X-ray structural studies on elemental liquids under high pressures

X-ray structural studies on several elemental liquids under high pressure are reviewed. Combination of synchrotron radiation sources and large-volume presses enables us to carry out in situ structural measurements on liquids at high pressures up to several gigapascals. The measurements have revealed that compressions of liquid alkali metals are almost uniform, whereas those of liquids that have covalent components in bonding are mostly anisotropic. For covalent liquids, the volume dependence of the nearest-neighbour distance deviates from (V/V₀)^1/3 behaviour (V being the molar volume and V₀ being the molar volume at zero pressure) and changes in coordination play important roles. In some elements, different types of volume dependence of the nearest-neighbour distances are observed in different pressure ranges. This behaviour suggests that the liquid phase can be divided into regions. Although most of the observed structural changes are continuous, a discovery of an abrupt structural change in liquid phosphorus, which is completed over a pressure range of less than 0.05 GPa around 1 GPa and 1050°C, supports the existence of a first-order liquid–liquid phase transition.