Crystal stability limits at positive and negative pressures, and crystal-to-glass transitions

The direct crystal-to-glass transformation, i.e., spontaneous amorphization, which was first observed by thermal annealing of stishovite SiO2 at ambient pressure, has now been observed as an isothermal phenomenon during both compression and decompression of initially stable crystals. While counterintuitive, and dependent on kinetically controlled metastable events, the phenomenon is of broad interest and potential importance in materials science and geophysics. In this paper we use a combination of molecular dynamics simulations and analyses of laboratory data to explore the metastable crystal ranges, including the negative pressure range, for key compounds such as the ices, silicas, and alkaline earth perovskites. Our focus is on the establishment of phenomenological patterns rather than on specific metastability-terminating mechanicsms. We find that a simple quadratic law, P-Ps∼(V-Vs)2 (where Ps and Vs are the values of the pressure P and volume V on the spinodal), well approximates the equations of state over much of the metastable and even the stable range-and implies the existence of an isochoric boundary line for stability to isotropic density fluctuations. We delineate the conditions under which amorphization occurs, usually substantially before the stability limit is reached. (c) 1995 The American Physical Society