  Pyroelectricity is a very promising phenomenon in three- and two-dimensional
(2D) materials, but first-principles calculations have not so far been used to
elucidate the underlying mechanisms. Here we report density-functional theory
(DFT) calculations based on the Born-Szigeti theory of pyroelectricity, by
combining fundamental thermodynamics and the modern theory of polarization. We
find satisfactory agreement with experimental data in the case of bulk
benchmark materials, showing that the so-called electron-phonon
renormalization, whose contribution has been viewed as negligible, is
important. We predict out-of-plane pyroelectricity in the recently synthesized
Janus MoSSe monolayer and in-plane pyroelectricity in the group-IV
monochalcogenide GeS monolayer. It is notable that the so-called secondary
pyroelectricity is found to be dominant in GeS monolayer. The present work
opens a theoretical route to study the pyroelectric effect using DFT and
provides a valuable tool in the search for new candidates for pyroelectric
applications.
