Pressure-induced structural changes in the network-forming isostatic glass GeSe4: An investigation by neutron diffraction and first-principles molecular dynamics

Abstract

The changes to the topological and chemical ordering in the network-forming isostatic glass GeSe4 are investigated at pressures up to ∼14.4 GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The results show a network built from corner- and edge-sharing Ge(Se1/2)4 tetrahedra, where linkages by Se2 dimers or longer Sen chains are prevalent. These linkages confer the network with a local flexibility that helps to retain the network connectivity at pressures up to ∼8 GPa, corresponding to a density increase of ∼37%. The network reorganization at constant topology maintains a mean coordination number n¯≃2.4, the value expected from mean-field constraint-counting theory for a rigid stress-free network. Isostatic networks may therefore remain optimally constrained to avoid stress and retain their favorable glass-forming ability over a large density range. As the pressure is increased to around 13 GPa, corresponding to a density increase of ∼49%, Ge(Se1/2)4 tetrahedra remain as the predominant structural motifs, but there is an appearance of 5-fold coordinated Ge atoms and homopolar Ge-Ge bonds that accompany an increase in the fraction of 3-fold coordinated Se atoms. The band gap energy decreases with increasing pressure, and midgap states appear at pressures beyond ∼6.7 GPa. The latter originate from undercoordinated Se atoms that terminate broken Sen chains.