Chemical Precompression in Hydrogen-Rich Materials

• Chemical precompression is the internal pressure generated by non-mechanical forces, such as charge transfer and geometric confinement, that mimics extreme external pressures.
• In hydrogen-rich hydrides, this phenomenon stabilizes metallic and superconducting phases by promoting charge transfer and orbital hybridization at lower applied pressures.
• It also facilitates the design of ultra-dense energetic materials by compressing guest molecules within host frameworks, yielding enhanced packing and thermal stability.

Chemical precompression is a phenomenon by which internal, non-mechanical forces—arising from chemical bonding, charge transfer, or host–guest interactions—generate an effective pressure on a molecular sublattice, typically hydrogen, enabling phase behaviors, electronic structures, and bond contractions otherwise accessible only under extreme external (mechanical) pressures. In hydrogen-rich hydrides and energetic crystals, chemical precompression acts as a foundational mechanism to stabilize metallic, superconducting, or ultra-dense forms at experimentally or technologically achievable conditions.

1. Fundamental Concept and Physical Origin

Chemical precompression denotes the “built-in” internal pressure exerted by a host lattice—metallic, ionic, or polymeric—on an encapsulated or embedded guest species, such as hydrogen, through ionic or covalent bonding, geometric confinement, or charge-transfer effects. Unlike external pressure, which is applied via mechanical means (e.g., diamond-anvil cell), chemical precompression emerges from the electrostatics and electronic structure intrinsic to the materials architecture (Yao et al., 2021, Jeon et al., 2020, Hilleke et al., 2021, Yan et al., 2018).

The origin typically involves:

• Charge transfer: Electron-rich metal frameworks donate electrons to hydrogenic sublattices, generating anionic hydrogen cages and cationic metal centers. The Coulomb attraction acts radially inward, mimicking external compression (Yao et al., 2021).
• Geometric entrapment: Hydrogen atoms or energetic molecules constrained within molecular cages, clathrates, or host–guest frameworks experience enforced short-range interactions and volume reduction (Hilleke et al., 2021, Yan et al., 2018).
• Orbital hybridization: Partial covalency and orbital mixing—e.g., Ce 5p/4f/5d with H 1s—add an additional bonding force that further contracts the guest sublattice (Jeon et al., 2020).

Quantitatively, the magnitude of chemical precompression can be inferred by correlating interatomic distances (such as H–H) in the given compound at moderate pressure with those realized in elemental hydrogen or the neat molecular form at much higher external stresses. For instance, an H–H bond length of 1.12 Å in CeH₉ at 100 GPa matches that of pure hydrogen at ∼250 GPa, indicating an internal compression equivalent to ∼150 GPa (Hilleke et al., 2021).

2. Chemical Precompression in Hydride Superconductors

Chemical precompression is central to the stabilization and metallization of hydrogen-rich phases, enabling high-$T_c$ superconductivity at reduced pressures compared to pure hydrogen. This is prominently realized in clathrate and sodalite-structured metal superhydrides, where large H cages encapsulate metal cations (Yao et al., 2021, Jeon et al., 2020, Hilleke et al., 2021).

Electronic Structure and Charge Localization

In ThH₁₀, ThH₉, and CeH₉, first-principles DFT calculations show:

• Isolated metal frameworks form “electride-like” states, i.e., spatially localized interstitial excess electrons in the absence of hydrogen (Yao et al., 2021).
• Upon hydrogenation, these electrons populate hydrogenic cages, generating large anionic charge on H sublattices.
• Bader charge analyses at 300 GPa reveal effective metal charges (Q_M): +1.486 e (ThH₁₀), +1.464 e (ThH₉), +1.199 e (CeH₉), +1.036 e (LaH₁₀) (Yao et al., 2021).

Coulombic Model of Internal Pressure

The internal Coulomb attraction is modeled as:

$$P_{\rm chem} = \frac{Q_{\rm M}^2}{16\pi\varepsilon_0 R^4}$$

where $Q_{\rm M}$ is the Bader charge of the metal and $R$ is the average metal–H bond length. Stronger internal pressure (i.e., higher $P_{\rm chem}$) results from increased charge transfer (larger $Q_{\rm M}$) and decreased $R$. Comparative DFT studies indicate that ThH₁₀ and ThH₉ exhibit chemical precompression roughly twice that of LaH₁₀, reflected directly in the critical pressure at which these phases become thermodynamically stable (Yao et al., 2021):

Material	$Q_{\rm M}$ (e)	$d_{\rm M-H}$ (Å)	$P_{\rm chem}$ (LaH₁₀ = 1.0)	$P_c$ (GPa)
ThH₁₀	+1.486	1.842	≈ 2.0	85
ThH₉	+1.464	1.803	≈ 1.9	86
CeH₉	+1.199	1.792	≈ 1.5	80
LaH₁₀	+1.036	1.840	1.0	170

Covalency Effects

DFT analysis of CeH₉ reveals strong Ce(5p, 4f, 5d)–H(1s) hybridization with significant charge transfer ($\Delta q_{\rm Ce} = 2.53$ e per atom) and high total charge densities at H–H bond midpoints. Including Ce 4f electrons as valence states contracts the lattice and stabilizes the high-pressure phase, indicating that partially delocalized $f$-electrons enhance chemical precompression along specific crystallographic axes (Jeon et al., 2020).

3. Host–Guest Chemistry and Chemical Precompression in Energetic Materials

Beyond hydrides, chemical precompression is realized in host–guest assemblies where energetic molecules are irreversibly compressed by a rigid or semi-rigid host. Notably, in HMX-TAGP hybrids, a two-dimensional triaminoguanidine–glyoxal polymer (TAGP) framework stacks via $\pi$–$\pi$ and H-bonding, “net-fishing” HMX molecules into interlayer voids (Yan et al., 2018).

• Single-crystal XRD of the trapped HMX reveals a compressed conformation and unit cell volume closely matching that of mechanically compressed (0.2 GPa) HMX.
• Packing densities surpass the ambient theoretical maximum for $\beta$-HMX (1.893 g cm$^{-3}$) and even exceed the most densely packed polymorphs obtained by high pressure (up to 2.13 g cm$^{-3}$).
• Thermal analysis shows suppression of polymorphic transitions and melting, indicating the chemical precompression effect kinetically traps the high-density conformation (Yan et al., 2018).

4. Quantitative Evaluation and Computational Approaches

First-principles calculations underpin the analysis and prediction of chemical precompression. The workflow typically involves:

• Crystal Structure Prediction (CSP): Global metaheuristic algorithms (USPEX, AIRSS, CALYPSO) generate candidate hydride lattices over a pressure grid, screened for enthalpic proximity to the thermodynamic convex hull.
• Relativistic Density Functional Theory (DFT): Relaxation, Bader charge population, and electron localization function (ELF) mapping identify the charge transfer and localization phenomena underpinning chemical precompression (Yao et al., 2021, Jeon et al., 2020, Hilleke et al., 2021).
• Phonon Dispersion and Electron–Phonon Coupling: Calculation of phonon spectra via DFPT, assessment of dynamical stability (absence of imaginary modes), and evaluation of the electron–phonon coupling constant $\lambda$ and Eliashberg function $\alpha^2F(\omega)$.
• Measurement of Effective Precompression: By comparing key structural parameters of hydrides (H–H bond lengths) with those of pure hydrogen at reference pressures, one deduces the chemical component of internal stress.

5. Structural Motifs and Materials Design

Hydrogenic frameworks exhibiting strong chemical precompression are categorized as follows (Hilleke et al., 2021):

• Mixed Molecular–Atomic H Frameworks: e.g., I4/mmm MH$_4$ (M=Sc, Y, Zr, La), where apical H$_2$ units are symmetrically compressed by neighboring metal ions; example: ScH$_4$ ($T_c \approx 100$–163 K at 120 GPa).
• Clathrate Cages: Im–3m, Fm–3m, or P6$_3$/mmc symmetry hydrides (e.g., CaH$_6$, YH$_6$, LaH$_{10}$, CeH$_9$, ThH$_{10}$) featuring extended H cages with central metal cations; these motifs offer large internal pressure and high $T_c$ ($>200$ K).
• Ternary and Complex Hydrides: Site substitution, cage filling (e.g., Li$_2$MgH$_{16}$, LaBH$_8$), and introduction of ternary elements further tune charge transfer and cage constraint, enabling stable hydrogenic frameworks at much lower applied pressures.

6. Practical Implications and Design Principles

Chemical precompression functions as a “materials design lever” for:

• Lowering the threshold external pressures for synthesizing metallic or superconducting hydrogen-rich phases, critical for practical experimental realization of high-$T_c$ superconductivity (Yao et al., 2021, Hilleke et al., 2021).
• Enabling ambient-stable, ultra-dense energetic materials through host–guest strategies (Yan et al., 2018).
• Rational selection of host/guest elements and structural motifs based on maximizing charge transfer, optimal orbital hybridization, and geometric encapsulation (Yao et al., 2021, Jeon et al., 2020, Hilleke et al., 2021).

In sum, chemical precompression extends the accessible phase space for extreme states of matter by embedding high local forces within the crystal lattice, set by electronic and chemical means rather than mechanical ones. This internal pressure is central to the rational design of novel hydride superconductors and energetic systems currently at the forefront of high-pressure physics and materials science.