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The Journal of Physical Chemistry, Vol. 83, No. 18, 7979
C. E. Stroud, J. M. Stencel, and L. T. Todd
to be little changed, so that the strong photodissociation feature near 400 nm should be observed throughout the series, and the photodissociation spectra are seen to follow this expectation. MINDO/3 calculations on the mixing of ring x orbitals and side-chain pseudo-x orbitals are not quantitatively reliable (for instance, the AI, x orbital is placed -0.8 eV too high in binding energy), but useful qualitative features can be discerned. In particular, some light is shed on the question of why the charge transfer transition appears with high intensity (in fact comparable to the intensities of the ring transition) even though the initial and final states are reasonably localized in quite different regions of the molecule. Regardless of the geometrical conformation of the alkyl side chain, the alkane pseudo-x orbitals in ethylbenzene and propylbenzene mix with the ring AI, x orbital to a sufficient extent to give transition dipole moments of several Debye to the charge-transfer transition (comparable to the x x transition moment of 5 D). The energy of the ring x x transition is not perturbed by more than 0.2 eV by this mixing, so that the calculations predict essentially a superposition of a strong ring-toside-chain charge transfer feature on top of an unchanged benzene-ion spectrum. The calculated transitions are shown in Figure 1 for ethylbenzene and propylbenzene ions; the x x transition represented by the dashed bar is almost entirely localized on the ring; the charge-transfer transitions (which are plotted for three orientations of the alkyl group) all involve a shift of more than half the positive charge onto the side chain.
The charge-transfer optical transition in toluene ion lies in the UV, probably near 4.5 eV, and is moreover calculated to have a small transition moment, so that if it is observed at all, it probably overlaps the intense T x* transition. For ethylbenzene, the charge-transfer transition should lie near, and be masked by, the x x transition at 3.1 eV. For propylbenzene and larger alkyl groups, it moves into the visible wavelength region, and becomes a dominant and observable feature of the photodissociation spectra.
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Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the National Science Foundation, and to the Air Force Geophysical Laboratory for support of this research. The hospitality of the Physical Chemistry Laboratory, Oxford University, is gratefully acknowledged during part of this work.
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References and Notes (1) R. C. Dunbar, J . Am. Chem. Soc., 95, 6191 (1973). (2) P. P. Dymerski, E. Fu, and R. C. Dunbar, J . Am. Chem. Soc., 96, 4109 (1974). (3) R. C. Dunbar, Chem. Phys. Lett., 32, 508 (1975). (4) H. H. Teng and R. C. Dunbar, J . Chem. Phys., 66, 3133 (1978). (5) D. W. Turner et al., "Molecular Photoelectron Spectroscopy", Wiley-Interscience, London, 1970. (6) J. N. Murrell and W. Schmidt, J . Chem. Soc., faraday Trans. 2 , 66, 1709 (1972). (7) J. J. Pireaux et al., Phys. Rev. A , 14, 2133 (1976). (8) P. Bischof et al., Helv. Chim. Acta, 52, 1745 (1969). (9) S.P. McGlynn et al., "Introduction to Applied Quantum Chemistry", Hold, Rinehart and Winston, 1972, Chapter 9.
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Infrared Spectra of Cathodochromic Sodalite C. E. Stroud,t J. M. Stencel," and L. T. Todd, Jr. Department of Nectrical Engineering, University of Kentucky, Lexington, Kentucky 40506 (Received Februaty 16, 1979)
Infrared spectra are reported for cathodochromic bromosodalite (Na6A16Si6024.L"d, A = Br) and for sodalite containing chemically substituted Ge and Ga. The infrared bands are related to the activity predicted from group theoretical analysis of the sodalite PZ3n crystalline space group. The 3000-4000-cm-' region contains infrared active modes which are due to structurally bound OH- introduced during the sodalite growth process. A possible site of this OH-, and the basic spectral differences of bromosodalite (A = Br) and nosean (A = OH.xH20) are discussed.
Introduction Powdered cathodochromic sodalite has received considerable attention as a screen material in storage cathode ray tube (CRT) display devices.'-3 This interest is a consequence of its coloration-bleaching characteristics when it is exposed to ultraviolet radiation or an electron beam. Coloration of sodalite is due to the generation of F centers during ultraviolet radiation (photochromism) or electron bombardment (cathodochromism). Both photochromism and cathodochromism possess an optical mode of coloration, which is erasable with light, and a thermal mode of coloration which is erasable only by heating the material to about 300 "C. This thermal mode of coloration is retained for an indefinite time at room temperature for cathodochromic bromosodalite. Various models have been proposed to explain these coloration proper tie^;^,^^^ how+ B e l l Laboratories, Naperville, Ill. 60540,
0022-3654/79/2083-2378$01 .OO/O
ever, no model has been successful in completely describing the donor and acceptor sites which produce these coloration-bleaching characteristics. Vibrational spectroscopy has been useful in identifying the origin and properties of trapped species and the perturbational effects of these species on the vibrational modes of the host lattice? We have therefore obtained and interpreted the infrared spectrum of cathodochromic bromosodalite from 4000 to 150 cm-I in an effort to more fully understand its physical properties. This report emphasizes bromosodalite, which shows a color contrast ratio as high as 35:l after sensitization in a hydrogen atmosphere at 700 O C and after exposure to a 20-kV electron beam. The origins of certain infrared bands have been identified by observing spectral changes as Ga is chemically substituted for A1 and Ge for Si. Spectra for the 3000-4000-~m-~ region are also discussed in relation to OH- band activity. 0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
I R Spectra of Cathodochromic Sodalites
2379
b
a
Figure 1. Sodalite structure. FREQUENCY(cm-')
Experimental Section Sodalite samples used in this investigation were grown in the form of powder either by a high-temperature hydrothermal technique417 or by a high-temperature water-balance hydrothermal technique.8 The chemicals used to grow bromosodalite were research grade A1203, S O z ,NaBr, and NaOH, which combine according to the following equation: 2NaBr
+ 6NaOH + 3Al2O3+ 6Si02
-+
Na6A16Si6024.2NaBr+ 3H20
The NaOH appears as the solvent in the growth process and is dissolved in distilled water to form a 10 M NaOH solution. This provides a 70% excess of NaOH above what is required by the above formula. The chemicals are mixed in a silver container which is sealed and placed in a high-temperature, high-pressure autoclave. A growth temperature of approximately 345 " C and a pressure of about 1.4 X lo2 kg/cm2 were used for a duration of 24 h to achieve a powder growth with an average particle size of about 13 pm. Chemical substitutions of Ga for A1 and Ge for Si were achieved with research grade Gaz03and GeOz to partially replace A1203 and Si02 in the above formula. In the case of Ge substitution, the percent of Ge in the starting chemicals was significantly greater than that actually incorporated in the grown powder. The actual percent of the substitution in the powder was determined with an electron microprobe.' Infrared spectra were obtained in the 4000-400-~m-~ region with a Perkin-Elmer 337G spectrometer and in the 800-150-~m-~ region with a Beckman IR-11 spectrophotometer. The sodalite was ground with ultrapure KBr in a Wig-L-Bug. The 1%by weight sodalite/KBr mixture was then pressed under 3.4 X lo4 kg/cm2 pressure to obtain transmission pellets for data acquisition in the 4000400-cm-l region. Sodalite and ultrapure CsI were mixed and pellets were pressed in the same manner to obtain suitable transmission pellets for the 800-150-~m-~ region. The band positions and relative intensities were identical in the spectrometer overlap region (800-400 cm-I).
Results Sodalite is represented by the P13n = Td4space group.g The Bravais lattice is body centered cubic with each lattice point occupied by a halogen (a Br atom in bromosodalite). Each halogen is surrounded tetrahedrally by four Na atoms as shown in Figure 1. The Na4Br tetrahedra is enclosed in an aluminosilicate cage structure forming the cubooctahedral Wigner-Seitz primitive unit cell for the bccub lattice. The cage consists of A104 and Si04 tetrahedral units which provide charge balance for the Na4Br inside the Si-0-A1 cage. To aid in the interpretation of the spectra, the correlation method was used to calculate the number and symmetry of the infrared and Raman active vibrational
Figure 2. Infrared spectra (4000-1300 cm-') of bromosodalite (a) and bromosodalite after heating at 285 "C for 18 h (b).
modes.1° This method leads to an irreducible representation for the vibrational modes as given by I':$$euc = 2A1
+ 3A2 + 5E + 8F1+ 9F2
Within this representation, the triply degenerate F2species are infrared and Raman active, while the E and A species are only Raman active; F1species are neither infrared nor Raman active. The infrared spectrum for the 4000-1300-~m-~region is shown in Figure 2a. Bands at 3480 and 1660 cm-l are due to the stretching and bending vibrational modes of H 2 0 , respectively.ll The presence of HzO is believed to result from both atmospheric adsorption and incorporation during the sodalite growth process. This is very likely since sodalite is, structurally, closely related to zeolites, commonly referred to as molecular sieves, and it is known that H 2 0 can easily enter into the zeolite cage structure. The spectra shown in Figure 2 indicate that most of the H 2 0 can be removed by drying sodalite at 285 "C. The spectra after the 285 " C dehydration, Figure 2b, show a significant reduction in the H 2 0 bands, while no other band intensities or positions in the 4000-400-~m-~ range change, thus indicating that the majority of the H20 is not structurally bound. Residual H 2 0 absorption a t 3480 cm-I can be removed only after heating at 450 " C for 4 h. The high temperature needed to completely remove the residual H 2 0 band indicates structurally bound units, perhaps in the form of hydrogen bound to the oxygen in the sodalite cage structure.12 The band a t 3640 cm-l is due to an OH- vibrational mode.13 It is generally agreed that OH- in zeolites is located inside the Si-0-Al cage,14-16with bands at 3750 and 3695 cm-l being attributed to SiOH and AlOH stretching modes, respectively. The OH- stretching mode of NaOH is located at 3640 cm-l.17 These frequencies indicate that the OH- band found in sodalite at 3640 cm-l is associated with the Na atoms; presumably the OH- partially substitutes for Br atoms during the growth process. This conclusion is supported by spectra of samples containing 3% Ga substituted for Al, and 10% Ge substituted for Si which do not show a shift in frequency or change in intensity of the 3640-cm-' band. Heating the material at 450 "C for 4 h removes this band. This result also suggests that the OH- is structurally bound with the lattice. The spectrum of cathodochromic sodalite in the 1200-400-~rn-~ region is shown in Figure 3. The band a t 992 cm-l is higher in frequency than the broad, undefined structure which we observed for A1203. It is, however, close to that found in the lllO-lOOO-cm-l region for the Si02 which was used in the growth process. Taylor et al.13 suggest that Si-0-Al vibrational mixing causes bands near 1000 cm-l in sodalite. For albite (NaAISiBOs),orthoclase
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The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
C. E. Stroud, J. M. Stencel, and L. T. Todd
z 6C E? v)
v, In
a c
ae 2c
I
FREQUENCY
(cm-')
Figure 3. Infrared spectrum of bromosodalite (1200-400 cm-').
)
I
I
600
400
FREQUENCY (cm-')
Flgure 5. Infrared spectra (800-400 cm-') of (a) bromosodalite, (b) bromosodalite with 3% Ga substituted for AI, and (c) bromosodalite with 10% Ge substituted for Si.
1200
1000
so0
FREQUENCY (cm-')
Figure 4. Infrared spectra (1200-800 cm-') of (a) bromosodalite, (b) bromosodalite with 3% Ga substituted for AI, and (c) bromosodalite with 10% Ge substituted for Si.
(KA1Si3O8),and leucite (KA1Si2O6),the lOOO-crn-' absorption has been attributed to the Si-0 asymmetric stretching mode of Si04tetrahedra.18 These crystals have an anion construction composed of Zhree-dimensional shells of Si04 and A104 tetrahedra which is similar to sodalite. It is known that the assignment of an absorption band to a vibrational mode of a given coordinated group is meaningful only if negligible interactions exist between neighboring groups.lg In the case of sodalite this would not seem to be the case because of the similar vibrational frequencies of Si-0 and A1-0 and the degree of crosslinking in the Si-0-A1 chains. However, a factor which is more important in a vibrational assignment is the coordination number n of the AO, groups. For example, silicon monoxide absorbs a t 1225 cm-1,20the stretching mode of SiOz appears in the 1210-1080-cm-' region, and Si04 and Siz07groups absorb in the 1050-950-cm-' region.21s22Similarly, A104 groups absorb in the 800-cm-' region, while increasing the coordination numbers to 6 in A106 lowers this frequency to the 650-cm-l regionalg In order to clarify whether the 992-cm-l mode in Figure 3 is due to Si04 or Si-0-A1 mixed vibrations, the spectra of sodalite with 3% Ga substituted for A1 and 10% Ge substituted for Si were obtained. These spectra are shown in Figures 4 and 5. The relative intensity of the 992-cm-l band does not change with Ga substitution (Figure 4b),
even though dramatic changes are observed below 800 cm-l. Ge substitution decreases the intensity of the 992-cm-l band (Figure 4c) but does not shift it in frequency. Simultaneously, a band appears at 880 cm-l which can be related to the GeO stretching mode.23 Thus, this indicates that the 992-cm-' band is due to Si-0 stretching, without major contributions from A1-0. As shown in Figure 5, the band at 740 cm-l in sodalite exhibits a behavior similar to the 992-cm-' band with respect to Ga and Ge substitution. Ge substitution decreases the intensity of the 740-cm-' band considerably while no significant change is found with Ga substitution. The correspondence of this band to the bending vibration of 0-Si-0 chains (730 cm-'P4 indicates that it is due to 0-Si-0 bending vibration. Since Si-0-A1 chains are formed by the cage structure in sodalite, one would expect that two bands should be found due to the difference in atomic weights of Si and Al. Therefore, the band at 715 cm-I could correspond to the bond-bending vibration due to A1 in the 0-A1-0 units. This assignment is not evident in the chemical substitution spectra shown in Figure 5, possibly due to the low Ga percentage substitution. However, the region for A1-0activity in feldspars and other aluminum silicates is from 780 to 720 cm-l;l8 this agrees reasonably well with our band assignment. Absorption at 670 cm-' in sodalite is absent with 100% Ga substitution for Al, yet there is evidence of its existence with 100% Ge substitution for Si.13 This would indicate that it is due to an A1-0 vibration. Although there has been controversy over the vibrational frequencies of the A104 tetrahedra,lg Schroeder and Lyons report medium to strong absorption in the 675-575-cm-' region due to the A104tetrahedraeZ0In sapphire, a strong band exists at 640 cm-' which is due to an A1-0 vibrational modea2' Therefore, an A1-0 vibrational mode due to the A104 tetrahedra appears to be the best way to describe the 670-cm-' band in sodalite. The bands in the region from 500 to 400 cm-' are identical in frequency with those bands in the same region of the spectrum of kaolin clay (A1203.2Si02.2H20)16and
IR Spectra of Cathodochromic Sodalites
The Journal of Physical Chemistry, Vo/. 83, No. 18, 1979 2381
70
c a
c
c a
20
30
53
40
60
70
28 300
Figure 6.
250 200 FREQUENCY ( c m-')
150
Figure 7. X-ray diffraction pattern of (a) bromosodalite containing no nosean and (b) bromosodalite containing nosean.
Infrared spectrum of bromosodalite (325-1 50 cm-').
TABLE I: Infrared Vibrational Frequencies and Band Assignments for Bromosodalite freq, cm-' 3640 3486 1660 992 740 715
band assign
0-H H*O Si-0 Si-0-A1
freq, cm-'
band assign
670 470 43 5 410 294 200
A1-0 Si-0 A1-0 Si-0-A1 Na-Br
identical in intensity with the exception of the band at 470 cm-l. The Si-0-Si bending vibration for SiOz is located at 470 cm-l and is relatively insensitive t o SiO, coordinationz1 Substitution of Ge for Si should decrease the intensity at 470 cm-l if this band can be described in terms of a Si-0-Si bending mode. This reduction is observed (Figure 5c), however, the 435-cm-' mode simultaneously broadens and shifts to lower frequency. Both the 470- and 435-cm-' bands are observed in albite, orthoclose, and leucite.16 The decrease in relative intensity of the 435-cm-' band with Ga substitution indicates that it is associated with A1-0 vibrations. These bands show small frequency shifts and intensity changes which show they are primarily associated with Si-0 (470 cm-l) and A1-0 (435 cm-') vibrations but may have contributions due to the Si-0-A1 chain structure. The spectrum for sodalite in the region from 325 to 150 cm-I is shown in Figure 6. The band at 200 cm-' is attributed to a Na-Br vibration of the Na4Br tetrahedra since it is known that NaBr is active in the region from 250 to 110 cm-1.17 The origin of the 294-cm-l band is unclear at this time; it may be due to the NalBr tetrahedra also, since the selection rules indicate that there should be two vibrational modes associated with Na and Br. One percent Ge substitution for Si does not produce any change in this band. The infrared band frequencies and a possible description of atoms involved for these bands are summarized in Table I.
Infrared Study of Sodalite Powder Growth Todd4 suggested that there exists a correlation between the degree of crystallinity and the coloration sensitivity of a given sample of sodalite. This correlation was supported by the fact that materials grown by solid-state sintering were not as well crystallized, as indicated by scanning electron microscope (SEM) photographs, and were much less sensitive than those materials grown by hydrothermal growth techniques. Todd4 also found that sodalite grown in NaOH concentrations below 10.1 M contained a second phase and that the amount of this second phase increased with decreasing NaOH concentrations. X-ray powder pattern spectra of bromosodalite
80
Infrared spectrum of bromosodalite containing basic nosean phase (1200-400 cm-'). Figure 8.
and bromosodalite containing a second phase are shown in Figures 7a and 7b, respectively. The second phase peaks occur at lower angles than their adjacent sodalite peaks, indicating a larger lattice parameter for the second phase. This rules out the possibility that the second phase is hydrosodalite since its lattice parameter is smaller than that of bromos~dalite.~~ The second phase was identified as basic nosean, Na&&Si6021-2NaOH.rHz0, because of the close agreement between the measured peak locations and intensities and those reported by Taylor26 for natural nosean. Barrer and Whitez7 point out that the X-ray spectrum of basic nosean is identical with that of naturally occurring nosean, with the exception of shifts in line spacings corresponding to a different lattice parameter. The lattice constant of the second phase calculated from Figure 7a is 9.15 A, in close agreement with the value of 9.10 A given by Barrer and Whitez7 for basic nosean. Nosean has essentially the same structure as sodalite, however, the stuffing of the aluminosilicate cage with OHand H 2 0 leads to a disordering of the cage in nosean.26 Powders containing the nosean phase exhibit a decrease in coloration sensitivity compared to that of sodalite containing no nosean. Figure 8 presents the infrared spectra of sodalite containing the basic nosean phase in the region from 1200 to 400 cm-'. By comparing this spectrum to that of sodalite containing no nosean (Figure 3), one observes a decrease in the intensities and broadening of the characteristic sodalite bands which indicates incomplete formation of the sodalite structure. The Si-0 stretching vibration at 992 cm-l has a much larger bandwidth and extends into the 1100-cm-' region in Figure 8. This indicates that a considerable amount of Si02exists due to Si04tetrahedra not being well formed. The intensity of the 670-cm-' band decreases similar to that of the 435-cm-' band, which was attributed to an A1-0 vibrational mode. Since the bands attributed to Si-0 vibrational modes do not decrease in
2382
C. E. Stroud, J. M. Stencel, and L. T. Todd
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
pretation of spectra obtained with chemically substituted Ge and Ga, the 992-cm-' band previously assigned to mixed Si-0-A1 vibrations13 has been shown to be due to Si-0 modes. The OH- and H20 absorptions, along with the fundamental vibrational bands in the 1200-400-~m-~ region, have provided an indication of the nosean co-ntent and the degree of crystallinity within bromosodalite. These results, and on-going research with infrared and Raman spectroscopy, are expected to contribute to an understanding of the coloring and bleaching mechanism of cathodochromic sodalite.
Acknowledgment. The authors thank Dr. E. B. Bradley for the use of his laboratory and equipment during part of the experimental work and E. F. Farrell for his assistance with the X-ray diffraction data. C. E. Stroud was supported by a University of Kentucky Graduate School Research Assistantship, and the supplies for this study were provided by funding from a US. Army Contract No. DAAG34-77-C-0011. References a n d Notes 4000
3500
3000
FREQUENCY Icm")
Figure 9. Infrared spectra (4000-3000 cm-') of (a) bromosodalite containing nosean and (b) bromosodalite containing no nosean both before (-) and after (- -) dehydration.
-
intensity to the extent of these two bands, the 670-cm-l band would be expected to be due to an A1-0 vibration, as suggested in the previous section. The HzO found in the nosean cage is indicated by the 3480-cm-l band, shown in Figure 9, which compares the infrared spectrum in the region from 4000 to 3000 cm-l for samples of sodalite with (Figure 9a) and without (Figure 9b) the nosean phase. The H20 content in the sodalite sample containing nosean is much greater than normal sodalite. However, it is important to note that the sodalite containing the nosean phase has almost no OH-, as indicated by the weak band at 3640 cm-l. Therefore, one could assume that disordering of the cage structure in nosean is due primarily to HzO. In order to better observe the OH- band in sodalite samples containing nosean, both samples were heated a t 285 "C for 18 h, in an attempt to remove the HzO; these spectra are also presented in Figure 9. Some of the OH- was removed, as can be seen by comparing the intensity of the OH- band of sodalite containing no nosean to the spectrum of the same sample before heating. However, the OH- content in the sodalite containing nosean is much less than that of normal sodalite.
Conclusion The origins of the fundamental vibrational modes of bromosodalite have been discussed. Through the inter-
I. Goroa. ADD/. ODt.. 9, 2243 11970). 8. W. Fiughnan, i. Gorog, P. M: Heyman, and I. Shidlovsky, Proc. 61, 921 (1973).
rm,
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