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 ZrO 2 films calcined at elevated temperatures in air or in N 2 atmospheres were examined to understand the metastability of the tetragonal phase and the mechanism of its phase transformation. The phase evolution of ZrO 2 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 N 2 atmosphere. The reduction of Zr 4+ 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 ZrO 2 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 ZrO 2 was maintained at elevated temperatures under N 2 atmosphere. In addition, regeneration of the oxygen vacancies via deoxygenation at high temperature results in the reformation of m-tetragonal ZrO 2 . 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.