Breakdown of intermediate-range order in liquid GeSe2 at high pressure

Studies of liquids with tetrahedral coordination, particularly during compr ession or quenching, have indicated the existence of distinct phases(1-3) i n the liquid state, distinguishable by density and local structure. In syst ems that exhibit critical phenomena in the supercooled state, anomalous beh aviour of the compressibility is also anticipated above the critical point, as revealed by simulations of water(4). Liquid GeSe2 is a potentially attr active system for studying both types of phenomena, given its two-dimension al tetrahedral structure and anomalous physical properties (including a den sity minimum near its melting point). Here we report in situ X-ray diffract ion measurements of solid and liquid GeSe2 at high temperature and high pre ssure, revealing that the structure of the liquid is sensitive to pressure and that anomalous compressibility is expected. During compression of liqui d GeSe2, the connectivity of the liquid changes from two- to three-dimensio nal, leading to a breakdown of the intermediate-range order. The gradual ch ange in structure above the melting line may develop to a first-order liqui d-liquid transition in the supercooled regime.