New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system

The application of pressure to materials can reveal unexpected chemistry. Under compression, noble gases form stoichiometric van der Waals (vdW) compounds with closed-shell molecules such as hydrogen, leading to a variety of unusual structures. We have synthesised Kr(H2)4 for the first time in a diamond-anvil high-pressure cell at pressures ≥5.3 GPa and characterised its structural and vibrational properties to above 50 GPa. The structure of Kr(H2)4, as solved by single-crystal synchrotron X-ray diffraction, is face-centred cubic (fcc) with krypton atoms forming isolated octahedra at fcc sites. Rotationally disordered H2 molecules occupy four different, interstitial sites, consistent with the observation of four Raman active H2 vibrons. The discovery of Kr(H2)4 expands the range of pressure-stabilised, hydrogen-rich vdW solids, and, in comparison with the two known rare-gas-H2 compounds, Xe(H2)8 and Ar(H2)2, reveals an increasing change in hydrogen molecular packing with increasing rare gas atomic number.

The application of pressure to materials can reveal unexpected chemistry. Under compression, noble gases form stoichiometric van der Waals (vdW) compounds with closed-shell molecules such as hydrogen, leading to a variety of unusual structures. We have synthesised Kr(H 2 ) 4 for the first time in a diamond-anvil high-pressure cell at pressures $5.3 GPa and characterised its structural and vibrational properties to above 50 GPa. The structure of Kr(H 2 ) 4 , as solved by single-crystal synchrotron X-ray diffraction, is face-centred cubic (fcc) with krypton atoms forming isolated octahedra at fcc sites. Rotationally disordered H 2 molecules occupy four different, interstitial sites, consistent with the observation of four Raman active H 2 vibrons. The discovery of Kr(H 2 ) 4 expands the range of pressure-stabilised, hydrogen-rich vdW solids, and, in comparison with the two known rare-gas-H 2 compounds, Xe(H 2 ) 8 and Ar(H 2 ) 2 , reveals an increasing change in hydrogen molecular packing with increasing rare gas atomic number. F amiliar concepts of chemical bonding and interactions in molecular systems can change dramatically upon compression, and new concepts can be revealed. At gigapascal pressures the atomic rare gases (e.g., He, Ar, Xe), diatomic molecules (e.g., H 2 , N 2 , O 2 ) and even full-shell molecules (e.g., CH 4 , SiH 4 ) can interact with each other and form compounds. The first reported, pressure-stabilized, stoichiometric van der Waals (vdW) solid was reported in the He-N 2 system, He(N 2 ) 11 1 . Since then a variety of stoichiometric vdW compounds have been synthesised under pressure for example, in the systems He-Ne 2 , Ar-H 2 3 , CH 4 -H 2 4 , and most recently Xe-H 2 5,6 . The study of hydrogen-rich binary mixtures has attracted particular attention because they are of interest to planetary science and astronomy, are potentially relevant technologically for hydrogen storage 7 , and, last but not least, offer the tantalising prospect to promote the pressure-induced insulator-metal transition in hydrogen at a lower pressure than required for pure hydrogen (e.g., 3,8).
Two hydrogen-rich rare gas RG binary mixtures have been investigated so far: Ar-H 2 and Xe-H 2 ; and two vdW compounds have been discovered and characterised in detail: Ar(H 2 ) 2 and Xe(H 2 ) 8 . Ar(H 2 ) 2 forms at 4.3 GPa and X-ray diffraction data showed Ar(H 2 ) 2 to be isostructural with the MgZn 2 Laves phase to at least 27 GPa 3 . The observation of a Raman active H 2 vibron confirmed the presence of molecular H 2 in the structure, and its disappearance around 175 GPa indicated the possibility of a pressure-induced molecular dissociation followed by metallization 3 . The prospect of metallic hydrogen stimulated extensive experimental and theoretical investigation of this molecular compound (e.g., [9][10][11][12] with the latest study in fact predicting a much higher metallization pressure for hydrogen mixed with argon than for pure hydrogen 12 . Stability of Ar(H 2 ) 2 to at least 220 GPa is supported by IR spectroscopic measurements 13 .
Xe(H 2 ) 8 was found to be stable above 5.4 GPa with a hexagonal crystal structure 5 . The Xe sublattice, as determined by X-ray diffraction, consists of Xe-Xe pairs oriented along the c axis of the unit cell. Raman and IR spectra composed of multiple H 2 vibrons confirmed the presence of molecular hydrogen in Xe(H 2 ) 8 and support that its structure can be viewed as a tripled, solid H 2 hcp lattice modulated by layers consisting of Xe dimers 5 . IR spectra showed the Xe(H 2 ) 8 compound to remain an insulator to at least 255 GPa 5 while its calculated metallization pressure is around 250 GPa 14 .
The Kr-H 2 binary system has not been investigated under compression experimentally and so the possible vdW compounds are unknown. A recent first-principles study suggested that Kr(H 2 ) 2 was a possible candidate for observing the pressure-induced metallization of hydrogen based on the similarities of RG(H 2 ) 2 to RG(He) 2 systems 15 . Here we present the first high-pressure single-crystal X-ray diffraction study of a Kr-H 2 solid synthesised at pressures $5.3 GPa in the diamond-anvil cell from a mixture of 8 vol% Kr and balance H 2 (Fig. 1).
Complementary Raman spectroscopic measurements were performed to verify the structural environments of the hydrogen molecules determined from single-crystal X-ray diffraction data and to characterize the evolution of the H 2 molecular bond under pressure.

Results
The structure of Kr(H 2 ) 4 and its evolution under pressure. The structure of the Kr-H 2 compound was solved from a single-crystal data set collected at 11.24 GPa (Tab. 1, Supplementary Fig. S1 and Tab. S2). The Kr sublattice is face-centred cubic (fcc), space group Fm-3m, and characterised by isolated Kr octahedra occupying the fcc sites (Fig. 2). Interestingly the intra-octahedral Kr-Kr bond distances (3.35 Å at 11.24 GPa) are comparable to the Kr bond distance in the pure krypton solid at the same pressure. The inter-octahedral distances are comparably long with the shortest inter-octahedral Kr-Kr distance being 4.89 Å at 11.24 GPa. Under pressure the volume of Kr(H 2 ) 4 compresses continuously to at least 51 GPa with the longer inter-octahedral Kr-Kr distance decreasing faster than the shorter intra-octahedral distance (Fig. 3). Compression of the intra-octahedral distance follows the compressional behaviour of the Kr-Kr distance in solid krypton. Above 15 GPa the fcc Kr sublattice indicates a small tetragonal distortion (space group I4/ mmm) deviating form the cubic unit cell by about 1% based on c tet /a!2 5 1.01. Cubic and tetragonal refinements of the Kr(H 2 ) 4 structure are of comparable quality to 51 GPa and for the purpose of volume determination we have refined the Kr substructure with cubic symmetry.
The low X-ray scattering cross-section of hydrogen, the contrast with heavier elements, and the micron-sized sample in the diamondanvil cell pose a major challenge in high-pressure X-ray diffraction experiments on hydrogen-rich samples for the determination of H 2 positions. In addition, single-crystals become increasingly strained under pressure and the reflection rocking curve deteriorates. Our structural model of Kr(H 2 ) 4 could be completed by locating the hydrogen molecules using difference-Fourier maps. The H 2 positions were identified at four different interstitial sites (Tab. 2) appearing with peak scattering densities from 0.9 to 1.23 eÅ 23 in the F calc -F obs difference electron density maps. Including the H 2 molecules in the refinement improved the refinement quality factor R 1 by 1.2% compared to a model with Kr atoms only. Further, the number of H 2 molecules refined confirms the stoichiometry Kr(H 2 ) 4 which was initially (with only the Kr substructure established) derived from volume considerations based on the separate equation of states of pure krypton 16 and hydrogen 17 .
High-pressure Raman spectra and the H 2 sublattice of Kr(H 2 ) 4 . The Raman spectra of Kr(H 2 ) 4 are characterised by roton peaks between 200-1200 cm 21 and by a maximum of four H 2 vibrons between 4200-4500 cm 21 confirming the presence of molecular hydrogen in the structure (Fig. 4, 5). The roton frequencies cannot be distinguished from those of pure hydrogen at the same pressure indicating rotational disorder of the H 2 molecules. The four H 2 vibrons are observed at higher frequencies than the Q 1 (1) vibron of solid H 2 at the same pressure. Attributing these vibrons to H 2 molecules in four different crystallographic sites is consistent with the four H 2 sites determined from the difference-Fourier maps.
Under compression the vibrons shift continuously to higher frequencies (Fig. 6) confirming the persistence of molecular H 2 and the absence of a major structural change in Kr(H 2 ) 4 to at least 51 GPa in agreement with the continuous volume compression. The two lower frequency vibrons (n 1 , n 2 ) remain intense to the highest pressure, the higher frequency vibron n 3 weakens significantly, and the fourth vibron, n 4 , is very weak and not uniformly observed across the pressure range studied. Interestingly, the two H 2 vibrons, n 1 and n 2 , show a striking reversal in their relative intensities under pressure: n 1 looses intensity while n 2 gains intensity. Around 15 GPa n 1 and n 2 There is a striking visible difference in shape between the crystal of experiment 1 (a) and experiment 2 (b) (squared, sharp edges versus rounded shape). Except for a difference in their crystallographic orientation with respect to the X-ray beam the X-ray diffraction and Raman spectroscopic data match showing both crystals as well crystalline. are of equal intensity, and above about 30 GPa their intensity ratio is constant. The changeover in dominant intensity from n 1 to n 2 occurs at the pressure above which a tetragonal distortion is detectable in the fcc Kr sublattice. Such coincidence suggests that the change in relative intensity of the H 2 vibrons can be associated with a change in the H 2 sublattice which in turn subtly affects the Kr sublattice at pressures .15 GPa. We can exclude a re-distribution of H 2 molecules between two different crystallographic sites because all four hydrogen sites are fully occupied.

Discussion
In order to provide additional insight and to enable a more systematic understanding of weakly-bonded systems under pressure we compare Kr(H 2 ) 4 with the two, other known RG-H 2 compounds, Xe(H 2 ) 8 and Ar(H 2 ) 2 . From the RG sublattice point of view a common characteristic of Kr(H 2 ) 4 , Xe(H 2 ) 8 and Ar(H 2 ) 2 is that the smallest RG-RG distance is comparable to the next nearest-neighbour distances in the pure RG solid at the same pressure. While Ar(H 2 ) 2 features only Ar-Ar distances close to those in solid Ar at the same pressure, Kr(H 2 ) 4 and Xe(H 2 ) 8 show a striking difference: Two different sets of atomic distances occur: intra-and inter-atomic RG-RG distances with the intra-atomic distances being comparable to the next nearest-neighbour distances in the pure RG solid at the same pressure. For Kr(H 2 ) 4 the intra-octahedral Kr-Kr distances are comparable to the Kr-Kr distances in solid krypton, and for Xe(H 2 ) 8 the Xe-Xe dimer separation was reported to match the Xe-Xe distance in solid xenon 5 .
Further, the pressure dependence of the intra-octahedral Kr-Kr distance follows closely that of the Kr-Kr distance in solid krypton making the Kr-octahedra appear as detached units, neither interacting with each other nor with the H 2 sublattice. A similar observation applied to the Xe-dimers in Xe(H 2 ) 8 5 . However, clearly compound formation and stability is a complex process dominated by a subtle interplay of van der Waals and weak covalent forces or possibly charge transfer interactions as argued for in Xe(H 2 ) 8 5,14 .
The hydrogen-lattice vibrational properties of Kr(H 2 ) 4 , Xe(H 2 ) 8 and Ar(H 2 ) 2 show strong similarities to solid H 2 . The rotational vibrations of all compounds are reported as indistinguishable from those of solid hydrogen indicating freely rotating H 2 molecules. The observation of multiple vibrons for Xe(H 2 ) 8   An interesting, well studied characteristic of the vibron in solid hydrogen is its turnover around 35 GPa above which its frequency decreases monotonically. This behaviour has been attributed to vibrational coupling between H 2 molecules; pressure-induced bond weakening does not occur until over 145 GPa 18 . We observe that the intense H 2 Raman vibron of Xe(H 2 ) 8 mirrors the behaviour of the vibron in solid hydrogen turning over around 40 GPa (inset Fig. 6). Starting from a lower frequency, it crosses the pressure-frequency trajectory of the pure hydrogen vibron at about 50 GPa and remains at a higher frequency to at least 142 GPa. The intense H 2 vibron we observe in Xe(H 2 ) 8 is the only vibron of the RG-H 2 compounds reported so far that on compound formation and at pressures ,50 GPa exhibits a frequency lower than the vibron of pure hydrogen at the same pressure.

and Kr(H 2 ) 4 contrasts
The H 2 vibrons of Kr(H 2 ) 4 and the single vibron of Ar(H 2 ) 2 are initially higher than the vibron of pure H 2 and harden continuously under compression over the investigated pressure range with the n 1 vibron of Kr(H 2 ) 4 appearing to flatten around 40-50 GPa so likely turning over at higher pressures. Continuously increasing H 2 Raman vibron frequencies have also been observed for H 2 dissolved in raregas matrices where there is no possibility of vibrational coupling due to mismatch of vibrational frequencies between matrix and H 2 molecules 19 . The RG-H 2 compounds appear therefore to show ''composite'' behaviour, with some of the H 2 vibrational properties dominated by H 2 -H 2 intermolecular interactions of near by H 2 and some more strongly modified by nearby RG structural units.
Details of the electronic band structure of Kr(H 2 ) 4 and the possibility of metallization present a challenge for future experimental and theoretical studies; experimental IR data in the megabar range would be particularly valuable. Furthermore, the low-pressure phase diagram of the Kr-H 2 binary system is unexplored and there may be other stoichiometries stabilized for different starting mixtures.

Methods
Diamond-anvil cell sample loading and crystal synthesis. In two separate experiments a single-crystal of Kr(H 2 ) 4 was grown in a diamond-anvil cell (DAC) from a pre-mixed gas of composition 8 vol% Kr and 92 vol% H 2 (certified accuracy 62% from Air Liquide). In both runs a DAC equipped with 0.3 mm culet diamonds and an opening angle of 76 deg (Betsa, France) was used. A Re gasket pre-indented to 25 mm thickness with a hole of 120 mm diameter formed the sample chamber and a ruby sphere served as pressure calibrant 20 . The pre-mixed Kr-H 2 gas was loaded in the DAC at 0.2 GPa 21 . Kr and H 2 are miscible in the liquid phase. By increasing the pressure delicately a crystal was grown from the mixture at a pressure $5.3 GPa. At slightly higher-pressure, ,5.5 GPa, solidification of the excess liquid was observed. 5.5 GPa is the known solidification pressure for pure hydrogen (e.g., 22,23), and Raman spectroscopic measurements confirmed the solidified excess liquid as solid hydrogen. For the high-pressure experiment hydrogen acts as a pressure-transmitting medium and ensures quasi-hydrostatic conditions up to the highest pressures reached in these experiments; 26.7 GPa in experiment 1 and 50.9 GPa in experiment 2. We cannot exclude the possibility that the starting stoichiometry was changed due to hydrogen dissolving into the gasket material under compression 24 . A possible loss of H 2 to the Re gasket would affect the total amount of pure hydrogen in the sample chamber but not the formation of the observed Kr-H 2 compound because this compound was grown in the presence of excess hydrogen and is in equilibrium with pure, solid H 2 at pressures $5.5 GPa.
Single-crystal X-ray diffraction. Single-crystal diffraction data have been collected on beamline I15 at Diamond Light Source (UK). The diffraction experiments were performed with a monochromatic beam (l 5 0.3647 Å ) collimated by a tungsten pinhole down to 20 mm in diameter. A MAR345 image plate (MarResearch) was used as a detector with sample-to-detector distances between 250 and 270 mm. The DAC,  positioned on a 6-circle Newport diffractometer with kappa geometry, could be rotated around the Q-axis, and data sets were collected using Q-scans over 60 and 73 deg, step sizes between 0.5 and 2 deg, and exposure times of 0.25 to 4.0 sec per step. The data were processed using CrysAlis Pro software (Agilent Technologies) and the WinGX version of SHELXS and SHELXL 25 .
Raman spectroscopy. Unpolarized Raman spectra have been recorded from the Kr-H 2 single-crystal and the excess, solid H 2 in 180u back-scattering geometry over the range 3800-4800 cm 21 (region of the H 2 vibron) with spot tests in the range 200-1200 cm 21 (region of the H 2 rotons). The instrument used was the Labram HR800 (Horiba Jobin Yvon) at beamline I15, Diamond Light Source (UK). It was equipped with 1200 g grating and an air cooled CCD detector. The spectra were excited by the 473 nm line of a 50 mW Cobalt Blues TM laser focused down to a 10 mm spot on the sample and collected through a 50 mm confocal aperture. The intrinsic resolution of the spectrometer is ,1.0 cm 21 and calibrations are accurate to 61 cm 21 . The frequency of each Raman band was obtained by Voigtian curve fitting using a leastsquares algorithm. In experiment 1 some spectra collected on the Kr(H 2 ) 4 and H 2 crystal each contained weak contributions from the other solid phase due to the small sample size but deconvolution was readily done.
The Raman spectroscopic data of Xe(H 2 ) 8 to 142 GPa were measured in 135 deg scattering geometry with a SPEX Triplemate equipped with a back-illuminated liquid-N 2 -cooled CCD detector at the Department of Earth Sciences, University of Oxford, UK. The spectra were excited by the 514.5 or 488-nm line of an argon-ion laser focused to a # 10 mm spot on the sample and collected through an adjustable confocal aperture closed down to about 10 mm in diameter .90 GPa. The intrinsic resolution of the spectrometer is 1.5 cm 21 and calibrations are accurate to 61 cm 21 .

Additional information
Supplementary information accompanies this paper at http://www.nature.com/ scientificreports