Washington, DC, 18 August 2010- Researchers from the University of Michigan and the Geophysical Laboratory have demonstrated a new method for quantitatively measuring the degree of pressure-induced atomic disordering in pyrochlore oxides using synchrotron x-ray diffraction, synchrotron infrared spectroscopy and Raman scattering techniques. 

Pyrochlore, A2B2X6Y, is an isometric structure with over 500 different compositions and a wide range of technological applications, from radiation resistant nuclear materials to being fast ionic conductors. Properties change dramatically depending on composition and the degree of ordering on the cation sites and of the anion vacancies. The La2Zr2O7 pyrochlore is a prominent photoelectrochemical catalyst, and the anomalous lattice expansion is attributed to the first report, in this study, of the incorporation of water into the structure.
 

The quantitative analysis of disordering at high pressure has always been a challenge. The team from University of Michigan and the Geophysical Laboratory, with the support of scientists from the diamond anvil cell x-ray (X17C beam line) facility at the National Synchrotron Light Source (NSLS) have found that the disordering on the cation and anion sites has different influences on the individual diffraction peaks, and their contributions to the diffraction intensities can be quantitatively distinguished by careful analysis.

Using the Rietveld method, the research team demonstrated that anion disordering occurs first at pressures below 5 GPa. As shown in the figure at the right, cation anti-site defects dominate above 10 GPa. An anomalous lattice expansion was confirmed in the lanthanum pyrochlore at 10 GPa by x-ray diffraction, Raman scattering, and infrared absorption measurements, in experiments where the pressure medium contains some water. Water intercalation in pyrochlore oxides may be common during the process of either pressurization or ion irradiation, and it is mainly caused by the cation anti-site defects that result from disordering. La2Zr2O7 is a catalyst, which can split water, during photon irradiation. The photochemical reaction may be closely related to the formation of anion disordering and the formation of Frenkel defects.
        
The feature picture is a rendering of the structure of La2Zr2O7. The thumbnail image shows the essential features of the pyrochlore structure.  The article can be found at: Zhang et al., Phys. Rev. Lett., 105, 015503 (2010).
 

Beamline U2A of the NSLS is a fully dedicated synchrotron infrared high-pressure/ microspectroscopy facility. The beamline was built by Geophysical Laboratory and became available for general users in 2000. The recently upgraded and expanded facility is supported by COMPRES (the NSF Consortium for Materials Properties Research in Earth Sciences), CDAC (Carnegie/DOE Alliance Center), and the Carnegie Institution.
 

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