Effect of electric field orientation on ferroelectric phase transition and electrocaloric effect

This paper demonstrates the influence of electric field orientation on the phase transition and the electrocaloric effect (ECE) in BaTiO3 single crystals by performing molecular dynamics simulation of first-principles-based effective Hamiltonian, where the ECE is directly characterized through the adiabatic temperature change (ΔT) of the microcanonical ensemble. Different orientation relationship between the electric field and the polarization direction of ferroelectric phases leads to abundant polarization states and various phase structures. If the electric field direction is collinear to the polarization direction of a ferroelectric phase, the stability of the corresponding phase will be improved, and its phase area in the phase diagram is also enlarged; whereas the noncollinear electric field has reverse impacts and distorts the lattice to a low symmetry monoclinic phase. The phase transitions produce large ECEs far beyond those caused by the domain switching. The tetragonal-cubic phase transition produces a positive ECE regardless of the electric field directions, while the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions induce positive or negative ECE, or even their coexistence, depending on the field-induced phase transitions under different electric field directions. The signs of ECE are determined by the lattice symmetry before and after the field-induced phase transition. Our simulated results help reveal a more comprehensive physical understanding of ECE and offer an instruction for the design of the refrigeration cycle by combining positive and negative ECEs.