO4e trigonal bipyramids and square pyramids (e = lone pair of electrons), with discrete Re oxoanions at the origin and body-centre of the unit cell. The infra-red spectrum and Re K-edge X-ray absorption spectrum were consistent with the presence of tetrahedral ReO4− and octahedral ReO65− species in the ratio of 3 : 1. This correctly provides charge balance in the overall structure. The phase is found to display high oxide ion conductivity, which may relate to the presence of the two different oxoanion species, and possible migration of O2− ions between them.
Fig. 1 FTIR spectrum of Bi28Re2O49. |
XPD suggested that the product was single phase with the tetragonal (I4/m) superstructure exhibited by Bi14SO241 and Bi14CrO24.3,4 The synthesis conditions require the formation of Re(VII) rather than lower valent ions, which suggests a formula of Bi14ReO24.5 and a unit cell content of Bi28Re2O49. Evidence from infra-red spectroscopy suggests the likelihood that 75% of the Re ions are present as ReO4− ions, and 25% as ReO65− ions. The absence of reduced Re species was confirmed by magnetization measurements, which demonstrated that the phase is diamagnetic.
High resolution NPD data were collected at ambient temperature to explore the structural details of Bi28Re2O49. As a result of the orientational disorder of the tetrahedral groups in the related Bi14MO24 compounds,4 and the consequent difficulty of locating the O atoms within these groups, it was considered that realistic information concerning the orientation of the mixed oxoanions in Bi28Re2O49 would not be attainable even at low temperatures. Structure refinement therefore focussed on the basic Bi–O structural framework at 298 K. Using the room temperature structure of Bi14CrO24 (I4/m)4 as a starting model, refinement proceeded in a straightforward manner with the exclusion of the O atoms bonded to Re. As expected, subsequent attempts to locate these atoms proved difficult. However, given that 75% of the Re was thought to be present as ReO4−, a successful refinement was achieved by making the approximation that only tetrahedral species were in the structure. On this basis, the final refinement confirmed the structure, and provided a bismuth oxide framework which is essentially the same as that found in the related Bi14MO24 compounds. Although less information can be obtained for the Re oxoanions, it appears that the ReO4− ions present have an orientation very similar to that reported previously for Bi14MO24, M = Cr, Mo, W.4 As observed for these phases, the Re is displaced off the origin along [001], by ∼0.17 Å. The fitted NPD profiles are shown in Fig. 2 and the refined atomic positions and other structural information are provided in Table 1. Even after allowing displacement of Re, its isotropic temperature factor remained rather high, as is the value for O5 which is bonded to it. It is likely that these values reflect disorder associated with the presence of both ReO4− and ReO65− ions in the structure, but attempts at splitting the Re site further, to allow for these species, did not improve the refinement. Fig. 3 shows the structural framework, highlighting the presence of the Bi2O4 distorted tetrahedra (trigonal bipyramids if we include the stereochemical lone-pair of electrons [e], Bi2O4e) and square pyramids (Bi1O4e and Bi3O4e with e at the pyramid apex). Owing to the orientational disorder of the ReO4− units (and the fact that the ReO65− units have not been incorporated), the O atoms bonded to Re (O4, O5, O6 and O7) have not been included in Fig. 3. Selected bond distances and angles are given in Table 2.
Fig. 2 Observed (+), calculated and difference NPD profiles of Bi28Re2O49 at 298 K. The reflection positions are marked (|). |
Fig. 3 The structure of Bi28Re2O49 showing the BiO4e trigonal bipyramids and the Bi–O bonds in the BiO4e square pyramids. The O atoms bonded to Re are not shown. |
Atom | Position | x | y | z | U iso × 100/Å2 | Cell occupancy |
---|---|---|---|---|---|---|
a I4/m; a = 8.7216(1) Å, c = 17.4177(2) Å; Rp = 0.0465, Rwp = 0.0548, Rexp = 0.0216. | ||||||
Re | 4e | 0 | 0 | 0.0100(6) | 4(2) | 2 |
Bi1 | 8h | 0.2107(3) | 0.4491(2) | 0 | 1.82(5) | 8 |
Bi2 | 16i | 0.2998(2) | 0.1056(2) | 0.1711(8) | 1.66(3) | 16 |
Bi3 | 4e | 0.5 | 0.5 | 0.1557(2) | 2.33(9) | 4 |
O1 | 8g | 0.5 | 0 | 0.1259(2) | 2.66(6) | 8 |
O2 | 16i | 0.0739(2) | 0.2517(3) | 0.2567(1) | 2.37(5) | 16 |
O3 | 16i | 0.3253(3) | 0.6302(3) | 0.0780(1) | 2.16(7) | 16 |
O4 | 16i | 0.020(2) | 0.232(2) | 0.015(2) | 0.3(2) | 2 |
O5 | 4e | 0 | 0 | 0.1126(6) | 4.1(3) | 2 |
O6 | 16i | 0.015(3) | 0.179(3) | 0.036(2) | 2.3(2) | 2 |
O7 | 16i | 0.995(2) | 0.206(3) | 0.012(3) | 2.3(2) | 2 |
Re–O4 | 2.03(2) | Bi2–O1 | 2.131(2) |
Re–O5 | 1.79(2) | Bi2–O3 | 2.584(2) |
Re–O6 | 1.76(3) | Bi2–O2 | 2.086(3) |
Re–O7 | 1.84(3) | Bi2–O2 | 2.202(3) |
O4–Re–O5 | 101.9(9) | O1–Bi2–O2 | 96.0(1) |
O4–Re–O6 | 106.2(9) | 93.0(1) | |
O4–Re–O7 | 97(2) | O1–Bi2–O3 | 83.7(1) |
O5–Re–O6 | 106.2(9) | O2–Bi2–O2 | 96.7(1) |
O5–Re–O7 | 102(1) | O2–Bi2–O3 | 79.1(1) |
O6–Re–O7 | 99(1) | 175.8(1) | |
Bi1–O3 | 2.221(3) [×2] | Bi3–O3 | 2.334(3) [×4] |
2.311(3) [×2] | |||
O3–Bi1–O3 | 72.0(1) | O3–Bi3–O3 | 70.31(9) [×4] |
72.7(1) [×2] | 109.0(2) [×2] | ||
75.4(1) | |||
114.9(2) [×2] |
The Bi/O framework is seen to be essentially the same as that previously reported for the M(VI) analogous materials4 and consists of linked BiO4 polyhedra with stereochemically active lone pairs of electrons. The Re–O bonds to O5,6,7 are typical of those in tetrahedral ReO4− ions (e.g. average 1.74 Å in Bi3ReO85), whereas the Re–O4 bond is significantly longer and is similar to distances observed for octahedral ReO65− species, e.g. 1.89 Å in Li5ReO6 and Na5ReO6,8 and La3ReO8.12,13 The coordination around Bi is typical for BiO4e arrangements and the Bi–O distances are very similar to those observed in related structures: bonds of 2.2–2.3 Å for the square pyramidal Bi1O4e and Bi3O4e with the trigonal bipyramidal Bi2O4e having the equatorial bonds (2.131(2) Å and 2.086(3) Å) significantly shorter than the axial bonds (2.584(2) Å and 2.202(3) Å).
Given the difficulties in locating O atoms bonded to Re in this phase, owing to orientational disorder and the presumed presence of both ReO4− and ReO65− ions, EXAFS data were collected and compared with reference materials KReO4 (tetrahedral Re) and Li5ReO6 (octahedral Re). The coordination around Re was modelled as a single species and the calculations revealed two different Re–O distances: 1.74(2) Å [3.1(5) bonds per Re, Debye–Waller factor 0.004(1) Å2] and 2.15(2) Å [1.5(2) bonds per Re, Debye–Waller factor 0.002(1) Å2] and a fit index, R = 32.6%.6Fig. 4 shows the experimental and calculated EXAFS k3χ(k) data and their Fourier transform. Although the longer bond distance of 2.15 Å is slightly longer than expected for octahedrally coordinated Re(VII), the 1.74 Å bond is highly typical of tetrahedral ReO4− ions. The EXAFS data therefore provide good support for the structural model involving 75% ReO4− and 25% ReO65−. Based on a single Re species, this mixture of ions would result in an average Re coordination of 4 × 0.75 short (tetrahedral) bonds and 6 × 0.25 long (octahedral) bonds, i.e. 3 short and 1.5 long bonds, in excellent agreement with the calculated local model.
Fig. 4 Experimental (solid line) and calculated (- - - ) Re K-edge EXAFS data for Bi28Re2O49: (a) k3 weighted EXAFS, (b) phase shifted Fourier transform of k3 weighted EXAFS. |
Reasonably high oxide ion conductivity has recently been reported in a bismuth oxide phase containing tetrahedral sulfate groups: Bi8SO15 or BiO11(SO4).14 In this material, a high level of disorder was apparent on the oxygen sublattice, and the conductivity in the range 400–600 °C was determined to be within an order of magnitude of that observed for the Y-stabilised δ-Bi2O3 phase, (Bi0.75Y0.25)2O3 or Bi3YO6. In Bi28Re2O49, although the bismuth oxide framework has no apparent disorder, the presence of both ReO4− and ReO65− ions in a random fashion suggests that a conduction path may exist for the support of oxide ion migration. Conductivity measurements were therefore made between 400 and 600 °C. The complex plane impedance plots could be fitted to a single semicircle, with a Warburg impedance at low frequency that was attributed to electrode/electrolyte interfacial effects. The response was similar to that observed by us for well characterised stabilised Bi2O3 oxide ion conductors (e.g. Bi3YO6), and the resistance was assigned to bulk effects. It was assumed that under the oxidising conditions used, the electronic contribution to the conductivity was negligible, as has been found for other oxide ion conductors based on Bi2O3.15Fig. 5 shows plots of log (σT/S cm−1 K) versus 1000/T (K−1) for Bi3YO616 and Bi28Re2O49, from which the Arrhenius activation energies, Ea, were calculated. Bi28Re2O49 produced a linear plot with Ea = 0.62 eV, which is slightly lower than the reported high temperature value for Bi3YO6 (Ea = 0.66 eV).16 For the temperature range studied, Bi28Re2O49 exhibits conductivity very similar to that observed for the disordered phase Bi8SO15, and is higher at low temperature: at 400 °C, the conductivities are 5.4 × 10−4 S cm−1 and 4.9 × 10−4 S cm−1, respectively, in comparison with 2.0 × 10−3 S cm−1 for Bi3YO6.16 The high conductivity, despite the high level of order within a large part of the structural framework, suggests that materials containing both tetrahedral ReO4− and octahedral ReO65− units may provide interesting future targets for highly conducting solids.
Fig. 5 Plots of log (σT) versus 1000/T for Bi28Re2O49 compared with Bi3YO6.16 The lines are linear fits to the data. |
This journal is © The Royal Society of Chemistry 2005 |