The structural transformation from the pyrochlore structure, A2B2O7, to the fluorite structure, AO2, studied by Raman spectroscopy and defect chemistry modeling

Pyrochlore oxides of the composition Y2Ti2-yZryO7,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 deficient fluorite 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 deficient cubic fluorite 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 fluorite samples is due to oxygen in seven fold coordination around Ti. Local structures are accordingly observed by Raman in these samples. We finally report a Raman spectrum of orthorhombic Y2TiO5. A defect chemistry model has been formulated, which qualitatively describes the observed pyrochlore-fluorite structural transition. (C) 2001 Academic Press.