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

• Applicant: Professor Dr. Harald Schmidt
• Subject Area: Synthesis and Properties of Functional Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426703838

In the LiNb1-xTaxO3 (LNT) solid solution system, ion transport and point defects significantly determine the macroscopic material properties, such as electrical conductivity, electromechanical properties, optical properties, high-temperature stability and Li stoichiometry changes. Their knowledge is therefore essential for tailoring properties and processes. The present subproject focuses on the understanding of the ion transport of the species Li, Nb/Ta, O and H in LNT crystals as a function of temperature, O2 partial pressure and Nb/Ta content. Experimental characterization is done as a function of temperature, oxygen partial pressure, and Nb/Ta content. The underlying point defects, defect equilibria and transport mechanisms are elucidated and the connection with the macroscopic material properties will be investigated. In the first project phase, basic diffusion properties were determined, relevant point defects and energies were derived and correlated with conductivity. The key results are: (1) a reduction of O diffusion, but not Li diffusion, as a function of Ta content at standard pressure, (2) a decrease of Li diffusion at strongly reduced O2 partial pressure in LiNbO3 and not in LiTaO3, and (3) an increased diffusion of Li ions along domain walls. (4) In addition to Li and electrons, H contributes to conductivity and can be used as a parameter to adjust this value depending on the temperature. In the second project phase, in addition to a more detailed exploration of these topics, the ion transport in thin LNT layers will be be analysed in addition to single crystals. Differences due to a different morphology and defect structure and as well as mechanical stresses and the resulting change in defect energies for the modification of materials properties will be demonstrated. LNT layers also offer the advantage that ferroelectric domains can be structured in the nanometre range, which increases the influence of the domain walls on ion transport. The production of isotope-pure 7LiNb1-xTaxO3 layers also increases the sensitivity of diffusion experiments by orders of magnitude and enables the elucidation of new mechanisms. Further focus points are ion transport in LNT crystals with variable Li content and in Mg doped crystals to elucidate the modification of defect types and conductivities. Stable tracer isotopes (2H2O, 6Li, 18O2,180Ta) are used for the diffusion experiments in combination with secondary ion mass spectrometry, infrared spectroscopy and neutron reflectometry. The results are incorporated into a defect model that explains the ionic and electronic transport processes and forms the basis for tailoring the material properties. The work is carried out in cooperation with all other sub-projects in the areas of sample synthesis (TP1, TP9), charge transport (TP3, TP4, TP7), domain (walls) (TP5, TP6) and atomistic modelling (TP8).

• DFG Programme: Research Units
• Subproject of: FOR 5044: Periodic low-dimensional defect structures in polar oxides