J. Phys. Chem. 1992, 96, 2010-2015
2010
pD 7.72 suggests some conversion to the imine free base. Experiment A. A stock solution of 33 mM Na2DP04in D20 containing 0.45 M NaCl and TSP (sodium trimethylsilyl-d4propionate) shift standard was prepared along with a separate stock of 0.15 1 M anabaseine dihydrochloride in 1 mL. of the above phosphate buffer. A second phosphate buffer consisted of 33 mM Na2DP04and 1.9 mM TSP but no NaCI. An 0.375 M SDS solution was made by dissolving solid SDS in the phosphate buffer free of NaCI. In turn, six samples containing 30 mM ANAB were prepared 11 h before making NMR measurements by mixing the ANAB with the SDS and phosphate buffer. Measurements were made with a Varian VXR-300 FT instrument at 25 f 1 O C using a 90’ pulse and a delay of 17.5 s (5 times the longest Tl) between transients. Samples were given at least 10 min to equilibrate to probe temperature. Each spectrum is the result of 64 transients. The reported ratio of amino ketone to imine is the average of the several values calculated using area ratios for several pyridyl protons of each form of the substrate, for example, the area for H-2’ of amino ketone to that for H-2’ of imine. The average deviation is of the order 5%. Chemical shifts for the two tautomers have been reported.14 The sample containing 120 mM SDS and 30 mM ANAB was measured again after 36 h. No significant change in composition was found. Experiment B. Buffer was prepared with 0.102 M KD2P04 and 0.019 M Na2DP04. Stock 0.15 M ANAB (from monohydrochloride) and 0.500 M SDS solutions were constructed using this buffer. Four samples for NMR analysis containing 32 mM (34) Batterham, T. J. NMR Spectra of Simple Heterocycles; Wiley-In-
terscience: New Yotk, 1973.
ANAB were prepared by mixing the solutions. The small amount of potassium dodecyl sulfate (presumably) which precipitated dissolved on warming. Spectra were recorded 30 min after preparation on a GE QE300 spectrometer using a 90’ pulse width and a delay between transients of 15 s; 64 transients were recorded. After standing overnight the sample with 0.200 M SDS was reexamined; no significant change was found in the spectrum. The pD values of the 0.400 M SDS mixture and that without SDS were 6.61 and 6.58, respectively, showing no real difference. Experiment C. Stock solutions of anabaseine monohydrochloride and SDS in D 2 0 without buffer were mixed with D 2 0 as needed to prepare 15 and 59 mM ANAB containing 0.300 M SDS. Proton NMR spectra were recorded as above 17 h after preparation. “Ultrapure” sodium dodecyl sulfate from U S . Biochemicals and D 2 0 (99.9%) from Merck & Co. were used. Deuteriated phosphate salts were made by dissolving the protio compounds in D 2 0 and then evaporating to dryness. Solution pD. Bates buffers35in H 2 0were used to standardize the Radiometer PHM 64 pH meter using a Radiometer glasscalomel electrode (Model GK 2321C). Meter readings were converted to pD by adding 0.40.36 Acknowledgment. The Division of Sponsored Research at the University of Florida kindly supported this work. Registry No. SDS, 151-21-3; anabaseine, 3471-05-4. (35) Bates, R. G. Determination of pH. Theory and Practice; Wiley: New York, 1954. (36) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960,64, 188.
Dielectric Relaxation of Intercalated Cd2P2S6 J. A. Read, C. Chick,and A. H.Francis* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09- 1055 (Received: August 2, 1991; In Final Form: October 21, 1991)
The dielectric loss of single crystals of pyridine-intercalated Cd2P2S6was examined over the temperature range -130 to +300 “C. The intercalation of amines involves a complex redox-disproportionation reaction leading to ionic and dipolar products that produce a large dielectric loss. Measurement of the dielectric loss provides information about the chemical and dynamical behavior of the intercalated species. Because the dielectric loss is sensitive to a different component of molecular motion than 2H-NMR spectroscopy, the information obtained is complementary to that extracted from magnetic resonance studies. Dielectric loss processes were observed at temperatures above =25 OC. The absence of measurable dielectric loss associated with the dipolar reorientation below 25 OC revealed previously by ESR and NMR spectroscopies indicates that the lowtemperature reorientation does not alter the projection of the intercalate dipole on the crystallographic c* axis. Above 25 OC, the fully rotationally averaged pyridine dipoles do not contribute to the dielectric loss. The rapid increase in the loss is due to pyridinium ions weakly bound to lattice cation vacancies. The activation energy for the high-temperature dielectric loss process was found to depend upon the extent of intercalation. The activation energies obtained are compared with the enthalpy of intercalationand deintercalation obtained from recent calorimetric measurements, and a model for the contribution of the dipolar reorientation to the dielectric loss was developed.
Introduction
The transition metal chalcogenophosphates are broad-band semiconductors with the general chemical formula M2P2X6(M = transition metal, X = S and Se) that crystallize with a layered structure.’,2 Because the repeating layers are held together only by weak van der Waals (vdW) interactions, it is possible to introduce a variety of atoms, ions, or molecules into the vdW in( I ) Klingen, V. W.; Ott, R.; Hahn, H. Z . Anorg. Allg. Chem. 1973,396, 271. (2) Wittingham, M.
S.Prog. Solid State Chem. 1978, 12. 41.
terstices to form intercalation compound^.^-^ In general, the intercalate layer is moderately to highly disordered, so that detailed structural determinations by diffraction methods are difficult or impossible. Because of the importance of this class of materials, a wide range of spectroscopic techniques have been employed to determine aspects of the intercalate layer (3) Schtjllhorn, R. Physica 1980, 994 89. (4) Brec, R.; Schleich, D.; Louisy, J.; Rouxel, J. Ann. Chimi (Puris) 1978, 3, 341. (5)
Clement, R.; Gamier, 0.;Mathey, Y. N o w . J . Chim. 1982, 6, 13.
0022-365419212096-2010$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 201
Dielectric Relaxation of Intercalated CdzP2S6 structure. ProtorP9 and deuterium solid-state NMR,Ibl3 ESR,I4-I6Raman," and IRISspectroscopies have been utilized. The intercalation of amines into lamellar chalcogenide lattices is thought to involve a complex redox-disproportionation reaction leading to a variety of ionic and dipolar products. The presence of ions and weakly bound dipoles within the vdW gap has a profound effect upon the dielectric properties of the intercalated crystals. Dielectric loss measurements may be used to determine the activation energy for thermal motion, the mean orientation, and the concentration of charged or dipolar intercalate species. Dielectric loss measurements are of particular utility because they are sensitive to a different component of molecular motion than 2H-NMR spectroscopy. Only the component of molecular diffusion that alters the orientation with respect to the direction of the applied electric field is sensed. Also, the dielectric loss is sensitive to the intercalate's average structure over a period of milliseconds, whereas NMR spectroscopy probes a more transient structure. This technique is particularly well suited to the study of amine intercalated MZP2S6lattices because the large dipole moments of amines (-1 D) produce large dielectric permittivities. Additionally, the layered structure of the MzP2S6lattice results in thin crystalline plates that are geometrically well suited for single crystal dielectric measurements. This paper reports measurements of the dielectric loss of Cd2P2S6intercalated with pyridine and pyridinium ion. The results obtained are compared with data from recent ZH-NMR and calorimetric studies of pyridine intercalated Cd2P2S6. Experimental Section Sample Preparation. The Cd2P2S6used in this work was synthesized by procedures described in detail e1~ewhere.l~Large single crystals were grown by the iodine transport method in sealed quartz tubes. Chemical microanalysis of individual crystals yielded a typical composition of 44.9 wt % Cd, 12.82 wt % P, and 42.18 wt % S. The composition indicates that the crystals characteristically contain 3 4 % excess sulfur. The nonstoichiometric sulfur is retained in the vdW interstices and may be removed by heating the crystals above 125 O C . The crystals were treated with pyridine as described below, to produce the samples for dielectric measurements. Crystals of Cd2P2S6,typically of dimensions 3 X 3 X 0.1 mm, were placed in dry pyridine or solutions of 10 vol % water in pyridine and heated to 75 OC for 24 h. The presence of water had a dramatic effect upon the rate of reaction. Pyridine did not appreciably intercalate into Cd2PzS6in the total absence of water. Even after months of reaction at room temperature with rigorously dried pyridine, crystals of Cd2P2S6 exhibited no measurable weight gain and no change in their dielectric behavior. It has been suggested that water is necessary for the reaction to take place, as rigorously dried amines do not react at a11.20,2' SchollhornZ2has proposed a redox-dispropor~
~~
(6) Silbernagel, B. G.;Gamble, F. R. Phys. Rev. Left. 1974,32, 1436. (7) Gamble, F. R.; Silbernagel, B. G. J . Chem. Phys. 1975,63,2544. (8) Silbernagel, B G.; Dines, M. B.; Gamble, F. R.; Gebhard, L. A.; Whittingham, M. S. J . Chem. Phys. 1976,65,1906. (9) Kleinberg, R. L.; Silbernagel, B. G.Solid State Commun. 1978,33, 21. (IO) McDaniel, P. L.; Liu, G.;Jonas, J. J . Phys. Chem. 1988,92,5055. (11) Lifshitz, E.; Vega, S.; Luz, Z.; Francis, A. H.; Zimmermann, H. J . Phys. Chem. Solids 1986,47, 1045. (12) McDaniel, P. L.; Barbara, T. M.; Jonas, J. J . Phys. Chem. 1988,92, 636
(13) Molitor, M.; Muller-Warmuth, W.; Spiess, H. W. 2.Nuturforsch.
1983,38A,237. (14) Lifshitz, E.; Francis, A. H. J . Phys. Chem. 1982,86,4714. (15) Lifshitz, E.; Gentry, A. E.; Francis, A. H. J . Phys. Chem. 1984,88, m i > 1 (low temperature, long relaxation time), we may write for the Debye loss et' = d ' D 5 (e, - e , ) / U T (9) and using eq 7 we obtain (27) Debye, P. Phys. Z.1912, 13,97; 1934, 36, 69. (28) Debye, P.;Sack, H. Hand. d. Radiol. 1934, 6 (2), 69.
+ In [(e,
- €,)/UT,)]
= n l ~ ~ ( ce o )s , m
em
us(e,)= -u,o
(11)
el
(12) where Usois the energy of the completely oriented dipole and 8, is the angle between the dipole and S. In order to determine the purely dipolar contribution to e, - e,, it is necessary to compute the thermally averaged dipole moment in the direction of the applied electric field or (COS
e), =
COS
j c o s 8 exp(-(UE + Us)/kT) d e jexpl-(v,
T
1/. = (1 / T o ) exP(-ED/kT)
= -ED/kT
where N is the number of identical, noninteracting dipoles, ( ) denotes a thermal average, and 8 is the angle between the applied field E and the dipole moment p. To examine the temperature dependence of e, - e,, we make use of a model introduced by Debye to account for the dipolar contribution to the permittivity of liquids. The model may be easily adapted to the present case of a two-dimensional arrangement of dipoles. Debye assumed the existence of long-range, nonbonded interactions that established a preferred direction for the solute dipole. In a similar manner, the intercalate dipoles will adopt a preferred orientation at sufficiently low temperatures as a result of interactions within the vdW gap. We define S as a direction of preferred orientation and eSas the angle between the applied field direction E and S. The energy of the dipole is
(6)
is the Debye relaxation time, e, and e, are the static and high-frequency dielectric permittivities, and w is the frequency of an oscillating electric field (E) directed perpendicular to the layers. In the Debye equations, the dependence of dDand e"D on frequency is given explicitly but the dependence on temperature arises through the variations of e,, E,, and T as discussed below. It is not uncommon, particularly in a heterogeneous material, to observe a range (dispersion) of relaxation times, and theories have been developed for dielectric relaxation and magnetic resonance that account for this effect with additional parameters. The behavior observed for the intercalated Cd2P2S6materials suggests that for the diffusionally averaged structure (millisecond time scale) a single relaxation time, whose value changes with intercalant concentration and heat treatment, is appropriate. We d a t e an activation energy with the relaxation time in the usual fashion:
In
(lo) Thus, an individual dipolar relaxation process is characterized by a linear region in the f(l/T) plot. The activation energy and preexponential factor can be extracted from the slope and intercept, provided that e, - e, is independent of temperature. The quantity e, - e, represents the dipolar contribution to the static (dc) permittivity and contains temperaturedependent terms. The magnitude of t, - e, is given by E
+ US)/kTl
de
(13)
where UE is the energy of the dipole due to the applied field UE= -WE COS 8 (14) For dipoles that adopt an angle 8 with respect to the applied field, we obtain
(a e)E = L b ) cos eS+ pE/kqcos* eS[l - 3LCy)/y - L2] + L/y] (15) where L b ) is the Langevin function and y = Uso/kT. If the preferred direction lies parallel to the layers (and, as always, the field is perpendicular to the layers), eS ~ / 2 while ; for a perpendicular orientation, 8s = 0 or ?r. Only the parallel orientation is considered further since the perpendicular orientation is not in agreement with NMR data. From eqs 15 and 11, we obtain for pyridine dipoles oriented parallel to the basal planes
(cos e)E = @/kT[Lb)/yl
(16)
and t,
- 6,
= Np'Lb)
/ Uso
(17)
Equation 17 is introduced into eq 10 to obtain the following relation for the temperature dependence of the dielectric loss f(l/T) = -ED/kT
+ In [NLb)p2/us0WT0]
(18)
If the concentration and structure of the dipolar centers do not change Over the range of temperatures investigated, eq 18 predicts a linear relation between In e" and 1/T. In order for eq 18 to apply, it is also necessary that, over the range of temperatures examined, WT >> 1. The activation energy for dipolar relaxation and the correlation time may be obtained from the slope and y-axis intercept, respectively. The temperature dependence of the dielectric loss is conveniently divided into two regions for discussion: the low-temperature regime 30 OC.
2014
l -1-]-[
Read et al.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
!
I I
(a> (b 1 (c> Figure 6. Orientation models for pyridine dipoles within the VWG: (a) as found by NMR spectroscopy in powders; (b) as found by NMR spectroscopy in single crystals; (c) an alternative model. The axis of thermally activated rotations is shown by the dashed line.
Low-Temperature Reorientation. EPR and 2H-NMR measurements indicate that pyridine in Cd2P2S6undergoes large amplitude rotational reorientation at temperatures as low as -25 OC. Manganese spin probes have been used to detect pyridine motion in Cd2P2S6(pyridine)crystals using EPR spectroscopy.14 An onset of molecular motion was detected at -35 OC from the increase in the EPR relaxation line width, and a phenomenological relaxation model yielded an activation energy of =13 kJ/mol for the process. Lifshitz et a1.l' examined the deuterium NMR spectra of selectively deuterated pyridines intercalated into single crystals of Cd2P2S6and concluded that all large-scale molecular motion was absent below -60 OC and that the pyridine intercalate adopted an orientation coplanar to the lattice planes (Figure 6b). Between -60 and 25 OC, the molecules initiated rapid rotational diffusion (on the NMR time scale) about an axis perpendicular to the molecular plane. Somewhat different conclusions were reached by Jonas and co-workers,10s12 who examined the deuterium NMR spectrum of pyridine-intercalated Cd2P2S6powders. Their analysis of the quadrupole splitting concluded that below -13 OC the pyridine adopted an orientation with the molecular plane perpendicular to the lattice planes and the C, axis parallel to the planes (Figure 6a). Rotational diffusion occurred about an axis perpendicular to C2 and in the molecular plane. A third model for the equilibrium orientation and rotational diffusion could not be ruled out by the NMR spectra and is shown in Figure 6c. Below =30 OC, the dielectric loss of 1 is constant. The absence of measurable dielectric loss indicates that there is no easy displacement of charges or dipoles in the direction of the applied field below this temperature. Therefore, the rotational motion revealed by ESR and NMR spectroscopies must occur either about the dipolar axis or about an axis that is perpendicular to the dipole and parallel to the applied field. All three low-temperature motions depicted in Figure 6 are consistent with the temperature independence of the dielectric loss below 30 OC. High-Temperature Relaxation, Above 30 OC, the rapid increase in the dielectric loss indicates the onset of field-induced charge displacements in the lattice perpendicular to the lattice planes. The 2H-NMR spectrum does not exhibit changes in this temperature range; the spectra are already fully motionally averaged with no evidence of a static structure component remaining. The function f( 1/ 7') is plotted in Figure 3 for two successive scans of the temperature from 25 "C to about 230 OC for I. In curve A, a quasi-linear plot is obtained with gradually increasing slope as 1/ T decreases. The repeat scan (curve B), obtained immediately after rapid cooling, exhibits a slope nearly equal to the hightemperature slope of curve A. The comparison of the dielectric loss of I with the TGA/DSC thermograms shown in Figure 5 shows that the dielectric loss increases steadily as neutral pyridine is deintercalated. Evidently, the loss mechanism does not involve the orientational polarization of the neutral pyridine directly. However, curve B, obtained after deintercalation of the netural pyridine, is more linear than curve A and has both greater slope and intercept. The lost pyridine was therefore associated with the dipolar center in such a manner as to effect its relaxation behavior.
The second endothermic peak in the DSC thermogram of I is due to the decomposition of the pyridinium ion, and the onset of this process has a dramatic and irreuersible effect upon the dielectric loss. We conclude that the increase in the loss between 30 and 200 OC is due to a relaxation process of the pyridinium ion "solvated" by neutral pyridine. The dielectric behavior is consistent with a model for the dipolar center in which the pyridme-solvated pyridinium cation is bound in the vicinity of a lattice cation vacancy thereby minimizing the crystal coulombic energy. Schematically, we represent this situation as (py),,(pyH+)(Vz-). Because the large pyH+ cation must be located in the vdW gap, Coulombic energy is minimized when the pyH+ cation occupies a position displaced from the vacancy along the stacking axis. Such an arrangement produces a local dipole directed parallel to the stacking axis that will interact maximally with the applied field. At temperatures below 30 OC, the pyH+ cation is tightly bound to the cation vacancy. As the temperature is raised above 30 OC, the cation undergoes thermally activated librational motion with a correlation time similar to that of molecular rotational diffusion. The thermally activated librational motion of pyH+ produces a very strong modulation of the dipole moment in the direction of the external electric field, leading to the large dielectric loss observed. Between 30 and about 100 OC, neutral pyridine solvent is deintercalated: As pyridine is lost, the environment around the dipolar center changes, which modifies the relaxation behavior. The increase in activation energy suggests that the solvent-free pyridinium cation binds more tightly to the nearby vacancy. At about 200 OC, the pyH+-V2- center is thermally decomposed
-
(pyH+)(V2-) PYt + (H+)(V2-) and the pyridine produced is deintercalated. The residue H+VZcenter is much less polarizable than the parent pyH+V2-center and is expected to have a much faster relaxation time. This reaction results in the observed rapid and irreversible decrease in the dielectric loss. This model of the dipolar center is supported by the dielectric loss behavior of Cd2P2S6 intercalated with pyH+ by cation exchange from aqueous solution. Figure 4 is a plot off( 1/T) for three successive temperature scans. The slope off( 1 / T ) decreases slightly with each scan and approaches the slope of the f(l/T) plot for the pyridine/pyridinium crystal (I). We believe that the decrease in slope arises in the following fashion. During the first heating cycle, pyridine is produced through pyridinium ion decomposition. At the end of the cycle, the crystal is allowed to cool to room temperature and some pyridine remains in the crystal, solvating the remaining pyridinium ions. The presence of solvating pyridine reduces the activation energy for dielectric relaxation of the pyH+ center. Activation energies for the dielectric relaxation processes were obtained from thefll/T) plots in Figures 3 and 4 and are given in Table I. As the concentration of neutral pyridine decreases, both the activation energy and the y-axis intercept increase. The activation energy increases from 23 W/mol for the pyridinesolvated pyridinium center to 38 kJ/mol after deintercalation of most of the solvating pyridine. From eq 18, the increase in the intercept may be associated with an increase in the local dipole moment or decreases in either the local crystal field (Uso)or the ) neutral pyridine is stripped from the lattice. correlation time ( T ~ as The correlation between EA and the intercept is seen by plotting E A vs the value of the intercept in Figure I. -cs of Pyridine Intercalntim The heat of reaction and energy of activation for the pyridine intercalation of Cd2P2S, An activation have been studied by Randzio and E~erieGoates.~~ energy of 58.1 kJ/mol was obtained for the deintercalationprocess. This result is included for comparison with the dielectric loss data in Table I. The enthalpies for intercalation and deintercalation (29) Randzio, S. L.;Boerio-Goates, J. J . Phys. Chem. 1987, 91, 2201.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 2015
Dielectric Relaxation of Intercalated Cd2P&
TABLE I: Activation Energies and Reaction Enthalpies for Some Amine Intercalation Compounds of Layered Chalcogenide Lattices ~~
intercalate CH,NH, (CH,),NH (CH3)JN (CH3)4N+ NH3 NH3 NH3 NH4+ NH3 NH4 CJHSN CJH5N CSHSN CSH5N C5H5N PYIPYH+ PYH+/ H20
host lattice NbS2 NbS; NbS2 NbS2 NbS2 Tis2 Tis2 TIS2 Tis2 Tis2 Cd2P2S6 Cd2P2S6
method'' NMR NMR NMR NMR NMR NMR DSC DSC VP VP
sc sc sc sc
Cd2P2S6
AH, kJ/mol
70, s 2 x 1044 2 x 10-14 1.5 X 3.0 x 10-14 6.0 x 1044
43.9 92.0 64.9 -54.4 75.8 -77.2 -73.3 58 13 23-38 35-46
Cd2P2S6
EPR DR DR
EA,
kJ/mol 36.6 30.8 23.0 22.5 23.3 55.6
Cd2P2S6 Cd2P2S6
Cd2P2S6
2.7
X
lo4
10-8-1 0-10 10-10-1 0-13
reaction
deintercalation deintercalation deintercalation reintercalation deintercalation intercalation intercalation deintercalation pyridine rotation deintercalation deintercalation
ref 30 30 30 30 30 31 26 26 26 26 28 29 29 29 14
ODR = dielectric relaxation, VP = vapor pressure, SC = scanning calorimetry, DSC = differential scanning calorimetry. 1
/
I
-20.9
41.7
-fS.S
44.3
-IZ.l
motion (rotational or translational) of NH4+ between 3 K and its deintercalation temperature. Thus, the NH4+cation, like the pyH+ cation, appears to be tightly bound to the lattice. The activation energy of NH3rotation at low temperatures was found to be 10 kJ/mol, which is only slightly less than the activation energy for pyridine rotation obtained from EPR measurement~.~~ The NH3 diffusional activation energy (55.6 kJ/mol) was comparable in magnitude to the deintercalation enthalpy of NH3 (43.9 kJ/mo1).26 Comparison of the thermodynamic and kinetic data for MX2 and Cd2P2S6intercalated lattices (Table I) indicate the activation energies and relaxation times are comparable. The activation energies and relaxation times measured using dielectric spectroscopy compare well to those measured by other methods.
L" NLop"
Gore
Conclusion
Figure 7. Activation energy variation with degree of intercalation.
Comparison of TGA/DSC thermograms and the dielectric lass function of pyridine/pyridinium intercalated Cd2P2S6lattices has are similar, with a mean value of 76.5 f 0.7 kJ/mol for pyridine. provided additional structural, chemical, and dynamical inforComparison with Other Amine IntercalationCompolmds. There mation about these materials. Pyridinium cations, solvated by are few studies of the dynamical behavior of amine intercalates neutral pyridine, rapidly intercalate CdzP2S6 via a cation exchange in lamellar materials with which to compare the present results. mechanism. The pyridinium cation is bound in a structurally The data that is available deals predominately with the layered well-defined fashion at cation vacancies and is thermally decomdichalcogenide lattices (MX,), whose structures are similar to the posed at temperatures above 200 OC. The pyridine solvent unCd2P2S6lattice. Molitor et examined the proton NMR dergoes rapid rotational diffusion at room temperature and spectra of the ammonium ion, methyl amine, and tetramethyldeintercalates slightly above room temperature. The effect of ammonium cation intercalates of NbS2. Relaxation times were pyridine solvation of the pyridinium cations is reflected in the obtained for rotation about the axis of the nonbonding orbital activation energies and relaxation times of both I and the pyri(dipolar axis), which was found to lie parallel to the lamella in dinium intercalated crystals. most cases. The reorientational dynamics in this regard are similar The activation energies and relaxation times measured using to those observed for pyridine in Cd2P2S6.The activation energies dielectric spectroscopy compare well with those measured using and correlation times for some of these materials are reproduced N M R techniques. The similarity in the activation energies and in Table I for comparison with the results of this work. relaxation times of the MX, and Cd2P2S6compounds would Recent NMR experiments on [(NH4+)o.22(NH3)o.37TiS2]o~22suggest that the interactions of amine intercalates in the vdW gap found that NH4+cations rotate randomly and rapidly on the NMR of these two host lattices are qualitatively similar. time~cale.~' No evidence was found for thermally activated (30) Molitor, M.; Miiller-Warmuth, W.; Spiess, H. W.; SchBllhorn, R. Z. Naturforsch. 1983. 38a. 237. (3 f) O'Bannon,' G. W.; Glaunsinger, W. S.;Marske, R. F. Solid State Ionics 1988, 26, 15.
Acknowledgment. We acknowledge support from the National Science Foundation (Grant DMR 8818371) and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
