Pyro- and dielectric properties as well as modelling

这里所描述的项目在柏林工业大学实验物理研究所是一揽子提案的一部分，该提案将与柏林工业大学矿物学小组和Ruhr-Universität波鸿晶体物理学小组一起实施。合作的目标是制定稀土氧硼酸盐晶体（简称RCOB）的结构和机电性能之间的相关性。这些不仅是基本的兴趣，而且对这些材料作为高温压电材料的潜在应用起着关键作用。1000°C。与高对称晶体相比，RCOB材料的单斜对称性意味着其性质的各向异性以及结构和性质之间的相关性具有更大的自由度。在自己的初步工作中观察到的不同性质随温度变化的不连续性表明可能存在结构不稳定性。在不同配位的阳离子位置上的大量替代可能性导致了一个很大的，直到现在几乎没有表征的化学变异性。对于一些代表，从熔体中生长大单晶的可能性已经得到证实。这里要求的项目将涵盖电气特性和从头算建模。重点研究RCOB材料的化学性质、结构参数与电导率、介电常数、热释电系数之间的关系。为此，需要澄清的是，在初步工作中观察到的与温度有关的比热和热膨胀的不连续是否也表现在电气参数中。在这种情况下，转变温度将被确定并与结构变化相关联。电气参数与显式结构特征的相关性是主要目标。从从头算模型中获得的理论性质将验证测量的性质，并确定不同种类晶体之间的系统联系。此外，将从理论上对替代、无序和混合占用的稳定性和材料性质进行陈述，以确认项目合作伙伴的实验工作。总的来说，将构建一个可能的结构变化的完整画面，同时允许预测与RCOB材料应用有关的特性。

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.