  Defects such as oxygen vacancy are widely considered to be critical for the
performance of HfO2-based devices, and yet atomistic mechanisms underlying
various exotic effects such as wake-up and fluid imprint remain elusive. Here,
guided by a lattice-mode-matching criterion, we systematically study the phase
transitions between different polymorphs of hafnia under the influences of
neutral and positively charged oxygen vacancies by mapping out the minimum
energy pathways using a first-principles-based variable-cell nudged elastic
band technique. We find that the positively charged oxygen vacancy can
substantially promote the transition of various nonpolar phases to the polar
phase kinetically, enabled by a transient high-energy tetragonal phase and
extreme charge-carrier-inert ferroelectricity of the polar $Pca2_1$ phase. The
intricate coupling between structural polymorphism kinetics and the charge
state of the oxygen vacancy has important implications for the origin of
ferroelectricity in HfO$_2$-based thin films as well as wake-up, fluid imprint,
and inertial switching.
