Abstract
A high pressure, low temperature differential scanning calorimetry (DSC) apparatus has been developed for the purpose of studying the transition enthalpy and transition entropy along the phase lines as a function of pressure. Measurements on mercury show that absolute values of the transition entropy as a function of pressure can be obtained within 0.3%. An interesting feature of the phase behaviour of ethane is the disappearance of the orientationally disordered (plastic) phase at 13 kbar. With our DSC apparatus enthalpy and entropy changes of the melting transition and the plastic-ordered solid transition of ethane have been determined with high precision up to 12 kbar. Our results show that above 6 kbar ΔS (melting) decreases as a function of pressure, while ΔS (Plastic-ordered solid) increases by the same amount. This indicates that the plastic phase becomes more liquid-like with increasing pressure and temperature, which finally results in the disappearance of the plastic phase at 13 kbar.
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