Emergent hidden order in ice: frustration and glassiness from slow hydrogen dynamics

Abstract

Frustrated systems can host hidden order, in which weak interactions select correlated structure from a highly degenerate manifold. Water ice Ih is the canonical example of such a manifold, yet whether its hydrogen disorder conceals local structure beyond the Bernal-Fowler ice rules has remained controversial. Here, using high-resolution inelastic neutron scattering on single-crystal heavy ice, we identify strongly anisotropic librational phonons dispersing uniaxially along the crystallographic c axis - a spectroscopic signature inaccessible to bulk-averaging probes. A physics-guided analysis reveals that these excitations encode a hidden partial order: correlated polar armchair chains driven by a shallow stereochemical bias that creates an imperfectly flat energy landscape. This bias promotes nanoscale polar domains, yet frustrated topology prevents their straightforward coarsening into the ice XI ground state, trapping the system in a rugged configurational landscape that, within the experimentally constrained model, retains finite residual entropy even in the limit of infinitely slow cooling. These findings show that ice Ih is not a simple disordered solid, but a frustrated, partially ordered hydrogen network with glass-like arrest on a crystalline lattice, providing a microscopic framework that reconciles thermodynamic theory with spectroscopic observations.