Origin of pyroelectricity in LiNbO${}_3$

We use molecular dynamics with a first-principles-based shell model potential to study pyroelectricity in lithium niobate. We find that the primary pyroelectric effect is dominant, and pyroelectricity can be understood simply from the anharmonic change in crystal structure with temperature and the Born effective charges on the ions. This opens an experimental route to study pyroelectricity, as candidate pyroelectric materials can be studied with x-ray diffraction as a function of temperature in conjunction with theoretical effective charges. We also predict an appreciable pressure effect on pyroelectricity, so that chemical pressure, i.e., doping, could enhance the pyroelectric and electrocaloric effects.

Figure 1

(a) Polarization and (b) proper pyroelectric coefficients for $P=-5$ (triangles), 0 (squares), and 5 (crosses) GPa. “Exp. 1, 2” labels the experimental values for a congruently melting composition and stoichiometric sample (Ref. 1), respectively; “Exp. 3” is from Ref. 37. (c) Zoom-in of (b). Our result for ${\Pi }^{\prime }$ does not go to zero at zero temperature as required by quantum mechanics since we use classical MD. (d) The pressure effect characterized by ${\gamma }_{{\Pi }^{\prime }}=\frac{1}{{\Pi }^{\prime }}\frac{\partial {\Pi }^{\prime }}{\partial P}$.

Figure 2

The average value of the internal structural parameters $z$ and $w$ (fractional displacement of Li and O along polar axis, respectively) from the MD simulations. We compare $\Pi$ computed with these parameters and the Born effective charges $Z^{*}$ (triangles), with experiment (circles; Ref. 1) and direct MD results (diamonds). The agreement shows that the pyroelectric effect arises almost entirely from the change in structure with temperature.