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1H relaxation study in (NH4)2SbF s

B B A N D Y O P A D H Y A Y , A G H O S H R A Y , R M U K H O P A D H Y A Y * a n d R M K A D A M *

Saha Institute of Nuclear Physics, 92, Acharya Prafulla Chandra Road, Calcutta 700009, India

• Nuclear Physics Division, tRadio Chemistry Division, Bhabha Atomic Research Centre, Bombay 400085, India

MS received 7 July 1989

Abstract. 1H spin-lattice relaxation rate (T~- ~) has been measured using inversion recovery technique in polycrystalline (NH4)2SbF s system in the temperature range 140-400 K, From the plot of log(M 0 -- M) against z, we have estimated two different TI corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence of 7"1 in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.

Keywords. Nuclear magnetic resonance; relaxation; molecular motion; (NH,)2SbF s.

PACS Nos 33-25; 61-50

I. I n t r o d u c t i o n

C o m p o u n d s of the c o m p o s i t i o n M 2 S b F s (M = N a , K, N H , , Rb, Cs) are k n o w n to exist in several phases (Avkhutskii e t a l 1983 a n d U r b o n a v i c i u s e t a l 1982). F o r example, a m m o n i u m pentafluoro antimonate, (NH4)2SbF 5 shows four p h a s e trans- f o r m a t i o n s at 292, 257, 168 and 142 K (Avkhutskii et al 1983) during h e a t i n g cycle.

T h e transition at 257 K has been identified as a superionic p h a s e transition a n d is associated with large hysteresis (transition occurs at 238 K during cooling cycle), T h e conductivity j u m p s by a factor of 106-107 on heating to 257 K. T h e X - r a y studies ( M a k a r o v a et al 1984) o f ( N H 4 ) 2 S b F 5 suggest that there exist t w o inequivalent ( N H 4 ) ions in the unit cell (figure 1). T h e first crystallographic type (NH4)! is positioned at a relatively short distance (2.75/~) from fluorine (F2) a t o m involving the h y d r o g e n b o n d a n d the o t h e r type, (NH4), is located at m o r e t h a n 3/~ f r o m the fluorine a t o m . F r o m N M R a n d N Q R (Avkhutskii et al 1983 a n d N a k a m u r a 1986) studies it h a s been revealed t h a t the successive phase t r a n s f o r m a t i o n s are connected with the different types of r o t a t i o n a n d / o r reorientation m o t i o n of v a r i o u s g r o u p s of ions present in the crystal. In particular, the phase transition at 168 K is believed to be connected with the complexities of reorientational m o t i o n of (NH4) t g r o u p s in the crystal. T r a n s i t i o n s at 292 and 257 K are believed to be connected with the d i s o r d e r of fluorine atoms. Specifically, the r o t a t i o n of I-SbF5] 2- a r o u n d C4-axis causes a phase transition at 257 K while the transition at 290 K is caused b y the free r o t a t i o n of [ S b F s ] 2- g r o u p s (resulting in substantial n a r r o w i n g of 19 F N M R lines (Avkhutskii 713

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714 B Bandyopadhyay et al C

0

I I H ( ~ )

I

I Hc3l

~ S b • F 0

N

o H

B

Figure l. Projection of structure of (NH4)2SbF s on coordinate plane BC (Makarova et al 1984).

e t a l 1983) in the 2 9 0 - 3 5 0 K interval). Recently, n e u t r o n incoherent quasielastic scattering technique has been applied to study the reorientational m o t i o n of NH~- ions at 300, 180 and 160K ( M u k h o p a d h y a y etal 1987). It has been suggested that the two N H ~ ions have similar reorientational times. Thus, so far dynamics of the system is not very clear. In the present paper, we have reported a detailed study of p r o t o n (~H) spin-lattice relaxation time (:/'1) in polycrystalline (NH4)2SbF5 system in the temperature range 140 K to 400 K. The results reveal some features of the dynamics of reorientation of N H 4 ion across the phase transitions. Experimental details are given in §2 and results and discussion in §3. A brief report was presented elsewhere in a symposium (Bandyopadhyay et al 1988).

2. Experimental

A m m o n i u m pentafluoro antimonate, (NH4)2SbFs, is prepared by mixing SbF 3 and N H 4 F in the molar ratio of 1:3 in 40% hydrofluoric acid, HF. Crystals are obtained by slow e v a p o r a t i o n from the solution kept in a teflon beaker. The required SbF 3 is obtained by dissolving S b 2 0 3 in H F in a platinum crucible and then heated to dryness.

The powdered samples were checked for the room t e m p e r a t u r e phase by X-ray diffraction.

T h e instrument used for measurements of ~H spin-lattice relaxation rate was a Bruker M S L 100 spectrometer. A varian V-7400 electromagnet system equipped with a field-dial regulator was used. The IH N M R spectra were o b t a i n e d by the Fourier transformation of free-induction decay (FID) signal observed after a 90 ° pulse. The H spin-lattice relaxation time (T~) was measured in the t e m p e r a t u r e range 140-400 K at 34 M H z using inversion recovery ( 1 8 0 ° - z - 9 0 °) technique. At low temperatures, where the resonance line-widths ( F W H M ) are comparatively larger than those observed at room temperature, solid-echo sequence was used as the m o n i t o r i n g pulse.

In this case the complete pulse sequence was 180 ° -- z -- 90 ° -- r t -- 90°~9oo~- ~'2-echo.

Thc spectra were obtained from the Fourier transformation of the F I D signal obtained after the monitoring pulse. The peak amplitude of each spectrum was measured as

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a function of time z, and from the change of magnetization, the spin-lattice relaxation time was determined.

The temperature of the sample was controlled to within ___ 1 ° by heating the flow of pre-cooled nitrogen gas using BVT-1000 temperature controller which also monitors the temperature at the sample using a copper-constantan thermocouple placed close to the sample.

3. Results and discussion

Figure 2 shows three typical 1H N M R spectra (obtained from Fourier transform of the FID after a ~ 9 0 ° pulse) at a resonance frequency of 34.0 MHz at three different temperatures. The line shapes and the line widths (FWHM) are dependent on temperature. A substantial narrowing of FWHM of 1H spectrum is observed in the temperature range of 350-400 K. This feature is also observed in IH NMR spectra obtained by CW method. However, it has not been possible to distinguish two inequivalent (NH2) ions (as observed from X-ray (Makarova et al 1984) studies) from the 1H spectra obtained by both the techniques. Temperature dependence of 1H NMR linewidth in (NH4)2SbF 5 monocrystal was earlier studied by Avkhutskii et al (1983) and was attributed to reorientational motion of NH4 groups. This sort of motion has profound influence on the spin-lattice relaxation rate (T~-I). According to well-known BPP theory, Ti-1 governed by the fluctuation of the nuclear-dipolar interaction due to the motion of NH 4 ions and/or other groups is given by

T [ " = C[{z~/(1 + co2z~)} + {4re/(1 + 4~2¢,2)}] (1) where zc is the correlation time of motion, to the angular resonance frequency and C a constant independent of c~ and To.

While measuring T~ of proton in polycrystalline (NH4)zSbFs, the change of magnetisation with z does not follow a single exponential behavi3ur in the entire range

••

T(K)

1 I I I I

0 4 0 8 0

kHz

Figure 2. Typical 1H NMR spectra at a resonance frequency of 34 MHz at three different temperatures.

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716 B Bandyopadhyay et al

3 0 0

~ooeeo

15( Oo

100 - - X x x ° ° O o

x

5 0 - - x

:E

I 0

1 0 - -

5 --

,l ,

0

e t l 0

0

x 0 0

0 0 0 0

x 0 0

x

239 K

o 4 0 0 K x 1 9 4 K

I I I I

0 . 4 0 . 8 1.2

"L" ( s )

Figure

3. Plot of log (M 0 -- M) vs r at different temperatures. Signature of two exponents is evident in the entire temperature range.

of t e m p e r a t u r e studied. This feature is shown in the plot of log ( M o - M) against z at different temperatures (figure 3). This sort of behaviour is c o m m o n l y observed if the spectrum contains signature of different species of p r o t o n s having different spin-lattice relaxation rates. The observation in the present case is not surprising as there are two inequivalent N H 4 groups in the unit cell as evidenced from X-ray studies ( M a k a r o v a et al 1984). Assuming two different reorientational rates (corres- ponding to two different correlation times) for the two N H 4 groups at any temperature, we have fitted the behaviour of log (M o -- M) vs • with two characteristic times Ttt and T n from initial linear portion for shorter T and the extreme linear p o r t i o n for longer values respectively. Simultaneous estimation of T~t and T~ have been possible in the t e m p e r a t u r e range of 180-400 K where the two relaxation rates are not widely different. Below 180 K, where T~ and T~ s differ at least by an o r d e r of magnitude, the estimation of longer T 1 a m o n g the two has not been reliable enough and has been omitted in o u r discussion.

Figure 4 shows the temperature dependence of relaxation times T~ and T~ of 1H in (NH4) 2 S b F 5 observed in the temperatures range 140-400 K. Apparently, it is not easy to conclusively associate any of these two relaxation times with any one of the two inequivalent N H 2 ions. However, we assign T~ and T~ with (NH,)I and (NH4)o groups respectively from consideration of structural aspects and different structural t r a n s f o r m a t i o n exhibited by (NH,)2SbF5 during t e m p e r a t u r e cycling. It has been discussed earlier that (NH4) t groups are located closer to F - a t o m s and are h y d r o g e n bonded. O n the o t h e r hand, (NH4), groups are located far away from F-atoms and are t h o u g h t to rotate freely in the whole t e m p e r a t u r e range studied. Regarding the successive phase transitions, it is k n o w n that the transitions at 142 and 1 6 8 K respectively are connected with the reorientational m o t i o n of the ( N H , ) t groups, whereas the disorder offluorine atoms causes transitions at 257 and 292 K respectively.

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I.--

1.0

0-5

0-2

0.1

0 " 0 5

0.02

o

•o

I I I I I

2-0 4-0 6 . 0

1/T x 103 (K - l )

Figure 4. Temperature dependence of the proton relaxation time T 1 in (NH4)2SbF S at 34 MHz. Two different Tl's (T~ and T~ t, shown as unfilledand filled circles respectively) are distinctly observed in the entire temperature range.

Therefore, it may be argued that the fluctuation of nuclear dipolar interaction arises due to different types of motions and would in all probability strongly influence the relaxation rate of (NH4) j groups. Nevertheless, the effect of disorder of fluorine atoms on the relaxation rate of (NH4)n groups c a n n o t b e neglected. Thus, we can think of the existence of at least two different correlation times (zc) for each ammonium ions. This can easily be visualized in (NH4)n groups. Since this ammonium group rotates freely even at the lowest temperature studied, the correlation time for its motion must be shorter than that of the motion of the hydrogen bonded ammonium group. Thus the monotonic increase of T~ ~ in the temperature region 140-220 K is characteristic of motional behaviour of (NH4hl groups in the ~o0z c << 1 region of eq.

(1). Experimental limitations prohibit us from searching for a possible minima at a lower temperature. As the temperature is raised above 220 K, the value of T~, instead of increasing further, exhibits a maximum at around 230 K and then decreases. The behaviour of T n in the 230-400 K region, including a broad minimum centered around 294 K, clearly indicates the occurrence of another type of motion over and above the already mobile (NHa) n ions. Thus the reorientational motion of [SbFs]- group associated with the superionic transition at 257 K, further facilitates the relaxation of (NH4)n ions through the fluctuation of nuclear dipolar interaction caused by the motion of F ions and this consequently results in the decrease of T~.

The results of Tit I studies for the ammonium group, (NH4)n can be summarized as follows. To the right and to the left of the maximum near 230 K the observed relaxation times are determined by two different correlation times associated with free ammonium and [ S b F s ] - reorientational motion respectively. To the right of the maximum, the

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718 B Bandyopadhyay et al

T a b l e 1. Activation energies at different temperatures.

Activation energies (kcal/mol) in the temperature range NH2 ion

type 140-220.K 250-300 K 300-400 K

I - - 5.5 _+ 0.5 4.3 _+ 0-5

II 2.9+0.2 5-5+0.5 3-6+0.5

correlation time for the rotation of ( N H , ) n g r o u p s is such that the m i n i m u m of ~r 1 at COoZ c = 0.62 should a p p e a r below 140 K. W h e r e a s at the left side of m a x i m u m the variation of correlation time due to mobile F - ions is such that the condition of m i n i m u m is satisfied at a r o u n d 294 K. This feature of d o u b l e m i n i m a has been observed in (NH4)2SO 4 (O'Reilly and T u n g Tsang 1967).

The t e m p e r a t u r e dependence of T~ which c o r r e s p o n d s to (NH4) I groups can also be explained in a f r a m e w o r k similar to the one we h a v e used T~ behaviour. T o the right of the m a x i m u m at 248 K, the reorientational m o t i o n of the hydrogen b o n d e d a m m o n i a , (NH,,) t is such that the condition of m i n i m u m in zc is observed near 200 K.

Left side of the m a x i m u m is again due to the m o t i o n of [ S b F 5 ] - groups. Therefore, we can say t h a t at the left of the minimum, T 1 for reorientational m o t i o n of (NH4)t is very long a n d thus T1 d u e to the m o t i o n of fluorine a t o m s seems to be p r e d o m i n a n t . At t e m p e r a t u r e in the m o t i o n a l region, the c o r r e l a t i o n time zc fits the Arrhenius relation zc = z o e x p ( E a / R T ) and Tt can be written as T1 = C i e x p ( + E J R T ) where C~ is a constant, Eo is the activation energy, R is the universal gas constant. T h u s a linear plot of In T~ as a function of 1/T provides an e s t i m a t e of the activation energy for the r o t a t i o n of N H , groups. Table 1 shows the a c t i v a t i o n energy of the two a m m o n i u m g r o u p s at different temperature ranges. In principle, the activation energies in the two regions (tOoZ , <,~ 1 a n d COo% >> 1) a b o u t the m i n i m u m should be identical.

However, in this case, the transition at 292 K m a y alter the barrier height a n d thus the activation energy in the 300-400 K region has been found to be different f r o m that in the 2 5 0 - 3 0 0 K region.

In conclusion, t H spin-lattice relaxation studies in ( N H , ) 2 S b F s revealed t w o distinct N H , ions with different reorientational rates. T h o u g h the variation of T~

with t e m p e r a t u r e in each case does not contain a n y distinct signature of phase transition, the results reveal some informations r e g a r d i n g the successive phase transitions. We h a v e mentioned earlier that the pecularities of the reorientational m o t i o n of the different groups of ions viz. N H , a n d S b F s are believed to be connected with the transitions. Effect of the different reorientational rates of the different g r o u p s exhibit p r o f o u n d influence on T1 via the m o d u l a t i o n of the m o t i o n of particular a m m o n i u m ions.

R e f e r e n c e s

Avkhutskii L M, Davidovich R L, Zemnukhova L A, Gordienko P S, Urbonavicius V and Grigas J 1983 Phys. Status Solidi BII6 483

Bandyopadhyay B, Ghoshray K, Ghoshray A, Mukhopadhyay R, and Kadam R M 1988 Solid State Phys.

(India) C31 313

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Makarova I P, Muradyan L A, Zavodnik V E, Tovbis A B and Orbeladze P V 1984 Soy. Phys. Crystallogr.

29 267

Mukhopadhyay R, Goyal P S and Rao K R 1987 Solid State Phys. (India) C30 319 Nakamura N 1986 Z. Naturforch 41 243

O'Reilly Donald E and Tung Tsang 1967 J.. Chem. Phys. 46 129

Urbnavicius V, Schneider V E, Grigas J and Davidovich R L 1982 Soy. Phys. JETP 56 151

References

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