Large Electrocaloric Effect in Relaxor Ferroelectric and Antiferroelectric Lanthanum Doped Lead Zirconate Titanate Ceramics

Both relaxor ferroelectric and antiferroelectric materials can individually demonstrate large electrocaloric effects (ECE). However, in order to further enhance the ECE it is crucial to find a material system, which can exhibit simultaneously both relaxor ferroelectric and antiferroelectric properties, or easily convert from one into another in terms of the compositional tailoring. Here we report on a system, in which the structure can readily change from antiferroelectric into relaxor ferroelectric and vice versa. To this end relaxor ferroelectric Pb0.89La0.11(Zr0.7Ti0.3)0.9725O3 and antiferroelectric Pb0.93La0.07(Zr0.82Ti0.18)0.9825O3 ceramics were designed near the antiferroelectric-ferroelectric phase boundary line in the La2O3-PbZrO3-PbTiO3 phase diagram. Conventional solid state reaction processing was used to prepare the two compositions. The ECE properties were deduced from Maxwell relations and Landau-Ginzburg-Devonshire (LGD) phenomenological theory, respectively, and also directly controlled by a computer and measured by thermometry. Large electrocaloric efficiencies were obtained and comparable with the results calculated via the phenomenological theory. Results show great potential in achieving large cooling power as refrigerants.

Polarization properties: Figure S4 shows the polarization as a function of temperature and external electric field for two samples.

(b) ceramics
Pyroelectric properties: Figure S5 shows the pyroelectric coefficient (dP/dT) as a function of temperature and external electric field for two samples.

Relaxor ferroelectric properties:
In general, the relaxation behavior of ferroelectric can be determined by the modified Curie-Weiss law S1 where and are the maximum dielectric constant and the corresponding temperature, and T the dielectric constant and corresponding temperature above , ′ the Curie-like constant.
is the critical exponent and associated with the type of ferroelectric. When = 1 and 2, the material is corresponding to an ideal normal ferroelectric and to an ideal relaxor ferroelectric, respectively. The relaxation behavior of the ferroelectric is gradually increasing with when is between 1 and 2. can be worked out by fitting the logarithmic plots of the reciprocal permittivity ( 1 − 1 ) measured at the same frequency as a function of temperature ( − where b and c are assumed to be temperature-independent phenomenological coefficients. For the parameter a a linear temperature dependence based on the Curie-Weiss law S3 , The Landau-Ginzburg-Devonshire (LGD) phenomenological theory has also been used to explain the phase transition and dielectric properties of the antiferroelectric PZT system S3-S5 . For the antiferroelectric with orthorhombic symmetry, the polarization is along the [110] direction.
It should be noted that the above relations are merely suitable for antiferroelectric single domains S4 . Based on them, the single-domain properties of PLZT can be determined and the intrinsic contributions to the properties understood. Hence, by neglecting extrinsic contributions (e.g. domain wall and defect motions), the theories can be used to further understand the properties of polycrystalline materials S4 .
For the antiferroelectric ceramics, the grains distribute randomly, which leads to disordered orientation of domains. When an electric field is applied on the polycrystalline ferroelectric ceramic, the distortions of at least some of the crystallites, initially randomly distribute, orient along the allowable direction along the poling electric field. Some literatures have reported the polarization of ferroelectric ceramics and crystals with the same composition at the same poling condition S6,S7 . The relationship between upper limits ̅ of the polarization of the ceramic and P of the antiferroelectric/ferroelectric single-domain is as follows S7 : tetragonal ceramic ̅ =0.831 P, rhombohedral ceramic ̅ =0.866 P, and orthorhombic ceramic ̅ =0.912 P. All of the coefficients of the Gibbs free energy function were independent of temperature, except for the antiferroelectric and ferroelectric dielectric stiffness coefficients σ 1 and α 1 , which were given as linear temperature dependences based on the Curie-Weiss law S3,S7,S8 . For the antiferroelectric orthorhombic phase, let σ 1 be β(T-T C ). Further, β, 2σ 11 +σ 12 , and σ 111 +σ 112 in the equation (S6) can be found from the first partial derivative stability conditions: where 3 and 3 are the electric field and the polarization components of a single-domain material along the coordinate axis. The electric field strengths, 5, 6 and 7 MV/m and their corresponding polarizations were selected respectively and substituted into Equation (S7) to procure the coefficient β. Then the reversible adiabatic changes in entropy (ΔS) and temperature (ΔT) can be obtained by using the relations as mentioned in Equations (S4) and (S5), and the polarization 3 as well.
The parameters ( ) used for the calculation of electrocaloric effect are listed in the Table S1.
where ℎ is the surrounding temperature and t the heat transfer time. More details about the test procedure and data analysis can be found in Ref. S9 and S10. During this test, an electric field of 3 MV/m was applied to the sample for 15 seconds to obtain temperature equilibrium first, then the electric field was released immediately. Meanwhile, the ECE signal appears as shown in Figure S6. The red curves are the fitted curves using equation (S8). is obtained by extrapolating the fitting toward the time of the fall of the step-like pulse. is measured in the temperature range from 303 K to 423 K at successive increments of 10 K in the temperature range of 303 K to 423 K. In the direct measurement of , one concern is the Joule heating in the samples, which will cause the enhancement of temperature when the field is applied. But in this test, the base line temperature T in Figure S6 is constant except while withdrawing the electric field, which indicates that the observed temperature change is due to ECE.