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Towards a single crystal Raman spectrum of kaolinite at 77 K
Spectrochimica Acta Part A 57 (2001) 163 – 175 www.elsevier.nl/locate/saa
Towards a single crystal Raman spectrum of kaolinite at 77 K R.L. Frost *, J.T. Kloprogge Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, 2 George Street, GPO Box 2434, Brisbane, Qld 4001, Australia Received 13 August 1999; accepted 13 July 2000
Abstract The Raman spectra at 77 K of the hydroxyl stretching of kaolinite were obtained along the three axes perpendicular to the crystal faces. Raman bands were observed at 3616, 3658 and 3677 cm − 1 together with a distinct band observed at 3691 cm − 1 and a broad profile between 3695 and 3715 cm − 1. The band at 3616 cm − 1 is assigned to the inner hydroxyl. The bands at 3658 and 3677 cm − 1 are attributed to the out-of-phase vibrations of the inner surface hydroxyls. The Raman spectra of the in-phase vibrations of the inner-surface hydroxyl-stretching region are described in terms of transverse and longitudinal optic splitting. The band at 3691 cm − 1 is assigned to the transverse optic and the broad profile to the longitudinal optic mode. This splitting remained even at liquid nitrogen temperature. The transverse optic vibration may be curve resolved into two or three bands, which are attributed to different types of hydroxyl groups in the kaolinite. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Kaolinite; Liquid nitrogen temperature; Raman microscopy; Structural order; Transverse and longitudinal optics
1. Introduction Raman spectroscopy is proving a very powerful technique for the study of minerals and in particular clay minerals [1 – 4]. Recent technical developments have meant that Raman spectra of mineral powders [4– 9] and single crystals [3,4,10] can be measured. Fig. 1 shows the structure of kaolinite (Al2(OH)4Si2O5) with the position of the inner and inner surface hydroxyl groups. It also shows * Corresponding author. Tel.: +61-7-38642407; fax: + 617-38641804. E-mail address: [email protected] (R.L. Frost).
the commonly accepted position of the axes of the unit cell. Wiewiora et al. [1] first used dispersive Raman spectroscopy to determine the Raman spectrum of kaolin. Raman bands were identified at 3620, 3650, 3667, and 3682 cm − 1 with a prominent shoulder at 3692 cm − 1. In highly crystalline kaolinites with large crystal sizes, the 3684 cm − 1 band predominates [8,9]. In other spectra, the band is weak [2,6,7,11]. Assignment of the 3620 cm − 1 band was made in terms of the inner hydroxyl group and the bands at 3650, 3667, 3684 and 3692 cm − 1 were assigned to the inner surface hydroxyl groups [11,12]. One proposition is that the band assignments were attributable to individ-
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ual OH oscillators rather than coupled OH units. Other studies suggest that the bands are attributable to coupled in-phase and out-ofphase inner surface hydroxyl groups [8,13,14]. Michaelian [5] proposed likely origins of the 3684 cm − 1 band in terms of uncoupled inner surface hydroxyl stretching and transverse-longitudinal splitting [14]. Farmer [14] further explains the existence of the 3684 cm − 1 band on the basis of the known physics of scattering systems. In Raman spectra observed with 90° scattering geometry, the incident radiation interacts with crystal vibrations of similar wavelength. If a unit cell vibration develops an oscillating polarisability tensor, two corresponding long wavelength crystal modes exist: (a) a longitudinal mode (LO) and a transverse mode (TO). In Raman spectroscopy both the TO and LO modes are active, the LO mode exists at a higher frequency because of induced dipoles. Therefore, the Raman active infrared inactive band at 3684 cm − 1 is ascribed to the transverse optic mode. The Raman band at 3692 cm − 1 is assigned to
the longitudinal optic mode and is of low intensity in the Raman spectrum but is strongly infrared active. The kaolinite polytype unit cells contain only four hydroxyl groups, one (OH1) of which lies in the ab plane and the other three (OH2–OH4) lie at angles between 65 and 73° to the ab plane [15]. The inner surface hydroxyl groups give rise to infrared bands at 3695 cm − 1, a band with the transition moment lying perpendicular to the ab plane, and two weaker absorptions at 3650 and 3670 cm − 1 with transition moments in the ab plane [16,17]. Farmer and Russell [16,17] showed the dichroic behaviour of these vibrations. This behaviour can only be described if the vibrations couple to give one in-phase and two out-ofphase vibrations [18]. A comparison may be made between the vibrations of kaolinite hydroxyls and ammonia. Ammonia has a C3v symmetry and, therefore, one in-phase and two out-ofphase vibrations. Kaolinite has a pseudo C3v symmetry and, therefore, also has one in-phase and two out-of-phase vibrations.
Fig. 1. Model of the kaolinite structure showing the inner and inner surface hydroxyls.
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Johnston et al. [2] showed the dependence of the Raman hydroxyl stretching frequencies of the aqueous suspension of kaolinites on pH. Kaolinites of different origin were shown to have different spectra [2]. Johnston et al. first presented the Raman spectrum of the single crystal of Keokuk kaolinite. However this spectrum is from the ab plane and spectra from the other faces are not presented [3,4]. More recently Frost and van der Gaast [9] classified kaolinites into two groups according to the ratio of the Raman active infrared inactive 3682 cm − 1 and Raman active/infrared active 3695 cm − 1 vibrations. Furthermore it was shown that there was a linear relationship between the ratio of the intensities of the 3684 and 3650 cm − 1 bands and also the 3695 and 3670 cm − 1 bands. The 3684 and 3695 cm − 1 bands were described as the in-phase vibrations and the 3670 and 3650 cm − 1 bands as the out-of-phase vibrations. A model for the two in-phase vibrations at 3684 and 3695 cm − 1 was proposed based on symmetry of the OH oscillation. Such an hypothesis was disputed by Farmer [14] where the 3686 and 3695 cm − 1 bands are described as the transverse and longitudinal optical vibrations, respectively. The Raman spectrum of dickite, a polytype of kaolinite, has also been obtained [4]. Although dickite has the same chemical composition as kaolinite (Al2(OH)4Si2O5), dickite differs from kaolinite because of a different stacking sequence. In this study we report the Raman spectra of the single crystals of a low defect kaolinite at liquid nitrogen temperature.
2. Samples and analytical techniques
2.1. Kaolinite sample The kaolinite used in this study is the Keokuk kaolinite from Iowa, USA. The kaolinite was used as supplied. Crystals of the kaolinite are large and some are ca. 1.0 mm. The crystals were perfectly hexagonal in shape and were of 1 mm in size. This value is above the spatial resolution of the Raman spectrometer. The crystals have a high aspect ratio. This enables the spec-
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tra along the a-axis to be obtained. Crystals were placed in the corner of a perfect cube and the crystal oriented parallel to the sides of the cube using a fine needle and an optical microscope. The rotation of the cube through 90° about the X, Y, and Z axes of the laboratory frame allows the kaolinite crystal to be aligned in relation to the laser beam direction. The long axis of the kaolinite crystal is defined as the a-axis (Fig. 1). The axis at right angles to the a-axis in the same plane is defined as the b-axis. Then the axis at right angles to the ab plane is the c-axis. The spectroscopic axes differ from the crystallographic axes.
2.2. Raman microprobe spectroscopy For spectra at 298 K, kaolinite clay minerals were placed on a polished stainless steel surface on the stage of an Olympus BHSM microscope, equipped with × 5, ×20 and × 50 objective lenses [9,10]. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a charge coupled device (CCD). Raman spectra were excited by a Spectra-Physics model 127 HeNe laser (633 nm), recorded at a resolution of 2 cm − 1 in sections of 1000 cm − 1. Repeated acquisitions using the highest magnification were accumulated to improve the signal to noise ratio in the spectra. For the 298 K spectra data were collected at 20 s intervals for 10 min at maximum magnification (× 50). Spectra were calibrated using the 520.5 cm − 1 line of a silicon wafer. The experimental method for obtaining single crystal spectra are based on the use of a perfect small stainless steel cube. A kaolinite crystal of ca. 1.5 mm size is placed in the corner of the cube with the 001 face pointing upwards. The crystal is then placed and positioned on the cube under an optical microscope. The XYZ axes of the cube then enable the rotation of the crystal along the three crystal faces. In the normal course of events a polariser is placed in the incident beam to ensure that the incident radiation is perfectly plane-polarised. An analyser is placed between the microscope objective and the entrance slit of
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the spectrograph to ensure the correct characteristics of the scattered radiation. Spectra at liquid nitrogen temperature were obtained using a Linkam thermal stage (Scientific Instruments, Waterfield, Surrey, UK) [10]. Samples were placed in a stainless steel cup fitted over the silver plate of the thermal stage. For spectra at 77 K, nitrogen gas from liquid nitrogen passed through a small hole in this plate immediately below the centre of the glass disc. It was found that the best method of obtaining 77 K was to rapidly cool at 50 K min − 1. Because of the increased optics used in collecting data at 77 K, spectra at 77 K are noisier and require longer accumulation times. Spectra were obtained using 12 s scans for 20 min using the special short × 50 (ULWD) objective. The intensity of the kaolinite spectra were found to be deceased in intensity by 0.3 using the ULWD objective compared with the normal × 50 objective. A lower Raman signal was obtained using this objective owing to the low numerical aperture of this long working distance objective. This, combined with the spherical aberration of the stage window, results in decreased signal. It should be noted that the use of the × 50 objective collects scattered light over a much wider angle than the × 20 or ×10 objective. There is less ‘polarisation leakage’ if × 20 objectives are used. Then, however, the intensity of the scattered light diminishes, making the collection of spectra more difficult. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package GRAMS® (Galactic Industries, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss – Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995.
3. Results and discussion
3.1. Raman spectroscopy The HeNe laser incident radiation is plane polarised. In the experiment 180° scattering geometry is used. The Raman intensity is a function of the shape of the polarisability tensor and the scattering geometry. The maximum intensity is obtained when the electric vector of the incident radiation and the ellipsoidal differential polarisability tensor of the hydroxyl bond are parallel [19]. Because of the scattering geometry only the transverse optical mode (TO) is being measured. To measure both the longitudinal and transverse optics 90° scattering geometry would need to be employed. If radiation is incident at 90° to the (001) plane and observed with 180° geometry, no intensity in either the TO or LO modes will be observed as the wave vector is at right angles to both the incident and scattered radiation. Furthermore, whilst the geometry of the incident radiation is tight, i.e. incident to the crystal face over a very small angle, this is not so for the scattered radiation as the scattered light is collected over an angle of ca. 30°. This means that not only will some of the TO modes be collected but also the LO modes as well. So in the spectra reported in this study we are simply looking at orientation effects. The kaolinite crystals are large \ 1 mm and are easily observed and the crystal faces positioned in the Raman microprobe. The spectra are reported according to the Porto notation [20]: the propagation directions of the incident and scattered light are described in terms of the crystallographic axes a, b and c. The notation may read, for example A(BC)D. Here A is the direction of the incident radiation and the D the direction of the collection of the scattered radiation. B is the direction of the polarisation of the electric vector of the incident radiation and C is the direction of the analyser [19–21]. In this paper we report only the spectra of the type C(AA)C. Thus the incident and collection optics are in the same direction and the direction of the polariser and analyser are also in the same direction. We are using 180° scattering geometry. This means, for example, that if the
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Fig. 2. Raman spectra of the hydroxyl-stretching region of Keokuk kaolinite along three axes perpendicular to the crystal surfaces at 77 K.
incident radiation is perpendicular to the 001 face of the kaolinite crystal and the electric vector is aligned with the direction of the polarisability tensor of the hydroxyl stretching vibration then the intensity of the Raman band will be maximised. If the electric vector of the incident radiation and the polarisability tensor are at right angles then the intensity will be at a minimum and approach zero.
3.2. Raman spectroscopy of the Keokuk kaolinite The Raman spectra for the Keokuk kaolinite at 77 K along three axes are shown in Fig. 2 and the results of the band component analyses is reported in Table 1. The three spectra obtained from Raman scattering at right angles to the kaolinite crystal surfaces are shown. All three spectra show considerable differences in the region attributed to the in-phase vibrations of the inner surface hydroxyls. The effect of obtaining the spectra at liquid nitrogen temperatures is to cause the frequency of the inner hydroxyl group to move to lower frequencies. The band is ob-
served in the room temperature spectra at 3620 cm − 1 and is at 3616 cm − 1 at 77 K. The inner hydroxyl shifts by 4 cm − 1 to lower frequencies. This data is in good agreement with previously published data [3,10]. Such frequency shifts have been reported in the infrared spectra of kaolinite where, at 5 K, the inner hydroxyl frequency was observed at 3612 cm − 1 [22]. The value of the infrared inner surface hydroxyl at 65 K was 3615 cm − 1 [22]. The bands attributed to the inner surface hydroxyls observed at 3695 (n1), 3684 (n4), 3670 (n2) and 3650 (n3) cm − 1 [9–11] at room temperature are now observed at 3708 and 3696, 3691, 3677 and 3658 cm − 1. Upon cooling to liquid nitrogen temperatures, the inner surface hydroxyl frequencies shift higher by ca. 8–10 cm − 1. Similar shifts have been reported for the infrared spectra of kaolinite at 5 K [22]. The effect of cooling to liquid nitrogen temperatures has meant that the attraction of the hydrogen of the inner hydroxyl has increased in strength. This is no doubt due to the closer proximity to the nearest oxygen. Upon cooling to liquid nitrogen temperatures the
Spectrum c, fit 3
Spectrum c, fit 2
Spectrum c, fit 1
Spectrum b, fit 3
Spectrum b, fit 2
Spectrum b, fit 1
Spectrum a, fit 3
C(AA)C
A(BB)A
A(CC)A
Spectrum a, fit 1
Spectrum a, fit 2
Spectroscopic experiment
Raman spectra
n1a Band 1 3704 22.3 15.0 3708 9.9 8.6 3710 4.5 6.2 3700 35.8 17.2 3707 9.8 10.5 3709 3.7 5.2 3698 32.2 16.1 3708 11.8 10.0 3709 6.2 7.8
Band Parameters Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1)
3702 13.9 9.6
3703 21.0 10.7
3703 11.0 11.3
n1b Band 2
3696 24.4 11.7 3695 16.0 7.8
3696 30.0 11.6 3697 11.8 7.5
3696 22.4 12.8 3695 20.4 8.8
n1c Band 3
3691 36.0 6.3 3691 30.7 6.0 3691 30.8 5.8
3692 19.1 6.4 3691 14.9 5.5 3691 20.0 5.0
3692 17.6 7.7 3691 7.4 4.9 3691 6.4 4.3
n4
3677 5.4 5.5 3677 7.0 6.4 3677 7.2 6.3
3677 7.0 5.1 3677 7.0 5.1 3677 7.1 5.3
3677 8.3 4.6 3677 8.3 4.4 3677 7.3 4.4
n2
3658 11.2 5.9 3658 11.2 5.9 3658 11.0 5.9
3658 14.9 4.9 3658 15.0 4.9 3658 14.6 5.2
3658 6.9 5.9 3658 6.9 5.9 3658 6.8 5.9
n3
3616 15.2 3.8 3616 14.8 3.8 3616 14.8 3.8
3616 23.0 3.4 3616 23.0 3.4 3616 21.8 3.4
3616 45.0 3.3 3616 45.0 3.3 3616 43.5 3.3
n5
Table 1 Results of the band component analysis of the Raman spectra at 77 K of the hydroxyl-stretching region of the ordered and disordered of Keokuk kaolinite along three axes perpendicular to the three crystal faces
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kaolinite layers are brought closer together, therefore, the bonding between the hydroxyls of the gibbsite-like surface to the next adjacent siloxane layer is weaker and, therefore, the bands occur at higher wavenumbers.
3.3. A(CC)A spectrum Spectrum a in Fig. 2 corresponds to the A(CC)A experiment. Spectrum a is the spectrum when the laser beam is parallel to the 001 and along the a-axis. Both the polariser and analyser are aligned in the c-direction. This experiment determines the hydroxyl stretching frequencies, which are from the hydroxyls, which are at small angles to the c-axis. The inner hydroxyl group of kaolinite lies parallel to the ab plane and the resultant hydroxyl stretching frequency will be determined in this experiment. In this spectrum the relative intensity of the band assigned to the inner hydroxyl is maximised and the relative intensities of the bands attributed to the inner surface hydroxyls are small. Strictly in this experiment only transverse optical vibrations should be observed and, therefore, no intensity should be observed in the bands between 3695 and 3710 cm − 1. However because of the leakage of the collection optics being over a wide angle, intensity in both the TO and LO modes are observed. The results of the band component analysis of this spectrum are reported in Fig. 3a –c where either a five, six or seven band fit was used for the inner surface hydroxyl stretching region. The differences in the band fitting procedure depends on the number of bands which are fitted in the spectral profile between 3685 and 3720 cm − 1. Upon using a five band fit (Fig. 3a) a squared correlation coefficient of 0.9955 was obtained. However the fit is not good and an extra band is required to fit the spectral profile. Upon using a three band fit, i.e. using three bands in the 3685 to 3720 cm − 1 region, an r 2 value of 0.9969 was obtained. Upon using a four band fit, an r 2 value of 0.9987 was obtained. The question arises as to whether there are three or four bands in the 3685–3720 cm − 1 region. One possible explanation is that if the three band model is used, then
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the reason for the additional band is ascribed to the longitudinal optics of different inner surface hydroxyls. If a four band model is used, then three of the bands observed at 3710, 3703 and 3695 cm − 1 may be attributed to three LO different inner surface hydroxyls and the band at 3691 cm − 1 to the transverse optical vibration. It is interesting to compare the relative intensities of the bands according to how many bands are fitted to the spectral profile. In lines labelled spectrum a fit1, fit 2, fit 3 of Table 1, the relative intensities of the n2, n3 and n5 bands are constant and any variation probably represents experimental error. The experimental error in the n5 band is 9 1.5%. The band fitting procedure does have a marked effect upon the relative intensity and bandwidth of the 3691 cm − 1 band attributed to the transverse optical vibration. For a two band fit of the transverse/longitudinal optic mode region, the relative intensity is 17.6% and the bandwidth 7.7 cm − 1. For a three band fit, the values are 7.4% and 4.9 cm − 1 and for the four band fit, 6.4% and 4.3 cm − 1. The differences in the parameters between the three and four band fit are small. It should also be noted that the band fitting procedure did not effect the position of the 3691 cm − 1 band. The bands observed in the 298 K spectrum are found at 3689, 3684, 3668, 3650 and 3620 cm − 1 with relative intensities of 22.3, 37.2, 9.1, 13.7 and 17.2%. These bands have bandwidths of 17.4, 9.0, 8.9, 7.8 and 3.7 cm − 1, respectively. A remarkable difference in the relative intensity between the 77 and 298 K spectra of the band attributed to the transverse longitudinal optic is observed. The value decreases from 37.2 to 17.6% (two band fit), 7.4% (three band fit) or 6.4% (four band fit). The question may be asked as to the reason for this large decrease. If the incident radiation is along the c-axis then the projection of the polarisability tensors of the inner surface hydroxyls would be small resulting in low relative intensities. Whereas the polarisability tensor of the inner hydroxyl would be large, hence the relative intensity of the inner hydroxyl vibration is large. The band fitting process also determines the bandwidths of the inner surface hydroxyls. If a two band fit is used to fit the 3685 and 3720 cm − 1
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region, then the band widths of the 3704 and 3691 cm − 1 bands are 15.0 and 7.7 cm − 1. For a three band fit the bandwidths of the 3708, 3696 and
3691 cm − 1 bands are 8.6, 12.8 and 4.9 cm − 1. If a four band fit is used then the bands at 3710, 3703, 3695, and 3691 cm − 1 are 6.2, 11.3, 8.8 and 4.3
Fig. 3. Band component analysis of the A(CC)A Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the ab plane with a five band fit, and (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the ab plane with a six band fit.
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cm − 1. The effect of the band fitting process is to have a sharp band at 3691 cm − 1 with one other broad band at either 3696 (three band fit) or 3703 cm − 1 (four band fit).
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3.4. A(BB)A spectrum When the laser beam is at right angles to the ac plane, the relative intensities of the inner surface
Fig. 4. Band component analysis of the A(BB)A Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a five band fit; (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a six band fit; and (c) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a seven band fit.
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Fig. 4. (Continued)
hydroxyls compared with that of the inner hydroxyl is increased significantly (Fig. 4). Spectrum b in Fig. 2 results from the A(BB)A experiment. The hydroxyl stretching frequency of the inner hydroxyl is observed at 3616 cm − 1, the identical position for all analyses. The bands attributed to the inner surface hydroxyls also are in identical positions but are a function of the number of bands used in the fitting process. Significantly the relative intensity of the 3691 cm − 1 band has increased. The bandwidth of the 3691 cm − 1 band in spectrum a for the four band fit is 4.3 cm − 1 and in spectrum b for the four band fit is 5.0 cm − 1. The relative intensity of the n5 band has decreased from 45% for spectrum a to 23% for spectrum b. Correspondingly there has been a substantial increase in the relative intensity of the 3703 cm − 1 band for the four band fit from 11 to 21%.
3.5. C(AA)C spectrum Spectrum c is the C(AA)C spectrum (Fig. 5). The predominant feature of spectrum c obtained
when the incident and scattered radiation is perpendicular to the ab plane is the intensity of the band at 3691 cm − 1 attributed to the transverse optic mode. The intensity of the band is 36% in the two band fit, 30.7% in the three band fit and 30.8% for the four band fit. The two bands at 3677 and 3658 cm − 1 have been described as the out-of-phase vibrations of the in-phase vibrations of the inner surface hydroxyls [8,13]. These two bands are observed in the room temperature spectrum at 3650 and 3668 cm − 1, i.e. there is a shift of 8 cm − 1 upon cooling to liquid nitrogen temperature. The bandwidths of these two bands are 8.9 and 7.8 cm − 1 respectively. In spectrum a the bandwidth of the 3677 cm − 1 band is 4.4 cm − 1, in spectrum b averaged at 5.3 cm − 1 and in spectrum c ca. 5.5 cm − 1. The bandwidths of the 3658 cm − 1 band in spectra a, b and c are 5.9, 5.3 and 5.9 cm − 1. The widths of these bands are very small at liquid nitrogen temperatures. By obtaining spectra of the inner surface hydroxyls at liquid nitrogen temperatures, the spectra of the longitudinal optic of the inner
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hydroxyls are being uncoupled, i.e. separate bands are being observed irrespective of whether a three or four band fit is used. Nevertheless the uncoupling of the out-of-phase vibrations is not occurring. The bands are simply becoming narrower.
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The question may be asked that if the in-phase vibrations become uncoupled or partially uncoupled, why then do the out-of phase vibrations not become uncoupled. This explanation is only one possibility. One proposition explaining the bands
Fig. 5. Band component analysis of the C(AA)C Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the ac plane with a five band fit; (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the ac plane with a six band fit; and (c) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along ac plane with a seven band fit.
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Fig. 5. (Continued)
found at 3710, 3703 and 3695 cm − 1 is the attribution to different inner surface hydroxyls arising from different parts of the kaolinite structure. Such a possibility although feasible depends on whether Raman spectroscopy measures molecular structure of surfaces. This does not seem likely. By obtaining spectra at liquid nitrogen temperatures, there may be separation of the coupled and uncoupled longitudinal optic vibrations. In this case, a three band fit of the in-phase hydroxyl stretching vibrations is favoured. The difficulty of using crystals which are at the limits of the spatial resolution of the spectrometer is the uncertainty in the positioning of the crystal in the beam. If the crystals are greater than the spatial resolution then there are no problems. The spectroscopic axes do not correspond to the kaolinite crystallographic axes and so, strictly, the spectra should be adjusted to the crystallographic axes. At this stage, such experimentation is not possible. A further difficulty is the actual selection of crystals. X-ray diffraction of single crystals has proved the existence of twinning [23]. Crystals of
kaolinite proved to be pseudo-twin intergrowths of three lattice orientations. The size of these intergrowths is submicroscopic. Twinning simply means that the dioctahedral vacancy is in different positions between two crystals. This will not effect the position of the kaolinite hydroxyls. Consequently, spectra at different orientations along three crystal axes can be obtained.
4. Conclusion The Raman spectra of the hydroxyl stretching region of Keokuk kaolinite were obtained at 77 K using a Raman microprobe and a thermal stage. The band attributed to the inner hydroxyl was observed at 3616 cm − 1 with a bandwidth of 3.3 cm − 1. The band profile of the 3685–3720 cm − 1 was curve fitted with two, three and four bands. The three and four band fits proved most satisfactory. The 77 K band at 3691 cm − 1 attributed to the longitudinal optical vibration proved to be
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strongly orientation dependent as did the other hydroxyl stretching bands.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Normandy Industrial Minerals through Mr L. Barnes, Chief Geologist, is thanked for financial support.
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[8] R.L. Frost, Clay Miner. 32 (1997) 73. [9] R.L. Frost, S.J. van der Gaast, Clay Miner. 32 (1997) 471. [10] U. Johannson, R.L. Frost, W. Forsling, J.T. Kloprogge, Appl. Spectrosc. 52 (1998) 1277. [11] V. Pajcini, P. Dhamelincourt, Appl. Spectrosc. 48 (1994) 638. [12] P.G. Rouxhet, N. Samudacheata, H. Jacobs, O. Anton, Clay Miner. 12 (1977) 171. [13] G.W. Brindley, K. Chih-Chun, J.L. Harrison, M. Lipsiscas, R. Raythatha, Clay Clay Miner. 34 (1986) 233. [14] V.C. Farmer, Clay Miner. 33 (1998) 601. [15] D.L. Bish, Clays Clay Miner. 41 (1993) 738. [16] V.C. Farmer, J.D. Russell, Spectrochim. Acta 20 (1964) 1149. [17] V.C. Farmer, J.D. Russell, Spectrochim. Acta 22 (1966) 389. [18] V.C. Farmer, The layer silicates, in: V.C. Farmer (Ed.), Infrared Spectra of Minerals, Mineralogical Society, London, 1974, pp. 331 – 363. [19] T.R. Gilson, P.J. Hendra, Laser Raman Spectroscopy, Wiley-Interscience, New York, 1970, pp. 97 – 99. [20] S.P.S. Porto, Spex Speak. 13 (1968) 1. [21] A. De Paepe, P.J. Hendra, Internet J. Vib. Spectrosc. 3 (1999) 25. [22] R. Prost, A. Damene, E. Huard, J. Draiard, J.P. Leydecker, Clays Clay Miner. 37 (1989) 464. [23] S.W. Bailey, Review of the structural relationships of the kaolin minerals in kaolin genesis and utilisation, in: H. Murray, W. Bundy, C. Harvey (Eds.), Special Publication No. 1, The Clay Minerals Society, Boulder, CO, 1993.
Towards a single crystal Raman spectrum of kaolinite at 77 K R.L. Frost *, J.T. Kloprogge Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, 2 George Street, GPO Box 2434, Brisbane, Qld 4001, Australia Received 13 August 1999; accepted 13 July 2000
Abstract The Raman spectra at 77 K of the hydroxyl stretching of kaolinite were obtained along the three axes perpendicular to the crystal faces. Raman bands were observed at 3616, 3658 and 3677 cm − 1 together with a distinct band observed at 3691 cm − 1 and a broad profile between 3695 and 3715 cm − 1. The band at 3616 cm − 1 is assigned to the inner hydroxyl. The bands at 3658 and 3677 cm − 1 are attributed to the out-of-phase vibrations of the inner surface hydroxyls. The Raman spectra of the in-phase vibrations of the inner-surface hydroxyl-stretching region are described in terms of transverse and longitudinal optic splitting. The band at 3691 cm − 1 is assigned to the transverse optic and the broad profile to the longitudinal optic mode. This splitting remained even at liquid nitrogen temperature. The transverse optic vibration may be curve resolved into two or three bands, which are attributed to different types of hydroxyl groups in the kaolinite. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Kaolinite; Liquid nitrogen temperature; Raman microscopy; Structural order; Transverse and longitudinal optics
1. Introduction Raman spectroscopy is proving a very powerful technique for the study of minerals and in particular clay minerals [1 – 4]. Recent technical developments have meant that Raman spectra of mineral powders [4– 9] and single crystals [3,4,10] can be measured. Fig. 1 shows the structure of kaolinite (Al2(OH)4Si2O5) with the position of the inner and inner surface hydroxyl groups. It also shows * Corresponding author. Tel.: +61-7-38642407; fax: + 617-38641804. E-mail address: [email protected] (R.L. Frost).
the commonly accepted position of the axes of the unit cell. Wiewiora et al. [1] first used dispersive Raman spectroscopy to determine the Raman spectrum of kaolin. Raman bands were identified at 3620, 3650, 3667, and 3682 cm − 1 with a prominent shoulder at 3692 cm − 1. In highly crystalline kaolinites with large crystal sizes, the 3684 cm − 1 band predominates [8,9]. In other spectra, the band is weak [2,6,7,11]. Assignment of the 3620 cm − 1 band was made in terms of the inner hydroxyl group and the bands at 3650, 3667, 3684 and 3692 cm − 1 were assigned to the inner surface hydroxyl groups [11,12]. One proposition is that the band assignments were attributable to individ-
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 0 ) 0 0 3 4 5 - 0
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ual OH oscillators rather than coupled OH units. Other studies suggest that the bands are attributable to coupled in-phase and out-ofphase inner surface hydroxyl groups [8,13,14]. Michaelian [5] proposed likely origins of the 3684 cm − 1 band in terms of uncoupled inner surface hydroxyl stretching and transverse-longitudinal splitting [14]. Farmer [14] further explains the existence of the 3684 cm − 1 band on the basis of the known physics of scattering systems. In Raman spectra observed with 90° scattering geometry, the incident radiation interacts with crystal vibrations of similar wavelength. If a unit cell vibration develops an oscillating polarisability tensor, two corresponding long wavelength crystal modes exist: (a) a longitudinal mode (LO) and a transverse mode (TO). In Raman spectroscopy both the TO and LO modes are active, the LO mode exists at a higher frequency because of induced dipoles. Therefore, the Raman active infrared inactive band at 3684 cm − 1 is ascribed to the transverse optic mode. The Raman band at 3692 cm − 1 is assigned to
the longitudinal optic mode and is of low intensity in the Raman spectrum but is strongly infrared active. The kaolinite polytype unit cells contain only four hydroxyl groups, one (OH1) of which lies in the ab plane and the other three (OH2–OH4) lie at angles between 65 and 73° to the ab plane [15]. The inner surface hydroxyl groups give rise to infrared bands at 3695 cm − 1, a band with the transition moment lying perpendicular to the ab plane, and two weaker absorptions at 3650 and 3670 cm − 1 with transition moments in the ab plane [16,17]. Farmer and Russell [16,17] showed the dichroic behaviour of these vibrations. This behaviour can only be described if the vibrations couple to give one in-phase and two out-ofphase vibrations [18]. A comparison may be made between the vibrations of kaolinite hydroxyls and ammonia. Ammonia has a C3v symmetry and, therefore, one in-phase and two out-ofphase vibrations. Kaolinite has a pseudo C3v symmetry and, therefore, also has one in-phase and two out-of-phase vibrations.
Fig. 1. Model of the kaolinite structure showing the inner and inner surface hydroxyls.
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Johnston et al. [2] showed the dependence of the Raman hydroxyl stretching frequencies of the aqueous suspension of kaolinites on pH. Kaolinites of different origin were shown to have different spectra [2]. Johnston et al. first presented the Raman spectrum of the single crystal of Keokuk kaolinite. However this spectrum is from the ab plane and spectra from the other faces are not presented [3,4]. More recently Frost and van der Gaast [9] classified kaolinites into two groups according to the ratio of the Raman active infrared inactive 3682 cm − 1 and Raman active/infrared active 3695 cm − 1 vibrations. Furthermore it was shown that there was a linear relationship between the ratio of the intensities of the 3684 and 3650 cm − 1 bands and also the 3695 and 3670 cm − 1 bands. The 3684 and 3695 cm − 1 bands were described as the in-phase vibrations and the 3670 and 3650 cm − 1 bands as the out-of-phase vibrations. A model for the two in-phase vibrations at 3684 and 3695 cm − 1 was proposed based on symmetry of the OH oscillation. Such an hypothesis was disputed by Farmer [14] where the 3686 and 3695 cm − 1 bands are described as the transverse and longitudinal optical vibrations, respectively. The Raman spectrum of dickite, a polytype of kaolinite, has also been obtained [4]. Although dickite has the same chemical composition as kaolinite (Al2(OH)4Si2O5), dickite differs from kaolinite because of a different stacking sequence. In this study we report the Raman spectra of the single crystals of a low defect kaolinite at liquid nitrogen temperature.
2. Samples and analytical techniques
2.1. Kaolinite sample The kaolinite used in this study is the Keokuk kaolinite from Iowa, USA. The kaolinite was used as supplied. Crystals of the kaolinite are large and some are ca. 1.0 mm. The crystals were perfectly hexagonal in shape and were of 1 mm in size. This value is above the spatial resolution of the Raman spectrometer. The crystals have a high aspect ratio. This enables the spec-
165
tra along the a-axis to be obtained. Crystals were placed in the corner of a perfect cube and the crystal oriented parallel to the sides of the cube using a fine needle and an optical microscope. The rotation of the cube through 90° about the X, Y, and Z axes of the laboratory frame allows the kaolinite crystal to be aligned in relation to the laser beam direction. The long axis of the kaolinite crystal is defined as the a-axis (Fig. 1). The axis at right angles to the a-axis in the same plane is defined as the b-axis. Then the axis at right angles to the ab plane is the c-axis. The spectroscopic axes differ from the crystallographic axes.
2.2. Raman microprobe spectroscopy For spectra at 298 K, kaolinite clay minerals were placed on a polished stainless steel surface on the stage of an Olympus BHSM microscope, equipped with × 5, ×20 and × 50 objective lenses [9,10]. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a charge coupled device (CCD). Raman spectra were excited by a Spectra-Physics model 127 HeNe laser (633 nm), recorded at a resolution of 2 cm − 1 in sections of 1000 cm − 1. Repeated acquisitions using the highest magnification were accumulated to improve the signal to noise ratio in the spectra. For the 298 K spectra data were collected at 20 s intervals for 10 min at maximum magnification (× 50). Spectra were calibrated using the 520.5 cm − 1 line of a silicon wafer. The experimental method for obtaining single crystal spectra are based on the use of a perfect small stainless steel cube. A kaolinite crystal of ca. 1.5 mm size is placed in the corner of the cube with the 001 face pointing upwards. The crystal is then placed and positioned on the cube under an optical microscope. The XYZ axes of the cube then enable the rotation of the crystal along the three crystal faces. In the normal course of events a polariser is placed in the incident beam to ensure that the incident radiation is perfectly plane-polarised. An analyser is placed between the microscope objective and the entrance slit of
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the spectrograph to ensure the correct characteristics of the scattered radiation. Spectra at liquid nitrogen temperature were obtained using a Linkam thermal stage (Scientific Instruments, Waterfield, Surrey, UK) [10]. Samples were placed in a stainless steel cup fitted over the silver plate of the thermal stage. For spectra at 77 K, nitrogen gas from liquid nitrogen passed through a small hole in this plate immediately below the centre of the glass disc. It was found that the best method of obtaining 77 K was to rapidly cool at 50 K min − 1. Because of the increased optics used in collecting data at 77 K, spectra at 77 K are noisier and require longer accumulation times. Spectra were obtained using 12 s scans for 20 min using the special short × 50 (ULWD) objective. The intensity of the kaolinite spectra were found to be deceased in intensity by 0.3 using the ULWD objective compared with the normal × 50 objective. A lower Raman signal was obtained using this objective owing to the low numerical aperture of this long working distance objective. This, combined with the spherical aberration of the stage window, results in decreased signal. It should be noted that the use of the × 50 objective collects scattered light over a much wider angle than the × 20 or ×10 objective. There is less ‘polarisation leakage’ if × 20 objectives are used. Then, however, the intensity of the scattered light diminishes, making the collection of spectra more difficult. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package GRAMS® (Galactic Industries, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss – Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995.
3. Results and discussion
3.1. Raman spectroscopy The HeNe laser incident radiation is plane polarised. In the experiment 180° scattering geometry is used. The Raman intensity is a function of the shape of the polarisability tensor and the scattering geometry. The maximum intensity is obtained when the electric vector of the incident radiation and the ellipsoidal differential polarisability tensor of the hydroxyl bond are parallel [19]. Because of the scattering geometry only the transverse optical mode (TO) is being measured. To measure both the longitudinal and transverse optics 90° scattering geometry would need to be employed. If radiation is incident at 90° to the (001) plane and observed with 180° geometry, no intensity in either the TO or LO modes will be observed as the wave vector is at right angles to both the incident and scattered radiation. Furthermore, whilst the geometry of the incident radiation is tight, i.e. incident to the crystal face over a very small angle, this is not so for the scattered radiation as the scattered light is collected over an angle of ca. 30°. This means that not only will some of the TO modes be collected but also the LO modes as well. So in the spectra reported in this study we are simply looking at orientation effects. The kaolinite crystals are large \ 1 mm and are easily observed and the crystal faces positioned in the Raman microprobe. The spectra are reported according to the Porto notation [20]: the propagation directions of the incident and scattered light are described in terms of the crystallographic axes a, b and c. The notation may read, for example A(BC)D. Here A is the direction of the incident radiation and the D the direction of the collection of the scattered radiation. B is the direction of the polarisation of the electric vector of the incident radiation and C is the direction of the analyser [19–21]. In this paper we report only the spectra of the type C(AA)C. Thus the incident and collection optics are in the same direction and the direction of the polariser and analyser are also in the same direction. We are using 180° scattering geometry. This means, for example, that if the
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Fig. 2. Raman spectra of the hydroxyl-stretching region of Keokuk kaolinite along three axes perpendicular to the crystal surfaces at 77 K.
incident radiation is perpendicular to the 001 face of the kaolinite crystal and the electric vector is aligned with the direction of the polarisability tensor of the hydroxyl stretching vibration then the intensity of the Raman band will be maximised. If the electric vector of the incident radiation and the polarisability tensor are at right angles then the intensity will be at a minimum and approach zero.
3.2. Raman spectroscopy of the Keokuk kaolinite The Raman spectra for the Keokuk kaolinite at 77 K along three axes are shown in Fig. 2 and the results of the band component analyses is reported in Table 1. The three spectra obtained from Raman scattering at right angles to the kaolinite crystal surfaces are shown. All three spectra show considerable differences in the region attributed to the in-phase vibrations of the inner surface hydroxyls. The effect of obtaining the spectra at liquid nitrogen temperatures is to cause the frequency of the inner hydroxyl group to move to lower frequencies. The band is ob-
served in the room temperature spectra at 3620 cm − 1 and is at 3616 cm − 1 at 77 K. The inner hydroxyl shifts by 4 cm − 1 to lower frequencies. This data is in good agreement with previously published data [3,10]. Such frequency shifts have been reported in the infrared spectra of kaolinite where, at 5 K, the inner hydroxyl frequency was observed at 3612 cm − 1 [22]. The value of the infrared inner surface hydroxyl at 65 K was 3615 cm − 1 [22]. The bands attributed to the inner surface hydroxyls observed at 3695 (n1), 3684 (n4), 3670 (n2) and 3650 (n3) cm − 1 [9–11] at room temperature are now observed at 3708 and 3696, 3691, 3677 and 3658 cm − 1. Upon cooling to liquid nitrogen temperatures, the inner surface hydroxyl frequencies shift higher by ca. 8–10 cm − 1. Similar shifts have been reported for the infrared spectra of kaolinite at 5 K [22]. The effect of cooling to liquid nitrogen temperatures has meant that the attraction of the hydrogen of the inner hydroxyl has increased in strength. This is no doubt due to the closer proximity to the nearest oxygen. Upon cooling to liquid nitrogen temperatures the
Spectrum c, fit 3
Spectrum c, fit 2
Spectrum c, fit 1
Spectrum b, fit 3
Spectrum b, fit 2
Spectrum b, fit 1
Spectrum a, fit 3
C(AA)C
A(BB)A
A(CC)A
Spectrum a, fit 1
Spectrum a, fit 2
Spectroscopic experiment
Raman spectra
n1a Band 1 3704 22.3 15.0 3708 9.9 8.6 3710 4.5 6.2 3700 35.8 17.2 3707 9.8 10.5 3709 3.7 5.2 3698 32.2 16.1 3708 11.8 10.0 3709 6.2 7.8
Band Parameters Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1) Band centre (cm−1) Area (%) Bandwidth (cm−1)
3702 13.9 9.6
3703 21.0 10.7
3703 11.0 11.3
n1b Band 2
3696 24.4 11.7 3695 16.0 7.8
3696 30.0 11.6 3697 11.8 7.5
3696 22.4 12.8 3695 20.4 8.8
n1c Band 3
3691 36.0 6.3 3691 30.7 6.0 3691 30.8 5.8
3692 19.1 6.4 3691 14.9 5.5 3691 20.0 5.0
3692 17.6 7.7 3691 7.4 4.9 3691 6.4 4.3
n4
3677 5.4 5.5 3677 7.0 6.4 3677 7.2 6.3
3677 7.0 5.1 3677 7.0 5.1 3677 7.1 5.3
3677 8.3 4.6 3677 8.3 4.4 3677 7.3 4.4
n2
3658 11.2 5.9 3658 11.2 5.9 3658 11.0 5.9
3658 14.9 4.9 3658 15.0 4.9 3658 14.6 5.2
3658 6.9 5.9 3658 6.9 5.9 3658 6.8 5.9
n3
3616 15.2 3.8 3616 14.8 3.8 3616 14.8 3.8
3616 23.0 3.4 3616 23.0 3.4 3616 21.8 3.4
3616 45.0 3.3 3616 45.0 3.3 3616 43.5 3.3
n5
Table 1 Results of the band component analysis of the Raman spectra at 77 K of the hydroxyl-stretching region of the ordered and disordered of Keokuk kaolinite along three axes perpendicular to the three crystal faces
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kaolinite layers are brought closer together, therefore, the bonding between the hydroxyls of the gibbsite-like surface to the next adjacent siloxane layer is weaker and, therefore, the bands occur at higher wavenumbers.
3.3. A(CC)A spectrum Spectrum a in Fig. 2 corresponds to the A(CC)A experiment. Spectrum a is the spectrum when the laser beam is parallel to the 001 and along the a-axis. Both the polariser and analyser are aligned in the c-direction. This experiment determines the hydroxyl stretching frequencies, which are from the hydroxyls, which are at small angles to the c-axis. The inner hydroxyl group of kaolinite lies parallel to the ab plane and the resultant hydroxyl stretching frequency will be determined in this experiment. In this spectrum the relative intensity of the band assigned to the inner hydroxyl is maximised and the relative intensities of the bands attributed to the inner surface hydroxyls are small. Strictly in this experiment only transverse optical vibrations should be observed and, therefore, no intensity should be observed in the bands between 3695 and 3710 cm − 1. However because of the leakage of the collection optics being over a wide angle, intensity in both the TO and LO modes are observed. The results of the band component analysis of this spectrum are reported in Fig. 3a –c where either a five, six or seven band fit was used for the inner surface hydroxyl stretching region. The differences in the band fitting procedure depends on the number of bands which are fitted in the spectral profile between 3685 and 3720 cm − 1. Upon using a five band fit (Fig. 3a) a squared correlation coefficient of 0.9955 was obtained. However the fit is not good and an extra band is required to fit the spectral profile. Upon using a three band fit, i.e. using three bands in the 3685 to 3720 cm − 1 region, an r 2 value of 0.9969 was obtained. Upon using a four band fit, an r 2 value of 0.9987 was obtained. The question arises as to whether there are three or four bands in the 3685–3720 cm − 1 region. One possible explanation is that if the three band model is used, then
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the reason for the additional band is ascribed to the longitudinal optics of different inner surface hydroxyls. If a four band model is used, then three of the bands observed at 3710, 3703 and 3695 cm − 1 may be attributed to three LO different inner surface hydroxyls and the band at 3691 cm − 1 to the transverse optical vibration. It is interesting to compare the relative intensities of the bands according to how many bands are fitted to the spectral profile. In lines labelled spectrum a fit1, fit 2, fit 3 of Table 1, the relative intensities of the n2, n3 and n5 bands are constant and any variation probably represents experimental error. The experimental error in the n5 band is 9 1.5%. The band fitting procedure does have a marked effect upon the relative intensity and bandwidth of the 3691 cm − 1 band attributed to the transverse optical vibration. For a two band fit of the transverse/longitudinal optic mode region, the relative intensity is 17.6% and the bandwidth 7.7 cm − 1. For a three band fit, the values are 7.4% and 4.9 cm − 1 and for the four band fit, 6.4% and 4.3 cm − 1. The differences in the parameters between the three and four band fit are small. It should also be noted that the band fitting procedure did not effect the position of the 3691 cm − 1 band. The bands observed in the 298 K spectrum are found at 3689, 3684, 3668, 3650 and 3620 cm − 1 with relative intensities of 22.3, 37.2, 9.1, 13.7 and 17.2%. These bands have bandwidths of 17.4, 9.0, 8.9, 7.8 and 3.7 cm − 1, respectively. A remarkable difference in the relative intensity between the 77 and 298 K spectra of the band attributed to the transverse longitudinal optic is observed. The value decreases from 37.2 to 17.6% (two band fit), 7.4% (three band fit) or 6.4% (four band fit). The question may be asked as to the reason for this large decrease. If the incident radiation is along the c-axis then the projection of the polarisability tensors of the inner surface hydroxyls would be small resulting in low relative intensities. Whereas the polarisability tensor of the inner hydroxyl would be large, hence the relative intensity of the inner hydroxyl vibration is large. The band fitting process also determines the bandwidths of the inner surface hydroxyls. If a two band fit is used to fit the 3685 and 3720 cm − 1
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region, then the band widths of the 3704 and 3691 cm − 1 bands are 15.0 and 7.7 cm − 1. For a three band fit the bandwidths of the 3708, 3696 and
3691 cm − 1 bands are 8.6, 12.8 and 4.9 cm − 1. If a four band fit is used then the bands at 3710, 3703, 3695, and 3691 cm − 1 are 6.2, 11.3, 8.8 and 4.3
Fig. 3. Band component analysis of the A(CC)A Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the ab plane with a five band fit, and (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the ab plane with a six band fit.
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cm − 1. The effect of the band fitting process is to have a sharp band at 3691 cm − 1 with one other broad band at either 3696 (three band fit) or 3703 cm − 1 (four band fit).
171
3.4. A(BB)A spectrum When the laser beam is at right angles to the ac plane, the relative intensities of the inner surface
Fig. 4. Band component analysis of the A(BB)A Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a five band fit; (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a six band fit; and (c) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the bc plane with a seven band fit.
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Fig. 4. (Continued)
hydroxyls compared with that of the inner hydroxyl is increased significantly (Fig. 4). Spectrum b in Fig. 2 results from the A(BB)A experiment. The hydroxyl stretching frequency of the inner hydroxyl is observed at 3616 cm − 1, the identical position for all analyses. The bands attributed to the inner surface hydroxyls also are in identical positions but are a function of the number of bands used in the fitting process. Significantly the relative intensity of the 3691 cm − 1 band has increased. The bandwidth of the 3691 cm − 1 band in spectrum a for the four band fit is 4.3 cm − 1 and in spectrum b for the four band fit is 5.0 cm − 1. The relative intensity of the n5 band has decreased from 45% for spectrum a to 23% for spectrum b. Correspondingly there has been a substantial increase in the relative intensity of the 3703 cm − 1 band for the four band fit from 11 to 21%.
3.5. C(AA)C spectrum Spectrum c is the C(AA)C spectrum (Fig. 5). The predominant feature of spectrum c obtained
when the incident and scattered radiation is perpendicular to the ab plane is the intensity of the band at 3691 cm − 1 attributed to the transverse optic mode. The intensity of the band is 36% in the two band fit, 30.7% in the three band fit and 30.8% for the four band fit. The two bands at 3677 and 3658 cm − 1 have been described as the out-of-phase vibrations of the in-phase vibrations of the inner surface hydroxyls [8,13]. These two bands are observed in the room temperature spectrum at 3650 and 3668 cm − 1, i.e. there is a shift of 8 cm − 1 upon cooling to liquid nitrogen temperature. The bandwidths of these two bands are 8.9 and 7.8 cm − 1 respectively. In spectrum a the bandwidth of the 3677 cm − 1 band is 4.4 cm − 1, in spectrum b averaged at 5.3 cm − 1 and in spectrum c ca. 5.5 cm − 1. The bandwidths of the 3658 cm − 1 band in spectra a, b and c are 5.9, 5.3 and 5.9 cm − 1. The widths of these bands are very small at liquid nitrogen temperatures. By obtaining spectra of the inner surface hydroxyls at liquid nitrogen temperatures, the spectra of the longitudinal optic of the inner
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hydroxyls are being uncoupled, i.e. separate bands are being observed irrespective of whether a three or four band fit is used. Nevertheless the uncoupling of the out-of-phase vibrations is not occurring. The bands are simply becoming narrower.
173
The question may be asked that if the in-phase vibrations become uncoupled or partially uncoupled, why then do the out-of phase vibrations not become uncoupled. This explanation is only one possibility. One proposition explaining the bands
Fig. 5. Band component analysis of the C(AA)C Raman spectrum at 77 K of: (a) the hydroxyl-stretching region of kaolinite, spectrum taken along the ac plane with a five band fit; (b) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along the ac plane with a six band fit; and (c) the inner surface hydroxyl-stretching region of kaolinite, spectrum taken along ac plane with a seven band fit.
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Fig. 5. (Continued)
found at 3710, 3703 and 3695 cm − 1 is the attribution to different inner surface hydroxyls arising from different parts of the kaolinite structure. Such a possibility although feasible depends on whether Raman spectroscopy measures molecular structure of surfaces. This does not seem likely. By obtaining spectra at liquid nitrogen temperatures, there may be separation of the coupled and uncoupled longitudinal optic vibrations. In this case, a three band fit of the in-phase hydroxyl stretching vibrations is favoured. The difficulty of using crystals which are at the limits of the spatial resolution of the spectrometer is the uncertainty in the positioning of the crystal in the beam. If the crystals are greater than the spatial resolution then there are no problems. The spectroscopic axes do not correspond to the kaolinite crystallographic axes and so, strictly, the spectra should be adjusted to the crystallographic axes. At this stage, such experimentation is not possible. A further difficulty is the actual selection of crystals. X-ray diffraction of single crystals has proved the existence of twinning [23]. Crystals of
kaolinite proved to be pseudo-twin intergrowths of three lattice orientations. The size of these intergrowths is submicroscopic. Twinning simply means that the dioctahedral vacancy is in different positions between two crystals. This will not effect the position of the kaolinite hydroxyls. Consequently, spectra at different orientations along three crystal axes can be obtained.
4. Conclusion The Raman spectra of the hydroxyl stretching region of Keokuk kaolinite were obtained at 77 K using a Raman microprobe and a thermal stage. The band attributed to the inner hydroxyl was observed at 3616 cm − 1 with a bandwidth of 3.3 cm − 1. The band profile of the 3685–3720 cm − 1 was curve fitted with two, three and four bands. The three and four band fits proved most satisfactory. The 77 K band at 3691 cm − 1 attributed to the longitudinal optical vibration proved to be
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strongly orientation dependent as did the other hydroxyl stretching bands.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Normandy Industrial Minerals through Mr L. Barnes, Chief Geologist, is thanked for financial support.
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