CRYSTALLOGRAPHY AND MICROSTRUCTURAL STUDIES OF PHASE-TRANSFORMATIONS IN THE DY2O3 SYSTEM

The crystallography, microstructures, and phase transformation mechani sms in dysprosia (DY2O3) have been studied. The lattice parameters of B and C phases were refined by x-ray diffraction (XRD). The modulated structures and decomposed structures in the CaO-doped samples were cha racterized by transmission electron spectroscopy (TEM). A new twin was observed in the modulated B phase. Contrary to the previous studies, the B to C transformation was induced by grinding. The A to B transfor mation was considered to be ferroelastic and the spontaneous strain wa s calculated. The major driving force for the B (monoclinic) to C (cub ic) transformation is suggested to be the release of lattice strains a nd cation charge repulsions in the B structure, which is analogous to the beta (monoclinic) to gamma (orthorhombic) transformation in Ca2SiO 4. This transformation can be displacive, if some conditions are provi ded to overcome the bonding energy of the interlayer oxygens in the B structure.