Materials Research Lecture

We review recent developments in the field of first-principles simulations of materials at extreme conditions, focusing mainly on 3d-transition metals, their alloys and compounds.  Besides structural and magnetic transitions upon compression, we investigate the effect of extreme stresses on the electronic structure. Specifically, we consider Os, one of most incompressible metals, compressed to over 770 GPa. In the experiments, we find anomalies in the behavior of the unit cells parameters ratio c/a at ~150 GPa and at ~440 GPa. Theoretically, we show that the former peculiarity is a signature of the topological change of the Fermi surface for valence electrons, the so-called electronic topological transition (ETT). However, the anomaly at 440 GPa might be related to a new type of electronic transition, the core level crossing (CLC) transition, associated with interactions between deep 4f and 5p core electrons of Os induced by pressure. In addition, we discuss applications of the first-principles theories to calculations of thermodynamic and mechanical properties of magnetic materials. The examples include fundamental problems, like importance of correlation effects and/or magnetism for understanding of properties of transition metals (Fe, Ni) and their compounds (NiO) at ultra-high pressure. Moreover, we show that our simulations are relevant for technological applications, e.g. for studies of phase stability, formation energies of vacancies, substitutional and interstitial defects, calculations of equations of state and elastic moduli for 3d-transition metal alloys and compounds, like CrN and steels.