Relationships between pyroelectric and ferroelectric parameters

We find empirically that although the pyroelectric coefficient p and the dielectric constant k vary strongly with temperature and from material to material, the ratio [image omitted] tends to be a constant ≃ (3 ± 1) × 10-9 coul/cm2 °C for many pyroelectric materials. From Devonshire's phenomenological theory of ferroelectricity it follows that [image omitted] should indeed be independent of temperature in a given material near the Curie temperature Tc. The same theory leads to [image omitted] where P0 is defined from the linear temperature dependence of P2s near Tc. We find also that [image omitted] is constant, (3.9 ± 1.6) × 10-9 coul/cm2 °C, for a wide range of ferroelectrics, where Pmax is the maximum observed polarization. As a particular example, the values of p, K, and Tc in Sr0.5Ba0.5Nb2O6 change considerably with small amounts of lanthanum addition, but the above relationships are obeyed.