Guodong Ren, Doctoral Candidate, Institute of Materials Science & Engineering

Ferroic materials are important components of modern day electronics. They can be described by an order parameter that gains a non-zero value upon undergoing a symmetry-lowering phase transition. Ferroic materials include ferroelectrics, ferromagnets, ferrotoroidal materials and ferroaxial materials. Their physical properties can be controlled using external stimuli such as temperature, strain or electric field. Their physical properties are highly sensitive to small changes in structure and composition. First-principles calculations based on density-functional theory (DFT) have proven instrumental in describing material properties right from the atomic scale. Concurrently, advanced scanning transmission electron microscopy (STEM) has enabled the simultaneous imaging and spectroscopy of materials’ atomic and electronic structures with unprecedented spatial and energy resolution. Therefore, the integration of theoretical modeling and microscopy provides an unparalleled approach to unraveling the intricate structure-property correlations in ferroic materials.

In this dissertation, I present our theoretical studies combined with quantitative STEM characterization of various ferroic systems and their physical properties. Specifically, I investigate a variety of structural distortions that undergo in quasi-one-dimensional A_xTiS₃ (A = Ba, Sr; x ≥ 1) chalcogenides, multiferroic hexagonal YbFeO₃ and fluorite-structure ferroelectrics BO₂ (B = Hf, Zr), along with the effect these distortions have on the optical, chiral and polar properties. The objective of this research is to delve into the underlying mechanisms driving the emergence of ferroic distortions in different materials, and providing guidance for manipulating their properties.

Dissertation Examination Committee:

Prof. Rohan Mishra, Chair
Prof. Sang-Hoon Bae

Prof. Erik Henriksen
Prof. Bryce Sadtler

Dr. Miaofang Chi, Oak Ridge National Laboratory