Epitaxial stabilization of kinetically frustrated metastable materials.

摘要

Developing synthesis techniques to form new materials has always fascinated material scientists. The challenge becomes even more interesting when the material under consideration is metastable and it is difficult to synthesize it using standard bulk approaches. Thin film deposition, particularly epitaxial stabilization of thin films is commonly used to synthesize metastable materials. In this technique one assumes the growth of compounds occurs on the surface with appreciable diffusion, no desorption, and fast reaction. In this work we will discuss cases where the above assumptions during epitaxial growth do not hold true and kinetics play an important role in phase selection during nucleation and growth.;The effect of surface diffusion on phase selection is discussed with respect to the growth of RE2Ti2O 7 (RE = rare earths) compounds. It has been shown that the growth of the (110)-layered perovskite polymorph of RE 2Ti2O7 compounds is kinetically frustrated and that the system can be captured in a previously-unknown, kinetically-preferred polymorph (gamma-polymorph). This is a unique observation as it shows that diffusion can direct phase selection kinetically during film growth. It has also been shown that epitaxial stabilization on isostructural polycrystalline substrates coupled with local characterization using electron back scattered diffraction (EBSD) is a powerful thin film technique for the growth of complex layered metastable compounds. This opens a whole new avenue for thin film research and can lead to the growth of novel compounds with particular compositions in specific crystal structures.;For compounds containing volatile components (e.g., Bi-based compounds), both desorption and reaction rates affect phase formation. Maintaining both global and local stoichiometry is a major challenge in the growth of such compounds. A systematic study was done to determine the effects that different deposition parameters have on the film composition of BiMnO3 and Bi2Ti2O7. Two novel growth approaches: the solid state epitaxy approach and the hybrid PLD approach, that overcome desorption and reaction rate issues and lead to stoichiometric epitaxial film growth of Bi-based compounds have been developed. The techniques are quite general and can lead to the synthesis of other Bi-based compounds as well.