Extreme pressure can change compounds radically: metals can turn into isolators, gases into metals, some substances even become superconductors. Usually, the crystal structure changes with rising pressure. The metallic element osmium, however, is a special case. Under ambient pressure, it has the highest known density of the naturally occuring elements and one of the highest melting points. It also has a very low compressibility.
Leonid Dubrovinsky, University of Bayreuth, Germany, Igor Abrikosov, National University of Science and Technology, Moscow, Russia, and colleagues have subjected osmium to extreme pressures of up to 770 GPA in a special double-stage diamond anvil. This pressure is over twice as high as that in earth’s inner core. The researchers found that while the volume of osmium’s unit cell shrinks under these pressures, its hexagonal close-packed crystal structure remains the same.
Detailed measurements revealed small anomalies in the lattice parameters at 440 GPa. Such anomalies are usually caused by changes in the valence electrons. In this case, however, calculations suggest that the cause is a pressure-induced interaction between the core electrons. Such a phenomenon had not been previously observed in highly incompressible materials, and according to the team, the results open up opportunities to search for new high-pressure states of matter.
- 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,