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Correlation of structure and properties in perovskite oxide thin films
Dissertation   Open access

Correlation of structure and properties in perovskite oxide thin films

Cole Richard Smith
Doctor of Philosophy (Ph.D.), Drexel University
01 Dec 2014
DOI:
https://doi.org/10.17918/etd-6388
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Abstract

Molecular beam epitaxy Crystallography Materials Science
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, La_[1-x]Eu_xNiO₃ (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 LaGaO₃ films strained to SrTiO₃ was solved. This program will be available for all perovskite thin film researchers after being thoroughly vetted.

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