Reconstructive phase formation of ZrO2 nanoparticles in a new orthorhombic crystal structure from an energized porous ZrO(OH)(2)center dot xH(2)O precursor

A high energy amorphous ZrO(OH)(2)(.)xH(2)O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr4+ cations in an aqueous solution in reaction with NH4OH at 5degreesC followed by aging and drying of the recovered gel at room temperature. At 20-40degreesC in air, the excess H2O evaporates leaving behind a porous ZrO(OH)(2)(.)xH(2)O powder with Phi similar to 37% porosity. X-ray diffraction characterizes its amorphous structure with two halos at 20.8 and 36.7 nm(-1) wavevectors. On heating, a reconstructive ZrO2 phase transformation occurs from a refined amorphous state at 500degreesC to form a new orthorhombic structure with a = 0.3340, b = 0.5535 and c = 0.6364 nm lattice parameters. Average crystallite size is D similar to 15 nm. This irreversibly transforms to m-ZrO2 at D greater than or equal to 23 nm at 800degreesC or above. A schematic energy level diagram for the precursor and the metastable ZrO2 polymorphs is proposed with the observation of the phase transformations. (C) 2003 Elsevier Ltd and Techna S.r.l. All rights reserved.