Applied Clay Science 13 Ž1998. 233–243
Low-frequency Raman spectra of kaoliniteralkali halide complexes K.H. Michaelian
a,)
, S.L. Zhang
a,1
, S. Yariv b, I. Lapides
b
a
Natural Resources Canada, CANMET Western Research Centre, 1 Oil Patch DriÕe, Suite A202, DeÕon, Alberta, Canada T9G 1A8 b Department of Inorganic and Analytical Chemistry, The Hebrew UniÕersity of Jerusalem, 91904 Jerusalem, Israel Received 17 March 1998; accepted 27 May 1998
Abstract The 50–1000 cmy1 region of the Raman spectra of five kaoliniteralkali haliderwater intercalation complexes is reported, and compared with both the Raman spectrum of a reference kaolinite and infrared spectra of the complexes. The alkali halides were KCl, KBr, RbCl, CsCl and CsBr. Most of the 26 Raman bands observed for the complexes can be correlated with Raman and infrared frequencies in uncomplexed kaolinite, permitting assignments based on previously published data. Some of these bands shifted or intensified due to the interaction of kaolinite with water and alkali halide. New Raman bands detected in the spectra of the chloride complexes probably arise from the keying of this ion through the ditrigonal hole into the TO layer. q 1998 Elsevier Science B.V. All rights reserved. Keywords: kaolinite; Raman spectroscopy; alkali halides; kaoliniteralkali halide intercalation complexes
1. Introduction The scarcity of Raman spectra of clay minerals Ž Griffith, 1974. has diminished recently as a consequence of both improved instrumentation and continued efforts by several research groups. Kaolinite, a common layer silicate of )
Corresponding author. Fax: q1-403-987-8676; E-mail:
[email protected] Current address: Department of Chemistry, University of Michigan, Ann Arbor, MI 481091055, USA. 1
0169-1317r98r$19.00 q Minister of Natural Resources, Canada, 1998. Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 2 6 - X
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empirical composition Al 2 Si 2 O5Ž OH. 4 , has been the subject of a number of investigations ŽWiewiora et al., 1979; Johnston et al., 1984, 1985; Michaelian, 1986; Frost et al., 1993, 1996, 1997; Pajcini and Dhamelincourt, 1994; Frost, 1995, 1997., as have several other clays Ž Frost et al., 1993, 1996; Coleyshaw et al., 1994; Frost, 1995, 1997. . Raman spectra of clays and their complexes Ž Frost et al., 1997. are complementary to generally more common infrared data, and afford the significant advantage of easy access to the 50–400 cmy1 region where many lattice vibrations occur. Recently, intercalation complexes of kaolinite with a series of alkali halides were obtained by a thermal solid state reaction between the kaolinite–dimethylsulphoxide intercalation complex and the appropriate alkali halide Ž Lapides et al., 1997.. The ground mixtures were pressed into disks that were gradually heated up to 2508C. X-ray diffractograms showed that alkali halide ions diffuse into the interlayers as a result of the thermal treatment, replacing the intercalated dimethylsulphoxide molecules. The extent of intercalation was observed to depend on the alkali halide and the maximum temperature. According to our infrared study, the complexes discussed in this paper were first obtained almost anhydrous. Before the Raman investigation, they were aged in ambient conditions for an extended period; according to their infrared spectra, the complexes can be assumed to be mostly hydrated. Infrared spectroscopy of such complexes confirms that intercalated water molecules are coordinated to the alkali and halide ions and that the inner surface hydroxyls of kaolinite form hydrogen bonds with intercalated water molecules and halides. Perturbation of the Si–O stretching bands of kaolinite also indicates that the inner surface oxygens form hydrogen bonds with intercalated water. Diffuse reflectance and photoacoustic far-infrared spectra of several kaoliniteralkali haliderwater complexes have already been reported Ž Michaelian et al., 1997.. Mid-infrared spectra of kaoliniterCsClrH 2 O ŽMichaelian et al., 1991a,b; Yariv et al., 1994., kaoliniterCsBrrH 2 O ŽMichaelian et al., 1991a,b; Yariv et al., 1991. , and kaolinite complexes with other salts Ž Michaelian et al., in press; Yariv et al., to be published. were also obtained during the course of this research; on the other hand, Raman data are still needed to complete the spectroscopic characterization of these complexes. In this paper, we present low-frequency Raman spectra of complexes between kaolinite and five different alkali halides ŽKCl, KBr, RbCl, CsCl and CsBr. , and compare the results to the far-infrared spectra of these complexes and the Raman spectrum of a reference kaolinite.
2. Experimental Well-crystallized Georgia kaolinite Ž KGa-1. was used in this work. Preparation of the kaoliniteralkali haliderwater complexes is described elsewhere
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ŽLapides et al., 1997.. Samples were studied as pellets, each containing a single complex Ž33 wt.%. dispersed in an excess of the appropriate alkali halide. This uncomplexed salt acts as a diluent that results in diminished Raman intensities, a situation different from that in infrared spectroscopy where the identity of the salt determines the low-frequency cut-off Ž Michaelian et al., 1997. . Notwithstanding their negative effect on the Raman measurements, no attempt was made to remove the excess alkali halides by washing with water, because this would have led to modification of the complexes. Raman spectra were recorded at a resolution of 4 cmy1 using a Bruker IFS 88 FT-IR Ž Fourier transform-infrared. spectrometer and FRA 106 FT-Raman module. Excitation was provided by a 1.06-mm Nd:YAG laser. Incident power was approximately 290 mW, and no evidence of sample heating or degradation was found during the experiments. Forty 100-scan spectra Ž total acquisition time about 2 h. were averaged for each complex except kaoliniterKBr, where 90 spectra were acquired to improve a somewhat lower signal-to-noise ratio.
3. Results Raman spectra of the five kaoliniteralkali haliderwater complexes are shown in Fig. 1. The top part of the figure displays the region between 50 and 950 cmy1 ; spectra have been normalized to constant intensity of the 636 cmy1 band, using the result for the CsBr complex as a reference. The top curve is the spectrum of KGa-1 kaolinite, while spectra of the complexes are labelled according to the alkali halide. The lower part of the figure shows the spectra above ; 200 cmy1 in the same sequence, expanded by a factor of approximately four and offset vertically for clarity. The hydroxyl stretching region will not be discussed because of reduced sensitivity of the spectrometer at greater frequency shifts Žlonger wavelengths. . It is readily apparent that the region below 190 cmy1 contains the strongest bands in spectra of both kaolinite complexes and the original clay, with a significant number of weaker peaks occurring at frequencies between 190 and 950 cmy1. Indeed, more than half of the bands identified in this study are not detectable by mid-infrared spectroscopy, consistent with the assertion that both Raman and far-infrared data are required for a complete study of the vibrational spectra of clay complexes. Attempts to record FT-Raman spectra of kaolinite complexes with CsI and KI were unsuccessful, although satisfactory infrared spectra of these samples were acquired ŽMichaelian et al., 1997, in press; Yariv et al., to be published. . This failure to obtain adequate Raman spectra may be attributable to sample heating or fluorescence caused by absorption of laser radiation. A colouration of the CsI and KI pellets, probably due to formation of small quantities of molecular iodine, suggests that these samples decomposed as they aged.
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Fig. 1. ŽUpper box. FT-Raman spectra of KGa-1 kaolinite Žtop curve. and kaoliniteralkali haliderwater complexes; labels indicate alkali halides. Spectra have been rescaled and offset by arbitrary amounts to facilitate comparison. ŽLower box. Same data as in upper box, expanded by a factor of about four and further offset for clarity.
Table 1 summarizes the frequencies and relative intensities of the Raman bands in Fig. 1. Assignments of these bands are discussed in the following subsections. 3.1. The 50–190 cm y 1 region The low-frequency region is dominated by a band at 142 cmy1, which is about an order of magnitude stronger than most of the peaks above 190 cmy1.
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This anomalously strong feature is observed in Raman spectra of some kaolinites ŽJohnston et al., 1984, 1985; Michaelian, 1986; Frost, 1995, 1997; Frost et al., 1997. , but is weak or absent in other samples such as Keokuk kaolinite. This variability suggests that the 142 cmy1 band does not arise from kaolinite: recent investigations ŽShoval et al., 1995; Murad, 1997. strongly indicate that this band is due to anatase, frequently observed to coexist with kaolinite. The constancy of the 142 cmy1 band in the current Raman spectra of alkali halide complexes is also consistent with its attribution to anatase rather than kaolinite. A low-frequency shoulder occurs between 119 and 130 cmy1 in most of these spectra; its dependence on intercalation implies that it arises from O–Si–O bending, which involves the oxygen plane at the surface of the TO layer. An alternative assignment to O–Al–O bending Ž Ishii et al., 1967. can be rejected because the latter groups are located inside the layer and should not be affected by intercalation. Table 1 shows that many of the low-frequency Raman bands observed for the complexes can be correlated with weak bands in uncomplexed kaolinite. For example, prominent bands occurring between 77 and 82 cmy1 in spectra of the complexes are significantly intensified with respect to the weak 83 cmy1 band in the kaolinite spectrum; the bands are stronger for the bromide salts than for the chlorides. This probably arises from ‘keying’ of the chloride ions into the tetrahedral sheet of the TO layer. The 77–83 cmy1 bands can thus be assigned to translation of Si or O atoms in the clay lattice. Weaker peaks at 57, 70 and 105 cmy1 in the complexes have analogues at 60, 68 and 106 cmy1, respectively, in the Raman spectrum of kaolinite. Ž The latter three bands were observed in dispersive Raman spectra obtained by coupling a third monochromator to a standard double monochromator, effecting a significant reduction in Rayleigh intensity that tends to mask weak lowfrequency bands.. To the best of our knowledge, these very low-frequency Raman bands of kaolinite have not been reported previously, although it can be noted that a band has been identified at 107 cmy1 in the far-infrared spectrum of kaolinite ŽZwinkels and Michaelian, 1985. . While no definitive assignments exist for these bands, it is reasonable to assume that the vibrations below 100 cmy1 involve translational motion of the heavy atoms Ž Al, Si, O. in the clay lattice ŽFripiat, 1982.. The intensification of these bands could be due to delamination of the clay, which occurs as a consequence of the intercalation reaction. The final Raman band in this region occurs at 182 cmy1 in the spectrum of kaoliniterCsClrH 2 O, and does not seem to originate in kaolinite. There is also a possible shoulder between 165 and 180 cmy1 in the spectrum of the CsBr complex. These frequencies are similar to what might be expected for hydrogen bond stretching; we therefore suggest that they arise from an O–H PPP O PPP Cs vibration in the hydrated alkali halides according to the structural model proposed previously ŽMichaelian et al., 1991a. . Observation of this band in the
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Kaolinite
Kaoliniteralkali haliderwater complexes CsBr
b
60 68b 83 106b 130c 142 – 195 – 243 273 – – 335
vw sh w vw sh vs – w – w w – – m
– – 82 – 126 142 – 196 – 246 266 – – 337
CsCl – – vs – sh vs – sh – w w – – m
57 – 77 – 120 142 182 200 – 246 268 – 320 337
RbCl w – m – s vs s sh – w vw – vw m
– 70 82 – 119 142 – 199 238 251 270 292 321 337
KBr – w m – vw vs – w w sh vw vw vw m
– 70 82 105 120 142 – 202 – 246 269 – – 333
KCl – w vs vw vw vs – w – vw vw – – m
– 70 82 – – 142 – 199 – 246 271 292 319 336
Assignment – w m – – vs – w – w w vw vw m
translational translational translational d
anatase e
d e
d e
K.H. Michaelian et al.r Applied Clay Science 13 (1998) 233–243
Table 1 Raman bands Žcmy1 . observed for kaolinite and kaoliniteralkali haliderwater complexesa
a
– m sh m m m s – vw w w w
– 395 417 437 468 509 636 680 709 753 783 902
– m vw w m m m w w w w w
354 395 422 437 468 511 636 688 720 753 785 901
vw m vw w m m m m w w vw w
354 395 – 434 469 514 636 687 724 753 788 902
vw m – m m m m w w vw vw w
vs: Very strong; s: strong; m: medium; w: weak; vw: very weak; sh: shoulder. From dispersive Raman spectrum observed with triple monochromator system. c Farmer Ž1979.. d Changes with alkali halide. e Insensitive to alkali halide; possibly due to uncomplexed kaolinite. b
– 395 – 434 473 514 636 682 708 753 788 906
– m – m m m m w w vw vw w
– 395 – 433 472 514 636 682 707 753 792 910
– m – w m m m vw vw vw w w
d
anatase O N
Al–OrSi–O Al–OrSi–O d
anatase L K J I AlO–H
d
d
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– 395 410 429 472 511 636 – 706 751 788 912
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caesium halide complexes might correspond to an extent of intercalation that is significantly greater than in the other complexes. The band could also be masked by the strong 142 cmy1 peak in the cases of the rubidium and potassium salts. 3.2. The 190–400 cm y 1 region Several Raman bands between ; 190 and 400 cmy1 in Fig. 1 agree with previously reported Ž Michaelian et al., 1997. infrared frequencies. In particular, the 195, 273 and 335 cmy1 bands of kaolinite were also detected in far-infrared spectra of the complexes. These bands may arise from uncomplexed clay ŽTable 1.. The 246–251 cmy1 Raman band in the complexes, although not detected in infrared spectra Ž Michaelian et al., 1997. , corresponds to a 248 cmy1 infrared band in kaolinite Ž Zwinkels and Michaelian, 1985. . Assignments of the 195, 243 and 273 cmy1 Raman bands of kaolinite to the three vibrations of an O–H–O grouping that includes neighbouring nonbridged oxygens have been proposed ŽFrost et al., 1993; Frost, 1997. based on earlier calculations Ž Loh, 1973. . However, this entity should be practically nonexistent in the complexes because of delamination of the kaolinite. The slight changes in the 273 cmy1 band with complexation might be attributable to hydrogen bonding of inner surface hydroxyls with intercalated anions or water. Other weak Raman bands in the complexes could not be detected in the far-infrared ŽMichaelian et al., 1997. or Raman spectra of the uncomplexed clay. Their occurrence in more than one chloride complex does, however, tend to confirm their authenticity; in this way, bands near 292, 320 and 354 cmy1 are identified. The latter two values are reasonably close to infrared frequencies of 324 cmy1 and 347 cmy1 ŽFarmer, 1979; Fripiat, 1982; Zwinkels and Michaelian, 1985. of kaolinite. 3.3. The 400–950 cm y 1 region Identification and assignment of bands in this region of the Raman spectra in Fig. 1 is facilitated by the existence of extensive Raman and mid-infrared data for kaolinite. Almost all of the kaolinite Raman bands in this region give rise to similar Raman Ž Table 1. and infrared ŽMichaelian et al., 1997, in press; Yariv et al., to be published. bands in the complexes. On the other hand, the complementarity of infrared and Raman spectra of kaolinite is typified by the strong Al–O infrared band at 550 cmy1 that does not appear in Raman spectra of kaolinite or its complexes. The prominent 636 cmy1 Raman band observed in the current investigation arises from anatase Ž Shoval et al., 1995; Murad, 1997. ; like its counterpart at 142 cmy1, it is unaffected by intercalation of kaolinite. Some of the Raman frequencies in this region agree reasonably well with known infrared bands, for which a labelling scheme exists. Thus, proceeding
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from high to low frequency, Raman bands of kaolinite at 912, 788, 751, 472 and 429 cmy1 are denoted I, J, K, N and O, respectively Ž Miller and Oulton, 1970. ; these frequencies tend to be a few cmy1 lower than their infrared counterparts. Table 1 shows that band I is perturbed in the Raman spectra of the kaoliniteralkali halide complexes. Infrared spectroscopy yields a similar result Ž Michaelian et al., in press; Yariv et al., to be published. . This perturbation is due to hydrogen bonding of the Al–OH group with the intercalated anion andror water. Hydrogen bonding of the alkali halide with the hydroxyl and oxygen planes also affects bands N and O in spectra of the complexes. As the discrepancy between Raman and infrared frequencies of kaolinite grows, their correlation becomes less certain. For example, in the region near 700 cmy1, two series of bands appear in the Raman spectra of the complexes, whereas infrared and Raman spectra of uncomplexed kaolinite each exhibit a single band. The most logical interpretation of these results is that infrared band L at 694 cmy1 is activated in the Raman spectra of the kaoliniteralkali halide complexes, where it occurs at 680–687 cmy1. Similarly, the 706 cmy1 Raman band of kaolinite gives rise to bands at 709–721 cmy1 in the complexes. The situation for band O Ž434 cmy1 in the infrared spectrum of kaolinite. is slightly more complicated. In addition to this well-known feature, photoacoustic infrared spectra of kaolinite Ž Michaelian et al., 1997. also contain weak bands at 411 and 421 cmy1, the former being Raman-active. The Raman spectra of the complexes display two series of bands in this region: those between 434 and 437 cmy1 agree closely with band O, while bands at 417 and 422 cmy1 in the kaoliniterCsBr and kaoliniterCsCl complexes could correspond to either of the two lower frequency bands.
4. Conclusions Raman spectra of kaoliniteralkali haliderwater complexes have been measured and compared with the Raman spectrum of a standard kaolinite, and with far- and mid-infrared spectra of uncomplexed and complexed kaolinite. Raman spectroscopy provides important data that are not available from infrared measurements, justifying its increasing use for the characterization of clays and their complexes. Disagreement between some Raman and infrared frequencies complicates interpretation of the spectra, and further work must be carried out before this phenomenon is understood. In agreement with our previous infrared study of intercalation complexes, some of the Raman bands of kaolinite shifted or intensified due to the intercalation of water and alkali halide. In our investigation of the OH stretching region of the infrared spectrum ŽYariv et al., to be published., we concluded that the chloride penetrated through the ditrigonal holes into the TO layers, forming H bonds with the framework
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inner OH groups. Bromide or iodide are too large and do not penetrate through the ditrigonal holes. In the present work, several new Raman bands were detected in the spectra of the chloride complexes. It is possible that the ‘keying’ of chloride gives rise to these new bands.
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