Mechanistic origin of the absence of pressure-induced amorphization in NH4F vs ice: The role of an intermediate rotator phase.

The absence of pressure-induced amorphization in NH4F stands in stark contrast to the behavior of ice, despite their isostructural characteristics. In this study, we developed a deep potential model for NH4F and constructed its phase diagram. Our molecular dynamics simulations reveal that the transitions from Phases Ih and Ic to Phase II proceed via an intermediate Phase IV (NaCl-type). Within this Phase IV lattice, NH4+ ions undergo active rotation, enabling a transient reconfiguration of the hydrogen-bond (HB) network. This intermediate rotator phase acts as a structural buffer, effectively releasing the topological constraints of the parent HB network and allowing the system to dynamically explore the transition pathway to the product phase. In contrast, the lack of such a dynamic intermediate in molecular ice necessitates rigid network preservation, which typically fails to find a viable crystalline pathway and triggers a collapse into high-density amorphous (HDA) ice upon low-temperature compression. Our findings elucidate how the emergence of an intermediate rotator phase prevents amorphization in the NH4F systems.