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(1-4), Bur recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water(5-8) and liquid carbon(9,10). Her we report an in sit 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 P-4 molecules-we have found a polymeric form at pressures above 1 Cpa, 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.