First-Principles Studies of Hafnia Ferroelectrics -- In a Class of Their Own

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.<br/><br/>Work done in collaboration with Hugo Aramberri, Sangita Dutta, Binayak Mukherjee and Natalya S. Fedorova (Luxembourg Institute of Science and Technolgoy). Funded by the Luxembourg National Research Fund through Grant INTER/NWO/20/15079143/TRICOLOR.