Charge Redistribution and Piezoelectric Constants

The contributions of internal strain and charge redistribution to the piezoelectric constants of tetrahedrally coordinated $A^N{B}^{8-N}$ crystals are discussed. It is shown that the reversal of sign of the piezoelectric constant on going from $N=1\mathrm{} \mathrm{and} 2$ to $N=3\mathrm{}$ can be explained quantitatively in terms of the dependence of charge redistribution on bond ionicity.