The high-pressure, high-temperature phase diagram of cerium.

We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to ∼22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the α-4-4-2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the4 phase, and 1-2 GPa lower than previously reported. We find the α-4 →2 phase boundary to have a positive gradient of 280 K (GPa), less steep than the 670 K (GPa)reported previously, and find the4 →2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2-3 GPa in the transition pressures at which the4 →2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the α-4 →2 is not second order at all temperatures up to 820 K.