Crystallographic, Electronic, and Phononic Properties of SrTiO₃-CaTiO₃ Superlattices Versus Layer Thickness

The rich phase-space of perovskite oxides offers a plethora of properties with technological importance that can be further tuned by combining multiple structures into a superlattice. This tunability has been attributed in some cases to coupling and competition of electronic and phononic structure between the layers of the superlattice. Perovskite oxide superlattices for example have exhibited thermal conductivity lower than solid solution values and exotic phases with non-transient negative capacitance. These properties suggest an evolution in the structure of the constituent layers away from their monolithic counterparts. In this thesis, it was found that crystal and electron structures in the layers of a symmetric SrTiO₃-CaTiO₃ superlattice converged from their distinct monolithic constituents to a similar and unique structure as the number of unit-cells in the layers was reduced from twenty-seven to one. The converged structure in the smaller-period superlattices displayed qualities reminiscent of the interface structure in the larger-period superlattices. It was also found that the vibrational response of the superlattices changed with the number of unit-cells in the layers, and the response was not consistent with theoretical predictions using the dipole scattering approximation and dipole-active phonon values for SrTiO₃, CaTiO₃, and NdGaO₃ from the literature. Therefore, the vibrational response of the superlattices was unique compared to the monolithic phases. These results demonstrate that superlattices can acquire unique structures and properties that are governed by interfaces as their sizes decrease below certain values and further, that these phenomena need to be quantified at the nanoscale to obtain a complete understanding of their behavior.