Modeling ferroelectric LiNb_(1-x)Ta_xO_3 solid solutions and their defect structure

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The proposal is part of the research group 'Periodic low-dimensional defect structures in polar oxides', which is dedicated to the correlation of defect structure, electron and ion transport and electromechanical properties using the model system lithium niobate-lithium tantalate (LiNb_(1-x)Ta_xO_3, LNT).The goal of the present project is the theoretical description of LNT solid solutions over the whole composition range. Macroscopic materials properties are calculated from first principles on the basis of the microscopic structure. Thereby, we combine a long-standing experience in the modelling of ferroelectrics with substantial expertise in the field of atomistic calculations. While LiNbO_3 is one of the most investigated ferroelectrics, our knowledge of LNT solid solutions is rather limited. In particular, they have been rarely investigated by atomistic theories. The main reason for this lack is that the microscopic modelling of disordered alloys and the quantitative determination of their properties are major challenges in the theoretical materials science. On the one hand, the correct description of disordered systems within the periodic supercell approach is not straightforward. On the other hand, the calculation of excited state properties such as the optical answer is very demanding due to the size of the involved supercells. In order to tackle these problems, we develop and extend computational algorithms for the ab initio modelling of solid solutions and their defect structures, and apply them to LNT.Our atomistic models offer two crucial advantages. The first advantage is the possibility to model artificial or experimentally not accessible structures to disentangle the complex physical processes that simultaneously act to determine the materials properties. Thus, we can singularly address the distinct contributions to physical properties, which simultaneously depend on many parameters. The second advantage is that, due to the broad methodological spectrum, we have access to a multitude of directly measurable quantities, which allow for the direct connection of the present proposal to each experimental project. With the determination of structural and thermodynamic properties (e.g. lattice parameters, activation energies, diffusion coefficients, excess enthalpies) as well as linear and non-linear optical signatures (e.g. optical, Raman- and second-harmonic-spectra) we lay the basis for the thorough understanding of the investigated model system.The aim of our proposal is the prediction of the composition dependent properties of the LNT ground state and its structural and electronic excitations. This enables us to understand the influence of structure, stoichiometry and doping on the macroscopic scale, and allows thereby the interpretation of the experimental results. This, in turn, puts us in the condition to propose optimal compositions of solid solutions with tailored electro-mechanical or electro-optical properties.