Ammonium Polyhydrides: Structure & Applications

• Ammonium polyhydrides are hydrogen-rich materials featuring NH₄⁺ cations and diverse hydrogenic sublattices stabilized under high pressures.
• They display varied structural motifs such as molecular host–guest frameworks, ionic solids, and metal–ammonium superlattices that influence their electronic and superconducting properties.
• The compounds have applications in high-pressure superconductivity, energy storage, and planetary science, with studies revealing key phase transitions and transport behaviors.

Ammonium polyhydride compounds are a class of hydrogen-rich materials incorporating ammonium (NH₄⁺) cations and diverse hydrogenic sublattices, often stabilized at high pressures. They display unique bonding, electronic properties, and phase behaviors, and have garnered significant interest for their implications in planetary science, materials under extreme conditions, energy storage, and high-pressure superconductivity. Recent theoretical and computational advances have led to a detailed understanding of their structures, stability ranges, transport properties, and electronic behaviors spanning from wide-gap insulators to metallic and superconducting phases (Srivastava, 2018, Wan et al., 2022, Wang et al., 19 May 2024, Qian et al., 2014, Villa et al., 26 Nov 2025).

1. Structural Diversity and Chemical Design

Ammonium polyhydrides form an extensive chemical family, characterized by general formulas NHₙ or NₙH₃ₙ₊₁⁺ for isolated cations. The design strategy for gas-phase and cluster cations follows stepwise substitution of H-ligands in NH₄⁺ by NH₄ units, yielding the NₙH₃ₙ₊₁⁺ “superalkali” series, represented as [NH₄·(n–1)NH₃]⁺ for n=1–5, and extending, e.g., to N₉H₂₈⁺ (Srivastava, 2018). In the extended solid state at elevated pressures, ammonium polyhydrides exhibit even greater structural variety:

• Molecular host–guest frameworks: Phases like NH₄ (Pc, C2/c symmetries) comprise NH₃ networks hosting nearly molecular H₂ (Qian et al., 2014).
• Ionic extended solids: Higher-pressure phases exhibit alternating NH₄⁺ and H⁻ layers or clusters, with stabilization via moderate hydrogen bonding and Coulombic attractions, as in P1-NH₄ or C2/c-NH₅ (Qian et al., 2014).
• Metal–ammonium polyhydrides: Compounds such as M N₂H₈ (M = Al, Mg, Ga, Zn, Cd, Hg) intercalate NH₄⁺ units into metal fcc lattices, with NH₄⁺ occupying tetrahedral sites and forming superlattices (Wan et al., 2022).
• High-hydrogen phases: Hydrogen-rich solids (NH₇–NH₂₄) feature motifs including H₂ molecules, H⁻ anions, polymeric hydrogen chains ([H₃^{0.5⁻}]_∞, H₅⁻), and cluster anions (H₇⁻) combined with NH₄⁺ (Villa et al., 26 Nov 2025, Wang et al., 19 May 2024).

Key geometric motifs include strong to medium-strength N–H···N hydrogen bonds (1.6–1.9 Å), molecular H₂ units (0.75–0.78 Å H–H) in channels, and variable arrangements of hydridic H⁻ centers (Srivastava, 2018, Qian et al., 2014, Wang et al., 19 May 2024, Villa et al., 26 Nov 2025).

2. Thermodynamic Stability and Pressure-Dependent Phase Behavior

Ammonium polyhydrides can be stabilized over broad pressure ranges, generally above ~50 GPa for most simple polyhydrides (e.g., NH₄, NH₅), and up to several hundred GPa for the hydrogen-rich or cluster-based phases (Qian et al., 2014, Wang et al., 19 May 2024).

• NH₄ is thermodynamically stable (on the convex hull) from ≈50 GPa to >800 GPa; NH₅ is on the hull from 55 GPa up to ≈162 GPa (Qian et al., 2014).
• Polyhydrides with n=7–24 are metastable within 60 meV/atom of the hull at 100–300 GPa, and are predicted to be synthetically accessible under high-pressure conditions (Villa et al., 26 Nov 2025, Wang et al., 19 May 2024).
• In M N₂H₈ phases, stability is achieved for Al, Mg, and Ga systems above ~10 GPa, whereas for Zn, Cd, Hg, dynamic and electronic stability persists down to ambient pressure (Wan et al., 2022).

Formation enthalpies for NH₄ and NH₅ are negative relative to NH₃ + H₂ at moderate pressures (ΔH_f ~ –0.25 eV/NH₄, –0.20 eV/NH₅ at 60 GPa), supporting their thermodynamic accessibility (Qian et al., 2014).

3. Bonding, Electronic Structure, and Superalkali Behavior

The electronic landscape of ammonium polyhydrides is governed by the interplay of NH₄⁺ cations, hydrogen-rich networks, and (where present) metal frameworks or guest elements.

• Superalkali cations: The NₙH₃ₙ₊₁⁺ series demonstrates decreasing vertical electron affinity (EAv) with increasing n, from NH₄⁺ at 4.39 eV to N₉H₂₈⁺ at 1.84 eV—well below alkali atom ionization energies (e.g., I_Li = 5.39 eV, I_Cs = 3.89 eV), confirming their “superalkali” designation. EAv scales linearly with the core N-atom NBO charge, and decreases exponentially with increasing n (Srivastava, 2018).
• Hydrogen bonding and networks: Stabilization arises from medium-strength, partially covalent N–H···N bonds and extensive H–H contacts, including hydridic (H⁻), molecular (H₂), and polymeric (H₃^{0.5⁻}, H₅⁻) motifs (Srivastava, 2018, Villa et al., 26 Nov 2025, Wang et al., 19 May 2024).
• Insulating and metallic phases: Phase electronic character spans wide-gap insulators (e.g., NH₄, NH₅ with E_g ≈ 2–3.5 eV at 60–200 GPa) to good metals and superconductors (e.g., NH₁₀, NH₁₁, NH₂₄ at >280 GPa) (Qian et al., 2014, Wang et al., 19 May 2024).
• Electrides: M N₂H₈ phases of Zn, Cd, Hg exhibit interstitial electrons at ambient pressure, acting as electrides with work functions as low as 2.78 eV (Wan et al., 2022).

4. Superconductivity and Metallization Mechanisms

Ammonium polyhydrides at sufficiently high pressure display strong electron–phonon coupling, enabling high-T_c superconductivity:

Phase	Pressure (GPa)	λ	ω_log (K)	T_c (K) (Eliashberg/ADM)
AlN₂H₈	40	2.35	744	118.4
MgN₂H₈	30	1.86	739	105.1
GaN₂H₈	50	1.43	1155	104.4
NH₁₀ (Pnma)	300	~1.6	1000–1200	179 (Eliashberg)
NH₁₁ (Cmc2₁)	300	1.2	1400–1600	114
NH₂₄ (C2)	300	1.3	1300–1500	134

In both metal–NH₄ polyhydrides and “pure” NHₙ phases, the mechanism involves pressure-induced charge transfer from metal or NH₄⁺ to the hydrogenic sublattice, “atomizing” H⁺ toward atomic H, and filling antibonding bands (Wan et al., 2022, Wang et al., 19 May 2024). Superconducting transition temperatures correlate linearly with the Bader charge on hydrogen (T_c = 731.58 q_H + 237.19 K) and inversely with the metal’s first ionization energy (Wan et al., 2022).

Hybridization of NH₄⁺-derived N–H levels and hydrogenic states at the Fermi level produces enhanced electron-phonon coupling; low-frequency rotations of NH₄⁺ and floppy lattice vibrations act as dominant contributors to λ (Wang et al., 19 May 2024). ZnN₂H₈, CdN₂H₈, HgN₂H₈ represent the first ambient-pressure electride superconductors, although with low T_c (Wan et al., 2022).

5. Hydrogen Superionicity and High-Pressure Phase Transitions

Upon heating under high pressure, ammonium polyhydrides undergo solid-to-superionic and superionic-to-liquid phase transitions, detectable via DFT–MD simulation by abrupt changes in internal energy, pressure, atomic msd, and diffusion constants (Villa et al., 26 Nov 2025). The temperatures of these transitions decline almost linearly with increasing proton fraction $x = n_\mathrm{H} / (n_\mathrm{H} + n_\mathrm{N})$. For $x \gtrsim 0.97$, the superionic window closes; such proton-rich phases exhibit direct melting (Villa et al., 26 Nov 2025).

Superionic transport arises from two primary mechanisms: (1) “paddle-wheel” exchanges involving rapid NH₄⁺ rotations and transient formation of cluster ions (e.g., H₇N₂), and (2) interstitial proton hopping—particularly along extended hydrogenic chains. The distinction between diffusive protons in NH₄⁺ and in chain environments becomes less pronounced at higher T (Villa et al., 26 Nov 2025).

Predicted transition temperatures, e.g., for Pnma-NH₁₀ at 300 GPa, are T_SI ≈ 450 K and T_m ≈ 2250 K; for C2-NH₂₄ at 300 GPa, T_SI ≈ 350 K and T_m ≈ 850 K (Villa et al., 26 Nov 2025).

6. Synthesis, Stability, and Prospects for Applications

Synthetic protocols typically employ diamond-anvil cell compression of NH₃/H₂ mixtures at pressures >50 GPa, yielding molecular or ionic polyhydrides (NH₄, NH₅) with moderate-temperature (300–600 K) annealing (Qian et al., 2014). Higher polyhydrides and metallic/superconducting phases may require extreme pressures (200–400 GPa), though metal–ammonium polyhydrides (e.g., ZnN₂H₈) are predicted to be stable at ambient pressure (Wan et al., 2022).

Energy densities for these compounds approach or exceed those of hydrazine, and reversible decomposition to NH₃ + H₂ is energetically favorable at moderate pressures—relevant for energy storage and high-energy-density material (HEDM) applications (Qian et al., 2014). Their unique phase behavior and hydrogenic conduction have direct implications for planetary interiors, suggesting that, unlike H₂O or NH₃, hydrogen-rich ammonium polyhydrides would exist as liquids rather than superionic solids in Uranus and Neptune mantles (Villa et al., 26 Nov 2025).

7. Outlook and Generalizations

The “superalkali” concept underlying NH₄⁺ applies to other low-ionization-potential molecular cations; theoretical studies propose that a broad class including FH₂⁺, OH₃⁺, organic cations, and mixed systems (e.g., Li(NH₃)₄·Hₙ) may yield similar hydrogen-rich metallic or superconducting phases under pressure (Wang et al., 19 May 2024). The tunability of hydrogen bond networks, charge localization, and molecular cation chemistry makes ammonium polyhydrides a versatile foundation for the design of high-T_c superconductors, ionic conductors, and HEDMs in extreme environments.