  First-principles calculations are made for the primary pyroelectric
coefficients of wurtzite GaN and ZnO. The pyroelectricity is attributed to the
quasiharmonic thermal shifts of internal strains (internal displacements of
cations and anions carrying their Born effective charges). The primary
(zero-external-strain) pyroelectricity dominates at low temperatures, while the
secondary pyroelectricity (the correction from external thermal strains)
becomes comparable with the primary pyroelectricity at high temperatures.
Contributions from the acoustic and the optical phonon modes to the primary
pyroelectric coefficient are only moderately well described by the
corresponding Debye function and Einstein function respectively.
