Ion transport and point defects in LiNb_(1-x)Ta_xO_3 solid solutions

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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 (LNT).Focus of this sub-project is the experimental characterization and basic understanding of the ion transport of the species Li, Nb/Ta, O and H and of the electrical conductivity as a function of temperature, oxygen partial pressure and Nb/Ta content in LNT single crystals with and without ferroelectric domains. The underlying point defects, defect equilibria and transport mechanisms have to be clarified. Defects and transport, as well as their interplay, have fundamental influence on the macroscopic material properties, such as electrical conductivity, electromechanical properties, optical properties etc. and their knowledge will allow a specific tailoring of properties.With regard to the overall goals of the research group, the results of this sub-project are of particular importance for an understanding of partial ionic conductivities and their contribution to total conductivity, for the identification of the temperature-dependent majority point defects and defect clusters as well as for the high temperature stability and stoichiometric modifications. Specifically, the influence of the paraelectric transition and of foreign atoms on diffusion should also be investigated. Of particular interest is the hitherto unexplored influence of the domain walls and the associated defects/charges on ion transport. The work also forms the base for an understanding of polaron dynamics in time-varying atomic structures. To determine the tracer diffusivities, stable tracer isotopes are used, which are either introduced from the gas phase (2-H_2O, 18-O_2) or by means of sputter deposition (6-Li, 180-Ta). The isotope-selective depth profile analysis will be carried out with secondary ion mass spectrometry (SIMS) and for selected systems with neutron reflectometry (NR) using a novel analytical approach. The necessary conductivities will be determined using impedance spectroscopy. The results should contribute to a defect model that explains the ionic and electronic transport processes and forms the basis for a tailoring of materials properties.