Universal insertion of molecules in ionic compounds under pressure

ABSTRACT Using first-principles calculations and crystal structure search methods, we found that many covalently bonded molecules such as H2, N2, CO2, NH3, H2O and CH4 may react with NaCl, a prototype ionic solid, and form stable compounds under pressure while retaining their molecular structure. These molecules, despite whether they are homonuclear or heteronuclear, polar or non-polar, small or large, do not show strong chemical interactions with surrounding Na and Cl ions. In contrast, the most stable molecule among all examples, N2, is found to transform into cyclo-N5− anions while reacting with NaCl under high pressures. It provides a new route to synthesize pentazolates, which are promising green energy materials with high energy density. Our work demonstrates a unique and universal hybridization propensity of covalently bonded molecules and solid compounds under pressure. This surprising miscibility suggests possible mixing regions between the molecular and rock layers in the interiors of large planets.


INTRODUCTION
Chemical substances are generally divided into two large categories: molecules and solid-state compounds.They distinctly differ in structures, bonding features, and properties and have been used in different areas.Molecules are generally formed from nonmetals and are held together by covalent bonds, whereas solid-state compounds often comprise metals or metalloids and can be characterized by ionic or metallic bonding.Hybrid materials, which consist of inorganic components and small molecules, have gained intensive attention owing to their unique chemical structure, physical properties, and potential applications in optics, electronics, mechanics, catalysis, and sensors as well as biomedical devices [1 -5 ].However, these unique characteristics also impose challenges on material synthesis, characterization, as well as the fundamental understanding of their chemical behavior.
The formation of most hybrid materials usually is caused by strong chemical interactions between inorganic species, such as ions and atoms, and small organic molecules.For instance, well-known hydrate compounds can be considered as a class of hybrids in which water molecules (H 2 O) are chemically integrated into inorganic crystal structures.Notably, CuSO 4 forms hydrates of the form CuSO 4 •n H 2 O where n can range from 1 to 7 [6 ].In these compounds, the lone-pair electrons in H 2 O form coordinate bonds with the d orbitals of transition metal hosts.NaCl, a classic ionic solid, can also form hydrates, especially under elevated pressure conditions.This phenomenon is driven by the screening of electrostatic potentials by highly polar molecules such as H 2 O.This interaction mirrors the solvation of NaCl in polar solvents, whereby water molecules surround and isolate individual sodium and chloride ions.
One of the more recent and notable examples of hybrid materials is organic-inorganic hybrid perovskites.These materials have gained significant attention due to their promising efficiency as photovoltaic materials.The formation of these perovskites is largely attributed to the strong ionic interactions between the negatively charged inorganic anions and the positively charged organic cations.This hybrid structure offers a promising route toward the design of next-generation energy materials [7 ].
In the current investigation, we aim to explore a distinctive new category of hybrid materials whose formation is not facilitated by strong chemical interactions between molecules and the encompassing inorganic species.Our study was inspired by recent research into the unique chemistry that emerges under high pressure.Under these conditions, the chemical properties of elements and the strengths of the homonuclear and heteronuclear bonds can change drastically, leading to the formation of many atypical compounds with non-intuitive compositions and structures, like NaCl 3 , Fe 3 Xe, and CsF 3 [8 -10 ].
Moreover, many recent theoretical and experimental studies have revealed that helium (He) can react with various ionic crystals to form stable ternary compounds under high pressure.In these compounds, He, despite being a noble gas, is integrated into the crystal structure without forming localized bonds with neighboring atoms.In this context, several pressure-stabilized He compounds have been predicted or synthesized, such as MgF 2 He, FeO 2 He, and Na 2 He [11 -13 ].These observations signal an unprecedented capacity of He to interact with other substances under high-pressure conditions, despite its known chemical inertness.
In this study, we aim to expand upon this concept of helium insertion w ith a v iew to investigating the potential for inserting small molecules, e.g.H 2 , N 2 , CO 2 , NH 3 , H 2 O, and CH 4 et al ., into ionic compounds under pressure.The examples are carefully chosen to represent homonuclear and heteronuclear molecules, non-polar and polar molecules, and molecules with different sizes.Unlike helium, which is highly resistant to forming chemical bonds, these small molecules generally have a much higher chemical reactivity.However, our comprehensive crystal structure search studies reveal a compelling general chemical trend under high-pressure conditions.Despite their higher chemical activity compared to helium, most of these small molecules surprisingly retain their integrity when inserted into prototypical ionic compounds, such as NaCl.This leads to the formation of thermodynamically stable hybrid compounds, opening up exciting new possibilities in exploring unconventional hybrid materials.
An immediate application of our study is the understanding of the composition and structure of planets' interiors.All large planets consist of both covalently bonded molecules and solid-state minerals, segregated into different layers with large dispersive regions.Mars and Venus consist of three layers: a rocky (iron-nickel/sulfide) core in the center, a silicate mantle in the middle and an outer composed mainly of gaseous carbon dioxide (CO 2 ) and nitrogen (N 2 ).The interiors of Uranus and Nep-tune are mainly composed of a rocky (silicate/ironnicke l) core in the center and icy mantle (water, ammonia, and methane, along with traces of other hydrocarbons, but not necessarily for these molecules).There are abundant ionic compounds and small molecules in the boundary of mantle-outer layer (Mars and Venus) and core-mantle (Uranus and Neptune) boundaries.Our investigations on structures and physical properties of salt-SM hybrid compounds wi l l provide key information to the understanding of the interior structure and dynamics of these planets.Especially, minerals and SM both exist in a variety of ocean exoplanets' interiors, namely salty high-pressure ice (NaCl-H 2 O) [14 ,15 ].
To this end, we performed extensive structure searches to examine the possibility of forming hybrid salt-SM materials under high pressure [16 ,17 ].Our simulations uncovered a variety of stable NaCl-SM compounds with various compositions at a wide pressure range of up to 200 GPa.Among all predicted structures, NaCl(H 2 ) 4 and NaCl(N 2 ) 5 compounds could be stable at pressures of 38 and 36 GPa, respectively.Our current findings not only establish a new family of hybrid salt-SM compounds for further design and discovery of intriguing materials, especially for the combination of inorganic compounds and small organic molecules under high pressure, but also provide crucial implications for the understanding of the interior of exoplanets.In addition, NaCl is often employed as a pressuretransmitting medium and thermal insulator in diamond anvi l cel l experiments due to its high compressibility, low strength, and limited chemical reactivity as well as its general ease of use.The present results also provide useful suggestions on the effectivity and the chemical limit of a solid NaCl pressure-transmitting medium.

RESULTS AND DISCUSSION
First, we conducted an extensive exploration on the high-pressure phase diagrams of NaCl-SM (SM = H 2 , N 2 , CO 2 , NH 3 , H 2 O, CH 4 ) hybrid compounds at high pressures by performing swarmintelligence based CALYPSO [18 ,19 ] structure searches.The thermodynamic stability of NaCl-SM hybrid compounds is evaluated from their formation enthalpies relative to the dissociation products of NaCl + SM.In principle, NaCl-SMs are ternary compounds and may have a great amount of different possible decomposition products.However, all structure searches show that there is no sign of SM dissociation at the pressures considered in this study, which allows us to treat SM as a single reaction unit.On the other hand, NaCl has the most stable stoichiometry among all metal halides at ambient conditions and the high-pressure range used in this study [20 ,21 ].Therefore, we reduce the stability evaluation to a pseudo-binary reaction of m NaCl + n SM → m NaCl (SM) n .For NaCl, the known body-centered cubic and face-centered cubic structures are considered in their corresponding stable pressure ranges.The phases of P 6 3 / mc , ε-N, I -42 d , P2 1 2 1 2, Pbcm and Pnma are considered for H 2 , N 2 , CO 2 , NH 3 , H 2 O, and CH 4 in their corresponding pressure, respectively.As a consequence, our simulations identified a series of hitherto unknown NaCl-SM (SM = H 2 , N 2 , CO 2 , NH 3 , H 2 O, CH 4 ) hybrid compounds under high pressure, as shown in Fig. 1 .In this work, we take hybrid NaCl(H 2 ) 4 compound as an example, since this structure is predicted to become thermodynamically stable at sub-megabar pressures of 38 GPa.
As shown in Fig. 1 , at 20 GPa, NaClH 2 is the only stable composition, but at a higher pressure of 100 GPa, both NaClH 2 and NaCl(H 2 ) 4 (containing four H 2 molecules) ( Fig. S1) become stable.As the pressure increases from 20 to 100 GPa, the formation enthalpy of NaClH 2 increases from −49 meV/atom to −192 meV/atom.These results indicate that pressure could significantly promote the formation of stable salt-H 2 compounds with a higher H 2 content.Besides, we also attempted to study other hybrid salt-H 2 stabilities under high pressure, such as KI-H 2 , RbI-H 2 and RbBr-H 2 ( Fig. S1).The stable pressure range of the hybrid salt-H 2 compounds are summarized in Fig. S1d.It is remarkable that KI forms a hybrid compound with H 2 at the lowest pressure of 1 GPa, and therefore the possibility of future experimentation is greatly stimulated.It is noted that the NaClH x compounds have been recently synthesized [22 ], in which our predicted NaClH 2 with P 6 3 / mmc symmetry [23 ] agrees well with the structure observed in the experiment ( Table S2 and Fig. S13).This greatly encourages us to study other NaCl-SM hybrid compounds, although our predicted NaCl(H 2 ) 4 with Pm symmetry was not observed.We found that the hybrid salt-H 2 compounds become less stable with increasing cation radius from NaCl to CsCl.In contrast to the trend of the cation, the increased size of the anion increases the stability of the hybrid salt-H 2 compounds.These results indicate that the difference in the sizes of the cation and the anion is a determining factor in the formation of stable hybrid compounds with H 2 molecules, which naturally explains the reason for the ultra-low-pressure threshold of KI-H 2 ( Fig. S1c).This offers an unexpected prospect for the storage of hydrogen in the salt, and other similar ionic compounds.
The above calculations fully include the effects of van der Waals (vdW) interactions and zero-point energies (ZPE), which may play critical roles in determining the stability of these predicted hydrogen-rich compounds.After considering these effects, the formation energies of the hybrid salt-H 2 compounds remain almost unchanged, while transition pressures of these compounds are slightly different from those without considering these effects.For example, the phase transition pressure for the Pc to P 6 3 / mmc structures is 14.6 GPa as compared to the static lattice PBE result of 16.5 GPa for NaClH 2 ( Fig. S27), and the formation pressures of NaClH 2 are 15.3 and 17.4 GPa with and without vdW interactions, respectively.Once the ZPE is included, however, the formation pressure for NaClH 2 becomes 19.7 GPa.
Our structure searches reveal unique, and in many cases surprisingly simple, structural features for the salt-SM hybrid compounds.Depending on the concentration of H 2 , our predicted structures represent three different ways of inserting H 2 molecules into the lattices of the salt: at atomic sites (Fig. 2 a, NaClH 2 with P 6 3 / mmc symmetry), inside tubes (Fig. 2 b, NaI(H 2 ) 2 with P 2 1 2 1 2 1 symmetry), and between layers (Fig. 2 c, NaBr(H 2 ) 2 with Pmmn symmetry) with increasing hydrogen content.In addition to the study on thermodynamic stability of these predicted structures, we have also investigated their dynamic stability, and the results indicate most of them are found to be recoverable when the pressure is partially or completely released.For example, the phonon calculations reveal no imaginary vibrational modes for the NaClH 2 in Pc phase ( Fig. S37a), KIH 2 in the P 6 3 / mmc structure ( Fig. S14a), and RbI(H 2 ) 2 in the Pmmn structure ( Fig. S14c) at ambient conditions.
We wi l l investigate the chemical interaction between SM and the ionic sublattices by various electronic structure analysis methods, including a rigid band structure analysis, Bader's quantum theory of atoms in molecules (QTAIM) [24 ], electron localization functions (ELFs) [25 ], projected density of states (PDOS), and crystal orbital hamilton population (COHP) [26 ].Our study wi l l focus on NaCl-H 2 compounds.The bonding features in all salt-SM compounds are quite similar.
The band structure of NaClH 2 at ambient pressure shows a large energy band gap of nearly 5.6 eV (Fig. 3 a).This value is close to the NaCl band gap under ambient pressure.In order to verify the effect of electroneutral H 2 insertion on the electronic structure of the ionic sublattice, we also constructed a model system of NaClH 0 , in which all the H 2 molecules are removed from the system.By comparing the band structures of NaClH 2 with NaClH 0 , as shown in Fig. 3 a, the results show that the H 2 molecules do not significantly interfere with NaCl bands around the Fermi energy, which also suggests weak interactions between the H 2 molecule and the ionic sublattice.Two groups of valence are heavily involved in H 2 insertion in NaClH 2 , the upper one ranges from the Fermi level (0 eV) to −2.5 eV, and the lower one ranges from about −4.9 eV to −7.1 eV.After checking the projected components, we found that the upper groups are mainly the Cl 3p orbitals, whereas the lower group are mainly the H 1s orbitals.The bands in the upper group correspond to the Cl 3p bands in NaClH 0 (dashed red lines).The insertion of H 2 into the NaCl lattice alters these bands to a considerable amount due to the occupation of the interstitial sites surrounded by Cl − ions.On the other hand, the H 2 bands only slightly overlap with Cl − and Na + states, indicating a weak interaction.This feature wi l l be further proved by the following ELF and COHP calculations.Interestingly, the similar band structure and PDOS in all the salt-(H 2 ) n and other NaCl(SM) n systems ( Figs S18-S29) confirmed the weak interaction between NaCl and SM, implying the insertion nature of molecules in ionic compounds.The Bader QTAIM charge [24 ] calculations ( Table S1) support a model of NaCl ionic compounds inserted by neutral H 2 molecules.The Bader charges on Na, Cl, and H atoms are found to be 0.88, −0.85, and 0.33 under 0 GPa, respectively.It is typical that the Bader charges are significantly smaller than the nominal charges.For example, the Bader charges of Na and Cl in NaCl crystal under 0 GPa are 0.82 and −0.82.For the same reason, the small residual charges found on H do not indicate the charge transfer between H 2 and NaCl lattices.As a matter of fact, the insertions of He into NaCl and elemen-tal Na also show a small charge of 0.83 under 50 GPa and of 0.81 under 100 GPa.
In conjunction with the Bader charge results, the ELF [25 ] calculations (Fig. 3 b-d) reveal that inserted H 2 molecules do not strongly bond with surrounding Na + and Cl − ions.The ELFs of Pm phase at 50 GPa (Fig. 3 c) and Cmmm phases for NaCl(H 2 ) 4 at 100 GPa (Fig. 3 d) unambiguously show the same bonding nature of Na-Cl and H-H.The low ELF values between Na and Cl confirm the ionic bonding nature (Fig. 3 b), whereas the high ELF values between neighboring H atoms are in accordance with the fact that neighboring H atoms are covalently bonded and form H 2 molecules.On the other hand, the low ELF values reveal very weak local bonding between H 2 molecular units and the surrounding ions in both compounds.Similar results are found for other salt-SM hybrid compounds predicted in this work.
To further examine the bonding strength, we calculated the COHPs [26 ] and the integrated COHPs (ICOHP) between neighboring atoms in these compounds.The results show the full occupation of the H-H bonding states and the empty H-H antibonding states, revealing that the bonding feature of H 2 molecules is not largely influenced while inserted into the NaCl crystal (Fig. 3 e).For comparison, we calculated the ICOHP values up to the Fermi level for H-H pairs in NaCl-H 2 compounds and in solid hydrogen at various pressures.The ICOHP value of −2.9 pairs/eV of the H-H bond reveals the strength of H-H in salt-H 2 systems.This is also in conformity with the fact that the bond length (0.72 Å) of H 2 in salt-H 2 is close to that (0.74 Å) in pure solid H 2 .
If the inserted molecules do not interact strongly with the surrounding ions, what mechanism drives the formation of these unusual hybrid compounds under pressure?We thoroughly investigate the origin of the thermodynamic stability of the salt-SM compounds, starting from the split of the reaction enthalpies of NaCl-H 2 compounds as shown in Fig. 4 , H , into the internal energy changes U and the pressure-volume terms PV.
For NaClH 2 , although U is positive at low pressure whereas PV is negative, both terms decrease with increasing pressure, contributing to the contin-uous decrease of H , leading therefore to the stabilization of the hybrid compound.In particular, the PV term decreases much more significantly than U , indicating that the formation of XY-H 2 hybrid compounds is primarily driven by volume reduction.For compounds with higher hydrogen composition, such as NaCl(H 2 ) 4 , the decrease in PV is less significant.However, it sti l l makes major contributions to the reaction enthalpy and the stabilization of inserted compounds.It is worth noting that many recently predicted and synthesized metal hydrides, such as NaH n [27 ], LiH n [28 ] etc , contain H 2 molecules in their crystal lattices.Many of these hydrides can be viewed as XH-H 2 hybrid compounds, further demonstrating that the small molecule inclusion is an extensive phenomenon under high pressure.
Most other molecules, such as H 2 O, NH 3 , CH 4 , and CO 2 , behave simi larly to H 2 whi le inserted into NaCl, i.e. they do not bond strongly with the neighboring Na + and Cl − ions.In contrast, while searching low enthalpy structures of N 2 -inserted NaCl, we found a striking phenomenon, namely, N 2 molecules wi l l decompose in NaCl and form pentazolate anions (cyclo-N 5 − ) (Fig. 2 d) inserted into the NaCl crystal at pressures ranging from 36 to 83 GPa ( Fig. S16a).Calculated ICOHP values for the N-N bond at 50 GPa is −1.56 eV/pair, indicating that there is a strong covalent bond of N-N in cyclo-N 5 − units ( Fig. S16c).The Bader QTAIM calculations show charges of 0.86, 0.28, and −0.114 for Na, Cl, and N atoms, which suggest a large charge transfer from Cl − to cyclo-N 5 − .Therefore, the formation of NaCl(N 5 ) 2 is due to the oxidation of Cl − by cyclo-N 5 − .The band structure and PDOS of C 2/ c -NaCl(N 5 ) 2 at 50 GPa ( Fig. S16b) show that the electron states of the cyclo-N 5 − contribute to both valence and conduction bands in a large energy range around the Fermi level and overlap with states of Na and Cl.Pentazolates are promising candidates for high-energy-density materials that do not release harmful products upon decomposition.However, its syntheses are hindered by low stability and usually involve additions of metal and organic stabilizers, forming complex structures.Our research shows that the commonly known ionic compounds such as NaCl that are easy to obtain and handle can be used to react directly with N 2 under pressure and produce compounds containing large compositions of pentazolate anions.As a matter of fact, the predicted NaCl(N 5 ) 2 structure has nearly 71% weight ratio of nitrogen in the form of cyclic N 5 units.The ambient-pressure decomposition of NaCl(N 5 ) 2 is estimated to possibly release 3.78 kJ•g −1 energy per NaCl(N 5 ) 2 unit.Moreover, our molecular dynamics simulations confirm the metastable feature of this predicted structure at ambient conditions ( Fig. S17).The present results may open a new avenue to discover high-energy-density materials of polynitrogen compounds.
Although the crystal structure search predicts that H 2 O molecules insert in NaCl similarly to H 2 , the potentially stronger interactions between the H 2 O molecule and surrounding ions merit an indepth analysis of its electronic structure.As shown in Fig. 1 and Fig. S16d, the predicted monoclinic structure of (NaCl) 2 H 2 O with Pnma symmetry (Fig. 2 e) can become stable at a high pressure of 102 GPa.Furthermore, the band structure and PDOS for the C 2/ c structure at 200 GPa ( Fig. S16e) show that the entire energy range is dominated by the bands of the Cl anion.A large band gap of nearly 4.2 eV appears between the valence and conduction bands.The calculated COHP ( Fig. S16f) clearly shows that the O-H bonding states are occupied, whereas the antibonding states are not.Correspondingly, the ICOHP value with −1.0 eV/pair of O-H bonds supports the strong covalent nature of O-H covalent bonding in H 2 O molecules.Despite the stronger interactions with the surrounding ions, H 2 O maintains its major molecular bonding features while inserted into NaCl.
Our results provide an important piece of knowledge for the understanding of the interior structure and dynamics of planets.Uranus and Neptune are called 'ice giants' in our solar system because they contain significant amounts of icy materials (mainly H 2 O, CH 4 , and NH 3 ).In addition, abundant ionic compounds exist in the rocky layer of Mars, Venus, Uranus, and Neptune.It is widely accepted that most exoplanets may assume similar multi-shell structures consisting of layers of molecular and ionic rocky materials.Thus, the molecule-rock interaction and its properties under pressure may govern the interlayer structures of most intermediate-mass exoplanets.While our study demonstrates that typical ionic compounds such as NaCl can react w ith ic y materials, the general chemistry it reveals can potentially be extended to other compounds, including the rock materials, such as binary and ternary oxides, in the planet's interior.Indeed, recent works uncovered that magnesium oxide-water compounds could become stable at high pressure and offered a renewed understanding of planetary interiors [29 ].

CONCLUSIONS
In conclusion, we demonstrate a unique chemical phenomenon under pressure using the firstprinciples calculations and the crystal structure search method: many covalently bonded molecules can form stable compounds by mixing into ionic crystals while maintaining their molecular integrity.The phenomenon is manifested in many striking predictions.For example, H 2 is predicted to react with NaCl to form NaClH 2 and NaCl(H 2 ) 4 at 20 and 38 GPa pressures, respectively.A similar reaction could happen at a much lower pressure of 1 GPa if H 2 is inserted into potassium iodide (KI), which facilitates future experimental synthesis and applications by bringing the reaction pressure down to the sub-GPa level.Similar insertions are also found for heteronuclear molecules such as H 2 O, NH 3 , and CH 4 , despite whether the molecule is polar or nonpolar.The crystal structure search also predicted the insertion of C 2 H 6 into NaCl under high pressure, indicating the wide range of the phenomenon and the possibility that large organic molecules might be chemically reserved inside the rocky interior of planets.Last, to our surprise, the most stable one among all molecules in this work, N 2, is found to transform chemical ly whi le inserted into NaCl and become cyclo-N 5 − which oxidizes NaCl.This reaction could become a new route to synthesize and stabilize pentazolates, green energy materials with high energy density.Our results suggest that the molecular and rock layers in the interiors of large planets may exhibit large regions in which the molecules and solid compounds diffuse into each other and are chemically mixed.

Crystal structure prediction
Our structure searching simulations are performed through the swarm-intelligence based CALYPSO method [18 ] via a global minimization of free energy surfaces merging ab initio total-energy calculations as implemented in the CALYPSO code [19 ] and random structure searching as implemented in the AIRSS code [30 ,31 ].These methods are specially designed for unbiased global structural optimization, and have been benchmarked on various known systems [31 -34 ].

Total energy calculations
Total energy calculations were performed in the framework of density functional theory within the Perdew-Burke-Ernzerhof [35 ] parameterization of generalized gradient approximation [36 ] as implemented in the VASP (Vienna Ab Initio simulation package) code [37 ].The projector-augmented wave (PAW) method [38 ] was adopted with the PAW potentials taken from the VASP library where d electrons are treated as valence electrons for alkali elements.The use of the plane-wave kinetic energy cutoff of 1200 eV (for H 2 molecules) and 800 eV (for NH 3 , H 2 O, CH 4 , CO 2 , N 2 molecules) and dense k-point sampling, adopted here, were shown to give excellent convergence of total energies (within ∼1 meV/atom).We explored the zero-point energy effects on the formation energy using the phonopy code [39 ].

NFigure 1 .
Figure 1.Phase stabilities of various NaCl-SM hybrid compounds.Enthalpies of formation of NaCl-SM (SM = H 2 , N 2 , CO 2 , NH 3 , H 2 O, CH 4 ) under several pressures.Dotted lines connect the data points, and solid lines denote the convex hull.

Figure 2 .
Figure 2. Structures of salt-SM hybrid compounds.Three different ways of inserting SM molecules inside the lattices of the NaCl: at atomic sites in NaClH 2 with P 6 3 / mmc symmetry at 20 GPa (a); inside tubes in NaI(H 2 ) 2 with P 2 1 2 1 2 1 symmetry at 50 GPa (b); and between layers in NaBr(H 2 ) 2 with Pmmn symmetry at 30 GPa (c).(d) NaClN 10 with C 2/ c symmetry at 50 GPa; (e) (NaCl) 2 H 2 O with C 2/ c symmetry at 300 GPa.The red, pink, grey, gold and green spheres represent O, H, N, Na and Cl atoms, respectively.

Figure 3 .
Figure 3. Calculated electronic properties for NaClH n at various pressures.(a) The electronic band structure and PDOS of P 6 3 / mmc phase for NaClH 2 at 20 GPa.In the left panel, the black solid lines are the electronic band structure of NaClH 2 ; the red dashed lines are those of NaClH 0 in which all the H 2 molecules are removed from the NaClH 2 structure.The black and red dashed lines show the Fermi energies of NaClH 2 and NaClH 0 .The right panel presents the projected DOS of NaClH 2 .(b-d) The calculated ELFs of NaClH 2 with P 6 3 / mmc phase at 20 GPa, NaCl(H 2 ) 4 with Pm symmetry at 50 GPa, and NaCl(H 2 ) 4 with Cmmm symmetry at 100 GPa.(e) Calculated COHP and ICOHP of NaClH 2 at 20 GPa.

ΔUFigure 4 .
Figure 4. Energy contributions to the formation of NaCl-H 2 hybrid compounds.Reaction enthalpies (top panel), internal energy changes (middle panel), and PV term changes (bottom panel) for the formation of NaCl-H 2 hybrid compounds at different pressures.