A New Lifshitz Transition and the Equation of State of Osmium

• Published in: Physical review letters 2003-11, Vol.93 (10)
• Main Authors: Occelli, F, Aracne, C M, Teter, D M, Hanfland, M, Canny, B, Couzinet, B, Chervin, J, Badro, J, Farber, D L
• Format: Article
• Language: English
• Subjects: ACCURACY; BONDING; COMPRESSIBILITY; COMPRESSION; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; DIAMONDS; ELECTRONS; FERMI LEVEL; HYDROSTATICS; MATERIALS SCIENCE; OSMIUM; PRESSURE RANGE; RESOLUTION; TRANSITION ELEMENTS; X-RAY DIFFRACTION; ZINC
• ISSN: 0031-9007; 1079-7114

We have measured the equation of state (EoS) of osmium to 75 GPa under hydrostatic conditions at room temperature using angle dispersive x-ray diffraction. A least-squares fit of the data using a third order Birch-Murnaghan EoS yields K{sub 0} = 411 {+-} 6 GPa and K'{sub 0} = 4.0 {+-} 0.2, showing osmium is in fact more compressible than diamond. Most importantly, we have documented an anomaly in the compressibility at 20.3 GPa associated with a large discontinuity in the first pressure derivative of the c/a ratio. This discontinuity likely arises from the collapse of the small hole-ellipsoid in the Fermi surface near the L point. There has been much interest in the possibility of a Lifshitz [1] or electronic topological transition (ETT) in zinc at high-pressure near 10 GPa. Interestingly, while the experimental data remain somewhat ambiguous [2-5], most simulations suggest the ETT exists in this pressure range [6-8]. Recently, Steinle-Neumann et al. [8] have shown that the transition arises from changes in the band structure near the high-symmetry point K where three bands cross the Fermi surface upon compression. Thus one might expect that other hcp metals should exhibit similar phenomena. The hcp 4d and 5d transition elements Re, Os and Ru are known to be among the densest and stiffest metals [9,10] suggesting that these might in fact be poor candidates in which to look for such effects. In osmium however, experimental and theoretical results [11,12] have shown the existence of small local maxima in the band structure just above the Fermi energy near the high-symmetry point L on the zone boundary [11]. These structures might potentially fall below the Fermi energy upon compression and give rise to an ETT. Osmium is of further interest as recent EoS measurements by Cynn et al. [13] have suggested that Os (K{sub 0} = 462 GPa and K'{sub 0} = 2.4) has the lowest known compressibility, lower even than diamond (K{sub 0} = 446 GPa and K'{sub 0} = 3) [14]. This conclusion has strong implications for the nature of the metallic bond in Os and paradoxically implies that the latter, where bonding electrons are delocalized, can be less compressible than the covalent bond, where bonding electrons are localized. The difficulty in supporting such a claim arises due to the fact that in all EoS studies of low compressibility materials, where the maximum experimental pressures are only a small fraction of the value of K{sub 0}, there exists a direct trade off between low