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Pressure Determination by RamanSpectra of Water in HydrothermalDiamond-Anvil Cell Experiments

YUPING YANG and HAIFEI ZHENG*MOE Key Laboratory of Orogenic Belts and Crustal

Evolution, Department of Geology, Peking University,

Beijing, China

Index Headings: Water; Raman spectroscopy; Pressure determination;

Diamond-anvil cells.

INTRODUCTION

High-pressure and -temperature experiments have a numberof important implications in geosciences. Their most significantcontribution has been to our understanding of the physico-chemical properties of earth’s interior materials and thestructure and the dynamic processes in the deep Earth.1 Inthe 1990s, the development of hydrothermal diamond-anvilcells (HDAC)2 provided great convenience in simulating theextreme conditions of high pressure and high temperature inthe deep Earth. The main methods to determine the pressure inHDAC experiments are based on the equations of state of somesubstances and the changes of vibrational properties ofminerals as a function of temperature, pressure, and compo-sition. Ruby has been commonly used to measure pressure, butit gives rise to strong fluorescence and is very slightly solublein water at high temperatures.3,4 Quartz is insoluble in waterunder certain temperature conditions and is a suitable pressuregauge in HDAC experiments,5 but if the experimental systemcontains K2CO3 solution, quartz will also dissolve and reactwith the solution.6 Other substances such as Sm:YAG, SrB4O7,and cubic boron nitride similarly have limitations for pressuredetermination.7–11 Diamond is totally insoluble, and 13Cdiamond has a Raman peak at a lower frequency than theusual 12C diamond and can be used as a pressure gauge.12

However, such diamond chips are not commercially available.Using the equations of state of water is also inconvenient anddifferent researchers give different calculated parameters.13–15

Besides, the correlations between pressure and crystal-cellvolume of some metals (Au, Ta, Cu, W, etc.) and minerals(NaCl, MgO, Al2O3, etc.) have been obtained accurately, andpressure can be determined by their P-V-T equations using X-rays.3 However, it takes several hours to measure a pressurevalue. With synchrotron X-rays, measuring time is greatly

shortened, but synchrotron facilities are limited for generalusers.6 Therefore, finding another method to determine thepressure in HDAC experiments is still very important for high-pressure research.

Water is the most abundant and common fluid in nature. It iswell known that many of the unusual properties of water areattributed to the result of the three-dimensional hydrogenbonding network formed between water molecules. Owing tothe anharmonicity of the hydrogen bonding and its relativelylow energy, the water structure is rather sensitive to changes inthe ambient environment.16–18 Previous studies demonstratethat the strength of hydrogen bonding increases with increasingpressure or decreasing temperature,19–22 resulting in corre-sponding changes in the OH stretching vibrations representedas a broad band in the Raman spectra in the range of 2800–3800 cm�1. A recent investigation, based on temperature-induced changes of the OH stretching band of water, provides anew method using Raman spectroscopy to determine thetemperature of supercooled and liquid water.16 Therefore, webelieve that water can also be a good candidate as a pressuregauge in HDAC experiments. However, literature studies ofRaman spectra of water at high pressures and temperaturesmainly focused on changes in the water structure and hydrogenbond interaction.17–39 Little has been done with the potentialapplication of water in determining pressure. In this paper, wepresent the results of a quantitative in situ Raman spectroscopicstudy of water at pressures from 0.1 MPa to approximately2200 MPa and temperatures from 298 to 594 K using HDAC.We show that the results can be used as a secondaryspectroscopic pressure standard for HDAC experiments.Although the focus of this study is the calibration of aspectroscopic pressure gauge, the data also provide goodinformation for better understanding the Raman spectra ofwater under high pressure and temperature conditions.

EXPERIMENTAL METHODS

The equipment for experimental study is a hydrothermaldiamond-anvil cell similar to a Bassett-type externally heateddiamond-anvil cell.2 Rhenium with a thickness of 0.3 mm wasused as a gasket and the sample chamber was 0.5 mm indiameter. Pure (Mili-Q) water was loaded into the chamber ofthe HDAC together with a single crystal of quartz (;0.2 mm, asa pressure gauge)5 and cubic boron nitride11 (;0.2 mm, as asecondary pressure gauge). The experimental pressure wasestimated according to the Raman shift of the 464 cm�1 peak ofquartz.5 The sample was heated by two small furnaces made ofNiFe alloy wires and the temperature was measured by aNi90Cr10:Ni95Si5 thermocouple, which was calibrated using themelting points of 1-naphthol (369 K), salicylic acid (432 K),hippuric acid (461 K), and phenolphthalein (538 K) atatmospheric pressure before the experiment. The uncertaintyof the temperature is 61 K in the temperature range 298–594 K.

Raman spectra were obtained using a confocal micro-Ramansystem (Renishaw 1000). The excitation wavelength was the

Received 22 February 2008; accepted 5 November 2008.* Author to whom correspondence should be sent. E-mail: hfzheng@pku.edu.cn.

120 Volume 63, Number 1, 2009 APPLIED SPECTROSCOPY0003-7028/09/6301-0120$2.00/0

� 2009 Society for Applied Spectroscopy

514.5 nm line of an Arþ ion laser. The measured laser powerwas 50 mW at the source and about 5 mW at the samplesurface in the HDAC. The spectra were recorded from 1500cm�1 to 4000 cm�1 with one scan time and accumulation timesof 30 s. The resolution was 61 cm�1. The peak position isdetermined by fitting the spectra using Jandel Scientific PeakfitV4.04. In order to eliminate error, a linear baseline between2800 and 3800 cm�1 was subtracted for the spectrum of waterand the same fitted parameters are used in all spectra-fittingprocesses. During the experiments, in order to attain thehydrostatic pressure distribution of the system, pressure wasapplied and kept for at least three minutes before Ramanspectra were measured.

RESULTS AND DISCUSSION

Figure 1 presents Raman spectra of liquid water in the OHstretching region with increasing pressure at four differenttemperatures. It can be seen from Fig. 1 that the shapes of theOH stretching band of water at a certain temperature do notshow obvious variation with increasing pressure, but this band

shifts slightly to a lower wavenumber; as the temperatureincreases, the band becomes narrower and the low-frequencyshoulder gradually disappears.

The interpretation of the spectra in Fig. 1 is stillcontroversial. It is commonly thought that the contour of thestretching band in the spectrum of dense water is composed ofthe m1, m3 modes, and the overtone 2m2 mode enhanced by theFermi resonance with m1. Due to the effect of hydrogenbonding, individual components of the total contour are verybroad and overlap each other.40

Traditionally, this broad band was decomposed by a set of afew Gaussian components. In this paper, for the purpose of thecalibration of a spectroscopic pressure sensor, we fit this bandinto just one Gaussian component (Fig. 2) and pay moreattention to the global effects of the Raman shifts of the OHstretching band with increasing pressure and temperature. Incomparison with multicomponent fitting, this procedure canreduce the uncertainties in the spectra processes and providesthe convenience of using water as a pressure gauge. Moreover,instead of using the frequencies obtained by Gaussian fittingdirectly, we used frequency shifts relative to the referencefrequency of the OH stretching band of water at 298 K and 0.1MPa in the text. According to our experimental conditions andthe spectral fitting parameters, the reference frequency is 3400cm�1. The frequency shift Dmi(P,T) of a mode i between thefrequency mi(P,T) at some pressure and temperature and thefrequency mi(Pref,Tref) at a reference pressure and temperature isdescribed by the following relationship:

DmiðP; TÞ ¼ miðP; TÞ � miðPRef ; TRefÞ ð1Þ

Such a procedure can efficiently reduce possible systematicerrors, e.g., errors related to the wavenumber calibration.13

Based on different experimental conditions and fittingparameters, the reference frequency needs to change corre-spondingly.

Table I shows the measured data of temperature and pressureand the frequency shifts (Dmp(OH)) of the OH stretching band.

FIG. 1. Raman Spectra of the OH stretching band of water measured at 298 K, 385 K, 499 K, and 594 K under several pressures.

FIG. 2. Schematic diagram of Gauss fitting of the Raman spectra of the OHstretching band of water.

APPLIED SPECTROSCOPY 121

Figure 3 shows the relationship between the frequency shift ofthe OH stretching band and pressure along isotherms from 298K to 594 K according to the data in Table I. At a giventemperature, by fitting the data with a linear equation atdifferent pressures we obtained the expressions for thefrequency shifts Dmp(OH) (cm�1) versus p(MPa) in Table II.

As can been seen in Fig. 3 and Table II, at each temperaturethe data show a good linear relationship. The slopes of theisotherms with temperature did not vary significantly. Theslope seems to be independent of the temperature conditionunder the present pressure and temperature conditions.

Figure 4 presents the temperature dependence of theintercept values (B). We fitted the present data by a nonlinearfunction and obtained the following relationship:

B ¼ �0:0179T2 þ 27:7981T � 6776:7908 ð2Þ

The correlation coefficient (R2) is 0.995.By taking the mean of the slopes at different temperatures,

we get a global slope of�32.24, and standard deviation is 0.64.Then the relationship among the frequency shift of the OHstretching band of water, pressure p, and temperature T can beexpressed as the following function:

P ¼ �32:24�DmpðOHÞ

�� 0:0179T2 þ 27:7981T � 6776:7908

ð3Þ

where 298 � T � 594 (K), �41.7 � Dmp(OH) � 98.1 (cm�1),0.1 � P , 2200 (MPa), and the pressure calculated using Eq. 3has an uncertainty of 659 MPa.

For the effect of ions in solution, although it is generallyassumed that ions dissolved in liquid water have a strong effecton the hydrogen-bond structure, the investigation of femtosec-ond pump–probe spectroscopy of three different solutions hasdemonstrated that the presence of ions has a negligible effecton the hydrogen bonding in liquid water.41 Therefore, thedominant influence on the structure should be associated to thechanges of pressure and temperature.

Due to the abundance and the fundamental importance of

TABLE I. Experimentally determined frequency shifts of the H2Ostretching vibration as functions of temperature and pressure.a

T (K) P (MPa) Dmp(OH) (cm�1) T (K) P (MPa) Dmp(OH) (cm�1)

298 373 �12.6 (60.4) 461 278 66.2 (60.4)298 539 �17.4 (60.1) 461 487 49.9 (60.1)298 373 �11.5 (60.2) 461 701 46.4 (60.2)298 717 �20.8 (60.2) 461 1118 34.2 (60.2)298 826 �24.0 (60.5) 461 1259 30.5 (60.5)298 1319 �41.7 (60.1) 461 1835 16.1 (60.1)347 398 5.5 (60.4) 499 351 75.1 (60.4)347 539 0.7 (60.1) 499 458 65.7 (60.1)347 479 6.0 (60.2) 499 782 55.6 (60.2)347 843 �7.2 (60.2) 499 1229 42.6 (60.2)347 939 �10.2 (60.5) 499 1364 39.9 (60.5)347 1479 �27.1 (60.1) 499 1935 24.8 (60.1)385 384 27.7 (60.4) 537 362 85.2 (60.4)385 573 19.3 (60.1) 537 472 76.5 (60.1)385 558 20.8 (60.2) 537 872 63.1 (60.2)385 941 12.2 (60.2) 537 1339 50.9 (60.2)385 1046 6.7 (60.5) 537 1483 48.3 (60.5)385 1599 �12.4 (60.1) 537 2038 33.8 (60.1)423 349 46.7 (60.4) 594 430 98.1 (60.4)423 538 36.4 (60.1) 594 506 86.0 (60.1)423 620 35.4 (60.2) 594 938 71.0 (60.2)423 1039 23.4 (60.2) 594 1494 56.8 (60.2)423 1168 20.7 (60.5) 594 1616 54.8 (60.5)423 1712 2.9 (60.1) 594 2184 41.8 (60.1)

a T ¼ temperature; P ¼ pressure; Dmp(OH) ¼ determined frequency of the H2Ostretching vibration at the experimental pressure and temperature minus thereference frequency at 298 K and 0.1 MPa, calculated errors in frequency aregiven in parentheses.

FIG. 3. The relationship between the frequency shift of the OH stretching bandand pressure along isotherms from 298 K to 594 K.

TABLE II. The functions between pressure and frequency shifts of theH2O stretching vibration at different temperatures.a

T (K) Function R2 6P (MPa)

298 P ¼ �32.13(Dmp(OH)) þ 5.91 0.986 630347 P ¼ �32.10(Dmp(OH)) þ 606.46 0.987 626385 P ¼ �31.15(Dmp(OH)) þ 1236.05 0.984 639423 P ¼ �32.60(Dmp(OH)) þ 1803.06 0.987 637461 P ¼ �32.18(Dmp(OH)) þ 2250.73 0.951 686499 P ¼ �32.43(Dmp(OH)) þ 2661.95 0.976 668537 P ¼ �33.42(Dmp(OH)) þ 3087.47 0.977 670594 P ¼ �31.91(Dmp(OH)) þ 3367.08 0.945 6114

a T¼ temperature; P¼ pressure; R2¼ correlation coefficient; 6P¼ calculatederror in pressure; (Dmp(OH) ¼ determined frequency of the H2O stretchingvibration stretching vibration at the experimental pressure and temperatureminus the reference frequency at 298 K and 0.1 MPa.

FIG. 4. The relationship between the intercept value and temperature.

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water in geological processes and chemical systems, water hasa great potential to be used as a pressure gauge in high-pressureand -temperature experiments with HDAC. For experimentswith hydrous systems, using water to estimate the pressure willbe very convenient for researchers and will not cause anycontamination to the studied systems.

CONCLUSION

The technique of using spectroscopic pressure gaugesprovides an efficient method for in situ pressure determinationin HDAC experiments. Based on the sensitivity of thehydrogen bonding structure of water to the changes of pressureand temperature, water can be used to determine theexperimental pressure. In this paper, in situ Raman spectro-scopic measurements of water in the region of OH stretchingvibration were conducted up to approximately 2200 MPa in thetemperature range of 298 K to 594 K with HDAC. We fit thebroad Raman stretching band into just one Gaussian curvehaving a reference value of 3400 cm�1. A formula fordetermining the pressure was obtained and the relationship isP ¼ �32.24(Dmp(OH)) � 0.0179T2 þ 27.7981T � 6776.7908,where 298 � T � 594 (K), �41.7 � Dmp(OH) � 98.1 (cm�1),0.1 � P , 2200 (MPa), and the error is 59 MPa.

ACKNOWLEDGMENTS

This research was supported by National Basic Research Program of China(Grant No. 2006CB403508 and 2009CB825007) and National Nature ScienceFoundation of China (Grant No. 40730314). The authors thank Jinqiu Ren forhelp in collecting the Raman spectra. The authors also thank Lifei Zhang andtwo anonymous reviewers for insightful comments and suggestions.

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