Letter pubs.acs.org/JPCL
Superconducting High-Pressure Phases Composed of Hydrogen and Iodine Andrew Shamp and Eva Zurek* Department of Chemistry, State University of New York at Buffalo, 331 Natural Sciences Complex, Buffalo, New York 14260-3000, United States S Supporting Information *
ABSTRACT: Evolutionary structure searches predict three new phases of iodine polyhydrides stable under pressure. Insulating P1-H5I, consisting of zigzag chains of (HI)δ+ and H2δ− molecules, is stable between 30 and 90 GPa. Cmcm-H2I and P6/mmm-H4I are found on the 100, 150, and 200 GPa convex hulls. These two phases are good metals, even at 1 atm, because they consist of monatomic lattices of iodine. At 100 GPa the superconducting transition temperature, Tc, of H2I and H4I is estimated to be 7.8 and 17.5 K, respectively. The increase in Tc relative to elemental iodine results from a larger ωlog from the light mass of hydrogen and an enhanced λ from modes containing H/I and H/H vibrations.
T
molecules held together by van der Waals (vdW) forces. Density functional theory (DFT) calculations have been undertaken to study the structural evolution of these systems up to 200 GPa.38 HI assumes a planar distorted hydrogenbonded diamond lattice, but its detailed structure is unknown.39 We therefore proceeded to find the global minimum structures of HI from 0 to 200 GPa in 50 GPa intervals using the evolutionary algorithm (EA) XTALOPT40 coupled to PBEDFT41 calculations carried out with VASP.42 More information about the computational details is provided in the Supporting Information, SI. The most stable structures that emerged in our EA searches were composed of 2D segregated layers of H2 and iodine, implying that HI is metastable with respect to these constituents. We also optimized geometries where HI assumed the experimentally determined and theoretically predicted lowtemperature structures of the lighter hydrogen halides.38 All of these were less stable than the H2/I segregated phases, suggesting that the impurities and side-products observed upon compression of HI above 70 GPa43 may be a result of decomposition. A previous theoretical study found that HBr was unstable with respect to decomposition into H2 and Br2 between 120 and 150 GPa.38 At these pressures bromine is monatomic, as is iodine, and it should not be unexpected that HI behaves similarly but at lower pressures. EA runs were carried out to find the most stable crystal lattices of HnI (n = 2−9) at 50, 100, 150, and 200 GPa. The structural motifs present in the hydrogen-rich iodine phases found fall into one of two categories: Iδ+, Hδ+, and H2δ− molecules as in H5I or Iδ+ and H2δ− molecules as in H2I and H4I. Density functionals approximating the effects of vdW interactions and zero-point-energy contributions to the enthalpies altered the transition pressures between the most stable HnCl phases slightly, but they did not affect the identity
he stoichiometries, electronic structures, and emergent behavior of materials are affected by pressure.1,2 Extreme pressures can be used to synthesize materials with unforeseen properties that would otherwise not be possible3−6 and adopt chemical bonding strategies that totally differ from what is observed at 1 atm.7,8 Recently, a number of unique phases have been theoretically predicted and experimentally synthesized. These include LiH2, LiH6,9,10 and HnS phases, some of which may have a superconducting transition temperature, Tc = 190 K at 150 GPa,11−15 surpassing those of the oxide-based hightemperature superconductors. The exceptionally high Tc that has been observed experimentally and theoretically predicted for HnS phases has sparked urgent renewed interest in the discovery of other hydrogen-rich pressure-stabilized superconducting phases, and this is part of the motivation for this work. Other predicted hydrogen-rich phases include sub- or polyhydrides containing an electropositive element9,16−24 and Group 14 hydrides.25−33 Some of these may be superconducting24 or contain unusual structural motifs such as the linear symmetric H3− anionthe simplest three-center-fourelectron bond.20 Recently, H2Cl and H5Cl were predicted to become stable with respect to decomposition into H2 and HCl between 60 and 300 GPa.34 All of the predicted HnCl phases contained zigzag [HCl]∞ chains reminiscent of those present within HCl. The three-center-two-electron (3c-2e) H3+ motif, which approached the ideal equilateral triangle configuration by 300 GPa, was incorporated within H5Cl. Herein, we theoretically study the iodine polyhydrides, HnI, because it has been proposed that pressure can be used to synthesize alloys of metallic hydrogen with the general formula (H2)1−xHIx that are potentially superconducting.35 HI undergoes a pressureinduced insulator to metal transition below 50 GPa,35 and iodine becomes metallic at 16 GPa36 and superconducting below 1.2 K at 28 GPa.37 The isomorphic low-temperature phases of HF, HCl, and HBr contain planar zigzag chains of hydrogen-bonded © XXXX American Chemical Society
Received: August 22, 2015 Accepted: September 25, 2015
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∼30 GPa, and it has the most negative ΔHf of any structures examined until 90 GPa. At this pressure the H2I stoichiometry becomes the lowest point on the hull and remains so until at least 200 GPa. H4I also comprises the 100, 150, and 200 GPa hulls. At 200 GPa the ΔHf of the phases continues to decrease. Even though some of the stoichiometries predicted to be stable in our calculations were also found in the hydrogen-rich H/Cl phase diagram under pressure,34 the structures adopted and their properties are distinct, highlighting the importance of the electronegativity (2.66 for I and 3.16 for Cl on the Pauling scale) and radius (1.98 Å for I and 1.75 Å for Cl) of the halogen. Importantly, whereas the HnCl structures were not superconducting,34 we predict using the Allen−Dynes−McMillan formula50,51 that electricity may pass without resistance through H2I and H4I below ∼7.8 and 17.5 K at 100 GPa, respectively. The most stable H5I phase possessed P1 symmetry up to 90 GPa and assumed Pmma symmetry at higher pressures. Above 150 GPa, another phase, containing only molecular hydrogen, became preferred but did not fall on the convex hull. Phonon calculations at 50 GPa verified the dynamic stability of P1-H5I. Just like the HnCl phases predicted in ref 34, H5I contained zigzag [HX]∞ (X = Cl, I) chains that resembled those found in HCl, HBr, and HI at low temperatures.38 These chains lay parallel to the crystallographic b axis in P1-H5I, illustrated in Figure 2a. At 50 GPa the H−I distances along the chains were nearly equivalent, 1.795 and 1.788 Å, indicative of pressureinduced multicenter bonding, which has also been observed in HX (X = F, Cl, Br)38 and H2Cl.34 The H−I−H and I−H−I angles measured 85.7 to 85.8° and 179.4 to 179.8°. The chains arranged to form sheets that were surrounded by layers of H2 units in which the H−H bond measured 0.747 Å, nearly identical to what we calculate for the isolated gas phase molecule at 1 atm, 0.750 Å. Actually, six H2 molecules surrounded half of the hydrogen atoms in the [HI]∞ chains in a “side-on” fashion. We previously found structures where H2
of the phases nor their stability.34 For this reason, we expect them to have a negligible effect on our results. Figure 1 shows the ΔHf of the predicted phases with respect to the elemental solids. The convex hull is the set of line
Figure 1. (a) Enthalpy of formation, ΔHf, for the reaction ((1/2)H2)n + I → HnI versus the H2 composition as a function of pressure. For H2 we used the enthalpy of the structures from ref 44 and for iodine the structure sequence Cmca → Immm → I4/mmm → Fm3m.45−48 The computed transition pressures of 18, 42, and 55 GPa are in good agreement with the experimental ones, 21, 43, and 55 GPa (neglecting the incommensurately modulated phase49). Fm3m iodine persists until at least 275 GPa.48 (b) ΔHf versus pressure of select HnI phases. ΔHf(H5I) becomes negative for P > 30 GPa. Above 90 GPa H2I has the most negative ΔHf.
segments below which no other points lie, and the phases whose ΔHf comprise the hull are thermodynamically stable. At 50 GPa H5I is the only species on the hull. This stoichiometry becomes stable with respect to decomposition into H2 and I at
Figure 2. (a) Supercell of P1-H5I at 50 GPa. Hydrogen atoms are white, and iodines are purple. The zigzag HI chains are a common motif in compressed hydrogen halides. (b) Contour plot of the ELF map of P1-H5I shown in a plane parallel to the b axis. (c) Total and projected DOS of P1H5I at 50 GPa. EF has been set to zero. 4068
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Figure 3. Supercells, total, and projected DOS plots of (a) Cmcm-H2I and (b) P6/mmm-H4I at 100 GPa.
exhibiting mainly Hδ+ s character, but because GGA functionals underestimate the pressure associated with band-gap closure, this phase likely metalizes at a pressure outside the stability range of the H5I stoichiometry. HCl and HBr were computed to be metallic by 130 and 80 GPa, and the bands at EF were primarily of halogen p character.38 Between 90 and 200 GPa a H2I phase with Cmcm symmetry had the most negative ΔHf, and phonon calculations revealed it is dynamically stable at 100 GPa. This was the most stable H2I structure in evolutionary searches performed at 50−200 GPa, and it does not bear any resemblance to the H2Cl phase predicted to be stable by 100 GPa.34 Two rows of H2 molecules arranged in a zigzag fashion run through channels formed by the iodine host within Cmcm-H2I (Figure 3a). At 100 GPa the hydrogen molecules measured 0.796 Å and the distance between them was 1.399 Å, compared with 0.732 and 1.495 Å in solid H2 at this pressure. The I−I contacts measured between 2.896 and 2.904 Å, very close to the 2.899 to 2.920 Å computed within Fm3m iodine at 100 GPa. The shortest H−I contacts were 2.097 Å, suggesting little interaction between the two lattices. High ELF values were calculated between the hydrogens comprising the H2δ− molecules. Similar to compressed solid iodine, a large ELF between the iodine atoms was not observed. A Bader analysis showed charge is transferred from iodine to hydrogen, (H−0.07)2 I0.13, at 100 GPa. By 200 GPa, the H2 bonds within Cmcm-H2I stretch to 0.817 Å, and the distance between them shrinks to 1.198 Å, suggesting that if this phase is stable at higher pressures, these chains may polymerize in a 1-D hydrogenic chain, as predicted for R3mSrH6 by 250 GPa.23 A P6/mmm symmetry H4I structure also comprised the 100 GPa convex hull and was found to be the most stable H4I phase between 90 and 200 GPa. Phonon calculations revealed dynamic stability at 100 and 150 GPa. This phase is composed
encircled the electropositive metal in this manner, for example in MgH12 and MgH16.21 A Bader analysis revealed the charges on the hydrogen and iodine atoms in the H−I chains are +0.03 and +0.12, respectively, as compared with −0.01 and +0.01 for the hydrogen and iodine atoms in a hypothetical HI phase at 50 GPa, whose Cmc21 symmetry was chosen because HF, HCl, and HBr assume this structure at ambient pressure. All of the hydrogens in the H2 molecules bore a slight negative charge, yielding an average of (H−0.04)2. The distance between the hydrogens comprising H2 and the those in the H−I chains ranged from 1.950 to 2.022 Å, and the closest iodine atom was at least 2.273 Å away. This suggests that the H2 molecules do not interact strongly with any of the atoms comprising the [HI]∞ zigzag chains. Indeed, the H2 vibron frequency ranged from ∼4150−4250 cm−1, comparing well with the 4161 cm−1 observed for gaseous H2 at 1 atm.52 The calculated electron localization function (ELF) for P1-H5I at 50 GPa (Figure 2b) reveals a high tendency of electron pairing within the H2δ− units and along the nearly symmetric [HI]∞ chains, reminiscent of the one calculated for symmetric HBr at 30 GPa.38 Cc-H5Cl was found to be stable in the hydrogen-rich H/Cl phase diagram between 100 and 300 GPa.34 By 300 GPa, H3+ motifs, whose bond lengths (∼0.89 Å) and angles (60°) approached those of the gas-phase species, emerged in this phase, and signatures of H3+ vibrations were observed in the phonon DOS between 1800 and 3400 cm−1. We do not see any evidence of the formation of H3+ within H5I up to at least 200 GPa. At 50 GPa, the PBE band gap of P1-H5I is ∼0.5 eV, Figure 2c. The I s states overlap primarily with the Hδ+ s states, whereas I p character is found in a region containing both H2δ− and Hδ+ contributions. Metalization occurs by 65 GPa within PBE: It results from pressure-induced broadening of the valence band, primarily of iodine p character, and the conduction band, 4069
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and Tc? At 100 GPa, ωlog becomes larger as the H/I ratio increases (Table 1) because of the light mass of hydrogen. In addition, the λ of H4I is slightly larger than H2I, and these are both significantly larger than pure iodine. The most significant contribution toward λ in the iodine polyhydrides stems from vibrational modes between 450 and 1300 cm−1 that involve hydrogen and iodine motions, with the hydrogen vibron contributing substantially toward λ (Figure 4). Vibrations
of 1-D chains of iodine atoms with an I−I distance of 2.812 Å and H2 molecules measuring 0.799 Å, both lying parallel to the c axis (Figure 3b). The intermolecular distances between the dihydrogen molecules (1.875 to 2.013 Å) and between H−I (∼2.13 Å) are too long for bonding interactions. Similar to H2I, the iodine donates electrons (the Bader charges are (H−0.05)4I0.21), and the ELF indicates high electron localization within the dihydrogen molecules. At 200 GPa, the H2 units are not yet close enough to polymerize. The total and projected DOS plots for Cmcm-H2I and P6/ mmm-H4I at 100 GPa resemble each other (Figure 3), which is not surprising considering their structural similarities. Both phases are good metals with the DOS at EF being 0.061 and 0.049 eV−1 states/(valence electron), respectively. The DOS can be split into three regions: < −13, −13 to −5 eV, and above −5 eV it is primarily of I s, H2 s, and I p character, respectively, but the H2 s states have a large degree of overlap with both iodine s and p. The metallicity is primarily due to iodine p states, but for P ≥ 100 GPa the hydrogenic states provide a substantial contribution to the DOS at EF. Pressure causes intramolecular I−I bonds in Cmca-I2 to stretch and intermolecular distances to decrease. At ∼16 GPa this phase undergoes an insulator-to-metal transition,36 superseded by an incommensurate phase at 23 GPa and the formation of a monatomic phase within which the I−I distances are “equalized” by 30 GPa.49 Experiments yielded a bond length of 2.75 Å under ambient conditions, whereas the nearestneighbor bonds in the fully dissociated monatomic crystal measured 2.89 Å.49 In our calculations bond equalization occurs near 35 GPa with I−I distances of 2.98 Å. We find that CmcmH2I and P6/mmm-H4I are metallic at 1 atm, and the DOS at EF exhibits primarily iodine p character. The metallicity results from the monatomic iodine present in these phases under ambient conditions, wherein the nearest-neighbor distances are found along 1-D chains measuring 2.93 Å. Experiments yield a Tc of 1.2 K at 28 GPa for iodine.37 Theory has shown that under hydrostatic conditions Tc decreases with increasing pressure, falling to 1). Phys. Rev. Lett. 2011, 106, 237002. (17) Hooper, J.; Zurek, E. Lithium Subhydrides Under Pressure and Their Superatom-Like Building Blocks. ChemPlusChem 2012, 77, 969−972. (18) Hooper, J.; Zurek, E. Rubidium Polyhydrides Under Pressure: Emergence of the Linear H3− Species. Chem. - Eur. J. 2012, 18, 5013− 5021. (19) Hooper, J.; Zurek, E. High Pressure Potassium Polyhydrides: A Chemical Perspective. J. Phys. Chem. C 2012, 116, 13322−13328. (20) Shamp, A.; Hooper, J.; Zurek, E. Compressed Cesium Polyhydrides: Cs+ Sublattices and H3− Three-Connected Nets. Inorg. Chem. 2012, 51, 9333−9342. (21) Lonie, D. C.; Hooper, J.; Altintas, B.; Zurek, E. Metallization of Magnesium Polyhydrides Under Pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 054107. (22) Hooper, J.; Altintas, B.; Shamp, A.; Zurek, E. Polyhydrides of the Alkaline Earth Metals: A Look at the Extremes Under Pressure. J. Phys. Chem. C 2013, 117, 2982−2992. (23) Hooper, J.; Terpstra, T.; Shamp, A.; Zurek, E. The Composition and Constitution of Compressed Strontium Polyhydrides. J. Phys. Chem. C 2014, 118, 6433−6447. (24) Wang, H.; Tse, J. S.; Tanaka, K.; Iitaka, T.; Ma, Y. Superconductive Sodalite-Like Clathrate Calcium Hydride at High Pressures. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 6463−6466. (25) Gao, G.; Oganov, A. R.; Bergara, A.; Martinez-Canales, M.; Cui, T.; Iitaka, T.; Ma, Y.; Zou, G. Superconducting High Pressure Phase of Germane. Phys. Rev. Lett. 2008, 101, 107002. (26) Chen, X.; Wang, J.; Struzhkin, V. V.; Mao, H.; Hemley, R. J.; Lin, H. Superconducting Behavior in Compressed Solid SiH4 with a Layered Structure. Phys. Rev. Lett. 2008, 101, 077002. (27) Martinez-Canales, M.; Oganov, A. R.; Ma, Y.; Yan, Y.; Lyakhov, A. O.; Bergara, A. Novel Structures and Superconductivity of Silane Under Pressure. Phys. Rev. Lett. 2009, 102, 087005. (28) Martinez-Canales, M.; Bergara, A.; Feng, J.; Grochala, W. Pressure Induced Metallization of Germane. J. Phys. Chem. Solids 2006, 67, 2095−2099. (29) Pickard, C. J.; Needs, R. J. High-Pressure Phases of Silane. Phys. Rev. Lett. 2006, 97, 045504. (30) Yao, Y.; Klug, D. D. Silane Plus Molecular Hydrogen as a Possible Pathway to Metallic Hydrogen. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 20893−20898. (31) Flores-Livas, J. A.; Amsler, M.; Lenosky, T. J.; Lehtovaara, L.; Botti, S.; Marques, M. A. L.; Goedecker, S. High-Pressure Structures of Disilane and Their Superconducting Properties. Phys. Rev. Lett. 2012, 108, 117004. (32) Kim, D. Y.; Scheicher, R. H.; Lebegue, S.; Prasongkit, J.; Arnaud, B.; Alouani, M.; Ahuja, R. Crystal Structure of the Pressure-Induced Metallic Phase of SiH4 from Ab-initio Theory. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 16454−16459.
transfer of charge from iodine to H2. The metallicity persists as H2I and H4I become dynamically and thermodynamically stable phases. The DOS at EF is primarily due to iodine p states, with an admixture of hydrogen s. At 100 GPa the Tc of H2I and H4I is estimated as being 7.8 and 17.5 K, respectively. The presence of H2 significantly enhances Tc compared with pure iodine because the light atomic mass of hydrogen increases ωlog and because vibrations involving H and I atoms, as well as the H2 vibron, contribute substantially to λ. As the quest for hightemperature superconductivity in hydrides gains momentum, we look forward to the eventual synthesis of these and other intriguing phases.
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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.jpclett.5b01839. Details of the computations, band structures, structural parameters, and DOS plots. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ezurek@buffalo.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the NSF (DMR-1505817) for financial support and the Center for Computational Research (CCR) at SUNY Buffalo for computational support. A.S. acknowledges financial support from the Department of Energy National Nuclear Security Administration under Award Number DENA0002006, and E.Z. thanks the Alfred P. Sloan Foundation for a research fellowship (2013-2015). We thank Duck Young Kim for fruitful discussions.
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