New Calcium Hydrides With Mixed Atomic and Molecular Hydrogen

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C: Energy Conversion and Storage; Energy and Charge Transport

New Calcium Hydrides With Mixed Atomic and Molecular Hydrogen Ajay K Mishra, Takaki S Muramatsu, Hanyu Liu, Zachary M. Geballe, Maddury Somayazulu, Muhtar Ahart, Maria Baldini, Yue Meng, Eva Zurek, and Russell J. Hemley J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05030 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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The Journal of Physical Chemistry

New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen Ajay K. Mishra1, Takaki Muramatsu1, Hanyu Liu1, Zachary M. Geballe1, Maddury Somayazulu1,2, Muhtar Ahart2, Maria Baldini1, Yue Meng3, Eva Zurek4,*, and Russell J. Hemley2,*

1 2

Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

Institute of Materials Science and Department of Civil and Environmental Engineering, The George Washington University, Washington DC 20052, USA 3

HPCAT, Carnegie Institution of Washington, Advanced Photon Source, Argonne, IL 60437, USA

4

Department of Chemistry, University at Buffalo, State University of New York, Buffalo, NY 14260, USA Email: [email protected], [email protected]

Abstract: Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca2H5 composition, which is characterized by a distinctive vibration at 3789 cm-1 at 25 GPa.

The

observed Raman spectrum is in close agreement with first-principles calculations of a Ca2H5 structure characterized by a lattice containing a central layer of H2 molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has composition CaH4 and a denser body centered tetragonal (bct) structure. This weakly metallic phase also contains molecular-like H2 units and its spectroscopic, as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the HH bond in these hydrides is a result of the Ca-H2 interaction, analogous to what occurs in molecular compounds where H2 binds side-on to a d-element, such as the Kubas complex. 1 ACS Paragon Plus Environment

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I. Introduction High pressure can be used to synthesize compounds with novel stoichiometries and crystal structures that are not stable at atmospheric conditions, some with exotic electronic structures and properties.1,2 Intense research has been devoted to high pressure hydrides because of their potential as hydrogen storage media3 or as high temperature superconductors,4 and because these compounds are likely to employ novel bonding strategies while adopting hitherto unobserved crystalline lattices.5 The last decade has witnessed an explosion in the application of a priori crystal structure prediction techniques coupled with first-principles calculations towards the phase diagrams of high-hydrides as a function of stoichiometry and pressure.6-9 By now, most binary systems have been studied theoretically. However, because experiments at very high pressure are challenging to carry out, only a handful of high-hydrides have been synthesized so far. These include the hydrides of lithium (LiH2 and LiH6), 10 sodium (NaH3 and NaH7),11 iron (FeH2, FeH3 and FeH5),12,13 platinum (PtH),14-16 tungsten (WH),17, 18 iridium (IrH3),19 rhodium (RH2),20 niobium (NbH2.5-3),21 and lanthanum (LaH10).22 Superconductivity at high temperatures has been observed in compressed samples of hydrogen sulfide; 23 theory and experiment appear to have converged on the superconducting phase as being Im3m H3S.24 Computations suggest that the superconductivity observed in compressed phosphine25 is due to several phases with PH, PH2, and PH3 stoichiometries.26-29 The synthesis of these hydrides requires extensive exploration of P-T space, and the development of new synthesis routes. The only known hydrides of the alkaline earth metals in group II are the dihydrides, AH2 (A=Mg, Ca, Sr and Ba), which adopt either an orthorhombic structure with the space group Pnma for Ca, Sr, and Ba, or a rutile-type structure with space group P42/mmm for MgH2.30,31 Earlier high pressure experiments demonstrated that CaH2 undergoes a phase transformation from an orthorhombic to a hexagonal phase (P63⁄mmc) around 18 GPa.30,31 First-principles calculations have predicted that the pressures at which the polyhydrides of the alkaline earths, AHn with n > 2, start to become stable with respect to the classic dihydrides and elemental H2 decrease going down the group from 100 GPa for Mg to 20 GPa for Ba.6 For Mg,32 Sr,33,34 and Ca35 the tetrahydride was the first stoichiometry found to resist decomposition into the classic dihydride and elemental hydrogen. SrH4 and CaH4 were predicted to assume an I4/mmm symmetry structure below 84 and 180 GPa, respectively. At higher pressures the lowest enthalpy SrH4 and CaH4 phases were isotypic with the Cmcm symmetry phase that was predicted for MgH4. Crystal structure prediction techniques have 2 ACS Paragon Plus Environment

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found that ScH4,36-39 YH4, CeH4 and PrH438 become stable under pressure and their preferred structures are isotypic with the I4/mmm geometry predicted for SrH4 and CaH4. Calculations also suggest that the tetrahydrides of Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu in the I4/mmm structure lie on the convex hull at experimentally attainable pressures.38 Therefore, the wealth of computational data available indicates that the I4/mmm structure is common to most alkaline earth and rare earth tetrahydrides under pressure. In order to verify the structures that have been predicted in the Ca-H2 phase diagram under pressure and to explore the synthesis of higher hydrides, we undertook a systematic high P-T study of this binary system using a combination of resistively heated and laser heated diamond-anvil cell (DAC) techniques combined with in-situ synchrotron x-ray diffraction and optical spectroscopy methods. Together with supporting theoretical techniques, we demonstrate the existence of new calcium polyhydride phases with unusual chemical bonding in a sequence of increasingly hydrogen-rich compounds CaH2, Ca2H5 and CaH4. Unlike hexagonal CaH2, an ionic stable compound to 50 GPa, which does not contain molecular hydrogen in its lattice, Ca2H5 and CaH4 contain both atomic (H) and molecular (H2) hydrogen in their structures, with the H2 units interspaced between CaH2 layers in the CaH4 lattice and for the Ca2H5 structure a central layer of H2 molecules is oriented along the (100) direction. A progressive weakening of the H2 molecule within these structures is found with pressure that reflects increased interaction between the Ca and H2 lattices with density. Such a weakening of the H-H bond upon coordination of H2 to a d-metal center has been observed in a number of molecular systems40, the most iconic being the W(CO)3(PR3)2(H2) Kubas complex.41 Electronic structure calculations illustrate that the bond lengthening mechanism under pressure in the solid state resembles the one responsible for lengthening the H-H bond in Kubas-like molecular complexes.

II. Methods A. Experimental Techniques Calcium metal (from Sigma Aldrich) was loaded in a sample chamber made up of a thin pre-indented tungsten sheet in a symmetric DAC in an Ar environment. H2 gas was loaded inside the sample chamber at 200 MPa pressures. Angle dispersive x-ray diffraction measurements were carried out at beam lines 16-BM-D and 16-ID-B of HPCAT at the Advanced Photon Source (APS), Argonne National Laboratory (ANL) using a 3 ACS Paragon Plus Environment


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monochromatic x-ray beam with a typical wavelength of 0.3066 and 0.4206 Å. In-situ pulsed laser heating experiments at megabar pressures were performed at 16 ID-B beamline at HPCAT at APS on 3-5 µm thick Ca samples loaded in symmetric type DACs in the presence of excess hydrogen. Pressure was measured from the Raman edge of the strained diamond anvil42 and from the hydrogen vibron.43 Diffraction images were integrated using the Dioptas program44 and the resulting one-dimensional patterns were fitted to structural models using the GSAS software.45 The temperature was measured by collecting the thermal radiation from the sample using Peltier cooled CCD and was quantified using a Planck fit to the black body radiation from the heated portion of the sample.46 In addition to powder x-ray diffraction, we have used custom built off-line micro Raman and IR spectroscopy to characterize the synthesized samples. Resistively heated diamond cell experiments used a Pt-Ir (80:20) wire heater wound around the diamonds.

B. Theoretical Techniques A priori crystal structure prediction calculations were carried out using the particle swarm optimization technique as implemented in CALYPSO47,48 and the XtalOpt evolutionary algorithm (EA) release 11,49,50 wherein the The XtalComp51 algorithm was used for duplicate matching and the RandSpg52 algorithm was used for random structure generation. Because it can never be guaranteed that a global optimization scheme has found the global minimum for all but the simplest systems, we employed two different methods to search for the ground state structures. The structure relaxations were performed using density functional theory using the Perdew-Burke-Ernzerhof (PBE)53 generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP) 5456

code, a plane wave code employing the projector-augmented wave (PAW) potentials.57,

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A cutoff energy of 500 eV was used in the structure searches, and 600 eV was used otherwise. Phonon calculations were carried out to verify dynamical stability using VASP combined with the supercell approach as implemented in phonopy. 59 The band structure and optical absorption of CaH4 at 120 GPa were computed using PBE as well as the HSE06 screened hybrid functional. 60 The bonding was analyzed by calculating the crystal orbital Hamilton populations (COHP) 61 and the negative of the COHP integrated to the Fermi level (-ICOHP) using the LOBSTER package.62

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III. Experimental Results We have synthesized CaH2 by heating a mixture of Ca and H2 at 3.2 GPa to ~770 K. The Raman modes for this compound agree with those reported in the literature30, 31 for CaH2. Further heating and/or compression of synthesized CaH2 in excess H2 does not make any new polyhydride. CaH2 loaded with H2 shows that it undergoes a phase transition to a hexagonal phase at 21.4 GPa with a volume drop of ~6% (Fig. S1) 63 instead of forming any new higher hydride. Because of the difficulty of synthesizing higher hydrides from mixtures of CaH2 and H2, we have used an elemental mixture of Ca and H2 to synthesize the higher hydrides of calcium at particular thermodynamic (P, T) conditions using resistive / laser heating and DACs as described in the following paragraphs.

Figure 1: Diffraction patterns of two new Ca-H compounds. (a) panel is for Ca2H5, lower and middle patterns show the diffraction pattern at 22 GPa after rapid compression and heating to 780 K at this pressure respectively. The top shows Le Bail fit of the diffraction pattern at room temperature after heating at 22 GPa. The symbol shows the observed diffraction pattern and the solid red line is the Le 5 ACS Paragon Plus Environment


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Bail fit to the observed data while the solid green line shows the residuals. The blue ticks indicate the diffraction peaks corresponding to the new monoclinic phase. The quality of fit parameters for this refinement are Rp = 10% and χ2 = 1.4. (b) The diffraction pattern of Ca+H2 before and after in-situ laser heating. The lower two traces show the diffraction pattern of Ca in the simple cubic phase at 76 GPa and in the tetragonal phase at 116 GPa before laser heating. The diffraction peaks marked with ’*’ in the third pattern from the bottom indicate the formation of a new compound with tetragonal symmetry after laser heating at 116 GPa to ~1600 K. The Le Bail fit to that diffraction pattern is shown at the top. The blue ticks show the diffraction peaks from the new synthesized phase. The quality of fit parameters is Rp = 15.4 % and χ2 = 2.9.

The calcium precursor was rapidly compressed in H2 to 22 GPa to avoid the formation of the dihydride CaH2. The diffraction pattern before heating at that pressure can be indexed to bcc Ca coexisting with the low pressure fcc phase as shown in the bottom panel of Fig. 1a. Upon heating, the Ca reacts with hydrogen and forms a new phase (middle panel of Fig. 1a). The reaction is observed to be complete by ~780 K. These new diffraction peaks do not match any of the known CaH2 phases. The recorded diffraction pattern after quenching the sample to room temperature can be indexed to a monoclinic lattice (space group C2/m) with lattice parameter as a = 11.452(4) Å, b = 3.373(3), c = 10.946(5) Å and β = 96.621° (4) as mentioned in Table 1.

Table 1: Comparison of structural parameters of CaHn compounds obtained from experiment and theory.

Compound

Lattice parameter

EOS Parameters

Ca2H5

a= 11.452 (4) Å b = 3.373 (3) Å, c = 10.946 (5) Å, β = 96.621 (3)° Monoclinic system

K0 = 158.3 (1) GPa, This work (exp.) K0’=4.0, (Synthesized at ~ 22 3 GPa, 780 K) V0=471.1 (16.1) Å

a= 11.213 Å b = 3.486 Å, c = 10.954 Å, β = 97.00°

Reference

This work (Theory at 25 GPa)

C2/m


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CaH2 (in H2)

RT phase-Pnma HP phase-P63/mmc

CaH4

a = 2.906 (5) Å c = 4.993(1) Å, Tetragonal system

K0 = 33.9(1.6) GPa, This work K0’=5.2(0.7), (Experiment) K0 = 29.2(2.1) GPa, K0’=4.7(1.2), K0 = 165 (4) GPa, This work (exp.) K0’ = 0.03 (0.01), (Synthesized at ~ K0'' = -0.04 116 GPa, 1600 K) V0 = 76.2 (2.4) Å3

a = 2.838 Å c = 5.059 Å I4/mmm

This work (Theory at 120 GPa)

Fig. 1b shows the diffraction pattern of Ca+H2 before and after laser heating. The sample was quickly pressurized to 40 GPa to minimize formation of any of the lower hydrides. At 76 GPa before laser heating the calcium is in the simple cubic phase as shown by the diffraction pattern with indexed diffraction peaks in the bottom panel of Fig. 1b. On compression Ca transforms to a tetragonal phase at 116 GPa before laser heating as shown by the second pattern in Fig 1b. At this pressure laser heating to ~1600 K was performed using short heating pulses. The diffraction pattern collected after quenching shows several new diffraction peaks marked with ‘*’ in the third pattern of Fig. 1b. The new diffraction peaks are indexed to a tetragonal system with lattice parameters a = 2.906 (5) Å, c = 4.993 (1) Å as noted in Table 1; the space group is determined to be I4/mmm. The stability range of each structure was constrained by varying the pressure after synthesis (mostly decompression runs were preferred since any further compression especially after laser heating usually led to diamond failure). The monoclinic phase is stable on decompression to 14.5 GPa but transforms to a new phase (or mixture of phases identified to be predominantly CaH2 and H2) below this pressure as shown in Fig. S2a.63 The lowest pressure at which this phase could be synthesized in our experiments was 22 GPa. The I4/mmm structure is stable from 116 to 135 GPa upon compression, and to 60 GPa upon decompression as show in Fig. S2b .63 Raman and IR spectra of the two phases show that they each contain quasi-molecular H2 units with anomalously weak H-H bonds. A sharp peak near 3789 cm-1 at ~ 25 GPa in the monoclinic phase (Fig. 2) can be identified as an H2 vibron whose frequency is 11% lower 7 ACS Paragon Plus Environment


than in pure H2 at the same pressure.64 Broad Raman modes are evident at a lower frequency (470 cm-1) indicative of Ca-H vibrational modes that overcome the weaker H2 rotational modes (cf. Fig. S3a).63 The presence of a weak Raman mode is evident when one zooms in on the 800-1200 cm-1 spectral region (cf. Fig S3a). However, one cannot expect to obtain excellent agreement between the calculated spectrum with the observed one because of the weak Raman scattering cross sections of some of the modes. On decompression, the 3789 cm-1 vibron stiffens

and disappears below 12 GPa (see Fig. S3b),63 in concordance to the observed change in diffraction pattern. The I4/mmm structure synthesized at 116 GPa contains an even softer H2 vibron at 3384 cm-1, which is 18% softer than the vibron of pure H2 at 116 GPa (Fig. S3b). The observed frequencies in these two compounds are plotted alongside the ones observed in several other compounds in Fig. S3d .63 The main point emphasized here is that the two phases synthesized display both Ca-H as well as a much softer H-H vibrational mode thereby indicative of encapsulation of molecular H2 in a CaH2 environment, albeit with a high degree of interaction as evidenced from the red shifted H-H vibrational frequencies. As discussed later in greater detail, the downshift of the H2 vibron has previously been observed in several molecular complexes with H-H bonds that have been stretched as a result of side-oncoordination to a d-electron metal.40

Figure 2: Raman characterization of the Ca-H hydrides. (lower panel) Comparison between the experimental Raman spectra collected after heating (red) for synthesized Ca2H5 and the calculated 8 ACS Paragon Plus Environment

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Raman spectrum for Ca2H5 (black sticks). (upper panel) Raman spectrum of synthesized CaH4 compound (blue) compared with the theoretically calculated Raman pattern (black sticks).

Figure 3: Pressure - volume equation of state for calcium and calcium hydrides. The experimental data for calcium and calcium hydrides are shown by symbols. The corresponding Birch-Murnaghan (BM) fits of the observed P-V data are shown by solid lines. Dash-dot lines are results from the density-functional theory calculations while the dashed lines represent various assemblage volumes such as Ca+2H2 (blue line) and Ca+(5/4)H2 (black line). The orange dotted line is data from Fujihiso et al. .71

The measured unit cell volumes constrain the stoichiometry of each phase. At ~30 GPa the volume per formula unit of monoclinic (C2/m) CaHn is ~1.2 Å3 larger than that of CaH2 (Fig. 3), suggesting 2 < n < 3. Thus, for the volume occupied per hydrogen atom to be ~ 2.4 Å3 the stoichiometry of the new compound turns out to be Ca2H5 (CaH2.5). The new tetragonal (I4/mmm) phase of CaHn shows a large expansion relative to pure Ca at 116 GPa. For example, the shortest Ca-Ca distance in the tetragonal phase is 2.91 Å, which is 28% larger than the shortest Ca-Ca bond length in pure simple cubic Ca. The volume per Ca atom is 9.14 Å3 larger than that of pure Ca. Assuming that this excess volume is occupied by n = 4 hydrogen atoms gives an effective atomic volume for hydrogen VH of 2.3 Å3/atom. This 9 ACS Paragon Plus Environment


value of VH is slightly larger than that determined for other hydrides such as IrH3 (2.0 Å3), FeH3 (1.8 Å3), AlH3 (1.5 Å3), and LaH10 (2.1 Å3) at 120 GPa22. In contrast, if we take n = 3 or n = 5 then it gives either 3.1 Å3/atom or 1.86 Å3/atom at this pressure. The former is larger than experimental systematics for related hydrides at these pressures. The latter is within the range found for iron hydride12 but lower than that found for most others; including alkali and alkaline earth hydrides.11 We conclude that the most consistent experimentally determined hydrogen stoichiometry for this phase is CaH4. Infrared absorption spectra obtained from this compound shows IR-active, H-H vibrational modes that are strong and the UV-VIS-MIR absorption measurements suggest that CaH4 is an optically transparent (dark red) material (Fig. S4).62 One of the most frequently used methods for characterizing thin films in the UVVIS absorption are so-called ‘Tauc’ plots .65 The extrapolation of the linear part of the onset of absorption is used to estimate the band gap (see Fig S4 of the SI). This method suggests a band gap of ~1.5 eV at 120 GPa. DFT calculations carried out with the non-hybrid PBE functional (Fig. S12),63 as well as the screened hybrid HSE06 functional (Fig. S13)63, on the other hand, suggest that at 120 GPa CaH4 is weakly metallic. Moreover, the absorbance spectrum obtained with both functionals (Fig. S15)63 shows the metallic nature of CaH4, and the gap opens with the release of pressure as shown in Fig. S14.63 The discrepancy can be attributed in part to the fact that the Tauc plot analysis strictly applies to disordered/amorphous materials and also to the approximations in the exchange and correlation functionals employed in the DFT calculations.

IV. Theoretical Calculations The lowest point on the convex hull at 25 GPa (Fig. S5)63 is the known P63/mmc CaH2 structure. At 20 GPa its shortest H-H contacts are 2.30 - 2.34 Å. The Ca2H5 structure shown in Fig. 4 lies 20 meV/atom above the 0 K convex hull at 25 GPa, but phonon calculations showed that it was dynamically stable (Fig. S7).63 Because a C2/m symmetry CaH4 phase was also a stable point on the convex hull, we calculated the change in Gibbs free energy associated with the decomposition of Ca2H5 into CaH2 and CaH4 (Fig. S6).63 The results suggest that even though Ca2H5 is metastable up to 1000 K at 20 GPa, higher temperatures tend to stabilize this stoichiometry. A unit cell of this C2/m symmetry structure contains 8 Ca atoms, 2 H2 molecules with bond lengths of 0.79 Å at 20 GPa, and 16 monoatomic hydrogens, so its formula can be written as (CaH2)8(H2)2. The monoclinic (C2/m) structure of Ca2H5 is a subgroup of the hexagonal (P63/mmc) structure of CaH2, and it 10 ACS Paragon Plus Environment

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consists of a central layer of hydrogen molecules oriented along the (100) direction. At 25 GPa it is semiconducting, consistent with experiment, with a PBE band gap of 1.66 eV (Fig. S8).63 The simulated diffraction pattern, and computed Raman spectrum of C2/m Ca2H5 (Fig. S3 and Fig. 2) were in good agreement with the experimental results. The calculated convex-hull of the Ca-H system at 140 GPa (provided in Fig. S10)63 shows that compounds with the Ca3H, Ca2H, CaH, CaH2, CaH4 and CaH6 stoichiometries are thermodynamically stable, and we find them to be dynamically stable as well. Previous calculations predicted the same CaH2, CaH4 and CaH6 structures identified in our study to be stable at 150 GPa.35 The preferred CaH4 structure adopts the I4/mmm space group. The tetragonal unit cell of CaH4 (Fig. 4) contains four monoatomic hydrogen atoms along with two molecular units (Ca2(H)4(H2)2) whose bond lengths at 120 GPa, 0.81 Å, are somewhat longer than those calculated for the C2/c phase of molecular hydrogen at this pressure (0.74 Å). The presence of molecular and monoatomic hydrogen atoms in this phase has previously been rationalized by Wang et. al. by considering the “formal” number of effectively added electrons (EAE) that would result from a full transfer of the valence electrons of Ca to the H2 molecules, 1e/H2. 35 I4/mmm CaH4 contains one H2 molecule per Ca atom. Thus, formally, the two valence electrons of the alkaline earth must be transferred to the σ* anti-bonding orbital of the other H2, concomitantly breaking the H-H bond and forming two H- units. The Bader charges, which typically underestimate the formal charge, illustrate that +1.1e is transferred from each Ca. The hydridic and molecular hydrogens have a charge of -0.5e and 0.05e, respectively. A comparison of the calculated Raman spectra for the I4/mmm symmetry CaH4 phase at 120 GPa, with the experimental spectra at 116 GPa (Fig. S3)63 further supports this structural assignment. In particular, we note the excellent agreement between the theoretical and experimental values obtained for the H2 vibron mode, as shown in Fig. 2. The simulated diffraction pattern for this phase was also in good agreement with the one obtained experimentally.



Figure 4: Conventional cells of CaH2, Ca2H5 and CaH4 at 20, 25 and 120 GPa, respectively. The big brown circles represent the Ca atoms while the small green circles correspond to the hydrogen atoms. The H-H distances in the dihydrogen molecules measure 0.79 Å in Ca2H5 and 0.81 Å in CaH4 at these pressures.

V. Discussion Fig. 5 illustrates that the H-H bond length, and therefore strength, in elemental molecular hydrogen, as well as within the H2 units in C2/m Ca2H5 and I4/mmm CaH4, becomes progressively weaker with increasing pressure. However, even by 200 GPa the nearest neighbor H-H distances in the binary phases are still ~0.3 Å shorter than in atomic hydrogen. This figure reflects two classes of hydride compounds, ones that have nearest neighbor H-H bond lengths resembling those that would be found in atomic hydrogen and another class of hydrides whose lattices contain molecular hydrogen. In the first class of compounds, the bond lengths in FeH5 and LaH10 lie close to those calculated for atomic metallic hydrogen.13 In the second class of compounds, doping with electropositive elements helps to promote the dissociation of the quasi-molecular H2 units. This is an example of chemical “precompression”5: in elemental hydrogen higher pressures are required to attain HH distances and densities than in a compound where the hydrogen has been chemically precompressed via its interaction with the “dopant” atom. As a result, the amount of external physical pressure required for metallization is postulated to be lower for the system where hydrogen has been chemically precompressed. Because the above reactions between calcium and hydrogen documented here generate new interactions between the valence electrons of


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Ca and H2 that may facilitate the dissociation and metallization of the hydrogen sublattice, one can consider the hydrogen in these systems to be chemically precompressed.

Figure 5: H-H nearest-neighbor distances in various hydrides compared to the calculated results for CaH4 and Ca2H5 (CaH2.5). The H-H distances for atomic solid hydrogen and molecular hydrogen, H2, are from the Cs-IV (I41/amd) and C2/c phases, respectively.67

Incorporation of H2 into the Ca2H5 and CaH4 structures is associated with a weakening of the H-H molecular bond. This is illustrated by the observation of softer vibron frequencies at ~3800 cm-1 and at ~3384 cm-1 in the Ca2H5 and CaH4 system at 25 and 116 GPa, respectively. These frequencies are low compared to the H-H vibron in pure H2, implying the H2 molecule inside the CaH2 lattice has been chemically precompressed. The theoretically calculated H-H distances also support this bond weakening: as compared to a bond length of 0.74 Å in pure H2 at ambient pressure the nearest neighbor H-H distance increases to 0.79 Å in the Ca2H5 structure at 20 GPa, and 0.81 Å in the CaH4 structure at 120 GPa. These H-H distances are not achievable in pure H2 up to very high pressures.66 The anomalously large H-H distances and small vibron frequencies show that the chemical 13 ACS Paragon Plus Environment


interaction of H2 with Ca lengthens and weakens the H-H bond, pushing the molecular like hydrogen nearer to the dissociated state. Indeed, the H-H distances in Ca2H5 and CaH4 are only 33 and 26% smaller than the H-H distance calculated for metastable monatomic metallic hydrogen at the same pressure.67 In contrast, the H-H distances in pure molecular H2 are 38 and 32% smaller at the same pressures. In this way, the Ca-H2 chemistry revealed here is a manifestation of chemical precompression of molecular hydrogen. Further work is needed to investigate this behavior at higher pressure and its relationship to metallization and possible superconductivity. The discovery of the Kubas complex, W(CO)3(PR3)2(H2),41 wherein an H2 molecule binds side-on to the metal center, was unexpected because it was assumed that such systems could not be isolated at ambient conditions. In this complex the H-H bond is weakened relative to the free H2 molecule, yielding an H-H distance of ~0.89 Å and a stretching mode of 2690 cm-1. By now hundreds of stable H2 containing compounds have been synthesized and characterized at atmospheric pressures.40 Some examples are WH4(H2)4 (with an H-H distance of ~0.88 Å and stretching mode of 2500 cm-1),41 Pd(H2)1,2,3, FeH2(H2)3, RuH(H2)4, and RuH2(H2)4.

68

In these complexes the H2 donates σ electrons to a vacant d-orbital, and

back bonding from an occupied metal d-orbital to the H2 σ* anti-bonding orbital also occurs.69 Both mechanisms weaken the H-H bond strength. Fig. 4 illustrates that in CaH4 each Ca atom is surrounded in a “side-on” fashion by four H2 molecules (and every H2 is surrounded by four Ca atoms) with Ca-H distances of 2.05 Å at 120 GPa. Pressure is known to induce s to d transfer in Ca70, which is evident in a plot of the projected densities of states (PDOS) of CaH4 (Fig. S16) wherein the amount of occupied Ca 3d character substantially exceeds the 4s character. The calculated crystal orbital Hamilton populations (COHPs) for CaH4 shown in Fig. S17c are indicative of strong intramolecular H-H σ-bonding interactions around -10 eV below the Fermi level (EF). Moreover, the feature in the COHP plot around -7 eV below EF is indicative of a partial filling of the H-H σ* anti-bonding orbitals. Indeed, the negative of the COHP integrated to the Fermi level (-ICOHP), which is a measure of the bond strength, for the H2 unit within CaH4 is somewhat smaller than the value calculated for an isolated H2 molecule with the same bond length (4.2 eV vs. 5.0 eV). Thus, the Ca-H2 interaction renders the intramolecular H-H bond within CaH4 at 120 GPa to be 16% weaker than in a hypothetical H2 molecule with an H-H bond length of 0.81 Å. The interaction between the Ca p semi-core states and both types of hydrogen atoms is stabilizing, whereas the interaction between the Ca d states and 14 ACS Paragon Plus Environment

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hydrogen is generally destabilizing (see the PDOS plot in Fig. S16, and the COHP plot in Fig. S17b). The –ICOHPs of -0.13/-0.24 eV between a Ca atom and a molecular/monoatomic hydrogen atom reveal that overall the Ca-H interaction is slightly destabilizing. The aforementioned features in the COHP plots suggest that the mechanisms leading to a lengthening of the H-H bond in the Kubas complex and other related molecular complexes play an important role in compressed I4/mmm CaH4. However, under pressure other factors also become important. The energy of the Ca p semi-core states increases and the Ca-H distances decrease thereby facilitating weak, but stabilizing, H-Ca-H interactions (cf. the COHPs around -20 eV below EF in Figs. S16b-d). Previous theoretical studies carried out by a priori crystal structure prediction searches found that SrH433, 34, ScH436-39, YH4, CeH4 and PrH438 are stable under pressure, and that these are isotypic with I4/mmm CaH4 in their respective pressure ranges. Moreover, theoretical predictions based on structural analogies suggested that all the remaining I4/mmm symmetry rare-earth tetrahydrides except La and Yb are stable species on the convex hull at pressures attainable in DACs38. Therefore, the CaH4 phase synthesized here is found to adopt the I4/mmm structure that has been theoretically predicted as being stable in the phase diagrams of the alkaline earth and many of the rare earth hydrides under pressure. By analogy, we hypothesize that some of the other predicted MH4 compounds can be synthesized using laser heated DAC methods such as those employed in this study. Moreover, it is likely that they will contain quasi-molecular H2 units that are elongated as a result of electron transfer from the electropositive element (because many of the rare earths can adopt formal oxidation states that are larger than +2), as well as via σ → d donation and d → σ* backdonation between H2 and the “dopant” metal via a Kubas-like mechanism.

VI. Conclusions High P-T experiments on the Ca-H system show that it forms two new stoichiometric compounds, Ca2H5 at 22 GPa and 780 K and CaH4 at 116 GPa and 1600 K. Global optimization techniques coupled with density functional theory calculations predict that these phases adopt C2/m and I4/mmm symmetry structures, respectively. Both phases contain mixed monoatomic and molecular hydrogen units, and their computed Raman spectra and xray diffraction patterns are in good agreement with those obtained experimentally. The H2 units within these phases are longer than those found within elemental hydrogen as a result of



the Ca-H interaction, which decreases the amount of external pressure required to attain H-H distances and densities that would be found within metallic hydrogen. First principles calculations illustrate that the elongation and weakening of the intramolecular H-H bonds in I4/mmm CaH4 are in part a result of H2-σ → Ca-d donation and Ca-d → H2-σ* backdonation, resembling the bond lengthening mechanism that occurs in molecular complexes where an H2 molecule is coordinated in a side-on fashion with a d-metal center, such as in the W(CO)3(PR3)2(H2) Kubas complex. Theoretical studies predict that most of the alkaline earth and rare earth metal tetrahydrides will adopt this I4/mmm symmetry structure with elongated H-H bonds under pressure. It may therefore be that a Kubas-like bond lengthening mechanism is another way to achieve chemical precompression of hydrogen, i.e. this mechanism can facilitate the realization of an atomic hydrogen lattice inside the cationic metal lattice at much lower pressures than those required to achieve metallic atomic hydrogen. Fundamental studies of the structures and electronic structure of the high pressure polyhydrides reveal just how different chemistry can be at high pressures, and help develop a chemical perspective that can be employed to rationalize structure and bonding at these extreme conditions.

Supporting Information Further experimental details, Raman spectra and x-ray diffraction patterns, pressure dependence of H2 vibron frequencies, and the IR absorbance of CaH4 at 116 GPa. Further computational details, calculated convex hulls, electronic and phonon band structures, Raman spectra, optical absorption of CaH4, projected densities of states, crystal orbital Hamilton populations and structural coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001057. The infrastructure and facilities used are supported by the U.S. Department of Energy / National Nuclear Security Administration (Award DE-NA-0002006, CDAC, and Award No. DE-NA0001974, HPCAT). YM acknowledges the support of DOEBES/DMSE under Award DE-FG02-99ER45775. HPCAT operation is supported by DOE16 ACS Paragon Plus Environment

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NNSA under Award No. DE-NA0001974, with partial instrumentation funding by NSF. The Advanced Photon Source is operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. E. Z. acknowledges the NSF (DMR1505817) for financial support and the Center for Computational Research (CCR) at SUNY Buffalo for computational support.



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New Calcium Hydrides With Mixed Atomic and Molecular Hydrogen

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