Metastable solid phase at the crystalline-amorphous border: The glacial phase of triphenyl phosphite

The fragile glass-forming liquid triphenyl phosphite (TPP) melts at 298 K, has a glass transition at about 186 K, and can undergo a first order transition to a metastable, solid, apparently amorphous phase (denoted the glacial phase) at about 230 K. Though apparently amorphous (on the basis of preliminary X-ray data), we have shown that the glacial phase is well described as a plastic crystal composed of nanocrystallites; it is thus not a second liquid or glass nor a poor ordinary molecular crystal. This picture of the glacial phase with its close connection to the supercooled liquid from which it forms is placed in the context of the frustration-limited-domain theory of supercooled liquids, and the glacial phase is associated with the defect-ordered phase predicted by that theory. In examining the applicability of this picture, we carried out extensive experimental studies, many of them by means of NMR at both ambient and high pressures, on the glacial phase and on the dynamics of the transformations to and from this phase.