Pyro- and dielectric properties as well as modelling

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The here described project at the Institute of Experimental Physics at TU Bergakademie Freiberg is part of a package proposal that will be carried out together with the groups of Mineralogy at TU Bergakademie Freiberg and Crystal Physics at Ruhr-Universität Bochum. The objective of the collaboration is the formulation of correlations between structure and electromechanical properties of rare-earth oxoborate crystals (short RCOB). These are not only of fundamental interest but play a key role for potential applications of these materials as high-temperature piezoelectrics for temperatures up to approx. 1000 °C. The monoclinic symmetry of the RCOB materials implies more degrees of freedom for the anisotropy of properties and, with that, for the correlation between structure and properties than in higher symmetric crystals. The discontinuities in the temperature-dependent development of different properties observed in own preliminary work indicate possible structural instabilities. Numerous substitution possibilities on the differently coordinated cation positions result in a large, until now only scarcely characterized, chemical variability. For some representatives the possibility of growing large single crystals from the melt has already been verified.The project requested here will cover the electrical characterization and the ab initio modelling. Focus of the investigations will be the relations between chemism, structural parameters and electrical properties (conductivity, permittivity, pyroelectric coefficient) of the RCOB materials. For this it needs to be clarified, if the discontinuities of temperature-dependent specific heat and thermal expansion observed in preliminary work also express in electrical parameters. In this case, transition temperatures will be determined and related with structural changes. The correlation of electrical parameters with explicit structural features is the main objective.Theoretical properties obtained from ab initio modelling will verify measured ones and identify systematic connections between different kinds of crystals. Furthermore, statements regarding stability and material properties regarding substitution, disorder and mixed occupation will be made theoretically to confirm the experimental work of the project partners. Altogether, a complete picture of possible structural changes will be constructed to simultaneously allow predictions on properties relevant for applications of the RCOB materials.