J . Phys. Chem. 1987, 91, 5361-5364
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Silicon-29 Magic Angle Sample Spinning Nuclear Magnetic Resonance Characterization of Sic Polytypes Jason R. Guth* and William T. Petuskey Department of Chemistry, Arizona State University, Tempe, Arizona 85287 (Received: April 29, 1987)
The 29SiMAS-NMR spectra of the 3C, 6H, and 15R silicon carbide polytypes are presented and interpreted. It is shown that there are a finite number of inequivalent silicon sites which are determined by the second- and third-order carbon and first- and second-order silicon coordinations. The spectra of any other S i c polytype may now be predicted.
Introduction High-resolution solid-state 29Simagic angle sample spinning nuclear magnetic resonance (MAS-NMR) spectroscopy has become a powerful and popular tool for investigating structures particularly in silicates and zeolites. This technique is equally useful for the study of non-oxide materials. To date, only 3C and 6H S i c polytypes have been studied by MAS-NMR. In this paper we present the spectra of the 3C, 6H, and 15R Sic polytypes and interpret the spectra from a structural standpoint. Assignment of spectral peaks to known types of silicon sites allows the spectra of other S i c polytypes to be predicted. The first application of 29SiMAS-NMR to solid-state inorganic materials was by Lippmaa et al.’ to study the correlation of chemical shift and polymerization in silicate crystals. Since then, 29SiMAS-NMR has been used extensively to study the structures of zeolites,2 silicate^,^ silicate g l a ~ s e sceramic ,~ material^,^ and many other silicon-containing amorphous6 and polymeric materials.’ Turner et al.,5 Finlay et a1.,8 and Inkrott et al.,9 have reported the 29SiN M R spectra of the 3C and/or 6 H S i c polytypes. The latter authors also discovered a relationship between synthetic processes and the MAS-NMR spectra. The basic theory of N M R lines in crystals was developed by Van Vleck’O and further treatments are given by Abragam” and Slichter.’* Oldfield and Kirkpatrick” have presented an overview of the experimental methods and recent results. As a polytypic material, silicon carbide exists in many crystalline modifications for which the crystal structures are known. The definition of polytypism and recommended nomenclature are given in the IUCr reports.I4*l5 The different forms of Sic arise because (1) Lippmaa, E.; Magi, M.; Samosan, A.; Engelhardt, G.; Grimmer, A,-R. J . Am. Chem. SOC.1980, 102, 4889. (2) Liu, X.;Kliniwski, J.; Thomas, J. M. Chem. Phys. Lett. 1986, 127, 563. (3) Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E.; Henderson, D. M. Am. Mineral. 1983, 68, 1206. (4) Dupree, R.; Holland, D.; Williams, D. S. J. Non-Cryst. Solids 1986, 81, 185. (5) Turner, G. L.; Kirkpatrick, R. J.; Risbud, S. H.; Oldfield, E. Am. Ceram. SOC.Bull. 1987, 66, 656. (6) Hayashi, S.; Hayamizu, K.; Yamasaki, S.; Matsuda, A.; Tanaka, K. J . Appl. Phys. 1986,60, 1839. (7) Schilling, F. C. Bovey, F. A,; Lovinger, A. J.; Zeigler, J. M. Macromolecules 1986, 19, 2660. (8) Finlay, G. R.;Hartman, J. S.; Richardson, M. F.; Williams, B. L. J. Chem. SOC.,Chem. Commun. 1985, 159. (9) Inkrott, K. I.; Wharry, S. M.; O’Donnell, D. J. Mater. Res. SOC.Symp. Proc. 1986, 73, 165. (10) Van Vleck, J. H. Phys. Rev. 1948, 74, 1168. (1 1) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, 1983. (12) Slichter, C. P. Principles of Magnetic Resonance; Springer-Verlag: New York, 1980. (13) Oldfield, E.; Kirkpatrick, R. J. Science 1985, 227, 1537. (14) Bailey, S. W.; Frank-Kamenetskii, V. A.; Goldsztaub, S.; Kato, A,; Pabst, A.; Taylor, H. F. W.; Fleisher, M.; Wilson, A. J. C. Acta Crystallogr., Sect. A 1977, A33, 68 1. (15) Gunier, A.; Bokij, G. B.; Boll-Dornberger, K.; Cowley, J. M.; Durovic, S.;Jagodinski, H.; Krishna, P.; DeWolff, P.M.; Zuyagin, B. B.; Cox, D. E.; Goodman, P.; Hahn, Th.; Kuchitsu, K.; Abrahams, S.C. Acta Crystallogr., Sect. A. 1984, A40, 399.
0022-3654/87/2091-5361$01.50/0
of the different possible stacking sequences of atomic layers. It has been tacitly assumedI6l8 that, in all polytypes, there are only two “types” of inequivalent silicon (and carbon) sites. The end members of the structure series, the cubic 3C (diamond or zinc-blende structure) and the hexagonal 2H (wurtzite structure), are composed entirely of only one of these types of site. A cubic site exists in an ABC stacking sequence and a hexagonal site exists in an ABA stacking sequence. MAS-NMR is ideally suited to determining the number of crystallographically inequivalent sites in a S i c polytype as is demonstrated below.
Experimental Section The spectra of the crystals and powders were obtained on a Bruker AM400 spectrometer with a Bruker MAS probe tuned to silicon at a frequency of 79.456 kHz. The samples were packed into DELRIN single air bearing rotors and spun between 4 and 5 kHz. In the case of crystals which did not fill the rotor, one or several crystals were supported in the rotor with BaC12 powder. Other powders such as sucrose and KC1 were used but it was found that a powder denser than the S i c was needed to prevent the crystals from moving in the rotor and upsetting its balance. The chemical shifts were measured relative to tetramethylsilane (TMS) which was contained in the same rotors at the “magic angle” without spinning. The measured 90’ pulse width for 29Siin TMS is 8 ps. It was assumed that this was the same for 29Siin solids. Inkrott et aL9 reported T I relaxation times to be on the order of 130 s in 3C S i c and 300 s in 6H Sic. The spectra were obtained by using a 90’ pulse with a 600-s delay between pulses for the 3C samples and an 1800-s delay for the 6 H and 15R samples. The origin and mass of the samples and data processing conditions are given in the figure captions. Inequivalent Sites As discussed above, it has been classically assumed that all layers within a polytype are equivalent, characterized by the dimensions a and c / n where n is the number of layers in the unit cell, and that all S i c structures can be described by using only cubic and hexagonal sites. It is now known that there can be more than two inequivalent sites in some polytypes. Choyke and P a t r i ~ k ’have ~ . ~ shown ~ that there are three inequivalent sites in 6 H S i c and four or, possibly, five in 15R Sic. Patrick2’ also discussed some ramifications of these inequivalencies. Examination of the known structures reveals that there exist only two basic silicon atom coordinations and only four basic carbon atom coordinations about any silicon atom. Combination of these would give rise to a total of eight possible environments about a silicon atom in all possible polytypes. In addition to differences in the (16) Mitchell, R. S. Z . Krismllogr. 1957, 109, I . (17) Verma, A. R.; Krishna, P.Polymorphism and Polytypism in Crystals; Wiley: New York, 1966. (18) Weltner, W. J. Chem. Phys. 1969, 51, 2469. (19) Choyke, W. J.; Patrick, L. Phys. Reu. 1962, 127, 868. (20) Choyke, W. J.; Hamilton, D. R.; Patrick, L. Bull. Am. Phys. SOC. 1962, 7 , 185. (21) Patrick, L. Phys. Reu. 1962, 127, 1878
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Guth and Petuskey
Figure 3. The (1120) plane of 15R Sic with the silicon sites as labeled in Table 11.
Figure 1. A projection onto (0001) of all possible nearest-neighbor coordinations about a silicon atom: (a) cubic and (b) hexagonal silicon neighbor coordinations. The neighboring atoms are in (e) the plane of the central silicon, (0) one plane above the central silicon, and ( 0 )one plane below the central silicon. (c) and (e) are the two possible “cubic” carbon neighbor coordinations, (d) and (f) are the two possible “hexagonal”carbon neighbor coordinations. The neighboring atoms are in ( 0 ) the plane directly above the central silicon, (0) the plane two planes above the central silicon, ( 0 )the plane directly below the central silicon, and ( + I the plane two planes below the central silicon. The vertical key for this figure is shown graphically in Figure 2.
a
1
0
Figure 2. The (1 120) plane of 6 H Sic giving the key for the vertical spacings of atoms in Figure 1 and the silicon sites as labeled in Table 11.
orientation of neighboring atoms, differences in the vertical spacings of atomic planes within a given p ~ l y t y p e and ~ ~ ,dif~~ ferences in the lattice parameters between polytypesZ4will also affect the chemical shifts of the possible peaks. Figure 1 shows all possible orientations of neighboring atoms about a silicon atom in any S i c polytype projected onto the (0001) plane. The diagrams are divided into the silicon neighbors and (22) Guth, J.; Petuskey, W. T. J . Phys. Chem. Solids 1987, 48, 541. (23) DeMesquita, A. H. G.Acra Crystallogr. 1967, 23, 610. (24) Tairov, Y . M.; Tsvetkov, V. F. Prog. Crystal Growth Charact. 1983,
7, 111.
TABLE I Four Types of Si Nearest-Neighbor Coordinations Shown in Figure 1 and the Number Ratios of Each Site in Several Common Polytypes type coordination 3C 6H 15R 4H 2H 1 0 0 I Sicub + Ccub 1 11 111
IV
Sicub + CcubZ Sihci+ Chex2 Sihel+ Cher
0 0 0
1 1
0
2 2 0
1 1 0
0 0 1
the carbon neighbors for clarity. The key to vertical spacings is shown in Figure 2. Figure 1, a and b, shows the orientations of silicon atoms about a cubic and a hexagonal site, respectively. The silicon neighbors included lie in the planes above, below, and in that of the silicon atom considered. In several cases there may be silicon atoms above or below the central atom at a distance of two (or more) plane spacings away. In these cases, however, there is always a carbon atom between these two silicons so the affect of this type of neighboring atom is neglected. For example, C(3) lies between Si(3) and Si(2) in Figure 2. Parts c and d of Figure 1 show the orientations of the nearest carbon atoms for a silicon atom in 3C (cubic) and 2H (hexagonal) S i c , respectively. Figure l e shows the carbon coordination about a “cubic” silicon site in an intermediate polytype such as Si(2) in 6 H S i c shown in Figure 2. Figure I f shows the neighboring carbon atoms about a “hexagonal” site in an intermediate polytype such as Si(1) in 6 H Sic shown in Figure 2. The neighboring carbon atoms included lie in the two planes above and the two planes below the silicon considered. Only four of the eight possible combinations of silicon and carbon coordinations are physically possible since the atoms in a (0001) carbon plane are always situated directly above the atoms in a (0001) silicon plane. This is clearly obvious if one tries to superimpose Figure I C on Figure lb. The solid circles in the carbon diagram do not lie above the solid circles in the silicon diagram as they should. Thus, there are four basic geometrically inequivalent silicon sites for all Sic polytypes. These combinations are given in Table I along with the relative numbers of each site in several common polytypes. The first type of silicon site is found in 3C, in 6H Sic as Si(3) (Figure 2), and in 15R as Si(3) (Figure 3). The second and third types of site correspond to Si(2) and Si( 1) in 6 H S i c , respectively. The fourth type of site is found only in 2H Sic. Since all Sic polytypes consist of two crystallographically equivalent interpenetrating sublattices, one of silicon atoms and one of carbon atoms, this discussion also applies to inequivalencies of carbon sites. Results The 29SiMAS-NMR spectra of 3C, 6H, and 15R S i c are shown in Figure 4. The peak position for 3C crystals is -16.1
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5363
Characterization of Sic Polytypes
,
,
-10
,
>
-20
I
!
PPM
Figwe 5. The 29SiMAS-NMR spectra of 3C SIC (a) single crystals and (b) Superior Graphite HSC Sic powder. IO-Hz LB.
1’
I
0
I
”
,
I
- 20
I
,
8
I -40
PPM
Figure 4. The 29SiMAS-NMR spectra of (a) CVD grown 3C crystals, 0.0255 g, (b) Sohio Co. 400-CS 6H powder, 0.2450 g, showing three peaks with area ratio l : l : l , (c) 15R crystal, 0.0217 g, showing three peaks with area ratio very near 1:2:2. 20-Hz exponential line broadening (LB). TABLE 11: Assignment of Site Type to the Observed Shifts for 3C. 6H. and 15R Sic polytype shift type site All 3c -16.1 I All I -18.4
6H
-14.3
-20.4 -24.9 15R
I I1 111
-14.6 -20.5
I I1
-24.1
111
MAS-NMR
Si(3) Si(2) (Figure 3) Si(1) Si(3) Si(2) Si(5) (Figure 4) Si(1) Si(4)
ppm and for a 3C powder is -18.4 ppm from TMS. The peak positions for 6 H are -14.3, -20.4, and -24.9 ppm and for 15R are -14.6, -20.5, and -24.1 ppm. Depending upon the amount of exponential line broadening applied to the free induction decay (FID), the central peak at -20 ppm in the 15R spectrum appears to be split into two peaks. Using resolution enhancement of the FID25shows that this center peak is indeed resolvable into two separate peaks. The integral area ratios of the three peaks in 6H Sic is 1:l:l and of those in 15R Sic is very nearly 1:2:2. Using Table I and comparing the 6H and 15R spectra shows immediately that the most positive peak (-14 ppm) is due to the type I silicon site 6 H Si(3) and 15R Si(3). Assignment of sites to the other peaks is not immediately obvious but, considering that the central peak in the 15R spectrum is indeed two separate peaks while the most negative peak is not, the most negative peak is assigned to the two type I11 sites in 15R and to 6 H Si(1). The center peak is therefore assigned to 6 H Si(2) and to the two type I1 sites in 15R. Examination of the 15R structure, Figure 3, reveals that the two “hexagonal” sites Si(1) and Si(4) are virtually identical but that the two “cubic” sites Si(2) and Si(5) may be slightly different even though they have the same type of nearest-neighbor coordination. The fact that N M R resolves these two latter sites may be an indication of differences in the absolute vertical spacings of the neighbors in these two equivalent coordination spheres. That is, the interplanar spacings along the c direction are slightly
different within the polytype structure. The two 15R “hexagonal” sites Si( 1) and Si(4) are seen to be identical within the resolution of this instrument. Table I1 gives the assignment of coordination types to the N M R peaks in the Sic polytypes studied. The type of nearest-neighbor coordination appears to have the major influence on chemical shift even though the energies (i.e,, energy gap26and electronic energies24are different for the various polytypes. This is demonstrated by the similarity of the 6 H and 15R spectra. The chemical shift has been shown2’ to be dependent upon both the electron orbital energies and distance. In this case, the type of coordination and silicon orbital energy may not be differentiable. The postulate that slight differences in chemical shift are due to slight deviations in layer spacings from c/n along the c axis is reinforced by the observation of small differences in the chemical shifts arising from the same site type in 6 H and 15R S i c . The difference in chemical shift due to site type I between 3C and 6 H or 3C and 15R (approximately 2 ppm) must be due to differences in both the a and c lattice parameters and, therefore, to the overall coordination sphere size. On proceeding through the series of Sic polytype structures from 3C to 2H, the a lattice parameter decreases while the average c direction layer spacing
increase^.^^^^^ Figure 5 shows the MAS-NMR spectra of two 3C SIC samples. Figure 5a is of 50-200-pm single crystals grown by chemical vapor deposition with a chemical shift of -16.1 ppm. Figure 5b is of a powder produced by siliconizing porous graphite particles with a chemical shift of -18.4 ppm. The difference in chemical shifts is apparently due to the different processing conditions. These two shifts were also observed by Inkrott et aL9 in different 3C Sic samples. The effect of processing and thermal history on the MAS-NMR spectra has been investigated and will be presented separately. Summary We have shown that, by analyzing the 29SiMAS-NMR spectra and known crystal structures, it is possible to assign the experimental spectral peaks to known silicon site types in Sic polytypes. Of the four possible geometrically inequivalent silicon sites in S i c plytypes, three have been observed by MAS-NMR. The spectra of the fourth type of site, found in 2H S i c , has not yet been recorded. It is now possible to predict the spectra of other Sic polytypes of known structure. For example, the 4H polytype will give a spectrum with two lines of equal intensity at approximately -20 and -25 ppm corresponding to the site types I1 and I11 in Table I. The peaks will, in all likelihood, not have the exact same chemical shifts as do peaks arising from the same site types in (26) Choyke, W. J.; Hamilton, D. R.; Patrick, L. Phys. Rev. 1964, 133,
A1163. ( 2 5 ) Fernge, A. G.; Lindon, J. C. J . Magn. Reson. 1978, 31, 337
(27) Ramsey, N. F. Phys. Rev. 1950, 78, 699.
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6 H or 15R Sic because of differences in lattice parameters. A further distinction between sites, other than that implied by the older notation of h (hexagonal) and k (cubic), must be made in order to explain the observed N M R signals. The system of notation in Table I distinguishes the four inequivalent sites possible in compounds such as Sic which exhibit polytypism.
Additions and Corrections Acknowledgment. We are grateful to W. J. Choyke of the University of Pittsburgh for supplying the 1SR Sic samples, to R. A. Nieman for assistance in the N M R studies, and to A. M. Yates for assistance in the X-ray diffraction studies. This work is supported by the National Science Foundation under Grants DMR-8512783 and CHE-8409644.
ADDITIONS AND CORRECTIONS 1987, Volume 91
J. M. G . Martinho and M. A. Winnik*: Transient Effects in Pyrene Monomer-Excimer Kinetics. Page 3642. Equation 17 should be as follows: ZoexP(t) = L(t) 8 kl(t)-lM(t)8 exp(-Ayt)
(17)
Page 3643. The correct version of Figure 4 appears below. The figure caption is correct as printed.
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