Electron microscopic studies of ferroelectric and ferroelastic Gd2(MoO4)3. I. General features of ferroelectric domain wall, antiphase boundary, and crystal defects

The ferroelectric domain structure and antiphase structure in Gd2(MoO4)3 crystals are studied by electron microscope, noting the contrast produced by the misorientation between the ferroelectric domains, by the moiré technique and by the dark field technique of the superlattice reflections. Besides the ferroelectric domain walls (FDW's) of the (110) and (110) planes, an unexpected type of FDW's of the {111} planes is found particularly in the thin (110) crystals. The dark field micrographs of the superlattice reflections very clearly contrast the antiparallel domains, the antiphase boundary (APB), and the two sorts of FDW's having different shifts of the neighbouring lattices. The dark field contrast between the antiparallel domains due to the failure of Friedel's law is proved to be absent in Gd2(MoO4)3. Dislocations and Frank type dislocation loops of complex natures are observed.