Robust and reversible polar topological center domains were found in BiFeO ₃ nanodots, which are individually controllable.

Correlations between the local electrocaloric responses and the domains are critical to understanding the mechanisms of electrocaloric effect and, thus, enhancing the electrocaloric responses in ferroelectrics, which have not been explored. Combining phase-field simulations and entropy analysis, we establish the correlations between local electrocaloric responses and domains for ferroelectric crystals and investigate the local electrocaloric responses in different domain structures. The results reveal that both initial 180° and 90° domain walls (referring to stable domain walls before electric excitation) exhibit large positive electrocaloric responses due to the increased polarization under the applied electric field, where the responses of the initial 180° domain walls are more significant. The final 180° domain walls (referring to stable domain walls after electric excitation) show large negative electrocaloric responses, since the polarization changes from nonzero to zero under the applied electric field. Good agreement between simulations and experimental measurements is observed. In addition, as the domain wall density increases, the macroscopic average electrocaloric response can be enhanced multiplicatively, suggesting that increasing domain wall density is an alternative pathway to enhance electrocaloric response in ferroelectrics.

It is well known that the performance of thermoelectric measured by figure of merit ZT linearly depends on electrical conductivity, while it is quadratic related to the Seebeck coefficient, and the improvement of Seebeck coefficient may reduce electrical conductivity. As a promising thermoelectric material, BiCuOCh (Ch = Se, S) possesses intrinsically low thermal conductivity, and comparing with its p-type counterpart, n-type BiCuOCh has superior electrical conductivity. Thus, a strategy for increasing Seebeck coefficient while almost maintaining electrical conductivity for enhancing thermoelectric properties of n-type BiCuOCh is highly desired. In this work, the effects of uniaxial tensile strain on the electronic structures and thermoelectric properties of n-type BiCuOCh are examined by using first-principles calculations combined with semiclassical Boltzmann transport theory. The results indicate that the Seebeck coefficient can be enhanced under uniaxial tensile strain, and the reduction of electrical conductivity is negligible. The enhancement is attributed to the increase in the slope of total density of states and the effective mass of electron, accompanied with the conduction band near Fermi level flatter along the Γ to Z direction under strain. Comparing with the unstrained counterpart, the power factor can be improved by 54% for n-type BiCuOSe, and 74% for n-type BiCuOS under a strain of 6% at 800 K with electron concentration 3 × 10²⁰ cm⁻³. Furthermore, the optimal carrier concentrations at different strains are determined. These insights point to an alternative strategy for superior thermoelectric properties.

Manipulation of topological structures has become one of the most interesting topics in ferroelectrics through multiple excitations due to their prospective applications in electro-mechanical-optic devices. Scanning probe microscopy (SPM) has been developed as a powerful tool to manipulate the polar state in ferroic materials, in which the electric field induced by charged SPM enables dynamic coupling between the switching of the polar states and electromigration of oxygen vacancies, resulting in unknown influences of oxygen vacancy on the polar state in ferroelectric topological structures. Here, we regulate the polar state by considering oxygen vacancies and a non-uniform electric field under the charged SPM experiment for flux-closure domain structures in PbTiO3 thin films. The charged SPM probe can excite the growth of newly flux-closure domains to improve the density of topological states. In contrast, oxygen vacancies are found to suddenly prohibit the evolution of the flux-closure domain structure, when their concentration jumps to a threshold value. Our results might give guidelines to increase and stabilize the memory unit through co-modulating the electric field and ions injection in the information field.