Abstract
A displacement in addition to that of the half lattice period was found to exist at the antiphase boundary of Gd2(MoO4)3 by means of electron microscopy. The displacement depends on orientation of the antiphase boundary, but is nearly in the a direction for the boundaries of all orientations except when it is in the b direction for the boundary parallel to the a axis. The displacement gives dilatation of the lattice at the boundary. These facts suggest that the antiphase boundaries have a finite width and a structure alike that of the tetragonal high-temperature phase.