Reconstructive phase formation of ZrO2 nanoparticles in a new orthorhombic crystal structure from an energized porous ZrO(OH)2·xH2O precursor

A high energy amorphous ZrO(OH)2·xH2O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr4+ cations in an aqueous solution in reaction with NH4OH at 5 °C followed by aging and drying of the recovered gel at room temperature. At 20–40 °C in air, the excess H2O evaporates leaving behind a porous ZrO(OH)2·xH2O powder with Φ ∼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 500 °C 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∼15 nm. This irreversibly transforms to m-ZrO2 at D⩾23 nm at 800 °C or above. A schematic energy level diagram for the precursor and the metastable ZrO2 polymorphs is proposed with the observation of the phase transformations.