High-pressure structural parameters and equation of state of osmium to 207 GPa

Abstract

The most incompressible transition metal osmium (Os) has been studied under high pressure. There is significant interest in Os because of the structural anomalies attributed to topological transitions in the Fermi surface for valence electrons in the hexagonal close-packed phase. We report on measurements of structural parameters and equation of state on Os metal to a pressure of 207 GPa at ambient temperature using platinum as a pressure standard. We obtained angle-dispersive X-ray diffraction data at a synchrotron source with closely spaced pressure intervals to observe any discontinuities or anomalies in the axial c/a ratio at high pressures. Rietveld refinements of X-ray diffraction data show a slowly varying axial ratio (c/a) with a broad minimum at 75 GPa. Our data do not provide any evidence of anomalous behavior in the c/a ratio in Os at 25 or 150 GPa as have been reported in previous studies. Our experimental results are in agreement with theoretical calculations that do not predict any anomalous behavior in c/a ratio in Os under extreme conditions. We present an equation of state for Os to 207 GPa (V/V₀ = 0.761) at ambient temperature and compare our results with the previously published data.

Keywords:

• transition metals
• high pressure
• equation of state
• X-ray diffraction

Public Interest Statement

The transition metal osmium is the most incompressible metal in the periodic table and its elastic stiffness rivals that of crystalline diamond. There is significant interest in high-pressure behavior of osmium as previous studies have reported structural anomalies that are attributed to changes in electronic structure induced by compression. The experimental studies on structural anomalies in osmium are particularly challenging as they require compression to several million atmosphere pressure in a diamond anvil cell. We present detailed structural analysis of X-ray diffraction data on osmium to over 2 million atmospheres (207 GPa) and found no evidence of structural anomaly at 150 GPa within the experimental errors inherent in the measurement. We present pressure–volume relationship or equation of state for osmium and show that even the most incompressible metal is compressed by 24% at the highest pressure of 207 GPa reached in our study.

Additional information

Notes on contributors

Yogesh K. Vohra

The high-pressure structural studies on transition metal osmium to multi-megabar pressures is carried out by Yogesh Vohra’s research group at the University of Alabama at Birmingham (UAB) and is supported by the US National Science Foundation (NSF). This is part of a larger study on structural, electronic, and magnetic transitions in transition metals, rare earth metals, and their alloys under extreme conditions of pressure and temperatures. Yogesh Vohra, PhD, is the founding director of UAB Center for Nanoscale Materials and Biointegration (CNMB) that houses a Diamond Microfabrication Laboratory. Yogesh Vohra’s research group has also pioneering several enabling technologies used in studies of materials under extreme conditions including diamond anvils with embedded sensors (designer diamonds) and nanocrystalline diamond micro-anvils grown by chemical vapor deposition techniques to study materials to pressures exceeding 500 GPa.