ARTICLE pubs.acs.org/JPCC
Pressure-Induced Formation of Noble Metal Hydrides Guoying Gao, Hui Wang, Li Zhu, and Yanming Ma* State Key Lab of Superhard Materials, Jilin University, Changchun 130012, P. R. China ABSTRACT: Noble metals (Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) do not form hydrides with hydrogen under ambient conditions originated from the extremely low solubility of hydrogen in them. Because of such limitation, finding these hydrides under ambient conditions is scarce. Here we use a combination of particle swarm optimization technique on crystal structure prediction and first-principles calculations based on density functional theory to explore the reactivity of hydrogen with noble metals under high pressure. We demonstrated that except for Au, other noble metals can commonly form monohydrides under high pressure even at zero temperature. We find universal high pressure structure features of these monohydrides the structures either are stable or eventually transform into closed packed fcc or hcp with hydrogen occupying the octahedral interstitial sites of the metal sublattices. Of particular interest by the introduction of hydrogen are the high pressure structural changes of these noble metals, which, however, remain remarkably stable for pure elements. Our research highlights the key role of pressure played in the formation of these noble metal hydrides.
I. INTRODUCTION The transition metals, especially the noble metals Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au, have been the central scientific focus due to their novel physical properties and important technical applications. For example, Os is known to be the densest and stiffest metal among transition metals, having a very large bulk modulus (395 462 GPa1,2) comparable to that of diamond, whereas Pt is even used as pressure standards, gaskets, or electrodes in diamond anvil cell experiments.3 Some of the noble metals are superconductors, and the transition-metal Nb holds the record for superconducting critical temperature (9.3 K) of an element at normal pressure.4 Under ambient conditions, the noble metals all adopt close-packed structures such as fcc or hcp and rarely undergo phase transformations up to very high pressures.5 For instance, no phase transitions are observed up to at least 304 GPa in Pt.5 Moreover, the noble metals are chemically very inert, resistant to form compounds with other elements (e.g., Pt only forms compounds with halogens, oxides, and chalcogenides).6 All of this structural simplicity and stability and chemical inertness make the noble metals very unique for scientific research. High pressure can reduce the interatomic distance and then change bonding patterns, causing profound effects on numerous physical and chemical properties and producing many new materials that cannot be formed under normal conditions. For instance, experiments have demonstrated that at high pressures fullshell molecules CH4, SiH4, GeH4, and even inert gas Ar and Xe can react with H2 molecules with the formation of intriguing van der Waals compounds.7 10 Recently, the inert noble metals Os, Ir, Pt, and Pd are experimentally observed to react with nitrogen and carbon at high-pressure and high-temperature conditions.6,11 13 These researches open up the possibility of finding other novel compounds (e.g., hydrides) made up of noble metals. r 2011 American Chemical Society
Hydrogen has a medium electronegativity value of 2.2, obviously larger than those (e.g., 1.22 for yttrium) of early transition metals but comparable or smaller than those (e.g., 2.2 for Ru, Pd, Os, and Ir; 2.28 for Pt; 2.54 for Au) of late transition metals. As a result of much different electronegativities, the early transition metals are found to form hydrides easily (e.g., ScH2/ScH3, YH2/ YH3, TiH2, ZrH2, HfH2, VH2, NbH2, etc.14) under ambient conditions. However, reaction of late transition metals (particular noble metals) with hydrogen is chemically unfavorable, and thus finding these hydrides is scarce at ambient pressure. High pressure is able to produce a steep increase in chemical potential of hydrogen, whose reaction with noble metals might become feasible. Previous experiments have indicated that high pressure makes it possible to synthesize Rh, Pd, and Pt hydrides.15 19 This quests us to perform a thorough search on noble metal hydrides under high pressure, which is of great scientific interest and could have the potential for hydrogen storage. We also target on whether the introduction of hydrogen into the noble metals can induce the lattice changes, where the pure noble metals remain remarkably stable under high pressure. To fulfill the task, we use our developed CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method20 on crystal structure prediction unbiased by any prior known information to predict the crystal structures of noble-metal monohydrides at high pressures. We found that except for Au, all other noble metals can commonly form monohydrides under high pressure, even at zero temperature. These monohydrides adopt ultimate high-pressure structures: either closed-packed fcc or hcp with hydrogen occupying the octahedral interstitial sites of the metal sublattices. Received: November 9, 2011 Revised: December 14, 2011 Published: December 15, 2011 1995
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This research highlights the major role of pressure played in the formation of noble metal hydrides.
II. COMPUTATIONAL METHOD The crystal structure prediction is based on a global minimization of free-energy surfaces merging ab initio total-energy calculations via the CALYPSO method, as implemented in the CALYPSO code,20 which has been successfully applied into the prediction of several high-pressure structures, which were then confirmed by independent experiments.21 23 The incorporation of hydrogen into d-band metals is a consequence of the hydrogen dissociation arising from the catalysis of the transition metals. Atomic hydrogen usually occupies the tetrahedral (T) or octahedral (O) interstices of the metal lattices.24,25 We therefore also consider those structures generated by educated hydrogen occupancies at T or O interstitial sites.25 The underlying ab initio structural relaxations were performed using density functional theory within the Perdew Burke Ernzerh (PBE) of parametrization of the generalized gradient approximation (GGA)26 as implemented in the Vienna ab initio simulation package (VASP)27
Figure 1. Predicted high-pressure structures for PtH (a d) and RuH (e). The Cmc21 structure for PtH is presented in the hexagonal form to compare with the hcp structure.
code. The all-electron projector-augmented wave (PAW)28 method was adopted with the PAW potentials taken from the VASP library. The energy cutoff 600 eV and appropriate MonkhorstPack29 k meshes were chosen to ensure that enthalpy calculations are well-converged to better than 1 meV/formula unit (f.u.). The phonon calculations were carried out by using a supercell approach30 through the PHONOPY code.31
III. RESULTS AND DISCUSSION We first consider two representative Pt and Ru monohydrides, where the elemental Pt and Ru adopt distinct fcc and hcp structures, respectively, at ambient pressure. Structural predictions for PtH and RuH have then been performed through CALYPSO code20 with simulation sizes ranging from one to eight formula units at 15 300 GPa. According to our simulations, PtH at 15 and 30 GPa possesses the stable structure of tetragonal P4/mbm (6 f.u./cell), where H occupancies at the center of the metal quadrilateral (Figure 1a). At 50 GPa, we predicted the formation of an orthorhombic Cmmm phase (2 f.u./cell) with one Pt atom distributed at cubic edges and one Pt atom and two H atoms in face centers of the lattice (Figure 1b). Notably, below 50 GPa, we also found metastable tetragonal structures with space groups P4/nmm and I-4m2, which are seen as the distortion of the bcc structure and actually can be obtained by partial H occupancy of T interstitial sites of the bcc metals. This raises an interesting argument that H occupancies at the center of the metal quadrilateral are energetically more favorable than those with H atoms sitting at the T sites. This finding is in contrast with a previous suggestion that H can only enter two types of interstitial sites O sites and T sites.25 In addition, our predicted tetragonal I-4m2 structure is the same as that predicted by Kim et al.32 at zero Kelvin, but it is metastable. At 80 GPa, we predicted an orthorhombic Cmc21 (4 f.u./cell) structure (Figure 1c), which can be viewed as the slight distortion of the P63/mmc (hcp) structure. Eventually, above 100 GPa, our simulations reached an hcp structure (Figure 1d). The Cmc21 and hcp structures are both regarded as the H occupancy of O sites of the metal sublattices. Remarkably, for RuH, our structural simulation gave one single fcc structure with H at O sites in all studied pressures above 10 GPa (Figure 1e).
Table 1. Lattice Parameters of the Predicted Various Structures for PtH pressure (GPa) 20
space group P4/mbm
lattice parameters (Å, deg)
atomic coordinates (fractional)
a = 5.451, c = 3.821
Pt 2a (0, 0, 0) Pt4h(0.23244,0.73244,0.5) H 2c (0, 0.5, 0.5) H4h (0.23064,0.73064, 0)
30
Cmmm
a = 4.776, b = 2.691, c = 2.698
Pt 2a (0, 0, 0) H 2c (0, 0.5, 0.5)
30
I-4m2
a = 3.469, c = 2.83
Pt 2a (0, 0, 0) H 2c (0, 0.5, 0.25)
30
P4/nmm
a = 3.414, c = 2.915
Pt 2c (0, 0.5, 0.23351) H 2b (0, 0, 0.5)
40
Cmc21
42.9
P63/mmc
50
Fm-3m
a = 3.288, b = 4.496, c = 4.494
Pt 4a (0,0.70731,0.58664) H 4a (0,0.91379,0.94632)
a = 2.795, c = 4.769
Pt 2c (1/3, 2/3, 0.25)
α = 90, β = 90, γ = 120
H 2a (0,0,0)
a = 3.991
Pt 4a (0,0,0) H 4b (0.5, 0.5, 0.5)
1996
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Figure 3. Phonon dispersion curves for hcp PtH at 50, 80, and 100 GPa, respectively. Figure 2. Enthalpy curves (per formula unit) of various structures as a function of pressure with respect to elemental decomposition for PtH (a) and RuH (b). The inset of panel a shows the transition sequence Cmmm f Cmc21 in more detail.
The lattice parameters of the predicted various structures for PtH are listed in Table 1. It is found that the lattice data for hcp-PtH at 42.9 GPa are in excellent agreement with experiments (e.g., our theoretical a = 2.795 Å and c = 4.769 Å versus experimental data of a = 2.773 Å and c = 4.713 Å with a maximum deviation of 1.1%).15 This fact strongly supported the choice of PAW potentials and GGA for the current study. Our current results also confirmed previous experiences that GGA provides better cohesive energies than the local density approximation in the cases of platinum metal (Pt, Ir, Os, Ru, Rh, and Pd) nitrides.33 The calculated enthalpy curves at a greater accuracy for our predicted structures relative to elemental decomposition for PtH and RuH as a function of pressure are presented in Figure 2. It is found that PtH hydride with P4/mbm structure can be actually formed above 3 GPa at zero temperature (Figure 2a). However, one has to take cautions that there might exist large kinetic energy barrier for the formation of PtH, leading to a much higher synthetic pressure.16,19 This situation is similar to that in the study of transition-metal nitrides.34 There, predicted formation pressures are 14 and 18 GPa for IrN2 and OsN2, whereas the actual synthetic pressure is ∼50 GPa. Our enthalpy calculations confirmed the phase transformation into Cmmm structure at 31 GPa and then Cmc21 phase at 42 GPa. Above 100 GPa, the hcp phase becomes most stable. Note that Cmc21 structure is a distortion of hcp phase and can be structurally optimized into hcp phase above 100 GPa. Experimentally, the observed formation of hcp-structured PtH is at 42 GPa,15,16,19 seemingly against the prediction of Cmc21 structure. However, our phonon calculations of hcp structure gave severe imaginary phonons below 100 GPa (Figure 3). This fact together with the obvious higher energy (Figure 2a) suggests that the observed hcp structure is in fact a metastable phase at low pressure and can only be stabilized by the anharmonic effects, sharing the similarity to the situation of simple cubic structure of Ca.35 Figure 2b confirms that RuH
Figure 4. Formation pressures of various transition metal monohydrides within different structures. The crystal structures of pure transition metals at ambient pressure are marked as superscript and designated as b (bcc), f (fcc), or h (hcp).
can be synthesized above 10 GPa and stabilizes in the fcc structure with H atoms occupying the O sites of the metal sublattices throughout the whole pressure range studied. It should be pointed out that the large zero-point energy of hydrogen is not able to alter the topology of the phase diagram, although a resultant change of transition pressure is possible. We have examined the zero-point energies of PtH for various structures at 20 and 200 GPa and found that the inclusion of zero-point motion in fact makes the formation of P4/mbm structure at 20 GPa and hcp structure at 200 GPa energetically more favorable. We also investigated the high-pressure reaction of hydrogen with other noble metals Rh, Pd, Ag, Os, Ir, and Au. The candidate 1997
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Figure 5. Total and partial electronic densities of states for hcp-Ru (a), hcp-RuH (b), and fcc-RuH (c) and the electronic band structure of fcc-RuH (d) at 100 GPa. The vertical dashed lines denote the Fermi level.
high-pressure structures are either borrowed from the currently predicted metastable and stable structures of PtH and RuH or designed by considering H occupancy of T or O sites of metal lattices. We found that all noble metals, except for Au, can form hydrides at pressures achievable within the current diamondanvil cell technique. One intriguing feature is that at the ultimate pressures, except for the formation of hcp PtH, other noble-metal monohydrides all stabilize in fcc structure (Figure 4). It is not occasional that our theoretical fcc structures for RhH and PdH agree well with the experimental observation.17,18 Our calculation strongly suggests the high-pressure formation of Ru, Os, Ir, and Ag monohydrides not yet experimentally observed, in good agreement with the theoretical data by Kim et al.32 However, it is noteworthy that we found that Au cannot form monohydride in view of the thermodynamical instability of AuH, in contrast with the result of ref 32, which only used the lattice dynamics as the criterion of structural stability. The unsuccessful formation of Au monohydride might be largely due to the true fact on the very large energy barrier for H2 molecules dissociating on the surface of Au36 and the larger electronegativity of 2.54 (the highest among the metallic elements) in Au than that (2.2) of H atom on the Pauling scale, which leads to the difficulty for Au to lose electrons. Notably, although Ag is in the same group of Au, the formation of AgH under high pressure is evident. This might be attributed to the much smaller electronegativity (1.93) of Ag, an indication of easier lose of electrons. To understand the physical origin of the stability of fcc structure for most monohydrides, we studied the total and partial density of states (DOS) for pure Ru, fcc-RuH, and hcp-RuH as an illustrative case (Figure 5). It is seen that for pure Ru (Figure 5a), the 4d electrons dominate as expected. The introduction of H atoms into Ru induces a formation
of low-lying hybridization bands between Ru 4d and H 1s states (Figure 5b,c). The stability of fcc-RuH rather than hcp-RuH is clearly energetically driven, as seen from the location at electronic pseudogap of the Fermi level in fcc-RuH (Figure 5d), having a lower density of state at Fermi level than that of hcp phase. This behavior might effectively lower the total energy of fcc phase. More interestingly, the crystal structures of pure noble metals are very stable and rarely undergo phase transitions under even extremely high pressures,5 whereas the introduction of H atoms into the noble metals makes the metal lattices easily change at very low pressures (e.g., hcp Ru and Os vs fcc RuH and OsH). This might be attributed to the fact that the introduction of H weakens the shear modulus and makes the noble-metal monohydrides subject to shear stress and easy to deform. From this study, we find that all noble-metal monohydrides are stable in or finally transform to either closed-packed fcc or hcp with hydrogen occupying the O interstitial sites of the metal sublattices. To extend this finding, we also studied monohydrides of other late transition metals such as Nb, Mo, Tc, Ta, W, Re, and Cu. We consistently found that the ultimate high-pressure structures of these transition-metal monohydrides are either the fcc or hcp structures with H atoms at O sites. By accident, the validity of our argument is supported by the excellent agreement with the experimental observations on MoH.17,18 Notably, our work focuses on the thermodynamical stability of monohydrides under pressure, but this does not exclude the possibility on the pressureinduced formation of nonstoichiometry or nonmonohydrides. However, these other possibilities do not revise our main conclusions on the concept of pressure-induced chemical reaction of noble metals with hydrogen. 1998
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Figure 6. Electronic band structures for RhH, PdH, AgH, OsH, and IrH in the fcc structure and PtH in the hcp structure at 100 GPa.
Figure 7. Total and partial electronic densities of states for RuH, RhH, PdH, AgH, OsH, and IrH in the fcc structure and PtH in the hcp structure at 100 GPa.
From the calculated electronic band structures and densities of state for all of the noble-metal monohydrides, as illustrated in Figures 5 7, it was found that all noble-metal monohydrides are metallic. For the fcc structure, we find the typical band structural feature that near the Fermi level the band along the X W direction is flat (the group velocities of the electrons approach zero), whereas some bands are quite dispersive along X W, X K, and
K Γ directions (Figure 6). These electronic features apparently satisfy the “flat band-steep band” scenario, which has been suggested to be a favorable condition for the occurrence of superconductivity.37 Therefore, these hydrides have a large possibility to be superconductive. Our argument is in good agreement with the observed superconductivity (Tc at 8 to 9 K)38,39 in PdH. Note for AgH that there exists only a rather small DOS at the Fermi 1999
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The Journal of Physical Chemistry C level, and the flat band stays far away from the Fermi level, possibly excluding the superconductivity, in good accordance with the results by Kim et al.32 The calculated partial DOS for all noble metal monohydrides (Figure 7) shows that they all contain the low-lying hybridization bands between noble metals 4d (or 5d) and H 1s states, which are the results of the introduction of H into the noble metals. It is also found that noble metals 4d (or 5d) states contribute most to the total DOS and dominate the DOS at the Fermi level. To check the dynamical stabilities of the currently predicted transition-metal monohydrides, we have also checked the full phonon dispersion curves for various monohydrides. The absence of any imaginary phonons gives direct proof of their dynamical stabilities at the corresponding stable pressure ranges.
IV. CONCLUSIONS In summary, we have used reliable theoretical approaches to predict the stabilization of noble-metal monohydrides at high pressure. We found common high-pressure structure features on these noble-metal monohydrides are the ultimate formation of either fcc or hcp structure with hydrogen occupying the O sites. This finding is generally applied to other transition-metal monohydrides studied in this work. It is also suggested that the ultrastable noble metals can experience high-pressure structural changes in the environment of hydrogen, which might be stemmed from the weakening of shear modulus of noble metals, thus subjected to shear stress and easy to deform. Our research highlights the major role played by pressure in the formation of noble-metal monohydrides and inevitably leads to future experimental synthesis on these technically important hydrides. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Professor John S. Tse for his valuable discussions and we acknowledge funding support from the National Natural Science Foundation of China (under grant nos. 91022029, and 11025418), the research fund of Key Laboratory of Surface Physics and Chemistry (no. SPC201103), and the China 973 Program under grant no. 2011CB808200. The calculations were performed in the High Performance Computing Center of Jilin University.
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(9) Strobel, T. A.; Chen, X. J.; Somayazulu, M.; Hemley, R. J. J. Chem. Phys. 2009, 133, 164512–164519. (10) Somayazulu, M. S.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Science 1996, 271, 1400–1402. (11) Crowhurst, J. C.; Goncharov, A. F.; Sadigh, B.; Zaug, J. M.; Aberg, D.; Meng, Y.; Prakapenka, V. B. J. Mater. Res. 2008, 23, 1–5. (12) Crowhurst, J. C.; Goncharov, A. F.; Sadigh, B.; Evans, C. L.; Morrall, P. G.; Ferreira, J. L.; Nelson, A. J. Science 2006, 311, 1275–1278. (13) Young, A. F.; Sanloup, C.; Gregoryanz, E.; Scandolo, S.; Hemley, R. J.; Mao, H. K. Phys. Rev. Lett. 2006, 96, 155501. (14) Smithson, H.; Marianetti, C. A.; Morgan, D.; Van der Ven, A.; Predith, A.; Ceder, G. Phys. Rev. B 2002, 66, 144107. (15) Degtyareva, O.; Proctor, J. E.; Guillaume, C. L.; Gregoryanz, E.; Hanfland, M. Solid State Commun. 2009, 149, 1583–1586. (16) Hirao, N.; Hiroshi, F.; Yasuo, O.; Kenichi, T.; Takumi, K. Acta Crystallogr. 2008, A64, C609. (17) Antonov, V. E. J. Alloys. Compd. 2002, 330 332, 110–116. (18) Somenkov, V. A.; Glazkov, V. P.; Irodova, A. V.; Shilstein, S. S. J. Less-Common Met. 1987, 129, 171–180. (19) Scheler, T.; Degtyareva, O.; Marques, M.; Guillaume, C. L.; Proctor, J. E.; Evans, S.; Gregoryanz, E. Phys. Rev. B 2011, 83, 214106. (20) Wang, Y.; Lv, J.; Zhu, L.; Ma, Y. Phys. Rev. B 2010, 82, 094116. (21) Li, P.; Gao, G.; Wang, Y.; Ma, Y. J. Phys. Chem. C 2011, 114, 21745–21749. (22) Lv, J.; Wang, Y.; Zhu, L.; Ma, Y. Phys. Rev. Lett. 2011, 106, 015503. (23) Zhu, L.; Wang, H.; Wang, Y.; Lv, J.; Ma, Y.; Cui, Q.; Ma, Y.; Zou, G. Phys. Rev. Lett. 2011, 106, 145501. (24) Besedin, S. P.; Jephcoat, A. P.; Hanfland, M. High Pressure Res. 2000, 17, 225–234. (25) Fukai, Y. Metal-Hydrogen System; Springer: Berlin, 2005. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. (27) Kresse, G.; Furthm€uler, J. Phys. Rev. B 1996, 54, 11169–11186. (28) Bl€ohl, P. E. Phys. Rev. B 1994, 50, 17953–17979. (29) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188–5192. (30) Parlinski, K.; Li, Z. Q.; Kawazoe, Y. Phys. Rev. Lett. 1997, 78, 4063. (31) Togo, A.; Oba, F.; Tanaka, I. Phys. Rev. B 2008, 78, 134106. (32) Kim, D. Y.; Scheicher, R. H.; Pickard, C. J.; Needs, R. J.; Ahuja, R. Phys. Rev. Lett. 2011, 107, 117002. (33) Aberg, D.; Sadigh, B.; Crowhurst, J.; Goncharov, A. F. Phys. Rev. Lett. 2008, 100, 095501. (34) Li, Y.; Wang, H.; Li, Q.; Ma, Y.; Cui, T.; Zou, G. Inorg. Chem. 2009, 48, 9904–9909. (35) Errea, I.; Rousseau, B.; Bergara, A. Phys. Rev. Lett. 2011, 106, 165501. (36) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238–240. (37) Ma, Y.; Tse, J. S.; Klug, D. D.; Ahuja, R. Phys. Rev. B 2004, 70, 214107. (38) Schirber, J. E.; Northrup, C. J. M. Phys. Rev. B 1974, 10, 3818. (39) Stritzker, B.; Buckel, W. Z. Phys. A: Hadrons Nucl. 1972, 257, 1–8.
’ REFERENCES (1) Kenichi, T. Phys. Rev. B 2004, 70, 012101. (2) Cynn, H.; Klepeis, J. E.; Yoo, C. S.; Young, D. A. Phys. Rev. Lett. 2002, 88, 135701. (3) Holmes, N. C.; Moriarty, J. A.; Gathers, G. R.; Nellis, W. J. J. Appl. Phys. 1989, 66, 2962–2967. (4) Cristina, B.; Kevin, R. Supercond. Sci. Technol. 2005, 18, R1. (5) McMahon, M. I.; Nelmes, R. J. Chem. Soc. Rev. 2006, 35, 943–963. (6) Gregoryanz, E.; Sanloup, C.; Somayazulu, M.; Badro, J.; Fiquet, G.; Mao, H. K.; Hemley, R. J. Nat. Mater. 2004, 3, 294–297. (7) Wang, S.; Mao, H. K.; Chen, X. J.; Mao, W. L. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 14763–14767. (8) Strobel, T. A.; Somayazulu, M.; Hemley, R. J. Phys. Rev. Lett. 2009, 103, 065701. 2000
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