Abstract
The melting curves of rhenium and osmium to megabar pressures are obtained from an extensive suite of ab initio quantum molecular dynamics (QMD) simulations using the Z method. In addition, for Re, we combine QMD simulations with total free energy calculations to obtain its phase diagram. Our results indicate that Re, which generally assumes a hexagonal close-packed (hcp) structure, melts from a face-centered cubic (fcc) structure in the pressure range 20–240 GPa. We conclude that the recent DAC data on Re to 50 GPa in fact encompass both the true melting curve and the low-slope hcp-fcc phase boundary above a triple point at (20 GPa, 4240 K). A linear fit to the Re diamond anvil cell (DAC) data then results in a slope that is 2.3 times smaller than that of the actual melting curve. The phase diagram of Re is topologically equivalent to that of Pt calculated by us earlier on. Regularities in the melting curves of Re, Os, and five other 3rd-row transition metals (Ta, W, Ir, Pt, Au) form the 3rd-row transition metal melting systematics. We demonstrate how this systematics can be used to estimate the currently unknown melting curve of the eighth 3rd-row transition metal Hf.
Highlights
Uncertainty in experimental stress conditions is an important source of error in measurements of phase equilibria and physical properties at deep Earth pressures
Full Free Energy Calculations on hcp-Re and fcc-Re In Figures 12 and 13 we show the phonon spectra of hcp- and fcc-Re, respectively, at one fixed density, namely, 25.1 g/cc (P ∼ 115 GPa), at five different temperatures calculated using the temperature-dependent effective potential (TDEP) method [42,43], which takes into account anharmonic lattice vibrations
We have run a total of about 2 million time steps in our quantum molecular dynamics (QMD) simulations on Os, and over 1 million time steps in those on Re; the high accuracy of the results and their importance to the field of phase diagram studies justifies the computational cost
Summary
Uncertainty in experimental stress conditions is an important source of error in measurements of phase equilibria and physical properties at deep Earth pressures. It should have no phase transitions over the experimental pressure range. The structures instability in Pt at high pressure ( P) and temperature ( T ) [2] limits the applicability of the Pt standard to low T Another consideration in pressure calibrant choice is overlap of diffraction peaks of the standard with those of the sample. That have fcc structure, their diffraction peak positions are similar at Mbar conditions In this respect, another pair of the third row transition metals, namely, Re and Os, that have hexagonal close-packed (hcp) structure, may offer potential to be a useful alternative to fcc standards. In what is following, we summarize the existing information on both Re and Os that can be found in the literature
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