Electron microscopic studies of ferroelectric and ferroelastic Gd2(MoO4)3. III. Image analysis of antiphase boundary and observation of phase transformation

Electron microscopic observation of the antiphase boundaries (APB's) which produce a residual contrast for the fundamental reflections shows the existence of an irrational lattice displacement ΔR in addition to the rational one Ro = [a/2, b/2, 0] across the boundaries. The displacement depends on orientation of the APB and gives dilatation of the lattice at the APB. The observed image profiles of the APB are compared with those which are calculated in terms of the many beam dynamical theory using non-systematic reflections with the best fit for the value of |ΔR| = 0.02a to 0.04a. It is proposed that the APB's have a finite thickness and a structure alike that of the tetragonal high-temperature phase. The thickness is estimated from the value of ΔR to be in the range 30 to 70 Å. Characteristic transformation behaviours closely connected with the APB structure were observed by the dark field image technique using the superlattice reflections.