Thermodynamic and electronic properties of niobium at finite temperatures

Sammanfattning: Niobium (Nb) is a fascinating element, that when it is in a solid state has remarkable properties. This is believed to be a result of its electronic configuration that has partially filled 4d and 5s sub-shells. Nb has a melting temperature of 2750 K, a high strength at high temperature, and a good wear resistance. Because of these properties, Nb is used as material for components of rockets and jet engines, and for strengthening steel. In the phonon dispersion relations, Kohn anomalies are experimentally observed to weaken with increased temperature, which is related to the superconducting properties of Nb. I include anharmonicity when I calculate the thermodynamic properties of Nb and relate this to the electronic structure. In this thesis I show that anharmonicity can not be neglected when considering thermodynamic properties of Nb. I observe broadening in the electronic band structure with increasing temperature, correlated with the gradual weakening of the Kohn anomalies in the phonon dispersion relations. Kohn anomaly in the phonon dispersion relation can be observed at 300 K and is completely absent at 1200 K. The observation of the Kohn anomaly's disappearance in the calculations is of great importance because it cannot be repeated by approaches that do not include anharmonic effects, meaning that properties that are directly related to phonon dispersion, like elastic constants, can be calculated more accurately with this approach.