Cation-dependent hydrogen vacancy stability in XNH ₄ (X = Li, Na, K): thermodynamic crossover, chemical potential, and implications for hydrogen release

In this theoretical study, we employ first-principles density functional theory calculations using the generalized gradient approximation to systematically investigate the structural, electronic, and thermodynamic properties of alkali–ammonium compounds XNH ₄ (X = Li, Na, K) intended for solid-state hydrogen storage applications. The clear aim of this study is to elucidate how the chemical identity of the alkali cation governs the progressive hydrogen removal mechanisms via single (1H) and double (2H) vacancy formations. The Gibbs free energy changes (Δ G) were evaluated as a function of the hydrogen chemical potential ( μ _H ), incorporating zero-point energy corrections and entropic terms at room temperature. Our findings reveal that LiNH ₄ exhibits a strong cooperative stabilization effect, consistently favoring full 2H removal across the entire studied μ _H range without any phase crossover. Conversely, NaNH ₄ and KNH ₄ demonstrate unique thermodynamic crossovers where defect preference flips between 1H and 2H states at specific threshold chemical potentials. Pressure–temperature mappings indicate that the transition in KNH ₄ is experimentally reachable under realistic high-vacuum conditions (1.8 × 10 ⁻⁷  atm at 300 K), whereas NaNH ₄ requires extreme, unfeasible conditions due to an inherent lattice structural bottleneck. Electronic structure analysis reveals an ionic-to-covalent bonding transition, showing that hydrogen release induces an evolution from a metallic ground state toward a narrow p-type semiconductor behavior with distinct band gap openings (1.37 eV for LiNH ₂ , 0.80 eV for NaNH ₂ , and 0.93 eV for KNH ₂ ). These comprehensive calculations provide quantitative principles for optimizing dehydrogenation and tuning release pathways in complex metal amides.