  First-order phase transitions are characterized by a discontinuous first
derivative of the Gibbs free energy, so that volumes and entropies are
discontinuous. Such transitions are common in the crystalline state, but
extremely rare in liquid substances and experimentally evidenced in only one
pure element, phosphorus. Here we report combined in-situ Raman scattering,
X-ray diffraction and density measurements that support the existence of a
first-order liquid-liquid transition in sulfur at high pressures and
temperatures. The transformation involves a sharp density jump between two
structurally and dynamically distinct liquids. The first-order phase transition
proceeds through an initial stage of temperature induced polymerization and a
final stage where the low-density liquid abruptly converts to a denser
polymeric state. This unique feature is explained by competing effects of
pressure and temperature.
