Annealing-Induced {011}-Specific Cyclic Twins in Tetragonal Zirconia Nanoparticles

Zirconia (ZrO2) nanocrystals with average size of 4 nm are fabricated by oxidation of the nonstoichiometric ZrC0.6 with ordered carbon vacancies at 450 degrees C under atmosphere. The nanocrystals are predominantly tetragonal (t) phase and spherical in shape, and their exposed surfaces are constructed by the {011} and {001} facets. After annealing at 700 degrees C under atmosphere, the coalescence of adjacent t-ZrO2 nanocrystals is observed, and most of the annealed t-ZrO2 nanoparticles are found to exhibit the {011}-specific twins. The dominant cyclic twins as well as a small number of the single and lamellar twins are recognized in the twinned nanoparticles. The cyclic-twinned nanoparticles are identified to have the 5-fold symmetry of either decahedron or icosahedron. In contrast to the single and lamellar twins which are formed via the coalescence of adjacent nanocrystals on the well-developed {011} surfaces, the cyclic-twinned nanoparticles are developed from the coalescence on the disoriented contact surfaces, in which the emission of partial dislocations and induced deformation are recognized to play the key role.