Abstract
Polyamorphism and isotropic liquid–liquid phase transition have been strongly linked to tetrahedral liquids with an open network of corner-sharing tetrahedra. Here, in situ high-pressure, high-temperature X-ray diffraction investigations of liquid and solid tetrahedral GeSe 2 are summarised. Ambient GeSe 2 is unique due to the presence of both edge- and corner-sharing tetrahedra giving the structure a 2D character. During compression the connectivity of the liquid changes from 2D to 3D due to conversion of edge- to corner-sharing tetrahedra. Moreover, the complete breakdown of intermediate range order is manifested by the loss of the first sharp diffraction peak. The transition from monoclinic 2D GeSe 2 to the stable high-pressure tetragonal (3D) GeSe 2 during isobaric heating was observed to depart from equilibrium at high pressures. The findings are rationalised by crossover from thermodynamic melting of 2D GeSe 2 towards a one-phase amorphisation process, where the transition is induced by the mechanical instability limit for 2D GeSe 2 as observed previously at ambient temperature. Finally, polyamorphism and possible liquid–liquid phase transition in supercooled liquid GeSe 2 is discussed in relation to the present findings.
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