Browsing by Author "Smith, AE"
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- ItemComparison between calculated and measured photoelectron diffraction patterns for Cu (001)(Australian Institute of Physics, 2008-01-30) Cousland, GP; Smith, AE; Riley, JD; Homolya, S; Stampfl, APJ; King-Lacroix, JWe compare the results of X-ray photoelectron diffraction experiments with simulations obtained using the EDAC multiple scattering computer simulation package. Comparisons are presented for Cu (111) at photon energies of ~ 600eV. With an intention to study Cu3Mn, our initial work considers experimental and simulated data for Cu (001) at photon energies from 100 to 380eV.
- ItemElectronic and vibrational properties of yttria-stabilised zirconia from first-principles for 10–40 mol% Y2O3(Elsevier, 2014-11) Cousland, GP; Cui, XY; Ringer, SP; Smith, AE; Stampfl, APJ; Stampfl, CMDensity-functional theory calculations are performed to investigate the electronic and vibrational density-of-states (DOS) for a series of recently predicted stable and metastable structures of yttria-stabilised zirconia (YSZ) with yttria (Y2O3) concentrations of 14, 17 and 20 mol%. Analogous quantities are also studied for the so-called δ-phase, which forms for 40 mol% yttria, as well as for model structures with ≈10.3 mol% yttria. These calculated results, together with those for the cubic, tetragonal and monoclinic phases of ZrO2, afford a comparison of structural, electronic and vibrational properties as a function of yttria concentration. With increasing yttria content, the electronic DOS exhibit a decrease in the valence band-width (of about 2.0 eV relative to the cubic phase) and a corresponding increase of the band-gap of 0.73 eV (from cubic to 40 mol% yttria containing ZrO2). Regarding the vibrational DOS (vDOS), the addition of yttria causes the peaks in the vDOS of ZrO2 to become less distinct, reflecting the more dense occupation of states due to the larger number of atoms in each primitive cell, and to the lower symmetry. The vDOS of the various YSZ structures appear qualitatively similar with contributions from O atoms spanning the whole frequency range and cation related contributions present for frequencies View the MathML source. With increasing yttria content, more Zr atoms become seven-fold coordinated as in monoclinic ZrO2, concominantly the vDOS increasingly resembles that of m-ZrO2, but with notable contributions from Y atoms, which has a main peak at about 17 meV, similar to that of Zr atoms. © 2014 Elsevier Ltd.
- ItemInvestigation of the vibrational properties of cubic yttria-stabilized zirconia: a combined experimental and theoretical study(Elsevier, 2014-03) Cousland, GP; Mole, RA; Elcombe, MM; Cui, XY; Smith, AE; Stampfl, CM; Stampfl, APJA combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO2 about the X-point. Compared to pure cubic ZrO2, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO2 at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO2, thus providing support for the structural model considered. Crown copyright © 2013 Published by Elsevier Ltd.
- ItemLow energy photoelectron diffraction analysis at high angular resolution of Cu and Mn/Cu surfaces(American Institute of Physics, 2009-11-01) Cousland, GP; Smith, AE; Riley, JD; Stampfl, APJX-ray photoelectron diffraction simulations using a real-space approach are Shown to accurately produce the extraordinarily detailed photoelectron diffraction pattern from Cu{111} at an electron kinetic energy of 523.5 eV. These same simulations show that most sensitivity is obtained when using low energy electrons at high angular resolution Structural differences are observed to be greatest around a kinetic energy of similar to 100 eV and many of the features observed in the photoelectron diffraction patterns may be directly related to phenomena observed in low energy electron diffraction patterns from the same Surface. For Cu{100}. simulations of buckled surfaces with a Mn overlayer predict that low energy photoelectron diffraction can easily discriminate chemical and structural differences. Even the effects of the relaxed Surface of Cu{100} is indeed observable along azimuthal scans around a kinetic energy of 100 eV Our results show that low energy photoelectron diffraction is extremely sensitive to changes in Surface Structure If high resolution patterns are acquired. © 2009, American Institute of Physics.
- ItemLow energy photoelectron diffraction at high angular resolution as a surface structure probe(Australian Institute of Physics, 2009-02-04) Cousland, GP; Smith, AE; Riley, DJ; Stammfl, APJThe EDAC multiple scattering computer package has been used to simulate X-ray photoelectron diffraction experiments of Cu(100) surfaces at kinetic energies of 303.5 eV and below. At 10 angular resolution EDAC produces visually distinct patterns for bulk truncated Cu(100) and buckled Cu(100) surfaces with and without an Mn overlayer. Changes to Cu(100) surface relaxation parameters have discernable effects at kinetic energies of ~100eV.
- ItemA medium-energy photoemission and ab-initio investigation of cubic yttria-stabilised zirconia(AIP Scitation, 2014-03-01) Cousland, GP; Cui, XY; Smith, AE; Stampfl, CM; Wong, L; Tayebjee, M; Yu, DH; Triani, G; Evans, PJ; Ruppender, HJ; Jang, LY; Stampfl, APJExperimental and theoretical investigations into the electronic properties and structure of cubic yttria-stabilized zirconia are presented. Medium-energy x-ray photoemission spectroscopy measurements have been carried out for material with a concentration of 8-9 mol. % yttria. Resonant photoemission spectra are obtained for a range of photon energies that traverse the L2 absorption edge for both zirconium and yttrium. Through correlation with results from density-functional theory (DFT) calculations, based on structural models proposed in the literature, we assign photoemission peaks appearing in the spectra to core lines and Auger transitions. An analysis of the core level features enables the identification of shifts in the core level energies due to different local chemical environments of the constituent atoms. In general, each core line feature can be decomposed into three contributions, with associated energy shifts. Their identification with results of DFT calculations carried out for proposed atomic structures, lends support to these structural models. The experimental results indicate a multi-atom resonant photoemission effect between nearest-neighbour oxygen and yttrium atoms. Near-edge x-ray absorption fine structure spectra for zirconium and yttrium are also presented, which correlate well with calculated Zr- and Y-4d electron partial density-of-states and with Auger electron peak area versus photon energy curve. © 2014, AIP Publishing LLC.
- ItemThe structure of yttria-stabilised zirconia: a combined medium energy photoemission and ab-initio investigation(Australian Institute of Physics, 2011-02-01) Cousland, GP; Wong, L; Tayebjee, M; Yu, DH; Triani, G; Stampfl, APJ; Cui, X; Stampfl, CM; Smith, AECubic zirconia-based materials are candidates for use in the nuclear fuel cycle. There are three phases of ZrO2, a room temperature monoclinic phase and higher temperature tetragonal and cubic phases. The cubic phase of zirconia, in comparison to the other phases, exhibits a very low thermal conductivity, allowing the material to be potentially used in high temperature fission and fusion environments. Interestingly, the cubic-phase may be stabilised at room temperature through the addition of small quantities of other oxides for example, Y2O3, CaO and Ce2O3. Recent ab initio calculations for yttria-stablised zirconia (YSZ) predict the atomic geometry for various oxygen-vacancy containing structures [1]. In particular, a set of “rules” is used to establish a structure for 6.25 Mol % [1,2]. This model is extended to a yttria content of 9.375 Mol % and compared with a sample of 9.5 Mol % yttria. Using this model, core-level shifts are estimated as changes in binding energy obtained from density-functional theory (DFT) calculations, due to the different chemical environments. The partial density-of-states of Y atoms differ depending upon whether there are oxygen vacancies at nearest-neighbour sites to the Zr atoms. Experimentally, a number of different core-levels and Auger-lines are acquired across the L-edges of Zr and Y. By measuring through the Y Ledge resonance, three distinct Zr environments and three distinct oxygen environments are observed in photoelectron peaks. The area under each peak is plotted against photon energy.
- ItemThe structure of yttria-stabilised zirconia: a combined synchrotron photoemission, neutron scattering and ab-initio investigation(Australian Institute of Physics, 2012-02-02) Cousland, GP; Mole, RA; Elcombe, MM; Cui, XY; Smith, AE; Stampfl, CM; Stampfl, APJZirconia-based materials have possible applications in high-temperature fission and fusion environments. Zirconia can be stabilised to room temperature by the addition of yttria to form yttria-stabilised zirconia (YSZ). YSZ is interesting because it retains strength at high temperature and is resistant to neutron bombardment. The structure of YSZ is investigated by comparing results from ab-initio calculations with those from x-ray photoemission and inelastic neutron scattering experiments. This analysis considers two candidate models for YSZ at 9.375 Mol % yttria, one with a supercell of 93 atoms and another with a 186 atom supercell. These structures are constructed using constraints based on findings from first principles calculations [1] and results from neutron scattering and x-ray measurements [2]. A method is used that studies the shift in Auger and photoemission core-levels. Predicted core energies are obtained from density functional calculations and are correlated to deconvoluted peaks of zirconium, yttrium and oxygen within photoemission spectra. This is done in order to corroborate the local structure of YSZ at this particular yttria concentration. Attention is also focused on any long range order of yttrium and vacancies. Inelastic neutron scattering experiments are conducted to determine the periodic nature of dopant and vacancy and then results compared to potential model structures. Work will be extended to investigate YSZ samples containing higher yttria concentrations of e.g. 14, 17, 20 and 40 Mol percent.
- ItemYttria-stabilised zirconia: a trend study of structural, electronic and vibrational properties(Australian Institute of Physics, 2015-02-03) Cousland, GP; Cui, XY; Smith, AE; Stampfl, APJ; Stampfl, CMYttria-stabilised zirconia (YSZ) is an important ceramic, due to its dielectric properties and its strength, which is maintained at high temperature. Finding the structure of YSZ has proven difficult because it changes with temperature, Yttria (Y2O3) concentration and sample preparation. Further, the scattering amplitudes of Zr and Y, for both x-rays and neutrons, are very similar and so cannot discriminate atom positions. Methods are used where state-of-the art DFT code (DMol3) simulations with 10.35 mol% Y2O3 are compared with x-ray photoemission spectroscopy to examine short-range order, and with inelastic neutron scattering to examine long-range order, for samples of YSZ with 8-9 mol%. Furthermore, YSZ with 10.35 mol% is used along with recently published models of 14, 17, 20 and 40 mol%, together with cubic, tetragonal and monoclinic zirconia (ZrO2), in a trend study which shows an increase in band-gap (0.73 eV) and decrease in valence band (2.0 eV) across the range of increasing Y2O3 concentration and relative to cubic ZrO2.