Berlin 2012 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 3: Nonlinear dielectrics, phase transitions, relaxors

DF 3.1: Talk

Monday, March 26, 2012, 15:00–15:20, EB 107

Ferroelectric transition in LiNbO₃ calculated from first principles — •Simone Sanna and Wolf Gero Schmidt — Lehrstuhl für Theoretische Physik, Universität Paderborn

Lithium niobate (LiNbO₃, LN) belongs to the most important ferroelectric materials. It is furthermore one of the most important optic materials, being the equivalent in the field of non-linear optics and optoelectronics to silicon in electronics [1]. Despite the extensive technological usage, the knowledge of the mechanisms underlying the ferroelectric phase transition is rather poor. The nature of the phase transition itself (displacive or order-disorder) is still argument of debate [2]. In this contribution we report on our first-principles theoretical studies of the ferroelectric transition, which is described in detail at the microscopic level. Total energy calculations as well as long-run molecular dynamics simulations are used to understand the driving forces and mechanisms of the phase transition. Our calculations show that the structural phase transition is not an abrupt event, but rather a continuous process occurring over about 100 K and involving different ionic species at different temperatures. Because of the different behavior of the Li and Nb sublattice, the ferroelectric transition displays both displacive and order-disorder character. In addition to pure materials, the role of the widely used dopant Ti is investigated, and the results of existing experiments explained in the light of our theoretical models. [1] A. Räuber, Chemistry and Physics of Lithium Niobate (North-Holland Publ. Company, Current Topics in Mat. Sci., 1978). [2] I. Inbar and R. E. Cohen, Phys. Rev. B 53, 1193 (1996).