Positive and negative electrocaloric effect inBaTiO3in the presence of defect dipoles

Abstract

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen vacancy-acceptor associates are described by fixed defect dipoles with orientation parallel or anti-parallel to the external field. By a combination of canonical and microcanoncial simulations the ECE is directly evaluated. Our results show that in the case of anti-parallel defect dipoles the ECE can be positive or negative depending on the density of defect dipoles. Moreover, a transition from a negative to positive ECE can be observed from a certain density of anti-parallel dipoles on when the external field increases. These transitions are due to the delicate interplay of internal and external fields, and are explained by the domain structure evolution and related field-induced entropy changes. The results are compared to those obtained by MD simulations employing an ab initio based effective Hamiltonian, and a good qualitative agreement is found. In addition, a novel electrocaloric cycle, which makes use of the negative ECE and defect dipoles, is proposed to enhance the cooling effect.

Electrocaloric effects in the lead-free Ba(Zr,Ti)O3 relaxor ferroelectric from atomistic simulations

“…1(a) is always continuous for any investigated field. It is worthwhile to know that the maximum of α at a certain field was also predicted to occur in Ba 0.5 Sr 0.5 TiO 3 [16] and defect doped BaTiO 3 [56], and that we also found this non-mononotic behavior of α in the paraelectric phase of BaTiO 3 (BTO) bulk -as evidenced in the Supplemental Material [54].…”

Impact of Polarization Dynamics and Charged Defects on the Electrocaloric Response of Ferroelectric Pb(Zr,Ti)O3 Ceramics

“…As anticipated, in the electric field range between 0.25 kV mm −1 and 1.25 kV mm −1 an inverse ECE is observed whereby the material cools down when the electric field is applied. The observed impact of defect complexes on ECE and the presence of inverse ECE at reversed electric fields obtained in Fe‐doped PZT ceramic is in agreement with reported Monte‐Carlo simulation and ECE behavior of ferroelectric single crystals …”

“…Because the electrocaloric response depends on the variation of polarization, i.e., dipolar entropy, as well as direction and strength of the applied electric field, it is conceivable that the presence of charged point defects and defect associates might have an impact on the ECE. However, very few studies have considered the influence of defects on the ECE …”

Origins of the Inverse Electrocaloric Effect

“…b, c) Data for ramping on the field taken from lattice‐based Monte Carlo simulations in Ref. . The initial states have been prepared by pre‐poling, that is, application and removal of a positive field under isothermal conditions.…”