Differential thermal measurements of phase transitions at high pressures and temperatures

Thermal measurements of high-pressure transitions in metals can provide thermodynamic insight into the properties of the high-pressure high-temperature phases and their transitions. However, these measurements have been essentially limited to a few large high-pressure facilities. An innovative pressure cell, designed for sensitive differential thermal analysis (DTA) measurements at elevated temperatures (300–1000 K) and high pressures (0–6 GPa) implemented in a ‘Paris–Edinburgh’ tabletop press, is presented. Differential measurements enable capture of small thermal signals, e.g., typical solid–solid transitions. The new cell’s capability is demonstrated via thermal measurements of transitions across the phase diagrams of indium, tin, and antimony. The melting transitions of these metals were identified based on the DTA measurements, together with the subtler transitions in the solid phases. The enthalpy of transition between solid phases was found to be significantly smaller than upon melting. Hence, the sensitivity of the experimental design is demonstrated. By consolidating the transitions obtained from the DTA curves during isobaric measurements, the high-pressure phase diagrams of the elements were reconstructed. For comparison, the high-pressure phase diagrams of In and Sb were determined by electrical resistance measurements. The phase diagrams obtained by high-pressure DTA measurements were found to agree very well with these and with previous experimental determinations using thermal and other methods.