J. G. Thompson

Structures of Ta22W4O67 and Ta74W6O203.I. Refined structural models using synchrotron radiation

The crystal structures of Ta 22 W 4 O 67 [M r =5788.19, a = 6.1485 (5), b = 47.6205 (12), c = 3.8559 (3) A, γ = 90.04 (1)°, space group = C112/m (non-standard setting), Z = 1, D x = 8.513 g cm -3 , F(000) = 2438] and Ta 74 W 6 O 203 [M r = 17741.06, a = 6.1664(5), b = 29.2717 (14), c = 3.8731 (2) A, space group = Pbam (no. 55), Z = 0.2, D x = 8.428 g cm -3 , F(000)=1494] were determined using synchrotron radiation at four different wavelengths below the TaL III edge [λ=1.2741 (-146eV), λ=1.2586 (-26eV), λ =1.2571 (-14eV) and λ =1.2563A (-8eV)]. The collection of data immediately below the TaL III edge at -8eV enabled resolution of Ta and W of up to eight electrons, which assisted in the refinement of Ta/W ordering for both structures. Bond valence arguments have been used to locate oxygen vacancies required by the formulae. From the largest data set for Ta 22 W 4 O 67 (λ=1.2741 A), a final value of 0.0481 for R 1 = Σ ∥F obs (h)| - |F calc (h)∥/ Σ|F obS (h)| was obtained for 3082 unmerged reflections with I(h) > 3σ[I(h)] and for Ta 74 W 6 O 203 (λ = 1.2563 A) a final value of 0.0571 for R 1 was obtained for 5675 unmerged reflections. The two structures are described from a conventional polyhedral perspective as 13- and 8-times superstructures occurring in the solid solution (1-x)Ta 2 O 5 .xWO 3 , 0 ≤ x ≤ 0.267.

Antiferroelectric Modulations in Sb2WO6 and Sb2MoO6

The structure of Sb 2 WO 6 [M r = 523.4, triclinic, F1, a = 11.132(1), b = 9.896 (4), c = 18.482(7) A, α = 90.20 (4), β = 96.87(8), γ = 90.21(5)°, D x = 6.88 g cm -3 , Z = 16, Mo Kα, λ = 0.7107 A, μ = 338.5 cm -1 , F(000) = 3536] has been refined as an enlarged 2a x 2b x 2c F-centred superstructure of the previously reported structure [Castro, Millan, Enjalbert, Snoeck & Galy (1994). Mat. Res. Bull. 29, 871-879] refined in the space group P1. The re-refinement follows the observation, initially by TEM, of satellite reflections at G ± 1/2 (111)*, where G represents a reflection of the P1 reciprocal lattice. A final value of 0.040 for R 1 = Σ h ∥F obs (h)|-|F calc (h)∥/Σ h ∥F obs (h)| was obtained for 3316 merged reflections with [(h) > 3σ[1(h)], compared with R 1 = 0.12 for the previous refinement. The refined structure is described in terms of an antiferroelectric modulation of a P12 1 /a1 underlying parent structure in the original setting. Twinning of the crystal was successfully modelled in the refinement. Synthesis of the previously unknown phase Sb 2 MoO 6 [M r = 435.5, triclinic, F1, a = 10.758(1), b = 9.673(2), c = 17.57(1) A, α = 90.00(5), β = 96.98 (3), γ = 90.05 (2)°, Z = 16, D x = 4.97 g cm -3 ] is also reported, along with evidence for its isostructuralism with Sb 2 WO 6 .

The structure refinement of compositely modulated Nb2Zrx−2O2x+1 (x = 12)

The room-temperature single-crystal X-ray structure refinement of Nb 2 Zr X-2 O 2x+1 , x = 12, as an incommensurate compositely modulated structure is reported. It consists of two component substructures, referred to as the metal substructure (M), a M = 5.108 (1), b M =4.997(1), c M =5.278(1)A, and the oxygen substructure (O), a o = 2.452A, b o = b M = b, c o = c M = c, respectively. The primary modulation wavevector of the M substructure is chosen to be q M = a O * - 2a M * + b * = 1/12a M * + b * . The overall superspace group symmetry is Amma(α10)0s0. Two sets of intensity data, recorded 8 (λ 1 =0.6892A) and 1458eV (λ 2 =0.7495A) below the Zr K absorption edge, were refined to an overall wR(λ 1 ) = 0.0406 and wR(λ 2 ) =0.0687. The displacive modulation wave amplitudes obtained from these refinements are in remarkable agreement with those obtained from a Fourier decomposition of the previously published, conventional superstructure refinement of the member x = 8 [Galy & Roth (1973). J. Solid State Chem. 7, 277-285]. Despite the enhanced scattering contrast between Zr and Nb at λ 1 , metal-ion ordering was not found to be significant. Comparative refinements as incommensurately and commensurately modulated structures gave distinctly better R-values for the former case.

Re-Refinement of Composite Modulated Nb2Zrx−2O2x+1 (x = 8) Using Synchrotron Radiation Data

The crystal structure of Nb2Zr_{x-2}O_{2x + 1}, x = 8, has been re-refined as a composite modulated structure using synchrotron radiation data measured at energies below the Zr K absorption edge. The structure comprises one-atom thick layers of two substructures stacked alternately along the b direction. The two component substructures are, in general, mutually incommensurable along their common a-axis directions and are referred to as the metal substructure (M), space group Amma, aM = 5.1348 (2), bM = 4.9590 (2), cM = 5.2908 (3) A, and the oxygen substructure (O), space group Imam, aO = 2.4164, bO = bM = b, cO = cM = c, respectively. The four basis vectors used to index reciprocal space, based on the more strongly scattering metal substructure, are a^{\ast}_{M}, b*, c* and qM. The primary modulation wavevector qM = ^{\ast}_{M} − 2a^{\ast}_{M} + b* - 1/8 a^{\ast}_{M} + b* for x = 8. The overall superspace-group symmetry is Amma(\alpha10)0s0. Two sets of intensity data recorded at room temperature, 8 eV (\lambda_n = 0.6892 A) and 1450 eV (\lambda_f = 0.7492 A) below the Zr K absorption edge, were refined to an overall R(\lambda_n) = 0.0377 and R(\lambda_f) = 0.0439. The displacive modulation wave amplitudes obtained from these refinements agree well with those previously obtained for an x = 12 member. Difference Fourier maps show that metal-atom ordering is virtually negligible.

A composite modulated structure approach to the lanthanide oxide fluoride, uranium nitride fluoride and zirconium nitride fluoride solid-solution fields

It is shown that the only generally applicable crystallographic approach to the anion excess, fluorite-related solid-solution fields reported in the zirconium nitride fluoride, uranium nitride fluoride and lanthanide oxide fluoride systems is a composite modulated structure approach. A TEM and powder XRD study has been made of the ZrN x F 4-3x (0.906 < x < 0.936) system

Structure Refinements of the Layered Intergrowth Phases SbIIISbV xA1−xTiO6 (x≃0, A = Ta, Nb) Using Synchrotron X-ray Powder Diffraction Data

The structures of the layered intergrowth phases SbIIISb^{\rm V}_xAl-xTiO6 (x \simeq 0, A = Ta, Nb) have been refined by the Rietveld method, using X-ray diffraction data obtained using a synchrotron source. The starting models for these structures were derived from those of Sb^{\rm III}_3Sb^{\rm V}_xA3−xTiO14 (x = 1.26, A = Ta and x = 0.89, A = Nb), previously solved by single-crystal X-ray diffraction. There were no significant differences between the derived models and the final structures, validating the approach used to obtain the models and confirming that the n = 1 and n = 3 members of the family, Sb^{\rm III}_nSb^{\rm V}_xAn−xTiO4n+2 are part of a structurally homologous series.

Structure refinement of commensurately modulated bismuth tungstate, Bi2WO6. Erratum

Owing to a printers error, Fig. 7 of the paper by Rae, Thompson & Withers [Acta Cryst. (1991), B47, 870--881] was published in the wrong orientation. The correct Fig. 7 is given. All relevant information is given in the Abstract.