FIRST PRINCIPLES TOTAL-ENERGY STUDY OF FERROELECTRIC TRANSITIONS IN LINBO3

Inspite of its uniaxially symmetric simple crystal structure, there ar e still dubious arguments on the types and origin of ferroelectric ins tability in LiNbO3. As a step toward first-principles theoretical stud ies of ferroelectric transitions, we carried out total energy calculat ions of LiNbO3 by using the full-potential linearized augmented-plane- wave (FLAPW) method based on the density functional theory within loca l density approximation. By performing many total energy calculations of LiNbO3 with various possible configurations of Li and O displacemen ts, we obtained total energy surfaces showing that the independent dis placements of Li and O atoms lead to a shallow potential well, while t he distortions of the coupled mode u(Li+O) leads to a deep double-well potential, the depth of which is significantly larger than the observ ed phase-transition temperature T-c. The Li-dipole induced oxygen pola rization is suggested to be the origin of such large energy gain by th e Li-O coupled distortions. Therefore, it is concluded that the Li dis placements coupled to oxygen polarizations may play a crucial role in the ferroelectric phase transition of LiNbO3.