J . Phys. Chem. 1992, 96, 5505-5509 (2) Reid, J. D.; Melroy, 0. R.; Buck, R. P. J . Electroanul. Chem. 1983, 147, 71.
( 3 ) Delahay, P. Double h y e r and Electrode Kinetics; Wiley-Interscience: New York, 1965; Chapter 2. (4) Verwey, E. J. W.; Niessen, K. F. Philos. Mug. 1939, 28, 435. (5) Chartier, P.; Mattes, B.; Reiss, H. J . Phys. Chem. 1992, 96, 3556.
5505
(6) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. G. Synth. Met. 1987, 18, 285. (7) Hansen, W. N . ; Hansen, G. J. In Electrochemical Surface Science; Soriaga, M. P., Ed. Kolb, D. M. Z . Phys. Chem. 1987, 154, 159. Kolb, D. M. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1175. Hansen, W. N . ; Henderson, D.; Ulstrup, J. Mol. Phys. 1989, 68, 401.
Low-Temperature (144 K) 12'Xe NMR Studies of Coadsorption of Xenon and Carbon Tetrachloride in NaY Zeolite T.T.P.Cheung Phillips Research Center, Phillips Petroleum Company, Bartlesville, Oklahoma 74004 (Received: January 22, 1992)
129Xenuclear magnetic resonance (NMR) of xenon coadsorbed in NaY zeolite with carbon tetrachloride at 144 K has been studied as a function of the xenon loading. The NMR spectra reveal two types of 129Xeresonances corresponding to xenon in supercages with and without CC14 molecules. The presence of CCl, molecules in the supercage suppresses the gas-liquid trancition of the xenon sharing the same cage. On the other hand, the gas-liquid transition of xenon in the supercages without CCll molecules can influence the distribution of the carbon tetrachloride in the zeolite.
Introduction The high sensitivity of the chemical shift of 129Xenuclear magnetic resonance (NMR) to the local electronic environment of the xenon atoms has made it a very useful tool for the studies of adsorption of molecules into microporous materials. Applications of this technique to the adsorption of water' and organic moleculesZ into Y zeolites have been documented. In this paper, we report the results of 129XeN M R studies of xenon coadsorbed with carbon tetrachloride in NaY zeolite. While others' works on coadsorption were done at room or elevated temperatures, our investigation concentrates mainly on coadsorption phenomena at low temperatures. We have shown prev i o u ~ l ythat, ~ * ~at 144 K, xenon adsorbed in the supercages of Y zeolites undergoes a quasi-gas-liquid phase transition when the xenon density inside the supercages is sufficiently high. (Typically, a supercage in Y zeolites can accommodate 9-10 xenon atoms.) However, such a transition is not observed in other zeolites with smaller pore structure^,^ where interactions among xenon atoms are very limited. Adsorption of other molecules in the supercages of Y zeolites may produce similar effects by reducing the void space available to the xenon, thus restricting the mutual attraction among the xenon atoms. Carbon tetrachloride, with a molecular diameter slightly larger than 7 A, can adsorb only into the supercages of Y zeolites. Since the supercage is 13 A in size, the adsorption of carbon tetrachloride will be influenced by the presence of coadsorbed xenon, because the supercage volume occupied by xenon will not be available to the carbon tetrachloride. This is particularly true at low temperatures where xenon atoms tend to form clusters within the supercages. Because of the large polarizability of carbon tetrachloride, Xe-CCl, interactions contribute significantly to the 129Xe chemical shift. In this paper, we show that coadsorption of carbon tetrachloride with xenon into NaY zeolite leads to two types of Iz9Xeresonances: one due to xenon in supercages containing CC14 molecules and the other due to xenon in supercages without CC14 molecules. We examine the effects of the xenon on the adsorption and distribution of carbon tetrachloride in NaY zeolite, and the influence of carbon tetrachloride on the gas-liquid phase transition of the adsorbed xenon. We shall show that these two phenomena are interrelated. Experimental Section Samples were prepared by first heating the NaY zeolite at 773 K for at least 10 h in a vacuum of 1 X 10-a Torr. After the sample
was cooled to room temperature, a known amount of carbon tetrachloride (equivalent to 1.9 molecules/supercage) was adsorbed into the zeolite. Then a known amount of xenon gas was introduced to the sample by chilling the sample region of the N M R tube with liquid nitrogen. After the introduction of xenon, the N M R tube was sealed and the sample was warmed back to room temperature. Details of the low-temperature '29Xe N M R experiments have been given el~ewhere.~-~ Briefly, the sample was equilibrated at 297 K for a few minutes before being cooled to 164 K at a rate of about 5 K/min and held there for 5 min. Then it was cooled to 144 K in 2 K steps with an equilibration time of 2 min between steps. Data acquisition began after the sample had been at 144 K for 5 min. lz9XeN M R chemical shifts are referenced to that of xenon gas extrapolated to zero pressure and room temperature by using the equation provided by Jameson et a1.6 We shall adopt the convention of a downfield resonance from the reference having a positive chemical shift. Xenon loading, nen, at 144 K is expressed in terms of the number of xenon atoms per supercage. It is computed from the amount of zeolite used (dry weight), the zeolite formula weight, and the amount of xenon adsorbed into the zeolite. The latter is obtained by subtraction of the amount of gaseous xenon remaining above the sample at that temperature from the total amount of xenon introduced to the sample. The amount of gaseous xenon above the sample is calculated from the xenon adsorption isotherm of the NaY zeolite, one that has absorbed the same amount of carbon tetrachloride (per unit weight of the zeolite). The xenon adsorption isotherm is measured at 143.3 K using a stirred liquid bath containing a solid-liquid mixture of n-pentane at its melting point. One observes that ncff represents the average xenon number density inside the supercage when all xenon atoms are adsorbed inside the zeolite. When the xenon loading exceeds that which can be held by the supercages, excess xenon begins to condense on the exterior surface of the zeolite microcrystallites. In that case, ncffis no longer the average number density. We shall refer to the maximum xenon loading that can be accommodated by the zeolite as the xenon end point of the xenon titration of the zeolite. The adsorption of organic molecules or water of hydration into Y zeolites at room temperature usually leads to heterogeneous distributions of the adsorbates within the zeolite Heat treatment may be required to assure a homogeneous distribution. In the case of carbon tetrachloride, heat treatment should be avoided since it leads to modifications of the zeolite by
0022-3654/92/2096-5505$03.00/00 1992 American Chemical Society
5506 The Journal of Physical Chemistry, Vol. 96,No. 13, 1992
Cheung
I
3
250
150
50
CHEMICAL SHIFT (PPM)
Figure 3. Comparison of the 129XeNMR spectrum of xenon adsorbed in pure NaY with that coadsorbed with carbon tetrachloride. The
chemical shift of the resonance of the former spectrum is the same as that of resonance B of the latter.
250
so
150
CHEMICAL SHIFT (PPM)
Figure 1. 129Xe NMR spectra of xenon coadsorbed in NaY zeolite with carbon tetrachloride at 144 K.
250
.-
1
.
-0
2 4 Xenon Atoma/Cop Mwrbad
6
Y
(kd~orptionIsotherm)
Figure 4. Comparison of the amount of xenon adsorbed (at I44 I() in
NaY zeolite containing carbon tetrachloridedetermined by two different methods: (1) xenon adsorption isotherm of the zeolite containing carbon tetrachlorideand (2) calculated from i29XeNMR and xenon adsorption isotherm of xenon in pure NaY zeolite.
1
.
Reeononce B
I
(XENON ATOMS/CAGE ADSORBED)
Figure 2. 129Xe NMR chemical shift of xenon coadsorbed in NaY zeolite with carbon tetrachloride at 144 K as a function of the xenon loading,
new
the carbon tetrachloride. In fact, at elevated temperatures, carbon tetrachloride can completely destroy the zeolite microcrystallites.
Results 129XeN M R spectra of xenon coadsorbed in NaY zeolite with carbon tetrachloride at a CC14loading of 1.9 molecules/supercage show only a single resonance at low loadings of xenon, as shown in Figure 1. At higher loadings of xenon, one observes two resonances, which we shall label as resonances A and B, with resonance A referring to the one with a larger chemical shift. Both resonances move to higher chemical shifts as the xenon loading increases. The zeolite is saturated with xenon at a nerfof 6.0 Xe atoms/cage. At loadings beyond the xenon end point, a third resonance corresponding to solid xenon condensed on the external surface of the zeolite begins to appear and the chemical shifts of resonances A and B become constant. In Figure 2, we plot the chemical shifts uA and uB of resonances A and B, respectively, as a function of nefP Resonance B can be assigned to xenon atoms adsorbed in supercages which do not contain any CC14molecules. This is based
on the similarities between resonance B and the 129Xeresonance of xenon adsorbed in pure NaY zeolite ( N a y zeolite without any carbon tetrachloride). First, they are broad and have similar line shapes a t low loadings of xenon. Second, they both collapse to narrow lines at high xenon loadings. Finally, they have the same chemical shift of 250 ppm at the xenon end point. In Figure 3, we compare the line shape of resonance B with that of the resonance of xenon in pure NaY zeolite, which has the same chemical shift as resonance B. The above assignment of resonance B implies that one can deduce the xenon loading of NaY zeolite with adsorbed carbon tetrachloride from the 129XeN M R and xenon adsorption isotherm of the pure NaY zeolite. For a pure NaY zeolite at xenon loadings smaller than the xenon end point, there are one-to-one correspondences between the lBXe NMR chemical shift and the xenon number density in the supercage, and between the xenon number density in the supercage and the xenon partial pressure above the sample. Therefore, from the chemical shifts of resonance B, one can determine the corresponding xenon number densities in the supercage (one which does not contain any CC14 molecules) from the experimental chemical shift versus xenon number density curve of the pure NaY zeolite. Then from the adsorption isotherm of the pure NaY zeolite, one obtains the amount of gaseous xenon above the sample for a given xenon number density in the supercage. The amount of xenon adsorbed in the NaY zeolite with carbon tetrachloride is simply the difference between the total amount of xenon introduced and the calculated amount of gaseous xenon. In Figure 4, we compare the calculated xenon loading with that determined from the xenon adsorption isotherm of the NaY zeolite with adsorbed carbon tetrachloride.
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5507
Coadsorption of Xe and C C 4 in NaY Zeolite
(XENON ATOMS/CAGE IN SUPERCAGES WmC CCI4)
(XENON ATOMS/CAGE IN SUPERCAGES WrrH CCI,) 0 3001
nA
nA
2
1
5
4
5
t
0.5 0.4
0
oo
2
4
6
8
10
s-74
3.61
* f ' 9
t
f
Figure 5. Key: solid circles, '29XeNMR chemical shift of resonance A as a function of nA, the number of xenon atoms per cage in supercages containing carbon tetrachloride (the upper abscissa);open circles, Iz9Xe NMR chemical shift of xenon in pure NaY zeolite as a function of the xenon loading (lower abscissa).
Resonance A is associated with xenon adsorbed in supercages which contain CC14molecules. Its larger chemical shift is partially due to the large polarizability of carbon tetrachloride, and to the fact that supercages containing CC14molecules have smaller free volume available to the xenon atoms, thus decreasing their mean free For a given neffbelow the xenon end point, the xenon number densities (in terms of numbers of xenon atoms per cage) nAand ne, respectively, for xenon inside supercages with and without CC14 molecules are different. It is more appropriate to express the results of resonance A in terms of the xenon number density nA. Let us denote the fraction of the xenon in supercages without CC14 molecules by f and the fraction of the supercages without CC14 molecules by g. Then at xenon loading below the xenon end point, we can write nB = v/g)neff
0
1
2
3
4
5
6
7
8
9
10
nB
XENON ATOMS/CAGE IN PURE NaY ZEOLITE
(1)
and nA
1
3.12
1
O
0.0
0
2;40
6
= [(l - f i / ( l - g)lncff
-n/u
= t(1 -fncff/nB)Incff (2) ne can be obtained from the chemical shift uBof resonance B using the chemical shift curve (as a function of the xenon number density) of xenon in the pure NaY zeolite. We estimate f from '29XeNMR spectra like those in Figure 1. It is given by the ratio of the area under resonance B to that of resonances A and B combined. Our procedure is to numerically fit the spectra by several Lorentzian and Gaussian peab using a nonlinear-square-fit routine. Typically, for resonance B, a sum of a Lorentzian and Gaussian peak is required for a good fit in the low xenon loading regime, where resonance B obviously consists of more than one component. At high xenon loadings, a single Lorentzian suffices, although sometimes a small Gaussian located within 3 ppm from the Lorentzian may be added to improve the fit at the wings of the resonance. Resonance A usually requires more than two peaks in order to give a good fit. We estimate that the overall uncertainty in the values off obtained in this way is about &lo%. In Figure 5 , the chemical shift uAof resonance A is depicted as a function of nAand is compared with that of xenon adsorbed in pure NaY zeolite.8 We also show the fraction of supercages without CC14 molecules, g, and the fraction of xenon adsorbed in the supercages without CC14 molecules,f, as a function of ne in Figure 6. Discussion From Figure 2, one observes that a dosage of 1.9 CCl, molecules/supercage reduces the xenon end point from 9.2 Xe atoms/cage of the pure NaY zeolite6 to about 6.0 Xe atoms/cage.
(XENON ATOMS/CAGE IN SUPERCAGES WITHOUT CCI4)
Figure 6. Fraction of xenon adsorbed in supercages without CCll molecules, and fraction of supercages without CC14 molecules, g, as a function of ng,the xenon loading in supercages without CCl, molecules. The solid lines are fourth-order polynomial fits to the data.
Therefore, at the xenon end point, the molecular volume of two CC4 molecules is equivalent to about three xenon atoms. This is surprising because according to the van der Waals volume9 of a CC14 molecule, which is 2.7 times larger than that of a xenon atom, one expects an equivalence to about 5.4 xenon atoms. There are two possible explanations for this discrepancy. First, not all the CC14molecules are adsorbed into the zeolite. Second, CCll molecules may be better accomodated in the supercages than the xenon atoms and, therefore, require less space. In a m h r p t i o n experiment, there is competition between CCl, molecules and xenon atoms for space inside the zeolite. The extent of the competition depends on both temperature and xenon loading. At high loadings of xenon, xenon atoms may displace some of the CC14molecules initially adsorbed in the supercages. The displaced CC14 molecules may migrate to the exterior surface of the zeolite crystallites. Since the actual amount of carbon tetrachloride adsorbed into the supercages is smaller than that calculated from the amount of carbon tetrachloride introduced to the zeolite, it creates an impression that the molecular volume of the CC14 molecule is smaller. However, we have indirect evidence showing that this explanation is probably invalid. The evidence is from the xenon adsorption isotherms of the zeolite with various amounts of adsorbed carbon tetrachloride. At xenon loadings larger than the xenon end point, there is an equilibrium among the xenon in the supercages, that at the exterior surfaces of the zeolite and that in the gas phase above the zeolite. It is obvious that, a t a fixed temperature (below the freezing point of bulk xenon), if all the CC14 molecules are adsorbed inside the supercages of the zeolite, the xenon end point will decrease with increasing C C 4 loading. On the other hand, if some of the carbon tetrachloride is displaced by the xenon to the exterior surface of the zeolite, thermodynamics dictates that, at the xenon end point, the molar ratio of the carbon tetrachloride to xenon inside the zeolite must be constant, independent of the CC14 loading. In other words, the xenon end point of the xenon adsorption isotherm will be independent of CC14 loadings. Xenon adsorption isotherm measurements of NaY zeolite show that, with increasing CC14 loadings of 0.9, 1.9, and 2.9 molecules/supercage, the xenon end point decreases from 9.2 Xe atoms/cage of the pure NaY zeolite to 7.1, 6.0, and 4.4 Xe atoms/cage, respectively. This suggests that all the adsorbed carbon tetrachloride is indeed inside the zeolite and that the apparently small CC14 molecular volume is due to other causes. The tetrahedral shape of the CC14 molecule makes it more suitable to fit along the wall of the supercage. For instance, although the van der Waals radius of a chlorine atom is about 1.8 A, making it too big to pass through the 2.6-A window of the
5508 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
j3 cage, the chlorine atom of the CCl, molecule can still protrude further into the j3 cage than a xenon atom, which has a radius of about 2.2 A. Also, one can envision that the packing of tetrahedral pyramids is more efficient than that of spheres in a fixed enclosure. A better understanding of how xenon atoms and CCl, molecules are accommodated by the zeolite probably requires some sort of computer simulation based on realistic models of the zeolite. In Figure 5 , there is an abrupt change in the slope of the chemical shift of resonance A at an nA of about 3 Xe atoms/cage. One possible explanation is that, at this xenon loading, the CCl, molecules suddenly move to different positions in the supercages. At the new position, the CCl, molecule may present less of a hinderance for the xenon atoms to interact among each other. The stronger interaction among xenon atoms leads to the larger change in the chemical shift with the xenon density nA. For instance, if the initial locations of the CCl, molecules are near the center of the supercage, their large molecular size practically isolates the xenon atoms from each other. This explains the small initial slope of uAin Figure 5 in comparison to that of xenon in pure NaY zeolite. If the CCl, molecules move closer to the cage wall at higher xenon loadings, closer contact between xenon atoms becomes possible. We have previously shown3 that the chemical shift of xenon in pure NaY zeolite increases abruptly at xenon number densities between 7 and 8 Xe atoms/cage (see Figure 5 ) . We attribute this change to a quasi-gas-liquid phase transition of the adsorbed xenon. It is, however, unlikely that the change in the slope of uA in Figure 5 is due to such a transition. First of all, it occurs at a much lower xenon number density of 3 Xe atoms/cage. While the C C , molecules may serve as attractive centers for the xenon atoms, their presence in the confinement of the supercage prevents close contact between xenon atoms, a prerequisite for many-body interactions and the formation of xenon clusters in a gas-liquid transition. Second, assuming that, at the xenon end point, the xenon density-the number of xenon atoms per supercage divided by the actual free volume of the supercage available to the xenon-is the same for the xenon in the supercages of pure NaY zeolite as that in the supercages containing CCl, molecules, a direct comparison of the shape of the uAcurve in Figure 5 with that of xenon in the pure NaY zeolite can be made. It is clear that the change in the slope of uAat nA 3 Xe atoms/cage is more gradual than that at the phase transition of xenon in pure NaY zeolite. Furthermore, the linear dependence of uAand nA after the change in the slope suggests that the xenon remains gaseous. Figure 6 shows that f and g are not constant but go through broad maxima as the xenon number density nB increases. The maxima are more pronounced iffand g are depicted as a function of the xenon loading nerfor nA (see the upper scale abscissa in Figure 6). This variation in f can be anticipated from Figure 1 from the relative intensities of resonances A and B. One observes from the chemical shift curve of xenon in pure NaY zeolite (see Figure 5 ) that these maxima occur in the plateau region (3-7 Xe atoms/cage) of the curve, prior to the rapid increase in the chemical shift. We believe that the maxima in f and g are related to the xenon gas-liquid transition in the supercages without CC14 molecules. As discussed the rapid increase in the chemical shift is due to the formation of liquidlike clusters of xenon in the supercages. Prior to this transition, one expects large fluctuations in the xenon number density in the supercages, as reflected by the increases in the inhomogeneous line broadening.' Since, at a given temperature, the rate of change of the internal pressure P of a gas as a function of the gas density p is inversely proportional to the fluctuation in the density of the gas, one can writelo
R
(dP/dp)/(dP/dp), = P/(AP)2
(3)
where ( A P ) ~is the square of the density fluctuation and the subscript o denotes the value of that of the ideal gas. For a gas away from the gas-liquid transition, R is about unity since Ap = p1I2. As one approaches the phase transition, R becomes infinitesimally small for a bulk gas as Ap becomes very large. For
Cheung the xenon gas in Y zeolite, R remains nonzero at the phase transition because the small cage size tends to suppress the fluctuation. Nevertheless, R is an order of magnitude smaller than unity. Let M be the maximum number of xenon atoms which can be accomodated by a supercage. At the phase transition, Ap is of the order of M , and R = n / M < 1/M. Here n represents the average number of xenon atoms in the supercage. For Y zeolite, M = 10 and R < 0.1. When the xenon in the supercages without CCl, molecules undergoes a gas-liquid transition while that in the cages containing CCl, molecules does not, in order to minimize the internal pressure P, any increase in the xenon loading will lead to preferential adsorption of xenon into supercages without CC14molecules. This process continues until the gas-liquid transition is complete. After that, any addition of xenon will go to both types of cages. The presence of a maximum in f-at xenon loadings where large fluctuations in the xenon number density occur in supercages without carbon tetrachloride-is simply a manifestation of the phase transition phenomenon. The maximum in g is also associated with the same gas-liquid phase transition. According to eq 3, the increase in P with xenon loading can be minimized by increasing the fraction of the supercages without CCl, molecules. However, an increase in g reduces the entropy of mixing, which tends to distribute the CCl, molecules and xenon atoms uniformly among the zeolite cages. Therefore, we have the situation that an increase in g at the xenon loading, where a gas-liquid transition occurs in supercages without C C , molecules, will decrease the free energy of the whole system by the amount of the enthalpy of the phase transition, but this reduction is partially canceled by the loss in the entropy of mixing. For a given xenon loading, there is an optimal g a t which the free energy is at its minimum. Results in Figure 6 suggest that when a xenon gas-liquid transition occurs, the gain in enthalpy by increasing g is more than compensating the loss in entropy. Others1s2have shown that the adsorption of organic molecules or water of hydration in NaY zeolite at room temperature may lead to heterogeneous distributions of the adsorbates in the zeolite on a macroscopic scale. The heterogeneity leads to room temperature lz9XeN M R spectra with two resonances. However, for the adsorption of carbon tetrachloride in NaY zeolite, we observe only a single resonance at low xenon loadings, which suggests that either the distribution of the CCl, molecules is rather uniform or the xenon atoms initially adsorb only into supercages containing the carbon tetrachloride. Regardless of the type of initial distribution of the carbon tetrachloride in the zeolite, our interpretation of the variations off and g as a function of the xenon loading remains valid because the mechanism we invoke-the phase transition of the xenon adsorbed in supercages without CCl, molecules-is independent of the heterogeneity, whether it is on the macroscopic or microscopic scale. In conclusion, we have shown that when xenon coadsorbs with carbon tetrachloride into NaY zeolite, two lZ9Xe resonances, corresponding to xenon in supercages with and without C C 4 molecules, are observed. There is an absence of a gas-liquid transition in xenon sharing the supercages with CCl, molecules. On the other hand, the gas-liquid transition of xenon in supercages without CCl, molecules can affect the distribution of the carbon tetrachloride in the zeolite. References and Notes ( I ) Gedeon, A,; Ito, T.; Fraissard, J. Zeolites 1988, 8, 376. (2) Ryoo, R.; Liu, S.-B.;de Menorval, L. C.; Takegoshi, K.; Chmelka, B.; Trecoske, M.; Pines, A. J . Phys. Chem. 1987, 91, 6575. de Menorval, L. C.; Raftery, D.; Liu, S.-B.;Takegoshi, K.; Ryoo, R.; Pines, A. J . Phys. Chem. 1990, 94, 21. (3) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J . Phys. Chem. 1988, 92, 5 170. (4) Cheung, T. T. P.;Fu, C. M. J . Phys. Chem. 1989, 93, 3740. ( 5 ) Cheung, T. T. P. J . Phys. Chem. 1990, 94, 376. (6) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J . Chem. Phys. 1973, 59, 4540. (7) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136, 314. Fraissard, J.; Ito, T.; Springuel-Huet, M.; Demarquay, J. Proceedings of the Seventh International Zeolite Conference, Tokyo; Elsevier: Amsterdam, 1986; pp 393-400.
J. Phys. Chem. 1992,96, 5509-5512 (8) The xenon loading at the xenon end point of the pure NaY zeolite is slightly smaller than that given in ref 3. In ref 3, the presence of gaseous xenon above the zeolite at the xenon end point is neglected. In this paper, we have taken this correction into account.
5509
(9) Weast, R.; Selby, S.M. Handbook of Chemistry and Physics, 48th ed.; The Chemical Rubber Co.: Cleveland, OH, 1967; p D-108. (10) See, for instance: Huang, K. Staristical Mechanics; John Wiley & Sons: New York, 1963.
High-pressure Phase Transition in y-Hexanitrohexaazaisowurtzitane T. P. Russell,* P. J. Miller, Naval Surface Warfare Center, White Oak, Maryland 20903
G . J. Piermarini, and S. Block Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: March 20, 1991; In Final Form: July 8, 1991)
A high-pressure phase transition in y-hexanitrohexaazaisowurtzitane (HNIW) has been described at 0.7 f 0.05 GPa and room temperature. The new high-pressure polymorph, f-HNIW, was observed in a diamond anvil high-pressure cell with (1) Fourier transform infrared spectrascopy (FTIR),(2) energy-dispersive X-ray powder diffraction (EDXD), and (3) polarized light microscopy. The y-f transformation is rapid and reversible in single crystals in a hydrostatic pressure transmitting medium (1:l n-pentane:isopentane) and can be detected by a sharp, well-defined change in birefringence with polarized light. FTIR spectra and EDXD data have been obtained for both the f and y phases as a function of pressure. The new high-pressure f polymorph could not be retrieved to ambient conditions.
Introduction Recently, considerable effort has been made in understanding the crystal and molecular structures and chemistry of hexanitrohexaazaisowurtzitane (HNIW), a new polycyclic nitramine compound.'-' The molecular structure of HNIW (C6H6NI2Ol2) consists of a basic isowurtzitane cage with one NO2 group appended to each of the six nitrogen atoms as shown in Figure 1. Four HNIW polymorphs (a,j3, y, and e) have been identified and confirmed by single-crystal X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) techniques.'+ Evidence for a fifth polymorph, 6, has been reported, but its existence has not been ~ e r i f i e d .The ~ crystal structures of a-,0-, y-, and eHNIW, which exist at ambient pressure and temperature, have been determine? The particular method of preparation or solvent recrystallization procedure used determines which phase is produced. Unlike the j3, y, and c polymorphs, which have been prepared in the pure state, the a-HNIW polymorph is directly prepared only as a clathrate compound. Several different clathrates have been identified, including an H 2 0clathrate with varying degrees of The H20 may be removed by heating the sample to 100 OC for several hours, giving various unit cell packings of anhydrous a-HNIW. However, to date, the completely anhydrous form has not been prepared directly. According to the phase rule, not all four of these polymorphs can be thermodynamically stable at ambient temperature and pressure. Furthermore, it is highly improbable that three phases are in equilibrium (an invariant triple point) or that even two phases are in equilibrium (a univariant line) in the region of ambient temperature and pressure. It is more likely that a bivariant system exists in this region and that only one polymorph is the stable phase. Hence, three of the polymorphs must be metastable, but it is not certain which ones are. An attempt to determine the most stable phase in HNIW has been made.' The a,8, and c polymorphs exhibit an endothermic transformation to the y phase when they are heated to temperatures in the range 155-198 O C . Differential scanning calorimetry (DSC) measurements were used to determine the heats of transition to the y phase based on the integrated area of the endothermic response for the a,i3, and e polymorphs. The d o - y transition was measured to have the largest heat of transition, 5.13 kcalfmol, excluding the a phase data becauseit includes the energy required to liberate
the clathrate water. Furthermore, the DSC measurements were made a t different heating rates, which affect the transition rate. Thus, activation energies for the transition could be calculated by treating the data according to Arrhenius kinetics formalism. The activation energy was highest for the c phase, 69 kcal/mol. The results gave the following order of stability: a(with water) > e > a(anhydrous) > j3 > y. The molecular symmetry of the c phase is the highest of the four polymorphs and thus it is thought to be the most stable. As far as we know, no high-pressure HNIW polymorphs have been reported. In this paper we report the discovery of a highpressure phase transition in y H N I W and the identification of the new high-pressure phase (f-HNIW) by infrared absorption. Experimental Section'z The experimental techniques employed in the present work have been reported and only a brief description will be given here. A high-pressure diamond anvil cell (DAC) for FTIR spectroscopy at elevated temperatures is employed. This pressure cell is fabricated from a high-temperature, high-strength Inconel 718 alloy and is designed for FTIR measurements with static heating up to 800 OC. Pressures are measured by the ruby fluorescence technique and are accurate to f0.05 GPa when the measurements are made in a hydrostatic environment at room temperature. For the FTIR measurements the accuracy in the pressure is lower, 0.15 GPa, in the absence of strictly hydrostatic conditions. A He-Cd laser operating at a maximum continuous wave power of 20 mW to avoid heating effects was used to excite the ruby fluorescence R lines. The focused laser beam is approximately 15 pm in diameter, which is roughly the same size as the ruby sphere used as the pressure sensor. The diamond anvil pressure cell can be mounted on a micrometer positioning device for (A) optical polarizing light microscopy (OPLM) and ruby fluorescence pressure measurements, (B) FTIR spectroscopy, and (C) EDXD measurements as described below. A. Optical Microscopy and Ruby Fluorescence Measurements. Our ruby fluorescence pressure measurement system, which is also used for optical polarizing microscopy studies, has been described earlier in great detaiL5 A massive micrometer x,y,z positioning device permits horizontal positioning of the sample (or ruby) in the DAC onto the optic axis of the microscope ( x q ) , while focusing
0022-3654/92/2096-5509%03.00/00 1992 American Chemical Society
