CRYSTAL STABILITY LIMITS AT POSITIVE AND NEGATIVE PRESSURES, AND CRYSTAL-TO-GLASS TRANSITIONS

The direct crystal-to-glass transformation, i.e., spontaneous amorphiz ation, which was first observed by thermal annealing of stishovite SiO 2 at ambient pressure, has now been observed as an isothermal phenomen on during both compression and decompression of initially stable cryst als. While counterintuitive, and dependent on kinetically controlled m etastable events, the phenomenon is of broad interest and potential im portance in materials science and geophysics. In this paper we use a c ombination of molecular dynamics simulations and analyses of laborator y data to explore the metastable crystal ranges, including the negativ e pressure range, for key compounds such as the ices, silicas, and alk aline earth perovskites. Our focus is on the establishment of phenomen ological patterns rather than on specific metastability-terminating me chanisms. We find that a simple quadratic law, P - P-s similar to (V - V-s)(2) (where P-s and V-s are the values of the pressure P and volum e V on the spinodal), well approximates the equations of state over mu ch of the metastable and even the stable range-and implies the existen ce of an isochoric boundary line for stability to isotropic density fl uctuations. We delineate the conditions under which amorphisation occu rs, usually substantially before the stability limit is reached.