  Accurate values for polarization discontinuities between pyroelectric
materials are critical for understanding and designing the electronic
properties of heterostructures. For wurtzite materials, the zincblende
structure has been used in the literature as a reference to determine the
effective spontaneous polarization constants. We show that, because the
zincblende structure has a nonzero formal polarization, this method results in
a spurious contribution to the spontaneous polarization differences between
materials. In addition, we address the correct choice of "improper" versus
"proper" piezoelectric constants. For the technologically important III-nitride
materials GaN, AlN, and InN, we determine polarization discontinuities using a
consistent reference based on the layered hexagonal structure and the correct
choice of piezoelectric constants, and discuss the results in light of
available experimental data.
