Crystallization and segregation in vitreous rutile films annealed at high temperature

Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m-TiO2 (monoclinic P21∕c space group) and α-TiO2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation shows that small m-TiO2 crystallites are able to form directly from vitreous TiO2 at 1273 K. However, the preferred mechanism for forming α-TiO2 is likely to be by epitaxial growth at crystalline rutile twin boundaries (secondary crystallization). Both phases are denser than crystalline rutile and reduce the average thermal stress in the films.