Matthew M. Armentrout

Thermoelastic properties of ReB2 at high pressures and temperatures and comparison with Pt, Os, and Re

We have measured the phase stability and thermoelastic equation of state of ultrahard rhenium diboride at pressures up to 30 GPa and temperatures up to 2500 K using a laser heated diamond anvil cell in conjunction with synchrotron X-ray diffraction. ReB2 is shown to be stable throughout this pressure and temperature region. The ratio of the c-axis to the a-axis provides a monitor of the annealing of plastic stresses during compression. We show that ReB2 has a small thermal anisotropy but a large mechanical anisotropy. Combining this new data set with previously existing results from a large volume press yields a thermoelastic equation of state with a Gruneisen parameter of 2.4 (0.08) and a q of 2.7. A comparison of ReB2 with other high electron density incompressible metals—Os, Re, and Pt—shows that ReB2 has the lowest thermal pressure and the highest bulk modulus.

High-pressure and high-temperature equation of state of cobalt oxide: Implications for redox relations in Earth’s mantle

Abstract The high-pressure and high-temperature equation of state of rock salt-structured cobalt oxide was measured up to 65 GPa and 2600 K using synchrotron X-ray diffraction in conjunction with the laser heated diamond-anvil cell. Fitting a Mie-Grüneisen-Debye model to the data we find best-fit parameters V0 = 77.4 (fixed) Å3, K0 = 190 (1) GPa, K′ = 3.49 (4), γ0 = 1.54 (4), q = 2.87 (15), and θ0 = 517.8 K (fixed). We use this newly determined equation of state in conjunction with existing measurements of the thermoelastic parameters of cobalt metal to calculate the Gibbs free-energy difference between the cobalt oxide and cobalt metal phases as a function of pressure and temperature. A comparison of the energetics of the Co/CoO system with the Ni/NiO system predicts that below 58 GPa CoO+Ni is stable relative to NiO+Co, while above 58 GPa the reverse is true. This tipping point in energy can be mapped as a crossing point in the electrochemical potential of the two metal ions, suggesting that cobalt becomes more siderophile than nickel with increasing pressure. This result is in qualitative agreement with existing measurements of nickel and cobalt partition coefficients between mantle and core materials.

A new high pressure and temperature equation of state of fcc cobalt

The high pressure and temperature equation of state of cobalt metal in the face-centered cubic phase was measured up to 57 GPa and 2400 K using the laser heated diamond anvil cell in conjunction with synchrotron X-ray diffraction. The measured region is bisected by a ferromagnetic to paramagnetic transition across the Curie temperature necessitating use of an equation of state that incorporates a 2nd order phase transition within its formalism. A third order Birch-Murnaghan equation of state with a Mie-Gruneisen-Debye thermal correction and a Hillert-Jarl magnetic correction is employed to describe the data above and below the Curie temperature. We find best fit parameters of V0 = 6.753 (fixed) cm3/mol, K0 = 196 (3) GPa, K′ = 4.7 (2), γ0 = 2.00 (11), q = 1.3 (5), and θ0 = 385 K (fixed).