Theoretical studies of the coupling between electronic, vibrational, configurational and structural effects in metal borides

Sammanfattning: This thesis addresses theoretical studies of the coupling between electronic, vibrational, configurational and structural degrees of freedom in metal borides. The effect of external conditions of temperature, pressure and composition on the interplay between internal degrees of freedom is investigated. The importance of excitations and disorder of the above types is well-established and known to dictate key materials science concepts such as phase stability, mechanical and electronic properties. Their mutual coupling composes the next level in complexity in understanding what parameters are to be necessarily included in the theoretical modelling of the system. The main tool used for making such predictions herein is density functional theory. It allows us to capture said excitations and disorder, and give accurate results with reasonable computational efficiency.Metal borides are chosen because of technologically interesting combination of both ceramic and metallic properties, like high hardness, melting point, fracture toughness and electrical conductivity, as well as previous reports of interesting fundamental physical phenomena, like the conventional superconductivity of MgB2 and the apparent off-stoichiometry of AlB2. The theoretical approach is chosen because of its ability to controllably couple and decouple different degrees of freedom to study their combined or isolated effect on the desired materials property. The level of theoretical modeling can be adjusted to fit what is reasonable in terms of efficiency, and still well be used for predictions with quantitative or semi-quantitative accuracy.The configurational aspect of phase stability of binary boron compounds has been believed to be trivial to understand as they most often can be constructed by stacking alternating layers of metal and boron atoms. However, a closer inspection of AlB2, the very name-giver of one of the most usual crystal structures within metal borides, shows a surprising existence of ambiguity regarding both its stable composition and the configuration of metal-site vacancies. Here theoretical approaches are used to reveal the configurational thermodynamics of these vacancies, the origin of their stability and coupling to the electronic structure of the compound. Furthermore, we answer the question why ideal stoichiometric AlB2 is unfavourable, why the vacancy stability window is so narrow, and how different arrangements of vacancies couple with the vibrational degree of freedom, including thermal expansion.If a second metal species is introduced the configurational problem becomes more complex. The arrangement of atoms on the metal sublattice is dependent on the bonding chemistry between the metal atoms and temperature driven thermodynamic effects, like entropy and lattice dynamics. The presented work on Ti1-xAlxB2 is an example of this, where mixing and clustering thermodynamics is considered to further investigate the potential for age-hardening of this ternary diboride. In it, the effect of lattice dynamics and configurational clustering on phase stability is discussed.The discovery of MgB2 being a superconductor in 2001 sparked many fruitful experimental and theoretical studies on the topic. It is generally agreed that MgB2 is a two-gap superconductor which originate in the σ (2D character boron px and py) and π (3D character boron pz) bands, respectively. Superconductivity in itself arises mainly from strong coupling between the E2g phonon mode, corresponding to in-plane bond-stretching vibrations of boron atoms, and electrons on the sigma bands. In the work presented, we use the coupling to the global structural parameters and external pressure to compare different theoretical models, with and without explicit treatment of electron-phonon coupling, and their ability to predict the superconducting transition temperature Tc of distorted MgB2. Moreover, a way to experimentally realize such lattice distorted MgB2 through clever nanostructure design is theoretically explored. Epitaxially growing an alternating out-of-plane ordered Mg/M diboride, with M atoms that naturally have clustering tendencies with respect to Mg, is proposed to being able to provide the necessary lattice distortions of both a- and c-parameters that can lead to an increase in Tc.All of the beforementioned covered compounds (TiB2, AlB2, Ti1-xAlxB2 and MgB2) crystallize in the AlB2-type structure with alternating hexagonal and metal 2D layers, that is most common in the diboride family. However, metal borides can also crystallize with puckered boron layers, while preserving the flat hexagonal metal layers, as in the case for ReB2. It is known for its exceptionally high Vickers hardness that varies from 30.1 ± 1.30 to 48.0 ± 5.6 GPa depending on indentation load. While boron-rich transition metal borides often are considered as potential candidates for hard and incompressible materials, rhenium borides with boron content higher than ReB2 have not been experimentally realized. Theoretical work has proposed that ReB4 adopts the same crystal structure as superhard WB4. In the presented papers, we report the successful synthesis of two novel ReB3 and ReB4 phases at high pressure that remain stable when decompressed down to ambient conditions. First-principles calculations are employed to characterize the electronic, dynamic and thermodynamic stability properties of these phases. Furthermore, novel complex modular Re2B5 and Re3B7 structures are synthesized and characterized by hexagonal boron networks interconnected by short B2 dumbbells.The aim of the in-depth investigations contained in this thesis, using state-of-the-art simulation techniques in collaborations with experimental work, is to further the understanding of how the coupling between electrons, vibrations, atomic configuration, disorder and external conditions influences the properties of materials and to share the results with the scientific community.