DEHYDROXYLATION AND THE CRYSTALLINE PHASES IN SOL-GEL ZIRCONIA

The formation and evolution with temperature of the crystalline phases in sol-gel ZrO2 was analyzed by using X-ray powder diffraction, refin ement of the crystalline structures, ESR, and UV-Vis spectroscopy. The precursor phase of crystalline zirconia was amorphous Zr(OH)(4) with the same local order as the tetragonal crystalline phase. This amorpho us phase dehydroxylated with temperature, generating nanocrystalline t etragonal zirconia, and producing point defects that stabilized the te tragonal structure, generated a paramagetic ESR signal with g values l ike the free electron, and had a light absorption band at 310 nm. When the sample was annealed at higher temperatures, it continued dehydrox ilating, and the point defects disappeared, causing the transformation of the nanocrystalline tetragonal phase into nanocrystalline monoclin ic zirconia. The two crystalline nanophases coexisted since the beginn ing of crystallization.