Decoding entangled transitions: Polyamorphism and stressed rigidity

There is much to learn from simulation studies of polyamorphism achieved for systems with different bonding environments. Chalcogenide glasses such as Ge-Se glasses undergo an elastic phase transition involving important changes in network connectivity. Stimulated by recent developments of topological constraint theory, we show that the concept of rigidity can be extended to a broader range of thermodynamic conditions including densified glasses. After having validated our structural first principles molecular dynamics models with experimental data over a broad pressure range for GeSe₄, we show that the onset of polyamorphism is strongly related to the constraint density measuring the degree of rigidity of the network backbone, while voids and cavities in the structure collapse at very small pressures. This leads to the identification that the progressive onset of higher coordinated species typical of high pressure phases is responsible for the onset of stressed rigidity, although the constraint analysis also indicates progressive stiffening of bonding angles. Results are compared to stoichiometric and stressed rigid GeSe₂ and to isostatic As₂Se₃ and then generalized to other compositions in the Ge-Se binary under pressure.