A Local Structural Basis to Resolve Amorphous Ices

Phases with distinct thermodynamic properties must differ in their underlying microscopic configurations.

While ordered phases are readily distinguished by unit cells and space groups, the local structural basis differentiating amorphous phases is less apparent.

Here, using a new probabilistic data-driven framework applied to molecular simulations of water, we identify local collective variables that discriminate low-density and high-density amorphous (LDA and HDA) ices and characterize pressure-induced transitions between them.

As expected, descriptors related to local density effectively distinguish LDA and HDA; however, phase identity is surprisingly encoded within the first coordination shell.

Furthermore, the pressure-induced LDA--HDA transformation proceeds through redistribution between LDA- and HDA-like local environments with no evidence for intermediate structures, consistent with a first-order-like phase transition.

This contrasts with the gradual structural evolution observed in other amorphous systems, such as metallic glasses.

Critically, local hydrogen density reveals pronounced structural hysteresis between compression and decompression pathways, which is not apparent in orientational order parameters, demonstrating that the microscopic interpretation of amorphous transformations depends fundamentally on descriptor choice.

These findings are robust across force fields and provide a general strategy for characterizing disordered phases lacking obvious distinguishing features.