Origin of ferroelectricity in LiTaO3 and LiNbO3; LAPW total energy calculations

Abstract

In order to better understand the origin of the ferroelectric instability in LiTaO3 and LiNbO3, a set of LAPW frozen phonon calculations were performed. Deep double wells, much deeper than the transition temperatures, Tc, are found as a result of the oxygen displacements against the transition metal atoms, whereas displacement of the Li atoms alone results in an anharmonic single well. This supports an order-disorder character for the oxygen atoms, contrary to current theories emphasizing the order-disorder character of the Li atoms as the mechanism for the ferroelectric phase transformation. Also, we find large hybridization between the transition metal atoms and the oxygens. The Li(Nb,Ta)O3 system is very similar to the perovskite ferroelectrics where the hybridization plays a major role in the transition to a ferroelectric phase.