5.5.4 Direct and Indirect Effect

Since all pyroelectric materials are naturally piezoelectric in nature, the thermal expansion/contraction of the material while heating or cooling induces thermal stresses in the material which in turn induce their own polarization and hence changing the overall polarization.
Since, change in the polarization can also be expressed as   , depending upon the sign of each of these term, the polarization will either decrease or increase with increasing temperature.