Visualization of the formation of the phase

The morphological transformations occurring in the precursor powders during the formation of the phase were investigated by scanning electron microscopy performed on the same location on the surface of a pellet, which underwent several heat treatments at for various times. Following the evolution of a given group of grains we have found that the formation of the crystals occurs simultaneously with a gradual decomposition of the intermediate phase. This result was confirmed by microprobe analysis as well as x-ray diffraction performed on the same samples. As a main result, we conclude that in the observed powder mixtures, the phase does not form from the pre-existing crystallites but through a nucleation and growth process. Based on the present observations, a model for the phase formation mechanism of is presented.