After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 μC/cm 2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

The high piezoelectricity found in PbZr 1− x Ti x O 3 (PZT) and related PbTiO 3 solid solutions materials originates in the existence of a monoclinic crystal structure and the formation of nanodomains at the morphotrophic phase boundary (MPB). [18][19][20] Ferroelastic nanodomain structures and low-symmetry monoclinic phases help accommodating the large elastic forces that develop at the MPB, providing elastic matching at the internal interfaces and inducing large piezoelectricity. In addition, the formation of ferroelastic nanodomains in thin fi lms gives rise to novel effects such as fl exoelectric polarization rotation in a/c domains, [ 1 ] enhanced piezoelectricity by the fl exoelectric effect in PZT nanostructures [ 21 ] or monoclinic areas of largely enhanced piezoresponse close to domain walls. [ 22 ] Thus, using epitaxial strain to induce different combinations of nanodomains with well-defi ned orientations and periodicities (domain engineering) opens a route to improved piezoelectrics, especially beneficial for the design of lead-free materials. [ 23,24 ] In classical lead-free ferroelectric BaTiO 3 single crystals, ferroelastic and ferroelectric 90° domains and nonferroelastic and ferroelectric 180° domains are observed at room temperature, [25][26][27][28][29] consistent with the tetragonal crystal structure of the bulk crystal. Predictions for epitaxially strained BaTiO 3 thin fi lms have resulted in different crystal symmetries and, thus, different domain structures. [30][31][32][33][34][35][36][37] Particularly interesting are the predictions for fi lms under low-strain values: even under zero nominal strain, the structure is modifi ed by the elastic constrains imposed by the substrate and the different thermal expansion of fi lm and substrate. Experimental realization of these phases is now becoming possible thanks to using substrates of the rare-earth scandate family. [ 38,39 ] In particular, using a Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory, Koukhar et al. reported the BaTiO 3 phase diagram shown in Figure 1 . [ 30 ] This and other related works [ 32,40,41 ] have predicted a rich and fl at energy landscape with large variety of single-, multi-, and metastable domain phases. Next to these many phase transitions (around which the piezoelectric responses are expected to greatly increase), monoclinic aa*/ca* and ca 1 /ca 2 phases, not present in single crystals, have been predicted for low-strain values. In this paper we test those predictions experimentally. We have stabilized two different domain structures with long and well-ordered domains: a room temperature monoclinic ca 1 /ca 2 phase and either an a/c or an aa*/ca* phase above 50 °C.Epitaxial strain in ferroelectric fi lms offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO 3 has been predicted to order in a variety of phases and domain confi gurations when grown under low strain on low mismatched substrates, but little ex...

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO 3 . A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro-and magnetocalorics.

Temperature-dependent changes in spontaneous polarization (i.e., the pyroelectric effect or PEE) [1] have the potential to impact applications in waste-heat energy conversion [2,3] and thermal imaging. [4] Similarly, the inverse thermodynamic effect (i.e., the electrocaloric effect or ECE) [1] where an electric field perturbs the dipolar order and therefore the entropy of the system can enable solid-state cooling devices. [5,6] Key to such applications is the ability to manipulate and control the temperatureand field-dependence of polarization and entropic changes in ferroic materials. In turn, research has focused on finding pathways to enhance the pyroelectric π = ∂ Ferroelectric Thin FilmsThe complex interplay of polarization (P), temperature (T), entropy (S), and electric field (E) in ferroic materials enables electrothermal susceptibilities useful for a range of applications.