- Email: [email protected]
Cu(II)-intercalated muscovite: An infrared spectroscopic study
Vibrational Spectroscopy 43 (2007) 427–434 www.elsevier.com/locate/vibspec
Cu(II)-intercalated muscovite: An infrared spectroscopic study Frank Friedrich *, Stefan Heissler, Werner Faubel, Rolf Nu¨esch, Peter G. Weidler Institute for Technical Chemistry, Department of Water and Geotechnology (ITC-WGT), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany Received 15 November 2005; received in revised form 6 March 2006; accepted 11 May 2006 Available online 30 June 2006
Abstract Diffuse reflectance infrared spectroscopy (DRIFT) has proved to be a useful tool for the investigation of migration paths of intercalated metal cations within the layered structure of phyllosilicates like muscovite. While the fixation of small cations like lithium or copper in clay minerals and their migration paths have been studied extensively, they never have been reported for muscovite. Therefore, the aim of this study was to investigate the effects of a treatment with a supersaturated Cu-nitrate solution on the structure of muscovite. Additional X-ray diffractometric (XRD) data showed the formation of new d(0 0 l) and d(0 0 2) peaks, and thus proved that the new cations already were intercalated into the muscovite interlayers, although the concentration of the original interlayer cation potassium was nearly not affected. This suggested the simultaneous occurrence of both potassium and copper in the interlayers, resulting in an expansion along the c-axis and in a decrease of the a and b parameters. The spectroscopic investigations proved a migration of the cations deep into the tetrahedral sheets. As all bands which can be assigned to vibrations of the octahedral sheet (e.g. at 713 or 903 cm 1) were strongly affected by the treatment, a fixation of Cu close to the OH-groups of the octahedral sheets was suggested. # 2006 Elsevier B.V. All rights reserved. Keywords: Cation migration; Copper; DRIFT; IR-spectroscopy; Muscovite
1. Introduction A layered structure is the characteristic feature of phyllosilicates like the mica mineral muscovite KAl2[Si3AlO10(OH)2]. The minerals of the mica group consist of layers of one octahedral Al(O/OH)6-sheet sandwiched by two tetrahedral SiO4-sheets. Due to substitutions in the central positions of the octahedra and tetrahedra, these layers have negative charges around one, which are compensated mostly by large monovalent cations like potassium or sodium (Fig. 1). Because of the resulting properties (e.g. anisotropic optical, electrical properties), muscovite is an important raw material in paint industries and in insulators or polymer composites. Important is on one hand a high aspect ratio, which will be enhanced by delamination of the stacks. On the other hand an exchange of the interlayer cations would change the chemical properties of muscovite, its refraction behaviour and the surface properties. But in contrast to clay minerals where such exchange
* Corresponding author. Tel.: +49 7247 82 6807; fax: +49 7247 82 3478. E-mail address: [email protected] (F. Friedrich). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.023
reactions are common procedures [1–4], the interlayer cations of micas, especially of muscovite are difficult to access and are not exchangeable under normal conditions [5–7]. A chemical modification of this mineral is difficult and needs increased pressure and temperatures [8,9]. Thus Friedrich [10] suggested an autoclave-based method for the incorporation of polyvalent cations into muscovite. With this method it was possible to intercalate Cu2+, Mg2+, Zn2+ or Al3+ for the first time. The main questions of the exchange and intercalation reactions in phyllosilicates are the migration paths and the positions of the introduced cations within the mineral structures. Therefore, the surfaces and interlayers of clay minerals have been investigated intensively. Two classic studies showed, that a diffusion of small ions like lithium deep into the ditrigonal holes of the tetrahedral layers of smectites is possible [11,12]. They even proposed a movement into the unoccupied octahedral cavities. In recent years a great deal of work has been done on the investigation of these mechanisms using Xray diffractometry and various spectroscopic techniques (e.g. [4,13,14]). For example, on the basis of magic angle spinning nuclear magnetic resonance (MAS NMR) measurements Trillo et al. [15]
428
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
filtered by vacuum filtration using cellulose-nitrate filters (pore size: 0.45 mm, obtained from Sartorius, Goettingen, Germany). Excessive salt precipitating during cooling was dissolved by adding additional water. Finally the samples were washed with 250 ml deionized water and dried at 60 8C for 24 h. This procedure supplied a fine-grained, silver shiny powder. 2.2. Sampling techniques
Fig. 1. Structure of the mica mineral muscovite.
proposed the migration of Li+ into the tetrahedral sheets of montmorillonite, but they could not confirm a further movement into the octahedral cavities. In contrast Jaynes and Bigham [16] postulated the movement of lithium into the octahedral sheet, and Mosser et al. [13] interpreted Cu(II) electron paramagnetic resonance (EPR) signals and extended X-ray absorption fine structure (EXAFS) data in a similar way, suggesting a substitution of Cu for Al/Mg in the octahedral sheets of smectites. While these techniques reveal informations on the crystallites (XRD) or on the atomic structure around the investigated atoms (NMR, EPR, EXAFS), IR-spectroscopy offers the possibility to investigate the effect of the incorporated cations on the latticeand OH-vibrations of the phyllosilicates and thus the migration paths of the cations can be observed excellently [1,8,17,18]. In contrast to the extensively investigated clay minerals, to date no discussion of vibrational spectra of di- or trivalent cations intercalated into muscovite are available in literature. Thus, the possibility of introducing polyvalent cations like Cu2+ into muscovite allows an extensive infrared spectroscopic investigation of the migration mechanisms of this high-charged cation and its interaction within the muscovite interlayers for the first time. 2. Experimental 2.1. Intercalation procedure For the experiments the <400 mm size fraction of an Indian muscovite (obtained from Merck, Darmstadt, Germany) was used. Also, the pro analysis grade copper nitrate (Cu(NO3)23H2O) was obtained from Merck, Darmstadt, Germany. According to the method of Friedrich [10] stainless steel autoclaves equipped with a Teflon compartment were filled with a mixture of 250 mg of mica and a super saturated nitrate solution (65 g copper nitrate + 20 ml H2O). The closed autoclaves were heated at 155 8C for a time series (24, 96, 144 and 240 h). Afterwards the suspensions were washed and
2.2.1. Infrared spectroscopy A Bruker IFS 66/s Fourier transform IR spectrometer, equipped with a DTGS detector was employed to obtain the IRspectra. Sixty-four scans in the 4000–400 cm 1 spectral range were recorded with a scanner velocity of 1.6 kHz and a resolution of 4 cm 1. For DRIFT measurements a diffuse reflectance accessory from Spectra-Tech Inc. was used with a polished aluminium-mirror for the background measurements. Sample material (10 mg) was slightly ground with 300 mg KBr then the powders were poured loosely into a sample cup and were supposed to be randomly orientated. 2.2.2. Chemical analyses The amounts of Cu2+ and K+ were analysed by X-ray fluorescence (XRF). The measurements were done at the laboratory for geochemistry at the RWTH Aachen using an energy-dispersive SPECTRO X-Lab 2000 spectrometer, equipped with a nitrogen-cooled Si(Li) detector. The finegrained sample powder (200 mg) was filled in plastic cylinders covered with a Mylar foil. To stabilize the powder, it was suspended in 1 ml Elvacit solution. To avoid the formation of bubbles during the evaporation of the acetone, the suspensions first were dried 1 h at room temperature and then 2 h at 40 8C. 2.2.3. X-ray diffractometry The XRD patterns were recorded over the range of 3–638 2Q, using a Siemens D5000 diffractometer equipped with a graphite secondary mono-chromator. Counting time was 2 s in ˚ ) was used for 0.0158 steps and Cu Ka radiation (l = 1.5418 A measurement. Because of the general difficulties in getting useful Rietveld refinements of clay mineral cell parameters from their XRD patterns [19], the parameters were calculated with the cell parameters refinement program CelRef [20], using single linefitted XRD-data. The peak fitting was done with the Jandel Peakfit software package. A Voigt function was chosen, with the minimum number of peaks used for the fitting process. A linear two-point background was chosen and fitting was undertaken until reproducible results were obtained with squared correlations of r2 > 0.997. CelRef V3 is a least square refinement method [20]. The refinements were undertaken by using at least eight fitted peaks until reproducible results were obtained with mean square deviations <0.01. As starting values for the cell parameter ˚ , b = 9.02 A ˚, calculations those of muscovite (a = 5.19 A ˚ , and b = 95.58) were chosen. The refinements c = 20.13 A were done in the two space groups C2/c and C2.
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
429
Fig. 2. X-ray patterns of untreated muscovite (MU-0) and of a Cu-muscovite sample after 240 h treatment. (a) The diffractograms over the entire range 38–608 2Q show strong changes due to the treatment, especially in the low-angle region. (b) Detailed view on the low-angle region with the new 0 0 l-peaks at about 2.2 and 1.2 nm.
Table 1 X-ray powder data of the untreated muscovite (MU-0) compared to literature data of a 2M1-muscovite d-Spacings
hkl
[21]
This study
1.014 0.500 0.448 0.446 0.439 0.429 0.411 0.397 0.389 0.374 0.350 0.335 0.321 0.299 0.287 0.280 0.259 0.258 0.256 0.251 0.246 0.245 0.240 0.238 0.225 0.224 0.220 0.218 0.215 0.213 0.205 0.201 0.198 – 0.174 0.170
1.035 0.502 0.448 0.446 – 0.432 0.416 0.399 0.391 0.378 0.352 0.326 0.322 – 0.288 0.278 0.262 0.259 – 0.251 – 0.243 0.241 0.233 0.226 0.225 – 0.215 – – 0.209 0.201 0.190 0.184 0.175 0.170
Spacings in nanometers.
002 004 110 1,1, 1 021 111 022 112 1,1, 3 023 1,1, 4 006 114 025 115 1,1, 6 1,3, 1 116 2,0, 2 008 1,3, 3 202 2,0, 4 133 040 041 221 2,2, 3 222 135 044 0 0 10 1,3, 7 1,3, 9 150
2.2.4. Auger electron spectroscopy (AES) The Auger electron spectra were recorded on a physical electronics Auger Nanoprobe Phi 680 with an electron beam energy of 10 keV. The mica sheets were removed by argon ion sputtering up to a depth of 60 nm, which corresponds to a thickness of about 50 layers. 3. Results 3.1. X-ray diffractometry The X-ray patterns of untreated (MU-0) and of Cu-nitratetreated muscovite are shown in Fig. 2a. According to this, MU0 is assigned as a muscovite-2M1 (Table 1), which has a monoclinic symmetry and crystallizes in the space group No. 15, C2/c [21,22]. Significant for this space group is the lack of the d0 0 1-peak. Thus, the first peak, which is visible in the range between 38 and 108 2Q is the d0 0 2 at 1.002 nm (8.88 2Q). The treatment with Cu-nitrate results in strong changes in the XRD patterns. Especially in the range between 38 and 108 2Q the Cu-muscovite pattern shows the development of two new broad peaks at 1.11 (7.98 2Q) and 2.21 nm (3.98 2Q), while the d0 0 2 of the original muscovite strongly decreases (Fig. 2b). Furthermore, most of the original muscovite peaks decrease their intensities and get broader during the treatment, while their 0 0 l-peaks slightly shift positions and decrease their intensities only to a minor degree. 3.2. Auger electron spectroscopy Fig. 3 displays two AES spectra measured on the surface of a 240 h treated muscovite and on the same position after the removal of about 50 muscovite layers. The measured ion concentrations (surface: K 3.7 at.%, Cu: 0.7 at.%; depth 60 nm: K 4.3 at.%, Cu 3.4 at.%) prove the simultaneous occurrence of potassium and copper on the surface as well as within the structure of the treated muscovite.
430
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434 Table 2 Observed changes of the IR-band positions, due to the Cu-intercalation, and their suggested assignments Band no.
Fig. 3. Auger electron spectra of a Cu-muscovite after 240 h treatment. Spectrum (a) is measured on the surface of the muscovite particle. Spectrum (b) on the same particle after the removal of about 50 layers (60 nm).
MU-0
Cu-treated
Suggested assignments
1
–
528
2
549
549
Tetrahedral Si–O–Al Cu affected Si–O stretching Al–OH-bending
3
651
667
Al–OH-bending, affected by repulsive forces, due to the fixation of Cu Al–OH
4 5
713 752
721 752
Al–O Cu? Al–O–Si Octahedral Al–O–Al bending
6
783
806
Al–O–Al of distorted octahedra AlAlOH
7
903
932
8 9
983 ?
(983) 1003
Al–Al–OH affected by Cu fixed within the ditrigonal holes of tetrahedral layer Si–O stretch Si–O stretch (modified phase) Si–O stretch
10
1033
1054
11
1062
(1062)
Si–O stretch (modified phase) Si–O stretch
3.3. DRIFT spectroscopy In general the spectral features in the MIR-region between 400 and 1200 cm 1 are attributed to lattice vibrations of the tetrahedral and octahedral units of phyllosilicates including OH-bending vibrations [17,18,23]. Fig. 4 and Table 2 show this
range, in which the spectra of the time-dependent treatment with Cu-nitrate are compared with that of the original muscovite. Due to the Cu-nitrate treatment distinct changes occur with increasing treatment time. A new strong vibration appears at 528 cm 1 (band 1), while the intensity of the muscovite band at 549 cm 1 (band 2) decreases. Band 3 at 651 cm 1 remarkably shifts to 667 cm 1, while it strongly decreases its intensity. Also the bands 4 and 6 at 713 and 783 cm 1 show a shift to higher wavenumbers. Additionally, the shoulder at 752 cm 1 (band 5) strongly increases its intensity. Band 7 at 902 cm 1 disappears already after 24 h of treatment and a shoulder at about 932 cm 1 occurs with slightly increasing intensity. As mentioned above, in the Si–O stretching region between 950 and 1100 cm 1 three strong bands occur at 983, 1033 and 1062 cm 1 (bands 8, 10, 11) in the spectra of the untreated muscovite. The treatment with Cu-nitrate results in the formation of band 9 at 1003 cm 1, while band 8 disappears in the shoulder of band 9 with increasing treatment time. Similarly the sharp vibration at 1033 cm 1 (band 10) strongly shifts to 1054 cm 1, while the muscovite band 11 at 1062 cm 1 disappears in the shoulder of band 10. 4. Discussion
Fig. 4. DRIFT spectra of the lattice region of original muscovite compared to spectra of samples after various times of Cu-treatment.
The successful treatment of muscovite with Cu-nitrate is indicated by the occurrence of new d(0 0 1) peaks in the lowangle-region of the XRD patterns (Fig. 2). The d-values of these
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
peaks show the increase of the interlayer spacings. Thus, the metal ions are not only adsorbed on the muscovite surfaces, but they are in fact introduced into the interlayers of the mineral. The occurrence of Cu-peaks in the Auger electron spectra after removal of 50 muscovite layers (60 nm) strongly confirms this assumption (Fig. 3). Surprisingly, the chemical and the AES data show, that the intercalation of the metal cations is no simple ion exchange mechanism of the interlayer potassium versus the Cu2+ cation. While the Cu concentration increases from 0 in the starting material to 2.4 wt.% CuO after a treatment time of 240 h, no significant decrease of the potassium concentration is detectable during the treatment (9.2 wt.% K2O in the untreated material, and 8.5 wt.% K2O after 240 h). Consequently, the potassium as well as the metal ions appear together in the interlayer region after the treatment, which is responsible for the increasing interlayer spaces. The necessity of a strong super saturation of the nitrate solution to reveal the intercalated phase suggests, that the main intercalation component is the Cu-cation. It is proposed, that the super saturation is necessary to create a high ionic potential to introduce the cations into the interlayers, although the potassium is located there. The fact, that the original muscovite d(0 0 2) peak at 1.002 nm does not disappear even after a Cu-nitrate treatment of 240 h, indicates that not all layers are intercalated with copper, thus the intercalation is not complete. Therefore, we assume that the treatment reveals a strongly modified muscovite structure with already Cu-intercalated, expanded layers and relatively unaffected ‘‘original’’ muscovite layers. These structural modifications are the result of changes in the crystal symmetry and in the extinction rules. According to that, a calculation of the cell parameters is only possible in a space group of lower symmetry. The calculations assuming the space group No. 5, C2 suggest, that the intercalation not only leads to an expansion along the c-axis, but also to a slight decrease of the a- and b-lattice-parameters (Table 3). While the expansion along the c-axis easily can be explained by the intercalation procedure and the occurrence of both copper and potassium in the interlayers, the changes along the other axes possibly occur due to distortions and rotations of the tetrahedral and octahedral units around the ditrigonal holes of the tetrahedral sheets during a migration of the copper ions from the interlayers into the muscovite lattice structure. Responsible for this migration are the strong super saturation of the Cu-solution and the high reaction temperature. A relatively similar behaviour of Li+ and Cu2+ cations in smectites is known as Hofmann–Klemen effect. In contrast, there much lower concentrations (0.1–1 M Table 3 Results of the calculations for the cell parameters of the intercalated mixedlayer phases Cell parameter
Muscovite (C2/c)
Cu-treated (C2)
a b c b
5.199 9.021 20.134 95.54
5.006 8.925 21.887 93.57
In parenthesis the space groups are given in which the calculations were done.
431
solutions) and temperatures (room temperature) are sufficient for the incorporation of the cations into the smectite lattice [4,14,16]. Furthermore, these reactions are cation exchange procedures, the heat treatment and thus the ion migration into the lattice occurs after the cation incorporation [4,11]. In contrast to the extensive investigations of this effect in smectites [11,24,25], the intercalation of metal cations like Cu2+ into muscovite to our knowledge never has been reported. Thus the smectite system was used for a comparative IR study of the effects of the copper cations on the muscovite lattice. As our data reveal no hints for the formation of copper oxides, hydroxides, or phases like chrysocolla (Cu2H2Si2O5(OH)4nH2O), or gerhardtite (Cu2(NO3)(OH)3) during the treatment, it can be assumed, that all the observed changes in the DRIFT spectra are attributable exclusively to changes in the molecular structure of muscovite. Farmer and Russell [26] pointed out, that Si–O stretching vibrations in clay minerals and micas are very sensitive to distortions in the tetrahedral sheets. Furthermore, Farmer and Russell [27] showed that lattice vibrations do not depend on the distance of the single layers and whether they are separated by a layer of water or other polar molecules. Therefore, changes in the interlayer space, for example due to the intercalation of cations with different radii and charge should only then affect these bonds, if the cations migrate to the surfaces of the tetrahedral sheets, move further into the ditrigonal holes and possibly even into vacant octahedral sites, as described for smectites [11,12]. The consequence is, that the observed changes in the spectra of the nitrate-treated muscovites are not caused by a simple intercalation of the divalent cations into the interlayers, as proven by our XRD results, but the cations in fact have to migrate further into the structure of the mineral. This behaviour is well known for Li+ or Cu2+ in smectites [2,28,29], but never has been reported for muscovite. The movement of the foreign cations within the lattice is proven by the infrared data displayed in Fig. 4. The Si–O stretching vibrations in the range between 980 and 1100 cm 1 (bands 8–11) and the Si–O bending modes around 540 cm 1 (bands 1 and 2) show remarkable changes, due to the cation movement. Their shifts to higher wave numbers (band 10) and partly strong intensity changes (bands 2, 8, 9 and 11) are similar to those of Li-treated montmorillonites [3,14]; e.g. a shift of the 1035 cm 1 band in smectite to 1047 cm 1 upon Li treatment was interpreted as a distortion of the silicon oxygen framework, when Li migrates into the ditrigonal holes [33]. Furthermore, Karakassides et al. [4] attributed similar effects in IR-spectra of Cu-exchanged montmorillonites to the fixation of the small Cucations in the ditrigonal holes, close to the Si–O bonds. Alba et al. [3] remarked, that Si–O stretching bands are rather insensitive to the nature of the interlayer cations. Thus the migration of cations within the lattice only reveals changes in IR-spectra, if the ions move into the tetrahedral sheet. This causes the same changes we observed in our study: broadening, high frequency shift and a decrease in intensity [3]. The interpretation of the mica bands in the 650–950 cm 1 region is much more difficult, as there exist only rare data with ambiguity in the assignments [18,23,30,31]. For example
432
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
McKeown et al. [23] attributed vibrational modes between 650 and 669 cm 1 (band 3) to stretching vibrations of bridging and non-bridging O-atoms of the tetrahedral layer. In contrast to this, Velde [30] assigned a band at 670 cm 1 to an OH-bending vibration. While others attributed these bands in montmorillonites to hydroxyls coordinated to three central atoms, confirming some trioctahedral character [32,33]. The shift of band 3 could be attributed to the repulsive forces on the proton of the OH-group, exerted by Cu2+, which is settled deep in the ditrigonal holes of the tetrahedral layers. Furthermore, McKeown et al. [23] attributed the band at 754 cm 1 (band 5) to an octahedral O–Al–O vibration and the band at 806 cm 1 (band 6) to a Si–O–Si mode, while Langer et al. [31] attributed both bands to tetrahedral Al–O–Al. In contrast to this, most authors assign the first band concordantly as an Al–O–Si mode and the second one as an octahedral Al– O–Al vibration [18,30,32]. As band 5 increases in intensity and band 6 shifts from 783 to about 806 cm 1, we suggest a strong distortion of the octahedron. This distortion is probably related to the JahnTeller effect [13], caused by Cu2+-ions migrating within the ditrigonal holes of the tetrahedral sheet. In general these cations are better stabilized in phyllosilicates whose octahedral cavities are not all occupied. The few known natural Cu-phyllosilicates reveal this phenomena too, all of them are dioctahedral [34]. Additionally, these distortions of the octahedra would also explain the strong shift of band 7 from 903 to 932 cm 1. In many dioctahedral phyllosilicates a band occurs at around 910 cm 1 which is generally assigned as an Al–Al–OHbending vibration. Due to the incorporation of small cations like Cu2+, Mg2+ or Li+ in smectites, this band shifts to higher wave numbers between 920 and 935 cm 1. Calvet and Prost [1] and White et al. [35] observed a band at 920 cm 1 in the spectra of a muscovite treated with fused LiNO3, too. In all cases this shift is explained by the fixation of the new cation within the ditrigonal holes close to the OH-groups [4,14,24,33]. Some authors even propose a fixation of the cations in the vacant
Fig. 6. XRD patterns of the five heated samples of the 240 h Cu-nitrate-treated muscovite. The d(0 0 1) value slightly decreases from 1.110 to 1.008 nm with increasing temperature.
octahedral sites [1,36]. As we found no hints for a local trioctahedral environment after the treatment, we do not suppose this for our samples. Madejova´ et al. [24] suggested, that the strong shift from 903 to 932 cm 1 in smectites can be attribute to repulsive forces between the migrating cations and the central octahedral atoms. These repulsive forces change the direction of the dipole moment of the OH-groups in smectites and therefore perturb their deformation vibration. As the direction of the OH-groups strongly depends on the localization of these additional cations [37], one can assume, that the migration of the cations leads to a switching of the hydroxyls into a position perpendicular to the ab-plane. Possibly the strong interactions between migrating cations and the OH-groups could cause a further effect: a partial deprotonation, which leads to the fixation of Cu at these sites very close to the octahedra [16,38]. The shift and intensity change of band 4 from 713 to 721 cm 1 possibly supports this assumption. Kelm et al. [34] observed a mode at the same position (720 cm 1) in the Cu-silicate chrysocolla and assigned it to a Cu–O–Si vibration. Additionally five portions of the 240 h Cu-nitrate-treated muscovite were heated at 100, 150, 200, 250 and 300 8C for 24 h. Afterwards DRIFT spectra and XRD patterns were recorded. The DRIFT spectra showed no further changes in the band positions of the Cu-intercalated material (Fig. 5), indicating that the cation migration through the interlayers into the muscovite lattice happened already during the Cunitrate treatment procedure. According to similar changes in XRD patterns of ion exchanged smectites [2,3,39], the d-value of the (0 0 1)-peak slightly decreased during the heat treatment (Fig. 6). This is attributed to an increasing dehydration of the interlayer cations with increasing temperature. 5. Conclusions and summary
Fig. 5. DRIFT spectra of five samples of the 240 h Cu-nitrate-treated muscovite heated to 100, 150, 200, 250 and 300 8C, showing no changes due to the heat treatment.
The incorporation of copper into the interlayers of muscovite and its migration within the mica structure can be followed by using a combination of X-ray diffraction and
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
433
Fig. 7. Molecular model of the intercalation of Cu into muscovite. The bonds which are affected by the migration of the Cu-ions are marked by fat lines.
spectroscopic methods. X-ray diffraction provides information on the unit cell, whereas the vibrational spectroscopy provides information on the effects of the migrating cations on the muscovite structure. The treatment with copper strongly affects the structure and the unit cell of muscovite, as determined by the occurrence of new d(0 0 l) peaks and the decrease and partly disappearance of all original muscovite peaks in X-ray diffraction patterns. Chemical data and Auger electron spectroscopy suggest the simultaneous occurrence of both potassium and copper in the interlayer region, thus the Cu-treatment is not only a simple ion exchange procedure, but also an intercalation process. This results in a remarkable distortion of the muscovite structure. Calculations of the cell parameters show a strong expansion along the c-direction and a slight decrease of the a and b parameters. This deformation of the entire muscovite structure can only be explained by a migration of the cations through the interlayer region deep into the ditrigonal holes of the tetrahedral sheet as displayed in Fig. 7. This is proven by a number of remarkable changes in the DRIFT spectra of a Cu-intercalated sample. Fig. 7 shows that all vibrations assigned to bondings of the atoms building the ‘‘cage’’ around the ditrigonal holes are affected by the Cu-ions along a suggested migration path. For example, the shifts and intensity variations of the Si–O modes at around 540 cm 1 and over the range from 980 to 1100 cm 1 can be explained by distortions of the Si–O tetrahedra, due to the cation movement into the ditrigonal holes. Furthermore the strong changes of the bending vibrations in the 700–950 cm 1 region suggest intense interactions of the migrating cations with the OH-groups of the octahedral sheet. This is only possible, if the new cations are located very close to the octahedral sheet. Especially the shift of the band a 903 cm 1 to around 932 cm 1 is explained by the fixation of the Cucations within the ditrigonal holes close to the OH-groups. Acknowledgements The authors wish to thank E. Nold (Forschungszentrum Karlsruhe) for performing the Auger electron spectroscopic
measurements and H. Stanjek and S. Sindern (RWTH Aachen) for the XRF measurements. References [1] R. Calvet, R. Prost, Clays Clay Miner. 19 (1971) 175. [2] L. Heller-Kallai, C. Mosser, Clays Clay Miner. 43 (1995) 738. [3] M.D. Alba, R. Alvero, A.I. Beccerro, M.A. Castro, J.M. Trillo, J. Phys. Chem. B 102 (1998) 2207. [4] M.A. Karakassides, J. Madejova´, B. Arvaiova´, A. Bourlinos, D. Petridis, P. Komadel, J. Mater. Chem. 9 (1999) 1553. [5] K. Jasmund, G. Lagaly, Tonminerale und Tone, Steinkopff Verlag, Darmstadt, Germany, 1993. [6] M.A. Osman, C. Moor, W.R. Caseri, U.W. Suter, J. Colloid Interf. Sci. 209 (1999) 232. [7] M.A. Osman, U.W. Suter, J. Colloid Interf. Sci. 214 (1999) 400. [8] H. Keppler, Am. Miner. 75 (1990) 529. [9] W.R. Caseri, R.A. Shelden, U.W. Suter, Colloid Polym. Sci. 270 (1992) 392. [10] F. Friedrich, Spectroscopic investigations of delaminated and intercalated phyllosilicates (Karlsruher Mineralogische und Geochemische Hefte, vol. 27, University of Karlsruhe), PhD Thesis, University of Karlsruhe, 2005. [11] U. Hofmann, R. Klemen, Z. Anorg. Chem. 262 (1950) 95. [12] R. Greene-Kelly, Clay Miner. Bull. 1 (1952) 221. [13] C. Mosser, M. Mestdagh, A. Decarreau, A.J. Herbillon, Clay Miner. 25 (1990) 271. [14] R. Alvero, M.D. Alba, M.A. Castro, J.M. Trillo, J. Phys. Chem. 98 (1994) 7848. [15] J.M. Trillo, M.D. Alba, R. Alvero, M.A. Castro, J. Chem. Soc.: Chem. Commun. 24 (1993) 1809. [16] W.F. Jaynes, J.M. Bigham, Clays Clay Miner. 35 (1987) 440. [17] E. Loh, J. Phys. C: Solid State Phys. 6 (1973) 1091. [18] V.C. Farmer, in: V.C. Farmer (Ed.), The Infrared Spectra of Minerals. Mineralogical Society, Monograph 4, London, UK, 1974, p. 331. [19] D.M. Moore, R.C. Reynolds Jr., X-ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd ed., University Press, New York, USA, 1997. [20] J. Laugier, B. Bochu, CELREF V3. Developed at the Laboratoire des Mate´riaux et du Ge´nie Physique, Ecole Nationale Supe´rieure de Physique de Grenoble (INPG), 2003, http://www.inpg.fr/LMGP. [21] S.W. Bailey, in: G.W. Brindley, G. Brown (Eds.), Crystal Structures of Clay Minerals and their X-ray Identification, Mineralogical Society Monograph 5, London, UK, 1980. [22] S.W. Bailey, in: S.W. Bailey (Ed.), Micas, Reviews in Mineralogy, vol. 13, 2nd ed., Mineralogical Society of America, Washington, USA, 1987. [23] D.A. McKeown, M.I. Bell, E.S. Etz, Am. Miner. 84 (1999) 1041.
434
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
J. Madejova´, B. Arvaiova´, P. Komadel, Spectrochim. Acta A 55 (1999) 2467. K. Emmerich, F.T. Madsen, G. Kahr, Clays Clay Miner. 47 (1999) 591. V.C. Farmer, J.D. Russell, Trans. Faraday Soc. 67 (1971) 2737. V.C. Farmer, J.D. Russell, Spectrochim. Acta 22 (1966) 389. R. Tettenhurst, Am. Miner. 47 (1962) 769. L.P. Meier, R. Nu¨esch, J. Colloid Interf. Sci. 217 (1999) 77. B. Velde, Am. Miner. 63 (1978) 343. K. Langer, N.D. Chatterjee, K. Abraham, Neues Jahrb. Miner. Abh. 142 (1981) 91. [32] J.D. Russel, A.R. Fraser, in: M.J. Wilson (Ed.), Clay Mineralogy: Spectroscopic and Chemical Determinative Methods, Chapman & Hall, London, UK, 1996, p. 11. [24] [25] [26] [27] [28] [29] [30] [31]
[33] J. Madejova´, J. Bujdak, W.P. Gates, P. Komadel, Clay Miner. 31 (1996) 233. [34] U. Kelm, V. Sanhueza, J. Madejova´, V. Sucha, F. Elsass, Geol. Carpath. 52 (2001) 111. [35] J. White, S.W. Bailey, C.B. Brown, J.L. Alrichs, Nature 190 (1961) 342. [36] E. Srasra, F. Bergaya, J.J. Fripiat, Clays Clay Miner. 42 (1994) 237. [37] A.S. Bookin, V.A. Drits, Clays Clay Miner. 30 (1982) 415. [38] J. Williams, J.H. Purnell, J.A. Ballantine, Catal. Lett. 9 (1991) 115. [39] C. Mosser, L.J. Michot, F. Villieras, M. Romeo, Clays Clay Miner. 45 (1997) 789.
Cu(II)-intercalated muscovite: An infrared spectroscopic study Frank Friedrich *, Stefan Heissler, Werner Faubel, Rolf Nu¨esch, Peter G. Weidler Institute for Technical Chemistry, Department of Water and Geotechnology (ITC-WGT), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany Received 15 November 2005; received in revised form 6 March 2006; accepted 11 May 2006 Available online 30 June 2006
Abstract Diffuse reflectance infrared spectroscopy (DRIFT) has proved to be a useful tool for the investigation of migration paths of intercalated metal cations within the layered structure of phyllosilicates like muscovite. While the fixation of small cations like lithium or copper in clay minerals and their migration paths have been studied extensively, they never have been reported for muscovite. Therefore, the aim of this study was to investigate the effects of a treatment with a supersaturated Cu-nitrate solution on the structure of muscovite. Additional X-ray diffractometric (XRD) data showed the formation of new d(0 0 l) and d(0 0 2) peaks, and thus proved that the new cations already were intercalated into the muscovite interlayers, although the concentration of the original interlayer cation potassium was nearly not affected. This suggested the simultaneous occurrence of both potassium and copper in the interlayers, resulting in an expansion along the c-axis and in a decrease of the a and b parameters. The spectroscopic investigations proved a migration of the cations deep into the tetrahedral sheets. As all bands which can be assigned to vibrations of the octahedral sheet (e.g. at 713 or 903 cm 1) were strongly affected by the treatment, a fixation of Cu close to the OH-groups of the octahedral sheets was suggested. # 2006 Elsevier B.V. All rights reserved. Keywords: Cation migration; Copper; DRIFT; IR-spectroscopy; Muscovite
1. Introduction A layered structure is the characteristic feature of phyllosilicates like the mica mineral muscovite KAl2[Si3AlO10(OH)2]. The minerals of the mica group consist of layers of one octahedral Al(O/OH)6-sheet sandwiched by two tetrahedral SiO4-sheets. Due to substitutions in the central positions of the octahedra and tetrahedra, these layers have negative charges around one, which are compensated mostly by large monovalent cations like potassium or sodium (Fig. 1). Because of the resulting properties (e.g. anisotropic optical, electrical properties), muscovite is an important raw material in paint industries and in insulators or polymer composites. Important is on one hand a high aspect ratio, which will be enhanced by delamination of the stacks. On the other hand an exchange of the interlayer cations would change the chemical properties of muscovite, its refraction behaviour and the surface properties. But in contrast to clay minerals where such exchange
* Corresponding author. Tel.: +49 7247 82 6807; fax: +49 7247 82 3478. E-mail address: [email protected] (F. Friedrich). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.023
reactions are common procedures [1–4], the interlayer cations of micas, especially of muscovite are difficult to access and are not exchangeable under normal conditions [5–7]. A chemical modification of this mineral is difficult and needs increased pressure and temperatures [8,9]. Thus Friedrich [10] suggested an autoclave-based method for the incorporation of polyvalent cations into muscovite. With this method it was possible to intercalate Cu2+, Mg2+, Zn2+ or Al3+ for the first time. The main questions of the exchange and intercalation reactions in phyllosilicates are the migration paths and the positions of the introduced cations within the mineral structures. Therefore, the surfaces and interlayers of clay minerals have been investigated intensively. Two classic studies showed, that a diffusion of small ions like lithium deep into the ditrigonal holes of the tetrahedral layers of smectites is possible [11,12]. They even proposed a movement into the unoccupied octahedral cavities. In recent years a great deal of work has been done on the investigation of these mechanisms using Xray diffractometry and various spectroscopic techniques (e.g. [4,13,14]). For example, on the basis of magic angle spinning nuclear magnetic resonance (MAS NMR) measurements Trillo et al. [15]
428
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
filtered by vacuum filtration using cellulose-nitrate filters (pore size: 0.45 mm, obtained from Sartorius, Goettingen, Germany). Excessive salt precipitating during cooling was dissolved by adding additional water. Finally the samples were washed with 250 ml deionized water and dried at 60 8C for 24 h. This procedure supplied a fine-grained, silver shiny powder. 2.2. Sampling techniques
Fig. 1. Structure of the mica mineral muscovite.
proposed the migration of Li+ into the tetrahedral sheets of montmorillonite, but they could not confirm a further movement into the octahedral cavities. In contrast Jaynes and Bigham [16] postulated the movement of lithium into the octahedral sheet, and Mosser et al. [13] interpreted Cu(II) electron paramagnetic resonance (EPR) signals and extended X-ray absorption fine structure (EXAFS) data in a similar way, suggesting a substitution of Cu for Al/Mg in the octahedral sheets of smectites. While these techniques reveal informations on the crystallites (XRD) or on the atomic structure around the investigated atoms (NMR, EPR, EXAFS), IR-spectroscopy offers the possibility to investigate the effect of the incorporated cations on the latticeand OH-vibrations of the phyllosilicates and thus the migration paths of the cations can be observed excellently [1,8,17,18]. In contrast to the extensively investigated clay minerals, to date no discussion of vibrational spectra of di- or trivalent cations intercalated into muscovite are available in literature. Thus, the possibility of introducing polyvalent cations like Cu2+ into muscovite allows an extensive infrared spectroscopic investigation of the migration mechanisms of this high-charged cation and its interaction within the muscovite interlayers for the first time. 2. Experimental 2.1. Intercalation procedure For the experiments the <400 mm size fraction of an Indian muscovite (obtained from Merck, Darmstadt, Germany) was used. Also, the pro analysis grade copper nitrate (Cu(NO3)23H2O) was obtained from Merck, Darmstadt, Germany. According to the method of Friedrich [10] stainless steel autoclaves equipped with a Teflon compartment were filled with a mixture of 250 mg of mica and a super saturated nitrate solution (65 g copper nitrate + 20 ml H2O). The closed autoclaves were heated at 155 8C for a time series (24, 96, 144 and 240 h). Afterwards the suspensions were washed and
2.2.1. Infrared spectroscopy A Bruker IFS 66/s Fourier transform IR spectrometer, equipped with a DTGS detector was employed to obtain the IRspectra. Sixty-four scans in the 4000–400 cm 1 spectral range were recorded with a scanner velocity of 1.6 kHz and a resolution of 4 cm 1. For DRIFT measurements a diffuse reflectance accessory from Spectra-Tech Inc. was used with a polished aluminium-mirror for the background measurements. Sample material (10 mg) was slightly ground with 300 mg KBr then the powders were poured loosely into a sample cup and were supposed to be randomly orientated. 2.2.2. Chemical analyses The amounts of Cu2+ and K+ were analysed by X-ray fluorescence (XRF). The measurements were done at the laboratory for geochemistry at the RWTH Aachen using an energy-dispersive SPECTRO X-Lab 2000 spectrometer, equipped with a nitrogen-cooled Si(Li) detector. The finegrained sample powder (200 mg) was filled in plastic cylinders covered with a Mylar foil. To stabilize the powder, it was suspended in 1 ml Elvacit solution. To avoid the formation of bubbles during the evaporation of the acetone, the suspensions first were dried 1 h at room temperature and then 2 h at 40 8C. 2.2.3. X-ray diffractometry The XRD patterns were recorded over the range of 3–638 2Q, using a Siemens D5000 diffractometer equipped with a graphite secondary mono-chromator. Counting time was 2 s in ˚ ) was used for 0.0158 steps and Cu Ka radiation (l = 1.5418 A measurement. Because of the general difficulties in getting useful Rietveld refinements of clay mineral cell parameters from their XRD patterns [19], the parameters were calculated with the cell parameters refinement program CelRef [20], using single linefitted XRD-data. The peak fitting was done with the Jandel Peakfit software package. A Voigt function was chosen, with the minimum number of peaks used for the fitting process. A linear two-point background was chosen and fitting was undertaken until reproducible results were obtained with squared correlations of r2 > 0.997. CelRef V3 is a least square refinement method [20]. The refinements were undertaken by using at least eight fitted peaks until reproducible results were obtained with mean square deviations <0.01. As starting values for the cell parameter ˚ , b = 9.02 A ˚, calculations those of muscovite (a = 5.19 A ˚ , and b = 95.58) were chosen. The refinements c = 20.13 A were done in the two space groups C2/c and C2.
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
429
Fig. 2. X-ray patterns of untreated muscovite (MU-0) and of a Cu-muscovite sample after 240 h treatment. (a) The diffractograms over the entire range 38–608 2Q show strong changes due to the treatment, especially in the low-angle region. (b) Detailed view on the low-angle region with the new 0 0 l-peaks at about 2.2 and 1.2 nm.
Table 1 X-ray powder data of the untreated muscovite (MU-0) compared to literature data of a 2M1-muscovite d-Spacings
hkl
[21]
This study
1.014 0.500 0.448 0.446 0.439 0.429 0.411 0.397 0.389 0.374 0.350 0.335 0.321 0.299 0.287 0.280 0.259 0.258 0.256 0.251 0.246 0.245 0.240 0.238 0.225 0.224 0.220 0.218 0.215 0.213 0.205 0.201 0.198 – 0.174 0.170
1.035 0.502 0.448 0.446 – 0.432 0.416 0.399 0.391 0.378 0.352 0.326 0.322 – 0.288 0.278 0.262 0.259 – 0.251 – 0.243 0.241 0.233 0.226 0.225 – 0.215 – – 0.209 0.201 0.190 0.184 0.175 0.170
Spacings in nanometers.
002 004 110 1,1, 1 021 111 022 112 1,1, 3 023 1,1, 4 006 114 025 115 1,1, 6 1,3, 1 116 2,0, 2 008 1,3, 3 202 2,0, 4 133 040 041 221 2,2, 3 222 135 044 0 0 10 1,3, 7 1,3, 9 150
2.2.4. Auger electron spectroscopy (AES) The Auger electron spectra were recorded on a physical electronics Auger Nanoprobe Phi 680 with an electron beam energy of 10 keV. The mica sheets were removed by argon ion sputtering up to a depth of 60 nm, which corresponds to a thickness of about 50 layers. 3. Results 3.1. X-ray diffractometry The X-ray patterns of untreated (MU-0) and of Cu-nitratetreated muscovite are shown in Fig. 2a. According to this, MU0 is assigned as a muscovite-2M1 (Table 1), which has a monoclinic symmetry and crystallizes in the space group No. 15, C2/c [21,22]. Significant for this space group is the lack of the d0 0 1-peak. Thus, the first peak, which is visible in the range between 38 and 108 2Q is the d0 0 2 at 1.002 nm (8.88 2Q). The treatment with Cu-nitrate results in strong changes in the XRD patterns. Especially in the range between 38 and 108 2Q the Cu-muscovite pattern shows the development of two new broad peaks at 1.11 (7.98 2Q) and 2.21 nm (3.98 2Q), while the d0 0 2 of the original muscovite strongly decreases (Fig. 2b). Furthermore, most of the original muscovite peaks decrease their intensities and get broader during the treatment, while their 0 0 l-peaks slightly shift positions and decrease their intensities only to a minor degree. 3.2. Auger electron spectroscopy Fig. 3 displays two AES spectra measured on the surface of a 240 h treated muscovite and on the same position after the removal of about 50 muscovite layers. The measured ion concentrations (surface: K 3.7 at.%, Cu: 0.7 at.%; depth 60 nm: K 4.3 at.%, Cu 3.4 at.%) prove the simultaneous occurrence of potassium and copper on the surface as well as within the structure of the treated muscovite.
430
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434 Table 2 Observed changes of the IR-band positions, due to the Cu-intercalation, and their suggested assignments Band no.
Fig. 3. Auger electron spectra of a Cu-muscovite after 240 h treatment. Spectrum (a) is measured on the surface of the muscovite particle. Spectrum (b) on the same particle after the removal of about 50 layers (60 nm).
MU-0
Cu-treated
Suggested assignments
1
–
528
2
549
549
Tetrahedral Si–O–Al Cu affected Si–O stretching Al–OH-bending
3
651
667
Al–OH-bending, affected by repulsive forces, due to the fixation of Cu Al–OH
4 5
713 752
721 752
Al–O Cu? Al–O–Si Octahedral Al–O–Al bending
6
783
806
Al–O–Al of distorted octahedra AlAlOH
7
903
932
8 9
983 ?
(983) 1003
Al–Al–OH affected by Cu fixed within the ditrigonal holes of tetrahedral layer Si–O stretch Si–O stretch (modified phase) Si–O stretch
10
1033
1054
11
1062
(1062)
Si–O stretch (modified phase) Si–O stretch
3.3. DRIFT spectroscopy In general the spectral features in the MIR-region between 400 and 1200 cm 1 are attributed to lattice vibrations of the tetrahedral and octahedral units of phyllosilicates including OH-bending vibrations [17,18,23]. Fig. 4 and Table 2 show this
range, in which the spectra of the time-dependent treatment with Cu-nitrate are compared with that of the original muscovite. Due to the Cu-nitrate treatment distinct changes occur with increasing treatment time. A new strong vibration appears at 528 cm 1 (band 1), while the intensity of the muscovite band at 549 cm 1 (band 2) decreases. Band 3 at 651 cm 1 remarkably shifts to 667 cm 1, while it strongly decreases its intensity. Also the bands 4 and 6 at 713 and 783 cm 1 show a shift to higher wavenumbers. Additionally, the shoulder at 752 cm 1 (band 5) strongly increases its intensity. Band 7 at 902 cm 1 disappears already after 24 h of treatment and a shoulder at about 932 cm 1 occurs with slightly increasing intensity. As mentioned above, in the Si–O stretching region between 950 and 1100 cm 1 three strong bands occur at 983, 1033 and 1062 cm 1 (bands 8, 10, 11) in the spectra of the untreated muscovite. The treatment with Cu-nitrate results in the formation of band 9 at 1003 cm 1, while band 8 disappears in the shoulder of band 9 with increasing treatment time. Similarly the sharp vibration at 1033 cm 1 (band 10) strongly shifts to 1054 cm 1, while the muscovite band 11 at 1062 cm 1 disappears in the shoulder of band 10. 4. Discussion
Fig. 4. DRIFT spectra of the lattice region of original muscovite compared to spectra of samples after various times of Cu-treatment.
The successful treatment of muscovite with Cu-nitrate is indicated by the occurrence of new d(0 0 1) peaks in the lowangle-region of the XRD patterns (Fig. 2). The d-values of these
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
peaks show the increase of the interlayer spacings. Thus, the metal ions are not only adsorbed on the muscovite surfaces, but they are in fact introduced into the interlayers of the mineral. The occurrence of Cu-peaks in the Auger electron spectra after removal of 50 muscovite layers (60 nm) strongly confirms this assumption (Fig. 3). Surprisingly, the chemical and the AES data show, that the intercalation of the metal cations is no simple ion exchange mechanism of the interlayer potassium versus the Cu2+ cation. While the Cu concentration increases from 0 in the starting material to 2.4 wt.% CuO after a treatment time of 240 h, no significant decrease of the potassium concentration is detectable during the treatment (9.2 wt.% K2O in the untreated material, and 8.5 wt.% K2O after 240 h). Consequently, the potassium as well as the metal ions appear together in the interlayer region after the treatment, which is responsible for the increasing interlayer spaces. The necessity of a strong super saturation of the nitrate solution to reveal the intercalated phase suggests, that the main intercalation component is the Cu-cation. It is proposed, that the super saturation is necessary to create a high ionic potential to introduce the cations into the interlayers, although the potassium is located there. The fact, that the original muscovite d(0 0 2) peak at 1.002 nm does not disappear even after a Cu-nitrate treatment of 240 h, indicates that not all layers are intercalated with copper, thus the intercalation is not complete. Therefore, we assume that the treatment reveals a strongly modified muscovite structure with already Cu-intercalated, expanded layers and relatively unaffected ‘‘original’’ muscovite layers. These structural modifications are the result of changes in the crystal symmetry and in the extinction rules. According to that, a calculation of the cell parameters is only possible in a space group of lower symmetry. The calculations assuming the space group No. 5, C2 suggest, that the intercalation not only leads to an expansion along the c-axis, but also to a slight decrease of the a- and b-lattice-parameters (Table 3). While the expansion along the c-axis easily can be explained by the intercalation procedure and the occurrence of both copper and potassium in the interlayers, the changes along the other axes possibly occur due to distortions and rotations of the tetrahedral and octahedral units around the ditrigonal holes of the tetrahedral sheets during a migration of the copper ions from the interlayers into the muscovite lattice structure. Responsible for this migration are the strong super saturation of the Cu-solution and the high reaction temperature. A relatively similar behaviour of Li+ and Cu2+ cations in smectites is known as Hofmann–Klemen effect. In contrast, there much lower concentrations (0.1–1 M Table 3 Results of the calculations for the cell parameters of the intercalated mixedlayer phases Cell parameter
Muscovite (C2/c)
Cu-treated (C2)
a b c b
5.199 9.021 20.134 95.54
5.006 8.925 21.887 93.57
In parenthesis the space groups are given in which the calculations were done.
431
solutions) and temperatures (room temperature) are sufficient for the incorporation of the cations into the smectite lattice [4,14,16]. Furthermore, these reactions are cation exchange procedures, the heat treatment and thus the ion migration into the lattice occurs after the cation incorporation [4,11]. In contrast to the extensive investigations of this effect in smectites [11,24,25], the intercalation of metal cations like Cu2+ into muscovite to our knowledge never has been reported. Thus the smectite system was used for a comparative IR study of the effects of the copper cations on the muscovite lattice. As our data reveal no hints for the formation of copper oxides, hydroxides, or phases like chrysocolla (Cu2H2Si2O5(OH)4nH2O), or gerhardtite (Cu2(NO3)(OH)3) during the treatment, it can be assumed, that all the observed changes in the DRIFT spectra are attributable exclusively to changes in the molecular structure of muscovite. Farmer and Russell [26] pointed out, that Si–O stretching vibrations in clay minerals and micas are very sensitive to distortions in the tetrahedral sheets. Furthermore, Farmer and Russell [27] showed that lattice vibrations do not depend on the distance of the single layers and whether they are separated by a layer of water or other polar molecules. Therefore, changes in the interlayer space, for example due to the intercalation of cations with different radii and charge should only then affect these bonds, if the cations migrate to the surfaces of the tetrahedral sheets, move further into the ditrigonal holes and possibly even into vacant octahedral sites, as described for smectites [11,12]. The consequence is, that the observed changes in the spectra of the nitrate-treated muscovites are not caused by a simple intercalation of the divalent cations into the interlayers, as proven by our XRD results, but the cations in fact have to migrate further into the structure of the mineral. This behaviour is well known for Li+ or Cu2+ in smectites [2,28,29], but never has been reported for muscovite. The movement of the foreign cations within the lattice is proven by the infrared data displayed in Fig. 4. The Si–O stretching vibrations in the range between 980 and 1100 cm 1 (bands 8–11) and the Si–O bending modes around 540 cm 1 (bands 1 and 2) show remarkable changes, due to the cation movement. Their shifts to higher wave numbers (band 10) and partly strong intensity changes (bands 2, 8, 9 and 11) are similar to those of Li-treated montmorillonites [3,14]; e.g. a shift of the 1035 cm 1 band in smectite to 1047 cm 1 upon Li treatment was interpreted as a distortion of the silicon oxygen framework, when Li migrates into the ditrigonal holes [33]. Furthermore, Karakassides et al. [4] attributed similar effects in IR-spectra of Cu-exchanged montmorillonites to the fixation of the small Cucations in the ditrigonal holes, close to the Si–O bonds. Alba et al. [3] remarked, that Si–O stretching bands are rather insensitive to the nature of the interlayer cations. Thus the migration of cations within the lattice only reveals changes in IR-spectra, if the ions move into the tetrahedral sheet. This causes the same changes we observed in our study: broadening, high frequency shift and a decrease in intensity [3]. The interpretation of the mica bands in the 650–950 cm 1 region is much more difficult, as there exist only rare data with ambiguity in the assignments [18,23,30,31]. For example
432
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
McKeown et al. [23] attributed vibrational modes between 650 and 669 cm 1 (band 3) to stretching vibrations of bridging and non-bridging O-atoms of the tetrahedral layer. In contrast to this, Velde [30] assigned a band at 670 cm 1 to an OH-bending vibration. While others attributed these bands in montmorillonites to hydroxyls coordinated to three central atoms, confirming some trioctahedral character [32,33]. The shift of band 3 could be attributed to the repulsive forces on the proton of the OH-group, exerted by Cu2+, which is settled deep in the ditrigonal holes of the tetrahedral layers. Furthermore, McKeown et al. [23] attributed the band at 754 cm 1 (band 5) to an octahedral O–Al–O vibration and the band at 806 cm 1 (band 6) to a Si–O–Si mode, while Langer et al. [31] attributed both bands to tetrahedral Al–O–Al. In contrast to this, most authors assign the first band concordantly as an Al–O–Si mode and the second one as an octahedral Al– O–Al vibration [18,30,32]. As band 5 increases in intensity and band 6 shifts from 783 to about 806 cm 1, we suggest a strong distortion of the octahedron. This distortion is probably related to the JahnTeller effect [13], caused by Cu2+-ions migrating within the ditrigonal holes of the tetrahedral sheet. In general these cations are better stabilized in phyllosilicates whose octahedral cavities are not all occupied. The few known natural Cu-phyllosilicates reveal this phenomena too, all of them are dioctahedral [34]. Additionally, these distortions of the octahedra would also explain the strong shift of band 7 from 903 to 932 cm 1. In many dioctahedral phyllosilicates a band occurs at around 910 cm 1 which is generally assigned as an Al–Al–OHbending vibration. Due to the incorporation of small cations like Cu2+, Mg2+ or Li+ in smectites, this band shifts to higher wave numbers between 920 and 935 cm 1. Calvet and Prost [1] and White et al. [35] observed a band at 920 cm 1 in the spectra of a muscovite treated with fused LiNO3, too. In all cases this shift is explained by the fixation of the new cation within the ditrigonal holes close to the OH-groups [4,14,24,33]. Some authors even propose a fixation of the cations in the vacant
Fig. 6. XRD patterns of the five heated samples of the 240 h Cu-nitrate-treated muscovite. The d(0 0 1) value slightly decreases from 1.110 to 1.008 nm with increasing temperature.
octahedral sites [1,36]. As we found no hints for a local trioctahedral environment after the treatment, we do not suppose this for our samples. Madejova´ et al. [24] suggested, that the strong shift from 903 to 932 cm 1 in smectites can be attribute to repulsive forces between the migrating cations and the central octahedral atoms. These repulsive forces change the direction of the dipole moment of the OH-groups in smectites and therefore perturb their deformation vibration. As the direction of the OH-groups strongly depends on the localization of these additional cations [37], one can assume, that the migration of the cations leads to a switching of the hydroxyls into a position perpendicular to the ab-plane. Possibly the strong interactions between migrating cations and the OH-groups could cause a further effect: a partial deprotonation, which leads to the fixation of Cu at these sites very close to the octahedra [16,38]. The shift and intensity change of band 4 from 713 to 721 cm 1 possibly supports this assumption. Kelm et al. [34] observed a mode at the same position (720 cm 1) in the Cu-silicate chrysocolla and assigned it to a Cu–O–Si vibration. Additionally five portions of the 240 h Cu-nitrate-treated muscovite were heated at 100, 150, 200, 250 and 300 8C for 24 h. Afterwards DRIFT spectra and XRD patterns were recorded. The DRIFT spectra showed no further changes in the band positions of the Cu-intercalated material (Fig. 5), indicating that the cation migration through the interlayers into the muscovite lattice happened already during the Cunitrate treatment procedure. According to similar changes in XRD patterns of ion exchanged smectites [2,3,39], the d-value of the (0 0 1)-peak slightly decreased during the heat treatment (Fig. 6). This is attributed to an increasing dehydration of the interlayer cations with increasing temperature. 5. Conclusions and summary
Fig. 5. DRIFT spectra of five samples of the 240 h Cu-nitrate-treated muscovite heated to 100, 150, 200, 250 and 300 8C, showing no changes due to the heat treatment.
The incorporation of copper into the interlayers of muscovite and its migration within the mica structure can be followed by using a combination of X-ray diffraction and
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
433
Fig. 7. Molecular model of the intercalation of Cu into muscovite. The bonds which are affected by the migration of the Cu-ions are marked by fat lines.
spectroscopic methods. X-ray diffraction provides information on the unit cell, whereas the vibrational spectroscopy provides information on the effects of the migrating cations on the muscovite structure. The treatment with copper strongly affects the structure and the unit cell of muscovite, as determined by the occurrence of new d(0 0 l) peaks and the decrease and partly disappearance of all original muscovite peaks in X-ray diffraction patterns. Chemical data and Auger electron spectroscopy suggest the simultaneous occurrence of both potassium and copper in the interlayer region, thus the Cu-treatment is not only a simple ion exchange procedure, but also an intercalation process. This results in a remarkable distortion of the muscovite structure. Calculations of the cell parameters show a strong expansion along the c-direction and a slight decrease of the a and b parameters. This deformation of the entire muscovite structure can only be explained by a migration of the cations through the interlayer region deep into the ditrigonal holes of the tetrahedral sheet as displayed in Fig. 7. This is proven by a number of remarkable changes in the DRIFT spectra of a Cu-intercalated sample. Fig. 7 shows that all vibrations assigned to bondings of the atoms building the ‘‘cage’’ around the ditrigonal holes are affected by the Cu-ions along a suggested migration path. For example, the shifts and intensity variations of the Si–O modes at around 540 cm 1 and over the range from 980 to 1100 cm 1 can be explained by distortions of the Si–O tetrahedra, due to the cation movement into the ditrigonal holes. Furthermore the strong changes of the bending vibrations in the 700–950 cm 1 region suggest intense interactions of the migrating cations with the OH-groups of the octahedral sheet. This is only possible, if the new cations are located very close to the octahedral sheet. Especially the shift of the band a 903 cm 1 to around 932 cm 1 is explained by the fixation of the Cucations within the ditrigonal holes close to the OH-groups. Acknowledgements The authors wish to thank E. Nold (Forschungszentrum Karlsruhe) for performing the Auger electron spectroscopic
measurements and H. Stanjek and S. Sindern (RWTH Aachen) for the XRF measurements. References [1] R. Calvet, R. Prost, Clays Clay Miner. 19 (1971) 175. [2] L. Heller-Kallai, C. Mosser, Clays Clay Miner. 43 (1995) 738. [3] M.D. Alba, R. Alvero, A.I. Beccerro, M.A. Castro, J.M. Trillo, J. Phys. Chem. B 102 (1998) 2207. [4] M.A. Karakassides, J. Madejova´, B. Arvaiova´, A. Bourlinos, D. Petridis, P. Komadel, J. Mater. Chem. 9 (1999) 1553. [5] K. Jasmund, G. Lagaly, Tonminerale und Tone, Steinkopff Verlag, Darmstadt, Germany, 1993. [6] M.A. Osman, C. Moor, W.R. Caseri, U.W. Suter, J. Colloid Interf. Sci. 209 (1999) 232. [7] M.A. Osman, U.W. Suter, J. Colloid Interf. Sci. 214 (1999) 400. [8] H. Keppler, Am. Miner. 75 (1990) 529. [9] W.R. Caseri, R.A. Shelden, U.W. Suter, Colloid Polym. Sci. 270 (1992) 392. [10] F. Friedrich, Spectroscopic investigations of delaminated and intercalated phyllosilicates (Karlsruher Mineralogische und Geochemische Hefte, vol. 27, University of Karlsruhe), PhD Thesis, University of Karlsruhe, 2005. [11] U. Hofmann, R. Klemen, Z. Anorg. Chem. 262 (1950) 95. [12] R. Greene-Kelly, Clay Miner. Bull. 1 (1952) 221. [13] C. Mosser, M. Mestdagh, A. Decarreau, A.J. Herbillon, Clay Miner. 25 (1990) 271. [14] R. Alvero, M.D. Alba, M.A. Castro, J.M. Trillo, J. Phys. Chem. 98 (1994) 7848. [15] J.M. Trillo, M.D. Alba, R. Alvero, M.A. Castro, J. Chem. Soc.: Chem. Commun. 24 (1993) 1809. [16] W.F. Jaynes, J.M. Bigham, Clays Clay Miner. 35 (1987) 440. [17] E. Loh, J. Phys. C: Solid State Phys. 6 (1973) 1091. [18] V.C. Farmer, in: V.C. Farmer (Ed.), The Infrared Spectra of Minerals. Mineralogical Society, Monograph 4, London, UK, 1974, p. 331. [19] D.M. Moore, R.C. Reynolds Jr., X-ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd ed., University Press, New York, USA, 1997. [20] J. Laugier, B. Bochu, CELREF V3. Developed at the Laboratoire des Mate´riaux et du Ge´nie Physique, Ecole Nationale Supe´rieure de Physique de Grenoble (INPG), 2003, http://www.inpg.fr/LMGP. [21] S.W. Bailey, in: G.W. Brindley, G. Brown (Eds.), Crystal Structures of Clay Minerals and their X-ray Identification, Mineralogical Society Monograph 5, London, UK, 1980. [22] S.W. Bailey, in: S.W. Bailey (Ed.), Micas, Reviews in Mineralogy, vol. 13, 2nd ed., Mineralogical Society of America, Washington, USA, 1987. [23] D.A. McKeown, M.I. Bell, E.S. Etz, Am. Miner. 84 (1999) 1041.
434
F. Friedrich et al. / Vibrational Spectroscopy 43 (2007) 427–434
J. Madejova´, B. Arvaiova´, P. Komadel, Spectrochim. Acta A 55 (1999) 2467. K. Emmerich, F.T. Madsen, G. Kahr, Clays Clay Miner. 47 (1999) 591. V.C. Farmer, J.D. Russell, Trans. Faraday Soc. 67 (1971) 2737. V.C. Farmer, J.D. Russell, Spectrochim. Acta 22 (1966) 389. R. Tettenhurst, Am. Miner. 47 (1962) 769. L.P. Meier, R. Nu¨esch, J. Colloid Interf. Sci. 217 (1999) 77. B. Velde, Am. Miner. 63 (1978) 343. K. Langer, N.D. Chatterjee, K. Abraham, Neues Jahrb. Miner. Abh. 142 (1981) 91. [32] J.D. Russel, A.R. Fraser, in: M.J. Wilson (Ed.), Clay Mineralogy: Spectroscopic and Chemical Determinative Methods, Chapman & Hall, London, UK, 1996, p. 11. [24] [25] [26] [27] [28] [29] [30] [31]
[33] J. Madejova´, J. Bujdak, W.P. Gates, P. Komadel, Clay Miner. 31 (1996) 233. [34] U. Kelm, V. Sanhueza, J. Madejova´, V. Sucha, F. Elsass, Geol. Carpath. 52 (2001) 111. [35] J. White, S.W. Bailey, C.B. Brown, J.L. Alrichs, Nature 190 (1961) 342. [36] E. Srasra, F. Bergaya, J.J. Fripiat, Clays Clay Miner. 42 (1994) 237. [37] A.S. Bookin, V.A. Drits, Clays Clay Miner. 30 (1982) 415. [38] J. Williams, J.H. Purnell, J.A. Ballantine, Catal. Lett. 9 (1991) 115. [39] C. Mosser, L.J. Michot, F. Villieras, M. Romeo, Clays Clay Miner. 45 (1997) 789.