Phase transformation in the oxides

Abstract

This paper reviews phase transformations in the oxides, and briefly examines the similarities and dissimilarities with the phase transitions in metals. A few topics selected for the discussion, include, displacive phase transitions, order-disorder transitions, and clustering and precipitation. The displacive transition involves the oxygen polyhedra, and is usually found in ferroelectric and antiferroelectric oxides, such as, BaTiO3, SrTiO3, PbZrO3, and other oxides of perovskite structure. In lattice dynamics studies, this transition has been associated with an instability of a long-wave optical phonon. The condensation of the ferroelectric soft mode causes the static distortion of the lattice at the phase transition. The order-disorder transformation in the oxides may occur in different forms. It may be the hydrogen ordering in KDP, the catio distributions in spinel MgFe2O4,the Li-Fe ordering of the octahedral sites in LiFe5O8, or the formations of layer structures in the double perovskites, such as Ba2CoWO6, Ba2CoReO6, and others. Clustering and precipitations are commonly observed phenomena in oxide systems. They result in phase separations in both the crystalline, and the glassy systems. Heat treatments to control the crystallization processes in glasses result in glass ceramics, which have been proven to be superior to both the original glass, and to the precipitated crystalline phase.