J. Phys. Chem. B 2007, 111, 4103-4108
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High-Pressure Studies of Cyclohexane to 40 GPa Michael Pravica,* Yongrong Shen,† Zachary Quine, Edward Romano, and David Hartnett Department of Physics and High Pressure Science and Engineering Center (HiPSEC), UniVersity of NeVada Las Vegas, Las Vegas, NeVada 89154-4002 ReceiVed: January 3, 2007; In Final Form: February 5, 2007
We present data from two room temperature synchrotron X-ray powder diffraction studies of cyclohexane up to ∼40 and ∼20 GPa. In the first experiment, pressure cycling was employed wherein pressure was varied up to ∼16 GPa, reduced to 3.5 GPa, and then raised again to 40 GPa. Initially, the sample was found to be in the monoclinic phase (P121/n1) at ∼8.4 GPa. Beyond this pressure, the sample adopted triclinic unit cell symmetry (P1) which remained so even when the pressure was reduced to 3.5 GPa, indicating significant hysteresis and metastability. In the second experiment, pressure was more slowly varied, and the monoclinic unit cell structure (P121/n1) was observed at lower pressures up to ∼7 GPa, above which a phase transformation into the P1 triclinic unit cell symmetry occurred. Thus, the pressure onset of the triclinic phase may be dependent upon the pressurizing conditions. High-pressure Raman data that further emphasize a phase transition (probably into phase VI) around 10 GPa are also presented. We also have further evidence for a phase VII, which is probably triclinic.
Introduction Cyclohexane (C6H12), a cycloalkane, is a common solvent and room temperature liquid (bp ) 81 °C, mp ) 7 °C) that is a component of petroleum and is an intermediate in the synthesis of nylon. It is a relatively simple and fundamental molecule that exhibits a high degree of polymorphism due in part to its boat, chair, and other conformations.1 Cyclohexane’s stability and conformational selectivity make it a common template for larger biological molecules, such as carbohydrates and steroids.1 Discerning the high pressure behavior of this relatively fundamental molecule benefits not only high-pressure science in which the material has been used as a quasihydrostatic medium,2-4 but also to better understand the molecule’s survivability, decomposition, and possible synthesis inside the Earth. Cyclohexane has been studied using shock compression methods.5-6 Connecting shock wave measurements with static high-pressure data is a long-desired goal that might be better achieved using molecules such as cyclohexane that survive extreme conditions of pressure and temperature.2 Finally, cyclohexane undergoes significant hysteresis effects,2,9-10 which merit further study. This paper represents a continuation of our prior high-pressure Raman spectroscopic study2 of cyclohexane to extend data on this important molecule far beyond the 10 GPa range of earlier studies.7-10 To the best of our knowledge, this is the first X-ray study of cyclohexane at high pressure. During the course of our Raman study, we noted the likely appearance of at least two previously unreported phases (phases VI and VII) of the molecule and were, thus, motivated to further understand these new phases using X-ray diffraction methods to complement our earlier Raman work. In that work, we gave evidence supporting the use of cyclohexane as a quasihydrostatic medium for highpressure experiments. In that spirit, it is also important to measure the X-ray diffraction spectra from cyclohexane as a * Corresponding author. E-mail:
[email protected]. † Deceased.
function of pressure to distinguish it from that of the pressurized sample under study. The incredibly high flux rates of X-rays available from X-ray synchrotron light sources offer a tremendous opportunity to interrogate low-Z/organic/hydrocarbon materials pressurized by diamond anvil cells (DACs) in which sample volumes are in the nanoliter range. X-ray diffraction patterns that might take days to acquire for samples in a DAC with conventional X-ray sources take a matter of seconds at the High-Pressure Collaborative Access Team Beamline (HP-CAT) at Argonne National Laboratory’s Advanced Photon Source (APS) where we performed our X-ray studies. This paper serves also as a demonstration of the tremendous utility of this X-ray synchrotron source facility for these types of measurements. Earlier static high-pressure studies on cyclohexane revealed five phases (I-V) via neutron powder diffraction,7 Raman spectroscopy,2,8-9 infrared spectroscopy,10-11 and X-ray diffraction.12 The sequence of phases at ambient temperature is liquid f phase 1 (cubic) f phase III (orthorhombic) f phase IV (monoclinic) f phase V (about which little is known8-9). Below 186.1 K, an orientationally ordered structure appears (phase II).8 Cubic, plastic phase I has unit cell parameters a ) 8.61(2) Å, Z ) 4, Fm3m space group.8 Phase III is orthorhombic with unit cell parameters a ) 6.587(3) Å, b ) 7.844(7) Å, and c ) 5.295(3) Å; Z ) 2; and Pmnn space group. Phase IV is monoclinic with unit cell parameters a ) 6.526(4) Å, b ) 7.597(6) Å, and c ) 5.463(5) Å; β ) 97.108(4)°; Z ) 2; and P121/n1. Recent Raman studies suggest that phase V is stable from 3.2 GPa to at least 10 GPa, with large hysteresis upon decompression at ambient temperature.9 The intention of the present set of measurements was to at least partially confirm via X-ray methods the above phase sequence at high pressure using X-ray powder diffraction methods and garner further knowledge about the phases, particularly phases V and VI.
10.1021/jp070052b CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007
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Figure 1. Typical XRD pattern of cyclohexane at high pressure.
Experimental Techniques We performed two angular dispersive X-ray powder diffraction (XRD) experiments at HPCAT’s 16ID-B beamline facility of the Advanced Photon Source at Argonne National Laboratory. The first experiment was performed in the range of 3.5-39.9 GPa, and the second study, focusing on the lower pressures, ranged from 0.5 to 20.4 GPa. Liquid cyclohexane (Aldrich Chemical) of 99.9+% purity and a few 5-10-µm-sized ruby crystals (pressure calibrant13) were loaded into a Mao-Bell-type DAC14 using diamonds with 300 and 480 µm culets, respectively. In the first experiment, the rhenium gasket was preindented to 66 µm thickness, and a sample hole with a diameter of 160 µm was drilled via electric discharge machining. In the second experiment, the rhenium gasket was preindented to 72 µm thickness, and a sample hole with a diameter of 200 µm was drilled via electric discharge machining. Pressures were measured with a ruby fluorescence measuring system located at HPCAT and determined using a quasihydrostatic fitting to the R1 line.13 Our data contains the results from two independent X-ray powder diffraction experiments on cyclohexane and an earlier
Raman experiment.2 All experiments were performed at room temperature, and no evidence of sample heating was seen in any of the experiments. The first experiment focused mainly on the high-pressure range from 7 up to 40 GPa and utilized X-rays of 0.4015 Å wavelength. The second experiment was conducted with the aim of investigating the lower pressure regime (0-7GPa) using X-rays of λ ) 0.3675 Å to connect with previously published data. Data were collected using an X-ray image plate detector (Mar325) for the first run and an X-ray CCD detector (MarCCD) for the second experiment. Typical exposure times ranged from 10 to 100 s each. For each of the runs, we used a collimated X-ray beam passing through a 20-30 µm pinhole. Data were first analyzed using the FIT2d software program15 to convert the 2-d images into graphs of intensity versus 2θ. The intensity plots were then analyzed using the JADE 7.0 commercial software program to ascertain the unit cell symmetry from which the determination of the lattice parameters as a function of pressure is possible and then to extract the volume versus pressure data of the molecular solid. We also present unpublished data on an earlier Raman experiment2 of cyclohexane. For this experiment, Raman spectra
High-Pressure Studies of Cyclohexane to 40 GPa
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Figure 2. Stacked intensity-vs-θ plots of cyclohexane at high pressure (initial data from the first experiment, λ ) 0.4015 Å). All plots are vertically stacked in order of increasing time.
were observed at ambient temperature with an ISA Jobin-Yvon U1000 double-grating spectrometer (resolution < 1 cm-1) equipped with a thermoelectrically cooled CCD detector (ISA SpectrumOne). We scanned the spectra from 80 to 1300 cm-1 and 2750 to 3400 cm-1 at 5-10 s per 100 cm-1. The samples were excited by a between 100 and 300 mW (CW), 514.5-nm line from a Spectra Physics BeamLok Ar-ion laser. We took great pains to load the sample initially at low pressure (about 0.6 GPa) and then monotonically to increase pressure in very small steps up to 10 GPa. Results For both of our X-ray synchrotron experiments, X-ray patterns were typically spotty, probably indicating preferred orientations in the sample due to the uniaxial stress conditions from the diamond cell. This precludes any accurate Rietveld refinement of the data to obtain atomic positions within the unit cell. A typical powder pattern is shown in Figure 1. The time between X-ray runs was usually between 1/2 and 1 h. During all of our experiments on cyclohexane, we noted that in the ∼4-10 GPa window, the sample pressure consistently increased by ∼0.25 GPa over the course of 1/2 hour. We would wait ∼1/2 h until we found no further increase in pressure. After ∼10 GPa, we found no further hysteretic effects. Throughout both experiments and for all rubies inside the samples, we were able to resolve both the R1 and R2 lines of ruby for all investigated pressures supporting our view that cyclohexane may serve as a practical, nonreactive, quasihydrostatic medium. First X-ray Experiment. In our first experiment, we initially pressurized the sample to ∼8.4 GPa during loading of liquid cyclohexane, which took ∼1-2 min. We then collected an X-ray powder pattern at this pressure and then took another pattern ∼40 min later at the same pressure to ascertain if there was any change in the pattern with time, which is displayed as an intensity-vs-θ plot in Figure 2. As can be seen, at this pressure, there is little difference in the two patterns. We then further increased pressure to 11.57 and 16 GPa, respectively, recording XRD patterns at these pressures. As can be seen in Figure 2, above 8.4 GPa, the XRD plots significantly alter, indicating a
Figure 3. Stacked intensity-vs-θ plots of cyclohexane at high pressure (subsequent data from the first experiment, λ ) 0.4015 Å). All plots are vertically stacked in order of increasing time.
phase transformation. We then reduced the sample pressure to 5.8, 3.8, and then 3.5 GPa, respectively, taking XRD patterns at each pressure. As is evidenced in Figure 2, no apparent phase transition is seen when the pressure is reduced, though lattice parameters clearly do change as the peaks shift to lower θ, suggesting an increase in unit cell volume, as expected. We
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TABLE 1: Lattice Parameter Data for Cyclohexane at the Various Pressures Investigated pressure (GPa) 8.41
volume per molecule (Å3) 101.49 ( 1.21
a (Å)
b (Å)
c (Å)
6.276 ( 0.044
First Run Monoclinic (Z ) 2) 6.480 ( 0.048 5.043 ( 0.031
3.45 3.8 4.56 5.8 5.92 6.57 7.3 8.69 10.64 11.68 16 19.3 25.51 28.6 32.28 35.05
112.43 ( 0.29 112.61 ( 0.32 110.06 ( 0.53 108.64 ( 1.29 107.88 ( 0.82 107.30 ( 0.39 105.77 ( 0.38 103.27 ( 0.40 99.68 ( 0.57 97.42 ( 0.38 93.97 ( 0.39 90.53 ( 0.99 87.64 ( 0.90 85.27 ( 0.85 84.31 ( 0.69 81.31 ( 0.47
4.800 ( 0.006 4.813 ( 0.008 4.761 ( 0.011 4.756 ( 0.036 4.722 ( 0.015 4.725 ( 0.009 4.712 ( 0.007 4.656 ( 0.008 4.608 ( 0.011 4.539 ( 0.014 4.509 ( 0.016 4.369 ( 0.017 4.331 ( 0.014 4.325 ( 0.021 4.304 ( 0.021 4.246 ( 0.009
First Run Triclinic (Z ) 1) 5.139 ( 0.003 5.119 ( 0.003 5.155 ( 0.004 5.111 ( 0.011 5.100 ( 0.013 5.078 ( 0.005 5.152 ( 0.039 5.004 ( 0.026 5.092 ( 0.025 5.024 ( 0.005 5.082 ( 0.003 5.017 ( 0.009 5.067 ( 0.006 4.950 ( 0.011 5.004 ( 0.007 4.941 ( 0.011 4.946 ( 0.012 4.868 ( 0.016 4.929 ( 0.008 4.823 ( 0.010 4.925 ( 0.008 4.737 ( 0.007 4.868 ( 0.011 4.728 ( 0.032 4.884 ( 0.010 4.640 ( 0.026 4.835 ( 0.011 4.546 ( 0.024 4.801 ( 0.012 4.448 ( 0.010 4.790 ( 0.004 4.456 ( 0.012
1.7 2.51 3.23 4.14 6.91
124.02 ( 0.36 118.58 ( 0.30 116.48 ( 0.36 111.72 ( 0.61 105.80 ( 0.89
6.401 ( 0.013 6.360 ( 0.013 6.338 ( 0.016 6.242 ( 0.029 6.252 ( 0.027
Second Run Monoclinic (Z ) 2) 7.353 ( 0.013 5.320 ( 0.006 7.113 ( 0.005 5.291 ( 0.006 7.063 ( 0.005 5.259 ( 0.008 6.886 ( 0.013 5.251 ( 0.011 6.694 ( 0.040 5.104 ( 0.020
6.91 12.3 15.1 20.4
105.31 ( 0.23 96.70 ( 1.11 95.61 ( 2.07 91.69 ( 1.40
4.681 ( 0.005 4.529 ( 0.028 4.448 ( 0.032 4.363 ( 0.022
Second Run Triclinic (Z ) 1) 5.043 ( 0.002 4.980 ( 0.006 4.920 ( 0.023 4.811 ( 0.032 4.934 ( 0.023 4.830 ( 0.066 4.892 ( 0.015 4.761 ( 0.044
then steadily increased the pressure of the sample up to ∼40 GPa, collecting XRD patterns at various pressures along the way. The results of this cycle of the experiment are displayed in Figure 3. The stacked intensity vs θ plots all appear to the have the same symmetry, and no phase transitions are immediately evident. The two plots at 8.41 GPa in Figure 2 were most easily fit with monoclinic P121/n1 symmetry (no. 14 space group, Z ) 2) which agrees with published literature.7-8 Above 8.41 GPa and for all of the plots in Figure 3, the triclinic (P1, Z ) 1) symmetry (spacegroup no. 1) better described the plots. The results of these fits are displayed in Table 1. Second X-ray Experiment. In the second experiment, we more carefully and montonically increased the sample pressure up to 20.4 GPa, beginning at 0.5 GPa. The results of this experiment are displayed in Figure 4. Above 4.14 GPa, a phase transition is evident in the plot beginning at 6.91 GPa, which we believe to comprise a mixture of phases. At higher pressures, the phase transition is complete up to 15 GPa. Beyond this pressure, a new phase likely exists, as seen in the 20.4 GPa plot, which is the same in character as data from the first experiment that is displayed in Figure 3. Below 6.91 GPa in Figure 4, the plots were most easily fit using the monoclinic with P121/n1 symmetry (no. 14 space group) which agrees with published literature.7-8 Above 6.91 GPa, the plots could be more easily fit to the triclinic P1 symmetry. We successfully fit the plot at 6.91 GPa as a mixture of the monoclinic and triclinic phases. The results of our fits for this experiment are also displayed in Table 1. Raman Experiment. In Figures 5 and 6, we include unpublished data from our earlier Raman experiment2 in the 2800-3200 and 100-600 cm-1 frequency ranges, respectively, to further emphasize the observed phase transitions. To the best of our knowledge, this is the first reported low-frequency Raman spectroscopic study of cyclohexane at high pressure. The most
R, deg
β, deg
γ, deg
98.230 ( 0.515 90.080 ( 0.488 90.295 ( 0.465 89.988 ( 0.491 90.350 ( 1.914 89.702 ( 1.443 89.523 ( 0.505 89.325 ( 0.247 88.749 ( 0.359 88.820 ( 0.528 88.866 ( 0.352 90.337 ( 0.934 87.810 ( 1.038 87.728 ( 0.825 88.516 ( 0.844 87.953 ( 0.763 87.033 ( 0.314
65.441 ( 0.190 65.372 ( 0.197 65.590 ( 0.349 66.011 ( 0.879 65.923 ( 0.285 65.872 ( 0.247 65.819 ( 0.239 65.964 ( 0.237 66.270 ( 0.340 66.747 ( 0.350 66.755 ( 0.553 67.114 ( 0.659 67.738 ( 0.575 67.555 ( 0.607 70.228 ( 0.623 68.362 ( 0.450
100.706 ( 0.285 101.329 ( 0.394 100.369 ( 0.330 103.028 ( 0.713 100.802 ( 0.793 101.211 ( 0.180 99.868 ( 0.228 99.273 ( 0.254 99.583 ( 0.391 99.305 ( 0.331 102.592 ( 0.633 100.181 ( 1.171 103.029 ( 1.118 102.204 ( 0.892 101.264 ( 0.800 102.658 ( 0.599
97.859 ( 0.140 97.808 ( 0.117 98.281 ( 0.219 98.193 ( 0.299 97.858 ( 0.355 89.196 ( 0.140 88.718 ( 0.851 87.739 ( 2.127 86.949 ( 1.650
66.168 ( 0.130 66.708 ( 0.547 66.742 ( 1.318 67.190 ( 0.927
100.348 ( 0.184 99.423 ( 0.751 98.947 ( 2.410 99.240 ( 1.720
notable change in the low frequency/lattice vibrational spectra and in the high frequency (C-H region) spectra occurs between 9.05 and 12.8 GPa, supporting our view of a phase transition (probably phase V f phase VI) around 10 GPa.2
Figure 4. Stacked intensity-vs-θ plots of cyclohexane at high pressure (second experiment, λ ) 0.3675 Å).
High-Pressure Studies of Cyclohexane to 40 GPa
Figure 5. Raman spectra of cyclohexane at high pressures (high frequency, 2800-3200 cm-1 window).
Figure 6. Raman spectra of cyclohexane at high pressures (lowfrequency window).
Discussion Our results can be summarized in the following way: Initially in the first X-ray experiment, the sample unit cell symmetry could be described as monoclinic at 8.4 GPa. Somewhere above this pressure, the sample unit cell changed to triclinic symmetry.
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Figure 7. Volume-vs-pressure curve of cyclohexane. The data point for phase III was obtained from ref 7.
All of the subsequent XRD patterns in this experiment were best described with P1 triclinic symmetry (spacegroup no. 1), which we will call the “high-pressure phase”, even when pressure was reduced to 3.5 GPa due to probable hysteresis. In the second experiment, pressure was more slowly varied. From 1.7 GPa up to ∼7 GPa, we ascribe the crystal unit cell symmetry as monoclinic. The pattern at 6.91 GPa (Figure 4) was fit as a mixture of monoclinic and triclinic phases. We believe that this would represent a sluggish transition from phase V (which was suggested as monoclinic9) into triclinic phase VI. In the second experiment, the “low pressure” monoclinic symmetry persisted to somewhere above 7 GPa but below 12.3 GPa, at which pressure the sample was more aptly described with triclinic symmetry above this pressure. Near 7 GPa, the sample was probably a mixture of the monoclinic and triclinic phases (V and VI). Our utilization of rhenium gaskets for high pressure use may be a partial reason for the sluggishness of the transition,2,9 but we also suspect that the rate of compression of the sample along with nonhydrostatic stresses may have a strong bearing on determining the various pressures around which phase transitions occur and determining the range of pressures through which a full phase transition occurs. We therefore believe that in the first experiment, the sample was rapidly brought into the “high-pressure phase” symmetry, and even when the pressure was reduced, it remained in the triclinic P1 unit cell configuration due to very large hysteresis, as suggested by Baonza, who found that phase V could be quenched down to 1.9 GPa.9 Upon the basis of the initial pressure of the first X-ray experiment, the sample would have begun in phase V (monoclinic) according to our and Baonza’s results.2,9 We suspect that both phase VI and phase VII possess triclinic unit cell symmetry. For the second experiment, the sample pressure was varied relatively slowly, and the sample thus had
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TABLE 2: Bulk Modulus and Pressure Derivative of the Bulk Modulus Parameters Derived from the Pressure-vs-Volume Fitsa
a
EOS
Bo (GPa)
Bo′
Vo (Å3)
Kumar Birch-Murnaghan Vinet Murnaghan
15.0502 ( 5.7551 12.0000 ( 13.6900 9.9367 ( 5.6033 22.9280 ( 5.0592
6.8823 ( 0.7552 14.5138 ( 11.9368 8.1350 ( 1.2517 4.7656 ( 0.3741
131.4878 ( 4.7820 130.8910 ( 8.0426 136.7397 ( 9.0156 127.9576 ( 2.9960
Listed errors were derived from the fits.
matter and dynamically investigate relaxation processes in the solid phase of hydrocarbons as a function of pressure and temperature.
Figure 8. Murnaghan EOS fit to our experimental volume vs pressure data.
Acknowledgment. We thank Malcolm Nicol for useful discussions and a critical review of the manuscript. We also thank Ravhi Kumar for some help with the measurements. We also wish to thank Maddury Somayazulu and the rest of the HPCAT staff at APS/ANL for technical assistance. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. HPCAT is a collaboration among the University of Nevada Las Vegas, Lawrence Livermore National Laboratory, the Carnegie Institution of Washington, and the Carnegie/DOE Alliance Center and is supported by DOE-BES, DOE-NNSA, NSF, and the W.M. Keck Foundation. We gratefully acknowledge the support from the U.S. Department of Energy Cooperative Agreement No. DEFC52-06NA26274 with the University of Nevada Las Vegas. References and Notes
a better chance to thermodynamically equilibrate. We thus saw a different sequence of phase transitions (1) between 4.14 and 6.91 GPa (phase IV f V), (2) between 6.91 and 12.3 GPa (phase V f VI), and (3) above 15.1 GPa (phase VI f VII2). Volume versus Pressure Analysis. Although we have observed significant out-of-equilibrium effects from cyclohexane under pressure that would not give a reliable equation of state (EOS), we nevertheless present volume-versus-pressure data in Figure 7. In this figure, we also include the cyclohexane shockwave data of Dick5 as well as a phase III data point from ref 7 for comparison. Molecular volume error bars are also included in our plot and were computed from the lattice parameter errors supplied by Jade. Following the spirit of a past high-pressure, lower-energy X-ray study of anthracene,16 we fit our pressure-vs-volume data in Figure 7 using the Murnaghan-type EOS,17 Birch-Murnaghan-type EOS,18-19 Vinetype EOS,20-22 and Kumar-type EOS.23-24 The Murnaghan fit seemed to give the lowest relative errors, and we include the fit curve in Figure 8(see also Table 2). Conclusions We have performed two X-ray powder diffraction experiments and one Raman experiment on cyclohexane as a function of pressure, which have revealed the importance of pressurizing conditions in determining the sequence of phase transitions that the material adopts as a function of pressure. In these three experiments, we have given further evidence that phase V is monoclinic and have for the first time provided supporting evidence that the higher-pressure phases (VI and VII) are of triclinic unit cell symmetry. We have convincing evidence for yet another phase (VII) of this interesting and important hydrocarbon. We have better-documented hysteresis of the molecule and, as a result, may have a unique opportunity here to examine the physics of metastable and frustrated phases of
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