NMR studies of metal-ammonia and ammonia in layered chalcogenide intercalation compounds

N M R STUDIES OF M E T A I ~ A M M O N I A AND A M M O N I A IN LAYERED C H A L C O G E N I D E INTERCALATION C O M P O U N D S E. WEIN, W. MOLLER-WARMUTH Institut J~r Physikalische Chemie der Universitdt Miinster, D-4400 Miinster, FederalRepublic of Germany

and R. SCH(~LLHORN lnstitut J~r Anorganische und Analytische Chemie der Technischen UniversitiitBerlin, D-1000Berlin, Germany

Received 10 June 1986

~HNMR spectraand relaxation times havebeen measuredoftbe intercalationcompoundsA~(NH3)rMS2and (NH4)x(NH3).~MS2 with A=Li, Na, K, Ca, Sr, Ba and M=Ti, Nb, Ta, Mo. Analysisof the structured line reveals a rotation of the NH3 groupsabout their Ca axes oriented parallel to the sheets. Upon increasingthe temperature an additional reorientational motion about an axis perpendicular to the layer planes occurs. The guest sublattices may be considered as two-dimensional electrolyte systems with either A÷ or NH~- as a central cation solvated by NH3 molecules. In the case of guest metal ions no proton exchangehas been observed, in contrast to the compoundswith ammonium ions, where the exchange appears to be rapid. The parameters of the molecular rcorientation as extracted from the spectra and relaxation rates are clearly different for systems containing alkali and alkaline earth cations. For (NH4)x(NH3)thiS2 in addition to the relaxation rate maximum due to the NH3 reorientation a peak at lowertemperature is observedwhich is assigned to NH~-rotation.

1. Introduction The formation of intercalation compounds by inserting guest species into a host matrix of suitable structure has attracted great interest in both basic research and technological application. Attention has focused particularly on layered host lattices with electronic conductivity such as binary transition metal chalcogenides with the stoichiometry MX2 (M = Ti, V, Nb, Ta, Mo; X = S, Se). Their structure is relatively simple and a great variety of guest species can be intercalated by topotactic redox and exchange processes [ 1,2]. Direct intercalation of alkali m©tal ions has been known for some time; due to the specific ability of layered systems to accommodate to the size of the intercalated species solvated phases, molecular ions, and Lewis bases can be inserted as well. The understanding of properties of these intercalation compounds and the ability to synthesize new materials depend primarily on the 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

knowledge of the host-guest and guest-guest interactions and on the motional behaviour of the guest phase. Nuclear magnetic resonance ( N M R ) techniques have proved to provide powerful tools for studying such phenomena in hydrated chalcogenides Ax(H20)~MX2 with alkali and alkaline earth ions [ 3-6] and in hydrogen bronzes HxMX2 [ 7 ]. A review which includes also host lattices with framework structure was published elsewhere [ 8 ]. In connection with NMR studies on intercalation compounds of layered chalcogenides with methylamines [ 9 ], ammonia as a guest species of fundamental importance has also been investigated with NbS2 as the host lattice. That work followed NMR wideline [ 10,11 ] and relaxation measurements [12-14] of Silbernagel et al. on (NHa)TaS2 and (NHa)TiS2. From all these studies it is clear that NHa is oriented in the interlayer space with the Ca axis parallel to the layer planes and participates in various types of motion. Evidence has been provided in

232

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

addition that these compounds have to be described as NH~ phases solvated by excess NH3, i.e. (NH+)x(NH3)y[MX2] [15]. There has been still some controversy, however, as far as the presence and molecular motion of ammonium ions and the role of proton exchange are concerned. The assignment of the two spin-lattice relaxation maxima observed for (NH 3) NbS2 was not unambiguously cleared [ 9 ], and the experimental results for samples which are not fully loaded with NH3 differ from each other [ 9,14 ]. In order to obtain further information about NH3 guests in layered MS 2 host lattices and to examine related solvated guest phases we have now prepared metal-ammonia intercalation compounds Ax(NH3)~VIS2 for NMR studies in combination with the(NH4) x(NH3 ) yMS2 system. From the standpoint of the preparation of these intercalation complexes there are, however, severe differences as far as the stability of the various members of this system is concerned. Intercalation compounds with the Lewis base NH3 can be obtained readily with TiS2, 2H-NbS2 and 2H-TaS2; in these cases the valence band of the host lattice is empty or partially filled. 2H-MoS2 is a semiconductor with a filled dz2 band and does not react with NH3, since the redox potential of the corresponding reaction 2NH3--,N2 + 6H ÷ + 6e- [ 15 ] is not sufficiently negative to allow a filling of the empty, higher-lying conduction band. The redox potentials of alkali and alkaline earth metals dissolved in liquid NH3 are, however, low enough to allow intercalation ofsolvated ions also into MoS2 [ 16 ] according to xA + yNH 3"~ MoS2 --+A+ ( NH3 ) y( MoS2 ) x - . The present NMR studies confirmed that these compounds are rather stable for long times (years) in closed systems. Similar reactions proceed readily with TiS2 and 2H-NbS2. It turned out, however, that the latter phases are rather unstable and the investigations of the solvated metal intercalation compounds have thus been concentrating mainly upon MoS2 as the host lattice. Major questions to be addressed in the present study are structure and motion as well as proton exchange i n t h e metal-ammonia and ammonia systems. We have also performed single crystal studies which are reported elsewhere [17 ].

2. Experimental The host lattices 1T-TiS2 and 2H-NbS2 were prepared isothermally at 750°C from the elements (purity>99.5%) in sealed quartz ampoules with a reaction time of 2-3 weeks. Excess sulfur-vapour pressure was required (metal/sulfur ratio 1:2.5) in order to avoid nonstoichiometric samples (MI +~$2). The crystals obtained were sieved and the fraction with a size below 0.25 mm used for the experiments. 2H-MoS2 was a commercial pure product with a Mo/S ratio of 1 : 2.0. It turned out to be necessary to wash MoS2 before the reaction with H 2 0 and acetone and to dry it at l03 Torr/100°C since we observed that on exposure to air sulfuric acid is formed in a thin layer at the crystallite surfaces by Mo catalyzed oxidation of the lattice anions. Lattice parameters of all binary phases agreed with the data reported in the literature. All synthetic operations were carded out under argon atmosphere on a high vacuum line, purification of NH3 was afforded by condensation on sodium metal. The preparation of the alkali and alkaline earth metal/ammonia intercalation compounds was achieved by adding the metal sulfide to solutions of the electropositive metals in liquid ammonia under normal pressure. The reaction proceeds within a few minutes as indicated by the decoloration of the liquid phase. The solid product was washed repeatedly with liquid NH3; the stoichiometries were adjusted to AI/6(NHa)j,[ MoS2 ] for alkali metals (in some cases A1/3) and A I / 6 ( N H 3 ) y [ M o S 2 ] for alkaline earth compounds. (NH3)NbS2 could be prepared directly at low temperatures by condensation of liquid NH3 onto NbS2. In the case of TiS2 it turned out to be necessary to perform the reaction with liquid ammonia in a pressure vessel at 300 K (reaction time 24 h). Samples were transferred via side arms of the reaction vessels into cylindrical glass tubes of appropriate size for the NMR measurements and sealed except for those samples which were carried out in equipment suited to change the NH3 vapour pressure in steps for a given sample. Analytical data were obtained by microanalysis (H,N), atomic absorption spectrometry (alkali, alkaline earth metals) and wet methods (sulfur and transition metals).

E. Wein et aL/NMR studies of metal-ammonm in intercalation compounds

X-ray powder diffraction data have been obtained by the Simon-Guinier-technique with samples sealed in lithium borate glass capillaries; for all intercalated samples the interlayer distances were found to be close to 9 A as expected for monolayers of NH3 molecules between the [ MoS2 ] x- sheets. The IH NMR spectra were either observed as derivative curves using a homemade solid state spectrometer operating at 8 MHz or as absorption lines with a FT-300 MHz spectrometer (Bruker CXP). Spin-lattice relaxation times were measured with a Bruker variable frequency pulsed NMR spectrometer at 15 and 30 MHz employing 9 0 ° - z - 9 0 ° pulse sequences. In general, the relaxation curves appeared to be purely exponential. In a few cases, however, we observed small deviations from exponentiality. All spectrometers were equipped with variable sample temperature control units.

3. Experimental results We first studied a series of intercalation compounds Ax(NH3)yNbS2 and Ax(NH3)yTaS2 with alkali and alkaline earth ions A. A typical spectrum obtained immediately after the preparation of the sample is shown in fig. 1. The spectrum consists of a central line and a pair of satellites with a separation of 0.41 mT. Upon coooling the spectrum broadens and the satellite separation increases to about 0.95 mT. After a few days the central line narrows and the structure disappears. This effect was observed for all samples tested independent of the degree of intercalation. We assume that slow irreversible solvolysis reactions of the layers with NH3 leading to the for-

,

, QlmT

J Fig.

1.

~H

NMR

spectrum

(derivative

Nax(NH3 )d~bS2 registered at room temperature.

curve)

of

233

mation of HzS and subsequent protolytic equilibria (rapid proton exchange) according to the formal processes solvolysis: [/XM==S]+NH3-* [/~M=NH ] +H2S, protolysis: H2S + NH3 ~ (NH4) ÷ + H S -

are responsible for this observation. This was confirmed by the analytical prove of H2S formation after extended storage of these compounds. Because of this instability it was decided to perform all further experiments with samples of the (kinetically) stable Ax(NH3)~MoS2 phases whose spectra displayed characteristics identical with those of the related intercalation compounds of 2H-NbS2 and 2H-TaS2 in the undecayed state. It is of interest to note here a characteristic difference between the aquo and the ammono systems of NbS2 and TaS2. In the ammono compounds the protolysis equilibrium mentioned above is such that obviously significant quantities of H S - / N H ~ are present resulting in rapid H ÷ exchange, signal narrowing and disappearance of satellites in the NMR spectra. In the aqueous system, similar irreversible solvolysis effects are observed on storage of the samples for extended time: [/xM=S] +H20-~ [~M=O] +H2S. However, due to the significantly lower basicity of H 2 0 (as compared to NH3) the corresponding protolysis equilibrium H2S+H20~-(H30) + + H S is far on the left side. The consequence of this effect is a rather slow proton exchange. This is convincingly demonstrated by the surprising spectrum of samples heated to about 500 K which display two two-spin signals (fig. 2 ). These belong to H20 and to H2S (different proton/proton distance) simultaneously present as neutral solvate species in the interlayer space and subject to anisotropic motion as discussed earlier [ 3-5 ]. As an example of the spectra obtained for Ax(NHa)yMoS2 with A=Li,Na,K at various temperatures, figs. 3 and 4 are presented. A certain asymmetry of the central line and a detailed structure of the satellites (fig. 4) disappears upon cooling to 236 K (fig. 3). As the temperature decreases fur-

234

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

,

,0.2 mT

(a)

I

' (~3mT

(b)

Fig. 2. (a) ~H NMR spectrum of the mixed solvate phase RbJH20),._:(H2S)..[NbS2]I_:/2[NbO2]=/2 and (b) of pure Csx( H2S)y[NbS2 ] (x= 1/3). The latter phase was prepared by the reaction of Cst/3NbS2 with H2S under the equilibrium pressure of liquid H2S at 300 K.

/~.~

296 K

Fig. 4. Same as fig. 2 (derivativei for 296 K, but the details of the spectrum are shown on a much larger scale. ther, all the lines b r o a d e n a n d the satellite separation finally becomes 0.95 m T at 90 K. In fig. 5 the b r o a d ening o f the central line is plotted versus temperature. The t e m p e r a t u r e d e p e n d e n c e o f the spectra with A = Li, N a looks very much the same, b u t the steplike decrease o f fig. 5 is slightly shifted to lower temperatures. Central line narrowing in c o m p o u n d s with A = S r (those with Ca a n d Ba are s i m i l a r ) is also indicated in fig. 5. It occurs m o r e s m o o t h l y a n d the rigid lattice limit could not be o b t a i n e d within the accessible range. As realized from fig. 6, at low temperatures the spectra o f systems with alkaline earth ions are also different, in such a way that the structure is better resolved. We also tried to heat the samples to t e m p e r a t u r e s o f about 470 K, where the c o m p o u n d s are not yet decomposed, a n d we observed that the spectra practically r e m a i n e d unchanged. In particular, no exchange narrowing has been observed. aB mT 0.3

236K

"---~-O.o\

Y

90

K 0.1

160 •

260

T/K=

0.5 m T

Fig. 3. ~HNMR spectra ofKx( NH 3)yMoS2measured at different temperatures (left) and their derivatives (right).

Fig. 5. Linewidth taken as the field separation between the extrema of the drivative central lines of the spectra of Kx(NH3 )yMoS2 and Srx(NH3)yMoS2plotted yersus temperature.

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

,

soo

, 0.5roT

%- %

2,so

235

22,o 2o0

,8o VK

l/T! S-1

/

o 15 MHz

~

.?

* 30 MHz

10 ¸

Fig. 6. ~H NMR spectrum of Cax(NHa)~MoS2 measured at 77 K and compared with a simulated spectrum (of. section 4.1 ) on the fight hand side.

o

. Fig. 7 shows the spectra o f layered a m m o n i a intercalation compounds without metal cation, which, for reasons to be explained later, we denote by (NH4)x(NH3)~MS2. Clearly, no satellites occur at room temperature and at low temperature broaden-

0.3-

300 K l.

5

103/T K-I

Fig. 8. Proton spin-lattice relaxation rates versus reciprocal temperature for Nax(NHs)~MoS2 measured at two different frequencies.

77K

ing and some unresolved structure is observed. The results o f some o f the spin-lattice relaxation rate measurements are given by the points o f figs. 8 and 9. The features o f the corresponding data for the other compounds are similar. Tt-results for (NH4) x(NH3 ) yTiS2 are reproduced in fig. 10.

4. Discussion 4.1. Wideline spectra

Fig. 7. tH NMR spectra of (NH4)x(NHs)yTiS2 at two different temperatures. Bottom: a simulated low temperature spectrum for an admixture of 800/0NHg20% NH4 ( solid line) is shown for the model explained in the text taking into account the intermolecular contribution by a Gaussian of0.17 mT width. The dotted line results if NHa only is considered.

The spectra of Ax(NHs)yMoS3 observed at low temperature (figs. 3 and 6) are characteristic of those o f an isolated triangle o f proton spins rotating about the normal to its plane with some intermolecular dipole-dipole interaction in addition. When the frequency o f rotation is larger than the rigid lattice splitting, in such cases the lineshape consists o f a

236

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

390 lIT, I

2,50

220

200

190

160 T/K

$-1

MHz 10

.

0.3-

/,

5

6

l"0a/T K-I

Fig. 9. Same as fig. 8, but for SrANH3) vMoS2.

central line flanked by a pair of side lines whose separation amounts to [ 18 ] Aco = ] ( # o / 4 n ) ( ? 2h/R3 ) ( 3cos2r/-- 1 ) .

( 1)

R is the proton-proton distance and r/ the angle between the external magnetic field and the C3 axis

1/1 $-1

o ISMHz

~..o..~

30 ¸

° 20MHz • 30MHz

_~,. "=,,<,ZO~o~ )'j"-'-...,.. - . . . . . - . L . e ~ o

10

K-I

Fig. 10. Same as fig. 8, but for (NH4)x(NH3),,TiS2.

of the NH3 rotor; the other symbols have their usual meaning. For polycrystalline materials, from the weighted average of the spectra for all angles ~/a distribution occurs where the intensity becomes significant for ~/=90 ° the most heavily weighted orientation. For NH3, eq. (1) with ~/= 90 ° and R ( H - H ) = 163 pm yields AB=Ato/7=0.98 mT. This value corresponds to the experimental splitting of figs. 3 and 6 at low temperature, if the smearing of the spectra due to intermolecular interaction is taken into account. Comparison between calculated and observed spectra is improved, if one does not only consider the positions of maximum intensity, but the whole shape. The curve on the right hand side of fig. 6 is the result of a computer simulation that folds eq. (1) with a Gaussian of width 0.13 mT to allow for the intermolecular proton interaction. It may be worth mentioning that in three-dimensional crystals usually the side lines cannot be recognized, since the details of the spectra are smeared by intermolecular interactions. On the other hand, in layered compounds with monolayers of guest molecules in the van der Waals gap, where intermolecular dipole coupling occurs in two dimensions only, we observed already in previous work well-structured spectra for H20 and CH3 groups [ 3,9 ]. The conclusions drawn are supported by single crystal studies of Ax(NH3)yMoS2 which show that the q=90 ° condition coincides with the external magnetic field being parallel to the hexagonal c axis of the crystal [ 17 ]. This proves that the C3 axes of the NH3 rotors lie in the plane of the metal disulfide sheets, making an angle of 90 ° with the c axis. Upon increasing the temperature the curves look better resolved and the side lines move more closely together, figs. 1, 3, and 4. This can be explained in terms of an averaging of the intermolecular couplings and by additional motions which are frozen in at 77 K. In eq. (1) then q varies with time, and in order to obtain the satellite separation it becomes necessary to replace 3cos2~/-1 by ½(3cos30'-1) ×(3cos2fl-1) [19,20]. fl is the angle which the direction for the C3 axes makes with the axis of an additional rotation or reorientation and 0' is the angle between the latter and the direction of the external field. If the axis of the additional reorientation would be oriented parallel to the crystalline c axis

E. Wein et al./NMR studies o f metal-ammonia in intercalation compounds

(fl = 90°), and if the reorientation is fast, this leads to a reduction of the side line splitting by a factor of two. The experimental splitting is reduced by 2.3 rather than 2.0. This does not make a great difference, the reason could be an angle fl shifted a little bit away from the 90 ° condition and an uncomplete ordering of the NH3 molecules in the plane. This could mean that the axes of reorientation are not exactly oriented parallel to the c axis, but distributed about this direction. Systems containing small cations should move more readily in this case, in agreement with experimental findings that the side line separation is here slightly smaller. A similar spectrum with a splitting of 0.42 mT has been observed for ammonia intercalated in graphite, Li(NH3) 2Cx [2 l]. Also rotating CH3 groups in layered intercalation compounds showed corresponding splittings [ 9,22 ]. The slight asymmetry of the central line and the structure of the side lines observed at room temperature (fig. 4) result from couplings to the nitrogen nucleus. Each line splits into three components ( I = 1 ) or into two components if 14N is substituted by ~SN ( I = 1/2) as observed much better in single crystals [ 17 ]. Fig. 3 shows that upon decreasing the temperature, before the large broadening occurs (cf. 90 K), the lines narrow first (236 K) and the structure disappears. This seems to be related to the relaxation behaviour of ~4N. The line narrowing process as a function of the temperature (fig. 5 ) appears to be rather similar for all monovalent cations on the one hand (not shown), and for bivalent cations on the other hand. The relaxation data reveal similar features. For the compounds A~(NH3)yMoS2 with A = Ca, Sr, Ba the linewidth reduction by the onset of reorientational motions starts already at lower temperature, but the averaging is less effective. Furthermore, the low temperature spectra of fig. 6 look better resolved than those of fig. 3. We come back to this difference in a later section. The linewidth data and the order of magnitude for the second moment at temperatures below 100 K are in agreement with the observation that rotation or tunnelling of the NHa groups about their C3 axes is still fast. Finally the NMR spectra of fig. 7 are to be considered, which are narrow and unstructured at room

237

temperatrure, but similar to those of Ax (NH3) ~MS2 at low temperatures, even if the structure is less visible. The absence of side lines and the narrow central line can be understood in terms of rapid intermolecular proton exchange, which occurs in the presence of NH~ cations as already discussed in an earlier paper [ 9]. At 77 K all types of motion except NH3 rotation are frozen. There is strong evidence for the presence of NH~cations in the systems (NH3)MS2 [ 15,23] so that an ionic formulation (NH~-)x(NH3)yMS2 would describe these materials much better. From the NMR results, especially as compared with those of the system Ax(NH3)yMS2, there are some indications of such a structure. Apart from the similarity of the 77 K spectra of figs. 3 and 7, where the intermolecular smearing is of course more important in fig. 7 due to the dipole interaction between NH3 and NH~- protons, the T~ data point at this direction. NH~- seems to adopt the role of the central cation solvated by NH3 molecules, and it may be oriented in such a way that one of the N - H bonds lies parallel to the sheets: NH~- is then free to reorient about the C3 axis, and such a configuration facilitates at elevated temperatures mutual conversion from NH~ in NH3 and vice versa via proton exchange. Theoretical spectra of NH4 groups rotating about a Ca axis do not possess any structure in contrast to those which rotate about a C2 axis [ 24 ]. The experimental spectrum of fig. 7 can be simulated assuming 80% NH3 and 20% NH4 and employing the configuration proposed in the last paragraph (solid line in fig. 7). The result is not conclusive, however, since a simulation of the proton resonance of rotating N H 3 only is not much different (dotted line). Some additional information has been obtained from a comparison of various layered compounds with ammonia as intercalant. Whereas in (NH4) x(NH3) yTiS2, in agreement with ref. [ 10], between 110 and 77 K lineshape and second moment do not change any more, in (NH4)x(NH3)yNbS2 in the wings some additional large lines become visible [25], which clearly increase the second moment. (NH4)x(NH3)yTaS2 lies in between as revealed by the second moment data [ 10]. The effect is most pronounced in the related compound (NH4)x(NHa)yRuC13. In fig. 11 supplemental extrema can he recognized whose large splitting of

238

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

methylamines [ 9 ] and hydrogen bronzes [ 27 ]. Following the arguments given in ref. [ 27] we have fitted the curves by a modified "BPP formula"

1 -C[ Tl

Fig. l 1. )H NMR spectrum of the layered intercalation compound (NH3)xRuCI3 measured at a temperature of 77 K.

"r~

4z~

1

l-[-(o90"~c) I + a + 1 +(2o9oZc)~+'~l '

(3)

where rc is connected with the activation energy Ea of the process by an Arrhenius law zc = r co exp(Ea/R T)

about 2.4 mT corresponds to those observed for NH~- in ammonium halides under rigid lattice conditions [ 26 ].

4.2. Relaxation rates The curves of figs. 8 and 9 for the system A~(NH3)yMOS2 display one maximum and can be

described by one motional process, while apparently two processes have to be considered in fig. 10 for (NH4)x(NH3)yTiS2. For the purpose of comparison, fig. 12 shows published data points for (NH4)x(NH3)yNbS2 [9] and for (NH4)x(NHa)~TaS2 [14]. In the second case we have interpreted the experimental results of Kleinberg and Silbernagel in the same way as those for M = N b and M = T i . Spin-lattice relaxation data about motions in intercalation compounds are distinguished from those of molecular crystals by asymmetries in the Ti-~ versus T -~ presentation and by different dependences on the frequency O9o and correlation time Zc [5,9,14,27]. Possible elements of lower dimensionality of the motion result in a characteristic frequency dependence in the high temperature region (Ogor~~ 1) [ 5,14 ]. This has not been detected during the present study, but cannot be ruled out, since the high temperature component of the T~ curves was only partially accessible (figs. 8,9 and 10). In the low temperature region, O9or¢>>1, the data show a pronounced deviation from an o96-2 proportionality as observed already for many other intercalated system~ [ 9,14,27 ]. This can either be explained by distributions of the correlation time or by the collective character of the motion and is further investigated. To describe the experimental data we have applied a similar treatment as that already employed for

(4)

and ot ~<1 is a parameter which characterizes the width of the distribution or the amount of collectivity; in isotropic crystals generally ot is equal to unity. The solid lines of figs. 8, 9, 10, and 12 are the result of this description with the model parameters listed in table 1. The relaxation of Ax(NH3)yMoS2 was explained by one process of the type of eq. (3), that 3.oo 2,sq,,,29o . . . .

1~o,

~o

vK

lit 5O 20 10: 5:

100: ° o'~"~o* 45 MHz

50 20 10:

I K-1 Fig. 12. Data points of the proton spin-lattice rc|axation rates for (NH4)x(NH3)yNbS2 [9] (above) and (NH4)x(NH3)~TaS2 [ 14] (below). The experimental results can be interpreted in terms of two motiona| processes (solid lines) as explained in the text.

E. Wein et al./NMR studies of metal-ammonia in intercalation compounds

239

Table 1 Numerical results for the relaxation strength, preexponential factor, activation energy and distribution parameter obtained from the best fitting of the experimental relaxation rates by eqs. ( 3 ) and (4). Guest phase

(NH4) x(NH3) yTiS2 (NH4)x(NH3)~qbS2 (NH4)x(NH3)yTaS2 Nax(NH3)yMoS2 Srx(NH3)yMoS 2

Fig. no.

10 12 12 8 9

"high" temperature peak

"low" temperature peak

Ca (109 s -2)

~co (10-13s)

Eo (kJ/mol)

a

CL (109s -2)

¢~0 (10-13s)

Eo (kJ/mol)

2.3 2.6 4.7 2.1 1.8

0.4 0.6 0.9 (2X 10 -4) 1.6

23 23 18 (44) 22

0.5 0.7 0.7 (0.5) 0.5

1.3 4.2 2.0

3.0 1.0 1.3 -

14 16 14 -

of (NH4) ~(NHa ) ~VIS2 by two processes. The "high temperature" relaxation peaks all occur in the temperature region between 200 and 250 K. Though somewhat too small (as usually if eq. (3) is applied [ 9 ] ), the order of magnitude of the relaxation strengths C corresponds to the reduced second moment of the rotating NH3 group; C = ~A£2ed gives 4.5 × 109 s- 2. The relaxation is therefore associated with a motion of NH3 which adds to the fast rotation. The activation energies are in agreement with those which may be estimated from the temperature at which the linewidth of the spectra begins to narrow and where the side splittings move closer together [ 28 ]; they will therefore be connected with the same process. For all these reasons the relaxation maximum may be assigned to the reorientation of the NH3 molecules about axes centred in a direction perpendicular to the sheets and the molecular axes. For Na~(NH3)yMoS2 and the compounds with A=Li, K (not shown), because of the prefactor anomaly, the fit is not without fail. But, as dicussed already in the last section, there are clear differences between mono- and bi-valent cations, where the latter look more comparable with the ammonia /ammonium systems. Since there is no dependence upon the enthalpy of solvation as observed for A~(H20)~V[S2 [4,5], diffusional jumps cannot be responsible for this difference; these would point into the other direction. The reason may be specific interactions between the various alkali or alkaline earth ions and ammonia which were already found in metal-ammonia complexes [ 29 ]. Our samples with bivalent cations are distinguished by the same degree of reduction as those with monovalent cations. The

0.3 0.5 0.7 -

larger electrical field strength of the smaller bivalent cations in combination with the lower concentration may lead to the reorientation of more defined and symmetric solvate complexes whose activation energy is lower. The same reasoning may lead to a higher order of the guest phase. In addition, the relative amount of solvated and non-solvated ammonia coordinated to the cation in the first and second sphere may play a role. It remains to explain the "lower field" relaxation peak which occurs between 150 and 180 K, but only for the (NH4) x( NHa ) ~MS2 systems. The position of this relaxation rate maximum, the associated activation energy, and a comparison with the spectra suggest to assign it to NH~ reorientations. Following the discussion of the last section we are tempted to combine both this relaxation peak and the large wings in the spectra, which become visible at very low temperatures, with the rotation of NH + about its C3 axis. The different ratios CL/CHof table l may reflect the actual compositions of the various samples. Due to its symmetry NH + is known to be very mobile in all classes of materials; there is no reason to believe that a rotation is too much hindered by the negatively charged MS2 sheets.

5. Conclusions The NMR experiments suggest that there are several similarities between layered intercalation compounds Ax(NH3)~MS2 and (NH4)x(NH3)~MS2. The NH~- ions may play the same role as the alkali or alkaline earth cations. Solvated complexes may be

240

E. Wein el al./NMR studies of metal-ammonia in intercalation compounds

f o r m e d in the v a n der Waals gap, a n d the NH3 molecules are aligned with their C3 axes i n the layer planes. W i t h N H ~ as cation, r a p i d i n t e r m o l e c u l a r p r o t o n exchange takes place, w h i c h is a b s e n t i n the case o f metal cations. R e o r i e n t a t i o n a l m o t i o n o f NH3 is activated a b o u t axes p e r p e n d i c u l a r to the C3 axes. NH~- is b e l i e v e d to r e o r i e n t a b o u t a C3 axis parallel to the layers, while in the s a m e t i m e there is a p r o t o n exchange b e t w e e n NH3 molecules a n d NH~- cations. At low t e m p e r a t u r e s all these m o t i o n s are f r o z e n i n a n d only the fast r o t a t i o n or t u n n e l l i n g o f NH3 a b o u t the C3 axes is left. At m u c h lower t e m p e r a t u r e s t h a n those a p p l i e d i n this s t u d y we have recently o b s e r v e d a n a d d i t i o n a l Ti- t - m a x i m u m which belongs to rotat i o n a l t u n n e l i n g o f NH3 [ 3 0 ] .

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