Grüneisen parameter of hcp‐Fe to 171 GPa

[1] We measured the phonon density of states (DOS) of hexagonal close‐packed iron ("‐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 "‐Fe’s total phonon DOS which, in turn, is directly related to the ambient temperature vibrational Grüneisen parameter (gvib). Fitting our individual gvib data points with gvib = gvib,0(V/V0) , a common parameterization, we found an ambient pressure gvib,0 = 2.0 ± 0.1 for the range q = 0.8 to 1.2. We also determined the Debye sound velocity (vD) from the low‐energy region of the phonon DOS and our in situ measured volumes, and used the volume dependence of vD to determine the commonly discussed Debye Grüneisen parameter (gD). Comparing our gvib(V) and gD(V), we found gvib to be ∼10% larger than gD at any given volume. Finally, applying our gvib(V) to a Mie‐Grüneisen 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 "‐Fe at Earth’s core pressures. Citation: Murphy, C. A., J. M. Jackson, W. Sturhahn, and B. Chen (2011), Grüneisen parameter of hcp‐Fe to 171 GPa, Geophys. Res. Lett., 38, L24306, doi:10.1029/ 2011GL049531.