Gruneisen parameter of hcp-Fe to 171 GPa

We measured the phonon density of states (DOS) of hexagonal close-packed iron (epsilon-Fe) with high statistical quality using nuclear resonant inelastic X-ray scattering and in situ X-ray diffraction experiments between pressures of 30 GPa and 171 GPa and at 300 K, with a neon pressure medium up to 69 GPa. The shape of the phonon DOS remained similar at all compression points, while the maximum (cutoff) energy increased regularly with decreasing volume. As a result, we present a generalized scaling law to describe the volume dependence of epsilon-Fe's total phonon DOS which, in turn, is directly related to the ambient temperature vibrational Gruneisen parameter (gamma(vib)). Fitting our individual gamma(vib) data points with gamma(vib) = gamma(vib, 0)(V/V-0)(q), a common parameterization, we found an ambient pressure gamma(vib,0) = 2.0 +/- 0.1 for the range q = 0.8 to 1.2. We also determined the Debye sound velocity (nu(D)) from the low-energy region of the phonon DOS and our in situ measured volumes, and used the volume dependence of nu(D) to determine the commonly discussed Debye Gruneisen parameter (gamma(D)). Comparing our gamma(vib)(V) and gamma(D)(V), we found gamma(vib) to be similar to 10% larger than gamma(D) at any given volume. Finally, applying our gamma(vib)(V) to a Mie-Gruneisen type relationship and an approximate form of the empirical Lindemann melting criterion, we predict the vibrational thermal pressure and estimate the high-pressure melting behavior of epsilon-Fe at Earth's core pressures. Citation: Murphy, C. A., J. M. Jackson, W. Sturhahn, and B. Chen (2011), Gruneisen parameter of hcp-Fe to 171 GPa, Geophys. Res. Lett., 38, L24306, doi:10.1029/2011GL049531.