Spectroscopic Study of the Effects of Pressure Media on High

Oct 15, 2010 - Structural phase transitions in natrolite have been investigated as a function of pressure and different hydrostatic media using micro-...
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J. Phys. Chem. C 2010, 114, 18819–18824

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Spectroscopic Study of the Effects of Pressure Media on High-Pressure Phase Transitions in Natrolite Dan Liu,† Weiwei Lei,‡ Zhenxian Liu,§ and Yongjae Lee*,† Department of Earth System Sciences, Yonsei UniVersity, Seoul 120-749, Korea, Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry Research, Campus Golm, 14424 Potsdam, Germany, and Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, United States ReceiVed: August 1, 2010; ReVised Manuscript ReceiVed: September 15, 2010

Structural phase transitions in natrolite have been investigated as a function of pressure and different hydrostatic media using micro-Raman scattering and synchrotron infrared (IR) spectroscopy. Natrolite undergoes two reversible phase transitions at 0.86 and 1.53 GPa under pure water pressure medium. These phase transitions are characterized by the changes in the vibrational frequencies of four- and eight-membered rings related to the variations in the bridging T-O-T angles and the geometry of the elliptical eight-ring channels under pressure. Concomitant to the changes in the framework vibrational modes, the number of the O-H stretching vibrational modes of natrolite changes as a result of the rearrangements of the hydrogen bonds in the channels caused by a successive increase in the hydration level under hydrostatic pressure. Similar phase transitions were also observed at relatively higher pressures (1.13 and 1.59 GPa) under alcohol-water pressure medium. Furthermore, no phase transition was found up to 2.52 GPa if a lower volume ratio of the alcohol-water to natrolite was employed. This indicates that the water content in the pressure media plays a crucial role in triggering the pressure-induced phase transitions in natrolite. In addition, the average of the mode Gru¨neisen parameters is calculated to be about 0.6, while the thermodynamic Gru¨neisen parameter is found to be 1.33. This might be attributed to the contrast in the rigidity between the TO4 tetrahedral primary building units and other flexible secondary building units in the natrolite framework upon compression and subsequent water insertion. Introduction Zeolites are hydrated framework alumosilicates which comprise an important class of low-density materials. Their frameworks are composed of corner connecting of TO4 (Si, Al, Ge, Ga, ...) tetrahedra to yield cavities and channels of molecular dimensions. Exchangeable nonframework cations occupy the cavities and channels along with absorbed water molecules at ambient conditions.1 As such, a plethora of structural studies have been carried out as a function of framework type, composition, and/or temperature to understand their relationship to catalytic, molecular sieving, and ion-exchange properties.2-5 In recent years, there has also been significant interest in pressure-induced structural and chemical changes in zeolites. In the case of natrolite, different structural responses have been reported as a function of the molecular size of pressure transmitting media. When nonpenetrating pressure transmitting liquid is used, the structural integrity has been reported to sustain up to 7 GPa before amorphization.6-9 On the other hand, natrolite has been shown to exhibit anomalous compressibility involving abrupt volume expansion when pore-penetrating pressure transmitting liquid is used.10-13 On the basis of Raman and NMR spectroscopic data, Belitsky et al.13 first reported two pressure induced phase transitions in natrolite at 0.75 and 1.25 GPa, respectively. Subsequently, Lee et al. have followed this investigation using synchrotron X-ray diffraction and Rietveld * To whom correspondence should be addressed, YongjaeLee@ yonsei.ac.kr. † Department of Earth System Sciences, Yonsei University. ‡ Max-Planck-Institute of Colloids and Interfaces Department of Colloid Chemistry Research. § Geophysical Laboratory, Carnegie Institution of Washington.

methods to confirm that these changes are due to the successive increase in the channel water content from alcohol-water pressure transmitting medium. This established pressure-induced hydration and the formation of ordered-paranatrolite (Na16Al16Si24O80 · 24H2O) around 1.0 GPa and fully superhydrated natrolite (Na16Al16Si24O80 · 32H2O) above 1.2 GPa.14,15 We can see that there exists ca. 25% difference in phase transition pressure from natrolite to ordered-paranatrolite between pure water and alcohol-water mixture as pressure media. In addition, we have experimentally observed that the formation of ordered-paranatrolite (and subsequently fully superhydrated natrolite) itself is only possible with a suitable volume ratio of alcohol-water medium and natrolite powder. Clearly the water content in the pressure media seems to play an important role in the phase transitions in natrolite. In the light of this, further chemical and structural modifications of the natrolite structure can be envisaged to occur as a function of the hydrostatic media for novel adsorption or molecular sieving applications under pressure. It is well-known that microspectroscopy is one of the most convenient and powerful tools for directly observing the pressure dependencies of water tracer diffusion through the change in the hydrogen bonds.16,17 Micro-Raman scattering and infrared spectroscopy can provide detailed information on the bonding properties of frameworks as well as the atoms and molecules in the nonframework sites, thereby yielding atomistic description of thermochemical properties in zeolites. In this paper, we present the detailed pathways of the phase transitions in natrolite using the combination of in situ micro-Raman scattering and synchrotron infrared spectroscopy to gain insights into the correlations between the chemical bonds and the structural

10.1021/jp107220v  2010 American Chemical Society Published on Web 10/15/2010

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Figure 1. Polyhedral representations of (a) natrolite, (b) ordered-paranatrolite, and (c) superhydrate natrolite shown along the c axis. TO4 tetrahedra are shown in two colors to illustrate the ordering of Al/Si over the framework tetrahedral sites. The water molecules and Na+ cations are shown as white and blue spheres, respectively, in the channels.

modifications under pressure, which is controlled by the water content in the pressure transmitting media.

following irreducible representations of phonon vibrational modes of natrolite at the Brillouin zone center (q ) 0) with a space group C2V19

Experimental Section The mineral sample of natrolite (ideally Na16Al16Si24O80 · 16H2O) was obtained from San Juan, Argentina, from OBG International. High-pressure Raman experiments were carried out at room temperature using a symmetric diamond anvil cell (DAC) equipped with a pair of low fluorescent background type-I diamond anvils. Pure water or alcohol-water (16:3:1 methanol-ethanol-water) mixture was used as a hydrostatic pressure medium up to 2.12 and 2.87 GPa, respectively. All Raman spectra were collected in a backscattering geometry using a customized PI (Princeton Instrument) Raman microscope system constructed in our lab. A Newport Excelsior-532 nm laser with maximum 150 mW was used as the excitation source. The laser spot was intentionally focused to ∼20 µm on the sample via the use of a 10× beam expander and a Mitutoyo 20× objective in order to avoid any laser-induced decomposition of the sample. The spectral resolution was about 1.5 cm-1. The average acquisition time for a single spectrum was about 90 seconds. High-pressure synchrotron IR experiments were performed at U2A beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The optical layout of the beamline facility has been described in detail elsewhere.18 The far-IR spectra of natrolite with a mixture of methanolethanol-water (16:3:1) were collected up to 2.56 GPa using a Bruker IFS 66v/S vacuum Fourier transform infrared (FTIR) spectrometer accommodated with the synchrotron source and a custom-made vacuum microscope system equipped with a bolometer (Infrared Laboratories) and a 3.5 µm thick Mylar beamsplitter. A spectral resolution of 4 cm-1 was applied to all spectra. High pressure was generated in a symmetric diamond anvil cell19 using a pair of type IIa diamond anvils. Pressure was determined from the frequency shift of the ruby R1 fluorescence line for all measurements.20 Results and Discussion The structural projections of natrolite phases established from the previous high-pressure crystallographic studies are shown in Figure 1.14,15,21,22 In the natrolite framework, an elliptical channel is formed by linkages of four chains of T5O10 building units along the c axis. Each channel contains two parallel chains of water and Na+ cations. Group theory predicts23-25 the

Γ ) 120A1(IR, R) + 120A2(R) + 120B1(IR, R) + 120B2(IR, R) (1) in which IR and R denote infrared and Raman activity, respectively. All the observed modes in this study are listed in Table 1, which is in good agreement with the peak positions reported in the literature for natrolite at ambient conditions.6,8,26-28 The relative intensity of the Raman peaks may vary based on the crystal orientations and the polarization of the laser source. Figure 2 shows the representative Raman and far-IR spectra of natrolite at ambient pressure. Three groups of Raman vibrational modes in natrolite observed around 1100-900, 900-420, and below 420 cm-1 are assigned to the stretching, deformation, and lattice modes, respectively. Similarly, the far-IR active modes observed below 230 cm-1 have been assigned as optical modes of the lattice vibrations. It should be pointed out that the IR active libration modes of water lie at 480-620 cm-1 with high intensity. The strongest Raman bands at 535 cm-1 are interpreted as the breathing modes of the four-membered ring. The second intensive Raman band located at 443 cm-1 (corresponding to far-IR bands in the range of 348-365 cm-1) is attributed to the breathing mode and corresponds to vibrations of a collapse mode of the eight-membered ring. Three Raman bands are observed in the range of 1500-3600 cm-1, the one at 1623 cm-1 represents the O-H bending vibrational mode, while the others peaks near 3328 and 3539 cm-1 are due to the O-H stretching of H2O. Figure 3 shows the representative high-pressure Raman spectra with pure water as a pressure medium. It can be seen that two phase transitions are well distinguished in Raman spectra taken at 0.86 and 1.53 GPa, which are in good agreement with the results of Belitsky et al.13 We can clearly see that the low-frequency lattice modes show a considerable change with pressure increase up to 0.86 GPa. Moreover, the vibrational peak of the four-membered ring bond abruptly decreases by 4 cm-1 at 0.86 GPa. This may be attributed to a relatively strong distortion of the TO4 building units and an increased spread of T-O-T bridging angles. On the basis of our previous high pressure X-ray study,14,15 we know that the volume expansion

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TABLE 1: Pressure Dependence of the Natrolite and Superhydrated Natrolite Vibrations natrolite ν0a

a

-1

(cm )

b

-1

ν0 (cm )

superhydrated natrolite dν/dP (cm-1 GPa-1)

γi

ν(O-H) Stretching Vibrations of H2O Molecule 4.10 0.06 3552.0(R) 3.20 0.05 3499.0(R) 3433.0(R)

-2.95 0.33 0.98

-0.040 0.010 0.010

dν/dP (cm

-1

-1

GPa )

γi

-1

ν0 (cm )

3542(R) 3331(R)

3540.3 3329.0

1633(R)

1627.0

H-O-H Bending Vibrations of H2O Molecule 2.60 0.08 1630.0(R)

2.43

0.070

1083(R) 1040(R) 978(R)

1092.6 1044.0 972.0

T-O Stretching Vibrations of SiO4 and AlO4 7.70 0.37 1099.0(R) 5.60 0.28 1044.0(R) 5.70 0.31 997.0(R)

-1.15 3.28 3.61

-0.050 0.150 0.180

727(R) 704(R) 534(R) 492(R) 443(R) 420(IR) 365(IR) 348(IR)

730.8 709.3 534.0 489.0 445.0 421.1 360.9 347.4

O-T-O Bending Vibrations of SiO4 and AlO4 6.70 0.49 741.0(R) 8.90 0.67 709.2(R) 2.60 0.26 529.9(R) 9.60 1.04 486.8(R) 1.60 0.19 435.1(R) 1.30 0.16 417.4(IR) 2.10 0.31 367.7(IR) -0.20 -0.03

8.53 -0.82 5.49 5.08 3.44 2.94 3.33

0.560 -0.060 0.510 0.510 0.390 0.350 0.440

620(IR) 600(IR) 580(IR) 540(IR) 510(IR) 490(IR) 442(IR)

625.0 600.0 578.3 543.3 510.2 485.3 442.0

6.30 6.50 2.60 4.40 -7.70 2.90 2.10

-4.90 2.75 3.33 1.57 1.37

-0.380 0.220 0.300 0.150 0.130

361(R) 345(R) 308(R) 272(R) 224(R) 206(R) 160(R) 160(R) 123(R) 330(IR) 290(IR) 278(IR) 264(IR) 240(IR) 208(IR) 205(IR) 186(IR) 168(IR) 143(IR) 136(IR) 122(IR) 110(IR)

360.7 334.0 308.5 275.0 242.9 210.7 166.4 146.3 125.0 327.5 289.4 277.3 263.3 241.5 219.9 204.4 183.4 162.1 146.7 135.5 122.6 107.5

Translation and Libration Vibration Modes 4.00 0.59 324.0(R) 2.70 0.43 269.0(R) 1.00 0.17 231.4(R) -1.9 -0.37 175.0(R) 3.40 0.74 329.4(IR) 12.70 3.19 290.7(IR) 6.10 1.92 272.4(IR) 5.70 2.06 247.3(IR) 6.70 2.84 194.8(IR) 1.80 0.29 175.6(IR) 1.80 0.33 154.0(IR) 1.43 0.27 138.6(IR) 3.80 0.76 119.3(IR) 0.90 0.20 3.40 0.82 6.30 1.63 0.40 0.12 5.00 1.63 3.40 1.23 0.90 0.35 0.90 0.39 0.40 0.20

2.13 0.92 7.50 10.80 0.02 14.90 -3.14 5.49 6.27 7.25 3.91 2.75 2.55

0.320 0.170 1.590 3.020 0.003 2.510 -0.560 1.090 1.580 2.020 1.240 0.970 1.050

Libration Vibration of the H2O 0.53 640.0(IR) 0.57 608.6(IR) 0.24 544.3(IR) 0.43 522.7(IR) -0.80 504.5(IR) 0.32 0.25

Reference 28. b this work.

in natrolite with alcohol-water medium around 1 GPa results from an expansion of the elliptical channels in the ab-plane via an abrupt increase in the T-O2-T bridging angle.15 At the same time, the breathing mode of the eight-membered ring exhibits a relatively large red shift, and the O-H stretching modes change dramatically during the transition at 0.86 GPa. The two bands at 3329 and 3540 cm-1 disappear at 0.86 GPa while a broad band around 3450 cm-1 becomes detectable starting from around 0.7 GPa. This broad band is associated with O-H stretching vibrations and reflects the dispersed water distribution in the most expanded elliptical channels upon the formation of the ordered-paranatrolite phase. This result reveals that the phase

transition starts around 0.7 GPa and is completed around 0.86 GPa. This demonstrates that Raman spectroscopy is a very sensitive tool to detect the onset of phase transitions in natrolite. The shape of Raman bands becomes narrower and the peak position of the collapse mode related to the eight-membered ring shifts back to high frequency with further increase in pressure up to 1.53 GPa. These phenomena are consistent with the contraction of the unit cell volume due to the second phase transition from ordered-paranatrolite to superhydrated natrolite. In addition, the ν(O-H) stretching vibration modes of H2O change significantly, i.e., the broad band around 3450 cm-1 disappears and three sharper bands appear at 3430, 3500, and

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Figure 2. (a) Raman and (b) far-infrared spectra of natrolite at ambient pressure. Figure 4. Pressure dependence of phonon frequencies of natrolite during compression from 0 to 2.12 GPa at room temperature. The inset shows the red shift of ν(O-H) stretching of H2O in the spectrum. 4MR and 8MR represent four-membered and eight-membered ring bonds, respectively. Three regions correspond to different phases.

Figure 3. Raman spectra of natrolite at different pressures using pure water as a pressure medium. For clarity the spectra have been divided into two parts: (a) 100-1200 cm-1 and (b) 1500-3550 cm-1. The top curve is the Raman spectrum at ambient pressure after decompression from 2.12 GPa.

3551 cm-1. This clearly indicates that a rearrangement of the water molecules occurs in the superhydrated phase with three possible hydrogen bond distances. This is consistent with the results from the previous neutron diffraction studies of superhydrated natrolite where four hydrogen bonds were identified with three different OD-O interatomic distances of 0.93(2), 0.96(2)-0.97(2), and 1.02(3) Å.29 Pressure dependence of the Raman phonon frequencies from 0 to 2.12 GPa is shown in Figure 4. It can be seen that the O-H stretching band at 3551 cm-1 exhibits an anomalous pressure-induced frequency shift with a negative slope (dν/dP ) -3.3 cm-1/GPa) as shown in the inset. This feature might be attributed to the change in the O-H · · · O hydrogen bond length with pressure. When pressure is released to ambient condition, the superhydrated phase is fully reverted as can be seen from the quenched spectrum on the top of Figure 3. To better understand the variations in the phase transition pressure from natrolite to ordered paranatrolite with water content in pressure media, we also measured both Raman and IR spectra of natrolite using different volume ratios between the alcohol-water pressure transmitting medium and natrolite. Figure 5 shows the Raman spectra of natrolite at different pressures using more than 1:1 volume ratio between the alcohol-water pressure medium and natrolite. Clearly, natrolite

Figure 5. Raman spectra of natrolite at different pressures using more than 1:1 volume ratio between alcohol-water pressure transmitting medium and the powder sample. The top curve is the Raman spectrum at ambient pressure after decompression from 2.87 GPa.

undergoes two phase transitions just like the changes observed in the pure water medium run. However, the phase transition from natrolite to ordered paranatrolite occurs at 1.13 GPa, which is about 25% higher than that observed in the pure water medium run. Moreover, the transition is completed in a very narrow pressure range (1.09-1.13 GPa) in the 1:1 alcohol-water medium and natrolite run. A complementary understanding of the high-pressure behavior of natrolite can be obtained from the evolution of far-infrared spectra shown in Figure 6. Overall, most vibrational modes show a discontinuous shift around 1.12-1.21 GPa with some modes showing a sudden change in slope. This signals the phase transition from natrolite to ordered paranatrolite. The pressure dependence of the IR bands of natrolite is shown in Figure 6b. Specially, the significant red shift of the elliptical eight-ring bands observed at 1.21 GPa is reminiscent of our Raman data. On the other hand, the shape of the lattice modes changes dramatically and some peaks merge into a broad band at 1.21 GPa. In addition, a change in the relative intensity of two libration modes of water is observed

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Figure 6. (a) Far-infrared spectra of natrolite at different pressures using more than 1:1 volume ratio between alcohol-water pressure transmitting medium and the powder sample. (b) Peak positions of the IR bands in natrolite as a function of pressure using alcohol-water as a pressure medium.

where KT is the isothermal bulk modulus, ν0 is the vibrational frequency at ambient pressure, and (dνi/dp) is the mode shift. The mode Gru¨neisen parameters for all the observed modes of natrolite are given in Table 1 based on the value of 53 GPa for KT.14 Generally, the overall average mode Gru¨neisen parameter for more than 40 measured modes is 0.594. For comparison, the thermodynamic Gru¨neisen parameter for natrolite is calculated from the definition:

γth )

Figure 7. Raman spectra of natrolite at different pressures using lower volume ratio of alcohol-water pressure medium and the powder sample than 1:1.

around 500 cm-1 with pressure increase. At pressures above 1.55 GPa, we notice that new peaks related to the translation and libration modes of water start to appear. These imply a successive rearrangement of hydrogen bonds upon the formation of superhydrated natrolite, which is, again, in good agreement with our Raman results. In contrast, the Raman spectra do not show any phase transition if lower volume ratio of the alcohol-water pressure medium and the sample was employed as shown in Figure 7. These results indicate that the phase transition pressure from natrolite to ordered paranatrolite strongly depends on the water content in the pressure media. A reasonable assumption is that water molecules could go into the channels in natrolite with pressure but the phase transition can only be trigged with enough water molecules squeezed into the channels. This explains the formation of the ordered paranatrolite at relatively lower pressure with pure water as a pressure medium compared to alcohol-water medium. The phase transition cannot be observed if natrolite was compressed by a pressure medium with insufficient water content. Finally, we calculate mode Gru¨neisen parameters, γi, based on the following formula:

γi )

KT(dνi /dp) ν0

(2)

RVKs FCp

(3)

where RV is the volumetric thermal expansion, Ks the adiabatic bulk modulus, F the density, and Cp the molar specific heat. The calculated γth at ambient pressure and temperature is 1.33 ((0.02) using an RV of 3.1 × 10-5 K-1, Ks of 48.5 GPa, F of 2260 kg m-3, and Cp at ambient condition of 357.7 J K-1 mol-1.30-32 The bulk thermochemical Gru¨neisen parameter is far more than the average mode Gru¨neisen parameter (0.594) of the natrolite phase. From the comparison between the thermal Gru¨neisen parameters, we can conclude that low frequency lattice vibrations are the main contribution to the Gru¨neisen parameters. Under pressure, the structure of the TO4 tetrahedral primary building unit undergoes remarkably little contraction. These results imply that the major compression in natrolite is achieved by changes in the linkages within (and between) the larger secondary building units such as the elliptical eight-ring channels and the T5O10 chain units involving the rearrangements in the water and Na+ cations distribution in the channels. Conclusion In summary, we have performed combined in situ highpressure Raman and synchrotron infrared spectroscopic investigations of natrolite using three different pressure-transmitting media, pure water and two different volume ratios between alcohol-water pressure transmitting medium and natrolite. The Raman spectra of natrolite show two phase transitions, which are in good agreement with the previous reports. The far-infrared spectra also showed the occurrence of two pressure-induced phase transitions in natrolite. In addition, we confirmed that the changes of O-H stretching band modes are associated with the

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successive insertion of H2O molecules into the new sites in the elliptical channels. We obtained a useful correlation between the O-H stretching frequency red shifts and that of the O-H · · · O hydrogen bond parameters, which is associated with pressure-induced hydration. In addition, our results reveal that the phase transition pressure from natrolite to ordered-paranatrolite strongly depends on the water content in the pressure media. Finally, the value of average mode Gru¨neisen parameters is 0.594, which is far less than the bulk thermochemical Gru¨neisen parameters (1.33). This might be attributed to the rigid TO4 tetrahedra units compared to the flexible linkages between the tetrahedra in the natrolite framework. Acknowledgment. This work was supported by the Global Research Lab Program of the Ministry of Education, Science and Technology (MEST) of the Korean Government. The use of the National Synchrotron Light Source beamline U2A is supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 06-49658 and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Research carried out in part at the NSLS at BNL is supported by the U.S. Department of Energy, Office of Basic Energy Sciences. References and Notes (1) Breck, D. W. Zeolite Molecular SieVes; Krieger: Malabar, FL, 1984. (2) Hazen, R. M. Science 1983, 219, 1065. (3) Fachini, A.; Vasconcelos, M. T. EnViron. Sci. Pollut. Res. Int. 2006, 13, 414. (4) Astala, R.; Auerbach, S. M.; Monson, P. A. J. Phys. Chem. B 2004, 108, 9208. (5) Sanchez-Valle, C.; Sinogeikin, S. V.; Lethbridge, Z. A. D.; Walton, R. I.; Smith, C. W.; Evans, K. E.; Bass, J. D. J. Appl. Phys. 2005, 98, 053508. (6) Goryainov, S. V. Eur. J. Mineral. 2005, 17, 201. (7) Ovsyuk, N. N.; Goryainov, S. V. JETP Lett. 2006, 83, 109. (8) Goryainov, S. Phys. Status Solidi 2005, 202, R25.

Liu et al. (9) Kholdeev, O. V.; Belitsky, I. A.; Fursenko, B. A.; Goryainov., S. V. Dokl. Akad. Nauk SSSR 1987, 297, 946. (10) Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Vogt, T. Am. Mineral. 2006, 91, 247. (11) Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Hanson, J. C.; Kim, S. J. Nature 2002, 420, 485. (12) Goryainov, S. V.; Kursonov, A. V.; Miroshnichenko, Yu. M.; Smirnov, M. B.; Kabanov, I. S. Microporous Mesoporous Mater. 2003, 61, 283. (13) Belitsky, I. A.; Fursenko, B. A.; Gabuda, S. P.; Kholdeev, O. V.; Seryotkin, Y. V. Phys. Chem. Miner. 1992, 18, 497. (14) Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Artioli, G. J. Am. Chem. Soc. 2002, 124, 5466. (15) Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Vogt, T. Am. Mineral. 2005, 90, 252. (16) Goryainov, V. S.; Belitsky, I. A. Phys Chem Miner. 1995, 22, 443. (17) Liu, D.; Lei, W. W.; Chen, X. H.; Hao, J.; Jin, Y. X.; Cui, Q. L.; Zou, G. T. J. Phys. Chem. B 2009, 113, 16479. (18) Liu, Z.; Hu, J.; Yang, H.; Mao, H.; Hemley, R. J. Phys.: Condens. Matter 2002, 14, 10641. (19) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res. 1986, 91, 4673. (20) Mao, H. K.; Bell, P. M.; Shaner, J.; Steinberg, D. J. Appl. Phys. 1978, 49, 3276. (21) Meier, W. M. Z. Kristallogr. 1960, 113, 430. (22) Baur, W. H.; Kassner, D.; Kim, C. H.; Sieber, N. H. Eur. J. Mineral. 1990, 2, 761. (23) Goryainov, V. S.; Smirnov, M. B. Eur. J. Mineral. 2001, 13, 507. (24) Goryainov, V. S.; Smirnov, M. B.; Shebanin, A. P. Dokl. Phys. Chem. 2000, 375, 263. (25) Pechar, F.; Gregora, I.; Rykl, D. Collect. Czech. Chem. Commun. 1981, 46, 3043. (26) Pechar, F.; Rykl, D. Can. Mineral. 1983, 21, 689. (27) Ovsyuk, N. N.; Goryainov, S. V. Bull. Russ. Acad. Sci.: Phys. 2007, 71, 233. (28) Goryainov, S. V.; Belitsky, I. A. Phys. Chem. Miner. 1995, 22, 443. (29) Colligan, M.; Lee, Y.; Vogt, T.; Celestian, A. J.; Parise, J. B.; Marshall, W. G.; Hriljac, J. A. J. Phys. Chem. B 2005, 109, 18223. (30) Lee, Y. Unpublished results. (31) Sanchez-Valle., C.; Sinogeikin, S. V.; Lethbridge, Z. A. D.; Walton, R. I.; Smith, C. W.; Evans, K. E.; Bass, J. D. J. Appl. Phys. 2005, 98, 053508. (32) Johnson, G. K.; Flotow, H. E.; O’Hare, P. A. G.; Wise, W. S. Am. Mineral. 1983, 68, 1134.

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