Chemical-composition-dependent metastability of tetragonal ZrO₂ in sol-gel-derived films under different calcination conditions

The chemical compositions at the surface and in the bulk of the sol-gel-derived ZrO₂ films calcined at elevated temperatures in air or in N₂ atmospheres were examined to understand the metastability of the tetragonal phase and the mechanism of its phase transformation. The phase evolution of ZrO₂2 in air followed the sequence of amorphous → m-tetragonal → monoclinic over 80-950°C, while the phase transformation of amorphous → m-tetragonal → monoclinic → m-tetragonal was observed under N₂ atmosphere. The reduction of Zr⁴⁺ to low-valent states and the generation of oxygen vacancies via dehydroxylation and deoxygenation play the crucial roles in stabilizing m-tetragonal ZrO ₂ in the sol-gel-derived films. The O/Zr ratios for stabilizing the m-tetragonal ZrO₂ ranged between 1.98 and 1.63. The m-tetragonal-to-monoclinic phase transformation primarily involves the segregation of lattice defects to grain boundaries and occupation of oxygen vacancies by the diffused O^2- ions that were converted from surface hydroxyl groups. In the absence of alternative oxygen donors, including water and oxygen molecules, the stability of m-tetragonal ZrO₂ was maintained at elevated temperatures under N₂ atmosphere. In addition, regeneration of the oxygen vacancies via deoxygenation at high temperature results in the reformation of m-tetragonal ZrO₂. The changes in the chemical compositions and crystallite sizes of the films depict that the m-tetragonal-to-monoclinic phase transformation starts from the core of tetragonal domains, while its retransformation begins from the boundaries of monoclinic grains.