Phase Transitions and Melting in Magnesium to 200 GPa and 4500 K

Magnesium is a ``simple'' nearly free-electron metal up to around 100 GPa. Despite similarly-simple group II metals being the subject of numerous studies that have revealed complex high-pressure behaviour, Mg has very few high-pressure diffraction studies, particularly above room temperature. Here we describe such studies to above 200 GPa at 300 K, combined with resistive- and laser-heating experiments to 4500 K and 100 GPa. The hcp-bcc transition at $\sim$50 GPa exhibits a large region of phase co-existence at all temperatures up to 800 K, and the transition pressure is found to decrease with temperature at the rate of $\sim$3.4 GPa per 100 K, somewhat smaller than the rate calculated by Mehta \textit{et al}., [1]. At lower pressures, below the melting curve at 10 GPa, we find the dhcp phase to be stable, in agreement with Errandonea \textit{et al.} [2]. Laser heating studies to 4500 K and 100 GPa show that Mg remains bcc up to the melting curve, our measurement of which is in good agreement with the previous ``speckle'' studies of Errandonea \textit{et al.} [3]. [1] S. Mehta, \textit{et al}., J. Chem. Phys. 125, 194507 (2006). [2] D. Errandonea, \textit{et al}., J. Phys.: Condens. Matter 15 (2003) 1277--1289 [3] D. Errandonea, \textit{et al}. Phys. Rev. B 65, 012108 (2001)