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

• Applicant: Professor Dr. Simone Sanna
• Subject Area: Synthesis and Properties of Functional Materials ; Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale ; Theoretical Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426703838

The present proposal describes a subproject for the 2nd funding period of the research group 5044 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 (LiNb1-xTaxO3, LNT). The scope of the present project is the theoretical description of the ground and excited state 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 atomistic modelling of ferroelectrics with the special quasirandom structures (SQS) developed in the first funding period. The SQS allow the atomistic modelling of disordered crystals within the supercell method. Owing to the manifold applied theoretical approaches, a multitude of materials properties and spectroscopic signatures could be quantified and compared with the outcome of the experimental projects in the first project phase. This laid the basis for a thorough understanding of the material system. Despite the substantial advances in the knowledge of this material class, a comprehensive theoretical characterization of the solid solutions is still not available and should be reached with the continuation of our theoretical investigation in the second funding period. Our models have revealed novel and surprising aspects of the mixed crystals that need to be further investigated. The non-linear dependence of the fundamental electronic gap on the composition and the correlation of the occupation of the oxygen octahedra with a measured jump in activation energy of the conductivity belong to these aspects. The atomistic modeling of ferroelectric domain walls and other interfaces shall be continued as well, and complemented by a phenomenological approach for the determination of the domain form. The doping of the solid solutions will be a central aspect in the second funding period. Moreover, we extend our method portfolio to efficiently model the LNT crystals at finite temperatures. The class of investigated systems shall be extended as well, in order to model free standing and strained thin films besides the crystal bulk. The final goal of the subproject is the prediction of composition dependent material properties of the LNT solid solutions and their comparison with the experimental results. This will allow both to interpret the experimental results and to quantify the impact of structure, stoichiometry and doping on the macroscopic properties. In turn, this allows us to predict LNT compositions that lead to the growth of crystals with tailored properties.

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