Effects of thermal and electrical hysteresis on phase transitions and electrocaloric effect in ferroelectrics: A computational study

Hysteresis, including thermal and electrical hysteresis, is a basic feature of ferroelectrics with a first-order phase transition and has consequences on properties, including the electrocaloric effect (ECE). Here we systematically study the influence of thermal and electrical hysteresis on the ECE by monitoring the history dependence of zero-field, and field heating and cooling, for three typical phase transitions in BaTiO₃ single crystals. We employ molecular dynamics simulations using an effective Hamiltonian based on first-principles calculations. We find that within the hysteresis regions of first-order phase transitions, where large ECEs occurs, the widest temperature span appears under zero field and this shrinks with increasing electric field. Moreover, the ECE is irreversible when one of the initial or final states with varying electric fields is located inside the hysteresis region. However, it is reversible when two states fully cross the hysteresis region. In continuous electric field cycles, the irreversible ECE has a large thermal response when the electric field is changed at first, but becomes smaller with subsequent changes in the field. The electrical hysteresis loss results in an increase of temperature under continuous field cycles regardless of positive or negative ECEs. Our work provides a basis for the efficient design of reversible electrocaloric cycles for refrigeration applications.