In this article, a recent discovery of a liquid-liquid phase transition in phosphorus is reviewed. By an in situ X-ray diffraction method, we have observed an abrupt pressure-induced structural change between a low-pressure molecular liquid and a high-pressure polymeric liquid at about 1 GPa. Experimental results strongly support that it is a first-order transition between two thermodynamically stable liquid phases. This is the first in-situ observation of such a transition in a liquid of pure substance.
