Electrocaloric effect in bulk and low-dimensional ferroelectrics from first principles

First-principles-based effective Hamiltonians are combined with nonequilibrium molecular dynamics to determine the electric-field-induced change in temperature in ferroelectrics. As a result, (i) the applied field can be ac in nature with a wide frequency range, (ii) bulks and nanostructures (with homogeneous or highly inhomogeneous dipolar configurations) can be investigated, and (iii) the influence of various phenomena on the electrocaloric effect can be revealed. This scheme yields an electrocaloric coefficient in excellent agreement with measurement and that strongly depends on the field-induced variation of volume in $\text{Pb}\left({\text{Zr}}_{1-x}{\text{Ti}}_x\right){\text{O}}_3$ bulks. An unusual phenomenon is also reported in dots under ac fields.

Figure 1

(Color online) Predicted time dependency of the temperature in bulk PZT under an ac electric field $E\left(t\right)=\left[0.6-0.5\text{cos}\left(2\pi \nu t\right)\right]\times {10}^9\text{V}/\text{m}$ with $\nu =100\text{GHz}$. Panels (a) and (b) display the unclamped and clamped cases, respectively. The filled dots show the raw data while the solid line represents $T\left(t\right)=T_0+\alpha E\left(t\right)$, where $\alpha$ is the time-independent coefficient that allows for the best fit of the raw data.

Figure 2

(Color online) Predicted time dependency of the temperature in a cubic dot of 4.8 nm lateral size and made of PZT under an ac electric field $E\left(t\right)=\left[0.6-0.5\text{cos}\left(2\pi \nu t\right)\right]\times {10}^9\text{V}/\text{m}$ with $\nu =100\text{GHz}$ for [panel (a)] $\beta =0.95$ and [panel (b)] $\beta =0.80$. The filled dots show the raw data while the solid line represents $T\left(t\right)=T_0+\alpha E\left(t\right)$ in panel (a) versus $T\left(t\right)=T_0+\alpha E\left(t\right)+\gamma t$ in panel (b), where $\alpha$ and $\beta$ are time-independent coefficients that allow for the best fit of the raw data. The insets display a cross section in the $\left(x,y\right)$ plane of the initial dipole configuration.