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Submitted on 1 Jan 1981
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ULTRASONIC ATTENUATION AND OCCURENCE
OF THE MARTENSITIC TRANSFORMATION IN
THE HIGH Tc A15 COMPOUND V3Si
A. Khoder, M. Couach, E. Bonjour
To cite this version:
ULTRASONIC ATTENUATION AND OCCURENCE OF T H E MARTENSITIC TRANSFORMATION IN T H E HIGH
T,
A 1 5 COMPOUND V3SiA.F. Khoder, M. Couach and E. Bonjour
Laboratoire de Cryophysique, Service des Basses Terrpe'ratuxes, Centre d 'Etudes NucZdaires de Grenobte, 85 X , 38041 GrenobZe Cedex, France
Abstract
-
The [I001 shear modulus C44 i s known to be nearly temperature independent in the case of V3Si, an extensively studied compound f o r wich many authors t r i e d t o associate the l a t t i c e i n s t a b i l i t y and the high super- conducting t r a n s i t i o n temperature.We report new sound velocity and attenuation measurements of t h i s [loo] shear mode. The temperature change of the sound velocity displays a
6 % r e l a t i v e decrease while a large increase i n the attenuation of t h i s mode occurs f o r T decreasing from 80 K t o Tc = 17 K.
This new observation gives a strong support f o r a very recently proposed approach t o the occurence of the structural transformation i n V3Si.
1. Introduction
The V3Si A15 compound i s known t o undergo a s t r u c t u r a l transfor- mation from a cubic symmetry t o a tetragonal one a t Tm = 21 K , a few degrees above the superconducting t r a n s i t i o n temperature (Tc = 17 K . ) .
Furthermore t h i s compound e x h i b i t s an anomalous e l a s t i c behaviour
1
as a strong softening of the acoustic shear mode Cs = (C11
-
C12) [ l l occurs on cooling despite the structural transformation i s observed o r not. Non trans- forming samples a r e believed t o have a "defect perturbed l a t t i c e " [2,31.The l a t t i c e softening of V3Si has been studied by sound velocity and attenuation measurements by Testardi and Bateman [41, the softening of the transforming samples being stronger than t h a t of the non transforming ones.
As shown by these authors, the attenuation as of the shear mode Cs
increases as temperature decreases below 100 K , while the attenuation of other acoustic modes increases only near Tm = 21 K.
Sound velocity and attenuation measurements have been performed on several V3Si single c r y s t a l s (both transforming and non-transforming). Shear waves being generaly more s e n s i t i v e t o a synnnetry break-down, even local one, than longitudinal waves, we investigated p a r t i c u l a r l y the shear waves along[1001 axis ( C q 4 ) .
The new r e s u l t s , reported below, allow us t o specify the common feature of the two Kinds of V3Si samples and to propose a new significance f o r Tm.
JOURNAL DE PHYSIQUE
2. Experiment
All V3Si samples studied have been prepared by the high frequency f l o a t i n g zone method-and submitted t o s p e c i f i c heat analysis C51. Two of these samples
(S2 and S5 M) e x h i b i t the s p e c i f i c heat anomaly a t Tm = 21 K commonly associated with the occurence of the transformation. The t h i r d one denoted S4M (V3,2 S i ) does not show such an anomaly and, therefore, i t i s considered as non transfor- ming. I t becomes superconducting a t 15 K while f o r the stoechiometric samples S2 and S5M the c r i t i c a l temperature i s near 17 K. Ultrasonic experiments were performed as a function of the temperature using an Y-cut quartz transducer
(10 MHz) bonded t o the sample with Nonaq, the pulse-superposition method of Mc Skimin 161 being used t o measure the wave v e l o c i t i e s .
3. Results
3.a
-
The velocity VS and the attenuation as of the acoustic shear mode CSalong
[ 1 1 O ] polarized [1'101 measured on the sample S f l
The r e s u l t s of velocity measurements obtained i n t h i s case a r e displayed on Figure 1 and compared with previous r e s u l t s obtained by Testardi and Bate- man 141 on transforming and non transforming samples denoted respectively "T" and "NTH.
Figure 1 - Comparison of the velocity VS of the acoustic shear mode CS
along [I101 polarized [ 1 5 ] measured on SqM with the r e s u l t s of Testardi (4) obtained respectively on transforming "T" and non transforming samples "NT"
As the temperature decreases, i t appears c l e a r l y t h a t the 3 samples S4M, "T" and "NT" behave similarly until 80 K i s reached.
:)
&HFigure 2
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Several measu- rements of the attenuation a s of the shear mode CS obtained on SqM and compa- red with the r e s u l t s of Testardi (4) f o r a trans- forming sample "T"3.b
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The velocity VT and the attenuation aT of the acoustic shear mode C4( along 11001 measured on the samples S2, SqM, S gThe ultrasonic velocity VT of t h i s mode show a s l i g h t r e l a t i v e decrease of 4-7 % between room temperature and 4.2 K i n agreement w i t h previous r e s u l t s ob- tained i n 141. The r e s u l t s of the ultrasonic attenuation aT measured on the three studied samples and on d i f f e r e n t p a r t s of them, a r e displayed on Figure 3.
The following observations can be drawn :
-
The increase of aT between 80 K and 25 K appears t o be a common f e a t u r e f o r a l l the samples investigated independent of t h e i r s t a t e whether transforming or not.-
For T < 25 K, the r e s u l t s are depending on the samples and a r e related t o the scanned p a r t of the sample.Thus the general behaviour of aT d i f f e r s markedly from t h a t measured by Testardi which reported a marked increase between Tm = 21 K and Tc.
4. Discussion
Two important features a r i s e from the r e s u l t s presented above :
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the attenuation aT behaves same a s as (see f i g u r e s 2 and 3)-
the increase of as and aT s t a r t s a t the same temperature 80+
5 K.The increase of
9
cannot be explained by the following c l a s s i c a l mechanisms :-
attenuation due t o electrons appears not s u i t a b l e t o explain such an increase considering t h a t aT remains nearly constant i n the superconducting s t a t e .-
according t o the c l a s s i c a l treatment 171 the reduction of the velocity V3 would imply an increase of the attenuation proportionnal t o 1/V
.
JOURNAL DE PHYSIQUE
Recent thermal c o n d u c t i v i t y measurements [ 8 ] on t h e two c l a s s o f V3Si samples show a common behaviour i n t h e temperature range 1.2
-
5 K o f b o t h sam- ples, which was i n t e r p r e t a t e d by t h e existence o f p l a n a r d e f e c t s associated i n t h e case o f non t r a n s f o r m i n g samples w i t h t h e boundary between t h e t e t r a g o n a l domain and t h e c u b i c m a t r i x .For t r a n s f o r m i n g samples, anomalies o f thermal expansion [9,10,111 extending up t o a t l e a s t 60 K were i n t e r p r e t e d as evidence f o r t e t r a g o n a l i t y .
The anomalous increase o f t h e a t t e n u a t i o n aT between 80 K and 20 K can be e x p l a i n e d by losses r e s u l t i n g from t h e d i f f r a c t i o n o f t h e u l t r a s o n i c waves by t h e t e t r a g o n a l domains developing below 80 K. T h i s temperature can be associated w i t h t h e onset o f s e n s i t i v i t y o f u l t r a s o n i c a t t e n u a t i o n t o t e t r a g o n a l i ty, i m p l y i n g t h a t t h e n u c l e a t i o n temperature would be h i g h e r than 80 K. The increase o f t h e a t t e n u a t i o n on c o o l i n g depends a l s o on t h e l e v e l o f t e t r a g o n a l i t y which would be lower i n t h e case o f non t r a n s f o r m i n g c r y s t a l as shown by sample S4M.
The upper p a r t o f the a t t e n u a t i o n aT f o r temperature lower than 25 K displays q u i t e a d i f f e r e n t behaviour ( F i g u r e 3).
the tetragonal domains even i n the presence of superconductivity. This obser- vation i s i n agreement with the r e s u l t s of Gibbs e t a1 Clll and Fawcett 1121.
The same Kind of analysis based on the occurence of t e t r a g o n a l i t y a t higher temperature may account f o r d i f f e r e n t anomalous e f f e c t s observed previously a s the second harmonic conversion i n ultrasonic wave propagation reported by Testardi 1131. The discontinuous behaviour of
3
(CS
= 112 ( c ~ ~ - c ~ ~ ) ) a s a function of pressure observed by Larsen and Ruoff 1141 near 90 K f o r a non trans- forming sample and the existence of a m i n i m u m of$
reported by Garcia and Barsch 1141 near 80 K f o r a transforming sample could be explained within the scope of tetragonal i t y .5. Conclusion
Anomalous attenuation of t h e ultrasonic shear mode C44 along [lOOl was interpreted a s the occurence of tetragonal i t y a t temperatures extending up t o and a minimum of 80 K, where i t i s i n agreement w i t h other observations.
The difference between transforming and non transformingsamples can be ex- plained by the level of t e t r a g o n a l i t y occuring i n each sample.
Studies a r e underway t o achieve a b e t t e r understanding of the development of t e t r a g o n a l i t y according t o a recently proposed approach t o the occurence o f v
the s t r u c t u r a l transformation i n V3Si 1161.
Acknowledgements
JOURNAL DE PHYSIQUE
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