Behavior of Os at High P,T: a Metal Analog of Fe in the Earth's Core

The transition metal osmium (Os) can be considered as a material analog for the high pressure behavior of the primary core metal iron since it is in the same group but has higher electron density. We examined the phase stability, density, and thermoelasticity of Os metal at pressures up to 50 GPa, and temperatures up to 2900 K in the laser-heated diamond anvil cell in conjunction with X-ray diffraction at beamline 12.2.2 at the Advanced Light Source. NaCl was used as a thermal insulator and internal pressure marker in these studies. Our data show that Os is stable in the hexagonal-close-pack phase throughout the study. Preliminary analysis of the thermoelastic parameters of Os suggests that the room pressure linear thermal expansion of the c-axis is ~ 2.9×10-6 (±1.2×10-6) K-1 and the linear thermal expansion of the a-axis is ~ 2.4×10-6 (± 0.9×10-6) K-1. The results suggest that both the linear thermal expansion parameters decrease by 2-3×10-8 K-1 GPa-1. Potential sources of error in the thermoelasticity measurements will be discussed in terms of thermal pressure in the laser-heated diamond anvil cell, pressure calibration with the NaCl standard and temperature measurement. The implications in terms of structure and thermoelasticity of Earth's core will be discussed.