Polyamorphism and pressure-induced metallization at the rigidity percolation threshold in densified GeSe4 glass

Chalcogenide glasses with tetrahedral networks can undergo significant densification under pressure owing to their open structures. The structural mechanisms of pressure-induced densification and the corresponding evolution of physical properties of glassy GeSe₄ alloy are studied over pressures ranging between ambient and 32.5 GPa, using X-ray scattering supplemented with 3D Monte Carlo structural modeling, Raman spectroscopy, electrical conductivity, and P-V equation of state measurements. The results demonstrate a pressure-induced, hysteretically reversible transition between low-density semiconducting and high-density metallic amorphous phases of GeSe₄ near ∼10-15 GPa. These two phases are characterized by their distinct P-V equations of state and structural mechanisms of densification. Densification in the low-density phase is dominated by large inward shifting of the second neighbors with a small amount of conversion from edge-sharing to corner-sharing GeSe₄ tetrahedra. On the other hand, densification in the high-density phase involves a gradual increase in the nearest-neighbor coordination numbers of Ge and Se atoms and the formation of Ge-Ge bonds between adjacent polyhedral units. These structural transformations are accompanied by a pressure-induced metallization that is reversible.