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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 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. (C) 2002 Elsevier Science B.V. All
rights reserved.