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].