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The most incompressible metal osmium at static pressures above 750 gigapascals

L. Dubrovinsky,N. Dubrovinskaia,E. Bykova,M. Bykov,V. Prakapenka,C. Prescher,K. Glazyrin,H.-P. Liermann,M. Hanfland,M. Ekholm,Q. Feng,L. V. Pourovskii,M. I. Katsnelson,J. M. Wills,I. A. Abrikosov

Nature 525,226–229(10 September 2015)

Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures. It is also very incompressible, but its high-pressure behaviour is not well understood because it has been studied so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells, with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.

Calculated electronic density of states (DOS) of Os as a function of energy E (relative to the Fermi energy E_F). DOS at pressures of 0 GPa (top) and 392 GPa (bottom). Experimental lattice parameters were used in the calculations. 6s, 6p, 5d, and 5f electrons form well-defined bands near the Fermi energy at all the pressures examined in this study. 5p electrons occupy 5p1/2 and 5p3/2 states, which are split owing to spin–orbit interaction; 4f electrons occupy 4f5/2 and 4f7/2 states. They behave as core electrons forming fully localized states at P=0 GPa. However, at P=392 GPa, the 5p states are broadened. Importantly, one clearly sees in b that 5p3/2 and 4f7/2 states start to interact with each other, and the CLC transition takes place. This interaction might be responsible for the peculiarity observed in our experiments at ultra-high pressure.

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Last updated: 09/27/15