  We report the formation of cubic phase, under ambient conditions, in thin
films of Zirconia synthesized by electron beam evaporation technique. The
stabilization of the cubic phase was achieved without the use of chemical
stabilizers and/or concurrent ion beam bombardment. Films of two different
thickness (660 nm, 140 nm) were deposited. The 660 nm and 140 nm films were
found to be stoichiometric (ZrO2) and off-stoichiometric (ZrO1.7) respectively
by Resonant Rutherford back-scattering spectroscopy. While the 660 nm
as-deposited films were in the cubic phase, as indicated by X-ray diffraction
and Raman spectroscopy measurements, the 140 nm as-deposited films were
amorphous and the transformation to cubic phase was obtained after thermal
annealing. Extended X-ray absorption fine structure measurements revealed the
existence of Oxygen vacancies in the local structure surrounding Zirconium for
all films. However, the amount of these Oxygen vacancies was found to be
significantly higher for the amorphous films as compared to the films in the
cubic phase (both 660 nm as-deposited and 140 nm annealed films). The cubic
phase stabilization is explained on the basis of suppression of the soft X2-
mode of vibration of the Oxygen sub-lattice due to the presence of the Oxygen
vacancies. Our first-principles modeling under the framework of density
functional theory shows that the cubic structure with Oxygen vacancies is
indeed more stable at ambient conditions than its pristine (without vacancies)
counterpart. The requirement of a critical amount of these vacancies for the
stabilization of the cubic phase is also discussed.
