Predicted Formation of H3+ in Solid Halogen Polyhydrides at High

Oct 15, 2015 - The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab ini...
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Predicted Formation of H3+ in Solid Halogen Polyhydrides at High Pressures Defang Duan,†,‡ Xiaoli Huang,† Fubo Tian,† Yunxian Liu,† Da Li,† Hongyu Yu,† Bingbing Liu,† Wenjing Tian,‡ and Tian Cui*,† †

State Key Laboratory of Superhard Materials, College of physics and ‡State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n > 1) are metastable up to 300 GPa, whereas for HnCl (n > 1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H3+ species are unexpectedly found in stoichiometries H2F with [H3]+[HF2]−, H3F with [H3]+[F]−, H5F with [H3]+[HF2]−[H2]3, and H5Cl with [H3]+[Cl]−[H2] above 100 GPa. Importantly, formation processes of H3+ species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H3+ species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.

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INTRODUCTION Since the H3+ molecular ion was originally discovered by J. J. Thomson in his “positive rays” in 1911,1 it had drawn considerable attention of physicists, chemists, and astronomers. Up to the 1930s, after the advent of quantum mechanics, Coulson predicted the equilateral triangular configuration of H3+ with H−H separation of 0.873 Å by means of theoretical studies.2,3 In 1980, the first infrared absorption spectrum of H3+ was observed by Oka using a liquid-nitrogen-cooled hydrogen plasma.4 Then in 1990s, H3+ has been successful detected in Saturn, Jupite, Uranus, and the interstellar medium.5,6 We found the H3+ species serendipitously during quest hydrogenrich structures of HnX (n > 1, and X = F, Cl, Br) under high pressure, where “n” is denoted as the simplest atomic stoichiometry between H and X. High pressure can effectively lower the barrier of chemical reaction and thus induce chemical reaction that hardly occurs at atmospheric pressure. Recently, design and synthesis of functional materials under high-pressure conditions with desirable chemical or physical properties by means of chemical reaction have attracted extensive attention. Experimentally, close shell systems (e.g., H2O, H2S, CH4, SiH4, GeH4, NH3BH3, Ar, Kr, and Xe) can reaction with H2 molecules under high pressure, forming potential hydrogen-storage materials, such as H 2 O(H 2 ), 7 (H 2 S) 2 H 2 , 8 CH 4 (H 2 ) 4 , 9 SiH4(H2)2,10,11 GeH4(H2)2,12 NH3BH3(H2)x,13 Ar(H2)2,14 Kr(H2)4,15 and Xe(H2)8.16 Theoretically, alkali metal polyhydrides MHn (n > 1 and M = Li, Na, K, Rb, Cs)17−22 and the alkaline earth metal polyhydrides MHn (n > 2 and M = Be, Mg, Ca, Ba)23−25 with high hydrogen contents were designed under high pressure. Moreover, some of these hydrides have been © 2015 American Chemical Society

predicted to be potential high-temperature superconductors, such as (H2S)2H2,26 SiH4(H2)2,27 GeH4(H2)2,28 KH6,19 and CaH6.23 Remarkably, the Im3̅m phase of (H2S)2H2 or H3S stoichiometry was predicted to have an outstanding high Tc of 191−204 K at 200 GPa26,29 by ab initio calculation, which has been verified by recent experiment.30 The exciting findings in high-pressure research have inspired us to theoretically explore compressed solid halogen polyhydrides HnX (X = F, Cl, Br, and I, n > 1). We accidentally discovered the triangular H3+ species in stoichiometries H2F, H3F, H5F, H5Cl, and H5Br at high pressure. However, there is no H3+ species in HnI up to 300 GPa.31 Because the highpressure properties of HnBr are different from that of HnCl, which have been presented in another paper.32 Although stoichiometry H5Cl was predicted,33 some physical and mechanical information are not clearly. In this study, we mainly focus on the formation mechanism of H3+ in halogen polyhydrides HnF and HnCl (n > 1) and give a clearly picture. Our studies on H3+ species in solid states are help for further understanding of isolated H3+. In addition, the lower synthetic pressure is within the reach of current diamond−anvil techniques, and our findings can provide guidance for experimental groups aiming to synthesize containing H3+ compounds under pressure. Received: August 22, 2015 Revised: October 15, 2015 Published: October 15, 2015 11059

DOI: 10.1021/acs.jpca.5b08183 J. Phys. Chem. A 2015, 119, 11059−11065

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The Journal of Physical Chemistry A

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COMPUTATIONAL METHOD To obtain stable structures for HnF and HnCl (n > 1), we conducted structure prediction implemented in CALYPSO (crystal structure analysis by particle swarm optimization).34,35 This method has successfully predicted the high crystal structures of various systems.36,37 For the H−F system, the structure predictions were performed within two and four HnF (n = 2−5) formula units (fu) at pressures of 50, 100, 200, and 300 GPa. For the case of the H−Cl system, the structure predictions were performed by considering simulation sizes ranging from one to four HnCl (n = 2−9) fu at pressures of 10, 50, 100, 150, 200, and 250 GPa. The structural relaxations, enthalpies, and electronic structures were calculated at temperature of 0 K using the projector augmented waves method,38 as implemented in the Vienna ab initio simulation package VASP code.39 The generalized gradient approximation (GGA) of Perdew− Burke−Ernzerhof (PBE)40 was adopted to describe the exchange-correlation potential. The energy cutoff 1000 eV and k-mesh of 2π × 0.03 Å−1 within the Monkhorst−Pack scheme were chosen to ensure that the total energy were well converged to better than 1 meV/atom. The Bader analysis and ELF were also calculated using VASP code. Lattice dynamics and Raman spectra were calculated using density functional perturbation theory41 and the plane-wave pseudopotential method with norm-conserving potentials,42 as implemented in the CASTEP code.43 Convergence tests gave a suitable value of 1000 eV kinetic energy cutoff. The q-point mesh in the first BZ of 2π × 0.05 Å−1 are used in the interpolation of the force constants for the phonon dispersion curve calculations. The Mulliken atomic charges were calculated with the CASTEP code.

S3. It can be seen that HCl has the most negative enthalpy of formation below 150 GPa, and then H2Cl has the most negative enthalpy, which is consistent with the fact that HCl exists at ambient condition. But the stability of other stoichiometries could not be distinguished from the Figure S3. To investigate the stability of HnCl more clearly, the formation enthalpies of HnCl (n = 2−9) for the reaction (n − 1)H2 + 2HCl → 2HnCl versus hydrogen atomic content at different pressures are illustrated in Figure 1. In addition, the pressure−composition

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RESULTS Thermodynamical Stability and Phase Graph. The convex hull is defined as the formation enthalpy per atom of the most stable phases at each stoichiometry versus the element proportions.44 The thermodynamic stability of different stoichiometric compounds at the given pressure and 0 K can be quantified by constructing the convex hull. In principle, any structure with an enthalpy located on the convex hull is supposed to be thermodynamically stable and can be synthesized experimentally. The structures of P63/m, C2/c, Cmca-12 for H2,45 C2/c for F2,46 Cmc21, Cmcm, Pnma for HF,47 and Cmc21, Cmcm, P1̅ for HCl47−49 are adopted in their stable pressure ranges. The convex hull of the H−F system versus the proportions of hydrogen atom at 100, 200, and 300 GPa are presented in Figure S1 of Supporting Information. It is shown that the formation enthalpies of HnF (n = 1−5) are negative, indicating that they can be synthesized at high pressure via the reaction nH2 + F2 → 2HnF. But only HF falls on the tie-line, suggesting that it is thermodynamically stable, whereas other stoichiometries, H2F, H3F, H4F, H5F, are metastable and may decompose into HF and H2 below 300 GPa. Furthermore, the formation enthalpies of HnF (n = 2−5) with respect to the HF and H2 are positive (Figure S2 in the Supporting Information), indicating that they are not synthesized experimentally using HF and H2 as precursors. It is fascinating that the triangular H3 unit was first observed in the stoichiometries H2F, H3F, and H5F. The formation enthalpies of HnCl (n = 1−9) for the reaction nH2 + Cl2 → 2HnCl at different pressures are depicted in Figure

Figure 1. (a) Formation enthalpies (ΔH in eV per atom) of HnCl (n = 2−9) for the reaction (n − 1)H2 + 2HCl → 2HnCl vs hydrogen atomic content at different pressures, as hf(HnCl) = [2h(HnCl) − 2h(HCl) − (n − 1)h(H2)]/2(n + 1). Dashed lines connect data points, and solid lines denote the convex hull. (b) Predicted pressure−composition phase diagram of HnCl (n ≥ 1).

phase diagram of HnCl (n = 1−9) is depicted in Figure 1b. It is shown that H3Cl can be synthesized above 12 GPa via the reaction HCl + H2 → H3Cl and it is stable in the pressure range 12−120 GPa. The H2Cl and H5Cl become stable at pressures of 40 and 120 GPa, respectively, and both remain stable up to 250 GPa, the highest pressure examined in this study. H7Cl is only stable in the pressure range 70−110 GPa. It is exciting that the triangular H3 unit also emerges in H5Cl. In addition, the stoichiometries having an even number of hydrogen (e.g., H4Cl, H6Cl, and H8Cl) and H9Cl are found to be energetically very unfavorable and are excluded in the discussions. Predicted Structures of HnF (n = 2, 3, 5). Although HnF (n > 1) are only metastable, it is instructive to examine the evolution of the crystal structure comparing with HnCl. Selected high structures of stoichiometries H2F, H3F, and H5F containing the H3 unit are shown in Figure 2. Above 70 11060

DOI: 10.1021/acs.jpca.5b08183 J. Phys. Chem. A 2015, 119, 11059−11065

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Figure 2. Selected structures of predicted HnF and HnCl (n > 1): (a) H2F-C2/c; (b) H3F-P21/m; (c) H5F-R3; (d) H2Cl-C2/c; (e) H3Cl-P212121; (f) H5Cl-Cc; (g) H7Cl-P21. Blue, green, and pink spheres represent F, Cl, and H atoms, respectively.

GPa, H2F with the simplest stoichiometry adopts a C2/c structure containing 24 H atoms and 12 F atoms in a unit cell (Figure 2a). Interestingly, a novel H3 unit emerges with distinctive [H3][HF2] complex. The H−H distances in the H3 unit are 0.844 and 0.880 Å, an isosceles triangle with vertex angle 62.8°. An analysis of the nearest neighbor distance of F− H reveals the existence of linear HF2 molecules (with equal H− F distance of 1.09 Å at 80 GPa comparable with the covalent bond length of 1.01 Å for HF-Cmc21 at 0 GPa47). The unexpected formation of the triangle H3 unit inspires us to perform a thorough analysis on the chemical bonding of the H2F-C2/c. The electron localization function (ELF) is calculated first, which is known to be an information tool to distinguish different bonding interactions in a solid. High ELF values indicate paired electrons (i.e., lone pairs, bonds, and core electrons), whereas values around 0.5 behave as in a uniform electron gas. The ELF values at areas between H atoms in the H3 unit are large (above 0.8), showing a covalent bonding character (Figure 2a). In addition, the ELF value at areas between the nearest neighboring H and two F atoms is near 0.75, a characteristic of covalent bonding. Based on analysis of ELF, it is distinctive that there exist H3 and H2F molecule units in H2F-C2/c. Furthermore, we calculated the charges using Bader’s quantum theory of atoms in molecules analysis50 for H2F-C2/c at 80 GPa. The approximate charge values of the H atom in the H3 molecule unit are +0.318, +0.318, and +0.118. The charge for one H atom and two F atoms in the HF2 molecule is +0.726, −0.740, and −0.740, respectively. In all, the total charge is +0.754 (2 × 0.318 + 0.118) for the H3 molecule unit and −0.754 (−0.740 × 2 + 0.726) for the HF2 molecule unit. It can be seen that the charge transfer from H3 to HF2 is about 0.75e at 80 GPa, illustrating the ionic nature with the notation of [H3]δ+[HF2]δ− (δ = 0.75). H3F adopts a P21/m structure with [H3]F complex at 200 GPa, as shown in Figure 2b. The H3 unit also forms an isosceles triangle with H−H bond lengths 0.804 Å and 0.907 Å and vertex angle 68.7°. The nearest H−F distance is 1.270 Å, which is much larger than the covalent bond length of 1.01 Å for HFCmc21, suggesting no covalent bonding between H and F. Similarly to H2F, the ELF values between H atoms in the H3 unit are more than 0.8, showing H3 molecule character. On the basis of Bader theory, the total charge is +0.729 for the H3 molecule and −0.729 for F at 200 GPa, illustrating the ionic nature with the notation of [H3]δ+[F]δ− (δ = 0.73).

The predicted crystalline phase of H5F at 160 GPa has a high-symmetry hexagonal R3 space group with the [H3][HF2][H2]3 complex. This H3 unit forms a perfect equilateral triangle with bond length 0.854 Å and also shows a molecular character (Figure 2c). The H−F bond length in the HF2 molecule is asymmetric with 1.032 and 1.106 Å. Besides, there are different ELF values (approximation 0.75 and 0.6) between H and F (Figure 3d), suggesting a strong and weak covalent bond in the HF2 molecule. In addition, the H−H bond length within the H2 pair for the structure is 0.708 Å, which is smaller than the intramolecular distance (0.741 Å) in pure C2/c-H2. The shortening H−H distance indicates that there may be a strong repulsive interaction between H3 and H2 molecules. The Bader charges for H3, HF2, and H2 are +0.707, −0.776, and +0.023, respectively. Predicted Structures of HnCl (n = 2, 3, 5, 7). The crystal structures and lattice parameters for stable stoichiometries H2Cl, H3Cl, H5Cl, H7Cl are shown in Figure 2d−g and Table S1 of the Supporting Information, respectively. H2Cl is stable in the C2/c structure up to 250 GPa, which consists of HCl zigzag sheets that trap H2 molecules in channels, as shown in Figure 2d. The HCl sublattice is similar to the pure HCl phase IV (Cmcm),48 with H atoms occupying midpoint of two neighboring Cl (hydrogen bond symmetrization). The predicted H3Cl adopts three phases with space group Cc (12−40 GPa), C2/c (40−60 GPa), and P212121 (60−120 GPa). The Cc structure consists of H-bonded HCl zigzag sheets and H2 molecules sheets forming sandwiched configuration (Figure S4a in the Supporting Information). Hydrogen bonds are formed along the HCl chains that roughly follow the c axis (Cl−H···Cl = 3.177 Å with Cl−H = 1.368 Å H···Cl = 1.769 Å at 20 GPa). With increasing pressure, the covalent bond distance increases corresponding to decrease of hydrogen bond. When the pressure was up to 40 GPa, the bond lengths of Cl− H and H···Cl are equal with 1.493 Å, and the hydrogen bond symmetrization happens. At the same time, the Cc structure is converted into a symmetrized C2/c structure (Figure S4b in the Supporting Information). Further compression to 60 GPa, orthorhombic P212121 structure occurs with zigzag H2 molecule sheets, as shown in Figure 2e. In addition, H7Cl is found to be stable in a structure with P21 symmetry consisting of symmetrization H−Cl zigzag chains and two H2 molecules sheets, as shown in Figure 2g. The predicted H5Cl prefers a Cc space group throughout the pressure range (120−200 GPa) and exhibits a distinctive crystal 11061

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Figure 3. Electron localization function (ELF) maps of (a) H2F-C2/c at 80 GPa (plane containing H3 and HF2 unit), (b) H3F-P21/m at 200 GPa (plane containing H3 and F unit), (c) H5F-R3 at 160 GPa (plane containing H3 unit), (d) H5F-R3 at 160 GPa (plane containing HF2 unit), (e)−(g) H5Cl-Cc at 60, 100, and 160 GPa (plane containing H3, H2, and Cl units), respectively.

structure (Figure 2f). Instead of forming a traditional [HCl][H2]2, it consists of a [H3][Cl][H2] complex. At 220 GPa, the H−H distances in the H3 unit measure 0.879, 0.876 and 0.880 Å with bond angles 59.8, 60.1, and 60.2°, approximate an equilateral triangle. In addition, the H−H distances in the H2 unit is 0.753 Å, close to the intramolecular distance of 0.750 Å in pure H2-C2/c. Furthermore, the two nearest neighbor H−Cl distances are 1.59 and 1.61 Å, which are much larger than the covalent bond length of 1.32 Å in pure HCl-Cmc21, suggesting that covalent bond between H and Cl disappears. Analysis of ELF values show that there exists covalent bond in H3 unit and H2 pair, and no-covalent bonding between H and Cl atoms. On the basis of Bader theory, the different charges for inequivalent hydrogen and chlorine ions are determined by their distinct chemical environments. It is seen that the charges for the H3, H2, and Cl units are +0.476, +0.015, and −0.490, respectively.

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DISCUSSION We clearly see the formation process of H3+ species in H5Cl-Cc, through comparing bond length, bond overlap population (BOP), and ELF as a function of pressure shown in Figures 3 and 4. At 60 GPa, there exist two type H2 molecules (H2−H3 = 0.77 Å, H4−H5 = 0.74 Å) and a HCl molecule (H1−Cl1 = 1.32 Å). With increasing pressure, the bond length of H1−Cl1 increases to 1.59 Å at 120 GPa, a characteristic of molecular dissociation (Figure 4a). In addition, the bond length of H2− H3 increases to 0.85 Å, while the distances of H1···H3 and H1···H2 decrease to 0.90 and 0.93 Å at 120 GPa, respectively, forming a distorted triangle H3 molecular unit. In addition, the bond length of H4−H5 changes little, showing H2 molecule nature in the whole pressure range. Moreover, the distances of H3···H4 and H3···H5 (1.27 and 1.30 Å at 120 GPa) are smaller than the nearest intermolecular distance in pure C2/c-H2, indicating a weak interaction between H3 and H2 molecules. To further demonstrate the mixed bonding behavior of H5Cl-Cc, we use Mulliken population analysis to explore the BOP. The BOP is widely used to assess the covalent or ionic bonding nature of particular bulk crystals. A high value of the BOP indicates a high degree of covalency in the bond, whereas a low value indicates a more ionic interaction. At 60 GPa, the

Figure 4. (a) Interatomic distances of H5Cl-Cc as a function of external pressure. (b) Bond overlap population of H5Cl-Cc as a function of external pressure.

calculated BOP value for H1−Cl1 is 0.63 |e| (Figure 4b), which sits between 0.87|e| of covalent Si and 0.22 |e| of ionic NaCl, indicating the covalent character. With increasing pressure, the BOP value of H1−Cl1 is reduced to 0.24 |e| at 120 GPa (0.05 | e| at 220 GPa), suggesting a more ionic bonding feature. On the contrary, the BOP value of H2−H3 is 1.09 |e| at 60 GPa and decreases to 0.72 |e| at 120 GPa, which means that covalent bonding weakens gradually with increasing pressure. Moreover, the BOP values of H1−H2 and H1−H3 are increased from 11062

DOI: 10.1021/acs.jpca.5b08183 J. Phys. Chem. A 2015, 119, 11059−11065

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The Journal of Physical Chemistry A Table 1. Bader Charge of H, Cl, H2, and H3 Ions in H5Cl-Cc at Different Pressures P (GPa)

H1

H2

H3

H4

H5

Cl

H1−H2−H3

H4−H5

60 80 100 120 140 160 180 200 220

−0.383 −0.385 −0.365 −0.274 −0.223 −0.200 −0.182 −0.165 −0.155

−0.061 −0.075 −0.089 −0.132 −0.171 −0.170 −0.160 −0.156 −0.139

0.005 −0.009 −0.035 −0.103 −0.111 −0.128 −0.150 −0.164 −0.183

0.007 −0.005 0.002 0.008 0.010 0.009 0.001 0.002 0.000

0.003 0.016 0.008 −0.002 −0.009 −0.013 −0.008 −0.014 −0.015

0.429 0.457 0.480 0.504 0.504 0.503 0.498 0.497 0.490

−0.439 −0.468 −0.489 −0.510 −0.505 −0.499 −0.491 −0.484 −0.476

0.011 0.011 0.009 0.006 0.001 −0.004 −0.007 −0.013 −0.015

0.17 |e| and 0.12 |e| at 20 GPa to 0.57 |e| and 0.41 |e| at 120 GPa, respectively, indicating a more covalent bonding feature under high pressure. It is noted that the BOP values of H1−H2, H1−H3, and H2−H3 are nearly equal with 0.68 |e|, 0.61|e|, and 0.63 |e| at 240 GPa, respectively. In agreement with above bond population analysis, our calculation of ELF also supports formation of the H3+ species. At 60 GPa, the highest ELF values are found in the H2 molecules, as shown in Figure 3e. In addition, the ELF value in the region of the H1−Cl1 is more than 0.8, suggesting covalent bonding, and in the region of H1−H2 and H1−H3 is close to 0.5, indicating no bonding. As the pressure is increased, the ELF value along the H1−Cl1 bond decreases, pushing the H1 atoms close to the H2 and H3 atoms, which results in the increase of the ELF values along the H1−H2 and H1−H3 direction (Figure 3d). At 160 GPa, the H1 and Cl1 ELF basins become virtually disconnected and the values in the region of H1, H2, and H3 are above 0.8, forming the H3 molecule unit (Figure 3f). This corroborates the notion that the application of high pressure leads to an increase of the ionicity of H−Cl bond and subsequent effective self-dissociation into Cl− anions and isolated [H3]+ cations. The variations of Bader charges for H and Cl atoms in H5ClCc at different pressures are explicitly shown in Table 1. The distinct charge transfer (about 0.38 |e|) from H1 to Cl, which suggests a polar covalent bonding feature between H1 and Cl below 100 GPa. However, the charge transfer from H1 to Cl decreases with increasing pressure, which leads to a reduction of the bond energy and an elongation of H1−Cl bond distance. In addition, the charge transfer from H2 and H3 to Cl are increased to 0.132 |e| and 0.103 |e| at 120 GPa, resulting in an elongated H2−H3 bond. Moreover, the charge transfers from H4 and H5 to Cl are very small, which can be negligible. It is concluded that the predicted charge transfer from H1, H2, and H3 to Cl significantly alters the competing attractive and repulsive forces between the nearest neighbors, leading to the eventual formation of the H3+ cation and Cl− anion. At the same time, the molecular phase converts into an ionic phase, with a sudden collapse in the H−Cl distance. The pressure-induced formation of ionic solid on molecular system has been predicted theoretically for B,51 H2O,52 and NH353 and experimentally for N2O.54 Partial ionization at high pressure has been claimed even for pure elements B forming covalent networks, such as [B2]+[B12]−. We also note that ammonia and ice have been predicted to disproportionate at high pressure into [NH4]+[NH2 ]− and [H3O]+[OH]−, respectively. It has been reported that an unusual ionic solid [NO]+[NO3]− was observed at pressures above 20 GPa and temperatures above 1000 K by laser heating N2O.

To further investigate the properties of H3+, we calculated the Raman spectra of stoichiometries H2F, H3F, H5F, and H5Cl, as shown in Figure 5. For isolated H3+, there exist three normal

Figure 5. Calculated Raman spectra of (a) H2F-C2/c at 80 GPa, (b) H3F-P21/m at 200 GPa, (c) H5F-R3 at 160 GPa, and (d) H5Cl-Cc at 160 GPa.

modes of vibration: totally symmetric breathing ν1 mode (3178.3 cm−1) and two wagging ν2 modes (2521.3 cm−1).6 From visualization of Raman peaks of stoichiometries H2F, H3F, and H5F lying above 2300 cm−1, they are assigned to H3+ wagging mode (ν2) between 2300 and 2780 cm−1, breathing mode (ν1) between 3700 and 4160 cm−1, and H2 stretch (ν) mode above 4630 cm−1. For H5Cl, the peaks between 1700 and 2200 cm−1 can be assigned to H3+ wagging modes (ν2) and in the region from 3060 to 3350 cm−1 is attributed to breathing mode (ν1). In addition, the peaks above 4000 cm−1 are assigned to the H2 stretch ν mode. These modes are shown schematically in Figure 5 and could serve as a fingerprint to characterize stoichiometries H2F, H3F, H5F, and H5Cl, particularly the H3+ breathing mode.

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CONCLUSION In summary, we systematically study the phase stabilities and structures of halogen polyhydrides HnX (X = F and Cl, and n > 1) between 10 and 300 GPa using ab initio calculations. Even though HnF are unstable with respect to decomposition into HF and H2 up to 300 GPa, a novel H3+ species is first observed in stoichiometries H2F, H3F, and H5F. In addition, H3Cl is 11063

DOI: 10.1021/acs.jpca.5b08183 J. Phys. Chem. A 2015, 119, 11059−11065

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The Journal of Physical Chemistry A

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found to become stable above 12 GPa and consists of zigzag H−Cl chains and H2 molecules. Above 100 GPa, H5Cl is stable and contains the H3+ cation and Cl− anion along with H2 molecule units. On the basis of bond length, BOP, ELF, and Bader charge, the formation processes of H3+ species in H5Cl are clearly seen. Moreover, a pressure-induced charge transfer from H to Cl results in forming H3+ and Cl− units. Our findings represent a significant step to detect H3+ in the solid state under high pressure, which can stimulate future high-pressure experiments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b08183. Convex hull of F−H, HF−H, and Cl−H systems at different pressures, selected structures of H3Cl, calculated phonon DOS, structure parameters of stable structures for H2Cl, H3Cl, H5Cl, and H7Cl, and list of reactions studied in this paper (PDF)

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AUTHOR INFORMATION

Corresponding Author

*T. Cui. E-mail: [email protected]. Tel./Fax: +86-43185168825. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2011CB808200), National Natural Science Foundation of China (Nos. 11204100, 51572108, 11504127, 11574109, 11404134), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Found for Fostering Talents of basic Science (No. J1103202). China Postdoctoral Science Foundation (2012M511326, 2013T60314, and 2014M561279). Part of calculations were performed in the High Performance Computing Center (HPCC) of Jilin University.

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REFERENCES

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DOI: 10.1021/acs.jpca.5b08183 J. Phys. Chem. A 2015, 119, 11059−11065