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<sup>27</sup>Al NMR Studies of the Aluminate Sodalites Sr<sub>8</sub>[Al<sub>12</sub>0<sub>24</sub>](Cr0<sub>4</sub>)<sub>2</sub> and Ca<sub>8</sub>[Al<sub>12</sub>0<sub>24</sub>](W0<sub>4</sub>)<sub>2</sub>

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27

Al NMR Studies of the Aluminate Sodalites Sr

8

[Al

12

0

24

](Cr0

4

)

2

and Ca

8

[Al

12

0

24

](W0

4

)

2

VAN DER KLINK, J. J., VEEMAN, W. S., SCHMID, Hans

Abstract

The 27Al NMR spectra of powders of strontium chromate aluminate sodalite (SACr) and calcium tungstate aluminate sodalite (CAW) by the magic angle spinning (MAS) technique and of single-crystal SACr by a static method. It is shown that the precision in the detn. of the quadrupole coupling const. (qcc) from MAS spectra is limited by residual broadening. Both in SACr and in CAW Loewenstein's rule is violated, and the qcc's are among the largest found for Al in a tetrahedral site of an aluminosilicate framework, due to the strong tetragonal distortion that accompanies this violation. MAS spectra in cubic and tetragonal SACr are indistinguishable, compatible with a phase transition mainly due to conformational shearing of the four-membered rings of the framework. In the orthorhombic phases of CAW and SACr, at least four different Al-sites can be distinguished: the MAS spectra suggest that the Al sites in the two compds. are very similar. Due to the local character of the NMR information, this does not necessarily imply that they have the same space groups. The empirical relation between 27Al chem. shifts and bond angles [...]

VAN DER KLINK, J. J., VEEMAN, W. S., SCHMID, Hans.

27

Al NMR Studies of the Aluminate Sodalites Sr

8

[Al

12

0

24

](Cr0

4

)

2

and Ca

8

[Al

12

0

24

](W0

4

)

2

. Journal of Physical Chemistry , 1991, vol.

95, no. 3, p. 1508-1511

Available at:

http://archive-ouverte.unige.ch/unige:31427

Disclaimer: layout of this document may differ from the published version.

1 / 1

(2)

1508 Reprinted from The Journal of Physical Chemistry, 1991, 95.

Copyright© 1991 by the American Chemical Society and reprinted by permission of the copyright owner.

27

AI NMR Studies of the Aluminate Sodalites Sr

8

[AI

12

0

24

](Cr0

4

h and

Ca

8

[AI

12

0

24

](W0

4

h

J. J .

van der Klink,*

Institute of Experimental Physics, Swiss Federal Institute of Technology. PHB-Ecublens, CH-1015 Lausanne, Switzerland

W. S. Veeman,

*

Department of Physical Chemistry, University of Nijmegen, 6525 ED Nijmegen. The Netherlands

and H. Schmid

Department of Mineral, Analytical, and Applied Chemistry, University of Geneva, CH-121 I Geneva 4, Switzerland (Received: June 4, 1990)

We have studied 27AI NMR spectra of powders of strontium chromate aluminate sodalite (SACr) and calcium tungstate aluminate sodalite (CAW) by the magic angle spinning (MAS) technique and of single-crystal SACr by a static method.

We show that the precision in the determination 6f the quadrupole coupling constant (qcc) from MAS spectra is limited by residual broadening. Both in SACr and inCA W Loewenstein's rule is violated, and the qcc's are among the largest found for AI in a tetrahedral site of an aluminosilicate framework, due to the strong tetragonal distortion that accompanies this violation. MAS spectra in cubic and tetragonal SACr are indistinguishable, compatible with a phase transition mainly due to conformational shearing of the four-membered rings of the framework. In the orthorhombic phases of CAW and SACr, at least four different AI sites can be distinguished: the MAS spectra suggest that the AI sites in the two compounds are very similar. Due to the local character of the NMR information, this does not necessarily imply that they have the same space groups. The empirical relation between 27 AI chemical shifts and bond angles established in (approximately) I

1

I aluminosilicates does not hold for these pure aluminates.

Introduction

N M R spectroscopy of 27 AI has been extensively applied to the study of zeolites and similar aluminosilicates, mainly for the purpose of determining AI coordination (tetrahedral vs octahedral) and degree of substitution (fraction of Si in the second coordination sphere). In most commercially important zeolites, the ratio Si/ AI is !::: I. This is a consequence of the empirical Loewenstein's rule, stating that in the tetrahedral network structure no direct Al-o-AJ bonds are allowed. In the sodalite structure however, both pure aluminates1•2 and pure silicates3 have been prepared; the natural mineral sodalite (sodium chloride aluminosilicate sodalite) has Si/ AI= I. Experimentally, it is found that the aluminate sodalites have strongly distorted Al04 tetrahedra; this is thought to be related to the violation of Loewenstein's rule.4 Here we discuss

~ 27 AI N MR spectroscopy, both static and magic angle spinning (MAS), of strontium chromate aluminate sodalite, Sr8[Al1r 024](Cr04

h

(SACr), in crystal and powder form and of calcium tungstate aluminate sodalite, Ca8[Al1

P

24](W04}z (CAW), in powder form.

The difference llv in N MR frequencies of an 27 AI nucleus placed in a crystal and of a .. bare" nucleus is usually determined by the chemical shift and by the quadrupole interaction. Both of these are tensorial quantities, and their effect varies with the orientation of the external magnetic field with respect to the crystal axes. The chemical shift has an isotropic (independent of ori- entation) and an anisotropic part; the quadrupolar interaction, an anisotropic part only. Theoretical expressions for 6.v are obtained as a perturbation on the Zeeman interaction for the bare nucleus: first-order perturbation theory is usually sufficient for the chemical shift, but rather frequently second-order theory is needed for the quadrupolar interaction. If the aluminum nucleus is in a site with the symmetry of one of the five cubic point groups,

(I) Dcpmcicr, W.; Schmid, H.; Setter, N.; Werk, M. L. Acta Crystallogr.

1987, C43, 2251.

(2) Depmcicr, W. Acta Crystallogr. 1988, 844, 201.

(3) Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V.; Dytryeh, W. J.; Bibby, D. M. J. Phy.f. Chern. 1988, 92, 243.

(4) Dcpmcicr, W. Acta Crystallogr. 1984, 840, 185.

both anisotropic interactions vanish, and only the isotropic chemical shift can be nonzero. For the eight point group sym- metries in the orthorhombic, monoclinic, and triclinic systems, the anisotropic interactions are determined by two constants each and are said to be asymmetric; for the remaining 19 point group symmetries of the hexagonal, tetragonal, and trigonal systems, the anisotropic interactions are axially symmetric around then-fold (n = 6, 4, 3) rotation (or rotation-inversion) axes and are therefore determined by one constant each. For half-integer spins, fast sample spinning around the magic angle (MAS) removes the first-order effect of all anisotropic interactions on the .. central"

transition of the NMR spectrum (them=

+

1/2 ++ m = -1/2 transition). In second order, corrections can be applied if the parameters of the quadrupolar interaction are known.

Both SACr and CAW undergo structural phase transitions of a rather subtle nature: 1.2.s orientational ordering of the tetrahedral chromate or tungstate ions is thought to drive positional changes in the aluminate framework. A similar lowering of framework symmetry by orientational ordering of hydroxyl groups has been suggested for hydrated sodium hydroxide aluminosilicate sodalite7 (but the dehydrated form shows no transition down to 8 K8). An interesting variant is bis(ethylene glycol)-silica sodalite Si12- 024·2C2H4(0Hh, where the phase transition may be driven by conformational changes in the ethylene glycol molecules.3

The phase sequences were given as follows: for SACrl.69

lm3m-(299 K)-4jmmm- (289 K)-+

mm2 (with 2 parallel to [ IIOJcub) (the space groups are not yet known); and for CA W5•10

143m-(653 K)- P4c2- (617 K)-

Aba2 (with 2 parallel to [00 I Jcub)

(5) Depmeier, W. Phys. Chern. Miner. 1988, I 5, 419.

(6) Rossignol, J.-F.; Rivera, J.-P.; Schmid, H. Ferroeltctrics 1989, 93, 151.

(7) Galitskii, V. Y.; Shcherbakov, V. N.; Gabuda, S. P. Kristallografiya 1973, /8, 988 [Sov. Phys.- Crystallogr. (Engl. Trans/.) 1974, /8, 620].

(8) Luger, S.; Felsche, J.; Fischer, P. Acta Crystallogr. 1987, C43, 1.

(9) Rossignol, J.-F.; Rivera, J.-P.; Tissot, P.; Schmid, H. Ferroe/ectrics 1988, 79, 197.

0022-3654/91/2095-1508$02.50/0 © 1991 American Chemical Society

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27AI NMR Studies of Aluminate Sodalites

N" 10 :X:

=..

;::: 0

:c

<ll -10

0 90 180

angle [degrees]

Figure I. Single-crystal rotation diagrams for the 27 AI central transition in cubic SACr in a 4-T field. The rotation axis is perpendicular to the magnetic field. Zero rotation angle corresponds to the field parallel to the [II 0] direction. The rotation is (a) around a 4-fold, (b) approxi- mately around a 3-fold, and (c) approximately around a 2-fold axis. The crystal was visually oriented on the basis of on natural facets. Fitted curves in (b) indicate a misalignment of a few degrees. For perfect alignment and no dipole-dipole interaction, (c) should show two lines instead of the observed three; the same is true at rotation angle 0° in all three panels. The likely explanation is dipole-dipole interaction. The fitted curves include a 73 ppm isotropic chemical shift (with respect to hydrated All+ in solution), as derived from MAS spectra. The quadru- pole coupling constant t2qQfh = 6.67 ± 0.04 MHz.

For the ferroelectric mm2 phase of SACr, it has been shown9 that the binary polar axis {P5 ~ 2.5 ILC/cm2) lies along a pseu- docubic [II 0] direction. For symmetry reasons, this result, combined with structural information on the cubic phase,1 requires that the space group of the prototype (parent phase) be !m"'Jm;

it is not certain whether this phase is realized below the melting point (in which case the high-temperature phase of SACr might be 143m). Because no single crystals of CAW are available, a similar determination of the polar axis in the low-temperature phase of this system has not been possible. One of the goals of the present work was to see whether differences in the ortho- rhombic CAW and SACr structures can be detected by NMR spectroscopy.

Results

SACr Crystal. The sample is a "yellow-type'" crystal of ap- proximately 5 mm3Rotation patterns of the central 27 AI NMR transition were taken in the cubic phase at 308 K in 4-T (see Figure I) and in 7-T (not shown) magnetic fields, with the rotation axis perpendicular to the field, and along (approximately) the [100], [I 10), and [I I I) directions. Crystal orientation was based on identification of natural (II O)c and (I I 2)c facets. In both fields, two-pulse sequences and Fourier transformation of one-half of the spin echoes were used to obtain the spectra. The pulse rep- etition time was 0.25 s, and the 90° pulse width 1.2 ILS. Com- parison of results in two fields in principle allows separate de- termination of chemical shift, dipolar, and quadrupolar effects.

A typical spectrum is shown in Figure 2a: the line widths (half-width at half-maximum) arc of the order of I kHz, about

(10) Depmeier, W. Acra Crysrallogr. 1984, 010, 226.

The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1509

15 10 5 0 -5

frequency [kHz]

Figure 2. 27Al NMR single-crystal spectra of SACr phases at fixed orientation (angle approximately 27° in Figure Ia) in a 7-T field: (a) cubic; (b) tetragonal; (c) orthorhombic. The noncubic phases are prob-

ably multidomain. These spectra are mainly determined by changes in~

orientation of relatively rigid Al04 tetrahedra. The low-field line position is least sensitive to small changes in angle (compare Figure Ia) and · therefore changes little on going from one phase to the other.

a)

b)

140 120 100 80 60 40 20 0 -20 -40 PPM

Figure 3. 27AI powder MAS spectra ofSACr phases in an 11-T field: (a) cubic.; (b) tetragonal. The spectra are indistinguishable; together with parts a and b of Figure 2, this shows that Mconformational shearing" is an important mechanism in this phase transition.

the size of the symbols in Figure I.

In agreement with the crystal structure, three equivalent AI sites are found, with symmetry axes parallel to the conventional unit cell axes. There is a complicating feature in these spectra: for the magnetic field along a [II 0) direction, two of the three lines in the N MR spectrum should coalesce into a single line;

actually the line shape is doublet like. The splitting in the doublet is -2.5 kHz, independent of field. We suppose that its origin is dipole-dipole coupling, but we have not attempted to evaluate it numerically. The nearest-neighbor Al-AI distance is 3.3

A;

at this distance the maximum magnetic dipole field of an AI nucleus is of the order of I G (I kHz). The observed line widths vary with crystal orientation and are probably also more deter- mined by dipolar couplings than by e.g. a distribution of quad- rupolar interactions. Within the precision of the measurements, no chemical shift anisotropy was detected.

The effect of the phase transitions on the N MR spectrum is shown in Figure 2 for one crystal orientation: the lines broaden and are badly resolved, no reliable rotation diagrams can be obtained, and the total width of a spectrum does not change significantly.

SACr and CAW Powders. MAS spectra were taken in I 1-T (see Figures 3 and 4) and 7-T (not shown) magnetic fields with

(4)

1510 The Journal of Physical Chemistry, Vol. 95, No.3, 1991

140 120 100 60 60 40 20 0 -20 -40 PPM

Figure 4. 27 AI powder MAS spectra of orthorhombic phases: (a) SACr;

(b) C/\ W. The spectral differ only in detail, showing that the defor- mations of Al04 tetrahedra are very similar in both structures. From numerical simulations it follows that these spectra indicate at least four inequivalent types of tetrahedra. An X-ray analysis10 has revealed seven types of tetrahedra in CAW.

b)

a)

90 80 70 60 so 40 30 20 10 PPM

Figure 5. 27AI powder MAS spectra of orthorhombic SACr: (a) actual;

(b) simulated by a superposition of four powder patterns. The parame- ters of the individual powder patterns have been chosen without consid- eration of structural data, and while the agreement between (a) and (b) is reasonable. no detailed interpretation of the parameters used in the simulation is warranted (see text).

an approximately 5 kHz spinning rate. This should be fast enough r'"'l to average the chemical shift anisotropy and dipolar couplings found from the single-crystal study. Comparison of the results in the two fields allows in principle separate determination of chemical shift and quadrupolar effects (but see also the Discus- sion). Fourier transformation of single-pulse free induction decays was used, with a pulse repetition time of 0.25 s and a pulse width of I p.s.

The 27 AI MAS spectrum of tetragonal SACr cannot be dis- tinguished from the cubic spectrum, but from numerical simu- lations ·we find that the orthorhombic SACr spectrum consists of (at least) four powder patterns with nearly identical chemical shifts, but different quadrupole interactions, equal to or smaller than those in the cubic and tetragonal phases. There is no clear asymmetry. Orthorhombic CAW has a similar MAS spectrum.

One possible simulation of the observed spectrum by a sum of four powder patterns is shown in Figure 5. This simulation did not use any of the available structural information, and we no not believe that its parameters have a physical significance beyond that stated earlier. The powder patterns in Figure 5 all have t1

= 0 and have relative intensity, isotropic shift, e2qQj h, and line broadening values of 0.61, I ppm, 6.10 MHz, and 200 Hz; 0.16, -3 ppm, 0 MHz, and 200Hz; 0.16, 0 ppm, 5.35 MHz, and 100 Hz; and 0.07, -1.5 ppm, 6.4 MHz, and 150 Hz. The fraction with zero quadrupole interaction is especially incompatible with the structure, and so is the assumption t1 = 0. This shows that the

van der Klink et al.

TABLE 1: 27 AI NMR Parameters for Cubic SaCr

a b

isotropic shift, ppm 68 ± 6 73 ±I anisotropic shift, ppm <5

e2qQfh, MHz 6.67 ± 0.04 6.54 ± 0.05

T/ <0.1 <0.1

• From crystal rotation diagrams in 4-T and 7-T fields. bfrom MAS spectra in 7-T and II-T fields.

9

N' ::c

~ 6 0

.r:.

" "

<I> r::r

<I>

3

110 115 120 125

angle [degrees]

Figure 6. Variation of 27 AI quadrupole coupling constant with 0-Al-0 angle. This graph shows that, in an isolated tetragonally distorted Al04 tetrahedron, the coupling constant is proportional to 1(3 cos a

+

I )I, where a is one of the 0-AI-0 angles and the Al-0 distance is assumed constant. The vertical scale has been calibrated to our experimental data for cubic SACr (circle). The cross is derived from data for I/ I alumi- nosilicate soda lite in ref II, as described in the text. The squares are for yttrium aluminum garnet (upper) and gadolinium aluminum garnet (lower), plotted from data cited in ref II.

parameters of such powder pattern simulations, even if they look very good, must be interpreted with extreme caution.

Discussion

From Table I it is seen that MAS spectra and single-crystal spectra for cubic SACr have the same estimated precision for e2qQj h, whereas determination of the isotropic chemical shift is clearly better by MAS. The relatively poor performance of the single-crystal experiments is due to uncertainty of the fits in Figure 1, caused by the dipolar interaction. The discrepancy between MAS and static values for e2qQj h, although not large (2%), is greater than the measurement error. In analyses of MAS spectra, it is usual to identify the positions of the maxima in the doublet (which can experimentally be determined to better than I ppm) with the positions of the divergences in the theoretical spectrum.

Any residual line broadening brings the maxima in the doublet closer together and therefore leads to an underestimation of e2qQj h, as shown in Table I. Since our MAS data at 7 T and at II T give the same e2qQj h, it is most likely that the source of the residual broadening is a spread in electric field gradients of about 2%, perhaps due to strains in the powder sample. We have not attempted to further identify the source of this broad- ening.

The striking similarity between the cubic and tetragonal SACr MAS spectra shows a fundamental property of NMR spectroscopy most important in solids: the main features of experimental spectra are determined by the local symmetry of the neighborhood of the nucleus, rather than by the full point group symmetry of its site (and even less by the space group of the crystal).

The quadrupolar interaction is among the largest for AI in a tetrahedral site of a framework aluminosilicatc. 11 This is certainly related to the strong tetragonal distortion of the Al04 tetrahedron, which in tum seems to be related to the violation of Loewenstein's rule. Indeed, comparable quadrupole interactions are found in the feldspath anorthite, 12 with a Si/ AI ratio of 0.67, while the

(II) Ghosc, S.; Tsang, T. Am. Mineral. 1973, 58, 748.

(12) Brinkmann, D.; Staehli, J. L. Helv. Phys. Acta 1968, 41, 274.

(5)

27AI NMR Studies of Aluminate Sodalites

feldspaths microcline and albite (both with a Sil AI ratio of 3) have much smaller quadrupole constants.11 In cubic SACr, the 0-AJ-o angles are 120.9° (two times) and I 04.1° (four times) 1 instead of 109.5° for the perfect tetrahedron (where the quad- rupolar interaction is zero by symmetry). Figure 6 gives the theoretical prediction for the variation of the quadrupolar in- teraction at the center of an isolated tetragonally distorted tet- rahedron; the vertical scale has been fitted to our experimental value (indicated by a circle). Also shown are values found in gadolinium aluminum garnet and yttrium aluminum garnet, which contain isolated, tetragonally distorted Al04 tetrahedra.11 The shift correction in the MAS spectrum of sodium chloride alu- minosilicate sodalite has been given as -D.3 ppm at 130.4 MHz;13 according to eq 4 of ref 13, this corresponds to a quadrupole interaction constant of e2qQI h = 0.9 MHz (for an asymmetry parameter.,= 0, as expected from the site symmetry). This value is plotted as the cross in Figure 6. The same system (together with its sodium iodide and bromide analogues and hydrated variants) has been studied by nutation NMR spectroscopy,'4 resulting in the values elqQI h ""' 0.6 MHz and 71 = 0.8-1.0. This value of the quadrupole interaction constant is in order-of-mag- nitude agreement with values in Figure 6, but the nonzero value of., is incompatible with the known crystal structure of sodium chloride aluminosilicate sodalite.'5 The 27 AI MAS spectrum of very low aluminum sodalite has been observed, but the quadrupole interaction has not been evaluated.16 If its 0-AJ-o angles are similar to 0-Si-0 in bis(ethylene glycol)-silica sodalite,3 Figure 6 predicts e2qQI h "" 0.5 MHz and a MAS quadrupole doublet splitting at 52.13 MHz of 0.7 ppm (actually, no splitting has

been

observed at this frequency16).

We interpret the differences in single-crystal spectra and the similarity of the MAS spectra in tetragonal and cubic SACr as indicating that the most important change in local symmetry is a change in orientation of the Al04 tetrahedra, without change of 0-AI-0 bond lengths or angles. Such a reorientation could result from "conformational shearing" of the four-membered rings in the framework10 (in a simple picture, the phase transition comes about through rotations in the AI-0-AI "hinges" between neighboring, quite rigid Al04 tetrahedra).

Two properties of the orthorhombic phase (both in SACr and in CAW) should affect the NMR spectra: the electric field gradient tensors must be asymmetric (but the degree of asymmetry may be arbitrarily small), and there might be contributions from the spontaneous polarization known to exist in these phases. The MAS spectra show qualitatively that the asymmetry is small (71

<

0.1; in fact they can be very well represented by a superposition

of lines that all have .,

=

0) and that the average quadrupole interaction is smaller in the orthorhombic phase than in the higher symmetry phases.

In an X-ray investigation of CAW, seven differently distorted Al04 tetrahedra have been found, 10 all of less than orthorhombic symmetry. For such a general distortion, a correlation has been found11 between the value of e2qQI h in a number of alumino- silicates and the angular (rather than the bond length) distortion, measured by a parameter 1'~~1 = L:;ltan(O;-00

)1,

where the sum runs over the six 0-AI-0 angles and 00 is their ideal value (I 09.5°) .. There is no corresponding relation for the asymmetry

(13) Lippmaa, E.; Samoson, A.; Miigi, M. J. Am. Chern. Soc. 1986, 108, 1730.

( 14) Janssen, R.; Breuer, R. E. H.: de Boer, E.; Geismar, G. Zeolites 1989, 9, 59.

(15) Uins, J.; Schulz, H. Acta Crystallogr. 1967, 23, 434.

(16) Meinhold, R. H.; Bibby, D. M. Zeolites 1986,6,427.

The Journal of Physical Chemistry, Vol. 95, No.3, /991 1511 parameter 'II· In this spirit, but with the angle a appearing in Figure 6 rather than

1'111,

suppose that the change in the MAS quadrupole doublet splitting was completely due to a change in angle a of a tetragonally distorted tetrahedron (see Figure 6): a eha~ge from a

=

I 09.5

+

11.4° as in cubic SACr to a

=

109.5

+

9.4° changes the doublet width from 18.2 to 12.3 ppm. (In CAW, the average of the large angle does indeed diminish, to a

= I 09.5

+

10° .) In view of this sensitivity, little can be said about possible ferroelectric effects (the transitions are strongly first order, and the spontaneous polarization varies little with temperature9).

The isotropic chemical shift from the MAS spectra is to within a few ppm the same for all sites of all phases studied and within experimental precision equal to that found in single-crystal SACr.

Lippmaa et al.13 have found a correlation between mean AI-o-Si angles and 27 AI chemical shifts in 1

I

I aluminosilicates. For an angle of,...., 148° (the value for AI-0-AI in SaCr), this relation gives a shift of ,....,60 ppm. In pure silica sodalite, the Si-0-Si anglel is !59. 7°, and the expected shift is 52 ppm. The experimental value in low-aluminum sodalites at 52.13 MHz is 40.1 ppm, without correction for quadrupolar effects.16 The latter should however not amount to more than I or 2 ppm, given the low distortion of the tetrahedron (if the assumption that the geometry of Al04 closely resembles that of Si04 is correct).

Conclusion

The extremely strong tetragonal distortion of the Al04 tet-~

rahedra that already exists in the cubic phase of the aluminate sodalites (see the comparison with natural sodalite in Figure 6) · - dominates the 27 AI NMR spectra to such an extent that spectral changes related to the phase transitions toward tetragonal and orthorhombic phases cannot be quantitatively interpreted. The single-crystal spectra of the low-symmetry phases are so badly resolved (see Figure 2) that powder MAS spectra (see Figures 3 and 4) actually contain more usable information. In powders of course, information about the orientation of electric field gradient (EFG) tensors is lost.

The rotation diagrams of the SACr single crystal show the expected three equivalent AI sites, with the unique EFG axes parallel to the cubic crystal axes (see Figure l). The spectra in Figure 4 show that the AI sites in orthorhombic CAW and SACr are very similar (and that at least four of them are inequivalent), but the local character of the NMR measurement and the lack of EFG orientation information do not allow conclusions about the space groups of these crystals. This latter point is also shown in a very nice way in Figure 3: the cubic to tetragonal phase transition in SACr remains completely undetected by MAS-NMR (although the N MR parameters do change; see Figure 2). The local symmetry hardly changes, except for a reorientation of the EFG tensors, in agreement with the proposed mechanism for this~

phase transition.10

The value of the chemical shift (in cubic SACr, but the other values are not very different) is some 10 ppm higher than expected from correlations found 13 between mean AJ-o-Si angles and 27 AI shifts in approximately I

I

I aluminosilicates. At the other extreme, literature data on very low aluminum sodalite16 indicate a shift that is I 0 ppm lower than that expected from the correlation. It follows that the correlation cannot be extended to compositions far from the I

I

I case for which it has been derived.

Acknowledgment. We thank Mrs. Gerda Nachtegaal of the SONINWO facility in Nijmegen for her assistance in taking the MAS spectra and J.-J. Bercier in Lausanne for the data appearing in Figure I. Partial support by the Swiss National Science Foundation is gratefully acknowledged.

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