First-principles phonon calculations for LiTaO3

Abstract

Abstract Frozen phonon calculations for LiTaO3 were performed using the Linearized Augmented Plane Wave (LAPW) method to study the energetics of the ferroelectric instability and its origin. Computations were performed in the R3c structure for O and Li displacements relative to the Ta, and a deep double well with a depth of 2739 K per 10 atom unit cell was found, much deeper than the 950K transition temperature, supporting strong order-disorder character for the transition. Displacements of the Li alone, with Ta and O fixed in the positions of the high temperature R3c structure yielded only an anharmonic single well potential, indicating that the ferroelectric instability is overwhelmingly due to the energetics of O motions relative to Ta, rather than to the Li displacements as has been widely assumed. These results indicate that the LiTaO3 and LiNbO3 families of ferroelectrics are more similar to the perovskite ferroelectrics than has been realized, and that again the ferroelectric instability is governe...