A simple tight-binding model for the study of 4d transition metals under pressure

Recently we have developed a simple tight-binding (TB) model of transition metals in the region near the middle of the 4d-series tuned to mimic molybdenum. The energetic, structural and melting properties deriving from this model are quite close to those obtained in previous first-principles work on Mo. TB approaches, reasonably accurate but computationally less demanding than first-principles calculations, therefore can be used to perform systematic analysis on the physical properties of transition metals across the 4d-series over wide thermodynamic ranges. Here we present a series of TB parametrizations designed to emulate the behavior of niobium, technetium, ruthenium, rhodium and palladium under extreme conditions of pressure and temperature. Our simple TB model is composed of two basic contributions to the energy: first, the pairwise repulsion due to Fermi exclusion, and second, the d-band bonding energy described in terms of an electronic density of states that depends on structure. The parameters of the model are adjusted to fit the dependence on pressure of the d-band width and the zero-temperature equation of state of the element in question. Calculated TB phonon spectra compare very well with ab initio results and experimental data, and the stable crystal structure in all transition metals at equilibrium is correctly predicted.