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Download Author Version (PDF) Chemical Science Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/chemicalscience Page 1 of 5 Chemical Science Journal Name RSC Publishing ARTICLE + Stabilization of H 3 in the high pressure crystalline structure of H nCl (n = 2-7) Cite this: DOI: 10.1039/x0xx00000x Ziwei Wang a, Hui Wang a,* , John S. Tse a,b,c,* ,Toshiaki Iitaka d and Yanming Ma a,c Received 00th January 2012, The particle-swarm optimization method has been used to predict the stable high pressure Accepted 00th January 2012 structures up to 300 GPa of hydrogen-rich group 17 chlorine (H nCl, n = 2-7) compounds. In comparison to the group 1 and 2 hydrides, the structural modification associated with DOI: 10.1039/x0xx00000x increasing pressure and hydrogen concentration is much less dramatic. The polymeric HCl www.rsc.org/ chains already present in the low temperature phase under ambient pressure persist in all the high pressure structures. No transfer of electron from the chlorine atom into the interstitial sites is found. This indicates the chemical bonding at high pressure in group 17 elements is Manuscript fundamentally different from the alkali and alkaline elements. It is found that almost perfectly + triangular H 3 ions can be stabilized in the crystalline structure of H 5Cl. Introduction and phase stabilities of hydrogen-rich HCl-H2 system. A major + finding is the stabilization of cationic (H 3) (H 2) species in + The suggestion that the hydrides of main group elements H5Cl. The geometry of H 3 becomes almost an equilateral at high pressure may be superconductors with high critical triangle under very high pressure. temperature has stimulated recent interest in the search of + possible existence of hydrogen-rich alloys by theoretical and The observation of triatomic hydrogen cation H 3 in the experimental means. 1 The former is greatly facilitated from the solids state is new and significant. The isolated molecule is Accepted recent developments in practical strategies for the prediction of important in various branches of sciences, such as physics, crystal structures. Although, the predicted hydrides yet to be chemistry and astronomy. For example, it is known that the + confirmed by experiment, the theoretical studies have revealed H3 ion with a triangular configuration is stable in the a myriad of novel structures for the high pressure hydrides and interstellar medium thanks to the low temperature and low enriched the understanding of the nature of chemical bonding in density of the interstellar space and the H 3 molecule is + the rarely explored high pressure regime. commonly formed from the neutralization reaction of H 3 and an electron but rather evanescent as a result of the repulsive 12 + The hydrides of Group 1 and 2 elements form from the nature of its ground state . Furthermore, the multicenter (H 3) (H 2) bond in Group 17 Cl compounds signifies a deviation in reaction of the metal and hydrogen molecules were the most Science studied. 2-6 Most of the structures and structural trend can be the nature of chemical bonding from the charge transfer explained from the simple concept of electron transfer from the interaction in Group 1 and 2 hydrides and covalent bonding in metal to the hydrogen due to the large electronegativity Group 14 hydrides. differences between the alkali and alkaline elements and hydrogen molecule. The predicted compounds display rich H Computational details species distinguished from the well-known H-ion in hydrides at traditional stoichiometric. Perhaps one of the most exciting The search for stable high pressure structures for the H nCl - predictions is the emergence of symmetric and linear H 3 at system was based on global minimization of free energy 7, 8 high pressures as observed in dense CsH 3 and BaH 6. surfaces using ab initio total energy calculations and the Moreover, the formation pressures of these compounds of just a Particle-Swarm-Optimization scheme as implemented in the few tens of GPa are accessible to experiment. In comparison, CALYPSO (Crystal structure AnaLYsis by Particle Swarm Chemical the bonding pattern is quite different for group 14 and transition Optimization) code 13, 14 . The performance and reliability of this elements. For example, a Van de Waas solid such with method has been demonstrated on many known systems. An 15-17 molecular H 2 units was found experimentally in SiH 4 at low example is the success on the prediction of an insulating 9 pressure. At higher pressure, the atoms of group 14 elements orthorhombic ( Aba2 , Pearson symbol o C40) structure of Li and, tend to aggregate to form a 2D layer structure decorated with more recently, two low-pressure monoclinic structures of Bi 2 10, 11 molecular like H 2 species as predicted for SiH 4 and SnH 4. Te 3. In both cases, the predicted structures were later confirmed In comparison, the high chemistry of hydrogen with electron- by experiments 18, 19 . Structural searching were performed at rich Group 17 halogens has not been investigated. In this 100, 200, and 300 GPa with simulation cell consisting of 1–4 paper, we presented results on a study of the crystal structures formula units. Ab initio electronic structure calculations and This journal is © The Royal Society of Chemistry 2013 J. Name ., 2013, 00 , 1-3 | 1 Chemical Science Page 2 of 5 ARTICLE Journal Name structural relaxations were carried out using density functional vdW effects may play an important role in stabilization of theory with the Perdew–Burke–Ernzerhof (PBE) exchange- a molecular solid. We have thus performed additional correlation20 implemented in the Vienna Ab Initio Simulation calculations on the H-Cl system with the vdW-DF2 method 26 . Package (VASP) code. 21 The predicted stable structures were The results show the differences between calculations with and carefully optimized at a high level of accuracy. A plane wave without vdW corrections on the formation enthalpies of the energy cutoff of 1000 eV was employed. Large Monkhorst- structures considered in Fig. 1 are small. The formation Pack k point sampling grids 22 were used to ensure that all the pressures were found to change slightly. For example, the enthalpy calculations are well converged to an accuracy of 1 stabilized pressure of H 2Cl was increased from 21.2 to 21.3 meV/atom. The atomic charges were obtained from Bader GPa, while for H 5Cl it increased from 50 to 60 GPa. Otherwise, topological analysis 23–25 with very large grids to ensure the energetic order remains the same. sufficient accuracy. We also performed additional calculations employing the DF-2 van der Waals (vdW) functional 26 to Now we examine the development of the high pressure validate the results, in particular for the low pressure structures. crystal structures in H nCl (n =2-7). The starting point is the Phonons were calculated with the supercell method 27 crystal structure of HCl under ambient pressure. At low implemented in the PHONOPY program. 28 In essence, from temperature, x-ray and neutron diffraction show HCl 30 finite displacements, the Hellmann-Feynman atomic forces crystallized in an orthorhombic structure (Bb 21m). In the computed at the optimized supercell by the VASP code were crystal, HCl molecules are linked via the H atoms forming input to the PHONOPY code to construct the dynamical matrix. zigzag chains running parallel to the crystallographic b axis. Diagonalization of the dynamical matrix gives the normal The nearest neighbour Cl-H and second nearest neighbour modes and their frequencies. Converged results were obtained Cl…H distances are 1.25 Å and 2.44 Å, respectively. The with the use of a 2 × 2 × 1 supercell and 4 × 4 × 6 k-meshes for Cl…Cl separation is 3.88 Å. And the H-Cl…H valence angle is o the Cc structure, and a 2 × 2 × 1 supercell and 4× 4 × 6 k- 93.6 . The predicted crystalline phase of H 2Cl at 100 GPa has a meshes for the C2/c structure. C2/ c space group and the structure is shown in Fig. 2. The crystal is formed from HCl chains interposed with H2 Manuscript Results and discussion molecules. In this case, the H in the HCl chain is midway between the two Cl atoms with an H-Cl distance of 1.45 Å. o Before embarking on a detail discussion of the structures The H-Cl-H angle has opened to 97.9 and the Cl…Cl of the predicted high pressure H nCl polymers, the relative separation is shortened to 2.90 Å. The H 2 units in the structure energetics of the H-Cl system with H-rich stoichiometry from all have a H-H distance of 0.74 Å, which almost identical to 100 to 300 GPa are summarized in the convex hull plot shown that of the isolated molecule.
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