Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides.

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the H2δ- units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the H2δ- bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to H2δ- and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH4, where the precompression of H2δ- units at 50 GPa results in bond lengths comparable to that of pure H2 above 275 GPa.

The application of high pressure provides a mechanical route to break covalent bonds and obtain otherwise unachievable chemical states. Hydrogen is the archetypal example: the 1935 prediction of a molecular–atomic transition at extreme compression has inspired decades of experimental research.[1−6] Current data suggests that hydrogen still retains its molecular bond up to pressures of at least 360 GPa,[7−9] pushing the limits of conventional diamond anvil cell experiments. Chemical dopants can assist in the dissociation of quasi-molecular H2 units by two mechanisms: electronic effects (charge transfer) and confining the molecule to small interstitial locations, the so-called chemical precompression.[10−12] This principle has been extensively used in the production of high temperature superconducting hydrides.[13,14] However, an understanding of the mechanisms involved in H2 precompression and how the H2 bonding distances are adjusted by chemical doping have yet to be achieved. In the majority of diamond anvil cell studies reporting the synthesis of metal hydrides, usually only the structure of the host metal can be experimentally determined. This is due to the comparatively weaker X-ray scattering of hydrogen and to the fact that the synthesis conditions of many interesting hydride candidates cannot be routinely reached in neutron diffraction studies. As such, often important information, such as the hydrogen content and the interatomic hydrogen distances, is reliant on theoretical calculations.[15−17] If the hydride contains H2 units, then Raman spectroscopy can be an efficient method to investigate the high pressure response of the H–H distances of the molecules. Out of the many known binary metal–hydrogen compounds (group I to XVII), only sodium trihydride and calcium tetrahydride are known to host quasi-molecular hydrogen.[18−20] In these cases, the Raman frequency of the H–H stretching mode (vibron) exhibits a downshift compared to pure molecular hydrogen, and this is related to a lengthening of the bond.

The alkaline-earth metals, except for radium, react readily with hydrogen, forming binary hydrides, MH2 (M = alkaline-earth metal).[21] CaH2, SrH2, and BaH2 undergo the same structural phase sequence on compression, from the cotunnite structure (Pnma, phase I),[22−25] to the hexagonal Ni2In structure (P63/mmc, phase II). A further transition to an AlB2-type structure (P6/mmm, phase III), has been experimentally observed in BaH2.[21,23,24,26] The structural transitions of the dihydrides are shifted to lower pressures for the heavier alkaline earth metals; e.g., phase III of CaH2 is predicted at 180 GPa, while phase III of BaH2 is observed between 40 and 50 GPa.[21−26]

Through a combination of both high temperature and high pressure, interesting compositions were observed in the alkaline-earth-metal–hydrogen system. Superconducting CaH6 was produced at pressures greater than 160 GPa.[27,28] Strontium polyhydrides have been reported above 70 GPa with a range of stoichiometries: SrH22, SrH9, Sr8H46, SrH6, Sr3H13, and Sr2H3.[29] Barium polyhydrides have also emerged, with Ba8H46 at 50 GPa[30] and BaH10, BaH6, BaH21–23, and BaH12 all above 90 GPa.[31]

Here, we synthesize CaH4, SrH4, and BaH4 in a series of diamond anvil cell experiments and study the influence that the metal host has on the H2 bond. While a combination of both temperature and pressure are required to transform the H2 embedded dihydrides into CaH4 (50 GPa, 1600 K) and SrH4 (40 GPa, 1600 K), BaH4 is formed on room temperature compression alone above 40 GPa. Synchrotron X-ray diffraction measurements of all three compounds revealed isostructural phases adopting I4/mmm symmetry. However, I4/mmm BaH4 appears as a metastable intermediate phase: both temperature and time lead to a transformation to the theoretically stable Ba8H46, while BaH4 forms upon decompression. All of the tetrahydrides share common Raman signatures indicating the presence of units within the metallic lattice. There is a more pronounced downshift of the vibrational Raman frequency with increasing size of the alkaline earth cation, and this can be related to an increase in the H–H intramolecular distance. Electron localization function (ELF) calculations suggest that at a given pressure, the radius of the ELF basin around the molecule becomes smaller with increasing atomic number of the alkaline earth cation, causing the lengthening of the intramolecular bond. As a result, the intramolecular hydrogen bond lengths of the alkaline-earth tetrahydrides at relatively low pressure (20–50 GPa), are comparable to those of ultradense elemental hydrogen at 220–275 GPa.

Elemental Ca (99.9%, Alfa Aesar), Sr (99.99%, Sigma-Aldrich), and Ba (99%,Sigma-Aldrich) (or BaH2 (99.5%, ABCR)) were loaded into diamond anvil cells in an argon atmosphere glovebox and subsequently gas loaded with research grade hydrogen (99.9995%) at 0.2 GPa (see Supporting Information for detailed methods). After sample loading, X-ray diffraction measurements indicated that dihydrides (MH2) were formed, adopting the Pnma space group. Upon compression, all dihydrides were observed to undergo a phase transformation to P63/mmc space group, with the transition pressures and phase sequences in agreement with the existing literature, Figure S1.[22,23,26] Laser heating P63/mmc CaH2 in an H2 atmosphere at 52 GPa to temperatures of 1800 K, yielded the previously reported tetragonal I4/mmm CaH4, see Figure a (and Figures S1–S4).[19] After laser heating SrH2 within excess H2 at a pressure of 44 GPa to temperatures of 1700 K, we observe a complete transformation to I4/mmm SrH4 upon quench; this phase persisted upon decompression to at least 4 GPa (Figure a and Figures S1, S2, S3, and S5). Both the structure, and the experimentally observed volumes as a function of pressure (Figure b), are in good agreement with our own (Tables S1–S4) and previous DFT calculations.[12,33] In a subsequent experimental run, we prepared a sample of Sr embedded in ammonia borane. Ammonia borane can act as a hydrogen source, decomposing at high temperature to produce H2 and BN.[34] After an initial laser heating at 75 GPa, SrH2 is formed, and after a subsequent heating, SrH2 transforms to another compound adopting an structure; see Figure S6. We find that the lattice parameters and volumes agree well with those for the recently reported cubic SrH9; see Figure S1.[29] Upon decompression, we find that SrH9 does not decompose to at least 30 GPa.

Figure 1

(a) Representative X-ray diffraction patterns of the alkaline-earth tetrahydrides. Tick marks indicate Bragg peaks from the labeled phases. CaH4, SrH4, and BaH4 are refined to tetragonal I4/mmm at 52, 44, and 45 GPa, respectively. Upon decompression, I4/mmm BaH4 (and Ba8H46)[30] undergoes a phase transition to BaH4. (b) Volume per alkaline-earth atom as a function of pressure. Filled symbols correspond to this work, CaH4 (brown circles), SrH4 (green diamonds), I4/mmm BaH4 (dark blue squares), and BaH4 (light blue squares). Brown crosses and brown asterisks correspond to CaH4 from Wu et al.[19] and Mishra et al.,[20] respectively. The open green square is the predicted value of SrH4 by Hooper et al.[33] The other open symbols correspond to our DFT calculated volumes. (c) c/a ratio as a function of pressure for the I4/mmm structural types.

On room temperature compression of BaH2 + H2, we observe the formation of BaH4I4/mmm above 40 GPa (Figure  and Figures S1–S3 and S5). Although CaH4 and SrH4 also adopt an I4/mmm structure, the c/a ratio of 2.17 for BaH4 is considerably larger than that of either SrH4 or CaH4, where c/a ≈ 1.6 (see Figure c). In all experiments, the transformation to I4/mmm BaH4 is incomplete, and a considerable fraction of BaH2 was present. Instead of a complete transformation to I4/mmm BaH4 with time, there is a kinetically sluggish transformation to the documented clathrate type-I Ba8H46. If the I4/mmm BaH4 sample is laser heated, then there is a complete transformation to Ba8H46.[30] Upon decompression of either I4/mmm BaH4 or Ba8H46, a new phase emerges below 27 GPa that can be indexed to an structure and is stable to at least 4 GPa (see Figures and S3 and S5). The volume per Ba atom as a function of pressure of the structure is similar to that observed for I4/mmm BaH4, Figure b.

Structural searches using AIRSS[35] result in a number of candidate structures for BaH4 with similar energy (all small bandgap insulators, Figures S7–S9). I4/mmm BaH4 is always competitive but never stable, while is found to be the most stable structure below 25 GPa. The instability of I4/mmm BaH4 in both DFT and experiments suggest that this is an intermediate phase formed as hydrogen migrates through the Ba lattice. The I4/mmm and predicted structures are subgroups of in which the symmetry-breaking is generated by the orientation of the H2 molecules. In the absence of detectable hydrogen scattering, the XRD pattern can be equally well indexed to either or . Starting from the structure, we observe in ab initio molecular dynamics calculations that H2 at 300 K went through a number of reorientation transitions on a picosecond time scale. Such reorientations indicate that BaH4 will have symmetry at 300 K, with rotationally disordered molecules.

To investigate the bonding, we study the ELF.[36] Free electrons have an ELF value close to 0.5, while higher ELF values indicate that the electrons are localized, e.g., covalent bonds, lone pairs, and atomic shells.[36] For the empty I4/mmm Ca/Sr/Ba structures, we identify two ELF maxima types, one centered on the 2b sites and elongated along the c axis (pseudo-octahedral interstices, Figures and S10) and another type with smaller ELF/volume located on the 4d sites, occupying tetrahedral interstices (Figures and S10). The Ba ELF topology is equivalent to that of I4/mmm Ba: octahedral ELF maxima with higher ELF values and tetrahedral sites of smaller ELF, Figures b and S10. The octahedral maxima of the metallic lattices correspond to the sites hosting the molecules in the tetrahydrides, and the tetrahedral maxima to the sites hosting the H– anions (Table S5). In -Ba and in I4/mmm-Ca/Sr the octahedral maxima are almost twice as high as for the tetrahedral ones, showing that these are good sites to localize electrons. However, I4/mmm Ba has a lower ELF at the octahedral site, with a value almost as low as the tetrahedral site, suggesting that the units are unfavorable; see Table S5. Indeed experimentally, over time, the I4/mmm BaH4 structure transforms to Ba8H46,[30] which has only tetrahedral sites, and to BaH4 in decompression, where the high-ELF octahedral site is more favorable for a molecular hydrogen unit.

Figure 2

(a) Top: ELF isosurfaces (ELF = 0.44, 0.38, 0.30) for the pure I4/mmm M sublattices (M = Ca, Sr, Ba) of MH4 at 50 GPa (in yellow). Ca, Sr, and Ba atoms are represented as purple, green, and blue spheres, respectively. The ELF values at the octahedral and tetrahedral maxima in Ba (0.477, 0.318) are similar to and lower than those of Ca (0.794, 0.472) and Sr (0.705, 0.406). Bottom: Conventional unit cell of I4/mmm MH4. (b) Top: ELF isosurfaces (ELF = 0.36) (in yellow) of the Ba sublattice at 10 GPa. The ELF values at the octahedral and tetrahedral maxima are 0.653 and 0.384, respectively. Bottom: crystal structure of BaH4 at 10 GPa. To analyze the experimental , BaH the space group is used. BaH4 appears as the most stable in calculations at low pressures, and would correspond to but with orientationally ordered H2 molecules.

All of the alkaline-earth tetrahydrides exhibit intense Raman activity, (see Table S6 for DFPT predictions[38]). Experimentally, we see the double-degenerate E2 mode between 400 and 1100 cm–1), which is attributed to modes associated with the angular degrees of freedom of the , referred as libration.[39,40] As seen in Figure a for all the tetrahydrides, the libration hardens substantially with pressure. Moreover, there is also a strong dependency of the frequency on the host alkaline earth metal atomic number, with Ba exhibiting the highest upshift. Previous works have calculated the effect on the Raman modes of a single H2 molecule when trapped in cages of various sizes.[39] By this means, compression of the cage leads to an upshift in the libron frequency.[39,40] This agrees well with the alkaline-earth tetrahydrides hosting H2 molecules within octahedral cages, and the libron frequency increases going from larger basin of Ca to the smaller basin of Ba, Figure and Table S5. Both I4/mmm and BaH4 exhibit similar libron Raman frequencies and pressure dependency, which is understandable as in both configurations the H2 molecules are hosted by octahedral cages.

Figure 4

(a) Raman shift as a function of pressure of the E2 librational modes. The dotted line is a guide to the eye. (b) Raman shift as a function of pressure of the A1 vibron. The dashed line corresponds to pure H2,[3] the dotted lines provide a guide to the eye. (c) Intramolecular H–H bond length estimates as a function of pressure. Symbols represent the bond lengths (r) calculated using the empirical relationship νr3, where ν is the Raman vibrational frequency.[37] The dashed line represents the corresponding values of pure H2.[3] Hollow symbols and asterisks correspond to H–H distances obtained from DFT calculations.

The most intense band, above 3000 cm–1 (Figure ), corresponds to the A1 mode resulting from the H–H intramolecular stretching of the , vibron (see Figure and Figure S11).[19,33] The vibron frequency and bandwidth depends strongly on the hosting cation. The Raman spectra presented in Figure were measured during decompression of the tetrahydrides (see also Figure S12). At 25 GPa, the intramolecular vibron has a frequency of 3800 cm–1 for CaH4, 3750 cm–1 for SrH4, and 3500 cm–1 for BaH4. The vibron broadens with pressure, an effect observed in pure H2 and attributed to the reduced lifetime of the molecule.[3]Figure b shows the Raman shift of the vibron as a function of pressure together with a comparison with pure H2.[3] In all the alkaline-earth tetrahydrides, the vibron softens rapidly in compression, more markedly as the alkaline earth atomic number increases.

Figure 3

Raman spectra of the alkaline-earth tetrahydrides upon decompression: (a) CaH4 + H2, (b) SrH4 + H2, and (c) BaH4 + H2. The A1 vibrons are marked with solid diamonds. Solid circles mark the E2 libron, while the empty circle indicates the E2 mode corresponding to an H2 libron coupled to a H– lattice mode observed only in BaH4. Crosses and asterisks mark the rotational modes and vibron of excess pure hydrogen, respectively.[3]

When pure molecular hydrogen is compressed to above 30 GPa, there is a turnover in the intramolecular vibron frequency and a lengthening of the H–H bond.[41−45] For instance, the H–H bond lengthens from 0.718 at 6 GPa in phase I to 0.84 Å when entering phase IV at 225 GPa,[3,46,47] and this is manifested by a 650 cm–1 drop in the H2-ν1 frequency.[3] Comparing the vibron in the alkaline tetrahydrides with the vibron of pure H2-ν1, we observe that pressures of only 60 GPa for CaH4, 40 GPa for SrH4 and 20 GPa for BaH4 are sufficient to reach values seen in H2 at 225 GPa (Phase IV). Using the empirical formula relating the Raman shift of the H–H vibron with the bond lengths (r), νr3 proportional to a constant, and the H–H bond length of pure H2 at 6 GPa as a reference,[37] we plot the H–H bond lengths as a function of pressure in Figure c, together with the bond lengths calculated from our DFT calculations. If the theoretical optimization of the internal coordinates of I4/mmm BaH4 is constrained to the experimental lattice parameters, the H–H distances are larger (e.g., 2.2% at 50 GPa) but still retain the molecular bond. Even at the lowest pressures reached experimentally in this study, all the tetrahydrides show H–H distances corresponding to those of pure H2 above 220 GPa. Compared to other metal hydride systems, NaH3 exhibits a downshift of the vibron, at around 4100 cm–1 in contrast to 4250 cm–1 in pure H2 at 50 GPa.[18] This further demonstrates that the effect of H2 chemical precompression is more pronounced in systems with heavier metallic hosts.

It has been long suggested that electron transfer from an electropositive metal to would contribute to the stretching and weakening of the H–H bond via the population of the σ* orbitals.[11,12] We have calculated the ELF basin charges and the Bader charges using the Quantum Theory of Atoms in Molecules (QTAIM[48]) for the different MH4 theoretically optimized structures, see Figure S13–S15. As shown as a function of pressure in Figure a the alkaline earth metal donates electrons to H– and . Ca and Sr charges are similar and higher in value than the Ba charge.[49−51] The charge of the hydrogen anion, , decreases in compression (in absolute numbers—toward neutral charge) regardless of the structure. Within the I4/mmm tetrahydrides, the charge is highest (the most negatively charged) in Sr and then decreases for Ca and furthermore for Ba, while BaH4 is smaller than its tetragonal analogue. Both the positive charge of the metallic cations and the negative charge of the must contribute to the population of the σ* orbitals, Figure a. However, within the I4/mmm structure, the of CaH4 is more negatively charged than that in BaH4. Consequently, the experimentally observed longest H–H bond of BaH4 cannot just be explained by the charge transfer from the metal to the σ* orbital of .

Figure 5

(a) Charges as a function of pressure based on the ELF topology of the metals (M, x+ is the charge), hydrogen atoms located in the tetrahedral sites (H, y– is the charge) and molecular hydrogen located in the octahedral sites (, δ – is the charge). b) Radius of the ELF basin along the line joining its middle point and the alkaline-earth metals across the I4/mmm ab plane, and the octahedral diagonal in . Insert: A metallic atom of the I4/mmm ab plane corner hosting one in the center perpendicular to the plane, blue is for Ba, green for Sr and purple for Ca, H2 molecule is pink. c) Radius of the ELF basin across the I4/mmm c axis. Insert: I4/mmm c axis with 2 metallic atoms on the corners hosting one in the center along the axis. The black arrows represent the ELF basin radius.

The topological analysis of the ELF can offer further insight into the chemical origin of the frequency downshift and associated bond elongation.[52] In particular, we explore the ELF basin radius of the molecule along the line joining its middle point and the alkaline-earth atoms (see Figure , parts b and c). The I4/mmm structures have two ELF radii to consider because of the pseudo-octahedral metallic coordination, one along the ab plane and one along the c axis (see Figure S10). The (used to model the experimental BaH4, Figure S10) has only one Ba-to- and therefore only one radius of the ELF basin of the molecule along the line joining its middle point in the pseudo-octahedral metallic coordination. This BaH4 Ba-to- distance is longer than the Ba-to- distance in the I4/mmm ab plane, but shorter than that in the I4/mmm c axis. Consequently, the resultant compression of the radius for the six equidistant Ba atoms is longer, justifying the similar Raman downshift of the vibron observed in compared to I4/mmm BaH4. Upon compression of the I4/mmm structure, the ELF basin radius in the ab plane decreases as the alkaline-earth atomic number increases (Figure b). Similarly, the ELF basin radius of the along the c axis also decreases upon compression, Figure c. We notice that, along the c axis, this distance is longer in BaH4 than in CaH4 and SrH4 (Figure b). As shown by the c/a ratio, c axis is not as compressible as a (Figure ). The shorter radius on the ab plane can be associated with a higher compression of the metals on that plane. As a result, there is an elongation of the bond along the c axis which increases with the atomic number. This correlates with the greater vibron downshift in BaH4 than for CaH4. Within the I4/mmm structures the unit lies along the c axis. Therefore, the upshift of the libron can be related to the increase in the c/a ratio in compression and with the atomic number of the cation. The larger the steric effect of the metal toward the , the more hindered the libration and the more upshifted the libron is.

In summary, our synchrotron X-ray diffraction measurements identify I4/mmm as a common structure adopted by the alkaline earth tetrahydrides, and all contain quasi-molecular hydrogen units within octahedral interstices. Comparison between the Raman spectra of the I4/mmm tetrahydrides demonstrates that the heavier alkaline earth cation causes a more pronounced downshift of the vibron Raman frequency, associated with a lengthening of the molecular hydrogen bond. Through the topological analysis of the ELF we find that the steric effect of the metal on the H–H bond is the dominating factor triggering the H–H bond lengthening. Although BaH4 has the largest lattice parameters, its ELF basin radius toward the alkaline-earth metal is the smallest along the ab plane, so the H2 is more strongly confined than in Ca or Sr. We propose that the H2 bond elongation is caused by H2 confinement in interstitial sites of the metals tearing apart the enclosed H2 molecule.