Correlation of Structure and Properties in Perovskite Oxide Thin Films

Rare-earth nickelate (RNiO3) materials exhibit metal-to-insulator transitions at given critical temperatures, which are highly correlated to the relative radii of the rare-earth elements to the radius of nickel. In this work, La1-xEuxNiO3 (LENO) single crystal thin films were grown by molecular beam epitaxy for the first time to understand how epitaxial strain correlates to the metal-to-insulator transition. These thin films showed transition temperatures that trended well with previous research on their bulk counterparts. However, it was found that films grown under tensile strain (~1%) transitioned at much higher temperatures than films grown under the same amount of compressive strain, while films under severe amounts of tensile strain (>2.5%) remained insulating from 1.8 K to 400 K. This is due to the elongation of the in-plane Ni-O bonds which dominate the electron conduction path.;During the course of this research the tolerance to cationic off-stoichiometry was also studied in LaNiO3 films. Typically it was assumed that films with defects were more resistive than more crystalline films, however it is rare to find a cationic composition study on complex oxide thin films. In this work Rutherford Backscattering Spectrometry and electronic transport measurements were utilized to show that LaNiO3 films with a La:Ni ratio of 0.75 can be more conductive than films with more stoichiometric cation ratios. This non-trivial behavior is attributed to an increase in electronic bandwidth brought on by a shortening of the Ni-O bonds. These Ni-O-Ni bond angles and lengths are heavily correlated to the macroscopic properties, and in all perovskites these B-O-B parameters are critical for understanding physical behavior.;A final effort of this work was to develop a program capable of solving for these angles and bond lengths through the use of synchrotron diffraction of half-order Bragg peaks. My program was shown to work with rhombohedral systems and more structurally complex orthorhombic perovskites, in which the origin of the half-order peaks arises from both octahedral rotations and A-site displacements. Using this program the structure of LaGaO 3 films strained to SrTiO3 was solved. This program will be available for all perovskite thin film researchers after being thoroughly vetted.