Hydrothermal synthesis of I:V perovskite thin films and powders

The purpose of this project was to study the development of the morphology and crystal structure of hydrothermally synthesized powders and heteroepitaxial films of the I:V perovskites. Specifically, the KTaO₃ and KNbO ₃ systems were synthesized in aqueous solutions containing 7 to 15M KOH at 200°C and below. It was observed that the formation of the perovskite phase was almost always preceded by an intermediate metastable phase.; For KTaO₃, this intermediate phase was a defect pyrochlore. Epitaxial KTaO₃ films formed in 7M KOH solutions contained epitaxial islands of this pyrochlore. As no intermediate pyrochlore phase was formed at 15M KOH, pyrochlore-free KTaO₃ films could be grown. Pyrochlore-free films were also grown at 7M KOH by delaying the introduction of the substrate until the pyrochlore phase was no longer nucleating.; Synthesis of the KNbO₃ phase proceeded via an intermediate hexaniobate ion. The high solubility of this complex ion accounted for the low yields of perovskite powder obtained for the shorter synthesis periods. Films synthesized in 15M KOH had two out-of-plane orientations because growth occurred at temperatures for which the orthorhombic structure was stable and the difference between the lattice mismatches of the two orientations were not very large. The film surface consisted largely of tower-like structures. These towers contained screw dislocations parallel to their long axes and grew by a spiral mechanism. It appeared that the advancement of spirals was impeded by impurities and it was proposed that lower impurity activity at the center of the spirals led to the development of the tower structures.; It was believed that protons incorporated into the pyrochlore and perovskite lattices reduced the binding forces of the crystal and caused an expansion of the lattice. It appeared that synthesis in solutions of higher KOH concentrations led to greater proton incorporation and subsequently to greater lattice expansion. In addition, removal of protons from the KNbO₃ lattice also caused the initial orthorhombic structure to collapse. This was attributed to the presence of free hydroxyl dipoles in the as-synthesized lattice, which upon removal with heat treatment, led to the collapse of the structure.