First-Principles Modeling of Thin-Film Ferroelectrics

The goal of this dissertation is to explore the properties of thin-film ferroelectrics using first-principles computational methods. Achieving this goal requires both the development of efficient methods for computing thin-film properties as well as the application of these methods to a variety of thin-film materials of interest. The research included in this dissertation is thus composed of a mix of these two efforts./nFirst, the structural properties, energetics, and polarizations of perovskite-based thin-film oxide systems are computed as a function of biaxial strain state and epitaxial orientation, employing an automated computational workflow based on density functional theory (DFT). A total of 14 compositions are considered, of the form ABO3, with A=Ba, K, Na, Pb, and Sr and non-magnetic B=Hf, Sn, Ti, Zr, Nb, Ta, and V site cations chosen to yield tolerance factors with values ranging between 0.95 and 1.1. Three biaxial strain states corresponding to epitaxial growth of (100)-, (110)-, and (111)-oriented films are considered, with misfit strains ranging between -4% to 4%. Results are presented for the series of perovskite-derived phases, and their corresponding symmetries, which are energetically favorable as a function of misfit strain, along with their corresponding equilibrium atomic positions, lattice parameters, and electric polarizations. The results demonstrate robust trends of in-plane polarization enhancement under tensile strain for all epitaxial orientations, and out-of-plane polarization enhancement with compression for the (100)- and (110)-oriented films. Strains corresponding to the (111)-growth orientation lead to a wider variety of out-of-plane polarization behavior, with BaTiO3 showing anomalous diminishing polarization with compression. Epitaxial orientation is shown to have a strong effect on the nature of strain-induced phase transitions, with (100)-oriented systems tending to have smooth, second-order transitions and (110)- and (111)-oriented systems more commonly exhibiting first-order transitions. The significance of this effect for device applications is discussed, and a number of systems are identified as potentially interesting for ferroelectric thin-film applications based on energetic stability and polarization behavior. Analysis of polarization behavior across different orientations reveals distinct groups into which compositions can be organized, some of which having polarization dependencies on misfit strain that have not been predicted previously./nFollowing the work described above, ground-state epitaxial phase diagrams are calculated by DFT for SrTiO3, CaTiO3, and SrHfO3 perovskite-based compounds, accounting for effects of antiferrodistortive and A-site displacement modes. Biaxial strain states corresponding to epitaxial growth of (001)-oriented films are considered, with misfit strains ranging between -4% and 4%. Ground-state structures are determined using a computational procedure in which input structures for DFT optimizations are identified as local minima in expansions of the total energy with respect to strain and soft-mode degrees of freedom. Comparison to results of previous DFT studies demonstrates the effectiveness of the computational approach in predicting ground-state phases. The calculated results show that antiferrodistortive octahedral rotations and associated A-site displacement modes act to suppress polarization and reduce epitaxial strain energy. A projection of calculated atomic displacements in the ground-state epitaxial structures onto soft-mode eigenvectors shows that three ferroelectric and six antiferrodistortive displacement modes are dominant at all misfit strains considered, with the relative contributions from each varying systematically with strain. Additional A-site displacement modes contribute to the atomic displacements in CaTiO3 and SrHfO3, which serve to optimize the coordination of the undersized A-site cation./nFurther, an effort is made to identify alternative vanadate perovskite-derivative systems similar to the well-studied pressure-stabilized PVO structure. To achieve this, the stability of perovskite-derivative thin-film structures of KVO3 and NaVO3 are studied under compressive biaxial strain. The electronic structure and polar properties of these compounds are computed as a function of biaxial strain, and the results are compared to those obtained for experimentally-observed PbVO3 structures. It is demonstrated that the substitution of Pb with monovalent K or Na cations increases the strength of the vanadyl bond due to the removal of the spatially extended Pb 6p states. Both KVO3 and NaVO3 exhibit epitaxially stabilized perovskite-derivative phases having large polarizations and low misfit strain energies. The calculated epitaxial phase diagram for KVO3 predicts a strain-induced phase separation from -4% to 1.5% misfit strain into a ferroelectric $Cm$ phase, having square-pyramidal coordination of the B-site, and a paraelectric Pbcm phase, having tetrahedral coordination of the B-site. The results show that strain-stabilized polar vanadate compounds may occur for other compositions in addition to PVO, and that changes in the A-site species can be used to tune bonding, structure, and functional properties in these systems.

Advisor: Asta, Mark D