J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2783-2790 EXAFS Data Analysis for Lanthanide Sesquioxides P. Malet, M. J. Capitan, M. A. Centeno, J. A. Odriozola and 1. Carrizosa Departamento de Quimica lnorganica e lnstituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-C.S.l.C., P.O. Box 553,41071 Sevilla, Spain The EXAFS spectra of lanthanide sesquioxides (LU,~,, Sm203, La203) at the lanthanide L,, ,-edge are analysed. The complex coordination polyhedron around the lanthanide cation is modelled with the minimum number of coordination shells that still allows the acquisition of coordination numbers and shell distances in agreement with the radial distribution functions from crystallographic data. Theoretical phase shifts and backscattering amplitudes with an amplitude reduction factor, S: = 0.73, are reliable for reproducing the experimental EXAFS data.A model with one Lu-0 and two Lu-Lu shells simulates the Lu coordination polyhedron in C-Lu,03, while for La,O, the model includes two oxygen shells for simulating the nearest neighbours and one longer distance that averages that of the La-La pairs. The coordination around Sm in Sm,O, is the most complex and two-shell models are needed to simulate the Sm-O and Sm-Sm absorber-backscatterer pairs. The models obtained are applied in the EXAFS analysis of dispersed Ln,O,/AI,O, samples, where X-ray diffraction fails to detect the structure adopted by the lanthanide phase. The results show that in an Sm,O,/AI,O, sample calcined at 800 "C, very small Sm,O, particles are formed.In an La,O,/AI,O, sample with low loading the analysis procedure allows the detection of the aluminium atoms that are present with the oxygens around the lanthanum ions, thus suggesting the incipient formation of a bidimensional LaAIO, phase. The local structure around the lanthanide atom is often of nation polyhedra around the lanthanide atoms in the Ln203 interest in non-crystalline and highly dispersed systems '-, phases with a limited number of shells, thus allowing the such as halide glasses (fibre optics and lasers), metallic glasses analysis of the EXAFS spectra of the unknown compound to (high strength, low-density materials) or, from a most general be made with a feasible number of parameters.point of view, disordered system^.^ However, examples of the In previous papers7-' we have reported EXAFS data application of the EXAFS technique to systems in which analyses for Sm,O,/Al,O, samples using phase and ampli- lanthanide atoms are involved are scarce in the literature. On tude functions calculated from McKale et uZ."*'~ for the the other hand, the structure of supported rare-earth-metal various contributions, which were shown to be reliable. oxides is a major concern in surface chemistry and cataly- However, their use has recently been discouraged12 and the sis.*-' Thus, the addition of small amounts of these oxides to use of the FEFF code', to calculate phase and amplitude y-Al,O, inhibits its sintering process at high temperatures. A functions is recommended. surface compound, usually amorphous in structure, has been In this paper we analyse the EXAFS spectra of Ln203 ses- suggested to be responsible for such behavio~r.'.~ EXAFS quioxides (LU,~,, Sm203, LU,~,) using theoretical phase spectroscopy is well suited to this structural problem.shifts and backscattering amplitudes calculated from FEFF Changes in the catalytic performance of supported rare-and propose a reasonable model that simulates the compli- earth-metal oxides in reactions such as the oxidative coupling cated structure of these compounds with the minimum of methane7** and hydrogenation over Ln,O,-promoted number of coordination shells, still yielding coordination rhodium catalysts' could be ascribed to changes in the struc- numbers and shell distances in agreement with the RDF ture of the highly dispersed or amorphous supported phase.obtained from crystallographic data. These models and the However, difficulties in the interpretation of the EXAFS parameters of the different coordination shells obtained to fit spectra of rare-earth-metal oxides have limited, to our know- the spectra could be used as inputs in the analysis of the ledge, the application of the technique to these systems. EXAFS spectra of materials with unknown local structure The analysis of the EXAFS spectra of crystalline com- around the lanthanide. pounds of lanthanide elements is not straightforward, even The physical meaning of the parameters obtained in the when the local structure around the lanthanide atom is analysis of the experimental spectra with a limited number of known.EXAFS spectra are measured at the lanthanide LI,,- shells and the errors introduced by approximating several edge, and the data range available is limited by the super- distances to only one shell, have been studied by analysing position of the L,,-edge, especially for elements with the theoretical EXAFS spectra including all the distances present lowest atomic numbers. The limited data range results in a in the crystalline structures. Alternative models, their sta- low resolution when using Fourier-transform (FT) techniques tistical significance and physical meaning, have also been to obtain the radial distribution function (RDF) around the considered. lanthanide.Moreover, RDFs deduced from crystallographic data for the oxides show that coordination polyhedra around Experimentalthe lanthanide atoms are complex, having short Ln-0 and Ln-Ln distances that extend over a rather wide range. Two X-Ray absorption spectra of La,O,, Sm,O, and Lu,O, main problems arise from the complex structures of the bulk from Sigma Chemical Co. calcined at 900 "C were recorded at compounds that are also expected in the dispersed systems. the L,,, absorption edge of the lanthanide (La, 5491 eV, Sm, Crystalline oxides cannot be used as EXAFS reference com- 6721 eV and Lu, 9250 eV). The Lu,O, spectrum and those of pounds to obtain experimental phase shifts and backscatter- the Ln,O,/Al,O, samples were measured at station EXAFS ing amplitudes for Ln-0 and Ln-Ln absorber-backscatterer I11 at LURE DCI, Orsay (France), while Sm203 and La203 pairs, and theoretical references have to be used.The second spectra were recorded at station 8.1 at the SRS, Daresbury problem is that it is necessary to model the complex coordi- Laboratory (UK). Monochromatization was obtained using double silicon crystals working at the (311) reflection at LURE and the (111) reflection at the Daresbury laboratory. The measurements were carried out in transmission mode using optimized ion chambers as detectors. Samples were placed on Kapton Sellotape with an absorbance of ca. 2.5 (Ap, < 1) just above the absorption edge, and measured at room temperature. Analysis and handling of the EXAFS spectra were carried out by using the program NEWEXAFS from the Eindhoven University of Technology.This program uses standard procedures to extract the EXAFS spectra from the measured absorption data. l4 Normalization was achieved by dividing by the height of the absorption edge. EXAFS data were analysed by multiple-shell fitting in k and R spaces using phase shifts and backscattering amplitudes calculated from FEFF.' Standard deviations of fitted parameters were calculated for a mean noise level of Estimated system- atic errors in R (* 1-3%) and N (f10-15%) are, in general, higher than the calculated standard deviations. In some cases, theoretical spectra including all the absorber-backscatterer distances in the crystallographic radial distribution functions were generated.Debye-Waller factors and AEo values were assumed to be zero in these calculations. Results and Discussion Radial distribution functions (RDF) around the lanthanide ions in the sesquioxides (Table 1) were calculated from crys- tallographic data."-" The simplest RDF corresponds to Lu203 which has a C-type structure under atmospheric pres- sure.15 In C-Ln203 structures six oxygen atoms appear around each Ln atom in an almost regular coordination J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 sphere, while two different Ln-Ln distances, each of them with a coordination number of six, form the second coordi- nation shell. As shown in Table 1 there are two types of cationic sites with slightly different coordination environment and a relative occupation of 1 :3.In the rather complex Sm,O, structure16 three types of Sm atoms with equal rela- tive abundance are found. In the RDF around the Sm atoms oxygen neighbours appear at distances of up to 2.755 A, with an 0:Sm coordination number of seven, while Sm neigh- bours appear in the range 3.3-4.2 A with an Sm :Sm coordi- nation number of 12. In La203, only one type of La atom is found with seven oxygen neighbours at the nearest distances and 12 La atoms between 3.76 and 3.94 A.17 Fig. 1 shows the unfiltered oscillatory EXAFS functions, Ak), at the lanthanide L,,-edge for Lu203, Sm203 and La,O,, as well as the imaginary part and the absolute value of their associated k3-weighted FTs. Owing to the presence of the L,,-edge, the available k range decreases in the order Lu > Sm >La, leading to a parallel decrease in the intensity and resolution of the associated FTs.In agreement with the radial distribution functions calculated from crystallographic data, peaks in the uncorrected FT at ca. 2 and 3.5 A have been assigned to oxygen and lanthanide coordination shells. Oxygen neighbours at distances longer than 3.5 A scarcely contribute to the whole EXAFS spectrum, which is domi- nated by the heavy lanthanide atoms at long distances.'*" EXAFSData Analysis LU203 As shown in Fig. 1, in the Lu case maxima are well resolved in the FT and therefore the oxygen coordination shell can be Table 1 Radial distribution functions around lanthanide atoms as calculated from crystallographic data (distances in A) LU~O~' 0 6 x 2.242 Lu 6 x 3.433 6 x 3.931 1 x 2.249 2 x 2.286 0 1 x 2.481 2 x 2.554 1 x 2.703 1 x 3.593 1 x 3.342 2 x 3.633 Sm 2 x 3.713, 2 x 3.736 2 x 3.854, 1 x 3.858 2 x 4.178 1 x 2.372 2 x 2.373 0 1 x 2.452 3 x 2.726 1 x 3.678 3 x 3.768 La 3 x 3.866 6 x 3.940 3 x 4.599 0 6 x 4.641 3 x 4.791 a Ref.15. 'Ref. 16. Ref. 17. Smz03* 4 x 2.213 2 x 2.276 2 x 3.922 2 x 3.433, 4 x 3.451 2 x 3.931, 4 x 3.947 1 x 2.288 2 x 2.317 1 x 2.375 2 x 2.485 1 x 2.755 1 x 3.884 1 x 3.342 2 x 3.633 2 x 3.692, 2 x 3.743 2 x 3.782 1 x 3.869 2 x 4.178 1 x 2.260 2 x 2.277 1 x 2.306 2 x 2.564 1 x 3.111 1 x 3.797 -1 x 3.618, 2 x 3.633 2 x 3.736, 2 x 3.743 2 x 3.782 2 x 3.854, 1 x 3.869 La2OJC J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.5 0.0 h3 x 4 -0.5 -1 .o 5 10 15 2 4 6 k/A-' RIA Fig. 1 A, Experimental EXAFS spectra for Lu203 (a), Sm203(b) and La,O, (c) and B, associated k3-weighted FT (Ak ranges are 2.0-16, 2.2-12 and 2.2-9.9 A-' for Lu203, Sm,O, and La203, respectively) isolated by Fourier filtering. A direct transformation was per- formed in the 1.9-14.9 k' range using a k2 weighting scheme followed by an inverse Fourier transformation in the 0.20-2.3 A interval. The isolated oscillations are plotted in Fig. 2 and can be fitted by a single Lu-0 coordination shell at 2.22 A, in good agreement with the bond length calculated from crystallographic data (Table 1).The fit was forced to be good in k and R spaces, and working with k' and k3 weigh-ting schemes, since it has been shownlg that strongly coupled parameters such as coordination number (N) and Debye- Waller factor (Aa2) or shell distance and AEo are better decoupled with this procedure. Theoretical phase shift and backscattering amplitude functions were calculated with the FEFF pr~grarn,'~ with an input value of ACT' = 0 (absolute Debye-Waller factor). Theoretically it is expected that an amplitude reduction factor, Sg, is required to correct the cal- culations for the decrease in the overlap of the passive elec- trons between the initial and final states of the absorbing atom, its value lying in the 0.6-0.8 range at k > 7 A for most of the atoms.'8b.20 This factor was calibrated by setting Sg = 1 as a first value to calculate the backscattering amplitude, yielding an 0 :Lu coordination number of 4.39 & 0.02.Since the crystallographic value is six, Sg should be set to 0.73. This value is close to that previously determined by the same procedure' when using McKale's phase shifts and back- scattering amplitudes, and lies within the range reported for most of the elements and close to those reported for other lanthanide elements (Pm, 0.729; Yb, 0.758;18'). Therefore an Si value of 0.73 will be employed in further calculations. Once the Lu-0 shell had been fitted the difference between the raw spectrum and the fitted Lu-O oscillation was calcu- lated (dotted lines in Fig.2). This difference mainly includes Lu-Lu contributions at 3-4 A. Lu-Lu contributions were isolated by performing a k2-weighted FT in the 3.6-15.6 A-' range and an inverse FT between 2.2 and 4.2 A, and are plotted in F@. 3. An FT of these isolated oscillations cor- rected by the Lu-Lu phase shift and backscattering ampli- tude [Fig. 3(b)] shows two clear maxima at ca. 3.5 and 3.9 A, very close to those determined from the diffraction data (Table 1). The oscillations can be fitted by employing two shells at 3.44 and 3.93 8, (Fig. 3), leading to a total Lu :Lu coordination number of 13, in agreement (within the preci- sion of the EXAFS technique) with the crystallographic value (12.0).Best-fit parameters and associated standard deviations are given in Table 2. In summary, a one-shell model for the Lu-0 and a two-shell model for the Lu-Lu contributions have been found to be adequate to simulate the mean coordi- nation polyhedron around Lu atoms in the Lu20, structure. Sm203 Since the k range for the analysis is limited by the presence of the Lredge, which lies 600 eV beyond the Lnredge, some overlapping occurs between the two main contributions in the uncorrected FT. The best way to isolate them has been found by generating theoretical spectra that include all the 1 A 41 nSO x .y -1 k/A-' 5 I0 15 Fig. 2 Lu203: Isolated EXAFS oscillations for the Lu-0 first shell klA-' and associated k2-weighted FT. (-) Experimental data; (---) fit Fig.3 Lu203: Isolated EXAFS oscillations for Lu-Lu shells and with a one-shell model; (. * .) difference spectrum (total experimental associated k2-weighted FT (Ak = 4-15.5 A-') corrected by Lu-Lu phase shift. (-) Experimental data; (---)spectrum minus fitted Lu-O shell). Table 2 coordination N 0 6.0 f 0.02 Lu 7.2 f 0.09 Lu 5.8 & 0.13 fitted data. EXAFS parameter values for crystalline compounds: Lu203 Aa2/A2 RIA AE'IeV 0.0053 f O.OOO1 2.22 f0.001 -7.5 f0.1 0.0046fo.Ooo1 3.44 f0.001 -4.1 f0.1 0.0047 f O.OOO1 3.93 f0.001 -7.4 f 0.2 S: has been set to 0.73 (see text). 2786 I I 0.2 0-4 0.3 0.10.2 E z-s 0.1 + x 0.0* 2 c,0.0 .-ij L -0.1 -0.1 93 -0.2 -0.3 L-L-2-L -0.2 2 4 6, 8 1012 k/A-' Fig.4 Sm,O,: (a) EXAFS oscillations for Sm-0 nearest-shell con- tributions and (b) associated k'-weighted FT (Ak = 3-11.5 A-'). (-) Experimental data; (---) fit with a two-shell model. distances in Table 1, and performing FTs with different k weighting schemes with or without correction by phase shifts and backscattering amplitudes. The Sm-0 contributions due to oxygen neighbours at dis- tances up to 3.04 A, were isolated by performing a k'-weighted Sm-0 phase-corrected FT (Fig. 4, Ak = 2.1-12.3 A-', AR = 0.96-3.10 A). Two alternative models (Table 3) will fit these Sm-0 oscillations. The one-shell model yields an 0 :Sm coordination number (6.2) below that known from X-ray diffraction (XRD) data (7.0) at the shorter Sm-0 dis- tance expected.The alternative two-shell model (Table 3, Fig. 4) gives a total 0 :Sm coordination number of 7.0 and Sm-0 distances in agreement with the values expected from XRD data. The reliability of the two-shell model for Sm-0 dis-tances up to 3.04 A has been checked by generating a theo- retical EXAFS spectrum that includes all the Sm-0 distances shown in Table 1 and groups the shells step by step, avoiding distortions of total coordination numbers and mean dis-Table 3 Sm203: different models for Sm-0 shells (a) Experimental EXAFS oscillations N Ao2/A2 RIA AE'jeV one-shell model O(1) 6.2 0.00822 2.34 -4.2 two-shell model W)O(2) 4.3 2.7 0.00575 0.02307 2.33 2.53 -3.5 -13.3 (b) Theoretical EXAFS oscillations N Ao/A2 RIA AE'IeV 3.66 O.OO0 99 2.291 0.2 2.35 0.001 61 2.527 -0.1 0.67 O.OO0 72 2.728 0.0 0.33 O.OO0 00 3.111 0.0 3.66 O.OO0 99 2.29 1 0.2 2.83 0.005 75 2.532 1.2 0.33 O.oO0 00 3.111 0.0 4.04 0.001 95 2.285 1.o 2.5 1 0.00378 2.506 3.5 4.98 0.009 93 2.276 5.2 Si has been set to 0.73 (see text). J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tances. Oxygen atoms between 2.249 and 2.375 A can be grouped in only one shell with an 0:Sm coordination number of 3.66 and a mean Sm-0 distance of 2.291 A, in agreement with XRD data. Sm-0 distances of 2.481-2.564 can also be grouped in one shell at 2.527 A with a total 0 :Sm coordination number of 2.35.This four-shell model (Table 3) reflects the deviations in Sm-0 distances around the mean values with increased Debye-Waller factors and slight deviations in AEo values. A three-shell model groups the central 0 shells at one distance leading to a slight under- estimation of the total coordination number, further increases in the Debye-Waller factors and deviations in the AEo value of the central shell. Errors in the mean Sm-0 distances determined with this model including only three shells are <0.04 A. Alternative models with one and two shells have also been considered. The third shell with an 0 : Sm coordi- nation number of only 0.333 is expected to be missed in the fitting of the experimental spectrum.A low 0 :Sm coordi- nation number is expected when fitting the whole spectrum with only one shell, as observed in the fitting of the experi- mental spectrum. Once the Sm-0 contributions had been fitted they were subtracted from the experimental spectrum. Sm-Sm contri-butions [Fig. 5(u)] were isolated from the difference spectrum by performing a k3-weighted Sm-Sm phase-corrected FT (Ak = 4.0-12.3 A-') and an inverse FT in the 2.7-4.5 A range. The analysis of the isolated oscillations yields two Sm shells, with a total Sm: Sm coordination number of 13.1 (expected value 12), and shell distances within the ranges observed by XRD. The analysis procedure is summarized in Table 4. A first shell is introduced at cu. 3.65 A. After refine- ment of this contribution, an Sm-Sm phase-corrected FT of the difference spectrum suggests the presence of, at least, one more Sm-Sm shell at cu.4.2 A. This new shell was introduced and the parameters of both shells refined, leading to the fit shown in Fig. 5(b). The reliability of this two-shell model has been checked by generating a theoretical EXAFS spectrum which includes all the Sm-Sm distances and groups the theoretical shells step by step (Table 4). In a five-shell model deviations in Sm-Sm distances around the mean values would be reflected only in increased Debye-Waller factors. The three-shell model aver- ages the central Sm distances leading to a slight underesti- mation of the total coordination number, a further increase in the Debye-Waller factor and deviations in AEo values.Errors in the mean Sm-Sm distances determined with this 1 ' LA-4-5 46 8 10122 4 6 k/A-' RIA Fig. 5 Sm,O,: (a) EXAFS oscillations for Sm-Sm contributions and (b), associated k3-weighted FT corrected by Sm-Sm phase shift (Ak = 4-11.5 A-'). (-) Experimental data. (---) Fitted data with a two-shell model. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Sm20, :different models for Sm-Sm shells (a) Experimental EXAFS oscillations one-shell model Sm(1) 9.2 0.00897 3.64 -1.3 two-shell model Sm(1) 8.5 0.00779 3.64 -1.4 Sm(2) 4.6 0.00697 4.16 -8.4 (b) Theoretical EXAFS oscillations ~~ 0.67 0.000 00 3.342 0.0 2.33 0.00003 3.631 0.0 5.35 0.OOO 86 3.740 0.0 2.33 0.00004 3.859 0.0 1.33 0.000 00 4.178 0.0 0.72 0.00251 3.320 6.0 7.90 0.006 01 3.682 4.4 2.72 o.oO0 75 4.122 8.2 8.68 0.006 28 3.666 6.6 2.43 0.000 16 4.130 6.5 model including only three shells are <0.06 A.Since the first Sm-Sm shell, with the lowest coordination number, seems to be missed in the fit of the experimental spectrum, an alterna- tive two-shell fit of the theoretical spectrum was performed (Table 4), a slight underestimation of the total coordination number results, while errors in Sm-Sm distances are kept within kO.07 A. The two-shell model for Sm-Sm and Sm-0 absorber-backscatterer pairs was considered to be adequate. To avoid errors in the coordination parameters that may be introduced during the isolation procedures, the set of parameters obtained in the analysis of the isolated EXAFS oscillations was used as input data to be refined in the analysis of the complete experimental EXAFS spectrum (Fig.6) filtered by performing an uncorrected k3-weighted FT (Ak = 2.12-1 1.96 A-') and an inverse FT (AR = 1.2-4.3 A). Refined param- eters are included in Table 5, and, taking into account the complexity of the system, their agreement with the RDF obtained from crystallographic data is excellent, providing a good model for the analysis of this type of compound when using theoretical phase shifts and backscattering amplitudes calculated from FEFF. A comparison of this model with that previously reported' for the analysis of the EXAFS spectrum of Sm203 with McKale's phase shifts and backscattering amplitudes shows that fit parameters obtained with both models are in agree- ment within the precision of the EXAFS technique.In general, higher Debye-Waller factors are calculated when using McKale's tables. The main differences are found in the parameters determined for Sm-Sm shells. The fit achieved 2 4 6 81012 2 4 k/A-' RIA Fig. 6 Sm20,: Fit of the whole EXAFS spectrum. (a)EXAFS oscil- lations; (b) uncorrected k3-weighted FT (Ak = 3-11.5 A-'). (-) Experimental data; (---) theoretical data (EXAFS parameters in Table 5). with McKale's tables requires three Sm-Sm shells to repro- duce the experimental ~pectrum,~ and overestimates the coor- dination number at the shortest distance (1.7 Sm neighbours at 3.35 A) when compared with the crystallographic value (0.67 Sm neighbours at 3.342 A).On the other hand, this short Sm-Sm distance is omitted when fitting with the phase shifts and backscattering amplitudes calculated from FEFF, which may be justified by its low coordination number. In this case the isolation of both contributions was not attempted, since a theoretical EXAFS spectrum including all the distances in Table 1 shows that there is severe overlap- ping between both maxima in the uncorrected FT. The pres- ence of the L,,-edge at 410 eV beyond the L,-edge leads to a limited data range in k space and thus to poorly resolved FTs. Moreover, contributions to the FT due to oxygen neigh- bours between 4.5 and 4.8 A are in this case alsounresolved, appearing as a weak shoulder at long R values, with the maximum at ca.3.8 A. A detailed analysis of the theoretical EXAFS spectrum generated by including all the La-0 and La-La distances in Table 1 shows that nearest oxygen shells give rise to two dis- tinct maxima in the 0.5-1.5 8, range (uncorrected), and can be modelled by two different La-0 distances at 2.38 and 2.72 A, with coordination numbers of four and three, respectively (Table 6). Destructive interferences were found between the EXAFS oscillations generated by these two La-0 subshells. The analysis of the theoretical spectrum also shows that the positions of La-La absorber-backscatterer pairs between 3.768 and 3.940 A can be approximated to only one distance at 3.90 and those of oxygen neighbours at longer distances can be approximated to one shell at 4.62 A.As shown in Table 6, a model including two shells for the shortest La-0 distances, and two more shells for La-La and long La-0 absorber-backscatterer pairs leads to mean distances and coordination numbers in agreement with XRD data within the precision of the EXAFS technique. Deviations in Table 5 EXAFS parameter values for crystalline compounds: Sm,O, coordination N Ao2/A2 RIA AE'IeV 0 4.3 f0.3 0.0057 f0.o006 2.32 f0.006 -0.8 f0.5 0 2.7 f0.3 0.0231 & 0.0010 2.49 f0.01 -15.0 f0.8 Sm 7.5 f0.1 0.0069 k0.0001 3.64 +_ 0.001 -0.5 & 0.1 Sm 4.6 f 0.1 0.0064 f0.0003 4.15 +_ 0.003 -7.2 f0.2 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 EXAFS parameter values for crystalline compounds: La,O, (a) Theoretical EXAFS oscillations four-shell model 0 4.0 0.001 56 2.38 0.4 0 3.1 O.OO0 90 2.72 1.o La 13.1 0.006 57 3.90 -1.4 0 11.9 0.00461 4.62 1.o three-shell model 0 4.0 0.001 56 2.38 0.4 0 3.1 O.OO0 90 2.72 1.o La 13.0 0.005 76 3.93 -4.3 (b) Experimental EXAFS oscillations three-shell model 0 4.1 f0.4 0.0112 fo.oO06 2.39 f0.01 2.0 f0.4 0 3.3 f0.5 0.0055 f0.0010 2.72 f0.01 3.2 f0.8 La 7.1 f0.1 0.0074 f O.OO01 3.87 f0.001 1.4 f0.1 four-shell model 0 4.1 f0.6 0.01 12 f0.0010 2.39 f0.01 2.0 0.6 0 3.3 f0.9 0.0055 f 0.0015 2.72 f 0.02 3.2 f1.5 La 7.2 f 0.1 0.0076 0.0002 3.86 f0.001 2.3 f0.1 0 4.8 f0.2 0.0096 kO.OO08 4.64 f 0.005 1.5 & 0.2 Sg has been set to 0.73 (seetext).absorber-backscatterer distances around the mean values are crystallographic distance, the low La :La coordination reflected in increased Debye-Waller factors and slight devi- number should be noted. An even lower La :La coordination ations in AEo values. Discarding the long La-0 distances in number was reported' when analysing the La203 EXAFS the three-shell model (Table 6) introduces larger errors, spectrum with McKale's phase shifts and backscattering although still tolerable, in the parameters determined for the amplitudes. Several possibilities have been considered to La-La absorber-backscatterer pairs. This analysis of a theo-explain this severe error.Interferences between the waves retical EXAFS spectrum leads to the conclusion that the originating from La-La shells and those originating from three-shell model should be adequate for the analysis of the La-0 shells at distances >3.5 A, not considered in the experimental data of La203. model, can be discarded. Long La-0 distances were not The analysis of the experimental EXAFS spectrum with included in the three-shell model used to analyse the theoreti- two La-0 and one La-La shell leads to the parameters col- cal spectrum, and the oxygen neighbour at 3.678 A was dis- lected in Table 6, which provide an excellent fit of the experi- carded in both the three- and four-shell models, leading in all mental data in k and R space (Fig.7). As expected from the cases to slightly overestimated La :La coordination numbers. analysis of the theoretical spectrum, the total 0 :La coordi- As expected, the inclusion of a fourth La-0 shell accounting nation number is 7.4 at the two closest distances (2.39 and for the 12 oxygen neighbours in the range 4.5-4.8 A (see 2.72 A), which agrees within experimental error with the crys- Table 6) does not improve significantly the La :La coordi- tallographic values. However, although the value for the nation number. In our view, the high reactivity towards H20 La-La distance (3.87 A) is also in agreement with the mean and C03 of the La203 when exposed to the atmosphere should not be forgotten since, although the La,O, sample was calcined immediately before its use, it is difficult to avoid contamination during handling and recording of the EXAFS 4 spectrum.Fig. 8(u) compares the EXAFS spectra for a just calcined La203 sample and an La203 sample that had been exposed 2 to the atmosphere for six months. Changes in the coordi- nation of lanthanum ions after exposure to the atmosphere n 30 are evident; thus, at long distances [see Fig. 8(b)], the m maximum at ca. 3.5 A in the uncorrected FT, ascribed to -Y La-La absorber-backscat terer pairs, decreases in intensity -2 and a new maximum appears at ca. 4 A. These changes may be ascribed to the formation of lanthanum hydroxy-carbonates. The crystalline structure of lanthanum hydroxy- -4 carbonate phases can be understood as a regular succession 246810 2 4 6 down the main crystallographic axis of a planar distribution k1A-l RIA of alternating [La(OH)2+], layers and C0,'-anions, Fig.7 La,O,: fit of the whole EXAFS spectrum. (a) EXAFS oscil-resulting in a separation between the [La(OH)2+], layers of lations; (b) uncorrected &'-weighted FT(A&= 3-9.5 A-1). (-) cu. 5 A.2' The hydration of the A-type Ln203 samples has Experimental data; (---) theoretical data (EXAFS parameters in been demonstrated to occur through the penetration of H20 Table 6). along the ternary axis of the oxide, which causes an increase J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 l*OI Fig. 8 La,03: changes in the EXAFS spectrum after exposure to the atmosphere; (-)just calcined sample; (---) sample exposed to the atmosphere for six months.(a)EXAFS oscillations; (b)associated k3-weightedFT at long distances (Ak = 3.0-9.9 A-'). in the cell volume.22 This topochemical mechanism is also present in the substitution of C1-, NO3-and/or SO4'-anions in lanthanide hydroxy Taking into account the above evidence, we propose that La,03 undergoes a trans- formation during the EXAFS experiment in which [Lao];' layers are separated by OH-and/or CO,'-anions, accord- ing to a model proposed earlier,24 resulting in a structure similar to that found in LaOCl (BiOCl type). If this is the case, and taking into account the fact that the layers are formed by [OLa,] tetrahedra sharing edges, we should expect that the La3+ ions are surrounded by eight oxygen ions at distances of around 2.40 A and four lanthanum ions at 3.90 A.The low La-La coordination number should then be ascribed to the incipient transformation of the La203 sample into an oxohydroxide during the recording of the EXAFS data. Application to Dispersed Systems The EXAFS parameters obtained in the analysis of the spectra of the bulk sesquioxides can be used to model the coordination polyhedra around the lanthanide ions in the analysis of the EXAFS spectra of unknown systems. The examples selected are two Ln,O,/Al,O, samples where X-ray diffraction fails to detect the structure adopted by the lantha- nide phases, since XRD patterns contain only lines that can be ascribed to y-Al,O, .Fig. 9(a) shows the EXAFS spectrum at the Sm L,-edge for a 5 mol% Sm,03/A1,03 sample, which was obtained by a -1.0 1 1" _-1 4 6 8 10 12 2 k/A-RIA Fig. 9 (a)Experimental EXAFS oscillations (-) and best-fit func- tion (---) for a 5 mol% Sm,03/Al,0, sample. (b)Uncorrected k3-weighted FTs for Sm2O3/Al2O3 (-) and bulk Sm203 (---, Ak = 2.1-12 A-' in all of the FTs). coprecipitation method' and calcined at 800 "C, and its associated k3-weighted FT [Fig. 9(b), solid line]. The uncor- rected k3-weighted FT for bulk Sm203 has also been included for comparison, showing that the local surroundings of the Sm atoms are similar in both compounds, although the maximum at cu. 3.5 A, ascribed to Sm-Sm absorber-backscatterer pairs, has almost vanished in the Sm,03/A1,0, sample, thus indicating the presence of small Sm,O, particles in the coprecipitated sample. The EXAFS spectrum of Sm,03/Al,03 was analysed by introducing, as a first guess, the set of parameters obtained for the bulk oxide.The ampli- tudes of the shells (Nand Aa') were first allowed to change to obtain the best fit of the experimental data, and then AEo and R were taken into the refinement. The set of parameters obtained and the best-fit function are included in Table 7 and Fig. 9(a) dashed line. Fig. 1qa) includes the EXAFS spectrum at the La Lmedge for a 10% w/w La,03/A1,0, sample, which was obtained by an impregnation method' and calcined at 900 "C. When com- paring its associated k3-weighted FT [Fig. lqb)] with that of bulk La203 [Fig.lqc), solid line], the main differences are found in the high4 region, where the maximum at ca. 3.5 A ascribed to La-La absorber-backscatterer pairs has almost disappeared in the supported sample. However, attempts to fit the EXAFS spectrum of La,O,/Al,O, with only two short La-0 distances cannot account for the peak at ca. 3 in the uncorrected FT, thus suggesting the presence of new neigh- bours in the lanthanum coordination sphere. These new neighbours have been identified as aluminium atoms by com- parison with the EXAFS spectrum of a 40% w/w Table 7 EXAFS parameters for Ln203/A1203 samples coordination N Aa2/A2 RIA AEo/eV ~ ~ ~~ ~~ 0 4.7 f 0.3 0.0106 f o.Oo04 2.38 f 0.004 -1.1 f 0.4 0 1.9 f 0.4 0.0267 f 0.0015 2.70 f 0.016 -2.6 f 1.5 Sm 2.7 f 0.1 0.0096 f 0.0003 3.72 0.002 2.3 f 0.3 10% La,O,/Al,O, 0 4.4 f 1.0 0.0094 f 0.0019 2.50 f 0.02 1.7 f 0.3 0 3.8 & 1.0 0.0055 f 0.0035 2.71 f 0.02 2.4 f 0.8 A1 3.9 & 0.1 0.0140 f 0.0002 3.34 f 0.002 -6.4 f 0.2 40% La,O,/Al,O, 0 5.0 & 1.4 0.0042 f 0.0019 2.48 & 0.02 4.4 f 0.5 0 7.4 f 1.9 0.0032 f 0.0030 2.70 f 0.02 1.5 f 0.6 A1 6.2 f 0.2 0.0015 f O.Oo04 3.30 & 0.004 -2.3 f 0.3 La 7.0 f 0.2 0.0068 f 0.0002 3.74 f 0.002 0.0 f 0.1 2790 1.a 1 00.5 E-1 5 h rc v)?5 0.0 w 55 U .-z L-0.5 JO9 -1 .o -5 Fig.10 (a) Experimental EXAFS oscillations (-) and best-fit function (---) for a 10% w/w La,O,/AI,O, sample.(b) Uncorrected k3-weighted FT for 10% w/w La,03/Al,03. (c) Uncorrected k3-weighted FT for bulk La,O, (-) and a 40% w/w La,O,/AI,O, sample (---, Ak = 2.6-9.8 A-' in all of the FTs). La,O,/Al,O, sample [Fig. lqc), dashed line] prepared by the same procedure, where the formation of an LaAlO, phase can be unambiguously stated by XRD. The analysis of the EXAFS spectra for the latter sample (Table 7) yields an 0 : La coordination number of 12.4 and the presence of La-A1 and La-La absorber-backscatterer pairs at 3.30 and 3.74 8, respectively, which are the main species responsible for the peaks at ca. 3 and 3.5 A in the uncorrected FT. These results are in agreement with XRD data and with the RDF, which can be calculated from crystallographic data for LaA10,,25 where A1 and La neighbours appear around the lanthanum atoms at mean distances of 3.28 and 3.79 A, respectively.Comparison of the uncorrected FTs for the 10% and 40% w/w La,O,/Al,O, supported samples allows us to ascribe the maximum at ca. 3 8 in the sample with low loading to A1 neighbours, thus suggesting the incipient for- mation of a bidimensional LaA10, phase around the lantha- num atoms in this sample. The best-fit function obtained by considering two La-0 and one La-A1 shell is included in Fig. 1qa) and corresponds to the EXAFS parameters given in Table 7. Conclusions We have shown that L,,,-edge EXAFS spectra of Ln,O, oxide systems, having a complex structure, can be modelled with a feasible number of shells even in the case of lantha- num, for which the energy difference between the L, and L,,, absorption edges is only ca.400 eV. Theoretical phase shifts and backscattering amplitudes calculated from FEFF, with an amplitude reduction factor of Sg = 0.73, are shown to be reliable in the modelling of the experimental spectra, thus overcoming the difficulty of finding EXAFS reference com- pounds with regular coordination around the lanthanide element. The obtained coordination polyhedra, which model the complex coordination around the lanthanide atoms in the oxides, are found to be useful as initial models for obtaining J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 information on the local order around rare-earth-metal ions in highly dispersed samples of unknown structure.In La,O,/AI,O, samples, the analysis procedure allows the detection of aluminium atoms, which are present around lanthanum ions alongside oxygen atoms. 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