Synthetic troegerite, phase II: Ferroelastic character and possible point groups

Abstract

Abstract It is known that H2(UO2)2(AsO4)2. 8 H2O (synthetic troegerite) undergoes several phase transitions, in particular a first order one, at room temperature, from a paraelectric phase I to a probably antiferroelectric phase II. The space group of none of the phases have been determined and although some reasonable guesses have been made taking as starting points the previously known facts and some macroscopic symmetry considerations, the observation that phase II behaves also as a ferroelastic justifies a new assesment of the possible point groups. From X-ray data, the point group of phase I is P 4/mmm. From these starting points and taking into account Aizu's criteria the number and orientation of permissible domains and both w and w' domain walls has been calculated for the possible point groups. Comparison with the observed domain structure gives 4/mmm F 2/m as the most probable transition. In an attempt to detect other than the optically observed four domain walls, and as TEM studies are precluded by the hydrated character of the sample, electron microscope observations of decorated cleavage surface replicas have been carried out, but only x = ± y domain walls have been observed.