Defects in oxides

T. Grande 
Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim, Norway

Piezoelectric/ferroelectric oxides have become irreplaceable materials in modern electronics. The polar nature and piezoelectric properties of these oxide materials can be understood from crystal structure and crystal symmetry, but defects play also an important role for the functionality and electrical properties. For example, defects are particularly detrimental for aging of electric properties and fatigue. In this lecture, an overview of different defects in oxides are presented with focus on point defects in polar oxides and their influence on electrical properties. 
The most successful description of point defects in oxide materials is based on point defect chemistry using Kröger-Vink notation, and the major point defects in prototype ferroelectrics are reviewed using this notation. The influence of temperature and partial pressure of oxygen on the defect chemistry is also covered. The role of point defects for the understanding of soft and hard PZT is described in detail, and recent progress in understanding point defects in important lead-free ferroelectrics is included. Finally, point defects due to exposure to moisture is reviewed briefly.
Defects possess a charge relative to the perfect crystal lattice, and diffusion of point defects will take place in potential gradients such as electrical field or differences in partial pressure of oxygen. We will discuss how the mobility of point defects are vital for understanding the electrical conductivity of semiconducting oxides and particularly the contribution from ionic conductivity.