The Structural Transformation from the Pyrochlore Structure, A2B2O7, to the Fluorite Structure, AO2, Studied by Raman Spectroscopy and Defect Chemistry Modeling

✍ Scribed by Marianne Glerup; Ole Faurskov Nielsen; Finn Willy Poulsen

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 421 KB
• Volume: 160
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2000.9142

✦ Synopsis

Pyrochlore oxides of the composition Y 2 Ti 2؊y Zr y O 7 , with y ‫؍‬ 0, 0.3, 0.6, and 0.9, were studied using Raman spectroscopy. Doping with Zr ؉4 , a homovalent ion, gradually induces the material to undergo a structural change from the perfect pyrochlore structure to a defect pyrochlore structure and ends at an oxygen de5cient 6uorite structure with the formula (Y, Ti, Zr) 1 O 1.75 . Invoking the bigger Zr ؉4 ion on the Ti ؉4 site causes this structural transformation. We have also investigated highly Ti-doped YSZ ceramic samples, which all have an oxygen de5cient cubic 6uorite structure. These samples have formula units between (Y, Ti, Zr) 1 O 1.675 and (Y, Ti, Zr) 1 O 1.90 . It is argued that a band observed at 750 cm ؊1 in both the Ti-doped pyrochlore samples and in the Ti, Y, Zr 6uorite samples is due to oxygen in seven fold coordination around Ti. Local structures are accordingly observed by Raman in these samples. We 5nally report a Raman spectrum of orthorhombic Y 2 TiO 5 . A defect chemistry model has been formulated, which qualitatively describes the observed pyrochlore+6uorite structural transition.