The Ignored Effects of Vibrational Entropy and Electrocaloric Effect in PbTiO 3 and PbZr 0.5Ti 0.5O 3 as Studied Through First-Principles Calculation

In an isothermal process, applying an electric field E to a ferroelectric material can change its entropy S through two distinct mechanisms: namely, through changing the dipole alignment or configurational entropy (ΔS_conf), and the vibrational entropy due to the intrinsic structure response (ΔS_vib). Previous numerical investigations yield only the total entropy change ΔS but cannot separate these two contributions. Here we develop a full first-principles method to extract ΔS_vib and the corresponding induced adiabatic temperature change ΔT_vib under E, i.e. the electrocaloric effect (ECE), and compare them to the total ΔS and ΔT from molecular dynamics simulation based on a first-principles effective Hamiltonian model. For both single crystal PbTiO₃ (PTO) and PbZr_0.5Ti_0.5O₃ (50/50 PZT), the calculation results show that for T far from the phase transition temperature T_pt, ΔS_vib plays an important and even dominant role in the ECE. On the other hand, for T close to T_pt, ΔS_conf dominates the ECE. Moreover, for PTO, we find that positive E can cause positive ECE, while negative E cause negative ECE. Therefore, by combining both positive and negative ECE in a "bipolar" Ericsson cycle, the cooling energy density can significantly increase as compared to that of a unipolar cycle.