Off-center impurities in a robust ferroelectric material: Case of Li in KNbO3

Abstract

The influence of Li impurities on the phase transition sequence of KNbO3 is investigated using an atomisticmodel with parameters fitted to first-principles calculations. We find that the replacement of K by Li ionsincreases local electric dipoles due to a large Li off-centering along [001] directions. Li doping stronglyaffects the transition temperatures of KNbO3 as a result of the coupling between the off-center Li relaxationdynamics and the macroscopic polarization of the host material. This coupling produces a decrease in thetemperatures for the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions, while thetemperature of the cubic-tetragonal ferroelectric phase transition increases. The Li relaxation mechanism inthe different phases of KNbO3 provides a simple framework to understand the transition temperature shifts.