X-ray topographic study of the antiferroelectric phase transition and antiphase boundaries in copper formate tetrahydrate

Copper formate tetrahydrate, Cu(HCOO)(2)center dot 4H(2)O, undergoes upon cooling below -38 degrees C a phase transition from a paraelectric (PE) into an antiferroelectric (AFE) phase, whereby its monoclinic point group 2/m is preserved and its lattice parameter c doubled. The boundary between the phases PE and AFE and the antiphase boundaries between the AFE domains have been imaged by X-ray diffraction topography (Lang technique), using a rotating-anode generator with Ag-K alpha radiation. A simple cooling chamber allowed the adjustment of the transition isotherm (i.e. the phase boundary) across the plate-shaped crystal and its stepwise movement across the probe. This has been done for slices of orientation (010) and (001) with several directions of the temperature gradient. The phase boundary appears on the topographs by strong kinematical contrast, indicating considerable lattice strain in the transition region. For (001) plates the X-ray diffraction positions of the PE and AFE phase regions on both sides of the phase boundary are different so that the two regions had to be imaged in separate exposures with a small angular readjustment between them. As expected, the domain boundaries, imaged by section topography, show fringe contrast typical for antiphase boundaries. The various findings are described and discussed in detail.