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

Studies of liquids with tetrahedral coordination, particularly during compression or quenching, have indicated the existence of distinct phases(1-3) in the liquid state, distinguishable by density and local structure. In systems that exhibit critical phenomena in the supercooled state, anomalous behaviour of the compressibility is also anticipated above the critical point, as revealed by simulations of water(4). Liquid GeSe2 is a potentially attractive system for studying both types of phenomena, given its two-dimensional tetrahedral structure and anomalous physical properties (including a density minimum near its melting point). Here we report in situ X-ray diffraction measurements of solid and liquid GeSe2 at high temperature and high pressure, revealing that the structure of the liquid is sensitive to pressure and that anomalous compressibility is expected. During compression of liquid GeSe2, the connectivity of the liquid changes from two- to three-dimensional, leading to a breakdown of the intermediate-range order. The gradual change in structure above the melting line may develop to a first-order liquid-liquid transition in the supercooled regime.