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Synthesis of Weaire–Phelan Barium Polyhydride

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Synthesis of Weaire–Phelan Barium Polyhydride pubs.acs.org/JPCL Letter Synthesis of Weaire−Phelan Barium Polyhydride Miriam Peña-Alvarez,* Jack Binns, Miguel Martinez-Canales, Bartomeu Monserrat, Graeme J. Ackland, Philip Dalladay-Simpson, Ross T. Howie, Chris J. Pickard, and Eugene Gregoryanz* Cite This: J. Phys. Chem. Lett. 2021, 12, 4910−4916 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, Ba8H46, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and first-principles calculations to determine that this compound adopts a highly symmetric Pm3̅n structure with an unusual 53 :1 hydrogen-to-barium ratio. This singular 4 stoichiometry corresponds to the well-defined type-I clathrate geometry. This clathrate consists of a Weaire−Phelan hydrogen structure with the barium atoms forming a topologically close- packed phase. In particular, the structure is formed by H20 and H24 clathrate cages showing substantially weakened H−H interactions. Density functional theory (DFT) demonstrates that ff cubic Pm3̅n Ba8H46 requires dynamical e ects to stabilize the H20 and H24 clathrate cages. trongly electropositive elements can readily donate 260 K at around 200 GPa.10,11,24 However, despite laying claim − electrons (E → E+ +e) into antibonding orbitals of to the discovery of high-temperature superconductivity, this S − molecular hydrogen (H2), weakening the H H covalent bond. study along with other recent discoveries of other hydrogen- 12 ff It has been proposed that the combination of electropositive bearing systems with high Tc su er from another elements and high pressures should promote the dissociation thermodynamic parameter, namely, pressure. These new of molecular H2 at lower pressures than in pure H2, often hydrogen-bearing materials require pressures in excess of 1−9 referred to as chemical precompression. Despite being 150 GPa for synthesis, a significant challenge to becoming 1 proposed in 2004, it is only in recent years that an abundance technologically relevant. Therefore, to achieve the ultimate of new hydrogen-bearing systems have been uncovered, goal of applicable room pressures and temperatures, one needs exhibiting superconductivity with high critical temperatures, first to find a pathway to nonmolecular hydrides at conditions 10−12 albeit at ultrahigh pressures. closer to the ambient. The alkaline and alkaline-earth metals are exemplary Electropositiveelementsathighpressurehavebeen − electropositive elements and can readily donate electrons to predicted to form highly symmetric hydrogen clathrates.1 9,26 σ* occupy the antibonding state ( ) of molecular hydrogen. Some alkaline and alkaline-earth elements form type-I − Early structural searches in the calcium hydrogen system (Ca clathrates structures with silicon (Si) with low-temperature H) predicted the formation of the hexhahydride CaH6 (space superconducting properties.22,23 However, these clathrates group Im43̅) based on a body-centered cubic (bcc) Ca lattice. have not yet been seen in its hydrogen analogue, neither Within this system the Ca atoms are enclathrated by hydrogen experimentally nor theoretically, so the effect of hydrogen cages interlinked, forming H4 units as building blocks of a substitution is yet to be explored.5,7,27 The pressures at which three-dimensional sodalite-like framework. The hydrogen the alkaline-earth polyhydrides stabilize correlate with their cages would present H−H distances significantly longer than 8 3,13,14 ionization potentials. Thus, electropositive elements are ideal those of molecular hydrogen. Subsequent theoretical to search for clathrate-based structures. Beryllium (Be) and H works predicted similar structural motifs in the rare-earth metal 2 form Be4H8(H2)2 crystallizing in a P63/mmc structure which hydrides YH6 (stable above 300 GPa) and LaH10 (stable above consists of corner-sharing BeH tetrahedra and H molecules 200 GPa).15,16 From the theoretical predictions, hydrogen 4 2 clathrate cages are motifs promoting superconducting proper- ties.17,18 Received: March 14, 2021 Clathrate cages appear in many diverse materials such as Accepted: April 16, 2021 − ices19,20 or silicon-based structures.21 23 In particular, hydro- Published: May 19, 2021 gen clathrates became relevant with the synthesis of LaH10 24,25 with H32 cages, and LaH10 is claimed to possess a superconducting critical temperature (Tc) between 250 and © 2021 American Chemical Society https://doi.org/10.1021/acs.jpclett.1c00826 4910 J. Phys. Chem. Lett. 2021, 12, 4910−4916 The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter occupying an interstitial site.28 Magnesium (Mg), less electropositive than Ca, was predicted to form MgH6 with the same sodalite-type structure as CaH6 at higher pressures of 300 GPa.29 The heavier alkaline-earth hydrides form with strontium (Sr) and barium (Ba), i.e., SrH6 and BaH6, at 150 and 50 GPa, respectively.5,6 However, the most favored structures for both did not resemble any clathrate-like 5,6 structure. In fact, BaH12 was recently synthesized at 90 − GPa, whose structure is characterized by H2, H3 units, and H12 − 5,27 chains with a Tc of 8 20 K. We explore the formation of highly symmetric hydrogen cages with barium atoms as the host, combining X-ray diffraction (XRD), Raman spectroscopy, density functional theory (DFT), and ab initio molecular dynamics. Heating Ba or BaH2 in hydrogen to 1200 K at 50 GPa leads to the formation of a novel compound. X-ray diffraction patterns indicate that the product adopts a highly symmetric Pm3̅n structure, which was stable on decompression to 27 GPa. Our approach shows its structure to be a type-I hydrogen clathrate with a Ba8H46 composition. Theoretical calculations reveal that the Ba8H46 hydrogen cage structure represents a Weaire−Phelan clathrate allowing the strange allocation of 53 hydrogen atoms per 4 30 fi barium. This unexplored con guration is a structure type Figure 1. Raman spectra on compression of BaH and H . Green formed by hydrogen polyhedral cages of 14 and 12 faces with 2 2 spectra are of a product formed just by compression of BaH2 in H2 at − ff H H distances varying between 0.86 and 1.4 Å. The room temperature, assigned as BaHx as the X-ray di raction does not 37 symmetric Pmn3BaH̅ 846structure is metallic, but it is allow its assignment. The red spectrum corresponds to Ba8H46 formed after laser heating at around 1200 K. The H2 rotons are dynamically unstable. Ab initio molecular dynamics demon- 35 strate its mechanical stability at room temperature and beyond. marked with a gray plus (+) symbol, while BaH2 excitations are marked with an asterisk (*) symbol. The spectral region between As the starting materials we use pure Ba (99%) or BaH − 2 1300 and 3250 cm 1 for the first- and second-order Raman spectra of (99.5%) loaded together with research grade H2 at 0.2 31,32 diamond are cut for clarity, and the whole spectra can be seen in GPa. Independent of the staring material we observe BaH2 Figure S3. and H2 after loading at below 1 GPa as seen by Raman spectroscopy and/or X-ray diffraction (Figures 1 and 2). Our results on BaH2 agree with those previously reported; at interestingly, the novel phase does not have any detectable ambient conditions BaH2 crystallizes in the contunnite Raman activity, a common feature among the metallic hydrides structure (Pnma), undergoing a transition to the hexagonal described to date.10,11,27 The were 10 repeated experimental − 34,35 Ni2In BaH2 II (P63/mmc), between 1.6 and 2.3 GPa, runs, using Ba and BaH with H as precursors, and the same − − 2 2 adopting a metallic AlB2-type structure, BaH2 III, at 40 50 structural changes were detected. The samples after laser 34,36 GPa. We did thorough searches to preclude contami- heating were thoroughly mapped by Raman and XRD analyses, nation, Figures S1−S3.InFigure 1 between 0.6 and 33 GPa, and no sign of presence of any other structure or hydrides the spectra show pure H2 and BaH2.H2 modes consist of the could be detected. 32,33 − 39 low-frequency rotational modes and E2g phonon and H H Structure solution of the novel phase by direct methods −1 31 stretching at around 4200 cm . BaH2 features are assigned reveals two Ba sites, 2a and 6d. Two-phase Rietveld refinement − − for BaH2 I at 0.6 GPa with Ba Ba stretching modes at 90 of the quenched diffraction patterns using only the Ba − − − cm 1 and Ba−H modes between 500 and 800 cm 134 36 and positions shows excellent agreement with the data, confirming 34−36 for BaH2 between 7 and 35 GPa. When pressure is the underlying Ba substructure (Figure 2(b, c)). The structure increased further, a novel phase referred to as BaHx appears. of this phase differs significantly from all postulated phases for fi Even though the changes in Raman spectra are quite signi cant barium polyhydrides including BaH6 (Fddd, P4/mmm, and (Figure 1), the X-ray pattern remains the same (Figure 2(a)). Imm2), BaH10 (C2c, Cm, and Cmmm), and BaH12 (Cmc21, 37 This will be described in a separate publication. Fm3̅m, and P2/m).5,27 The novel hydride has no detectable BaH2 in H2 was further compressed to 45 GPa and laser Raman activity (Figure 1 and Figure S3). These observations heated using a YAG laser (λ =1064nm),reaching are analogous to the changes observed during the formation of 10 27 temperatures of 1200 K and leading to dramatic changes in the lanthanide superhydrides or BaH12. The low X-ray the observed Raman and diffraction patterns (Figures 1 and 2). scattering power of hydrogen precludes direct determination of All observed peaks could be uniquely indexed to a mixture of atomic positions and exact stoichiometry of this novel BaH2 and a second phase with a primitive cubic unit cell a = compound.
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