The 9th International Conference on Matter and Radiation at Extremes (ICMRE2026) ( ICMRE2026)

Hydrogen, the simplest and most abundant element in the universe, displays unexpectedly rich structural behavior under extreme compression. For decades, theory has predicted a cascade of symmetry-breaking phase transitions in solid hydrogen, yet direct experimental confirmation has remained elusive. Here, we present high-resolution in-suit nuclear magnetic resonance spectroscopy in diamond anvil cells up to 286 GPa combined with quantum-chemical density functional theory calculations. We resolve a sequence of structural transitions --- from P63/mmc to P6122, C2/c, and ultimately Pc--- in agreement with theoretical predictions. These findings provide the first direct crystallographic evidence for a long-hypothesized continuous symmetry lowering in dense molecular hydrogen, tracing its structural evolution toward a metallic monatomic state.

hydrogen, high-resolution nuclear magnetic resonance, structural transition