The Influence of Oxygen Vacancies on Domain Patterns in Ferroelectric Perovskites

Citation

Xiao, Yu (2005) The Influence of Oxygen Vacancies on Domain Patterns in Ferroelectric Perovskites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5QSX-9Y68. https://resolver.caltech.edu/CaltechETD:etd-01032005-140446

Abstract

This thesis investigates the role of oxygen vacancies in determining ferroelectric properties and domain patterns of ferroelectric perovskites. Being non-polar (paraelectric) above their Curie temperature but spontaneously polarized (ferroelectric) below it, ferroelectric perovskites offer a tantalizing potential for applications: large actuation through domain switching and memory storage via switchable electric polarization. Oxygen vacancies, commonly present and mobile at high temperature, are the primary defects and thus play a central role in these applications.

We develop a model that combines the ferroelectric and semiconducting nature of ferroelectric perovskites. Oxygen vacancies act as n-type dopants and thus affect the semiconducting properties. We show that the ferroelectric and semiconducting features interact and lead to the formation of depletion layers near the electrodes and double layers at the 90° domain walls. We find a potential drop across 90° domain walls even in a perfect crystal. This potential drop marks the essential difference between a 90° and an 180° domain wall, drives the formation of a space charge double layer in a doped crystal, promotes electronic charge injection and trapping, and leads to the redistribution of oxygen vacancies at 90° domain walls. The rearrangement of oxygen vacancies near 90° domain walls may form a basis for domain memory and provides a potentially new mechanism for large electrostriction.

We also rigorously justify the continuum theory by calculating the Coulomb energy of a spontaneously polarized solid starting from a periodic distribution of charges based on the classical interpretation of ferroelectrics and with a definite choice of polarization per unit cell. We prove that in the limit where the size of the body is large compared to the unit cell, the energy of Coulombic interactions may be approximated by a sum of a local part and a nonlocal part. The local part depends on the lattice structure, but is different from the Lorentz formula for a lattice of dipoles. The nonlocal part is identical to the Lorentz formula.

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