Pyro- and dielectric properties as well as modelling

这里描述的在弗莱贝里工业大学实验物理研究所的项目是一揽子建议的一部分，该计划将与弗莱贝格工业大学的矿物学小组和波鸿鲁尔大学的晶体物理小组一起实施。合作的目标是阐明稀土氧硼酸盐晶体(简称RCOB)的结构和机电性能之间的相关性。这些材料不仅具有根本的意义，而且在这些材料作为高温压电材料的潜在应用中发挥着关键作用，温度可达约30℃。与高对称性晶体相比，RCOB材料的单斜对称性意味着性质的各向异性以及结构和性质之间的相关性具有更多的自由度。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。不同配位的阳离子位置上的许多取代可能性导致了很大的化学可变性，到目前为止还很少被表征。对于一些代表来说，已经验证了从熔体中生长大单晶的可能性。这里要求的项目将包括电学特性和从头计算模型。研究的重点将是RCOB材料的化学性质、结构参数和电学性能(电导率、介电常数、热释电系数)之间的关系。对于这一点，需要澄清的是，如果在初步工作中观察到的随温度变化的比热和热膨胀的不连续性也表现在电参数中。在这种情况下，转变温度将被确定，并与结构变化相关。电学参数与明确的结构特征的关联是主要目标，从头算模拟获得的理论性质将验证测量结果，并识别不同类型晶体之间的系统联系。此外，还将从理论上阐述关于取代、无序和混合占据的稳定性和材料性质，以确认项目合作伙伴的实验工作。总之，将构建可能结构变化的完整图景，以同时预测与RCOB材料应用相关的性能。

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.