  Two-dimensional polarity is intriguing but remains in the early stage. Here a
structural evolution diagram is established for GeS monolayer, which leads a
noncollinear ferrielectric $\delta$-phase energetically as stable as the
ferroelectric $\alpha$-phase. Its ferrielectricity is induced by the phonon
frustration, i.e., the competition between ferroelectric and antiferroelectric
modes, providing more routes to tune its polarity. Besides its prominent
properties like large band gap, large polarization, and high Curie temperature,
more interestingly, the morphotropic phase boundary between $\alpha$- and
$\delta$-phases is highly possible, which is crucial to obtain giant
piezoelectricity for lead-free applications.
