Flat interface between amorphous ices and the role of MDA-like intermediate states in the LDA-HDA transformation

Abstract: The pressure-induced transformation between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) is a prototypical polyamorphic transition, yet its microscopic mechanism - and in particular the nature of intermediate amorphous (IA) states - has long remained unresolved. We present the first characterisation of a flat LDA||HDA interface, determining its equilibrium thickness and showing its link to medium-density amorphous ice (MDA). Interestingly, the thickness of the interfacial layer remains constant under compression, while its position shifts reversibly into the LDA region - an elastic response that exhibits kinetic hysteresis upon decompression, reminiscent of memory effects. For high-precision discrimination of amorphous ices, we use a neural network classifier based on SOAP (Smooth Overlap of Atomic Positions) descriptors. The systematic description of the local structure emphasizes the importance of orientational information from the hydrogen bond network. Furthermore, this approach enables molecular dynamics analysis of the LDA-to-HDA transformation, directly mapping the spatial distribution of intermediate structures relative to growing HDA domains and the surrounding LDA matrix, and revealing that MDA-like IA configurations are not a distinct bulk phase but rather localise at the LDA-HDA interface.