Graham King

The High-Temperature Polymorphs of K3AlF6

The crystal structures of the three high-temperature polymorphs of K(3)AlF(6) have been solved from neutron powder diffraction, synchrotron X-ray powder diffraction, and electron diffraction data. The β-phase (stable between 132 and 153 °C) and γ-phase (stable between 153 to 306 °C) can be described as unusually complex superstructures of the double-perovskite structure (K(2)KAlF(6)) which result from noncooperative tilting of the AlF(6) octahedra. The β-phase is tetragonal, space group I4/m, with lattice parameters of a = 13.3862(5) Å and c = 8.5617(3) Å (at 143 °C) and Z = 10. In this phase, one-fifth of the AlF(6) octahedra are rotated about the c-axis by ∼45° while the other four-fifths remain untilted. The large ∼45° rotations result in edge sharing between these AlF(6) octahedra and the neighboring K-centered polyhedra, resulting in pentagonal bipyramidal coordination for four-fifths of the K(+) ions that reside on the B-sites of the perovskite structure. The remaining one-fifth of the K(+) ions on the B-sites retain octahedral coordination. The γ-phase is orthorhombic, space group Fddd, with lattice parameters of a = 36.1276(4) Å, b = 17.1133(2) Å, and c = 12.0562(1) Å (at 225 °C) and Z = 48. In the γ-phase, one-sixth of the AlF(6) octahedra are randomly rotated about one of two directions by ∼45° while the other five-sixths remain essentially untilted. These rotations result in two-thirds of the K(+) ions on the B-site obtaining 7-fold coordination while the other one-third remain in octahedral coordination. The δ-phase adopts the ideal cubic double-perovskite structure, space group Fm ̅3m, with a = 8.5943(1) Å at 400 °C. However, pair distribution function analysis shows that locally the δ-phase is quite different from its long-range average crystal structure. The AlF(6) octahedra undergo large-amplitude rotations which are accompanied by off-center displacements of the K(+) ions that occupy the 12-coordinate A-sites.

The effect of the B-site cation and oxygen stoichiometry on the local and average crystal and magnetic structures of Sr2Fe1.9M0.1O5+y (M = Mn, Cr, Co; y = 0, 0.5)

Six compounds with formula Sr2Fe1.9M0.1O5+y (M = Mn, Cr, Co; y = 0, 0.5) were synthesized in air and argon, exhibiting surprisingly different properties depending on the B-cation type in spite of the low (5%) doping level. All argon synthesized phases, y ∼ 0, have long range brownmillerite ordering of oxygen vacancies with Icmm symmetry as shown by neutron diffraction (ND). All show long-range G-type antiferromagnetic order with Neel temperatures, TN, from variable temperature ND of 649(3)K, 636(2)K and 668(5)K for Cr, Mn and Co-compounds, respectively, compared with Sr2Fe2O5, TN = 693 K. Competing ferromagnetic interactions may be responsible for the anomalously low value in the M = Mn case. The air synthesized phases with y ∼ 0.5 show surprising variation with M as investigated by X-ray, TOF and constant wavelength neutron diffractions. The M = Co compound is isostructural with Sr4Fe4O11 (Sr2Fe2O5.5), Cmmm, while the M = Cr phase is cubic, Pm-3m, and that for M = Mn appears to be cubic but the reflections are systematically broadened in a manner which suggests a local Cmmm structure. NPDF studies show that the local structure of the Cr phase is better described in terms of a Cmmm ordering of oxygen vacancies with Fe–O coordination numbers of five and six. The M = Co material shows C-type antiferromagnetic long-range magnetic order at 4 K as found for Sr4Fe4O11. TN ∼ 230 K is inferred from a ZFC-FC magnetic susceptibility divergence compared with TN = 232 K for un-doped Sr4Fe4O11. The M = Cr and Mn compounds show no long-range magnetic ordering down to 4 K, but the divergence of ZFC and FC susceptibility data indicative of spin glass-like transitions occur at ∼60 K and ∼45 K for Cr and Mn, respectively. ND shows both diffuse and sharp Bragg magnetic reflections at positions consistent with a Cmmm cell for the M = Mn phase. For the M = Cr material, a very weak magnetic Bragg peak indexed as (1/2 1/2 1/2), consistent with a G-type AF order, is found at 4 K. These results rule out a spin glass-like ground state for both materials.

The Incommensurately Modulated Structures of the Perovskites NaCeMnWO6 and NaPrMnWO6

The structures of the doubly ordered perovskites NaCeMnWO(6) and NaPrMnWO(6), with rock salt ordering of the Mn(2+) and W(6+)B-site cations and layered ordering of the Na(+) and (Ce(3+)/Pr(3+)) A-site cations, have been studied by transmission electron microscopy, electron diffraction, neutron and synchrotron X-ray powder diffraction. Both compounds possess incommensurately modulated crystal structures. In NaCeMnWO(6) the modulation vector (with reference to the ideal ABX(3) perovskite subcell) is q ≈ 0.067a* (∼58.7 Å) and in NaPrMnWO(6)q ≈ 0.046a* (∼85.3 Å). In both compounds the superstructures are primarily the two-dimensional chessboard type, although some crystals of NaCeMnWO(6) were found with one-dimensional stripes. In some crystals of NaPrMnWO(6) there is a coexistence of chessboards and stripes. Modeling of neutron diffraction data shows that octahedral tilting plays an important role in the structural modulation.

Magnetic and nuclear structure of goethite (α-FeOOH): A neutron diffraction study

The magnetic structure of two natural samples of goethite (α-FeOOH) with varying crystallinity was analyzed at 15 and 300u2005K by neutron diffraction. The well crystallized sample has the Pb′nm color space group and remained antiferromagnetic up to 300u2005K, with spins aligned parallel to the c axis. The purely magnetic 100 peak, identifying this color space group, was clearly resolved. The nanocrystalline sample shows a phase transition to the paramagnetic state at a temperature below 300u2005K. This lowering of the Neel temperature may be explained by the interaction of magnetic clusters within particles. The nuclear structure, refined with the Rietveld and pair distribution function methods, is consistent with reports in the literature.

Local structure of the vacancy disordered fluorite Yb3TaO7 from neutron total scattering

The local structure of the defect, vacancy disordered fluorite Yb3TaO7 has been investigated using neutron total scattering methods. The average structure of Yb3TaO7 is well described in space group Fmm, with unit cell constant a = 5.1872(1) A, in which Yb and Ta occupy, randomly, the single cation site in a fluorite structure and there is a vacancy concentration of approximately ⅛ on the anion site. Examination of the neutron diffraction data shows many features in the background which can be ascribed to local order of Yb, Ta, and O atoms. The pair distribution function, G(r), was analyzed using both crystallographic models and Reverse Monte Carlo (RMC) methods. From both perspectives the local structure resembles very strongly the cation and vacancy ordered model expected by extrapolation from the structure of Ho3TaO7, space group C2221. As well, this local structure model provides a superior fit to the G(r) out to ∼30 A compared to the average structure model. This result may have implications for the understanding of phenomena such as catalytic activity which are reported to depend on order/disorder structural transformations in the series, RE3TaO7 and RE3NbO7 where RE is a rare earth element.

Slip casting of sol–gel-synthesized barium strontium zirconium titanate ceramics

The sol–gel method was used to synthesize two different Ba0.75Sr0.25Ti0.95Zr0.05O3 powders: one of high purity and the other of low purity. These two sol–gel-synthesized powders show two distinct particle sizes and surface areas. The slip casting method was applied to these two sol–gel powders followed by a pressureless sintering, which shows large differences in sintered density and grain size for the pressureless sintered disks. Neutron powder diffraction shows a transition to the nonpolar cubic Pm–3m space group at higher temperatures for both materials. Pair distribution function analysis was used to examine the local displacements of the Ti4+ and Zr4+ cations. The dielectric constant, loss tangent, and bias were measured on these two materials.

Cation and anion ordering in Sr2Si7Al3ON13 phosphors

A series of photoluminescent Ce3+ doped samples with compositions close to Sr2Si7Al3ON13:Ce have been studied by neutron powder diffraction to determine the Si4+/Al3+ and N3−/O2− site ordering. Contrary to a commonly held assumption that the edge sharing tetrahedral sites in this structure are occupied exclusively by Al3+, we find a partial occupancy of Al3+ on these site but also an unexpected preference for Al3+ to occupy 2 other tetrahedral sites which are only corner sharing. From the crystal structures and local structures, as determined by pair distribution function (PDF) analysis, we also find evidence for alternating Si–Al site ordering within the edge sharing chains as well as dimerization of the Si4+ and Al3+ cations within these chains. The O2− are found to be partially ordered onto 2 of the anion sites, although small amounts of O2− are found on other sites as well. The cation and anion ordering found by neutron diffraction is supported by theoretical calculations. Understanding cation and anion ordering is essential for optimizing the photoluminescence properties of this promising class of phosphor materials.

Processing of crack-free high density polycrystalline LiTaO3 ceramics

This work has successfully achieved high density (99.9%) polycrystalline LiTaO3. The keys to the high density without cracking were the use of LiF-assisted densification to maintain fine grain size as well as the presence of secondary lithium aluminate phases as grain growth inhibitors. The average grain size of the hot pressed polycrystalline LiTaO3 is less than 5xa0μm, limiting residual stresses caused by the anisotropic thermal expansion. Dilatometry results clearly indicate liquid phase sintering via the added LiF sintering aid. Efficient liquid phase sintering allows densification during low temperature hot pressing. Electron microscopy confirmed the high-density microstructure. Rietveld analysis of neutron diffraction data revealed the presence of LiAlO2 and LiAl5O8 minority phases and negligible substitutional defect incorporation in LiTaO3.

Local Structure of Zr(OH) 4 and the Effect of Calcination Temperature from X-ray Pair Distribution Function Analysis

Analysis of X-ray pair distribution function data has provided a detailed picture of the local structure of amorphous Zr(OH)4 and its thermal decomposition into ZrO2. In the untreated phase, the Zr atoms tend to be coordinated by six or seven oxygen atoms. The Zr centered polyhedra connect to each other primarily by sharing edges, but also with a significant amount of corner sharing, to form two-dimensional sheets in which the Zr are connected to an average of about five other Zr. This local structure is related to the structure of monoclinic ZrO2 and can be derived from it by removing certain Zr neighbors to form sheets and reduce the corner to edge sharing ratio. The maximum correlation length in Zr(OH)4 is about 12 Å. Heating up to 125 °C results in significant water loss but does not alter the network of Zr and bridging O atoms. Additional water loss caused by heating to 250 °C triggers a reorganization into a new type of amorphous phase with a three-dimensional network and a greater number of Zr-Zr neighbors. Further heating to 330 °C causes crystallization into a mixture of tetragonal and monoclinic ZrO2, with the minor tetragonal phase having a smaller average domain size. The tetragonal component vanishes by 900 °C.