Mechanism of Solid Ammonia Stabilization at Ambient Temperature: Insights from Thermodynamics, Phonon Calculation,...

The stabilization of low-temperature solid-state ammonia at ambient temperature was recently achieved by confining fine crystals within a boric acid glass matrix through freeze-drying using liquid nitrogen, opening a new frontier for hydrogen storage materials. We determined the stabilization mechanism using a thermodynamic model supported by first-principles phonon calculation and pressure analysis based on micromechanics. Phonon calculation revealed vibrational densities of states (DOS) for the intermolecular translational and rotational modes of NH₃ molecules, as well as intramolecular librational and bending modes of hydrogen atoms. Simulations incorporating the computed DOS into a thermodynamic model reveal that sub-GPa pressure is exerted on ammonia crystals by the surrounding glass matrix. Micromechanical analysis confirms that this hydrostatic stress originates from the difference in thermal expansion coefficients. The results suggest that ammonia crystals are stabilized by this pressure. Thus, our findings provide a new strategy for stabilizing nonequilibrium phases at ambient temperature, opening new frontiers for hydrogen storage materials.