NMR studies in single crystal and dispersed phase lithium iodideo

Solid State lonics 18 & 19 (1986) 557-561 North-Holland, Amsterdam

557

NMR STUDIES IN SINGLE CRYSTAL AND DISPERSED PHASE LITHIUM IODIDE°

J.L. BJORKSTAM, D. BRINKMANN+, M. MALl+, J. ROOS+, J.B. PHIPPS* and P.M. SKARSTAD* Department of E l e c t r i c a l Engineering, University of Washington, Seattle, WA 98195. +Physik - I n s t i t u t , University of Zurich, 8001 Zurich, Switzerland. *Medtronic, Inc., 6700 Shingle Creek Parkway, Brooklyn Ctr., MN 55430.

We have begun an NMR study of dispersed phase systems o f L i l by focussing a t t e n t i o n upon properties of the s i n g l e c r y s t a l . In addition we present some preliminary results on dispersed phase, LilwSiO 2 and LIIWAI203 t o i l l u s t r a t e the substantial differences which are evident in these systems.

I . INTRODUCTION Since i t was demonstrated by Liang I t h a t s t a b l e , dispersed phase samples o f L i I * A l 2 0 3 , showed s u b s t a n t i a l l y improved Li + c o n d u c t i v i t y , i n t e r e s t in such materials has increased. While studies of i n t e r f a c i a l transport have provided f u r t h e r d e t a i l s on the nature o f the c o n d u c t i v i t y , 2 a f u l l understanding is s t i l l lacking. A previous 7Li i n v e s t i g a t i o n in L i I was conf i n e d t o p o l y c r y s t a l l i n e and dispersed phase samples.3 In o r d e r t o separate f e a t u r e s which are c h a r a c t e r i s t i c of the bulk from those r e l a ted t o d e f e c t and i n t e r f a c i a l e f f e c t s we have begun w i t h a study o f s i n g l e c r y s t a l s . Our experiments i n c l u d e s p i n - l a t t i c e (T1) and s p i n spin (T2) r e l a x a t i o n , l i n e w l d t h and p u l s e d magnetic-field-gradient (pfg) diffusion measurements. In addition t o the s i n g l e crystal data we present some p r e l i m i n a r y measurements on d i s persed phase samples to. i l l u s t r a t e the substant i a l sample dependence o f the data. These d i f ferences w i l l be e x p l o i t e d t o learn more about the i n t e r f a c i a l effects. Some o f our dispersed phase samples are o f g r a i n s i z e as small as 300 m2/g so t h a t an appreciable f r a c t i o n of a L i I c r y s t a l l l t e w i l l be composed of the surface charge layer. We b e l i e v e t h a t some o f the d i f f e r e n c e s in NMR r e s u l t s , which w i l l be presented, are a consequence of Li nuclei spending an appreciable f r a c t i o n of t h e i r time in t h i s l a y e r .

2. SAMPLE PREPARATION 2.1. Single Crystals The L i l was p u r i f i e d by p a s s i n g HI gas through the m e l t t o remove LiOH and o t h e r hydrides. After zone r e f i n i n g , the c l e a r e s t part of the crystal was selected, ground and placed in the NMR tube where i t was grown into a s i n g l e crystal by the Bridgman technique. 2.2. Dispersed Phase Samples The L i I was Puratonic Grade LiI*3H20 obtained from Johnson - Matthey. I t was dehydrated under vacuum f o r one week at temperatures reaching 420°C. An IR spectrum i n d i c a t e d l i t t l e or no water and/or LiOH a f t e r dehydration. A l l manipul a t i o n of the samples was done in a Helium filled d r y box. The c e r a m i c powders were dehydrated at 500°C under vacuum f o r one day. The L i I was ground with a mortar and pestle and mixed w i t h ceramic powders in the mortar. The mixture was transferred to the sample tubes and packed l i g h t l y , then melted at about 550°C f o r 15 minutes. The tubes were then sealed and removed from the glove box. There was some disc o l o r a t i o n o f the samples due t o carbon from back d i f f u s i o n o f pump o i l during the dehydrat i o n procedure and iodine, evolved from the decomposition o f L i l near the p o i n t where the tubes were sealed. Neither of these contaminants should adversely e f f e c t the NMR results.

Supported in part by the US Dept.of Energy Grant #DE-FGO6-84ER45065, in part by the Center for Process Analytical Chemistry (CPAC) at the Univ.of Washington, and in part by the Swiss National Science Foundation. 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

558

J.L. B/orkstarn et al. / N M R studies in single crystal

3. NMR DATA ON SINGLE CRYSTALS 3.1. Spin Lattice Relaxation The T 1 data was c o l l e c t e d in a very conventional way with the usual 180 ° - ~ - 90 ° pulse sequence, following which the magnetization recovery was recorded as the free-induction-decay (lid). For simple exponential recovery an appropriate plot of the natural log of the fid amplitude vs ~ should be a straight line projecting back through unity at • = 0. While 7Li has a substantial quadrupole moment, the cubic environment leads to zero quadrupolar splitting. Thus one should expect the spin system to show a simple exponential recovery if the sample is homogeneous. In contrast we find a substantially non-exponential recovery, for'even "pure" single crystals. The departure from simple exponential recovery is evident at all temperatures. While it is s o m e t i m e s p o s s i b l e to decompose such nonexponential behavior into 2, or more, relaxation processes, such decomposition was unconvincing in our results. We have instead simply plotted in Fig. l the e-I recovery time, denoted as Tle. The "saturation" behavior at low temperatures is to be expected from the presence in ppm of Mn 2+ and Ca 2+. At higher temperatures more "fundamental" relaxation processes become dominant. Dipolar relaxation was suggested in ref. 2 as the dominant mechanism at these higher temperatures. Even though there is no static quadrupolar splitting, motion of the spins can modulate the quadrupolar interaction, leading to relaxation. Diffusive motion through the inhomogeneous environment of grain boundaries is also expected to modulate the quadrupolar interaction. While the dipolar and quadrupolar mechanisms may be comparable, we b e l i e v e that the quadrupolar mechanism should not be ruled out. There are several features of the single crystal data which deserve comment. It is quite clear that the results are not described by the simple, symmetrical minimum predicted by BPP theory. 4 While no c o n d u c t i v i t y measurements have been performed on the samples described herein, the temperature designated as T A is that at which the transition from extrinsic to intrinsic behavior was evident in previous single crystal conductivity studies. 5 While this "knee" In the conductivity moves to lower tem-

/

~

o ~

~

~

.....

_ - o

c

o~ -0.89

eV

•

/,

/

r~_ Hie V / / ~ jo/ / / / / •

~ iI II

I

I/ jI? ,i t~ jlt i

m

~, ~

~%oo°

/ g,f

~

iI

.o,

i

L 2

0.32 e V /-,~ 0 . 2 5

/

~/ / L/. /t

i

,

o ~ : o i~

"rA

0 . 2 0 eV

/i

It

'

t

tl

-

eV

0.3 g e'¢

SI ~ 3 5 • Ft ]

o

,,~ ~

.

u'L/

~ E

I 3

M H Z

C "~ 77.7 M H Z

l L 4 5 T-I( LO-3K -[)

]

I 6

L 7

FIGURE l Temperature dependence of the 7Li s p i n - l a t t i c e r e l a x a t i o n time Tie in several samples.

peratures with greater sample purity, we believe that the anomalous NMR behavior near T A is related to this transition. While the slight curvature in magnetization recovery evident at low temperatures may result from competition between paramagnetic and more fundamental relaxation processes, we b e l i e v e that the curvature evident for much shorter recovery times at high temperatures is associated with the fact that the spins sample a dlstributlon of environments, thus leading to a distribution of recovery times. It may seem presumptuous to show dotted lines with indicated slopes of 0.89 eV at high temperature, and 0.20 eV at low temperature. The rationale for doing so will become evident in connection with a discussion of the diffusion and line narrowing results. Concern over the danger of melting our single crystal prevented an extension of T I data to higher temperatures. 3.2. Spin-Spin Relaxation and Line Narrowing

The s p i n - s p i n r e l a x a t i o n t i m e (T2) r e s u l t s in Fig. 2 were obtained w i t h the usual 90 ° - T _180 °

ZL. B/orkstam et al. / N M R studies in single crystal pulse sequence a f t e r which the echo appears at time T. For T- I > 1.9(10-3 ) K- I the echo amplitude shows simple e x p o n e n t i a l r e c o v e r y f o r an approximately 2-decade range over which data was taken. At temperatures above TA non-exponential recovery is very evident in much less than the f i r s t decade. We have decomposed t h i s non-exponential r e c o v e r y i n t o 2-components and show in Fig.2 both the s h o r t and long components, as well as the e-1 recovery time, T2e. At T < 400 K T2 is too short for the echo t o be observed. Conductivity measurements in the temperature range f o r which a s i n g l e T2 value is observed give migration enthalpies from 0.33 eV f o r fine p o l y c r y s t a l l i n e L i l , t o 0.39 eV f o r single cryst a l s . 6 Implications of the fact t h a t the s i n g l e crystal T2 value f o r t h i s temperature range is close t o the c o n d u c t i v i t y value f o r fine p o l y c r y s t a l l i n e material is yet to be assessed. The same i s t r u e w i t h r e s p e c t t o t h e even l o w e r T 2 a c t i v a t i o n energies a t high temperatures, where the l a r g e f o r m a t i o n e n t h a l p y g i v e s an even h i g h er measured v a l u e f o r c o n d u c t i v i t y . 5 Below 350 K the resonance l i n e undergoes the usual broadening associated w i t h f r e e z i n g out o f m o t i o n . The f u l l w i d t h a t h a l f maximum o f t h e a b s o r p t i o n l i n e reaches t h e r i g i d l a t t i c e v a l u e o f j u s t o v e r 5 kHz b e l o w 150 K. The u s u a l BPP

1000

559

i

i

I

/ E:0.335eV~ / lOO

!°;/

I--1 i

~N

IC

i i iD

,,

•

\T"

i

i 2.o 1000 [K-t] T

1.5

i 2.5

FIGURE 2 Temperature deDendence o f the s p i n - s p i n r e l a x a t i o n time in s i n g l e c r y s t a l L i l at 35 MHz.

analysis of t h i s motional narrowing regime gives an apparent a c t i v a t i o n energy of 0.2 eV near T-1 = 3.5(10 -3 ) K- I . This is consistent with the T1 slope in t h i s temperature range, as is shown in Fig. 1. 4 3.3 PFG Diffusion Measurements The r e s u l t s o f c o n v e n t i o n a l p u l s e d - f i e l d gradient (pfg) diffusion measurements are shown in Fig.3. The s t r a i g h t l i n e through the data points f o r T > TA corresponds t o an a c t i v a t i o n energy o f 0.89 eV, not t o o d i f f e r e n t from the value o f 0.96 eV from p r e v i o u s s i n g l e c r y s t a l c o n d u c t i v i t y data. 5 The dotted l i n e passing through the high temperature single crystal T1 data points in Fig. 1 was chosen t o have the same slope as t h a t of the high temperature d i f fusion r e s u l t . The break in the NMR d i f f u s i o n data occurs where c o n d u c t i v i t y studies show the t r a n s i t i o n from low temperature " e x t r i n s i c " conductivity t o the high temperature " i n t r i n s i c " regime. I t was not p o s s i b l e t o o b t a i n r e l i a b l e diffusion results at lower temperatures.

n

i~'

2'0

25 '-

l o o o [K-~] T FIGURE 3 P u l s e d - f i e l d - g r a d i e n t d i f f u s i o n measurements f o r Li in the s i n g l e c r y s t a l S1 and a d i s persed phase sample (G),taken a t 35 MHz.

560

J.L. Bjorkstam et al. / NMR studies in single crystal

4. PRELIMINARY DISPERSED PHASE NMR RESULTS The major thrust of the study presented here is to demonstrate the extent t o which NMR data can show d e t a i l e d features associated with Li + transport in single crystal L i I . We present some results on dispersed phase materials which show how s e n s i t i v e NMR is t o the material properties. We are i n v e s t i g a t i n g a range of samples with Al203 and/or SiO2 of grain size from 0.5 t o 300 m2/g, and of various volume % ceramic, as well as pure p o l y c r y s t a l l i n e L i l . Some samples are packed and sintered, while some are only packed. We present here some l i m i t e d data from four samples: Sample "H" is a m e l t formed, coarse g r a i n p o l y c r y s t a l o f pure L i I . Samples "C" and "G" c o n t a i n , r e s p e c t i v e l y , 50% and 10% by v o l ume of 15 m2/g Al203. Sample "E" contains 50% by volume o f 300 m2/g Al203. A l l were packed and melt formed. 4.1. Spin L a t t i c e Relaxation At the risk of unduly complicating Fig. 1 we have i n c l u d ~ a substantial portion of the p o l y c r y s t a l l i n e r e l a x a t i o n data which we have a v a i l able at t h i s intermediate stage of i n v e s t i g a t i o n . In each case i t is p o s s i b l e t o e x t r a c t an a c t i v a t i o n energy from the slope. As mentioned e a r l i e r , the T1 minimum evident f o r the s i n g l e crystal sample is c l e a r l y not of the simple BPP form. Nevertheless, the low temperature behavior is expected to satisfy the usual condition at temperatures below such a minimum; i.e., ~T >>I. This corresponds t o the s o - c a l l e d " h i g h f r e q u e n cy regime" where t h e n u c l e a r spins are sampling the dynamics o f s h o r t range motion, r a t h e r than t h e " l o w f r e q u e n c y r e g i m e " where t h e dynamics o f long range t r a n s p o r t are important. I t i s worth n o t i n g t h a t , p r o g r e s s i n g from t h e most o r d e r e d s a m p l e t o t h e most d i s o r d e r e d ; i.e., single crystal (S 1) -> H -> C -> E, t h e activation energy increases in sequence as 0.20 -> 0.25 -> 0.32 -> 0.39 eV. We w i l l return t o a d i s c u s s i o n o f these data a f t e r presenting some l i n e narrowing r e s u l t s . 4 . 2 . Line Narrowing Temperature dependence o f the resonance l i n e narrowing is shown in Fig. 4. D i f f e r e n c e s in t h e f r e q u e n c y and method o f t a k i n g t h e s e d a t a a r e not significant. The o n l y i m p o r t a n t m a t t e r i s t h e dependence upon t e m p e r a t u r e . A c t i v a t i o n e n e r g i e s were e x t r a c t e d from t h e s e " m o t i o n a l narrowing" r e s u l t s in the usual way, 4 y i e l d i n g

6

I

I

I

I S I - - 0.Z0 eH --0.35 x C --0.33 \ "x I E - - 0 . 3 4 •

N

5

t i

Fa

-o,,

3

~

N

\\ \\

\

~z

x \

,lx

LLI

z

t

_i

o

I

I

200

Z50

"~1

T(K)

300

I

35O

4OO

FIGURE 4 Temperature dependence o f l i n e narrowinq. Numbers by the sample d e s i g n a t i o n s r e p r e s e n t the a c t i v a t i o n energy in eV e x t r a c t e d from the raw data by a conventional BPP a n a l y s i s . The SI and H sample data was taken at 35 MHz. t h a t f o r the o t h e r samples was a t 77.7 MHz.

the values 0.20, 0.35, 0.32 and 0.35 eV f o r samp l e s SI , H, C and E, r e s p e c t i v e l y . Except f o r sample H, i t may be noted t h a t t h e v a l u e s are in reasonable agreement w i t h those e x t r a c t e d from t h e T1 data. In each case the l i n e shape shewed a composite c h a r a c t e r in the t e m p e r a t u r e range where n a r r o w i n g was most p r o n o u n c e d , w i t h a narrow p o r t i o n centered upon broader s h o u l d e r s . This was p a r t i c u l a r l y e v i d e n t in the case o f sample H, making i t d i f f i c u l t to uniquely define the line width throughout the entire narrowing r e g i m e . Whether t h i s , o r some more f u n d a m e n t a l process, is the o r i g i n o f the a n o m a l l y remains t o be determined. 5. DISCUSSION I t i s c l e a r t h a t d e t a i l s o f t h e dynamic and structural properties of single crystal and composite samples have a s u b s t a n t i a l e f f e c t upon t h e NMR r e s u l t s . In a d d i t i o n t o t h e f e a t u r e s which have been presented, i t has been p o s s i b l e t o go t o temperatures above t h e T I minimum i n s e v e r a l samples. As may be expected on the basis o f t h e d a t a h e r e i n , t h e p o s i t i o n and shape o f these minima are v e r y sample dependent. We focus a t t e n t i o n here upon t h e low temperature region. 5.1. A c t i v a t i o n Enthalpies W h i l e T 1 minima in f a s t ion c o n d u c t o r s a r e seldom o f the s i m p l e BPP t y p e , i t i s o f t e n poss i b l e t o r e l a t e t h e b e h a v i o r on the two sides o f t h e minimum in a r a t h e r s t r a i g h t f o r w a r d way. 4

J.L. B]orkstam et al. / NMR studies in single crystal This is complicated in L i I by the aforementioned f e a t u r e s . Except f o r the e x t r i n s l c / i n t r i n s i c t r a n s i t i o n , one might expect a high temperature slope of 039 eV in s i n g l e c r y s t a l s , corresponding t o the migration enthalpy, hm.6 The low temperature value is often about h a l f as large.4 I t is i n t e r e s t i n g t h a t in t h e s i n g l e c r y s t a l t h e low temperature value ist within experimental e r r o r , Just h a l f the migration enthalpy. Because o f t h e "narrow" TI minimum, i t was possible t o gather a substantial amount o f high temperature data in sample H. I f the high temperature slope of 1.1 eV can be considered t o be [ ( i / 2 ) h f + hm], with hf the formation enthalpy, and 0.25 is h a l f the low temperature m i g r a t i o n enthalpy f o r t h i s strained p o l y c r y s t a l , one ext r a c t s hf = 1.2 eV, in agreement with the s i n g l e crystal v a l u e .6 In view o f the lower m i g r a t i o n energy along an interface,6 one might expect the low temperature slopes t o e x h i b i t reduced a c t i v a t i o n energy with increasing sample disorder. We have, as yet, no explanation f o r the inverse behavior. 5.2. NMR Correlation Times We consider f i r s t the internal consistency of the NMR dynamic information, a f t e r which we w i l l make a comparison between diffusion and conduct i v i t y results. From the condition t h a t ARLT = 1 when r i g i d l a t t i c e l i n e narrowing is h a l f complete,4 we find from the s i n g l e crystal data t h a t T = 3.2(10 -5 ) sec a t T- I = 3.28(10 -3 ) K-1. We have taken as the r i g i d l a t t i c e linewidth, ~RL= 5 kHz. The usual BPP condition predicts a TI minimum when ~o z= 1. With ( ~ o / 2 ~ ) = 35 MHz t h i s corresponds t o T = 4.5(10 -7 ) sec. Assuming an activated process, with migration enthalpy of 0.4 eV, the BPP TI minimum should occur when l ~1 =2.4(10 -3) K- I . Wlth the t y p i c a l departures from BPP behavior found in fast ion conductors, ox = 5 at the T1 minimum. With t h i s c o n d i t i o n one expects the minimum at T-1 = 2.7(10-3 ) K-1. These two v a l u e s e x a c t l y bracket t h e s i n g l e c r y s t a l minimum of Fig. I , thus showing t h e i n t e r n a l consistency of the l i n e narrowing and TI data i n t e r p r e t a t i o n , and v e r i f y i n g t h i s as a "conventional" T1 minimum.

561

5.3. Conductivity and Diffusion The Nernst-Einstein r e l a t i o n allows a comparison of conductivity with the NMR d i f f u s i o n measurement. At T-~ = 0.0015 K-1, f o r example, DNMR = 10-1~ m2/s and the single crystal cond u c t i v i t y is 6 ~ = 0.135 (Ohm m)-~. Setting DNMR = (akT/q2C), where C is the concentration of c a r r i e r s with charge q, leads to the r e s u l t C=4.9.1027 m-3. This is about 3.3 times small e r than t h e L i concentration in L i l (which has the NaCl structure) as calculated from the spec i f i c density of 3.494.103 kg/m 3. While our res u l t indicates that not a l l Li ions are mobile, a d e f i n i t e conclusion can only be drawn a f t e r measuring conductivity and d i f f u s i o n in the same specimen. Such a measurement is in progress. 6. CONCLUSIONS We have" demonstrated that NMR in single crystal and dispersed phase L i l material is very sample dependent. For the most part, the NMR results show a very nice internal consistency; a preliminary estimate of the number of mobile ions has been obtained. We a n t i c i p a t e that f u r t h e r investigations w i l l resolve some of the remaining questions and, in so doing, provide a d d i t i o n a l in s ight with respect to fast ions transport in disordered systems. 7. ACKNOWLEDGMENTS We wish to thank Dr. Gary DrobnyandMr. Eric Shankland of the University of Washington Chemi s t r y Department f o r technical assistance and data gathering with the Bruker CXP 200.

REFERENCES I. C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289. 2. J.B. Phipps and D.H. Whitmore, Sol. State Ionics 9/10 (1983) 123. 3. R. Dupree, J.R. Howells, A. Hooper and F.W. Poulsen, Sol. State Ionics 9/10 (1983) 131. 4. See Bjorkstam et a l . , this Volume. 5. B.J.H. Jackson and D.A. Young, Chem. Solids 30 (1969) 1973.

J.

Phys.

6. J.B. Phipps, Ph.D Thesis, Dept. of Materials Science and Eng.,Northwestern Univ. (1983).