Elastic properties and structural stabilities of elements and compounds

Abstract: A first principles study of the elastic and structural properties of selected elements and compounds is presented. The theoretical basis for these investigations is the density functional theory, in combination with a full potential linear muffin-tin orbital method.The elastic constants have been calculated for the hexagonal 4d and 5d elements and for the compounds ZnO and TiSi2; these agree with measurements within 30%. For the hexagonal elements in the 4 d and 5 d series it is shown that the elasticity is more isotropic than the cubic elements inthe same series.A study of the recently measured pressure induced isostructural phase transition in Zn is presented. The lattice parameter a, in the hexagonal plane, has been observed to exhibit a highly anomalous behavior. The effect is reproduced by theory and the mechanism causing the anomaly is demonstrated to be a change in the topology of the Fermi surface (FS) with decreasing volume.Uranium metal exhibits three temperature driven phase transitions in the range of 47K to 30K. These transformations are usually referred to as changes in the charge density wave states, with the most dominating one called α1. The transition to the α1 phase is investigated and it is shown that the driving mechanism is a Peierls distortion of the uranium lattice.Several ground state properties of the elements in the lanthanide series have been investigated, such as the cohesive energy, equilibrium volume and bulk modulus. In addition a novel way of calculating the valence stability for lanthanide systems is presented.Some technologically important materials have also been considered. The quantity hardness is coupled to the bulk modulus for the compound RuO2; it is shown that this system is almost as hard as diamond.