A neutron diffraction study of alkali cation migration in ... - CiteSeerX

Phys Chem Minerals (2008) 35:49–58 DOI 10.1007/s00269-007-0197-z

ORIGINAL PAPER

A neutron diffraction study of alkali cation migration in montmorillonites D. Gournis Æ A. Lappas Æ M. A. Karakassides Æ D. To¨bbens Æ A. Moukarika

Received: 30 March 2007 / Accepted: 12 October 2007 / Published online: 30 October 2007 Ó Springer-Verlag 2007

Background

Keywords Montmorillonite Neutron diffraction Lithium migration Infrared reflectance Hoffman Klemen effect

D. Gournis (&) M. A. Karakassides Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece e-mail: [email protected] A. Lappas Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Vassilika Vouton, 71110 Heraklion, Greece D. To¨bbens Berlin Neutron Scattering Center, Hahn-Meitner-Institu¨t, Glienicker Strasse 100, D-14109 Berlin, Germany A. Moukarika Department of Physics, University of Ioannina, 45110 Ioannina, Greece

The clay mineral montmorillonite, a member of the dioctahedral smectite group, is widely used as a raw material due to its powerful catalytic (Pinnavaia 1983; Konta 1995) and sorbent (Madsen 1998) properties. Montmorillonites are also important in areas of environmental concern and, for such applications, a precise knowledge of the interaction mechanisms between clay surfaces and heavy metal cations, such as Cu2+, Cd2+, Zn2+ etc., is greatly desirable as these cations are often present in various forms in polluted soils (Baker and Senft 1995). Smectites are 2:1 layered silicates, which have a structure (MacEwan 1951) shown in Fig. 1 consisting of essentially four layers: (1) a gallery in which the intercalated species together with molecular H2O reside, (2) a tetrahedral sheet consisting of SiO4 tetrahedra joined at the corners to form a hexagonal arrangement, (3) an octahedral sheet composed of edgesharing AlO4(OH)2 octahedra, also in a hexagonal drawing, and (4) another tetrahedral sheet symmetrically disposed to the first with respect to the octahedral sheet. The basal oxygen sheets are arranged in a Kagome´ lattice whose hexagonal ‘‘pockets’’ form triangular lattice gallery sites. Such clays have a cation exchange capacity, which depends upon the substitution of lower valent atoms e.g., Mg2+ for Al3+ in the octahedral sheet and Al3+ for Si4+ in the tetrahedral sites. As a consequence, the sheets have a fixed negative charge and neutrality is provided, for example, by hydrated cations that are present in the galleries and on the outer surfaces. The intercalation process in these compounds is equivalent to ion exchange and, unlike the intercalation compounds of graphite, it does not involve charge transfer between the guest and the host species. The charge on the sheets affects many fundamental properties of the clays including cation exchange capacity, cation

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Fig. 1 Idealized structure of montmorillonite

fixation, swelling ability, water holding and specific surface area. To a greater or lesser degree, these materials have the natural ability to adsorb and exchange cations from solutions and it is this cation ‘‘storage’’ that makes them such important components in industrial applications. A striking feature of montmorillonites is that upon saturation with small cations, such as Li+, and subsequent heating to 200–300°C, the charge on the negative layer reduces and the systems lose their important natural ability to expand (swelling) the interlayer space during hydration (Hoffmann and Klemen 1950; Komadel et al. 1996, 2005). This phenomenon was tentatively attributed to the migration of the Li+ cations from the interlayer space to vacant lattice octahedral sites, where they are trapped providing the necessary local charge compensation (Srasra et al. 1994; Karakassides et al. 1999a; Hroba´rikova´ et al. 2001). On the contrary, in the case of larger cations, such as Cs+, no fixation was observed. Other researchers proposed a different location for Li+ cations at the bottom of the hexagonal cavities of the basal oxygen surfaces after heat treatment i.e., in the Kagome´ pockets (Tettenhorst 1962; Luca et al. 1988; Alvero et al. 1994; Theng et al. 1997), while some others claim that lithium ions migrate to both hexagonal cavities and vacant octahedral sites (Calvet and Prost 1971; Sposito et al. 1983; Madejova´ et al. 1999, 2000; Karakassides et al. 1999b; Ebira et al. 1999). Evidence for the first hypothesis has come almost entirely form infrared studies (Srasra et al. 1994; Karakassides et al. 1999a; Hroba´rikova´ et al. 2001; Calvet and Prost 1971; Sposito et al. 1983; Madejova´ et al. 1999, 2000, 2006; Karakassides et al. 1999b; Ebira et al. 1999), with most of them based on the variation of the stretching and librational modes of hydroxyl groups found between 3,300 and 3,800 cm-1 and 650–1000 cm-1, respectively. In

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Phys Chem Minerals (2008) 35:49–58

addition, some studies examine also the Si–O vibrational pattern in 400–1300 cm-1 region (Heller-Kallai and Mosser 1995; Hroba´rikova´ et al. 2001; Madejova´ et al. 1999, 2000; Karakassides et al. 1999b). Komadel et al.1996 supported that gradual decrease in the layer charge due to a ‘virtual’ Li migration shifts the Si–O stretching bands to higher frequencies and leads to formation of non-swelling pyrophyllite sheets, while solid state NMR studies (Gates et al. 2000) strengthen these interpretations of the IR data on Li-fixation. A possible explanation for the discrepancies on the location of lithium cations is provided by most recent investigations (Madejova´ et al. 2000; Karakassides et al. 2000; Hroba´rikova´ et al. 2001), indicating that the kind of preferred sites depends on the degree and type of substitution in the parent clay mineral. More specifically, migration towards octahedral sites is mostly favorable when the degree of substitution is high in the octahedral sheet and low in the tetrahedral, whereas in the opposite case the cation prefers almost exclusively the hexagonal cavities. Our previous research studies, based mainly on IR (Karakassides et al. 1997, 1999a) and ESR measurements (Karakassides et al. 1999b), pointed towards an intriguing inter-relationship between the valence and size, on one hand, of the exchangeable metal cations of unheated montmorillonite and, on the other hand, on the geometrical properties of the host lattice site after heating. Deconvolution of the IR absorption profiles of the Si–O vibrations revealed after heating large variations in the normalized IR absorption area of one of the component bands, \A[2, in the case of Li-montmorillonite, besides its systematic decrease with increasing size of the exchangeable cations (Na \ K \ Cs \ Rb). This band arises from the asymmetric stretching vibrations of the silicon-apical oxygen units and is correlated with the decrease in the interlayer charge due to Li withdrawal. However, the migration process itself remains unclear. The geometrical considerations for the sheets inferred from IR or ESR alone are not adequate to address fully the possible cation location. Later, Stackhouse and Coveney (2002) employed density functional methods to examine the phenomenon at the electronic structure level. They found that it is energetically preferable for lithium cations to reside in the vacant octahedral sites as opposed to the hexagonal cavities due to the closer proximity to the negative charge sites in the octahedral sheet. In addition, they argue that the occupation of octahedral sites causes perpendicular orientation of the hydroxyl groups with respect to the ab plane, while a comparison between the experimental and calculated infrared spectra suggested that lithium ions do not migrate to hexagonal cavities when tetrahedral aluminium is not present. Thus, it is obvious that the subject still holds some open problems and direct experimental evidence describing the crystal structure itself


could be invaluable. This motivated us to study the migration of exchangeable cations in montmorillonite upon heating by employing neutron powder diffraction measurements. Neutron, as compred to X-ray diffraction, is preferred when light atoms, like Li, are involved in the materials under consideration, since the scattering lengths do not depend on the atomic number and can therefore trace their location in the crystal structure. On the other hand, the fact that the neutron scattering factors are almost invariant with sin h/k (Bacon 1975) means that the intensity of the data does not drop off at high scattering angles (2H), as is the case with X-ray patterns. So, a neutron experiment is expected to provide considerably more structural information. Several studies showed that neutron diffraction measurements with isotopic substitution (e.g., H/D) have been successfully applied to unravel mainly the structure of interlayer water in Na-, Li- and Ni-montmorillonite clays (Powell et al. 1998a, 1998b; Pitteloud et al. 2001). The results showed that the interlayer water adopts a liquid-type structure and also a tendency to form hydrogen bonds with the oxygen of the internal clay surfaces. The present work was directed towards establishing an extensive neutron diffraction study on two montmorillonite clays saturated with Li or Cs. The reason for choosing Li and Cs as exchangeable cations is that the Li migrates, whereas Cs does not. The purpose of this study was: (1) to refine the crystal structure of montmorillonite, which still remains an open problem in the field of clay mineralogy and (2) to investigate the possible steps in the migration of the cations exchanged in the interlamellar space and in particular to determine the actual sites where these cations can effectively reside. To resolve these questions, neutron powder diffraction (NPD) profiles of air-dried Li- and Csmontmorillonite samples, before and after heat treatment at 300°C were collected on a high-resolution powder diffractometer. Diffraction profiles were also measured at various temperatures between ambient and 300°C on a Li-montmorillonite material, having its interlayer water removed by mild preparative routes.

Experimental section Materials The natural smectite clay, a dioctahedral montmorillonite (mont), with structural formula: M0:62 ½Al3:01 FeðIIIÞ0:41 Fe(II)0:04 Mg0:54 ðSi7:8 Al0:2 ÞO20 ðOHÞ4 ; M ¼ Li; Na; Cs; . . . and cation exchange capacity (CEC) 75.2 meq/100 g clay (Gournis 1998) was supplied by the Clay Minerals Society

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(from the source SWy-1, Crook Country, WY, USA). The samples were obtained by purification of the clay according to the following method: 10 g of clay was dispersed in 250 mL of distilled and de-ionized water; the suspension was first stirred for 1 day and then placed in a volumetric cylinder of 1 L and left to rest for 5–6 h. The suspension, except the last 5 mL at the bottom of the volumetric cylinder, was removed by siphoning, and thus contained particles with diameter less than 2 lm. The aqueous clay dispersion was centrifuged and the solid was treated with 150 mL of 1 M buffer solution (pH*5) of CH3COONa and CH3COOH in order to remove carbonates absorbed by the clay mineral. The mixture was heated to 70–80°C in a water bath with continuous stirring and cooled immediately afterwards, centrifuged, re-suspended in 100 mL of water, re-centrifuged and finally immersed into 1 M solutions of the corresponding metal chlorides in order to prepare the Li and Cs exchanged samples. The cation exchange was completed after washing and centrifuging five times with the appropriate solutions. The samples were then washed with distilled de-ionized water, transferred into dialysis tubes to remove the Cl- ions and dried at room temperatures. These samples will be referred hereafter as Li-mont and Cs-mont. The samples were heated to 300°C for 24 h in air, since these are the most common conditions used in literature for a complete migration of Li cations in the clay structure (Komadel et al. 2005). Those samples produced after heating at 300°C for 24 h in air will be called Limont-300 and Cs-mont-300. In addition, a 5 g Li-mont sample was subjected to a mild dehydration procedure at low temperature (60–80°C) using an appropriate doublewalled reflux apparatus (Gournis 1998). The sample was introduced at the center of the double-walled glass container. In the outer wall, n-butanol was refluxed. Adsorbed water was removed by vacuum pumping the sample over P2O5 as the dehydrating agent. The corresponding sample is named as dLi-mont. All samples were sealed in ampoules immediately after they were made and kept till they were loaded, inside an argon-purged glove box, in the vanadium holder just before the neutron experiments.

Methods Neutron diffraction data were collected on the high-resolution E9 powder diffractometer of the research reactor BER II in Berlin (BENSC/HMI) using wavelength ˚ . About 4.5 g of each sample was placed in a k = 1.7978 A cylindrical vanadium ([8 mm) holder, sealed with indium, and used for the neutron scattering experiments. The highresolution data were obtained at two measuring temperatures, 300 K and 10 K, over a wide range of scattering angles (0.016° B 2H B 158°) in 2H steps of 0.101°; every

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data set accumulation lasted for 10 h. In an additional experiment, neutron diffraction measurements at various temperatures between ambient and 300°C (100, 160 and 263°C) were carried out on the dLi-mont sample in order to study in situ the changes that occur in the structure of this sample due to heating and therefore identify the possible migration of Li through the crystal lattice. This sample was heated at each specific temperature for a duration of 2 h and subsequently, at the end of this period, data accumulation was commenced. Infrared reflectance spectra were measured with a Nicolet 550 infrared spectrometer, in the region of 400– 4000 cm-1, equipped with a DTGS detector. The samples were ground and pressed in pellet form, inside an argonpurged glove box, in order to obtain regularly flat surfaces suitable for specular reflectance measurements. Each spectrum was the average of 200 scans collected at 2 cm-1 resolution by means a SPECAC variable-angle attachment. All spectra were measured at room temperature, using unpolarized radiation at an incidence angle of 10° off normal, against a high reflectivity aluminum mirror.

Data analysis All powder diffraction data were analyzed with the Rietveld method (Rietveld 1969) employing the program FULLPROF (Rodriguez-Carvajal 1993). First, we had to verify if our diffraction data could suggest any deviation from the so far known space-group symmetry for the structure of this particular clay mineral, and, second, to study the interlayer cation migration as a result of thermal treatment. We employed all possible space groups belonging to the monoclinic symmetry, since most published data referred to 2:1 phyllosilicate family crystallize in this system. To fit the experimental points, free parameters for evaluation were the unit cell dimensions (a, b, c, b) and the coordinates (xi, yi. zi) of the atoms together with their site occupancies. For an acceptable convergence, other parameters such as peak broadening, peak shape, background, etc., had also to be fitted. With respect to the peak shape, the Thompson-Cox-Hastings pseudo Voigt function was used because it gave a better matching to the experimental diffraction lines, without extending the convergence time of this multi-parametric system. Note that special care is usually needed when molecular water is present in samples studied by neutrons. The reason is that the hydrogen of the water molecule is responsible for the observation of diffuse (incoherent) scattering and hence for the loss of structural information from the region of H2O. In such cases, hydrogen is usually replaced by deuterium leading to the desired coherent scattering. In the present study, such an

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isotopic substitution was not performed in the cases where interlayer water was present, as for example in samples Li-mont and Cs-mont materials. It would have provided enhanced contrast for the location of the exchangeable cations if it had been done. Of course, in the rest of the samples, the water molecules had already been removed either by heating or due to the specific apparatus used for dehydration. Any possible moisture re-absorption was avoided by keeping the samples in evacuated containers. However, hydrogen is also present in the hydroxyl groups, which altogether with a non-ideal crystallinity, a common feature of natural clay samples, is responsible for the considerable background seen in our neutron diffraction patterns. Various methods to fit the background were attempted, but the best results were obtained when the Fourier filtering technique was chosen. The results of the Rietveld refinements were then fed into the crystallographic modeling program ‘ATOMS’. Using the specific crystal system, the atomic positions and their occupancies, as they resulted from the best fit (evaluated by visually inspecting the difference profile as well as the R-factors and the v2) to the data, allows the drawing of a 3D representation of the unit cell contents (Fig. 3). The results of the crystal structure analysis are acceptable only if two criteria are fulfilled. The first criterion is the v2-goodness of fit, a measure of the deviation between the observed and calculated profile. In the present study, the acceptable values were around v2= 1.9–2.5. Sometimes, however, the Rietveld program on its effort to minimize the v2 gave a unit cell configuration that was not compatible with the common features of the 2:1 phyllosilicate structure. So the extra condition that the output of program ‘‘ATOMS’’ should have a physico-chemical meaning gave us the final confidence for the acceptance of the Rietveld analysis results.

Results and discussion The results of the Rietveld analysis from the diffraction patterns of Li and Cs montmorillonite, before and after heat treatment at 300 °C, are summarized in Table 1, while Tables 2 and 3 contain the atomic coordinates, Wyckoff positions and occupancies for these four samples. The corresponding diffraction patterns and unit cell representations appear in Figs. 2 and 3, respectively. From this study, we conclude that all samples crystallize in the C/2m space group of the monoclinic system. The Al, Mg and Fe cations of the unheated samples are located in the 4h octahedral sites, while the 2c octahedral sites are empty. The exchangeable cations Li or Cs (as expected) occupy the 2b site in the interlayer space. Also, some quartz (SiO2) exists as naturally occurring impurity phase in all samples


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Table 1 Crystallographic data for lithium and cesium montmorillonite samples before and after heat treatment at 300°C Sample

˚) a (A

˚) b (A

˚) c (A

b (degrees)

˚ 3) Unit cell volume (A

Position Li/Cs

Li-mont

5.175(1)

8.896(2)

12.45(1)

99.69(4)

565.1(4)

0, 0.5, 0

87(2)

Quartz

4.894(1)

4.894(1)

5.388(2)

a

111.74(3)

–

13(3)

Li-mont-300

5.171(1)

8.957(2)

9.74(1)

96.1(1)

448.5(6)

0, 0, 0.5

33(2)

Phase-B

5.196(1)

9.000(1)

10.96(2)

101.10(6)

505.8(8)

0, 0.5, 0

49(3)

Quartz

4.917(1)

4.917(1)

5.410(1)

a

113.36(2)

–

18(3)

Cs-mont

5.181(1)

8.945(1)

12.34(1)

99.62(3)

563.8(2)

0, 0.5, 0

84(2)

Quartz

4.906(1)

4.906(1)

5.404(1)

a

112.65(3)

–

16(3)

Cs-mont-300

5.191(1)

8.976(2)

11.32(1)

100.56(5)

518.5(4)

0, 0.5, 0

78(2)

Quartz

4.906(1)

4.906(1)

5.402(1)

a

112.62(3)

–

22(3)

a

Fraction (%)

a = b = 90° and c = 120°

Table 2 Atomic coordinates, Wyckoff positions and occupancies for Li-mont and Cs-mont Li-mont

Cs-mont

Atom

Wyckoff position

x

y

z

x

y

z

Occupancy

Si

8j

0.148(2)

0.163(1)

0.309(1)

0.140(1)

0.171(1)

0.302(1)

0.975

Al1

8j

0.148(2)

0.163(1)

0.309(1)

0.140(1)

0.171(1)

0.302(1)

0.025

Li or Cs

2b

0.000

0.500

0.000

0.000

0.500

0.000

0.08

Al2

4h

0.000

0.384(2)

0.500

0.000

0.374(2)

0.500

0.38

Fe

4h

0.000

0.384(2)

0.500

0.000

0.374(2)

0.500

0.05

Mg

4h

0.000

0.384(2)

0.500

0.000

0.374(2)

0.500

0.07

O1 O2

4i 8j

0.218(4) 0.406(2)

0.000 0.226(1)

0.261(2) 0.262(1)

0.217(4) 0.397(2)

0.000 0.218(1)

0.271(3) 0.248(1)

0.50 1.00

O3

8j

0.201(2)

0.215(1)

0.437(1)

0.199(2)

0.229(1)

0.430(1)

1.00

OH

4i

0.627(4)

0.000

0.387(3)

0.628(4)

0.000

0.392(2)

0.50

Table 3 Atomic coordinates, Wyckoff positions and occupancies for phase-A of Li-mont-300 and for Cs-mont-300 Li-mont-300 Atom

Wyckoff position

x

Cs-mont-300 y

z

x

y

z

Occupancy

Si

8j

0.179(2)

0.151(1)

0.283(1)

0.153(1)

0.171(1)

0.305(1)

0.975

Al1

8j

0.179(2)

0.151(1)

0.283(1)

0.153(2)

0.171(1)

0.305(1)

0.025

Li

2c

0.000

0.000

0.500

–

–

–

0.08

Cs

2b

–

–

–

0.000

0.500

0.000

0.08

Al2

4h

0.000

0.376(3)

0.500

0.000

0.369(2)

0.500

0.38

Fe

4h

0.000

0.376(3)

0.500

0.000

0.369(2)

0.500

0.05

Mg O1

4h 4i

0.000 0.107(2)

0.376(3) 0.000

0.500 0.192(2)

0.000 0.278(5)

0.369(2) 0.000

0.500 0.287(5)

0.07 0.50

O2

8j

0.442(4)

0.230(3)

0.236(3)

0.406(2)

0.240(1)

0.247(2)

1.00

O3

8j

0.195(4)

0.204(2)

0.444(1)

0.188(2)

0.238(2)

0.440(1)

1.00

OH

4i

0.713(6)

0.000

0.402(6)

0.660(4)

0.000

0.415(4)

0.50

(space group: P3221; ICSD #63532). The main diffraction line of SiO2 appears at 2H = 31.08°. In the unheated samples, the exchangeable cations Li and Cs occupy the (2b) Wyckoff positions in the above-

mentioned space group. The unit cell volume of the two samples differs by less than 0.2%, while their lattice dimensions are not similar (Table 1). The major deviation concerns the c-axis of the cells, which is directly related to

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Phys Chem Minerals (2008) 35:49–58 6000

8000

6000

Yobs Ycalc Yobs -Ycalc Bragg position

Li-mont-300

5000

Intensity (arb. units)

10000


Yobs Ycalc Yobs-Ycalc Bragg position

Li-mont

4000

3000

2000

4000

ph-A ph-B

1000 2000

0 20

40

60

80

100

120

140

160

30

180

60

150

180

Yo bs Ycalc Yo bs -Ycalc Bragg position

Cs-mont-300

7000 6000


10000


120

8000

Y obs Y calc Yo bs - Ycalc Bragg position

Cs-mont

12000

90

2 Θ (deg.)

2 Θ (deg.)

8000

6000

4000

5000 4000 3000 2000 1000

2000

0 0 20

40

60

80

100

120

140

160

180

2 Θ (deg.)

20

40

60

80

100

120

140

160

180

2 Θ (deg.)

Fig. 2 Rietveld analysis of neutron diffraction patterns for Li and Cs montmorillonites before and after heat treatment at 300°C. A naturally occurring quartz impurity phase is represented by the

second set of tick marks except in the case of Li-mont-300, which is shown in the third line down

the hydration sphere around the exchangeable cation; it is ˚ in Li-mont and 12.34 A ˚ in Cs-mont. found to be 12.45 A These values suggest a simple hydration layer around the metal-cations (Me´ring 1946). Differences are also observed, even before heating, in the a and b dimensions of the cells, with the values of Cs-mont being slightly larger than those of the Li sample. The explanation might be attributed to the different ionic radius r of the two cations ˚ for Li and 1.67 A ˚ for Cs). The size of the Cs atom (0.68 A is much bigger and the atoms are fixed near the hexagonal cavities leading to the well-known ‘‘cation keying’’ (Theng 1974), thus deforming the unit cell dimensions and resulting in the observed increase in the a and b-axis. The diffraction patterns of the samples, after heating to 300°C, give evidence for several changes that occur due to thermal treatment. Attempts to fit the diffraction patterns using only one phase were not satisfactory (v2 = 3.83, Rwp = 30.2, number of refined parameters: 31). In the case of Li-mont-300, we realized that the sample comprises a mixture of two phases. In an effort to fit the profile, various

configurations of the atomic positions for these two components were tried out. The model (v2 = 2.47, Rwp = 26.1, number of refined parameters: 56) that satisfied the abovementioned criteria of acceptance demanded the existence of two phases having similar structures, but with Li atoms lying in different crystallographic positions: phase-A (33(2)% wt), in which the recorded changes in the structure are the expected ones (e.g., contraction owing to heating and removal of H2O) and phase-B accounting for the 49(3)% wt. In the A-phase of Li-mont-300, the ion of Li migrates through the whole structure and occupies the 2c site. In other words, it moves through the Kagome´ cavity to the octahedral sheet and occupies the available vacant 2c octahedral sites of the di-octahedral clay structure. In the B-phase, Li atoms remain in the same 2b sites, as in the unheated sample. This assumption seems to be reasonable if we consider the existence of a degree of substitution also in the tetrahedral sheet of montmorillonite. In this case, the charge is localized and the exchangeable cations prefer to stay in the interlayer space (near the hexagonal cavities) in

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Li-mont

Li-mont-300 (phase-A)

Cs-mont

Cs-mont-300

Fig. 3 Unit shell representations, based on Rietveld results, of Li and Cs montmorillonites before and after heat treatment at 300°C

order to balance the localized negative charge. We would also like to note that the discrepancy in the Rietveld refined quartz volume fractions (Table 1), for as-obtained and heated samples, is due to a possible enhanced crystallization of the naturally occurring mineral at the elevated temperatures of the neutron diffractions experiments. In this case, the Rietveld fit appears to be able to determine the phase fractions more efficiently. A few other changes observed also in the structure of the materials after heating indicate a higher or lower degree of relation with the movement of lithium cations. For example, in the case of Li-mont-300, in phase-A a decrease is observed in the value of b angle followed by a considerable reduction in the c dimension, while the b dimension is increased. The interlayer space largely collapses inasmuch as the c dimension, describing the basal spacing d001 ˚ . On the contrary, in phase-B the values decrease to 9.74 A of b dimension and b angle increase, while the c dimension ˚ ), indicating that the is decreased to a lesser extent (10.96 A interlayer space does not collapse but is somehow retained by the lithium cations lying in the interlayer space of the mineral. These unit cell changes constitute a reduction in

the volume equal to 21% for the A-phase and 10% for the B-phase. The diffraction pattern of Cs-mont-300 was fitted using one phase with the Cs atoms remaining at the same (2b) crystallographic site after heating. In the case of Cs-mont, the decrease in the interlamellar space due to heating is partial, and it is possible that the large Cs ions are stacked into the Kagome cavities. Hence, the compensating charge remains still far from the octahedral sheet conserving a weak electric attraction. The values of a and b dimensions and the b angle are increased, compared with the unheated Cs-sample, while the c dimension is slightly decreased to ˚ ; a reduction of the unit cell volume equal to 8% is 11.32 A observed. The resultant values of crystal structure refinement for dLi-mont sample heated and measured in situ at different temperatures (RT, 100, 160 and 263°C) are compiled in Table 4. The corresponding diffraction patterns are shown in Fig. 4. The pattern of dLi-mont at RT was fitted using one phase having the Li atoms at the 2b Wyckoff position, while the patterns of the heated samples were impossible to model under the same condition (dLi-mont-100 v2 = 3.44; dLi-mont-160 v2 = 3.22; dLi-mont-263 v2 = 3.28). Thus, two-phase refinements were employed, namely: one phase with Li atoms placed in the 4g site, with coordinates (0, y, 0) or Li in 2c site, and a second phase with lithium atoms left over at the 2b (0, 0.5, 0) site. From the Rietveld analysis of the diffraction data and the evaluation of the quality of the fit factors, we conclude that Li ions, already from 100°C, quit the (0, 0.5, 0) position of C2/m space group and appear to be move along the y-axis until the temperature reaches 263°C. Around this temperature regime, the Li ions seem to acquire adequate thermal energy to be able to ‘‘hop’’ on the c axis and finally occupy the 2c lattice site (0, 0, 0.5). The above findings, for the diffusion mechanism, are in agreement with infrared (IR) measurements for the dLimont sample heated to the same temperatures as before for the NPD experiments. Specular reflectance spectra are sensitive to changes in the Si–O stretching vibration due to migration of small interlayer cations upon heat treatment of montmorillonites (Karakassides et al. 1997, 1999). Figure 5 shows the IR specular reflectance spectra of dehydrated Li+ and Cs+ montmorillonites before and after heat treatment at 100, 160 and 263°C for 3 h in air. The spectra exhibit two main reflectivity maxima at 1,050 and 1,130 cm-1, which have been assigned to characteristic vibrations of the Si–O bonds of the clay mineral structure (Karakassides et al. 1997, 1999). After heat treatment, small changes are observed only in the reflection spectra of the Li+ sample (Fig. 5a). Especially, the band at 1,130 cm-1 increases in intensity upon heat treatment, whereas the high frequency side of the 1,050 cm-1 band

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56


Table 4 Crystallographic data for a dehydrated lithium-montmorillonite sample heat treated at various temperatures between ambient and 263°C Sample

˚) a (A

˚) b (A

˚) c (A

b (degrees)

˚ 3) Unit cell volume (A

Position Li/Cs

Fraction (%)

(v2

Rawp

dLi-mont

5.182(1)

8.904(3)

10.942(2)

100.28(6)

496.8(2)

0, 0.5, 0

88(2)

2.42

28.8

12(1) 1.98

31.0

1.93

30.1

1.94

30.6

Quartz

4.913(1)

4.913(1)

5.404(2)

b

112.96(4)

–

dLi-mont-100

5.177(1)

8.942(2)

9.98(1)

95.31(8)

460.3(6)

0, 0.27, 0

46(3)

Phase-B

5.202(2)

8.995(3)

10.97(1)

100.54(8)

504.8(7)

0, 0.5, 0

37(3)

Quartz

4.925(1)

4.925(1)

5.411(1)

b

113.68(3)

–

17(1)

dLi-mont-160

5.178(2)

8.953(2)

9.86(1)

95.64(8)

454.8(7)

0, 0.18, 0

42(3)

Phase-B Quartz

5.210(2) 4.929(1)

8.991(2) 4.929(1)

10.96(1) 5.414(1)

100.58(7)

504.6(7) 113.91(3)

0, 0.5, 0 –

41(3) 16(1)

b

dLi-mont-263

5.181(2)

8.952(3)

9.86(1)

95.68(8)

455.2(7)

0, 0, 0.5

43(3)

Phase-B

5.209(3)

9.004(4)

11.02(2)

100.58(9)

508.1(9)

0, 0.5, 0

41(3)

Quartz

4.939(1)

4.939(1)

5.421(1)

b

114.51(4)

–

16(1)

a

Reliability factors for points with Bragg contributions for pattern

b

a = b = 90° and c = 120°

5000

Yobs Ycalc Yobs-Ycalc Bragg position


dLi-mont 4000


dLi-mont-100 4000


5000

3000

2000

3000

2000

1000

1000

20

40

60

80

2

100

120

140

160

20

180

40

60

80

2

(deg.)

100

120

140

160

180

(deg.)

5000

dLi-mont-160

4000


4000



3000

2000


dLi-mont-263

3000

2000

1000

1000

20

40

60

80

2

100

(deg.)

120

140

160

180

20

40

60

80

100

2

120

140

160

180

(deg.)

Fig. 4 Rietveld analysis of neutron diffraction patterns for a dehydrated lithium-montmorillonite sample heat treated at various temperatures between ambient and 263°C (RT, 100, 160 and 263°C)

123


57

undergoes small changes upon heating without changing its intensity. In contrast to the Li-montmorillonite, the Csmontmorillonite spectra remained unaffected by heating, as shown clearly in Fig. 5b. The lithium cations after moving from the interlayer space to the vacant octahedral sites can cause local charge compensation. The decrease in the layer charge due to the Li migration can affect the restoring forces in the SiO4 tetrahedron and in turn the infrared spectrum in the region of the Si–O vibrations (Karakassides et al. 1997, 1999). In these positions, the Li+ cations, by interacting with the negative charge of the octahedral sheet, reduce the magnitude of this charge on the octahedral sheet (Jaynes and Bigham 1987) and cause a weakening of the restoring forces in the M–O bonds (M = Al, Mg, Fe). The decrease in the octahedral sheet charge must influence the Si–Oapical vibrations, since the apical oxygens are shared by octahedral and tetrahedral sheets. Thus, in the bridging M–Oapical–Si configuration, a reduction in the total negative charge and a transposition of the remaining charge is expected. From the gradual increase in the intensity of the 1,130 cm-1 band, we conclude that Li ions are

(a)

Reflectivity

263 °C 160 °C

displaced from their original position after heating to 100°C and keep moving until the temperature rises to 263 C, in agreement with neutron diffraction findings.

Summary Neutron powder diffraction demonstrates that montmorillonite crystallizes in the monoclinic C/2m space group, with the Al, Mg and Fe cations residing in the 4h octahedral sites, whereas the 2c octahedral sites are vacant; the exchangeable cations, Li or Cs, occupy the 2b site in the interlayer space. Furthermore, we have shown that when Li-mont is heated, the interlayer space collapses as a fraction of the exchanged small cation diffuses into the clay structure and finally resides in the empty 2c octahedral site (compensating the charge deficit due to Al+3 substitutions by divalent ions), while the rest remains in the interlayer space of montmorillonite. The Li migration into the octahedral sheets describes a possible mechanism for the wellknown fixation of Li-mont after heating. In the case of Cs-mont, the reduction of the inter-lamellar space due to heating is partial and the large Cs ion is found immobilized in the Kagome´-like cavity of the phyllosilicate structure. As a result, the compensating charge remains far from the octahedral sheet, in effect conserving a weak electric attraction in the layers and therefore permitting re-hydration and re-expansion of the clay lattice under the appropriate conditions.

100 °C

Acknowledgments The experiments at BENSC in Berlin were supported by the European Commission under the Access to Research Infrastructures Action of Human Potential Programme (contract: HPRI-CT-1999-00020).

RT

References

(b)

Reflectivity

1050

1300

1130

1200

1100

1000

900

800

-1

Wavenumbers (cm )

Fig. 5 Infrared reflectance spectra of dehydrated materials: (a) dLimont and (b) dCs-mont, before and after heat treatment at different temperatures (100, 160 and 263°C)

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