Effective material properties in twinned ferroelectric crystals

Without external fields, ferroelectric materials will have multidomain conf iguration in the ferroelectric state. Detailed analysis found that twinning may not be treated as random since the number of orientations for the doma in walls are limited in a given symmetry change during a ferroelectric phas e transition. In each finite region of a large crystal or in small crystall ites, a particular set of twins is favored under certain boundary condition s, which consists of only two of the low temperature variants. Statistic mo dels of random distribution of domains do not apply for calculating the phy sical properties of such twin structures. However, one could derive the two domain twin properties by using the constitutive equations and appropriate mechanical boundary considerations. This paper presents a theoretical anal ysis on such a two-domain twin system, including its global symmetry and ef fective material properties resulting from different twinning configuration s. Numerical results are derived for LiNbO3 and BaTiO3. (C) 1999 American I nstitute of Physics. [S0021-8979(99)01214-1].