A first-order liquid-liquid phase transition in phosphorus

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon. Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid-liquid phase transition.