Comparison of the electronic structures and energetics of ferroelectric LiNbO3 and LiTaO3

Extensive linearized augmented plane-wave frozen phonon calculations were performed in order to understand the origin of ferroelectricity in LiTaO3 and LiNbO3. Displacement of the Li atoms alone results in an anharmonic single well, whereas displacements of oxygen and lithium together result in deep double wells, much deeper than the transition temperatures, T-c. This is contrary to current theories which model the underlying potential as a triple well potential for the lithium atoms. Our results support an order-disorder model for the oxygen atoms as the driving mechanism for the ferroelectric instability. Oxygen displacements alone against the transition-metal atoms result in shallower double wells as a result of oxygen-lithium overlap so that the lithium and oxygen displacements are strongly coupled. We find large hybridization between the oxygens and the transition-metal atoms. Thus ferroelectricity in the Li(Nb,Ta)O-3 system is similar in origin to ferroelectricity in the perovskites. We also find that the electronic structures of LiTaO3 and LiNbO3 are very similar and hardly change during the phase transition.