High-pressure phases of calcium and their finite-temperature phase boundaries

The phase diagram of Ca up to 100 GPa and 3500 K is studied using first-principles density-functional theory. We propose two solid phases with orthorhombic $Cmcm$ and $Pnma$ structures and determine their finite-temperature phase boundaries. In molten Ca, we present predictions for liquid transitions under compression. Our results describe significant electronic and structural changes that are qualitatively different from those found in dense alkali liquids. The predicted liquid and solid transitions provide a consistent description of the Ca phase diagram and insight into the expected properties of other alkaline-earth metals.