Über die elektronische Struktur funktioneller Festkörpermaterialien und ihre Beschreibung mittels lokaler Bindungsindikatoren

• On the electronic structure of solid-state functional materials and their characterization using local bonding indicators

Ertural, Christina; Dronskowski, Richard (Thesis advisor); Raabe, Gerhard (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

In this work, several applications for local bond descriptors are demonstrated. The benefit lies in the structural and electronic determination of functional materials, such as thermoelectrics or electrode materials for (metal ion) batteries, which are essential parts of our everyday life. With ever-growing demand for efficient, safe and clean technology, the challenge of designing materials for specific applications and with other desired properties that meet this demand arises. A necessary step in this is chemical bond analysis, using various local bond indicators from the LOBSTER (Local-Orbital Basis Suite Towards Electronic-Structure Reconstruction) software package - partially (co)developed during the doctorate phase - to explore atomistic properties, using comparison with established methods (e.g. Bader charge analysis) and literature values. Local means that the immediate chemical environment, or chemical bonding of an atom, or individual orbital interactions are considered to determine stability, structure, magnetism, among others. Among the electrostatic descriptors there are the wave function or electron density based charges or the Madelung energy. Among the electronic descriptors, the density of states (DOS) or the crystal orbital Hamilton population (COHP) are worth highlighting, reflecting the covalency and thus the chemical bonding strength. Ionic and covalent properties can co-exist, so it is worth looking at the combination of different bonding indicators. In my work, I present my development, implementation and application of plane wave (PAW, projector augmented waves) based Mulliken and Löwdin population analyses and my preliminary development, implementation and application of polarization calculation of ferroelectrics. In addition, I address the co-development and application of Madelung energies, the crystal orbital bond index (COBI, bond order and multi-center bonding), and the co-development, co-implementation and application of band-resolved COHP using LOBSTER. A wide variety of functional materials are investigated, always considering the chemical perspective using appropriate bond descriptors. The focus lies on intermetallic and Zintl phases with application as thermoelectrics as well as electrode materials for use in (metal ion) battery technology. The focus lies on nitrogen-based compounds as potential new cathode materials. Finally, preliminary results on electrides, ferroelectrics, and alkaline earth metal and lanthanide hydrides or fluorides are presented. The new PAW-projected wavefunction-based Mulliken and Löwdin population analyses are partially superior to the electron-density-based Bader charge analysis in some aspects. For example, the charge trends from Bader analysis do not agree with electronegativity trends of the presented alkali metal halide salts, Zintl phases, or graphite-based anode compounds. In case of the latter, an unexpected Bader charge distribution also occurs. LOBSTER calculations are about an order of magnitude faster in many cases and consumes less storage space. In addition, chemically questionable charges are obtained by the Bader method, such as integer charges on carbon and nitrogen (in dicyanamide or carbodiimide salts), although these elements are known to have partial charges, or even negative charges on alkali metals (Li, Na) in nanoporous compounds where cationic or metallic behavior is expected and experimentally demonstrated. Application of the population analyses and other routines in LOBSTER to various electrode materials confirms the known good intercalation behavior of LiCoO2 . Li and Na dicyanamide salts exhibit Löwdin charges similar to commercial cathode materials, with the two Li-based dicyanamides studied exhibiting voltages around 4 V. This indicates their potential application as transition metal-free cathodes. In view of the results and the known thermal and chemical stability of dicyanamide salts, further experimental investigations are suggested. The application of the COBI method to classical two-center bonds (here the ICOBI corresponds to the classical bond order) and multicenter bonds, in combination with Mulliken population analysis, offers new insights regarding structure and electronic structure, especially for polyionic compounds, Zintl and intermetallic phases, where an application of classical methods such as the Zintl-Klemm concept is difficult. Multi-center bonds are typical in this case. The structural difference between Cs2Te5 and Ga2Te5, in which Te5 unit connectivity is different due to their respective chemical natures, is further determined by the COBI method. In ionically characterized Cs2Te5, the Te5 units are linked to each other, while in Ga2Te5 the units are linked via Ga. The chemically intuitive interpretation of COBI of multicenter interactions beyond three-center interactions remains to be clarified and how they fit with other chemical concepts, such as the occupation of bonding, nonbonding, and antibonding states. First insights into the study of electrides, barium titanate (as an example of ferroelectrics), and lanthanide hydrides/fluorides reveal: The electronic structure of electrides cannot yet be adequately described with the basis functions implemented in LOBSTER. For BaTiO3, the polarization increases as expected with increasing distortion of the unit cell compared to the cubic system. The correct electronic description of the fluoride and hydride compounds of europium and ytterbium requires the addition of f electrons, reproducing experimental volumes. In summary, the further development of the LOBSTER software and the application of its features to the structural and electronic determination of functional materials has been successful.

Institutions

• Department of Chemistry [150000]
• Chair of Solid-State and Quantum Chemistry [151110]