Experimental vibrational Grüneisen ratio values for ϵ‐iron up to 330 GPa at 300 K

Experimental values for the vibrational Grüneisen ratio, γ υ , for hexagonal close‐packed (hcp) iron, called epsilon (ϵ)‐iron, were obtained over a large compression range corresponding to Earth's inner‐core pressure. This was done by measuring the intensity change in x‐ray diffraction lines under pressure, P . X‐ray structural refinement using the Rietveld method on powder diffraction data yielded information on mean square displacement used to obtain the Debye temperature, Θ, as a function of compression, V/V 0 . The value of Θ was determined from the measured intensity of the diffraction lines, which arises from the mean squared amplitude < u ² > measured in‐situ in the diamond cell. From the Debye relationship, γ υ =−(∂ ln Θ/∂ ln V ) T , we determined experimental data on γ υ versus V of Fe (hcp) up to 330 GPa at 300 K. The measured value of γ υ ( V ) is 1.18 at 330 GPa and 1.41 at 135 GPa.