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Proposing Institution

Fakultät für angewandte Naturwissenschaften und Mechatronik,
Project Manager

Prof. Dr. Alfred Kersch
Lothstraße 34
80335 München
The aim of this project is to investigate the piezoelectric properties of the HfxZr1-xO2 systems for various doping species and concentration ranges using density functional theory (DFT) calculations. We are mainly interested in the giant piezoelectric effect arising due to a crystallographic phase transition which is well known for perovskites. In contrast, the proper piezoelectric effect originate from a volume change in the same crystal phase under external electric field. In HfxZr1-xO2 systems this effect can emerge, when the tetragonal crystal phase P42c/nmc (other phases are also possible) undergoes an electric field induced phase transition into the ferroelectric, orthorhombic crystal phase Pca21. The ferroelectric phase is highly sensitive to an external electric field and is therefore mandatory in such a phase transition. If the volumes of both phases are significantly different, the giant piezoelectric effect can be high and can overcome the conventional piezoelectric effect by an order of magnitude.Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. Recently, ferroelectric properties have been found in HfxZr1-xO2 thin films including doped films for various dopants. The competing crystal phases in HfxZr1-xO2 are usually (i) the monoclinic P21/c low energy phase, (ii) the tetragonal P42/nmc high-k phase, (iii) the orthorhombic Pca21 ferroelectric phase, (iv) the orthorhombic Pbca phase, (v) the cubic Fm-3m phase (known from YSZ-zirconia) and (vi) the orthorhombic Pbcn phase (intermediate). The tetragonal and the ferroelectric phases are not the preferred phases from the bulk total energies giving rise to another excess energy (surface/interface energy). Furthermore, the ferroelectric phase is not related to perovskites but can be similarly induced from perturbations of the crystal lattice caused by dopants and stress. One way to manipulate the volumes of the competing phase on a phase transition is by using different dopants or rather chemical elements. In addition, the doping concentrations can significantly alter the volume and the crystal morphology leading either to a enhancement or deterioration of the piezoelectric properties. Clarifying this correlation is the main task of this proposal.Besides, manipulating the piezoelectric properties of HfxZr1-xO2 by doping other crystal properties such as ferroelectricity, pyroelectricity and dielectricity are also modified which can be of high interest for other applications. From RWTH Aachen which is a project partner in the Inferox DFG project first values for piezoelectric coefficients of about 10 pm/V have been obtained from laser interferometric measurements (c.f. 300 pm/V for PZT). These values are surprisingly high taking into account that the investigated materials have not yet been optimized for the piezoelectricity. Besides the collaboration with the Namlab in Dresden and the RWTH Aachen due to the Inferox II DFG project, a cooperation with the Fraunhofer IPMS which investigates piezoelectric micro-harvester experimentally containing (so far non-optimized) piezoelectric hafnia is scheduled.For the investigation of the piezoelectric properties of the crystal system HfxZr1-xO2 we will use mainly two DFT programs: FHI AIMS and Quantum Espresso. Our physical values of interest in doing so are especially the ground state energies and their volumes.

Impressum, Conny Wendler