Density-driven defect-mediated network collapse of GeSe2 glass

The evolution in structure of the prototypical network-forming glass GeSe2 is investigated at pressures up to similar to 16 GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The neutron diffraction work at pressures <= 8.2 GPa employed the method of isotope substitution, and the molecular dynamics simulations were performed with two different exchange-correlation functionals, the Becke-Lee-Yang-Parr (BLYP) and the hybrid Heyd-Scuseria-Ernzerhof HSE06. The results show density-driven structural transformations that differ substantially from those observed in common oxide glasses such as SiO2 and GeO2. Edge-sharing tetrahedra persist as important structural motifs until a threshold pressure of similar to 8.5 GPa is attained, whereupon a mediating role is found for homopolar bonds in the appearance of higher coordinated Ge-centered polyhedra. These mechanisms of network transformation are likely to be generic for the class of glass-forming materials where homopolar bonds and fragility-promoting edge-sharing motifs are prevalent in the ambient-pressure network