http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:11427 Structural evolution as a function of temperature through the Pnma↔incommensurate (IC) phase transition in Pr₀.₄₈Ca₀.₅₂ perovskite has been analyzed from the perspectives of symmetry and strain. The structure and stability of both phases are shown to depend on combinations of order parameters which have symmetries associated with irreducible representations M⁺₃, R⁺₄, M⁺₂, Γ⁺₃ and Σ₂ of space group Pm¯3m . The physical origin of these can be understood in terms of octahedral tilting, cooperative Jahn-Teller distortions and charge order/Zener polaron ordering. The M⁺₂ order parameter describes the Jahn-Teller ordering scheme which develops in LaMnO₃ while the Γ⁺₃ order parameter relates to an ordering scheme in which the unique axes of the distorted octahedra are all aligned in the same direction. Irrep Σ₂ contains two components with gradient coupling and provides the symmetry-breaking mechanism by which the IC transition can occur. Each order parameter couples with macroscopic spontaneous strains in a manner that depends strictly on symmetry and this leads to specific interactions between the order parameters through their coupling with common strains. In order to establish the extent and importance of this coupling, symmetry-adapted strains have been extracted from a new set of lattice parameters obtained by high-resolution powder neutron diffraction in the temperature interval 10–1373 K. It is found that the predominant strain of the incommensurate structure (up to ~2.5%) is a tetragonal shear strain which arises by bilinear coupling with the Γ⁺₃ order parameter. This combination is probably responsible for most of the energy reduction accompanying the Pnma↔IC transition and also gives it some characteristics typical of a pseudoproper ferroelastic transition. Strain coupling promotes mean-field behavior and the evolution of the symmetry-breaking order parameter can be described by a standard Landau tricritical solution, q⁴^∝(T_c-T) with T_c=237±2K. Octahedral tilting at high temperatures is closely similar to tilting in the Pnma structure of other perovskites, such as SrZrO₃. This is accompanied by a degree of Jahn-Teller ordering on the basis of the M⁺₂ scheme below ~775 K but is replaced by the Γ⁺₃ scheme below T_c. In contrast with the tilting and Jahn-Teller effects, magnetic ordering at the Néel temperature (~180 K) is accompanied by only the slightest volume strain and is not likely to influence the evolution of the other order parameters to any significant extent, therefore. An additional change in the volume strain below ~85 K is perhaps related to changes in magnetic structure at lower temperatures. Line broadening in powder diffraction patterns collected in the temperature interval ~150–260 K appears to be related to the presence of ferroelastic twins arising from octahedral tilting and draws attention to the fact that the Pnma↔IC transition takes place in a material which already contains heterogeneities. Finally, correlation of the repeat distance of the IC structure with Γ⁺₃ distortions of MnO₆ octahedra shows that the nature of the IC structure itself is also determined essentially by geometrical factors and strain. 2012-09-09T22:45:29.295Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:10653 An investigation has been conducted to examine the effect that ultrasonication has on the synthesis of manganese dioxide used as a pseudocapacitive electrode material in supercapacitors. Using the reaction between KMnO4 and MnSO4, the layered birnessite polymorph of manganese dioxide was produced with and without ultrasonication (control). X-ray diffraction analysis of the materials indicates that ultrasonication has a minimal effect on the crystallite size, although the crystallite dimensions are small anyway (8.6 vs 14.1 nm within the layers, and 9.4 vs 8.9 nm between the layers for the control and ultrasonicated samples, respectively). However, ultrasonication has had a profound effect on morphology, essentially fusing these small crystallites together to form much larger needles 100–200 nm in length. This fusion process has also dramatically decreased the Brunauer, Emmett, and Teller surface area (from 170 to 10 m2/g with ultrasonication), and furthermore electrochemical performance has also been diminished, although by not as much as the drop in surface area; e.g., 106 vs 64 F/g. These phenomena are discussed in terms of the pore volume available for charge storage. 2012-04-16T03:42:48.824Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9619 The thermal stability of the relaxor ferroelectric Pb(Zn₁/₃Nb₂/₃) O₃ (PZN) upon heating was studied using high-temperature X-ray diffraction (XRD) and scanning electron microscopy. It was found that single-phase PZN is stable below 700°C, whereupon the first decomposition phase, the pyrochlore Pb₁.₈₃Nb₁.₇₁Zn₀.₂₉O₆.₃₉ was observed. With the increasing temperature, the amount of pyrochlore increased until at 1100°C there is no PZN remaining. Pyrochlore formation was accompanied by the precipitation of ZnO above 800°C and after holding at 1100°C, the formation of Nb₂ZnO₆. Rietveld refinements based on the XRD patterns have allowed the relative phase proportions as well as the atomic site occupancies of the metal ions in PZN and the pyrochlore to be estimated. The Zn site occupancy within the perovskite PZN decreases upon heating in parallel with the pyrochlore formation. An elemental mass balance based on the XRD results shows that PZN decomposition is well under way before any Pb is lost from the sample. This indicates that Zn egress rather than Pb volatility is the determining factor in the instability of PZN, contrary to prior published work. 2011-12-07T00:20:06.787Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9344 The first example of a heteropolyoxomolybdate containing palladium(IV) was isolated and characterized by X-ray crystallography. The palladium(IV) hexamolybdate, K₀.₇₅Na₃.₇₅[PdMo₆O₂₄H₃.₅]·17H₂O, was isolated from an aqueous solution at pH 4.5 in the space groupPī,a10.790(2),b12.244(3),c14.086(3) Å,α113.77(1),β90.41(1), γ 107.86(1)°, and the structure was determined using X-ray diffraction methods, refining to a residual of 0.0301 for 5334 reflections. A formal“[PdMo₆O₂₄H₃]⁵⁻” subunit exhibits the basic Anderson structure, with two [PdMo₆O₂₄H₃]⁵⁻ cluster anions in the structure bridged by a hydrogen atom (formally an H⁺) situated on a center of symmetry to give a “[Pd₂Mo₁₂O₄₈H₇]⁹⁻” dimeric anion. The palladium(IV) atom occupies a slightly distorted octahedral environment, with Pd–O distances ranging from 1.968 to 2.009 Å. 2011-11-13T22:40:40.600Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8651 A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 2011-08-17T02:40:15.437Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8051 The kinetics of Mn₂O₃ digestion in various H₂SO₄ solutions (0.5–2.0 M) and at various temperatures (ambient to 80 °C) to form solid γ-MnO₂ and soluble Mn(II) have been examined using X-ray diffraction. Using a modified first-order Avrami expression to describe digestion kinetics, rate constants in the range 0.02–0.98 h⁻¹ were found for Mn₂O₃ disappearance, and 0.03–0.42 h⁻¹ for γ-MnO₂ formation, with higher H₂SO₄ concentrations and temperatures leading to faster conversion rates. Also, for a particular set of experimental conditions, the rate of γ-MnO₂ formation was always slower than Mn₂O₃ disappearance. This was interpreted in terms of the solubility and stability of the soluble Mn(III) intermediated formed during the digestion. Activation energies for Mn₂O₃ dissolution and γ-MnO₂ formation were also determined. 2011-07-04T06:30:09.240Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3332 Ti₃SiC₂ exhibits a unique combination of ceramic and metallic properties suitable for both electrical and mechanical application. With high-temperature stability, high electrical and thermal conductivity and resistance to oxidation, Ti₃SiC₂ has proven promising as a contact layer for high power SiC semiconductors. However, until recently, synthesis of this material has proven difficult without appreciable quantities (<2 vol{%}) of impurity phases, namely TiC₁₋x and Ti₅Si₃Cx. As such, many properties of this compound are as yet unknown. In this paper, a comparable analysis of Ti₃SiC₂ and associated compounds, TiC and Ti₅Si₃Cx has been performed using both x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Assessing impurity sensitivities for each technique, XRD was shown to readily identify impurities of TiC and Ti₅Si₃Cx within Ti₃SiC₂ at <2 wt{%}. Although XPS could not independently resolve these impurities, its use resulted in the detection of a complex oxide structure on Ti₃SiC₂. It was speculated that it was composed of mixed C-Ti-C-O and Si-Ti-C-O bond chemistries. In a comparison of TiC, Ti₅Si₃Cx and Ti₃SiC₂, differences in oxide states suggest that oxidation is chemically dissimilar for all the three compounds. However, upon etching, the binding energies of Ti₃SiC₂ and Ti₅Si₃Cx were shown to be very similar. It may be concluded that a concurrent analysis of both XRD and XPS was essential for identifying the overall surface chemistry of Ti₃SiC₂. 2010-04-27T05:26:08.257Z ]]>