Phase Diagrams of Calcium and Strontium at High Pressures

Abstract

The melting temperatures for both Ca and Sr increase rapidly with pressure, reaching nearly 1200°C at 40 kbar. The only solid-solid transition in Ca below 35 kbar is that between the body-centered cubic and face-centered cubic polymorphs; the slope of the phase boundary is about 3.3°C/kbar. The phase boundary between the homologous Sr polymorphs has an initial slope of about -10°C/kbar becoming much steeper at higher pressures. Also, the hysteresis associated with the transition is a function of pressure and becomes very large above 22 kbar. The face-centered cubic Sr shows a large increase in resistance with pressure and a negative temperature coefficient of resistance prior to the polymorphic transition. The resistance discontinuity for the fcc-bcc transformation is a sharp increase at lower pressures, which progressively decreases in magnitude and sharpness and reverses sign at the higher pressures.