ARTICLES PUBLISHED ONLINE: 22 NOVEMBER 2009 | DOI: 10.1038/NCHEM.445 Pressure-induced bonding and compound formation in xenon–hydrogen solids Maddury Somayazulu1 *, Przemyslaw Dera2, Alexander F. Goncharov1, Stephen A. Gramsch1, Peter Liermann3, Wenge Yang3, Zhenxian Liu1, Ho-kwang Mao1,3 and Russell J. Hemley1 Closed electron shell systems, such as hydrogen, nitrogen or group 18 elements, can form weakly bound stoichiometric compounds at high pressures. An understanding of the stability of these van der Waals compounds is lacking, as is information on the nature of their interatomic interactions. We describe the formation of a stable compound in the Xe–H2 binary system, revealed by a suite of X-ray diffraction and optical spectroscopy measurements. At 4.8 GPa, a unique hydrogen-rich structure forms that can be viewed as a tripled solid hydrogen lattice modulated by layers of xenon, consisting of xenon dimers. Varying the applied pressure tunes the Xe–Xe distances in the solid over a broad range from that of an expanded xenon lattice to the distances observed in metallic xenon at megabar pressures. Infrared and Raman spectra indicate a weakening of the intramolecular covalent bond as well as persistence of semiconducting behaviour in the compound to at least 255 GPa. ydrogen occupies a unique position in the periodic table as a result of its quantum nature and simple electronic structure, and the prediction of unusual chemical, electronic and dynamical properties at very high pressures. There is great interest in the behaviour of hydrogen-rich materials over a broad range of thermodynamic conditions1,2. In addition to the novel phenomena predicted in their dense metallic phases3,4, the introduction of electronic levels in the bandgap by doping with impurity atoms, thereby facilitating changes in the electronic properties at lower pressures, has been predicted5. Xenon has the lowest measured metallization pressure among the rare gas solids, and optical measurements have established bandgap closure and concurrent metallization of xenon at 130–150 GPa (refs 6–8). Evidence for xenon doping of hydrogen in matrix isolation experiments at ambient pressure has been reported 9,10, but the nature of the bonding in these metastable phases is not well understood. There has been growing interest in the chemistry of xenon to form bulk compounds at high pressures11 as well as recognition of the biological effects of xenon at high pressures12. During the course of investigating the Xe–H2 system, we have discovered novel compound formation in Xe and H2. High-pressure van der Waals compounds formed in simple molecular systems were reported in the early 1990s (refs 13,14). In addition to revealing new compounds involving xenon, our data provide the first experimental signatures of pressure-induced bonding states in these materials. A series of H2–Xe gas mixtures was prepared and loaded into diamond anvil cells using a high-pressure gas loading system. Single crystals of the H2-rich mixtures were grown using a combination of varying pressure and temperature. At 4.1 GPa, a xenonrich solid was observed to form. Transitions were optically observed at 4.4, 4.9 and 5.4 GPa with accompanying changes in stoichiometry as inferred from relative volume changes of the phases in the cell (see Supplementary Information). Synchrotron X-ray diffraction data were collected at selected pressures. In the diffraction pattern taken at 4.9 GPa, a total of 201 reflections (Fig. 1a) could be indexed on a hexagonal unit cell with cell constants a ¼ 8.654(3) Å and c ¼ 12.357(7) Å and the systematic absences yielded the space H group R3. The structure of the xenon sublattice could be successfully solved using direct methods, which resulted in an excellent refinement (Rl ¼ 3.78%) for 120 unique observed reflections (see Supplementary Information). A remarkable feature of the xenon sublattice is the presence of two different sets of Xe–Xe distances. The six xenon atoms in the unit cell are arranged into three Xe–Xe pairs oriented along the c axis of the unit cell, giving rise to an array of dimers (Fig. 1b). At 4.9 GPa the distance between the xenon atoms within each pair is 3.875(1) Å, whereas the closest Xe–Xe inter-dimer distance is 4.915(1) Å. Remarkably, the Xe–Xe distance in the dimer at this pressure is close to that of neutral dimers in the gas phase (3.84 Å; ref. 15), which is also close to the nearest-neighbour Xe–Xe distance of 3.83 Å in solid fcc xenon at 5 GPa and room temperature16. In comparison, the nearest-neighbour distance at 4 K (at ambient pressure) determined from X-ray diffraction measurements is 4.34 Å in the LT, fcc phase (ref. 17). The structure of the xenon sublattice determined from an analysis of the X-ray diffraction data at 4.9 and 7.1 GPa remained unchanged across the transitions. The transitions therefore arise from changes in the amount of hydrogen assimilated into the material. Raman and infrared (IR) spectra provide further information about the nature of the high-pressure phases. A total of five vibron modes for H2 were observed in the Raman spectrum (Fig. 2a), indicating that the H2 molecules are intact. The lowfrequency Raman spectrum is found to be indistinguishable from that of pure solid H2 at the same conditions, indicating rotational disorder of the molecules. No signature of Xe–H bonding is observed in the vibrational spectra9. A total of five IR-active vibron modes are observed (Fig. 2b). Of these, two are simultaneously Raman and IR active. A 3  3  3 superstructure based on the hcp lattice of solid hydrogen would give rise to similar activity and result in the appearance of additional vibron bands compared to pure H2 as a result of Brillouin zone folding18,19. The IR spectrum shows that the compound remains an insulator at 255 GPa. The stoichiometries of the compounds were estimated as follows. The change in hydrogen content occurs at discrete pressures 1 Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA, 2 Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA, 3 HPCAT, Carnegie Institution of Washington, Advanced Photon Source, Argonne, Illinois, USA. * e-mail: zulu@gl.ciw.edu 50 NATURE CHEMISTRY | VOL 2 | JANUARY 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved. NATURE CHEMISTRY ARTICLES DOI: 10.1038/NCHEM.445 a b z x y Figure 1 | X-ray single-crystal diffraction of Xe–H2 compound. a, Oscillation photograph of a single crystal of the Xe–H2 compound obtained at 4.9 GPa (see Supplementary Information for details of indexing). b, Corresponding structure of the xenon sublattice deduced from these data. a Vibrons Raman spectra Intensity Rotons 24.6 GPa x8 x20 4.9 GPa x8 200 400 600 800 1,000 1,200 4,150 4,350 Raman shift (cm−1) b 24.6 GPa 0.2 OD Absorbance 4,000 5,000 Infrared spectra 6,000 255 GPa Vibrons + Rotons 210 GPa 1.0 OD Vibrons 2,000 3,000 4,000 5,000 6,000 Wavenumber (cm−1) Figure 2 | Raman and infrared spectra of Xe–H2. a,b, Representative Raman spectra (a) and IR spectra (b) of the Xe–H2 compound. The Raman spectrum of pure H2 at 4.9 GPa is shown in a as a dotted curve. The vibron spectrum of the Xe–H2 compound shows a multiplet structure that becomes evident at higher pressures, in contrast to the behaviour of pure H2. The lowpressure IR spectrum is shown in the inset of b. The IR spectra at the highest pressure show no evidence of Drude absorption. The small increase at the longest wavelengths is indicative of diffraction effects due to decreasing sample size. Transmission at the longest wavelengths places an upper bound on the possible carrier density, with a corresponding upper bound for the plasma frequency of 0.2 eV (ref. 29). The frequency range of the high twophonon absorption of the diamond is blocked. (4.9 and 5.4 GPa), and there is no evidence for major changes in stoichiometry at higher pressures based on the continuity of the vibrational frequency shifts to 255 GPa and the volume compression obtained from X-ray data to 50 GPa. The hydrogen content was bounded at these pressures based on the equations of state of the component elements Xe (ref. 16) and H2 (ref. 20). The space group R3 constrains the number and placement of H2 molecules in the compound formed at 4.9 GPa. The observed unit cell volume of 801.4 Å3 is 2% smaller than the sum 6Xe þ 42H2 , suggesting a stoichiometry Xe(H2)7 for this phase. In comparison, the observed volume of 831.7 Å3 at 7.1 GPa suggests a stoichiometry Xe(H2)8 for all pressures above 5.4 GPa. The systematic absences observed at 7.1 GPa suggest that the space group changes from R3 to P3, with the accompanying change in hydrogen stoichiometry. Simulated annealing reverse Monte-Carlo calculations21,22 were performed with the diffraction intensity data used in the single-crystal refinement to further constrain the H2 positions. Xenon atoms were fixed at the positions (in R3) obtained from the original refinement, and H2 locations were optimized from an initial random placement. Simulations with 42 H2 molecules (Xe(H2)7) yielded a structure that improved the refinement quality factor (in comparison to the model with only xenon atoms) R1 by 0.5%. The structure is shown in Fig. 3. The assumption of 42 H2 molecules in the unit cell agrees qualitatively with the highest peaks in the difference Fourier map obtained from a refinement that excluded hydrogen atoms. The structural refinement provides direct information on the origin of the stability of this compound. Making use of the observed structure factors, we calculated the electron density maps. The single crystal at 4.9 GPa was immersed in a hydrostatic liquid hydrogen medium, and the use of Boehler–Almax-type seats with an opening angle of 708 allowed data to be collected over a large angular range. The resulting inherent high quality of the data and the span of the reciprocal space covered gave rise to electron density maps of very good quality. As described above, the xenon atoms are distributed as dimers rather than distinct atoms, with a uniform Xe–Xe distance. Examination of the electron density maps shows no significant distortion of electron density between the xenon atoms at low pressures and therefore no interaction between them (Fig. 4a,b). However, a striking spread of electron NATURE CHEMISTRY | VOL 2 | JANUARY 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved. 51 ARTICLES NATURE CHEMISTRY z x y Figure 3 | Model structure of Xe(H2)7. The xenon atoms are surrounded by dumbbell-shaped, freely rotating hydrogen molecules represented by the spherical shells. density into the interstitial space between the xenon atoms and from the Xe2 pairs towards the surrounding hydrogen molecules is observed at lower cutoff (Fig. 4c). We interpret these changes in electron density with pressure as arising from increasing chemical a DOI: 10.1038/NCHEM.445 interaction between the xenon atoms in each pair as well as between Xe2 pairs and the surrounding array of hydrogen molecules. The spread of electron density from the xenon atoms (located on 3a positions) to the surrounding hydrogen molecules serves to stabilize the Xe2 pairs by depopulating a fully filled s* molecular orbital of the Xe2 unit, creating a Xe–Xe bond. In xenon– halogen compounds, a gradual transition from van der Waals to covalent bonding between xenon atoms and the X2 halogen molecules is inferred and this depends on the extent of the overlap of the valence orbitals of Xe with the antibonding orbitals of the neighbouring molecule23. In a similar way, a gradual transition from a neutral Xe2 to an ionic Xe2þ seem to take place with pressure in the Xe–H2 system24. In other systems, this change in ionicity is initiated by either chemical means25 or xenon partial pressure26. Our data indicate that the Xe2Xe bond length in the dimers attains the value observed in Xe2þ at 50 GPa. The formation of ionic xenon dimers is expected to give rise to charge transfer to the high-lying s* antibonding states of the H2 molecules, and results in additional weakening of the H–H bond. Because the bond weakening (inferred from the observed decrease in vibrational frequencies) is similar to that found in pure H2 (ref. 18), the charge must be localized elsewhere (see Supplementary Information). This conclusion is consistent with the unusual spread in electron density distribution deduced from the refinement of X-ray data at lower pressures (Fig. 4c). The lowest H2 vibron frequency decreases and shows a monotonic decrease with increasing pressure above 60 GPa. The decreasing vibron frequencies indicate a weakening of the covalent 80 0.8 60 0.6 y 40 0.4 20 Electron density (eÅ3) 1.0 0.2 0 0 0 −0.4 0.8 0.4 1.2 x b o y c z x z o x y Figure 4 | Changes in the electron density of xenon. a, Electron density calculated from the observed structure factors (Beever–Lipson maps)30. The projection along y obtained at the first level (a) shows the xenon pairs. b,c, Three-dimensional equal-energy contours obtained at different cutoff levels, 15e (b) and 5e (c), show the spread of electron density between the Xe atoms and in the direction of coordinated H2 molecules lying within the first coordination sphere of the Xe atoms. 52 NATURE CHEMISTRY | VOL 2 | JANUARY 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved. NATURE CHEMISTRY ARTICLES DOI: 10.1038/NCHEM.445 intramolecular bond at the highest pressures (that is, above 120 GPa), as observed in pure, solid hydrogen18,27. The observed vibron shift under pressure is similar to that found for H2 molecules dispersed in rare gas matrices under pressure28. The high-pressure compound formation reported here contrasts with the photochemically induced Xe–H bond formation documented as impurities in cold rare gas matrices near ambient pressure9,10, because no spectroscopic evidence for the formation of Xe–H bonds has been observed at any of the pressures studied. The IR measurements at the highest pressure show no evidence of Drude absorption (vp , 0.2 eV) and therefore provide no indication of metallization (volume compression V/V0  0.2). Nevertheless, the measured variation of the structure on compression suggests the existence of xenon dimers forming a one-dimensional metal at still higher pressures. 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Additional information The authors declare no competing financial interests. Supplementary information accompanies this paper at www.nature.com/naturechemistry. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressed to M.S. NATURE CHEMISTRY | VOL 2 | JANUARY 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved. 53