Browsing by Author "Tayebjee, M"

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    A 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, APJ
    Experimental 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.
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    Single crystal neutron diffraction on the clathrates of Sr8Ga16Ge30, Ba8Ga16Ge30 and Sr4Ba4Ga16Ge30
    (Australian Institute of Physics, 2006-12-05) Tayebjee, M; Piltz, RO; Yu, DH; Cai, K; Wong, L; Stampfl, APJ
    The key consideration in the development of more efficient thermoelectric materials is to design materials that conduct heat like a glass but maintain good crystal-like electrical conductivities, that is to design so called “phonon glass and electron crystals (PGEC’s), as proposed by Slack [1]. One potential PGEC are clathrates that consist of a cage framework of the group 13/14 elements and alkali, alkaline earth or rare earth atoms trapped within the cages. The properties of such clathrates strongly depend on the details of their structures, the dynamic disorder of the trapped atoms as well as the distribution of framework atoms. The structure of single crystals of Sr8Ga16Ge30, Ba8Ga16Ge30 and Sr4Ba4Ga16Ge30 are characterized by single crystal neutron diffraction. The general gallium-doped germanium cage structure with strontium or barium guest atoms inside is confirmed. The lattice constants are 10.704(1)Å, 10.759(2)Å and 10.757(2)Å for Sr8Ga16Ge30, Ba8Ga16Ge30 and Sr4Ba4Ga16Ge30 respectively. Each unit cell contains two 20-atom cages and six 24-atom cages containing strontium/barium atoms. Analysis of the Sr8Ga16Ge30 cage structure shows that gallium atoms preferentially occupied the 6c site and avoided the 16i site. At the 6d site in the 24-atom cage, large atomic displacement parameters (ADP) from guest-atoms are determined for all structures. Further experiment at low temperature is in progress. [1] G. A. Slack, in CRC Handbook of Thermoelectrics, Edited by D. M. Rowe (CRC Boca Raton, FL, 1995), pp 407-440
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    The 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, AE
    Cubic 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.