Probing cation disordering in Gd2Ti2O7 pyrochlore by X-ray Photoelectron Spectroscopy
EMSL Project ID
6295
Abstract
An order-disordered structural transition occurs in the pyrochlore structure type as a result of disordering at cation sublattices as well as anion disorder, which mainly leads to the dramatic increase in oxygen ion conductivity and radiation tolerance in ordered pyrochlore-structured oxide materials. X-ray photoelectron spectroscopy (XPS) has been used to study the structural variation and the degree of disorder in pyrochlore structure due to its surface sensitivity as a result of short inelastic mean free path (IMFP) of the photoelectrons and sensitivity to chemical surroundings of the photoelectrons. Our initial XPS analyses of the Ti 2p and O 1s electron binding energy shifts of Gd2Ti2O7 upon ion-irradiation showed a decrease in the coordination number of titanium, the main cause for ion-irradiation-induced disorder, and simultaneous cation and anion disordering. The O 1s XPS spectra for Gd2Ti2O7 exhibits a broad feature with two peaks centered at binding energies of ~526 and ~531 eV. This can be attributed to two distinct oxygen species corresponding to the unique oxygen sites in the ordered pyrochlore structure. However, XPS is a very sensitive method for analyzing surfaces, and the experimental results could be affected by surface preparation, heterogeneity of the samples, charging effects, instrument resolution, etc. A recent XPS investigation by Pacific Northwest National Laboratory (PNNL) reported a symmetrical single peak for O 1s electron at 529.6 eV with a FWHM of ~1.5 eV characteristic of Ti-O in Gd2Ti2O7. No evidence for an anisotropic distribution of Ti and O sites in Gd2Ti2O7 pyrochlore has been observed. This result is inconsistent with our initial XPS observation showing the simultaneous cation disordering – anion disordering mechanism. The validity of the split vacancy model in pyrochlore materials needs to be re-examined. We propose to investigate the structure information in the highly ordered titanate pyrochlores using the XPS facility at PNNL by acquiring the O1s and Ti 2p spectra of single crystal and polycrystalline Gd2Ti2O7 samples. The objective of this proposal is to understand the discrepancy in the experimental results of the O 1s spectrum of Gd2Ti2O7, and to probe cation antisite disorder occurring in pyrochlore structure. Furthermore, the O 1s and Ti 2p spectra also will be acquired for the amorphous Gd2Ti2O7 created by ion implantation to understand the mechanism responsible for the ion beam-induced order-disorder structural transformation and amorphization. This study may also have significant implications for enhancing ionic conductivity in the Gd2(ZrxTi1-x)2O7 system with the substitution of Zr for Ti used as solid electrolytes in the application of solid oxide fuel cells.
Project Details
Project type
Exploratory Research
Start Date
2004-01-10
End Date
2006-03-13
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Nachimuthu P, S Thevuthasan, MH Engelhard, WJ Weber, DK Shuh, NM Hamdan, BS Mun, EM Adams, DE McCready, V Shutthanandan, DW Lindle, G Balakrishnan, DM Paul, EM Gullikson, RC Perera, J Lian, LM Wang, and RC Ewing. 2004. "Probing Cation Antisite Disorder in Gd?Ti?O? Pyrochlore by Site-Specific Near-Edge X-ray-Absorption Fine Structure and X-ray Photoelectron Spectroscopy." Physical Review. B, Condensed Matter and Materials Physics 70(10):100101(R), 1-4.
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