Because of its compatibility with semiconductor-based technologies, hafnia (HfO2) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO3) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material.

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown directly on (111)-Nb:SrTiO3 substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO2 films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm2 with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO3/ZrO2/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO2. Overall, the rhombohedral ferroelectric ZrO2 films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.

The structural, electronic and optical properties of bulk bismuth oxyhalides, BiOX (X = F, Cl, Br, and I), were studied using state-of-the-art density functional theory (DFT)-based calculations. The effects of compressive and tensile strains on the in-plane lattice parameters were analyzed to better understand their good performance in photo-catalytic applications. Our present first-principles calculations show that at least 4% in-plane bi-axial compressive strain over the experimental lattice parameters of BiOF is needed for phonon stability of this material, whereas other BiOX systems can accept up to 2% in-plane bi-axial compressive strain and retain their dynamical stability. On the other hand, 2% in-plane tensile strain breaks the structural stability of all bulk BiOX structures. Tuning the electronic band structures with such external compressive strain indeed helps to enhance the separation of charge carriers due to larger electron-hole effective mass differences in the BiOBr and BiOI structures. The optical properties are discussed from their calculated absorption spectra and optical conductivity within independent particle approximations. The average values of the calculated optical band gaps are in the range of 3.8-3.9 eV, 3.3-3.4 eV, 2.7-2.8 eV and 1.7-1.8 eV for the unstrained and compressive strained structures, respectively, of the BiOF, BiOCl, BiOBr and BiOI materials, which are reasonably good compared to their known experimental ultra-violet visible spectroscopy measured data.