Theoretical investigation of the ferroelectric transition in BaTiO3 and LiNbO3

Barium titanate (BaTiO3, BT) and lithium niobate (LiNbO3, LN) are the most important ferroelectric materials. The first is the prototypical ferroelectric oxide, as many features and properties caused by or related to ferroelectricity have been found for the first time in BT. The second is one of the most important optic materials, being the equivalent in the field of optics, non-linear optics and optoelectronics to silicon in electronics. Despite the extensive interest in BT and LN, the knowledge of the mechanisms underlying the ferroelectric phase transitions is rather poor. The nature of the phase transition itself (displacive or order-disorder) is still argument of debate. In this contribution we report on our first-principles theoretical studies of the ferroelectric transition in the two systems. Total energy calculations, molecular dynamics and frozen-phonon calculations are used to understand the mechanisms driving the transition at microscopic level. The Curie temperatures are estimated by ab initio thermodynamics. Similarities and differences between the two systems are discussed.