All-electron GW calculations for perovskite transition metal oxides

Ever since the middle of the 1920s, when compounds from the perovskite structure family played a key role in the groundbreaking work of Goldschmidt on material synthesis, perovskite transition-metal oxides (TMOs) have repeatedly stimulated new activities in fundamental research as well as in the development of new technical applications. Thus, many perovskite TMOs, for example the ferroelectrics BaTiO3 and PbTiO3, have become an integral part of present-day technologies. Intriguing properties of other perovskite TMOs such as ferromagnetic LaMnO(3+delta) or the high-k dielectric BaZrO3 and their potential use in future applications are currently investigated. As a large variety of A and B cations can be combined to form the perovskite crystal structure with stoichiometric composition ABO3, compounds with a wide range of material properties can be synthesized. This makes perovskite transition-metal oxides a unique formal laboratory to test models and theories pertaining to solid state physics. The goal of this thesis is to shed light on the structure-composition-properties relation of this fascinating class of materials from the electronic-structure point of view. The investigations are based on density functional theory (DFT), the most successful theory for the description of ground-state electronic properties from first-principles in combination with the GW approximation (GWA) from many-body perturbation theory which has emerged as the method of choice to describe single-particle excitation spectra of solids. In this work, the full-potential linearized augmented plane-wave method (FLAPW) is used in all calculations. As an all-electron scheme it is particularly suitable to describe d and f states of transition metals and rare earths contained in the perovskite TMOs. Trends in the electronic structure of three series of prototypical perovskite TMOs including BaTiO3, BaZrO3, and PbTiO3 in the high-temperature cubic crystal phase are investigate to relate changes in the single-particle excitation spectra and band gaps to changes in the composition of the materials. In addition, the effect of symmetry-breaking relaxations from the cubic crystal phase on the electronic structure occurring at room temperature is investigated. The first-principles results emphasize the importance of including these effects in the calculations in order to quantitatively reproduce band gaps measured in experiment. Furthermore, trends in the positions of high-lying core states are analyzed. The calculated positions of the core states agree well with results from photo-emission experiments. Secondly, GW calculations for the three band insulators LaCrO3, LaMnO3, and LaFeO3 are carried out. Results from DFT calculations employing the generalized-gradient approximation with the PBE functional or results obtained from calculations using the hybrid-functional HSE are used as starting point to apply many-body perturbation theory. The analysis of photo-emission spectra focuses specifically on the position of the partially filled d states of the transition metals yielding finite spin-magnetic moments at the transition-metal site of all three compounds which order antiferromagnetically. Whereas the HSE result lead to a general improvement of the PBE results for the spin-magnetic moments, band gaps and photo-emission spectra are best described by the combination of HSE+GW in the case of LaCrO3 and by PBE+GW calculations for LaMnO3 and LaFeO3. For all three compounds, good quantitative agreement with experimental data is attained. The last part of the thesis focuses on the question how accurately a Hubbard model can reproduce the spectrum of a subspace of the full Hilbert space, as this kind of model allows to gain insight into the electronic structure of materials even if first-principles approaches are not applicable. A simple test system is constructed to simulate partially filled valence states whose single-particle excitation spectrum can be calculated exactly. These results are compared with the description for a subspace of the full system obtained from a Hubbard model. The model is designed to yield the best possible approximation for the exact spectrum. The investigations reveal that the Hubbard model cannot reproduce the spectrum exactly as soon as the wave functions of states inside the subspace exhibit a finite overlap with wave functions of states not contained in the subspace. This limits the applicability of the Hubbard model to the description of subspaces with a small degree of hybridization between states inside and outside the subspace.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2011

Author dc:creator

• Gierlich, Andreas

Contributors dc:contributor

• Blügel, Stefan

• info:eu-repo/classification/ddc/530
• Übergangsmetalloxide
• Perowskitstruktur
• Ab-initio-Rechnung
• Zustandsdichte
• Bandstrukturberechnung
• Elektronische Eigenschaft
• Bariumtitanat
• Bleititanat
• Bariumzirkonat
• Dichtefunktionalformalismus
• Störungstheorie
• Hubbard-Modell
• Physik
• transition metal oxides
• perovskite
• energy-band theory of solids
• density functionals
• GWA
• BaTiO3
• PbTiO3
• BaZrO3
• LaCrO3
• LaMnO3
• LaFeO3
• FLAPW
• FLEUR
• SPEX

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-121165%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59376