Perovskite-Film ANtiferroelectrics for DieLectrics (PFANDL)

Perovskites are materials with a special, cube-shaped basic pattern at the atomic level. In each cube, 3 different elements are arranged in the middle, at the corners and on the side faces of the cube. In oxide-based perovskites, the first and second elements are each a metal, which is present as a positively charged ion, and the third element is oxygen, which is negatively charged. In many perovskite oxides, the positively and negatively charged ions can shift against each other, so that the "atomic cubes" are polarized. The polarizations can either have a random orientation (paraelectric behavior), point in the same direction (ferroelectric behavior), or alternate "up" and "down" (anti- ferroelectric behavior). The ferroelectric variants are already used in high-performance capacitors with high energy density for the efficient conversion of direct current into alternating current. However, they have one disadvantage: energy losses occur during the conversion. These losses can be minimized with anti-ferroelectric variants. Anti-ferroelectric perovskites already exist, but they contain lead, which is undesirable due to its toxicity. Lead-free variants would be an attractive alternative, but anti- ferroelectric behavior is very difficult to achieve with them and has only been implemented in a few cases. This is particularly the case when the materials are not manufactured as ceramics, but as thin layers with a layer thickness in the range of 100 nanometers. These thin layers, in turn, would be particularly attractive for technical use because very powerful capacitors can be obtained by "stacking" many thin layers. This is where PFANDL comes in: It investigates how the special conditions of thin films influence the (anti-) ferroelectric behavior. This includes the extremely small layer thickness as well as the special microstructure and the residual stresses that arise when a thin film grows on the substrate. To this end, PFANDL develops and applies new simulation methods from the atomic scale up to the micrometer scale, which make it possible to investigate different scenarios and to examine the influence of each condition on the electrical behavior separately. In parallel, experimental thin films are produced and characterized from the atomic scale to the macroscale. The special combination of state-of-the-art simulation methods and experiments will be used to investigate the mechanisms that enable or prevent anti-ferroelectric behavior. This will ultimately lead to the derivation of recipes for realizing anti-ferroelectric behavior in lead-free perovskite thin films.

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