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The contributions of internal strain and charge redistribution to the piezoelectric constants of tetrahedrally coordinated A^NB^8-N crystals are discussed. It is shown that the reversal of sign of the piezoelectric constant on going from N=1 and 2 to N=3 can be explained quantitatively in terms of the dependence of charge redistribution on bond ionicity.
Phillips et al. (Mon,) studied this question.