  The metastable nature of the ferroelectric phase of HfO$_2$ is a significant
impediment to its industrial application as a functional ferroelectric
material. In fact, no polar phases exist in the bulk phase diagram of HfO$_2$,
which shows a dominant non-polar monoclinic ground state. As a consequence,
ferroelectric orthorhombic HfO$_2$ needs to be kinetically stabilized. Here, we
propose an alternative approach, demonstrating the feasibility of
thermodynamically stabilizing polar HfO$_2$ in superlattices with other simple
oxides. Using the composition and stacking direction of the superlattice as
design parameters, we obtain heterostructures that can be fully polar, fully
antipolar or mixed, with improved thermodynamic stability compared to the
orthorhombic polar HfO$_2$ in bulk form. Our results suggest that combining
HfO$_2$ with an oxide that does not have a monoclinic ground state generally
drives the superlattice away from this non-polar phase, favoring the stability
of the ferroelectric structures that minimize the elastic and electrostatic
penalties. As such, these diverse and tunable superlattices hold promise for
various applications in thin-film ferroelectric devices.
