Origin of relaxor behaviour in Ba-based lead-free perovskite

Most of the currently used materials for dielectric applications, solid-state refrigeration and actuators contain a toxic element: lead. For this reason, researchers all over the world work hard recently to find lead-free alternatives that can perform equally well to their lead-based counterparts. The clue is to find a way to tune chemical composition in order to obtain the desired macroscopic properties. Barium-based relaxors are a class of highly disordered ceramic materials that have the potential to replace lead-based compositions in many of the aforementioned applications. However, it is not yet clear how the local structure can be modified by doping so that the properties of relaxors - which are essentially electrical in nature - can be controlled. Our project aims to explain the origin of the peculiar dielectric and piezoelectric properties of relaxors by an integrated computational and experimental approach. We will make use of Raman spectroscopy, a technique that can measure lattice vibrations in materials. Since lattice vibrations are influenced by any modification of the local structure of materials, Raman spectra contain information on static chemical arrangements or dynamic electrical dipoles, which are at the basis of relaxor behaviour. To extract structural information from Raman spectra, we will need an exquisite procedure: We will simulate the spectra with atomistic modelling based on fundamental physical principles. In other words, we will construct, for each investigated composition, several possible material structures and calculate a Raman response out of them. By comparing the calculated spectra with the measured ones, we will be able to determine which structure the material assumed upon doping. Moreover, for the same material we will measure the macroscopic dielectric and piezoelectric properties, and well be able in this way to find out which structure has to be induced by doping the material so that the desired performances are obtained in view of application. Our investigations will be done under the influence of both temperature and electric field amplitude and frequency, thus mimicking in-service conditions. In a word: We will find out how to obtain the best material candidates for specific applications. The approach we will follow is unprecedented in the scientific community and will provide concrete answers to relevant industry partners worldwide, whose need is to find a tool to tune the material properties. In addition, it will boost the popularity of environmentally friendly materials for several advanced applications by understanding the mechanisms linking the atomic structures with the macroscopic response.

Most of the currently used perovskite-based materials for dielectric applications, solid-state refrigeration and actuators contain a toxic element: lead. For this reason, researchers all over the world have been working hard recently to find lead-free alternatives that can perform equally well to their lead-based counterparts. The clue is to find a way to tune chemical composition in order to obtain the desired macroscopic properties. In this context, the so-called relaxors are an important class of materials. Barium-based relaxors are highly disordered ceramic materials that have the potential to replace lead-based compositions in many of the aforementioned applications. However, it was not yet clear how the local structure can be modified by doping so that the properties of relaxors - which are essentially electrical in nature - can be controlled. Our project explained the origin of the peculiar properties of relaxors by an integrated computational and experimental approach. We used Raman spectroscopy, a technique that can measure lattice vibrations in materials. Since lattice vibrations are influenced by any modification of the local structure of materials, Raman spectra contain information on static chemical arrangements, which influence the atomic-scale electric polarization and thus trigger relaxor behaviour. To extract structural information from Raman spectra, we simulated the spectra with atomistic modelling based on specific atomic arrangements. By comparing the calculated spectra with the measured ones, we were able to determine that in heterovalent-substituted barium titanate charge compensating defects largely disrupt the long-range ferroelectric polarization correlation. By comparing our structural results in both homovalent- and heterovalent-based relaxors with the macroscopic dielectric properties, we were able to conclude that the local atomic arrangement triggered by chemical substituents is the defining factor of relaxor properties. The approach we followed is unprecedented in the scientific community and provides a tool to investigate and tune relaxor properties, thus paving the way for controlled chemistry-based adjustment of dielectric properties. In addition, the results of our project may boost the popularity of environmentally friendly materials for several advanced applications by understanding the mechanisms linking the atomic structures with the macroscopic response.