The massive transformation in metallic alloys has been investigated in considerable detail over the past three decades, both theoretically as well as experimentally. Rapid atomic transport, however, usually makes it difficult to experimentally investigate the kinetics of the massive transformation in metallic alloys under isothermal conditions. In ceramics, on the other hand, diffusion is generally much slower and can be manipulated through defect chemistry by the addition of an aliovalent dopant (that is, a dopant of a valence different from that of the host ion), and/or by altering the atmosphere. This, in principle, makes it possible to investigate the kinetics of the massive transformation in ceramics under isothermal conditions. The present manuscript reports on a study of the massive transformation in Bi 2 O 3 -based ceramics. Samples in the Bi 2 O 3 -RE 2 O 3 systems, where RE is Y or a rare-earth ion, were fabricated in a dense form by consolidating requisite powder mixtures and sintering compacts in air. The high-temperature form of Bi 2 O 3 is isostructural with CaF 2 , with 25 pct of the oxygen sites being vacant. This makes it an excellent oxygen ion conductor, thus affording the measurement of oxygen ion conductivity as a technique to investigate the kinetics of the massive transformation. The kinetics of the massive transformation was investigated under isothermal conditions using a number of techniques, including X-ray diffraction (XRD), reflected optical microscopy, transmission optical microscopy, transmission electron microscopy (TEM), electron-probe microanalysis (EPMA), and the measurement of ionic conductivity. The studies demonstrated that the cubic → rhombohedral transformation (during cooling) in Bi 2 O 3 -based ceramics is a massive transformation, and the rhombohedral → cubic reverse transformation (during heating) is also a massive transformation. A study of the Gd 2 O 3 -Bi 2 O 3 system also showed that a massive transformation occurs inside a twophase field, thus possibly shedding some light on a controversial point, much debated in the literature on metallic alloys. The sluggishness of the transformation kinetics suggests that ceramic systems are perhaps better suited to investigate salient features of the massive transformation (and, for that matter, many other diffusional transformations), theories for which have been developed for metallic alloys.
