T-p phase diagrams and the barocaloric effect in materials with successive phase transitions

Gorev, M. V.; Bogdanov, E. V.; Flerov, I. N. JOURNAL OF PHYSICS D-APPLIED PHYSICS, 50 (38): Doi: 10.1088/1361-6463/aa8025 SEP 27 2017

An analysis of the extensive and intensive barocaloric effect (BCE) at successive structural phase transitions in some complex fluorides and oxyfluorides was performed. The high sensitivity of these compounds to a change in the chemical pressure allows one to vary the succession and parameters of the transformations (temperature, entropy, baric coefficient) over a wide range and obtain optimal values of the BCE. A comparison of different types of schematic Tp phase diagrams with the complicated $T(\,p)$ dependences observed experimentally shows that in some ranges of temperature and pressure the BCE in compounds undergoing successive transformations can be increased due to a summation of caloric effects associated with distinct phase transitions. The maximum values of the extensive and intensive BCE in complex fluorides and oxyfluorides can be realized at rather low pressure (0.1–0.3 GPa). In a narrow temperature range around the triple points conversion from conventional BCE to inverse BCE is observed, which is followed by a gigantic change of both $\vert\Delta S_{\rm BCE}\vert$ and $\vert\Delta T_{\rm AD}\vert$ .