INFLUENCE OF THE MARTENSITIC PHASE TRANSFORMATION ON THE ULTRASONIC ATTENUATION AND VELOCITY IN SINGLE CRYSTAL LITHIUM

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INFLUENCE OF THE MARTENSITIC PHASE

TRANSFORMATION ON THE ULTRASONIC

ATTENUATION AND VELOCITY IN SINGLE

CRYSTAL LITHIUM

J. Trivisonno, A. Slotwinski, M. Johnson

To cite this version:

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INFLUENCE OF THE MARTENSITIC PHASE TRANSFORMATION ON T H E ULTRASONIC

ATTENUATION AND VELOCITY IN SINGLE CRYSTAL LITHIUM

J . T r i v i s o n n o , A.R. S l o t w i n s k i and M.P. Johnson

Department of Physics, John Cam022 University, neveZand Ohio 44118,

U.S.A.

A b s t r a c t . - A g a t e d c o h e r e n t p h a s e s e n s i t i v e u l t r a s o n i c t e c h n i q u e was e m p l p e d

t o measure t h e u l t r a s o n i c a t t e n u a t i o n and v e l o c i t y i n s i n g l e c y r s t a l s of L i and 7 ~ i i n t h e v i c i n i t y of t h e m a r t e n s i t i c p h a s e t r a n s f o r m a t i o n t e m p e r a t u r e . To a v o i d s t r a i n i n g t h e s a m p l e s , e l e c t r o m a g n e t i c t r a n s d u c e r s were used t o gen- e r a t e and d e t e c t t h e a c o u s t i c a l s i g n a l s . The v e l o c i t y and t h e a t t e n u a t i o n were measured a t lOMHz f o r t e m p e r a t u r e s between 20K and 180K f o r waves asso- c i a t e d w i t h t h e t h r e e i n d e p e n d e n t e l a s t i c c o n s t a n t s CL=(CI I+C1 2+2C44) / 2 , C k 4 and C'=(Cll -C12) / 2 . T r a n s i t i o n t e m p e r a t u r e s between 60K and 80K were ob- s e r v e d . I n e v e r y c a s e f o r a l l modes s t u d i e d , t h e t r a n s i t i o n was v e r y a b r u p t and n o p r e m a r t e n s i t i c e f f e c t s were o b s e r v e d . The t r a n s f o r m a t i o n was accom- p a n i e d by a l a r g e i n c r e a s e i n t h e u l t r a s o n i c a t t e n u a t i o n . The a t t e n u a t i o n t y p i c a l l y changed by 5dB/cm, 15dB/cm and 55dB/cm f o r t h e CL, C 4 4 and C' modes r e s p e c t i v e l y . The v e l o c i t i e s a s s o c i a t e d w i t h t h e e l a s t i c c o n s t a n t s CL

and C ~ Q d e c r e a s e d by 8% and 20% r e s p e c t i v e l y a t T whiule t h e v e l o c i t y asso- c i a t e d w i t h C' i n c r e a s e d by a b o u t 10% t o 20% a t T:. For C and C 4 4 t h e ve- l o c i t y c o n t i n u e d t o d e c r e a s e a s t h e t e m p e r a t u r e was lowere2 t o 20K. Gener- a l l y on r e v e r s i o n t h e c r y s t a l s r e t u r n t o t h e o r i g i n a l body-centered o r i e n t a - t i o n . The r e s u l t s w i l l b e compared w i t h o t h e r s t u d i e s of t h e m a r t e n s i t i c t r a n s f o r m a t i o n i n l i t h i u m . I n t r o d u c t i o n . - The m a r t e n s i t i c phase t r a n s f o r m a t i o n i n l i t h i u m h a s b e e n e x t e n s i v e l y 1 s t u d i e d u s i n g a v a r i e t y of d i f f e r e n t e x p e r i m e n t a l t e c h n i q u e s . X-ray s t u d i e s show t h a t l i t h i u m t r a n s f o r m s from a body-centered c u b i c s t r u c t u r e t o a f a u l t y hexagonal- c l o s e d packed s t r u c t u r e a t a p p r o x i m a t e l y 72K. The e x t e n t of t h e t r a n s f o r m a t i o n i n - c r e a s e s a s t h e t e m p e r a t u r e i s lowered and about a 50% t r a n s f o r m a t i o n o c c u r s on c o o l i n g t o 55K. Upon h e a t i n g , r e v e r s i o n t o t h e b . c . c . p h a s e o c c u r s a t approxi-

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m a t e l y 90K. Thermal s t u d i e s of t h e m a r t e n s i t i c t r a n s f o r m a t i o n show a l a r g e spe- c i f i c h e a t anomaly i n t h e r e v e r s i o n r e g i o n , and t h e s p e c i f i c h e a t i n t h e two-phase r e g i o n i s dependent on t h e t h e r m a l h i s t o r y of t h e sample. Debye t e m p e r a t u r e s of 390K and 371K a r e r e p o r t e d f o r t h e h . c . p . and b . c . c . phases r e s p e c t i v e l y a t a tem- p e r a t u r e of 55K. O t h e r e x p e r i m e n t a l i n v e s t i g a t i o n s i n c l u d e d s t u d i e s of t h e e l e c -

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t r i c a l r e s i s t i v i t y and t h e m e t a l l o g r a p h i c c h a r a c t e r i s t i c s .

The i n f l u e n c e of t h e t r a n s f o r m a t i o n on t h e mechanical p r o p e r t i e s h a s been

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determined by V e r d i n i , who measured t h e f l e x u r a l v i b r a t i o n s of t h i n p o l y c r y s t a l - l i n e p l a t e s which w e r e clamped and e x c i t e d e l e c t r o s t a t i c a l l y a t a f r e q u e n c y of 20 H z . The measurements showed t h a t t h e t r a n s f o r m a t i o n h a s a profound e f f e c t on t h e v e l o c i t y and a t t e n u a t i o n . The t r a n s f o r m a t i o n was accompanied by a volume con- t r a c t i o n and a c o u s t i c e m i s s i o n . The t r a n s f o r m a t i o n t e m p e r a t u r e i n l i t h i u m depends

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markedly on t h e mechanical and thermal h i s t o r y of t h e sample. TC1s between 68K and 82K have been r e p o r t e d . The range i n t r a n s i t i o n temperatures i s u s u a l l y a t t r i b u t e d t o g r a i n s i z e ; t h e l a r g e r t h e g r a i n s i z e t h e h i g h e r t h e Tc. With t h e e x c e p t i o n of t h e m e t a l l o g r a p h i c s t u d y , a l l p r e v i o u s i n v e s t i g a t i o n s employed p o l y c r y s t a l l i n e sam- p l e s . The p r e s e n t s t u d y employed l a r g e s i n g l e c r y s t a l samples of l i t h i u m . The a c o u s t i c specimens were o r i e n t e d a l o n g t h e [I101 d i r e c t i o n , and t h e specimens were t y p i c a l l y one c e n t i m e t e r i n diameter and s i x - t e n t h s of a c e n t i m e t e r i n l e n g t h .

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Zener was t h e f i r s t t o s u g g e s t t h a t c r y s t a l s w i t h a b .c.c. s t r u c t u r e and a s m a l l v a l u e of C' should tend t o b e u n s t a b l e and undergo a l a t t i c e t r a n s f o r m a t i o n a t low temperatures. I n t e r e s t has focused on t h e n a t u r e of t h e t r a n s f o r m a t i o n i n l i t h i u m . I f t h e t r a n s f o r m a t i o n i s r e l a t e d t o t h e d i s p l a c i v e type, t h e n some phonon modes should e x h i b i t a soft-phonon mode b e h a v i o r . This l a t t i c e s t a b i l i t y can then g i v e r i s e t o p r e m a r t e n s i t i c e f f e c t s i n t h e u l t r a s o n i c a t t e n u a t i o n and v e l o c i t y .

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I n a p r e v i o u s s t u d y we r e p o r t e d some preliminary u l t r a s o n i c measurements f o r t h e l o n g i t u d i n a l modes and no p r e m a r t e n s i t i c e f f e c t s were observed. No s h e a r d a t a was o b t a i n e d because of t h e high a t t e n u a t i o n . We r e p o r t i n t h i s paper a t t e n u a t i o n and v e l o c i t y r e s u l t s f o r t h e s h e a r modes achieved by using a n u l t r a s o n i c g a t e d coherent t e c h n i q u e employing phase s e n s i t i v e d e t e c t i o n and box-car i n t e g r a t i o n . The g a t e d coherent technique e n a b l e s one t o measure r e l a t i v e v e l o c i t y changes with only one echo even i n t h e p r e s e n c e of n o i s e .

Experimental D e t a i l s . - The u l t r a s o n i c s i g n a l s were generated and d e t e c t e d by e l e c - tromagnetic a c o u s t i c t r a n s d u c e r s . ENAT t r a n s d u c e r s were employed t o e l i m i n a t e t h e need f o r a c o u s t i c bonds, which can be a f f e c t e d by t h e t r a n s f o r m a t i o n , and t o avoid s t r a i n i n g t h e sample. The EMAT t r a n s d u c e r s were of t h e s o l e n o i d type which gener- a t e l i n e a r l y p o l a r i z e d waves. By changing t h e o r i e n t a t i o n of t h e s t a t i c magnetic f i e l d , e i t h e r l o n g i t u d i n a l o r s h e a r waves could b e generated. The s t a t i c magnetic f i e l d , t y p i c a l l y 14kG, was s u p p l i e d by a 12" electromagnet.

A b l o c k diagram of t h e system used f o r coherent phase s e n s i t i v e d e t e c t i o n of

t h e u l t r a s o n i c s i g n a l s is shown i n Fig. 1. A s t a b l e o s c i l l a t o r s i g n a l i s g a t e d and a m p l i f i e d i n t o a h i g h v o l t a g e r . f . p u l s e of l u s e c d u r a t i o n which i s a p p l i e d t o an EMAT t r a n s d u c e r . The r e c e i v e d s i g n a l s a r e a m p l i f i e d and phase d e t e c t e d . The out- put of t h e phase d e t e c t o r i s f i l t e r e d and s e n t t o a box-car i n t e g r a t o r . An ampli- tude d e t e c t e d s i g n a l from t h e a m p l i f i e r i s a l s o s e n t t o an o s c i l l o s c o p e and used f o r t h e a t t e n u a t i o n measurements.

The experiments were performed i n a t h r o t t l i n g dewar which was suspended between t h e p o l e f a c e s of a 12" electromagnet. I n t h i s dewar, l i q u i d helium is

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it-

O U T r-, 8 10 CHANNEL 1 VlOEO OUT CHANNEL 2 In

1

1 MASTER OSCILLATOR 7 OSCILLOSCOPE 2 DIGITAL DIVIDER AMPLIFIER 3 UNIT PULSE GENERA104 9 SAMPLE

r COUNTER 10 BOXCAR INTEGRATOR 5 PHASE SENSITIVE DETECTOR I t DIGITAL VOLTMETER

6 GATED AMPLIFIER 12 OSCILLOSCOPE o

20 ro 60 80 m l a , lro TEMPERATURE IN K

F i g . 1: Schematic diagram of t h e F i g . 2: Temperature dependence of s y s t e m used f o r v e l o c i t y and a t t e n - t h e r e l a t i v e a t t e n u a t i o n f o r t h e Cb4 u a t i o n measurements. s h e a r wave.

e l a s t i c c o n s t a n t C,+,+. The sample was s l o w l y c o o l e d from 130K a t a r a t e of 0.25 d e g r e e s p e r minute. The a t t e n u a t i o n was e s s e n t i a l l y c o n s t a n t down t o 64K where a n a b r u p t and l a r g e i n c r e a s e i n t h e a t t e n u a t i o n o c c u r r e d . The w i d t h of t h e t r a n s i t i o n was l e s s t h a n 0.05K. As t h e t e m p e r a t u r e was lowered below 64K, a d e c r e a s e i n t h e a t t e n u a t i o n o c c u r r e d . During t h e h e a t i n g c y c l e , t h e a t t e n u a t i o n remained r e l a - t i v e l y c o n s t a n t , and between 90K and l l O K a l a r g e d e c r e a s e i n t h e a t t e n u a t i o n o c c u r r e d . Above l l O K , t h e a t t e n u a t i o n was e s s e n t i a l l y t h e same a s t h e i n i t i a l a t - t e n u a t i o n . The c o r r e s p o n d i n g v e l o c i t y d a t a i s shown i n F i g . 3 . A l a r g e d e c r e a s e i n t h e v e l o c i t y (22%) was observed a t Tc. The v e l o c i t y c o n t i n u e d t o d e c r e a s e r e a c h i n g a minimum a t 35K and t h e n i t i n c r e a s e d s l i g h t l y a s t h e t e m p e r a t u r e was lowered t o 4.2K. As t h e t e m p e r a t u r e was r a i s e d , t h e v e l o c i t y remained c o n s t a n t u n t i l a p p r o x i m a t e l y 90K was r e a c h e d . Between 90K and 130K a l a r g e i n c r e a s e i n v e l o c i t y o c c u r r e d . The v e l o c i t y t h e n e x h i b i t e d t h e normal t e m p e r a t u r e dependence a s t h e t e m p e r a t u r e was f u r t h e r i n c r e a s e d . The sample was t h e n h e a t e d t o room tem- p e r a t u r e , removed from t h e p r o b e and x-rayed. The c r y s t a l was found t o b e s i n g l e w i t h t h e same [I101 o r i e n t a t i o n . Both t h e v e l o c i t y and a t t e n u a t i o n of t h e l o n g i -

t u d i n a l waves e x h i b i t e d a t e m p e r a t u r e dependence upon c o o l i n g and h e a t i n g which was q u i t e s i m i l a r t o t h a t observed f o r t h e f a s t s h e a r waves. The v e l o c i t y d e c r e a s e d by a p p r o x i m a t e l y 8%. The a t t e n u a t i o n i n c r e a s e d by 5dB/cm a t Tp.

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55dB/cm. The v e l o c i t y , i n c o n t r a s t t o t h e C44 mode, i n c r e a s e d by a b o u t 22% a t Tc. Both t h e v e l o c i t y and a t t e n u a t i o n remained r e l a t i v e l y c o n s t a n t a s t h e t e m p e r a t u r e was r a i s e d , t h e n d e c r e a s e d r a p i d l y between 95K and l l O K . The v e l o c i t y d a t a o v e r a w i d e r t e m p e r a t u r e i n t e r v a l is shown i n F i g . 6 f o r t h e C' mode. For t h i s r u n a s m a l l e r i n c r e a s e , a p p r o x i m a t e l y 7%, was o b s e r v e d a t 70K; t h e v e l o c i t y t h e n c o n t i n - ued t o i n c r e a s e r a p i d l y a s t h e t e m p e r a t u r e was lowered t o 40K. The t o t a l change i n t h e v e l o c i t y was, however, a b o u t t h e same a s t h a t shown i n F i g . 5.

I n one e x p e r i m e n t a l r u n a pancake EMAT, which i s s e n s i t i v e t o a l l p o l a r i z a - t i o n d i r e c t i o n s , was used a s a r e c e i v i n g t r a n s d u c e r . A s o l e n o i d c o i l was employed t o g e n e r a t e t h e C44 mode. Above Tc, t h e normal f a s t s h e a r echo p a t t e r n was ob- s e r v e d ; below Tc, however, two s m a l l e c h o e s were o b s e r v e d , one a s s o c i a t e d w i t h C44 and t h e second assumed t o b e C'. The v e l o c i t y a s s o c i a t e d w i t h t h e C 4 4 echo de- c r e a s e d w h i l e t h e v e l o c i t y a s s o c i a t e d w i t h t h e C' echo i n c r e a s e d a s t h e t e m p e r a t u r e was lowered. I n t h e r e v e r s i o n c y c l e , t h e s i g n a l s a l s o e x h i b i t e d t h e b e h a v i o r char- a c t e r i s t i c of t h e C b 4 and C' modes. The two echoes c o a l e s c e d a t 120K and t h e r e - s u l t i n g echo e x h i b i t e d t h e t e m p e r a t u r e dependence of C 4 4 . The v e l o c i t y and t h e a t - t e n u a t i o n changes f o r t h e C4,+ mode were e s s e n t i a l l y t h e same a s measured w i t h t h e s o l e n o i d - - s o l e n o i d geometry.

For a l l t h e s e e x p e r i m e n t a l r u n s t h e c r y s t a l was found t o b e s i n g l e w i t h t h e same [I101 o r i e n t a t i o n . We d i d h a v e s e v e r a l e x p e r i m e n t a l r u n s f o r which t h e c r y s - t a l was e i t h e r a b i c r y s t a l o r a s i n g l e c r y s t a l w i t h a n o r i e n t a t i o n s e v e r a l d e g r e e s from t h e o r i g i n a l [I101 d i r e c t i o n ; t h a t i s , a s t r u c t u r a l r e o r i e n t a t i o n o c c u r r e d . Summary and Discuss i o n .

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The p r e s e n t s t u d y shows t h e f o l l o w i n g :

1. The phase t r a n s f o r m a t i o n o c c u r s v e r y a b r u p t l y and t h e t r a n s i t i o n is e x t r e m e l y narrow.

2. No p r e m a r t e n s i t i c e f f e c t s were o b s e r v e d i n e i t h e r t h e v e l o c i t y o r t h e a t t e n u a t i o n f o r any of t h e t h r e e i n d e p e n d e n t e l a s t i c modes.

3 . Tc is n o t a w e l l - d e f i n e d t e m p e r a t u r e i n l i t h i u m . A r a n g e i n T c l s ex- t e n d i n g from 61K t o g r e a t e r t h a n 77K h a s been observed. S i n c e s i n g l e c r y s t a l s were employed, t h e p r e s e n t s t u d y shows t h a t t h e r a n g e of Tc i s

n o t a s s o c i a t e d w i t h g r a i n s i z e a s was p r e v i o u s l y b e l i e v e d .

4 . G e n e r a l l y t h e c r y s t a l s r e t r a n s f o r m a s s i n g l e c r y s t a l s w i t h t h e o r i g i n a l b . c . c . o r i e n t a t i o n . The s t r u c t u r a l r e o r i e n t a t i o n t h a t sometimes o c c u r s

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i s s i m i l a r t o t h e r e s u l t s r e p o r t e d f o r sodium.

5. The v e l o c i t y changes i n t h e p r e s e n t s t u d y a r e a n o r d e r of magnitude g r e a t e r t h a n t h e r e s u l t s on p o l y c r y s t a l l i n e samples. T h i s f e a t u r e , t o - g e t h e r w i t h a b r u p t t r a n s f o r m a t i o n , s u g g e s t s t h a t a l a r g e m a r t e n s i t e i s formed a t Tc.

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t h e same e l a s t i c modes a s above Tc s i n c e t h e low t e m p e r a t u r e p h a s e i s non-cubic and t h e o r i e n t a t i o n of t h i s p h a s e is unknown. N e v e r t h e l e s s , t h e u l t r a s o n i c d a t a does g i v e a n i n d i c a t i o n of t h e b e h a v i o r of t h e e l a s t i c p r o p e r t i e s a s s o c i a t e d w i t h t h e t r a n s f o r m a t i o n . The v e l o c i t y and a t t e n u a t i o n changes a r e a l s o s u r p r i s i n g l y r e p r o - d u c i b l e from one r u n t o a n o t h e r . It i s i n t e r e s t i n g t o p o i n t o u t t h a t t h e d e c r e a s e i n t h e v e l o c i t y a s s o c i a t e d w i t h CL and C4,+ modes and t h e i n c r e a s e i n t h e v e l o c i t y a s s o c i a t e d w i t h t h e s l o w s h e a r c o n s t a n t C' w i l l l e a d t o a 0 below T which i s

D c

l a r g e r t h a n

'aD

above Tc i n agreement w i t h t h e s p e c i f i c h e a t r e s u l t s .

9

The Au-50% Cd s y s t e m undergoes a m a r t e n s i t i c phase change which i s a l s o of f i r s t o r d e r p h a s e change. The t e m p e r a t u r e dependence of Cb4 and CL i s normal b u t C' d e c r e a s e s a s t h e t e m p e r a t u r e is lowered. C b 4 and CL d e c r e a s e a t Tc w h i l e C' in- c r e a s e s d r a m a t i c a l l y a t Tc. The t r a n s i t i o n is b e l i e v e d t o b e a s s o c i a t e d w i t h a . s o f t mode.

S i n c e we have o b s e r v e d n o p r e m a r t e n s i t i c e f f e c t s f o r any o f t h e modes, t h e p r e s e n t measurements s u g g e s t t h a t a s o f t l a t t i c e mode i s n o t t h e b a s i s of t h e t r a n s f o r m a t i o n i n l i t h i u m . More measurements a r e needed, however, i n l i t h i u m t o d e t e r m i n e t h e n a t u r e of t h e t r a n s i t i o n . Thermal e x p a n s i o n measurements and x-ray d a t a on s i n g l e c r y s t a l s would b e d e s i r a b l e . A d d i t i o n a l n e u t r o n d i f f r a c t i o n mea-

1 0

s u r e m e n t s a r e a l s o needed. While t h e p r e v i o u s n e u t r o n d i f f r a c t i o n measurements i n l i t h i u m i n d i c a t e d no phonon f r e q u e n c y s h i f t s o r peak b r o a d e n i n g , t h e s e e x p e r i - ments may n o t have b e e n done c l o s e enough t o Tc, which is n o t a w e l l d e f i n e d tem- p e r a t u r e i n l i t h i u m . Measurements below Tc and i n t h e r e v e r s i o n c y c l e would a l s o b e u s e f u l .

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