  Although dopants have been extensively employed to promote ferroelectricity
in hafnia films, their role in stabilizing the responsible ferroelectric
non-equilibrium Pca21 phase is not well understood. In this work, using first
principles computations, we investigate the influence of nearly 40 dopants on
the phase stability in bulk hafnia to identify dopants that can favor formation
of the polar Pca21 phase. Although no dopant was found to stabilize this polar
phase as the ground state, suggesting that dopants alone cannot induce
ferroelectricity in hafnia, Ca, Sr, Ba, La, Y and Gd were found to
significantly lower the energy of the polar phase with respect to the
equilibrium monoclinic phase. These results are consistent with the empirical
measurements of large remnant polarization in hafnia films doped with these
elements. Additionally, clear chemical trends of dopants with larger ionic
radii and lower electronegativity favoring the polar Pca21 phase in hafnia were
identified. For this polar phase, an additional bond between the dopant cation
and the 2nd nearest oxygen neighbor was identified as the root-cause of these
trends. Further, trivalent dopants (Y, La, and Gd) were revealed to stabilize
the polar Pca21 phase at lower strains when compared to divalent dopants (Sr
and Ba). Based on these insights, we predict that the lanthanide series metals,
the lower half of alkaline earth metals (Ca, Sr and Ba) and Y as the most
suitable dopants to promote ferroelectricity in hafnia.

Materials Science