Synthesis and structural and optical properties of metastable ZrO2 nanoparticles with intergranular Cr3+/Cr4+ doping and grain surface modification

Nanoparticles of stabilized ZrO2 in a single cubic (c) phase are obtained with an intergranular doping of 4 to 20 at.% Cr3+/Cr4+ additives by a chemical method using a high energy amorphous precursor with polymer molecules of sucrose and polyvinyl alcohol. In the polymer, the metal cations disperse and rearrange in a specific network structure with local symmetry probably similar to that in c-ZrO2. On heating at 250 to 800°C in air, the polymer network decomposes and burns out spontaneously (with a strong exothermic peak over 350 to 500°C in thermal analysis) in a refined microstructure in 10 to 20 nm diameter particles of near spherical shape. Those are identified to be of c-ZrO2 by x-ray diffraction. A modified microstructure of 15 to 30 nm crystallites of dispersed tetragonal (t) and/or monoclinic (m) phases in stabilized c-ZrO2 develops on a prolong heating at 900 to 1000°C from a polymer precursor for 2 h or longer. Particles in t-ZrO2 are in acicular shape, as long as 450 nm with aspect ratio φ ∼5 to 20, while in the shape of platelets in m-phase in an average 300 nm size. It is found that the Cr3+/Cr4+ additives promote formation of c-ZrO2 by a controlled decomposition and combustion of precursor in small particles at 250 to 800°C temperature. Part of the additives form a thin amorphous surface layer in individual c-ZrO2 grains so that it prevents them to grow or transform in the equilibrium m-ZrO2 bulk structure as long as the temperature lies below 900°C. The x-ray diffraction in light of the optical spectrum reveals that part of the Cr4+ cations occupy Zr4+ sites in a distorted c-ZrO2 lattice.