J. Phys. Chem. l!J94,98, 4673-4676
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Structure and Spectroscopy of Sodalite Containing MnOd- Ions V. I. Srdanov,**+W. T. A. Harrison,* T. E. Gier,? and G. D. Stucky'vt Department of Chemistry, University of California at Santa Barbara, Santa Barbara, California 931 06, and Department of Chemistry, University of Houston, Houston, Texas 77204
A. Popitsch,s K. Gatterer,l D. Markgraber,l and H. P. Fritzerl Institute of Inorganic Chemistry, University of Graz, A-8010 Graz. Austria, and Institute of Physical and Theoretical Chemistry, Graz University of Technology, A-8010 Graz, Austria Received: December 29, 1993"
Permanganate ions were encapsulated in sodalite cages during the hydrothermal synthesis of basic sodalite in the presence of excess sodium permanganate. Both spectroscopic and structural studies of the solid solutions of basic sodium-permanganate-aluminosilicate sodalite focus on a particular stoichiometry: Na4[AlSi04]~(Mn04),(0H)~, with x = 0.73. The most important spectroscopic findings include 8-cm-l splitting of the u3 (908 cm-l) asymmetric stretch and 300-cm-l splitting of the orbitally degenerate 'Tz state of the MnO4ion. These results provide evidence for departure of MnO4- in sodalite cages from Td symmetry, in contradiction with the X-ray Rietveld analysis, which suggests a cubic lattice with a = 9.0992(7) A and preserved P43n crystal symmetry. The apparent discrepancy between structural and spectroscopic data is explained, and the reasons for the lowered symmetry are discussed.
I. Introduction Most sodalites with an aluminosilicate framework have cubic symmetry' and predominantly2 crystallize in thePd3nspacegroup. The sodalite structure is a space-fillingarrangement of identical cages in the form of truncated octahedra with inner diameters of -6.5 A. The aluminosilicate framework is made of regularly alternating tetrahedrally coordinated A1 and Si atoms (T-atoms) which are connected through oxygen atoms. Each cage has six eight-atom rings and eight twelve-atom rings, containing four and six T-atoms, respectively. Unlike some nonaluminosilicate sodalites,3 the middle of the cage is generally occupied by a negative ion tetrahedrally coordinated to the four nearest sodium atoms. The only known exceptions are Naa(AlSiO& sodalite4 and the fascinating Nas(AlSi04)6 "black sodalitew5in which an excess elecron plays the role of a central ion. Thus the common sodalite stoichiometrycan be represented by Nas [AlSi04]6(2X); Xisa central monovalent negativeionand theactual water content is disregarded. Structural and physical properties of sodalites depend to a large extent upon the nature of the central ion. Sodalites can be synthesized with a variety of different central ions: OH-, C1-, B r , I-, C103-, ClO4-, etc.6 Among the most extensively studied have been halogen (X = C1, Br, I) sodalites because of their important cathodochromic and photochromic properties.7 Although the choice of the central ion is restricted by certain limitations with respect to its charge and size! an incorporation of relatively large tetrahedral molecules in sodalites has been reported previo~sly.~ During the course of this work, synthesis of a sodalitecontaining Mn04- ions has been briefly reported.1° Species such as Mn04are particularly suitable for spectroscopic investigationsbecause they contain a transition metal ion with an open d shell. Because of this a number of electronic transitions in the visible spectral region are easily accessible to spectroscopic studies." In addition, the molecules of nominal Td symmetry have a certain number of degenerate vibrational and electronic states. Removal of de-
* Author to whom correspondence should be sent.
t University of California, Santa Barbara.
t University of Houston. t University of Graz. I Graz University of
Technology. Abstract published in Advance ACS Absrrucfs, March 15, 1994.
0022-3654/94/2098-4673%04.50/0 , I
,
generacy, if detected, is a sensitive probe to the symmetry of the surrounding environment. Motivation for this work was to examine theinfluenceof theNa4[AlSi04]3(Mn04)x(0H)I-xsolidstate solution on the electronic and vibrational spectra of Mn04ions encapsulated in sodalite cages. 11. Experimental Section
A. Synthesis. The Na4[AlSi04]3(Mn04),(0H)1-xsodalite (Mn04-SOD) was synthesized in the 0.2 < x < 0.9 stoichiometric range using a standard hydrothermal technique. The procedure for x = 0.73 was as follows: 2.80 g of NaOH pellets, 7.86 g of NaMn04.3H20, 15 mL of H20, and 5 mL of freshly prepared 4 M Na2A1020H were added into a 60-mL Teflon bottle. After -30 min, 3.86 g of LUDOX LS-30 (DuPont) was added and vigorously stirred to obtain optimum homogeneity. The sealed mixture was held at 100 OC for 3 weeks. The crystalline product was recovered by filtration and rinsed with distilled water until a colorless filtrate was obtained. This procedure gave 3.80 g of pink crystallites containing Mn04- ions in the sodalite cages. A routine X-ray powder diffractogram showed the familiar sodalite pattern from which an initial value for the unit cell size a0 = 9.102 8, was obtained. Different permanganate contents in Mn04-SOD were prepared by varying the NaMn04.3H20 concentration in the reaction mixture. The actual permanganate content x was determined by standard chemical analysis of the product. The unit cell size in the 0.2 < x < 0.9 range changes only slightly with x; 9.085 < a0 < 9.110 A. In spite of many attempts we were unable to make Mn04SOD with x = 1, as reported elsewhere.10 During the synthesis of Mn04-SOD, both Mn04- and OH-ions are present in the solution competing for the cage template, so that incorparation of a certain amount of OH- is unavoidable. It is also not possible to make Mn04-SOD with x < 0.2 because, in a basic dilute solution of permanganate, reduction to manganate(V1) takes place, leading to crystallization of a green manganate cancrinite. The diffuse reflectance spectra in the visible region confirmed the presence of Mn042- ions in these materials.12 B. Crystallography. The crystal structure of Mn04-SOD was established by X-ray Rietveld refinement. X-ray powder data were collected on a Scintag PAD-X automated powder diffrac0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
tometer operating in 8-8 geometry, using Cu K a radiation for 20 < 28 < 100'. The data analysis was carried out using the program GSAS13 running on a DEC FVAX-I1 computer. The framework structure of the sodium-hydroxide-aluminosilicate sodalite Na8(AlSi04)6(0H)214was used as the starting model in space group P43n, assuming perfect silicon/aluminum alternation over the framework tetrahedral nodes. To avoid biasing the model, initial cycles of refinement included only the framework species (Si, Al, and 0). The usual profile variables (scale factor, cubic lattice parameter, and pseudo-Voigt peak shape width variation coefficients) and atomic positional and isotropic thermal parameters were optimized to convergence; then i~-;-: I ,. , I the extraframework species were located from difference Fourier U 20 30 40 50 60 70 80 90 100 synthesis. Strong peaks at (O,O,O)and ( x , x , x ) with x 0.40 and 2-Theta, deg x 0.22 were assigned to Mn, 0, and Na, respectively. To Figure 1. Final observed (crossed),calculated (line),and differenceprofile derive independent information about the Mn04-/OH- ratio, a plots for the Rietveld refinement of Mn04-SOD with 73% of the cages joint site population refinement for Mn04- and OH-, subject to occupied by permanganate. Allowed reflection positions are marked. the constraint of maintaining full occupancy of the (O,O,O) crystallographicsite, was carried out. Recent workl5 has revealed TABLE 1: Structural Data for Na4(MnO4)o.n(OH)o.n(AISi04)3 (Cubic: Space Group P43n that Na8(AlSi04)6(0H)2.2H20 contains a novel 02H3- hydrogen (No.218), 8 = 9.0992(7) A) dihydroxide unit. There was no evidence for such a species in the present study. atom Wa x Y Z Uiwb P The refinement for 19 variables and 799 observations resulted 0.00 0.50 0.007(9) Si(1) 6c 0.25 in final agreement factors of R, = 8.32% (RP= cb0- Cycl/~yo) 0.50 0.00 0.008(9) Al(1) 6d 0.25 0.4631(10) 0.1559(13) 0.004(3) and RWP=10.61% (Rwp= [ ~ w ( ~ ~ - C y ~ ) ~ / ~ ~ y ~ ~ ] ~ ~ ~ ) ; C0(1) i s a s24i c a l0.1476(14) e Na(1) 8e 0.2238(7) 0.2238(7) 0.2238(7) 0.047(5) factor. Difference Fourier synthesis at the end of the refinement 0.00 0.00 0.036(6) 0.73(4) Mn(1) 2a 0.00 revealed no significant regions of extraatomic electron density. 0.4008(8) 0.4008(8) 0.036(6) 0.73(4) 0(2)d 8e 0.4008(8) No corrections for extinction and absorption were applied. 0.00 0.00 0.036(6) 0.27(4) 0(3)c 2a 0.00 To check for possible phase transitions, additional lowa Wyckoff site notation. Temperature factor (A2) = exp(-8rZ(Ub) temperature X-ray measurements on a Philips PW 1820 difsin2O/h2).e Fractional site occupancy. Permanganate ion oxygen atom. fractometer attached to a helium closed-cycle cryostat were Hydroxide ion oxygen atom. performed in the 5 < 28 < 80' region at several temperatures between 20 and 250 K. TABLE 2 Selected Bond Distances (A) and Angles (Deg) for C. Spectroscopy. Infrared spectra in the region 300-4000 cm-' were measured as KBr pellets on a computer-interfaced 1.730(14) X 4 Si(l)-O(l) 1.627(15) X 4 Al(l)-0(1) Perkin-Elmer 325 grating spectrometer. Low-temperature mea2.863(8) X 3 Na( 1)-0(1) 2.368(10) X 3 Na(l)-O(l)' surements were performed in a helium closed-cycle cooling system Na( 1)-0(2) Mn(l)-O(2) 1.563(11) X 4 2.791(16) (Model 20 Cryodyne cryocooler). Far-IR spectra in the region O(l)Si(l)-O(1) 106.1(3) X 4 O(l)-Si(l)-O(l) 116.5(7) X 2 50-650 cm-1 were obtained on a Nicolet 20F spectrometer. O(1)-Al(l)-O(l) 106.9(4) X 4 O(l)-Al(l)-0(1) 114.8(7) X 2 Raman spectra were collected at room temperature using Al(l)-O(l)Si(l) 146.7(7) 0(2)-Mn(l)-0(2) 109.5 X 6 equipment consisting of a SPEX 1301 double monochromator, an RCA C3 1034 photomultiplier tube, photon-counting electron114.8'. Both Si and A1 atoms have site symmetry 4. A notable ics, and a Spectra-Physics Model 2016 Ar+ laser (A = 514.5 nm). feature of the framework geometry is the large Si-&A1 angle The sample was mixed with KBr and pressed into a disk which of 146.7', which approaches the one found in the 'expanded" was rotated during excitation to minimize heating effects.16 sodaliteNa6[AlSi04]6,4wheretheSi-0-Albondangleis 156.3'. Illumination geometry with the laser beam impinging the surface The lattice constants of these materials are also quite similar: at a grazing angle was employed. The laser power was kept 9.0992(7) A for Mn04-SOD and 9.122(1) A for Na6[A1Si04]6. below 300 mW at the sample. The permanganate anion occupies the center of the sodalite The diffuse reflectance spectra in the visible region at room cage. For a Mn atom, which has exact tetrahedral coordination temperature and 20 K were obtained with a Cary 17D specin the free Mn04- ion, the site symmetry reduces to 23 in the trometer with a type I reflectance unit and a helium closed-cycle sodalitecage. The Mn-O(2) distanceof 1.563Aisslightly smaller Air Products Displex cryostat. Bas04 was used as a reference. than found in KMn04, where the average bond length is 1.607 A.18Each Mn-O(2) bond axis points toward. one of the four twelve-atom rings that are not occupied by Na+ species in the 111. Results and Discussion same cage. There was no evidence from the diffraction data for The quality of the final cycle of the Rietveld refinement of the a tetragonally-distorted M n 0 4 tetrahedron, as was suggested for Mn04-SOD structure is illustrated in Figure 1. Atomic positions M4(ABO&X04 phases by Depmeier.19 As shown in Figure 2, and thermal parameters are listed in Table 1, and selected sodium cations approximately occupy the twelve-atom rings and geometrical data are given in Table 2. The Mn04-SOD are closely coordinated by three framework oxygen atoms with framework consists of the usual face-sharing truncated octahedra, three further Na-O( 1) contacts in the same ring. They are also built up from an ordered array of alternating S i 0 4 and A104 bound to an oxygen of the permanganate in the adjacent cage. tetrahedral units. This is indicated by the bond distance results This gives rise to a distorted 'hexagonal-pyramidal" geometry listed in Table 2; d(Si-0) = 1.627(2) A and d(A1-0) = 1.730(2) for the Na+ ions. A are in good agreement with the expected distance from ionic In spite of the number of repeated X-ray diffraction measureradii s ~ m s . 1 ~ ments at various temperatures (the lowest point was 20 K), we could not observe any significant changes in the Mn04-SOD The S i 0 4and A104 0-T-0 tetrahedral angles are somewhat distorted from their regular Td geometry: M i - 0 values are powder pattern that would unambiguously indicate a phase 106.1 and 1 16.5', while for 0-A1-0 bond values are 106.9 and transition. I
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; 1,
-'
I
I
I
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4675
Structure of Sodalite Containing Mn04- Ions
600
500
400
330
200
la,
Wavenumbers (cm-1) Figure 4. Far-infrared spectrum of MnO4-SOD.
4500
4000
)sMI
3000
3100
same cage.
1500
2000
Raman Shill ( cm.‘
1000
500
)
Figure 5. Resonance Raman spectrum of MnO4-SOD at room temperature. The spectrum is dominated by the strong progression of the YI stretching vibration of the MnO4- ion.
TABLE 3: Electronic and Vibrational Constants of Mn04in W t e ~~
EO(cm-I) (1) 17551
ITz ‘Ai
930 1200
1000
910 800
890
600
400
Wavenumbers (cm-1) Figure 3. Mid-infrared spectrum of Mn04-SODshowing the framework
vibrations and the asymmetric stretching vibration of the MnO4tetrahedra. The inserted spectrum shows the u j mode at 20 K. The mid-infrared spectrum of Mn04-SOD, shown in Figure
3,containssodaliteframeworkpeaksat1000,719,692,654,451, 423, and 395 cm-l. These areconsistent with thevaluescalculated from an empirical relation between thecell edge and thevibrational frequencies of aluminosilicate soda1itesa2O In addition, the asymmetric stretching vibration u3 of Mn04- a t -912 cm-l is clearly visible in the infrared spectrum. The asymmetric bending vibration u4, which is also infrared active, is obscured by the strong framework absorption band at 395 cm-l. At low temperature the framework vibration bands show no significant changes, but the asymmetric stretching vibration u3 is split by 8 cm-1 (see insert in Figure 3). This experimental detail unambiguously shows that the degeneracy of the 19mode which belongs to the T2 symmetry species of the Td representation has been removed. In other words, we find decisive evidence at low temperature that Mn04- in Mn04-SOD is not an ideal tetrahedral ion. We will discuss this important point in more detail. The far-infrared spectrum in Figure 4 reveals the intracage cation and anion translational and librational motions. The Mn04-SODspectrum below 300 cm-1 is very similar to that found for Na8[AlSi04]6(C104)2 sodalite.21 The bands at 280 and 245 cm-I are due to framework vibrations, and the absorption peaks at 202 and 115 cm-1 are due to sodium translations. We assign the weaker peaks at 152 and 60 cm-I to the anion motions based on a comparison with the Na8[AlSi04]6(C104)2 spectrum. The
(2) 17863 0
ul (cm-1)
XII
(cm-I)
766
1.o
842
1.3
small difference, however, is the absence of the splitting in the anion-related absorption band a t 152 cm-l in Mn04-SOD. The resonance Ramanspectrumof Mn04-in sodalitez2exhibits a strong progression in u1 with an origin a t 842 cm-I. Five bands belonging to this progression could be easily observed in Figure 5. Theground-state anharmonicity constant xl1 for thisvibration is 1.3 cm-1 (see Table 3). These constants are very close to the values found for MnO4- in other hosts.23 The v3 progression appears to be much weaker due to the fact that the u3 mode at 908 cm-1is significantly broadened, which we ascribe to the partial removal of its degeneracy. The bending vibrations a t v2 = 350 cm-I and v4 = 395 cm-l are observed in the spectrum with much weaker intensities compared to the stretching modes. The diffuse reflectance spectrum of Mn04-SOD in Figure 6 shows four absorption features in the 300-700-nm region among which only the transition in the vicinity of 520 nm has a partially resolved vibrational structure. On the basis of general similarity with the well-known KMn04/KC104 spectrum,24the absorption band at -20 000 cm-l (500 nm) is ascribed to the first electric dipole allowed tl(lA1) 2e(lT2) transition in Mn04-. This charge-transfer transition involves excitation of an electron from the predominantly oxygen-located tl to the metal-located 2e molecular orbital. Charge-transfer transitions typically exhibit large displacement in the potential curve minima which are likely to produce favorable Franck-Condon overlap for a long vibrational progressions. Based on the intensity pattern and the energy separation between the adjacent peaks, the fine structure is attributed to the ul”=O vl’=n vibrational progression of the tI(IA1) 2e ( ~ T zelectronic ) transition. The series of absorption maxima in this progression, quite broad at room temperature, considerably narrow at 20 K when the splitting into two distinct sets of vibrational peaks with uneven intensities becomes apparent. Since no phase transition has been detected in the low-temperature X-ray measurements, it is likely that the splittingof thevibrational manifold is present also at room temperature but is obscured by phonon broadening. The splitting of the vibrational manifold in
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The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
Srdanov et al. orientations of the distorted central ion which sums up to an undistorted tetrahedron. Statistical fluctuations of thelocal cage environment are also responsible for the inhomogeneous broadening of the spectral features which are obviously present in the electronic and vibrational spectra of MnO4- ions in sodalite.
.-Ye
-
*
U
I
700
600
500
Wavelength ( nm )
400
-
300
Figure 6 . Diffuse reflectance spectrum of MnOh-SOD in KBr disk at room temperature and at 20 K. The tl(IA1) 2e(IT2) transition of the Mn04- ion is split by approximately 300 cm-I at 20 K.
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IV. Conclusion A careful spectroscopic and crystallographic study of a solid solution comprising a mixture of Mn04- and OH- ions in sodalite cages is presented. A significant size difference of the two central ions is believed to be responsible for the local strains and distortions in the sodalite lattice, causing the departure of Mn04- ions from the expected Td symmetry. This information is solely derived from the spectroscopic data and is in apparent disagreement with the crystallographic analysis. This study is an important contribution to possible pitfalls in the X-ray structure determination of pseudosymmetric sodalites26and suggests that spectroscopic studies are a prudent supplement to the structural investigations. Acknowledgment. We are grateful to B. Koppelhuber and A. Mautner for the low-temperature X-ray measurements. D.M. thanks the Bundeswirtschaftskammer and the Austrian government for support of a 3-month scientific visit in Santa Barbara. H.P.F. thanks the Graz University of Technology for support, and G.D.S.and V.I.S. acknowledge the Office of Naval Research for support.
the tl(lAl) 2e(lT2) transition strongly indicates removal of the degeneracy in the IT2 state, which is a sign of departure of Mn04- from Td symmetry. The two vibrational manifolds can be represented by a simple energy expression involving two terms References and Notes for electronic origins, the excited and the ground state frequencies (1) Pauling, L. Z . Kristallogr. 1930, 74,213. L h s , J.; Schulz, H. Acta of the symmetric V I mode, and corresponding anharmonicity Crystallogr. 1967, 23, 434. constants x11. Substitution of these constants from Table 3 into (2) Depmeier, W. Z . Kristallogr. 1992, 199, 7 5 . theexpression E = EO vl’(v’+O.5) -x~l’(~’+o.~)~-vl’’(V’+0.5) (3) Surprisingly, the Al-Ge, Ga-Si, and Ga-Ge extended family of IIIIV sodalites crystallize without the central negative ion: Nenoff, T.; et ai., X I I”(U”+0.5)2 reproduces the spectrum (see Table 3). submitted to J. Inorg. Chem. An important outcome of this study is an apparent controversy (4) Felsche, J.; Luger, S.;Bearlocher, Ch. Zeolites 1986, 6, 367. between the optical data and the structural information derived ( 5 ) Srdanov, V. I.; Haug, K.; Metiu, H.; Stucky, G. D. J. Phys. Chem. 1992,96,9039. Haug, K.;Srdanov, V. I.; Stucky, G. D.; Metiu, H. J. Chem. from the Rietveld analysis. While the structural analysis implies Phys. 1992,96,3495. Barrer, R . M.; Cole, J. F. J.Phys. Chem.Solids 1968, that Mn04- in sodalite preserves its tetrahedral symmetry, the 29, 1755. 8-cm-1 splitting of the v3 vibrational mode a t low temperature (6) Barrer, R. M.; Cole, J. F. J. Chem. SOC.A 1970, 1516. (7) Faufhnan, B. W.; Heyman, P. M.; Gorog, I.; Shidlovsky, I. In Adu. indicates distortion of the MnO4- tetrahedra in the ground Image Pickup Display 1981,4, 87. electronic state. Furthermore, the 312-cm-I splitting of the (8) Taylor, D.; Henderson, C. M. B. Phys. Chem. Minerals 1978,2,325. orbitally threefold degenerate ‘Tz state also indicates considerable (9) See, for example: Depmeier, W. Acta Crystallogr. 1984, C40, 226. Mn04- distortion in the excited electronic state. This splitting Levasseur, M. M. A.; et ai. J. Solid Stare Chem. 1976, 16, 167. (10) Weller, M. T.; Harworth, K. E. Chem. Commun. 1991, 734. is only 20 cm-1 in isomorphous KMnOd/KC104 crystals24 but is (1 1) For an extensive Iist of references covering the subject, see: Buijse, of similar magnitude (500 cm-l) to the splitting between the ‘E M. A.; Bearends, E. J. J. Chem. Phys. 1990, 93, 4129. and ’Al components of the IT2 state in LiMn04-3HzO/ (12) Srdanov, V. I.; McLeiland, W.; Stucky, G. D., unpublished data. (13) Larson, A. C.; Von Dreele, R. B. GSAS User Guide; Los Alamos LiC104.3H20 where the Mn04- site symmetry is reduced to C3”.25 National Laboratory: Los Alamos, NM, 1985-1991. An explanation for the apparent discrepancy between the (14) Luger, S.; Felsche, J.; Fischer, P. Acta Crystallogr. 1987, C43, 1. structural and optical data must be found in the solid solution (15) Wiebecke, M.; et al. J. Phys. Chem. 1992, 96, 392. (16) Kiefer, W.; Bernstein, H. J. Appl. Specrrosc. 1971, 25, 500. nature of the Mn04-SOD crystals. Each cage with Mn04- ion (17) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. is surrounded by 14 cages containing OH- and Mn04- central (18) Palenik, G. J. Inorg. Chem. 1967, 6, 503. ions with the statistical occupancy ratio of 3:7. The asymmetric (19) Depmeier, W. Phys. Chem. Miner. 1988, 15, 419. (20) Henderson, C. M. B.; Taylor, D. Spectrochim. Acta 1977,33A, 283. local environment is likely to cause distortions of both zeolite (21) Godber, J.; Ozin, G. A. J. Phys. Chem. 1988, 92,4980. cages and the Mn04- tetrahedra in a purely random fashion. As (22) Popitsch, A.; Fritzer, H. P.; Srdanov, V. I.; Stucky, G. D. Proceedings a consequence, the translationalsymmetry of thedistorted Mn04of the 13th Conference on Raman Spectroscopy, Wurzburg, 1992, p 732. (23) Clark, R. J. H.; Dines, T. J. J . Chem. Soc., Faraday Trans. 2 1982, tetrahedra is lost. While spectroscopic techniques are a sensitive 78, 723. probeof the symmetry of a local environment, theX-ray diffraction (24) Holt, S.L.; Ballhausen, C. J. Theorr. Chim. Acta 1967, 7, 313. technique requires long-range order. In the case of Mn04-SOD, (25) Johnson, L. W.; McGlynn, S . P. Chem. Phys. Lett. 1971, 10, 595 the X-ray powder diffraction data reflect an average of all possible (26) Hu, X.;Depmeier, W. 2. Kristallogr. 1992, 201, 99.
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