Extensive linearized augmented plane-wave frozen phonon calculations were performed in order to understand the origin of ferroelectricity in ${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3$. 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, ${\mathit{T}}_{\mathit{c}}$. 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)${\mathrm{O}}_3$ system is similar in origin to ferroelectricity in the perovskites. We also find that the electronic structures of ${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3$ are very similar and hardly change during the phase transition. © 1996 The American Physical Society.

• Received 3 August 1995

DOI:https://doi.org/10.1103/PhysRevB.53.1193