Pure orthorhombic zirconia islands grown on single-crystal sapphire substrates

This study reports on the occurrence of pure orthorhombic zirconia obtained for the first time at ambient pressure. Thin films of pure zirconia are composed of isolated islands which generally present heteroepitaxial relationships with the underlying sapphire substrate. Epitaxial growth develops at high temperature in the stability domain of the tetragonal phase of zirconia. On cooling, most of the tetragonal islands transform into monoclinic, but numerous islands present an orthorhombic structure with Pbc21 and Pbca space groups. It is first suggested that these two orthorhombic phases are formed because the tetragonal → monoclinic reconstructive phase transition was impeded due to the pre-existing heteroepitaxial relationships that developed between the tetragonal zirconia phase and the substrate. However, the orthorhombic phases are much better interpreted as alternative phases that arise as a consequence of some constraints produced by the substrate. Thus, these phases represent metastable structures when appropriate external anisotropic constraints are applied.