Metastable melting and pressure-induced amorphisation of GeSe2

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 GeSe2 are summarised. Ambient GeSe2 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 GeSe2 to the stable high-pressure tetragonal (3D) GeSe2 during isobaric heating was observed to depart from equilibrium at high pressures. The findings are rationalised by crossover from thermodynamic melting of 2D GeSe2 towards a one-phase amorphisation process, where the transition is induced by the mechanical instability limit for 2D GeSe2 as observed previously at ambient temperature. Finally, polyamorphism and possible liquid–liquid phase transition in supercooled liquid GeSe2 is discussed in relation to the present findings.