ARTICLE pubs.acs.org/JPCC
Phase Behavior of H2 þ H2O at High Pressures and Low Temperatures Timothy A. Strobel,* Maddury Somayazulu, and Russell J. Hemley Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, United States ABSTRACT: Whereas several clathrate-like structures are known to exist from mixtures of H2 þ H2O under pressure, the combined high-pressure and low-temperature region of the phase diagram remains largely unexplored. Here we report a combined Raman spectroscopy and synchrotron X-ray diffraction study on the low-temperature region of the phase diagram. Below ∼120 K, the H2 vibron originating from the clathrate 2 (C2) phase splits into two distinct components, yet X-ray diffraction measurements reveal no structural change between room temperature and 11 K. We suggest that the two vibrons of the C2 phase at low temperature originate from vibrational transitions of hydrogen molecules in the ground and first excited rotational energy levels. At ∼1 GPa we observe the clathrate 1 (C1) phase to persist to the lowest temperature measured (80 K). Upon decompression from the C2 phase we observed the appearance of cubic ice (Ic), which converted to a new phase before transforming to the C1 phase. The structure of the new phase is consistent with a water framework similar to R-quartz; the structure could also be related to the tetragonal clathrate phase reported previously for nitrogen and argon guests.
I. INTRODUCTION Under conditions of high pressure and/or low temperature, mixtures of water and small molecules may crystallize into structures called clathrate hydrates.1 True clathrate hydrates consist of hydrogen-bonded water molecules arranged in polyhedral cavities which trap small guest molecules such as argon or methane. Similarly, small molecules may stuff the void spaces of ice phases, forming “filled ices.” In general, open-network clathrate hydrate structures go through a series of structural transformations with pressure, eventually transforming into dense filled ices.2-4 Filled-ice phases, while not true clathrate hydrates, are often referred to as clathrates due to the similar guest-host relationship; herein, this nomenclature is maintained. In 1993, Vos et al.5 reported the formation of two filled-ice clathrates. At room temperature, H2 þ H2O mixtures crystallize into the C1 (clathrate 1) phase at ∼0.7 GPa. The C1 structure is based upon the framework of ice II (R3). When viewed with hexagonal axes, the unit cell of the C1 phase contains 36 water molecules, in a channel-like arrangement, which host six hydrogen molecules. The lattice constants at 2.1 GPa are a ≈ 12.7 Å and c ≈ 6.0 Å. The C1 phase is unique for water-based clathrates in the sense that the protons are ordered. This H2 þ H2O C1 phase is isostructural with the helium-filled-ice phase reported by Londono et al.6 Further compression of the H2 C1 structure at room temperature results in the formation of a second filled-ice clathrate (C2) at 2.3 GPa. The C2 phase is based on the diamond structure of cubic ice (Ic) (Fd3m) and may be viewed as two interpenetrating fcc lattices (as in ice VII) with points of one lattice occupied by proton-disordered H2O and the other by rotationally disordered H2. The unit cell of C2 contains eight water molecules and eight hydrogen molecules with a ≈ 6.4 Å at 3.1 GPa. At ambient temperature, the C2 structure is stable to at least 40 GPa, above which a denser structure was suggested to persist to at least 80 GPa.7 r 2011 American Chemical Society
Aside from the C1 and C2 phases, structure two (sII) clathrate hydrate is the only additional clathrate phase currently known for H2 þ H2O mixtures. The unit cell of sII consists of 136 water molecules arranged into eight hexakiadecahedral (51264) and sixteen pentagonal dodecahedral (512) hydrogen-bonded polyhedral cavities. The formation of the sII phase was originally inferred from phase equilibrium data8,9 and later confirmed through spectroscopic and diffraction measurements.10 Completely filled sII clathrate contains 48 H2 molecules in the a ≈ 17.1 Å Fd3m unit cell.11 The stability of this phase extends from ∼150 K at ambient pressure to ∼274 K at 0.36 GPa. Above ∼0.36 GPa available experimental evidence suggests this phase transforms into C1.8-10,12-15 Structural parameters for the various H2 þ H2O clathrate phases are provided in Table 1. While the ambient temperature phase behavior of H2 þ H2O mixtures has been widely studied, low-temperature measurements under pressure are extremely limited. We are aware of only one preliminary Raman result for the C2 phase at 77 K.12 Studies at low temperature may reveal additional phases and structural complexity observed in clathrate-forming systems with similar guests. These results are pertinent to hydrogen-rich interstellar ices and may reveal novel high-density hydrogen storage materials. In this work we performed a combined Raman and synchrotron X-ray diffraction study of the H2 þ H2O system from 0.3 to 5 GPa between 11 and 300 K.
II. EXPERIMENTAL METHODS Mixtures of H2 þ H2O were prepared in copper-beryllium (CuBe) piston-cylinder diamond anvil cells containing 300 μm Received: December 24, 2010 Revised: January 30, 2011 Published: February 28, 2011 4898
dx.doi.org/10.1021/jp1122536 | J. Phys. Chem. C 2011, 115, 4898–4903
The Journal of Physical Chemistry C
ARTICLE
diameter anvil culets and CuBe gaskets. A drop of ultrapure H2O (HPLC grade, Alfa Aesar) was placed into the ∼150 μm diameter gasket hole with a ruby sphere for pressure calibration. The cell was assembled, and the piston was oscillated several times within the cylinder until an air bubble (∼100 μm) was trapped. The cell was then placed inside a high-pressure gasloading apparatus, and the air bubble was displaced by H2 at ∼0.2 GPa. All measurements were performed on samples with excess hydrogen to avoid the formation of any pure H2O ice phases. Once the H2 bubble was contained in the cell, the compressive force was transferred to a lever-arm assembly to enable pressure change within a cryostat. Raman spectra were obtained on the C2 phase between 3.4 and 5.1 GPa from 300 to 77 K. Temperature was controlled by the regulation of liquid nitrogen in an open-flow cryostat (Cryoindustries) with a cooling rate of approximately 1.5 K/ min. Spectra were obtained using a Princeton Instruments spectrograph (SP2300, Trenton, NJ) using the 488 nm line of an Arþ laser as an excitation source with ∼20 mW power. Laser light was focused on the sample using a 10 long working distance objective lens, and Raman light was collected in the backscatter geometry through a 50 μm spatial filter. Scattered light was passed though a 50 μm slit and dispersed off of a 1500 gr/mm grating onto a liquid nitrogen cooled charge-coupled device (CCD) detector. Powder X-ray diffraction measurements were performed at the High Pressure Collaborative Access Team (HPCAT), beamline 16-IDB, of the Advanced Photon Source, Argonne National Table 1. Structural Parameters for H2 þ H2O Clathrate Phases ideal unit cell phase
structure
stoichiometry (H2:H2O)
sII C1
cubic (Fd3m), a ≈ 17.1 Å trigonal (R3), a ≈ 12.7 Å, c ≈ 6.0 Å
48:136 6:36
C2
cubic (Fd3m), a ≈ 6.4 Å
8:8
Laboratory. Temperature was controlled by the regulation of helium in an open-flow cryostat, and pressure was determined by fluorescence from a ruby standard using an online measurement system. A monochromatic beam (∼5 5 μm2) with λ = 0.36821 Å was focused on the sample, and data were recorded using a MAR image plate. Diffraction images were processed using the FIT2D16 data analysis program.
III. EXPERIMENTAL RESULTS When the sample was compressed to 3.0 GPa at 300 K, Raman spectra obtained from the hydrogen roton, OH stretching, and hydrogen vibron regions were consistent with previous reports of the C2 phase.3,5,17,18 The sample was spatially homogeneous, and we observed no spectral characteristics originating from ice VII. The existence of excess hydrogen was verified by the presence of bulk H2 roton and vibron modes, but we were able to spatially resolve the pure clathrate phase. Upon initiation of cooling, the sample pressure increased due to the thermal contraction of the cell. Down to ∼120 K, we observed no significant changes in the Raman spectra of the C2 phase other than the sharpening of the Raman peaks associated with thermal fluctuations. At ∼120 K, a small shoulder developed at ∼11 cm-1 higher frequency than the hydrogen Q1(1) vibron. The intensity of this contribution increased with decreasing temperature and was clearly resolved at 77 K (Figure 1). While we observed no indication of a structural change in other regions of the Raman spectra (e.g., OH stretch, Figure 1), the appearance of the high-frequency vibron may be indicative of a structural transformation (i.e., hydrogen within a new chemical environment). To probe this possibility, we performed synchrotron X-ray diffraction measurements on the C2 phase at 4.1 ( 0.3 GPa from 300 to 11 K. Figure 2 shows X-ray diffraction patterns obtained from the C2 phase as a function of temperature. At 288 K and 4.1 GPa the diffraction pattern was indexed to the C2 Fd3m structure with a = 6.393(1) Å, in good agreement with previous reports.5,17 Upon cooling to 11 K, we observed no indication of a structural phase transition and observed the persistence of the C2
Figure 1. Raman spectra of the H2 vibron (left) and OH (right) regions of the C2 phase with decreasing temperature. A high-frequency shoulder appears in the vibron spectra marked by the arrows. Between 178 and 77 K the pressure increased from 4.2 to 5.1 GPa. 4899
dx.doi.org/10.1021/jp1122536 |J. Phys. Chem. C 2011, 115, 4898–4903
The Journal of Physical Chemistry C
Figure 2. X-ray diffraction patterns of C2 from 288 to 11 K at 4.1 ( 0.3 GPa. Miller indices are indicated, and gasket contributions are labeled “g”.
phase over the entire temperature range. We obtained additional data points at 5 GPa (56 and 82 K) and at 3 GPa (100 K), only observing the C2 phase. The sample was decompressed from 3.0 GPa and 100 K to obtain additional information about the low-temperature phase diagram. While we attempted to decompress in a controlled manner, the pressure dropped rapidly to 0.3 GPa at 110 K. Under these conditions, the diffraction peaks broadened and shifted to higher angles. The decrease in d spacing with decreased pressure indicates a phase transformation as decreased pressure is generally concomitant with lattice expansion. At 0.5 GPa and 145 K, the diffraction pattern was indexed to the space group Fd3m with a = 6.24(2) Å consistent with the formation of Ic, which is contracted relative to the C2 lattice (Figure 3). The pressure increased to 0.6 GPa as the sample continued to warm to 168 K, and several new diffraction peaks were observed. At 0.73 GPa and 172 K the Ic phase disappeared completely. We were unable to index the patterns obtained at these conditions to any common ice or clathrate phases. The observed reflections were successfully indexed to a trigonal lattice with a = 6.24 Å and c = 6.18 Å. These reflections can also be described by a tetragonal lattice with a = 6.25 Å and c = 10.67 Å, similar to reports of the high-pressure P42/mnm tetragonal clathrate structure observed previously for argon19,20 and nitrogen2 guests (a ≈ 6.3 Å, c ≈ 10.6 Å). We were unable to collect additional data on this phase as the temperature and pressure were stabilizing during this period in the experiment and small pressure-temperature (PT) changes prompted another phase transition. As the pressure and temperature stabilized to 0.73 GPa at 170 K, many new peaks appeared in the diffraction patterns. These new peaks were successfully indexed to the C1 structure with a = 12.986(6) Å and c = 6.205(4) Å. As time progressed the intensity of the diffraction peaks from the new phase decreased relative to that of the C1 contributions. From 0.73 GPa and 170 K, we cooled the sample again to explore the phase diagram further.
ARTICLE
Figure 3. Diffraction patterns obtained before and after C2 decompression. Indexing of the new phase is shown for the trigonal and tetragonal lattices, and simulated patterns are shown assuming the C022 and Rquartz23 structures (see the text). Tick marks for the various phases are displayed in the respective patterns, and contributions form the gasket material are indicated by “g”.
Upon cooling to 80 K, the sample consisted almost entirely of C1 with a small amount of the new phase (Figure 4). The pressure increased to 1.2 GPa upon cooling. At 80 K the C1 diffraction pattern was described well by a simulated pattern with a = 12.851(3) Å and c = 6.140(2) Å using oxygen atom positions from Londono et al.21
IV. DISCUSSION Diffraction data obtained in this study help to extend the phase stability limits into the low-temperature regime for both the C1 and C2 structures. At 4.1 ( 0.3 GPa C2 is stable at room temperature and persists to the lowest temperature studied (11 K). At 1.05 ( 0.15 GPa the C1 phase persists to the lowest temperature reached (80 K). These observations constrain the P-T phase diagram of H2 þ H2O. On the basis of our observations, we propose an extended phase diagram shown in Figure 5. During decompression to 0.3 GPa we observed the disappearance of the C2 phase and the formation of ice Ic. This process occurred over a period of minutes and is likely a metastable phenomenon. We expect that the true equilibrium structure at 0.3 GPa and 110 K is sII on the basis of previous observation of this phase at similar conditions.12 We speculate that the C2 phase transformed directly to ice Ic upon rapid decompression without going through the intermediate C1 phase (or other phases). Ice Ic may be viewed as the same structure as C2 without hydrogen and likely represents a low-barrier pathway to a local energy minimum. Indeed, isothermal decompression of C2 from 2.3 GPa at 77 K resulted in decomposition at 0.35 ( 0.05 GPa in a previous Raman study.12 Subsequent to the structural collapse of C2 to ice Ic and prior to the recrystallization of C1, we observed diffraction from an unidentified phase in a small P-T interval near 0.7 GPa and 170 K. We are unable to confidently assign this phase at present, but 4900
dx.doi.org/10.1021/jp1122536 |J. Phys. Chem. C 2011, 115, 4898–4903
The Journal of Physical Chemistry C
Figure 4. Diffraction patterns obtained from the (predominantly) C1 phase upon cooling at 1.05 ( 0.15 GPa. The lower pattern is a simulated profile for the C1 structure, and tick marks indicate allowed reflections for the C1 phase. Asterisks show reflections remaining from the new phase, and “g” shows gasket contributions.
indexing is consistent with a trigonal or tetragonal lattice. In the case of the trigonal lattice, we are unaware of any known clathrate or ice-based structures that can reproduce the observed diffraction pattern. In the tetragonal case, this structure might be similar to previously reported clathrate structures for argon and nitrogen; however, several diffraction lines were not observed, and calculated diffraction intensities were not well matched. Pressure-induced phase transformations in clathrates typify structural densification through application of pressure. At low pressure, small clathrate-forming guest molecules take on the most open structures (sI, sII), gradually transforming to dense filled ices upon pressure increase.2-4 In general, the order by which structural transitions proceed is directly related to the size of the guest molecule: the smaller the molecule, the lower the filled-ice transition pressure. The diameter of the hydrogen molecule dictated the common consensus that its lowest pressure structure was filled ice until it was realized that multiple occupants of a single cavity could stabilize sII.10 The native low-pressure structure for both Ar and N2 clathrates is sII. The slightly smaller Ar atom gives rise to transitions from sH, sT, and hexagonal filled ice at approximately 0.5, 0.7, and 1.1 GPa, whereas the larger N2 molecule goes though the same transitions at 0.8, 1.3, and 1.6 GPa, respectively.3 On the basis of comparisons with other clathrate-forming systems, and our observations of new diffraction lines, it is reasonable to suggest a region of stability for a new clathrate structure of hydrogen. This region would logically exist between the least dense native sII and the first filled-ice phase— exactly where we observed new diffraction features. Efimchenko et al.22 recently reported the formation of a new H2 þ H2O phase identified from quench-recovered samples formed at 253 K and ∼0.5 GPa. On the basis of similarities in formation conditions and experimental X-ray diffraction patterns, this phase appears to be identical to the one we report here. Additionally, the independent observation using different experimental techniques and P-T paths for synthesis suggests that this phase has a field of thermodynamic stability.
ARTICLE
Figure 5. Proposed phase diagram for H2 þ H2O (>50 mol % H2). Small open circles represent experimentally determined phase boundaries compiled from various sources;5,8,9,12-15,28 solid lines through these data guide the eye. Larger filled symbols show P-T conditions where diffraction data were obtained in this study: red triangles, C2; blue squares, ice Ic; yellow tilted squares, new phase; green circles, C1. The dotted orange arrows trace the P-T path followed in this study. Dashed-dotted lines extrapolate phase boundaries on the basis of available data. The hatched region of the phase diagram indicates uncertainty and the possible location of a new phase. L = liquid, F = fluid, and S = solid.
Efimchenko et al.22 suggested the structure of the new “C0” phase has trigonal symmetry (P3112) with a = 6.33 Å and c = 6.20 Å (ambient pressure), and three unique oxygen atom positions to describe the preliminary structure: O1 at the 3a positions with x = 0.23 and full site occupancy, O2 at the 3b positions with x = 0.75 and full site occupancy, and O3 at the 3a positions with x = 0.1 and half site occupancy. While the structure proposed by Efimchenko et al.22 is capable of reproducing diffraction features observed in both experimental studies, variation in observed diffraction intensities (i.e., texture) precludes a definitive description. Additionally, the proposed structure is unusual with respect to the half-occupied oxygen positions. We find that the general characteristics of the diffraction patterns are reproduced well by assuming a structure based on the framework of R-quartz23 (Figure 3). Reflections allowed for the R-quartz structure (P3221) are identical to those of the proposed P3112 structure. While we are unaware of any ice-based analogues to the R-quartz structure, it readily satisfies the tetrahedral bonding requirements of water. Unfortunately, our patterns were not suitable for Rietveld analysis; nevertheless, this model serves as a basis for future structural studies. Our initial structural study was prompted by the appearance of a new H2 vibron at higher frequency than the Q1(1) vibron of the C2 phase (Figure 1). However, our low-temperature diffraction data clearly rule out the possibility of a low-temperature phase transition at 4 GPa. To reconcile these observations, we must consider the nature of the H2 Q branch at high pressure and low temperature. For the isolated H2 molecule at room temperature (and at low density), the Q branch of the Raman spectrum of hydrogen consists of four sharp lines (with appreciable 4901
dx.doi.org/10.1021/jp1122536 |J. Phys. Chem. C 2011, 115, 4898–4903
The Journal of Physical Chemistry C intensity). These excitations represent transitions from the ground to first excited vibrational energy levels (ν) for molecules in each of the four most populated rotational energy levels (J) at room temperature. Namely, QΔν(J), where Δν = (ν = 0 f ν = 1) = 1 and J = 0, 1, 2, and 3. At room temperature, these sharp lines broaden nonlinearly with increasing pressure until only a single peak (Q1(1)) is observed at ∼1.0 GPa; above this pressure rotational fine structure is not observed.24,25 This situation, however, is not the case at low temperature. In the low-temperature fluid and solid H2 phases, both the Q1(1) and Q1(0) contributions are observable with Raman spectroscopy.26 Assuming a rigid quantum rotor model and Boltzmann energy distribution, the populations of the first and second rotational energy levels account for >99% of all occupied states at 77 K, and higher energy rotations may be neglected. For the isolated molecule, separation between the Q1(0) and Q1(1) lines is ∼6 cm-1. Studies on pure H2 at 77 K revealed the divergence of the Q1(1) and Q1(0) lines with pressure, reaching ∼20 cm-1 separation at ∼3 GPa.26 Additionally, the intensity ratio of Q1(0) to Q1(1) drops rapidly from 0.25 at ambient pressure to 0.025 at ∼2.5 GPa.26 This behavior is rationalized by considering that a J = 0 impurity within a J = 1 lattice induces its neighbors to oscillate out of phase, due to its higher vibrational energy, thereby reducing the overall polarizability.27 The appearance of a higher energy vibron band in the low-temperature C2 phase is consistent with the behavior of bulk hydrogen in this PT range.26 Therefore, like bulk H2, the second vibron can be attributed to a vibrational transition of hydrogen in the lowest lying rotational energy level, i.e., Q1(0). The weak intensity of this band and energy separation from Q1(1) (∼11 cm-1) are both consistent with observations in bulk hydrogen.26 We note that unassigned high-energy vibron peaks were also observed at 77 K in a related low-temperature C2 Raman study.12
V. CONCLUSIONS A combined Raman spectroscopy and synchrotron X-ray diffraction study was performed to explore the high-pressure and lowtemperature region of the H2 þ H2O phase diagram. Below ∼120 K, the H2 vibron originating from the C2 phase splits into two distinct components. The appearance of a second higher energy vibron is consistent with vibrational transitions of hydrogen molecules in the ground and first excited rotational energy levels, and diffraction measurements indicate that the C2 phase persists to 11 K. At ∼1 GPa we observe the C1 phase to persist to 80 K. This information helps to extend the stability limits of the C1 and C2 phases on the H2 þ H2O P-T phase diagram. Upon decompression from the C2 phase we observed the appearance of Ic, which transformed to a new phase before transforming to C1. This new phase is consistent with a water framework based on the structure of R-quartz, but could also be related to the tetragonal clathrate structure reported previously for nitrogen and argon guests. These results are directly applicable to PT conditions of hydrogen-rich interstellar ices and may help to reveal novel high-density hydrogen storage materials. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank S. Sinogeikin for assistance with the X-ray diffraction measurements. This work was designed and executed under the
ARTICLE
U.S. Department of Energy, Office of Basic Energy Sciences (DOE-BES) (Grant DE-FG02-06ER46280). Funding from the National Science Foundation, Division of Materials Research (NSF-DMR) (Grant DMR-0805056), and DOE National Nuclear Security Administration (NNSA) (Grant DE-FC5208NA28554) is acknowledged for instrumentation support during the experiments. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT is supported by the Carnegie Institution of Washington (CIW), University of Nevada, Las Vegas (UNLV), and Lawrence Livermore National Laboratory (LLNL) through funding from DOE-NNSA, DOE-BES, and NSF. APS is supported by DOE-BES under Contract DE-AC0206CH11357.
’ REFERENCES (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC, Taylor Francis: Boca Raton, FL, 2008. (2) Loveday, J. S.; Nelmes, R. J.; Klug, D. D.; Tse, J. S.; Desgreniers, S. Can. J. Phys. 2003, 81, 539. (3) Hirai, H.; Tanaka, T.; Kawamura, T.; Yamamoto, Y.; Yagi, T. J. Phys. Chem. Solids 2004, 65, 1555. (4) Loveday, J. S.; Nelmes, R. J. Phys. Chem. Chem. Phys. 2008, 10, 937. (5) Vos, W. L.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Phys. Rev. Lett. 1993, 71, 3150. (6) Londono, D.; Kuhs, W. F.; Finney, J. L. Nature 1988, 332, 141. (7) Machida, S.; Hirai, H.; Kawamura, T.; Yamamoto, Y.; Yagi, T. J. Chem. Phys. 2008, 129, 224505. (8) Dyadin, Y. A.; Larionov, E. G.; Manakov, A. Y.; Zhurko, F. V.; Aladko, E. Y. Mendeleev Commun. 1999, 5, 209. (9) Dyadin, Y. A.; Larionov, E. G.; Aladko, E. Y.; Manakov, A. Y.; Zhurko, F. V.; Mikina, T. V.; Komarov, V. Y.; Grachey, E. V. J. Struct. Chem. 1999, 40, 790. (10) Mao, W. L.; Mao, H. K.; Goncharov, A. F.; Struzhkin, V. V.; Gou, Q. Z.; Hu, J. Z.; Shu, J. F.; Hemley, R. J.; Somayazulu, M.; Zhao, Y. S. Science 2002, 297, 2247. (11) Lokshin, K. A.; Zhao, Y. S.; He, D. W.; Mao, W. L.; Mao, H. K.; Hemley, R. J.; Lobanov, M. V.; Greenblatt, M. Phys. Rev. Lett. 2004, 93, 125503. (12) Mao, W. L.; Mao, H. K. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 708. (13) Lokshin, K. A.; Zhao, Y. S. Appl. Phys. Lett. 2006, 88, 131909. (14) Efimchenko, V. S.; Antonova, V. E.; Barkalov, O. I.; Klyamkin, S. N.; Tkacz, M. High Pressure Res. 2009, 29, 250. (15) Antonova, V. E.; Efimchenko, V. S.; Tkacz, M. J. Phys. Chem. B 2009, 113, 779. (16) Hammersley, A. P. ESRF Internal Report ESRF97/HA02T; European Synchrotron Radiation Facility: Grenoble, France, 1997. (17) Hirai, H.; Ohno, S.; Kawamura, T.; Yamamoto, Y.; Yagi, T. J. Phys. Chem. C 2007, 111, 312. (18) Vos, W. L.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Chem. Phys. Lett. 1996, 257, 524. (19) Kurnosov, A. V.; Manakov, Y. Y.; Komarov, V. Y.; Voronin, V. I.; Teplykh, A. E.; Dyadin, Y. A. Dokl. Phys. Chem. (Transl. of Dokl. Akad. Nauk) 2001, 381, 303. (20) Hirai, H.; Uchihara, Y.; Nishimura, Y.; Kawamura, T.; Yamamoto, Y.; Yagi, T. J. Phys. Chem. B 2002, 106, 11089. (21) Londono, D.; Finney, J. L.; Kuhs, W. F. J. Chem. Phys. 1992, 97, 547. (22) Efimchenko, V. S.; Kuzovnikov, M. A.; Fedotov, M. K.; Simonov, S. V.; Tkacz, M. J. Alloys Compd. 2011 in press. (23) Lager, G. A.; Jorgensen, J. D.; Rotella, F. J. J. Appl. Phys. 1982, 53, 6751. (24) Moulton, N. E.; Watson, G. H.; Daniels, W. B.; Brown, D. M. Phys. Rev. A 1988, 37, 2475. 4902
dx.doi.org/10.1021/jp1122536 |J. Phys. Chem. C 2011, 115, 4898–4903
The Journal of Physical Chemistry C
ARTICLE
(25) Sharma, S. K.; Mao, H. K.; Bell, P. M. Phys. Rev. Lett. 1980, 44, 886. (26) Eggert, J. H.; Hemley, R. J.; Mao, H. K.; Feldman, J. L. AIP Conf. Proc. 1994, 309, 845. (27) James, H. M.; Van Kranendonk, J. Phys. Rev. 1967, 164, 1159. (28) Diatshenko, V.; Chu, C. W.; Liebenberg, D. H.; Young, D. A.; Ross, M.; Mills, R. L. Phys. Rev. B 1985, 32, 381.
4903
dx.doi.org/10.1021/jp1122536 |J. Phys. Chem. C 2011, 115, 4898–4903
