The spontaneous polarization as evidence for lithium disordering in LiNbO3

Abstract

The ferroelectric to paraelectric phase transition in lithium niobate is examined. The present study focuses on the microscopic mechanism for this phase transition. Literature reports that give insight into this mechanism are reviewed. Two alternate mechanisms for this second order transition have been discussed previously. The phase transition has been proposed to occur by either (a) cooperative displacement of Li ions or (b) statistical disordering of Li between two octahedral sites in the structure. The present study develops a general Landau expression for the second order phase transition. The spontaneous polarization of the lattice is used as an indicator of the extent of transformation. Then both displacement and disordering models are explored. These are compared with the observed spontaneous polarization data. It is concluded that the spontaneous polarization data are indicative of a Li disordering model, rather than a Li displacement model.