J. Phys. Chem. B 2001, 105, 6805-6811
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Evidence of Edge-Sharing TiO5 Polyhedra in Ti-Substituted Pollucite, CsTixAl1-xSi2O6+x/2 Nancy J. Hess,* Yali Su, and M. Lou Balmer† Physical Chemistry of Materials, Pacific Northwest National Laboratory, Richland, Washington 99320 ReceiVed: January 24, 2001; In Final Form: May 2, 2001
The compositional series CsTixAl1-xSi2O6+x/2, 0 e x e 1, was investigated using X-ray absorption and Raman spectroscopy. The data indicate the formation of TiO5 edge-sharing polyhedra at relatively low Ti concentrations (x ) 0.3). The presence of TiO5 polyhedra in the compositional series is evident from the intensity and energy value of characteristic preedge features in the near-edge structure of the X-ray absorption spectra (XANES). The edge-sharing geometry of the TiO5 polyhedra is determined by the analysis of the extended X-ray absorption fine structure (EXAFS) that indicates short (3.0 Å) Ti-Ti interatomic distances. The appearance of features in the Raman spectra at 645 and 717 cm-1 also support the existence of edge-sharing Ti polyhedra at low levels of Ti substitution. The EXAFS and Raman results suggest a nonrandom pairing of TiO5 polyhedra on symmetrically equivalent sites and the formation of edge-sharing relationship between adjacent TiO5 polyhedra.
Introduction Silicotitanates have generated much interest as a result of their interesting structures and catalytic properties. In an effort to understand the chemical activity of silicotitanates the structures of these materials have been thoroughly investigated. The vast majority of these materials are framework structures consisting of corner-sharing silica and titania polyhedra. Silica is almost always present as SiO4 tetrahedra. On the other hand, titania can occur in three polyhedral configurations, TiO4, TiO5, and TiO6. However, TiO6 polyhedra predominate in silicotitanates and TiO5 polyhedra are rarely observed. The aluminosilicate pollucite, CsAlSi2O6, has been considered as an alterative waste form to borosilicate glass for the encapsulation of 137Cs isolated from waste streams generated by the reprocessing of spent nuclear fuel.1 Since crystalline silicotitanates are highly selective for separating Cs from Narich waste streams, CsTiSi2O6.5, is a particularly attractive material.2,3 Pollucite belongs to a family of cubic aluminosilicates based on a SiO4 and AlO4 tetrahedral framework. The framework consists of tetrahedra that are linked by sharing corners to form four-, six-, and eight-membered rings that form cages capable of accommodating large ions such as Cs+. Limited ordering of the AlO4 and SiO4 tetrahedra among topologically equivalent tetrahedral sites in cubic pollucite has been indicated in the NMR investigations of Teertstra et al.4 The authors saw indications of Loewenstein Al-O-Al avoidance5 and the extended Loewenstein rule6 restricting Al-O-Si-O-Al linkages in pollucite. However, powder X-ray diffraction7 and highresolution neutron powder diffraction8 investigations of cubic pollucite have not revealed evidence of ordering of Al and Si on the symmetrically equivalent tetrahedral sites. Initial studies on Ti-substituted pollucite have demonstrated that Ti4+ can substitute for Al3+ and the charge is balanced by the incorporation of extra O2- anions into the structure.9-12 * Corresponding author. E-mail:
[email protected]. Fax: 509/3721632. † Current address: Caterpillar Inc., Technical Center - E/845, P.O. Box 1875, Peoria IL 61656-1875.
Neutron and X-ray diffraction results10 identified three possible sites for the additional oxygen atoms and suggested that Ti substitution resulted in the formation of TiO5 polyhedra. Fivecoordinate Ti is consistent with earlier Ti K-edge XANES analysis of CsTiSi2O6.5 samples.13 However, the connectivity of the TiO5 polyhedra to the rest of the structure remained undetermined. Subsequent 29Si NMR experiments on CsTiSi2O6.5 samples suggested the existence of edge-sharing Ti polyhedra based on a detailed analysis of the possible stable bonding configurations.11 A recent Raman investigation of a series of silicotitanates of known structure demonstrated that different structural motifs have distinctive Raman signatures.14 The Raman study also suggested the possible existence of edgesharing Ti polyhedra in the Ti-substituted aluminosilicate pollucite.14 A complete solid solution series, CsTixAl1-xSi2O6+x/2, where 0 e x e 1, has been synthesized.15 The lattice parameters and enthalpies of drop solution were measured for this suite of Ti-substituted pollucite compositions. The measured lattice parameters indicate that the CsAlSi2O6 endmember is only slightly distorted from the cubic structure (c/a ≈ 1.0014). However, the structure becomes strictly cubic with substitution of Ti4+ for Al3+. The cell parameter, a, and the cell volume, V, increase with increasing Ti substitution, which is consistent with the replacement of Al3+ cations with a larger Ti4+ cation and the incorporation of extra O2- anions in to the structure. However, the variation of a and V as a function of Ti substitution is not linear. Instead, there is a negative deviation from linearity (a decrease in cell volume) for sample compositions in the range 0.3 < x 0.3), the oxygen atom distribution has reversed between the two Ti-O bond distances. The approximate 0.15 Å difference between the two Ti-O bond lengths is smaller than the 0.3 Å difference determined for other structures in which the TiO5 polyhedra adopts a squat square pyramidal configuration (Na2TiSiO5 and Ba2TiSi2O8)23-26 or a highly distorted trigonal bipyramid configuration (K2Ti2O5).27 This observation suggests the formation of a more equidimensional TiO5 polyhedra in the Ti-substituted pollucite. The 0.15 Å difference between Ti-O bond lengths is comparable to the 0.14 Å difference between the two Al-O bond lengths in the AlO5 polyhedra found in andalusite,20 Al2SiO5, suggesting that an equidimensional 5-coordinate polyhedra is structurally reasonable. Commensurate with changes in the oxygen coordination environment are changes in a minor peak in the Fourier transform (Figure 4), at an apparent 2.4 Å distance, that represents the cationic interatomic distance between adjoining polyhedra. Fits to the EXAFS, summarized in Table 1, indicate that this peak is composed of two interatomic distances with backscattering atoms of different chemical compositions. These two interatomic distances consist of Al or Si backscattering atoms at ∼2.65 Å and a Ti backscattering atom at ∼3.0 Å. It is not possible to distinguish between Al and Si backscattering atoms because the backscattering amplitude and phase functions for each element depends on the atomic number.28 As a result, the Al and Si backscattering amplitudes and phase functions are similar. However, since the backscattering amplitude and phase function for a Ti atom differs significantly from those of Al and Si, it is possible to distinguish Ti from either Al or Si backscattering atoms. The relationship between these two interatomic distances and chemical composition are more easily understood in the r-space representation shown in Figure 6. With increasing Ti content, Ti backscattering becomes the dominant contributor to the Fourier transform peak. Since Ti backscattering atoms are at a longer interatomic distance than the Al/Si
TiO5 Polyhedra in Ti-Substituted Pollucite
Figure 6. r-space representation of the Ti-Al/Si and Ti-Ti interatomic distances that comprise the minor peak in the Fourier transforms in Figure 4. The thinner line represents the contribution from the shorter Al/Si backscattering atoms, and the thicker line represents Ti backscattering atoms. At high Ti concentration the Ti-Ti backscattering is the main contributor to the minor peak, but at low Ti concentration Al/Si backscattering dominates.
backscattering atoms there is a shift in the position of the minor peak in Figure 4 to longer distances as the Ti content increases. This shift indicates a transition in the local Ti environment with increasing Ti content from a state with adjoining Al/Si tetrahedra at 2.65 Å to a state with adjoining Ti polyhedra at about 3.0 Å. Within the CsAlSi2O6-pollucite structure, the shortest Al/ Si-Al/Si interatomic distance is ∼3.0 Å, a distance that corresponds to adjacent corner-sharing tetrahedra. Therefore, the existence of Al/Si atoms at a 2.65 Å interatomic distance from Ti is not consistent with corner-sharing tetrahedra. Al/ Si-Al/Si interatomic distances on the order of 2.7 Å are present among edge-sharing silicate and aluminosilicate polyhedra in the structures of stishovite29 (SiO6-SiO6) and andalusite20 (AlO5-SiO4). Therefore, the presence of the short Ti-Al/Si interatomic distance in the Ti-substituted pollucite structure suggests that an edge-sharing relationship exists between Ti-O and Al/Si-O polyhedra. Similarly, the ∼3.0 Å Ti-Ti interatomic distance is not consistent with known structures of corner-shared Ti polyhedra, but it is consistent with edge-shared TiO5 polyhedra. Structures with adjacent 5-coordinate Ti atoms are relatively uncommon. One such structure, K2Ti2O5, contains sheets of both corner- and edge-sharing TiO5 polyhedra.27 In K2Ti2O5, the average Ti-Ti interatomic distance in the cornersharing geometry is 3.8 Å whereas the edge-sharing geometry interatomic distance is 3.0 Å. Structures with neighboring TiO6 polyhedra are more common and are found in the titanate rutile and anatase structures.19 In these structures the Ti-Ti interatomic distances for corner- and edge-sharing configurations are 3.56-3.78 and 2.96-3.04 Å, respectively. Thus, the Tisubstituted pollucite EXAFS results are consistent with an edgesharing relationship between Al/Si and Ti polyhedra adjacent to TiO5 polyhedra. The existence of a Ti-Ti shell at low Ti concentration is significant because it suggests nonrandom distribution of Ti atoms within the pollucite aluminosilicate framework. If TiO5 polyhedral substitution for AlO4 occurred randomly in the pollucite structure, then the likelihood of occurrence of two adjacent TiO5 polyhedra would increase with increasing Ti
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6809
Figure 7. Raman spectra of the compositional series CsAl1-xTixSi2O6+x/2 where 0 < x < 1. As the Ti content increases, the intensity of the 474 cm-1 mode, which is characteristic of the Si-O bonds in CsAlSi2O6, decreases. The Raman features at 645 and 717 cm-1 are indicative of an edge-sharing relationship between Ti polyhedra.
substituion. However, this is not observed. The EXAFS fitting results in Table 1 suggest that there is little change in the number of paired TiO5 polyhedra at Ti substitution compositions greater than x ) 0.3. A possible reason for preferential TiO5 pairing is described below. RAMAN SPECTROSCOPY. The Raman spectra of the Tisubstituted pollucite compositional series are shown in Figure 7. The spectrum of CsAlSi2O6 (x ) 0) has one prevalent band at 474 cm-1 and a minor band at 392 cm-1, both of which are typical of Si-O bonds. The Raman spectrum of Ti-substituted pollucite (x ) 1.0) has at least four more bands than the aluminosilicate pollucite, presumably due to the distorted Ti and Si local environments. The spectral features and corresponding structural units of titanosilicate pollucite are discussed in detail in a separate publication.14 In summary, bands at 645 and 717 cm-1 indicate the presence of edge-shared TiO5 units. Bands at 915 and 960 cm-1 are due to symmetric siliconoxygen stretching motions [O3Si-O]δ--[TiO4]δ+. The strong band at 510 cm-1 is assigned to a Si-O-Si stretch. It can be seen from Figure 7 that with increasing Ti substitution, the prominent band indicative of the aluminosilicate pollucite at 474 cm-1 diminishes and features typical of the Ti endmember increase. Bands representative of edge-shared TiO5 units (645 and 717 cm-1) and Ti-O-Si bonds (915 cm-1) can clearly be seen across the compositional range 0.1 < x e 1. At the lowest Ti substitution (x ) 0.1) the band at 915 cm-1, which is characteristic of Ti-O-Si bonding, is clearly present; however, bands at 645 and 717 cm-1 due to edge-shared TiO5 polyhedra are not discernible. It cannot be determined from examination of the Raman spectra alone whether these latter bands are not perceptible over the background due to the low concentration of Ti and low Raman scattering cross-section or whether edge-sharing TiO5 polyhedra do not exist at dilute Ti substitutions. Discussion Analysis of the XANES indicates that Ti maintains a 5-coordinate local environment throughout the compositional series. However, analysis of the EXAFS shows that changes in the geometry of the Ti-O coordination environment occur with changing pollucite chemical composition. One might envision
6810 J. Phys. Chem. B, Vol. 105, No. 29, 2001 that at the Ti endmember composition, the Ti atom defines its own lowest energy configuration, consisting of two oxygen shells at 1.80 and 1.96 Å, whereas at the Al compositional extreme state Ti substitutes into a coordination environment that is more suited for Al. However, substitution within the compositional series requires that edge-sharing, 5-coordinate Ti replace corner-sharing, 4-coordinate Al in the aluminosilicate framework. Therefore, even at low Ti concentrations, the presence of Ti may require a significant rearrangement of the Al/Si tetrahedra. At low Ti concentration the adjacent edgesharing polyhedra is likely to be an Al/Si tetrahedra. Since formal charge considerations dictate that over-bonded, negatively charged 5-coordinate Ti and 4-coordinate Al avoid each other (similar to the Loewenstein Al-Al avoidance rule), it is likely that the TiO5 polyhedra are coordinated by SiO4 tetrahedra. The distortion of the Ti-O coordination environment observed at low Ti-substitution may result from the need to accommodate an edge-sharing geometry with SiO4 tetrahedra. At higher Ti substitutions (x > 0.3) the EXAFS results suggest that 5-coordinate Ti forms an edge-sharing relationship with an adjacent 5-coordinate Ti atom. While the structural information from Raman spectra is not as extensive as that determined from XAS studies, the trends are similar. At compositions where x > 0.1, the features are consistent with edge-sharing TiO5 polyhedra. At all Ti concentrations no new features appear that would indicate the presence of 4- or 6-coordinate Ti. Features due to Ti-O-Si bonds are clearly present even at x ) 0.1; however, due to the low intensity of the peaks, it is not clear if edge-sharing TiO5 polyhedra are present at this concentration of Ti. Observations by XAS that edge-sharing Ti configurations exist over the compositional range 0.3 < x 0.3), the adjacent TiO5 polyhedra are edge-sharing (Figure 8c). Conclusions X-ray absorption and Raman spectroscopic investigation of the compositional series CsTixAl1-xSi2O6+x/2 indicates the
Hess et al.
Figure 8. (a) Corner-sharing tetrahedra linkages typical of the Al endmember. (b) Ti substitution resulting in the formation of an edgesharing geometry between adjacent TiO5 and Al/SiO4 polyhedra. (c) Edge-sharing relationship between adjacent TiO5 polyhedra.
formation of TiO5 edge-sharing polyhedra at relatively low Ti substitution levels (x > 0.3). The occurrence of neighboring TiO5 polyhedra suggests ordering of TiO5 on symmetrically equivalent sites in the cubic pollucite structure. The determination of Ti-Ti interatomic distance of ∼3.0 Å is consistent with the formation of an edge-sharing relationship between neighboring TiO5 polyhedra, which is an unusual occurrence especially in framework aluminosilicates. Acknowledgment. This work was supported by the Environmental Management Science Program (EMSP). XAS data were collected at the Stanford Synchrotron Radiation Laboratory (SSRL), which is operated by the Department of Energy (DOE), Office of Basic Energy Sciences. The SSRL Biotechnology Program is supported by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and by the DOE, Office of Biological and Environmental Research. The research was performed in part at the Environmental Molecular Sciences Laboratory, a national scientific user facility supported by the Department of Energy (DOE), Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830. References and Notes (1) Strachan, D. M.; Schulz, W. W. Am. Ceram. Soc. Bull. 1979, 58, 865.
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