Geometric Structures of Lanthanide Ions within Layered Clays as

9850

J. Phys. Chem. 1994,98, 9850-9860

Geometric Structures of Lanthanide Ions within Layered Clays As Determined by EXAFS: From the Lu(II1) Hydrate to the Disilicate Adela Muiioz-Ptiez,*gtMaria Dolores Alba, Miguel Angel Castro, Rafael Alvero, and JosC Maria Trill0 Departamento de Quimica Inorghnica, Instituto de Ciencia de Materiales, Universidad de Sevilla, Consejo Superior de Investigaciones Cientijkas, P. 0. Box 553, 41012 Sevilla, Spain Received: April 14, 1994; In Final Form: July 8, 1994@

The interaction of multivalent rare earth cations with clay minerals is of importance for the design of new materials based on layer silicates. The effect of thermal and hydrothermal treatments upon the local structure around lutetium ions intercalated within the interlamellar space of montmorillonite has been studied by X-ray absorption spectroscopy (EXAFS). The initial Lu(II1) hydrate (7.7 Lu-0 at 2.30 A), formed after the ion exchange process, loses one water molecule after air-heating at 300 "C (6.5 Lu-0 at 2.29 A) and is fully disrupted by air heating at 500 "C. After this treatment oxide- and hydroxide-type environments are detected, characterized by higher shells (Lu-Lu at 3.3 A and Lu-0 at 4.1 A) and distortions in the first Lu-0 contribution (5.9 Lu-0 at 2.22 A). At 700 "C the transformation concerns all lutetium atoms that now show the oxide structure (5.4 Lu-0 at 2.22 A and 2.8 Lu-Lu at 3.33 A). The spectra recorded after thermal treatment at higher water vapor pressure (100 atm) can be well fitted with only three shells corresponding to the disilicate phase. It is concluded that the local structure around interlamellar Lu(II1) changes from that of the hydrate, in the untreated sample, to that of the disilicate, the latter quantitatively and at a temperature of 400 "C. This temperature is considerably lower than the lowest one previously reported (900 "C). EXAFS, frequently used in other fields, has been shown as a unique technique to study the evolution of intercalated Lu(III), thus complementing the results obtained by other techniques in previous studies.

Introduction Smectite clays consist of negatively charged sheets of aluminosilicates separated by an interlayer space where chargebalancing, exchangeable hydrated cations are located and whose thickness changes upon absorption of polar solvents. Knowledge of the structural effects occumng upon heating smectites intercalated with cations and polyoxocations is incomplete and not well founded. Even in the notable case of the Greene-Kelly mineralogical test,l results leading to a definitive explanation of the lithium migration mechanism in the montmorillonite matrix have only very recently been reported2 at the laboratory where its first experimental evidence was observed by F. Gonzalez more than four decades ago.3 In addition to its academic interest, the interaction of multivalent rare earth cations with clay minerals is of importance for the design of new materials, such as solid acid catalysts4 and components for highly radioactive nuclear waste repositor i e ~ .The ~ effects of heating treatments on the local structure of interlamellar lanthanum6,' and lutetium8 intercalated within the layer of smectites have already been reported. They have proved to be more remarkable than e ~ p e c t e d .In ~ the case of lutetium, even the generation of a disilicate phase has been observed after hydrothermal treatment at 400 "C, from X-ray powder diffraction, energy dispersive X-ray microanalysis, and magic-angle spinning nuclear magnetic resonance.8 The structural study of layered silicates intercalated with diverse cations is a classical topic in solid state physical chemistry. However, the application of extended X-ray absorption fine structure and near edge structure (EXAFS and XANES) spectroscopies (techniques frequent in other fields) is not widespread within it. These techniques are quite appropriate t e-mail: [email protected]. @

Abstract published in Advance ACS Abstracts, August 15, 1994.

to observe the local environment of interlamellar cations because of their sensitivity to local order. Moreover, the tunability in energy provided by Synchrotron sources makes these techniques specific for the target element. The much lower contribution to the absorption coefficient of the lighter elements comprising the silicate matrix provides sensitivity to the intercalated element even when it is in minor concentration. For these reasons, EXAFS andor XANES have been used to probe the local structure of transition metal cations intercalated within clays. Thus, preliminary measurements of ironexchanged vermiculites were carried out more than 10 years ago.1° In that article, the only one that explored the effects of heating treatments, analysis of the XANES region gave information about changes in the oxidation state of iron. EXAFS has been used in several works, generally focusing on the ionic species just after the sorption process. The most recent publication includes a study of cupric ions within a smectite clay' and considers the clay as merely a support for the hydrated ion. In agreement with this, Manceau and Calas'* found that Ni(I1) ions retain the hydrate structure when intercalated in several phyllosilicates. Polynuclear species were identified both in the pillaring solution and in the chromia pillared clay catalyst by Bornholt et al.13 As in the previous cases, the first coordination shell around the metal cation was formed by an octahedron of oxygen atoms. In comparison, ion exchange of silicates with silver led to cation environments with much smaller coordination number, approaching two oxygens, resembling the immediate environment of Ag in A g ~ 0 . l ~ X-ray absorption spectra of intercalated rare earth ions are more scarce. This may be due to two intrinsic difficulties in the study of these ions: the singularity of the expected structure, based on irregular polyhedra of 6 to 9 oxygen atoms around each cation, and the spread of distances for each shell. To our knowledge, in addition to our previous publication^^^^ there are

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0022-365419412098-9850$04.50/0 0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 39, 1994 9851

Geometric Structures of Lanthanide Ions only two works using EXAFS to study rare earth cations intercalated within silicates, and in neither of them has the effect of heating treatments been studied. The first one includes the analysis of the local structure around interlayer ions in lanthanide-exchanged vermiculite as a function of ion size;15 the second, the study of the uranyl ion, U022+, in solution and sorbed onto silica and montmorillonite, as a function of the pH of the solution.I6 The aim of this paper is the EXAFS study of the local environment of Lu(II1) ions intercalated within the interlayer space of a montmorillonite, a dioctahedral member of the smectite clays. The effects of thermal treatments in air at low water pressure, which are of importance in the demand of new acid catalysts, have been studied. Additionally, a study of hydrothermal treatments under water pressures close to those expected in the projected radionuclide repositories with clay backfill has been included.

EXAFS Data Analysis. A general expression for the measured EXAFS ~ ( khas ) been given by Stem et aLZo X(k) = x A j ( k ) sin(2kRj

+ #@))

i

The EXAFS function is a superimposition of contributions from different coordination shells; the index J refers to the jth coordination shell; Rj is the average coordination distance between the absorbing atom and the neighboring atoms in the jth coordination shell, @(k) is the phase shift which the photoelectron experiences during the scattering process, and A,( k ) is the amplitude function, which is expressed as in the following equation:

Experimental Section Material. The fraction of a Trancos montmorillonite mineral from Gador, Almerfa (Spain), with particle diameter smaller than 2 p,was selected (MT). The structural formula of the obtained montmorillonite, together with the result of the GreeneKelly test, has already been published by Trillo et al.17 After removal of carbonates and organic matter, the sample was saturated with sodium. This Na-montmorillonite, called NaMT, was used as a starting material for the preparation, by ionic exchange, of the montmorillonite saturated with Lu(II1) according to the method described by Miller et al.18 The sample thus prepared was dried at room temperature and will be called untreated or LUMTRT. Other portions of the lutetium-saturated sample were submitted to heating treatments in air for 6 h at increasing temperatures: 300 "C (hereafter called LUMT300), 500 "C (LUMTSOO), and 700 "C (LUMT700). An additional portion of the sample was submitted to hydrothermal treatment consisting of heating under a water vapor pressure of 10 MPa at 400 "C for 24 h (hereafter called LUMT400HT). These last conditions have been chosen to approximate the real ones at the projected nuclear wastes repositories. EXAFS Measurements. X-ray absorption spectroscopy (XAS) measurements were carried out at the EXAFS Station 8.1 of the SRS at Daresbury Laboratories (U.K.). The storage ring was operated at 2.0 GeV with a maximum stored current of 250 mA. The station was operating with a double-crystal monochromator Si[220], which was detuned 80% in intensity to minimize the presence of higher harmonics. In the absence of a lutetium metallic foil, the monochromator was calibrated using a foil of a transition metal with the closest value of the K edge to that of the Lu LIIIedge, that is, copper (Cu K edge = 8979 eV, Lu LIIIedge = 9250 eV). Resolution was estimated to be -1 eV at the Cu K edge by a well-defined shoulder in the main edge jump of the Cu foil. Data were collected in the transmission mode using ionization chambers filled with Ar/ He mixtures (IO= 68.7 Torr of Ar, It = 495.4 Torr of Ar; He was added to bring the total pressure to atmospheric) as detectors. Each data point was collected for 0.75 s, and at least four scans were averaged, thus minimizing high- and lowfrequency noise. Samples were pressed into thin self-supported wafers using BN when necessary. The thickness of the wafers was chosen to give an absorbance (p)of 2.5, ensuring an optimum signal to noise ratio. The spectra were recorded at liquid-nitrogen temperature in an EXAFS sample cell that allowed in situ thermal treatment of the sample under a controlled atmosphere. l9

N, is the average number of scatterer atoms in the Jth coordination shell; a, is the mean square deviation about the average coordination distance R (caused by thermal motion a n d or static disorder); Fj(k) is the backscattering amplitude function characteristic of a particular type of neighboring atom; So2(k) is a correction for the relaxation of the absorbing atom and multielectron excitations; A is the mean free path of the photoelectron; A is a correction term (A % R1) in the mean free path concept, which is used because So2(k)and FJ@)already account for most of the photoelectron losses in the first coordination shell. The expression is valid for the case of the small disorder with a Gaussian pair distribution function. The EXAFS functions, ~ ( k )of, the Lu Lm edge were obtained from X-ray absorption spectra by subtracting a Victoreen curve followed by a cubic spline background removal. Normalization was performed by division by the height of the edge.21 EOwas defined as the maximum in the first derivative of the absorption edge. EXAFS parameters N (coordination number), A d (DebyeWaller factor), R (coordination distance), and AEo (inner potential corrections) were obtained from the (un)filtered EXAFS function by nonlinear least squares fitting procedures using the program NEWEXAFS from the University of Eindhoven. No deglitching of the EXAFS function was carried out. Although the use of the phase shifts and backscattering amplitude functions obtained from experimental spectra is more desirable,22 these functions have been obtained theoretically because no compounds with well-defined Lu-X (X = 0, Si, Lu) bonds were available. Of the algorithms proposed, we have used that of J. Rehr et al.23 (FEFF program) because it has been shown to be the most reliable in general, and particularly in the case of L edges. Errors in the structural parameters were calculated from the covariance matrix, taking into account the statistical noise in the experimental EXAFS spectra and the correlation between the refined parameters. The quality of the fit is quantitatively expressed by means of the values of the goodness of fit ( E ~ ~ ) calculated as outlined in the report on standards and criteria in EXAFS spectro~copy:~~

€1'v3=-c 1

NP &i(exptl)-Xi(model))2

i=l

Y = degrees

aj(exptl)2

of freedom; N p = number of independent points.

,

9852 J. Phys. Chem., Vol. 98, No. 39, 1994

Raw data of some of the samples were separated from the remaining background by Fourier filtering. The range of this filtering process (Ak, AR) determines the number of independent points Np in the spectrum and thus the number of shells according to the Nyquist theorem:25

Np= 2AkhRIn

+1

The degrees of freedom, u,calculated taking into account the number of fitting parameters, P , and the number of independent points, Np (v = N p - P), determine the goodness of fit (cv2) values, as outlined above.

Results Figure 1 shows the unfiltered EXAFS functions of the thermal and hydrothermal samples. Although the signal/noise ratio is far above detection limits, the EXAFS data could be used only up to k = 12 A-1 due to the appearance of the white line of the Lu LII edge at this energy. Remarkable differences, in both amplitude and node positions, between the thermal and hydrothermal samples can be appreciated in this figure. The simplest and most intense spectrum is that of the untreated sample. The thermal treatment at 300 "C (Figure lb) causes a general decrease in the EXAFS spectrum, which is still very simple, probably due to a unique contribution. Node positions remain unchanged, thus indicating that the sample has undergone only small changes in the structure around Lu(II1) ions. Comparison of the uncorrected Fourier transform of both spectra (Figure 2a,b) leads to the same conclusion: there is a single peak in both cases, centered at the same value (-1.8 A), with the only difference being a lower intensity in the sample heated at 300 "C. In samples heated at 500 and 700 "C, together with the decrease in signal intensity, some interferences appear at 5 and 7 Figure lc,d, indicating the existence of additional contributions. Confirming this hypothesis, in the uncorrected Fourier transform, Figure 2c and 2d, a second peak appears at -3.0 A and the main peak is slightly shifted to lower distances. The hydrothermal treatment causes a general increase in the intensity of the EXAFS function (Figure le) and changes in the imaginary part of the Fourier transform, as well as shifting of the second peak (Figure 2e). Considering the oxidation state of lutetium and the position of the main peak in the Fourier transform, the principal contribution to the EXAFS spectrum should correspond to backscattering from oxygen atoms. Although less intense, the second peak can still be quantitatively analyzed. Given the complexity and singularities of the geometrical arrangement around lanthanide ions in general and Lu(II1) in particular, the stable coordination environments for Lu(1II) ions in several compounds showing Lu-0 bonds will be presented in a first stage. There are several possibilities for stable coordination environments around Lu(II1) ions involving oxygen atoms: the hydrate or aquocomplex, the hydroxide, and the oxide. As Lu(111)ions are intercalated within a silicate network, the formation of a compound containing silicon has also been considered. The aquocomplex is of particular interest in this study since it is the stable form of the Lu(II1) ions in aqueous solution and, at a first approach, its structure is not expected to be disrupted during the impregnation process. In this complex the first coordination sphere around Lu(1II) ions is formed by an average of eight water molecules at a distance of 2.33 A.26 In contrast with La(II1) ions, the coordination around Lu(II1) ions in the hydroxide (see Figure 3) is quite different from that of the aquocomplex, which allows discrimination between the two

Muiioz-PBez et al.

TABLE 1: Crystallographic Data of Lu3+ Crystalline Compounds compound model ABS-SCA pair" R (4 N [L~(Hz0)d~+ 1 Lu-0 7.9 2.33 6.0 2.24 Lu(OH)3 I1 Lu-0 LUzo3

Lu~Si207

I1 I11 I11 I11 IV IV IV

Lu-0 Lu-0 Lu-Lu Lu-Lu Lu-0

Lu-Si Lu-Lu

4.03 2.23 3.44 3.94 2.24 3.49 3.53

6.0 6.0 6.0 6.0 6.0 6.0 2.0

ref 26 27 28 9

Absorbing-scatterer pair. structures. That of Lu(OH)3 can be approximated as a slightly distorted octahedron of oxygen atoms around the central Lu(111) ion, linked to another six octahedra (three within the unit cell),27where each oxygen atom is coordinated to two lutetium atoms. Note that the structure is very open, and thus (see Table 1) no Lu-Lu contributions appear below 4.0 A. Three nonequivalent Lu atoms at the center of slightly distorted octahedra form the oxide (shown in Figure 4). Each oxygen is shared by three lutetium atoms, forming a structure more compact than the previous one: the basic octahedra are smaller and are closer to each other. There are 12 Lu-Lu contributions below 4 The Lu-silicate compound investigated (the disilicate LuzSi207 plotted in Figure 5 ) is the only member of the family of disilicate structures stable from room temperature up to the melting point of the c o m p ~ u n d .It~ can be described as a type C structure, a hexagonal, close packing of the oxygen atoms containing lutetium cations in the octahedral holes and silicon in the tetrahedral holes, in alternating parallel layers (001). The (SiO4) tetrahedra show a very low degree of distortion compared to other disilicate configurations. In Figure 5a, where the view from the z axis has been plotted, the six Lu-0 bonds have been drawn (dotted lines from lutetium atoms to the comer of tetrahedra). In the same scheme, a coordination number of 2 for Lu-Lu contributions is clearly seen. Table 1 presents a summary of the crystallographic data of model compounds LuzO3, Lu(OH)3, and LuzSi207, as well as the structure of the aquocomplex. It can be seen that all the compounds imply a basic polyhedron of six to eight oxygen atoms around Lu(1II) ions. The structures of the solid compounds included in Table 1 have been simplified by averaging the nearest distances. Data analysis of the EXAFS spectra of lutetium-exchanged montmorillonite samples (LUMT) has been carried out in two steps. In the first, they were compared with spectra calculated with the structural parameters of the crystalline or aqueous compounds described above. Once it was determined which compound they were more similar to, the iterative fitting procedure was carried out leaving the structural EXAFS parameters floating around the values in the known compounds. Debye-Waller factors and inner potential corrections were left free, although care was taken to keep them similar for similar bonds (see below). Fitting ranges, degrees of freedom, u,and the goodness of fit (€2)values of the samples LUMmT, LUMT300, LUMT500, LUMWOO, and LUMT4HT are listed in Table 2. The final results of the EXAFS data analysis are presented in Table 3. The standard deviations given for each coordination parameter are calculated from the covariance matrix, including the noise level obtained from estimates of the signallnoise ratio present in the scans. Comparison of the fit with the raw data in k space and in R space (k2 weighted) for samples LUMTRT and LUMT300 is

. I Phys. . Chem., Vol. 98, No. 39, 1994 9853

Geometric Structures of Lanthanide Ions

0.4 I

I

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Ir:

*

h

v

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0.0

4

6

-

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8

10

-0.6

2

n

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12

t 2

I

1

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I

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k (A-') Figure 1. EXAFS spectra ( k weighted) of lutetium montmorillonite samples submitted to the following treatments: untreated (a), heated in air at 300 "C (b), 500 "C (c), and 700 "C (d) and heated at 400 "C at a pressure of 100 atm for 24 h (e).

shown in Figures 6 and 7. As expected from visual examination of the EXAFS spectrum, the first two samples can be fitted satisfactorily by a single shell. The untreated sample closely resembles the aquocomplex (model I), in both coordination

distance (2.30 vs 2.33 A) and coordination number (7.7 vs 7.9). In the sample heated at 300 "C, all the fitting parameters are similar, the only remarkable change being the decrease in coordination number. As usual in EXAFS, higher accuracy is

9854 J. Phys. Chem., Vol. 98, No. 39, 1994

Muiioz-P6ez et al. 0.2

0.3

E

2:

0.0

0.0

-0.2

-0.3 0

1

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R (A) Figure 2. Fourier transform of EXAFS spectra included in Figure 1 (I?, Ak = 3.2-12.0

attained in coordination distance (error