Theoretical discussion of the exotic intrinsic properties of hafnia ferroelectrics*

Hafnia ferroelectrics hold great promise for the development of nano-devices that may take advantage of their permanent and switchable electric polarization. Hence, their technological interest has made them a focus of attention. But that is not all: surprisingly, these materials are proving to constitute a ferroelectric class of their own, displaying many intrinsic behaviors that seem genuinely unprecedented and certainly lay outside the "standard model" of soft-mode ferroelectricity in perovskite oxides. In this talk I will present first-principles results to discuss and explain some of those intriguing properties. In particular, I will discuss the evidence for (im)proper ferroelectricity -- and a possible soft-mode behavior -- in these compounds. I will also address the question of whether hafnia should be consider a triaxial, biaxial or uniaxial ferroelectric, explaining how the last option may be sufficient (and convenient) to account for the behavior of many woken-up samples. I will then show that, within such a uniaxial picture, it is possible to identify a centrosymmetric reference phase that yields a straightforward description of all the known low-energy polyrmorphs of these materials, evidencing how they are structurally connected. Finally, I will discuss the piezoelectric response of hafnia ferroelectrics, emphasizing the various unique features it presents (e.g., sign tunability). Time permitting, I will summarize other recent results of my group, e.g., on hafnia-based superlattices.