CRYSTAL TO GLASS TRANSFORMATIONS UNDER PRESSURE

Over the last ten years or so about four dozens of crystalline materia ls have been shown to become amorphous when subjected to a pressure of the order of about 10 GPa. What drives materials to adopt amorphous s tructure under compression has intrigued scientists all over the world . Further, these transformations may be reversible as well as irrevers ible. Even some of these reversible transformations become irreversibl e beyond a threshold pressure. For many years these crystal to amorpho us (c --> a) transformations have been explained in terms of pressure induced melting. Our group at BARC has tried various approaches to und erstand different aspects of these fascinating phase transformations. With the help of X-ray diffraction studies, molecular dynamics simulat ions as well as in terms of geometric steric constraints due to molecu lar interactions, a substantial success has been achieved in understan ding these transformations. Thermodynamical approach suggests that man y of these transformations are due to the kinetic hindrance on the pat h to a high pressure equilibrium phase. Use of the theory of kinetics of phase transformation also suggests that some transformations which are reversible under slow unloading of pressure such as in static expe riments may become irreversible under fast decompression, characterist ic of shock unloading. This field of c --> a transformations is briefl y reviewed and the work done at Trombay presented: