Bidirectional Transformations of n-Pentadecane between Low-Pressure and High-Pressure Phases

Abstract

We optically observed the transformation between the solid phases of SII and SI of n-pentadecane of which a p–T diagram has a parallel and wide gap between the coexistence lines between liquid-SI and between SI–SII. We were able to grow an SII crystal in the liquid by cooling or compression, and subsequently filled the high-pressure specimen vessel with the SII crystal. The transformation to SI crystals in the SII crystal started after successive cooling or compression. Final compression transformed again all the SI crystals to the SII crystal. At the growth of an SII crystal in the liquid, we measured the SII-liquid equilibrium pressures and temperatures, which were located below the coexisting lines of the SI and liquid phases in the p–T diagram. We found that the SII crystal of n-pentadecane is stable even below the coexisting line between the SI and liquid. We elucidate thermodynamically the stabilities of the SI and SII phases based on the variations of some thermodynamic functions calculated from th...