RESUMO
The phase diagram of oxygen is investigated for pressures from 50 to 130 GPa and temperatures up to 1200 K using first-principles theory. A metallic molecular structure with the P63/mmc symmetry (η' phase) is determined to be thermodynamically stable in this pressure range at elevated temperatures above the ε(O8) phase. Crucial for obtaining this result is the inclusion of anharmonic lattice dynamics effects and accurate calculations of exchange interactions in the presence of thermal disorder. We present analysis of electronic, structural, and thermodynamic properties of solid oxygen at 0 K and finite temperature with hybrid exchange functionals, including a comparison with available experimental data.
RESUMO
The low-temperature crystal structure of elemental lithium, the prototypical simple metal, is a several-decades-old problem. At 1 atm pressure and 298 K, Li forms a body-centered cubic lattice, which is common to all alkali metals. However, a low-temperature phase transition was experimentally detected to a structure initially identified as having the 9R stacking. This structure, proposed by Overhauser in 1984, has been questioned repeatedly but has not been confirmed. Here we present a theoretical analysis of the Fermi surface of lithium in several relevant structures. We demonstrate that experimental measurements of the Fermi surface based on the de Haas-van Alphen effect can be used as a diagnostic method to investigate the low-temperature phase diagram of lithium. This approach may overcome the limitations of X-ray and neutron diffraction techniques and makes possible, in principle, the determination of the lithium low-temperature structure (and that of other metals) at both ambient and high pressure. The theoretical results are compared with existing low-temperature ambient pressure experimental data, which are shown to be inconsistent with a 9R phase for the low-temperature structure of lithium.