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Some aspects of mechanical energy dissipation phenomena in yttrium superconductors.
0038-1098/9255.00+.00 Pergamon Press Ltd
) Solid State Communications, Vol. 83, No. 10, pp. 793-797, 1992. Printed in Great Britain.
SOME ASPECTS OF MECHANICAL SUPERCONDUCTORS.
ENERGY DISSIPATION
PHENOMENA
IN
YTTRIUM
M . G a z d a , B.Kusz, R . J . B a r c z y ~ s k i and L . M u r a w s k i D e p a r t m e n t of P h y s i c s and M a t h e m a t i c s T e c h n i c a l U n i v e r s i t y of G d a ~ s k , P o l a n d O . G z o w s k i , I . D a v o l i and S . S t i z z a D i p a r t i m e n t o di M a t e m a t i c a e F i s i c a U n i v e r s i t a di C a m e r i n o , I t a l y ( R e c e i v e d 21 J u n e
1991 by M.
Tosi)
The r e s u l t s of m e a s u r e m e n t s of i n t e r n a l f r i c t i o n a n d Y o u n g ' s modulus in yttrium 1-2-4 superconducting ceramics are reported. The internal friction maxima observed in the t e m p e r a t u r e r a n g e f r o m 30 K to i00 K are of r e l a x a t i o n n a t u r e . T h e a p p r o x i m a t e v a l u e s of a c t i v a t i o n e n e r g y a n d r e l a x a t i o n t i m e of t h e s e p h e n o m e n a are given. T h e a n e l a s t i c p r o p e r t i e s of the 1-2-4 and 1-2-3 superconductors are compared. An a l t e r n a t i v e m e c h a n i s m for one of t h e r e l a x a t i o n p r o c e s s e s is proposed.
Introduction.
p o s s i b l e c h a n g e s in the m e c h a n i c a l energy s p e c t r a w h i c h may be induced by the second Cu-O chain. These m i g h t give some clues as to the origin of r e l a x a t i o n e f f e c t s p r e s e n t in the y t t r i u m s u p e r c o n ductors.
The YBa2Cu40 ~ s u p e r c o n d u c t o r was first s y n t h e s i z e d by K a r p i n s k i et al. in 1988 I. It r e q u i r e d a p p l i c a t i o n of h i g h p r e s s u r e t e c h n i q u e to obtain a single p h a s e b u l k superconductor. Recently, several p a p e r s d e s c r i b i n g the s y n t h e s i s m e t h o d w h i c h r e q u i r e d oxygen p r e s s u r e of the a r d e r of one a t m o s p h e r e have been p u b l i s h e d 2'3. Consequently, the studies of the p r o p e r ties of this m a t e r i a l b e c a m e m u c h m o r e a c c e s s i b l e to m a n y laboratories. The crystal s t r u c t u r e of the 1-2-4 c o m p o u n d is in many r e s p e c t s s i m i l a r to that of the 1-2-3 material. The m a i n d i f f e r e n c e is a d o u b l e Cu-O c h a i n w h i c h is p r e s e n t only in the 1-2-4 phase. Each oxygen atom in these chains is b o u n d to three copper atoms. It r e s u l t s in the 1-2-4 s u p e r c o n d u c t o r h a v i n g stable o x y g e n s t o i c h i o m e t r y . The 1-2-4 unit cell exh i b i t s smaller o r t h o r h o m b i c i t y t h a n t h a t found in the 1-2-3 s t r u c t u r e (0.8% in the 1-2-4 phase and 1.8% in the 1-2-3 phase) 4' M a n y p a p e r s on the a n e l a s t i c effects in the 1-2-3 s u p e r c o n d u c t o r s have b e e n ~ u b l i s h e d in the last four y e a r s e.g. 5'6'''~. A number of r e l a x a t i o n p h e n o m e n a have been o b s e r v e d in o r t h o r h o m b i c and t e t r a g o n a l materials. Several m e c h a n i s m s w h i c h m i g h t be r e s p o n s i b l e for t h e s e p h e n o m e n a have b e e n proposed. T h e s e inc l u d e a v a c a n c y and oxygen atom jumps 7,n, the S n o e k - t y p e p r o c e s s 23 and the d i s t o r t e d Cu-O c h a i n r e l a x a t i o n 12.13.As two r e l x a t i o n p h e n o m e n a h a v e b e e n a s s o c i a t e d w i t h the r e l a x a t i o n of c h a i n atoms it s e e m e d int e r e s t i n g to us to study the p r o p e r t i e s of 1-2-4 c o m p o u n d and to o b s e r v e the
Experimental. The synthesis procedure of the YBa2Cu40 ~ s a m p l e s was based on the m e t h o d p u b l i s h e d by S . J i n et al. 3. A s t o i c h i o m e t ric 1-2-3 p o w d e r was m i x e d w i t h an app r o p r i a t e a m o u n t of CuO d i s s o l v e d in a small v o l u m e of 60% HNO 3 (i.e. the m i n i m u m v o l u m e r e q u i r e d to d i s s o l v e the CuO). T h e n the m i x t u r e was dried, g r o u n d and p r e s s e d into pellets. Three sets of 1-2-4 samples, f u r t h e r r e f e r r e d to as A, B and C, were p r e p a r e d . The s i n t e r i n g was carried out at 830°C in flowing o x y g e n at a p r e s s u r e of one atmosphere. The g r i n d i n g and s i n t e r i n g of the B and C samples was r e p e a t e d t w i c e w h i l e sample A was proc e s s e d t h r e e times. The total time of s i n t e r i n g was 70h, ll0h and 190h, respectively. A p a r t of samples A was a n n e a l e d at 660 ° C in a r g o n atmosphere. The time of a n n e a l i n g was lh (sample AI), 2h (A2) and 10h (A3). The s y n t h e s i s p r o c e d u r e of 1-2-3 s a m p l e s (orthorhombic - D and tet r a g o n a l - E) has been p u b l i s h e d elsew h e r e ~. The X - r a y tests were p e r f o r m e d with a Philips X-ray diffractometer. The m e a s u r e m e n t s of a n e l a s t i c effects w e r e c o n d u c t e d by the v i b r a t i n g reed technique. The m e a s u r e m e n t frequency was in the r a n g e of 95Hz - 660Hz. The s a m p l e s had a shape of a thin plate (30mm
793
MECHANICAL ENERGY DISSIPATION PHENOMENA
794
x 5mm x 0.4mm). The a m p l i t u d e of o s c i l l a tions was about 10#m w h i c h c o r r e s p o n d s to a the strain amplitude of 5 x 10 .6. The rate of temperature increase was 0.6 K/min. The decrement of the a m p l i t u d e d ecay was calculated by the least s q u a r e s method, fitting to a few h u n d r e d s u b s e quent amplitudes, simultaneously, the s h i e l d i n g effect was m e a s u r e d in o r d e r to d e t e r m i n e the transition t e m p e r a t u r e of the studied samples.
0 0012
Vol. 83, No. 10
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Results. The experimental r e s u l t s are presented in figures I, 2, 3 and t a b l e i. Several internal friction m a x i m a can be seen in all the spectra. The n u m b e r s i n d i c a t i n g the particular p e a k s are consistent with those u s u a l l y used for describing the 1-2-3 internal friction maxima s'~J2. The frequencies at w h i c h the spectra were obtained are shown in the figures. All the p h e n o m e n a o b s e r v e d in the 1-2-4 material in the t e m p e r a t u r e range studied here proved to be t h e r m a l l y a c t i v a t e d relaxation processes. The narrow frequency range available in our e x p e r i m e n t s did not allow us to d e t e r m i n e the e x a c t values of their activation e n e r g y and relaxation time. The q u a n t i ties p r e s e n t e d in table I, show the r a n g e of t he i r p o s s i b l e values. A l s o s h o w n in table i, for comparison, are the r e l a x a tion p a r a m e t e r s of the i n t e r n a l f r i c t i o n m a x i m a found in the i-2-3 m a t e r i a l ~J°. All the d i f f r a c t i o n p e a k s d e t e c t e d in the X-ray spectra of s a m p l e s A c o u l d be indexed on the basis of the k n o w n o r t h o r h o m b i c unit cell of YBa2Cu40~ 16, thus c o n f i r m i n g the s i n g l e - p h a s e n a t u r e of the
0002 ........ ~ : ~ " 2"0
.~
40
I -
~3
80 ,60 120 Ii0 160 i80 200 Temperature (K)
Figure I. Temperature dependence of internal friction in the 1-2-4, 1-2-3 orthorhombic and 1-2-3 tetragonal m a t e r i a l . The spectra were t a k e n w i t h the v i b r a t i o n f r e q u e n c y of 246Hz, 212Hz and 230Hz, r e s p e c t i v e l y .
sample. Its t r a n s i t i o n t e m p e r a t u r e was 76K. S a m p l e D was a s i n g l e - p h a s e 1-2-3 o r t h o r h o m b i c c e r a m i c s with the t r a n s i t i o n t e m p e r a t u r e of 90 K. Fig.l p r e s e n t s the r e s u l t s of internal f r i c t i o n m e a s u r e m e n t s v e r s u s t e m p e r a t u r e o b t a i n e d for the pure 1-2-4 (A), 1-2-3 o r t h o r h o m b i c (D) and 1-2-3 t e t r a g o n a l (E) ceramics. As one can see, the s p e c t r a of 1-2-4 and 1-2-3 ort h o r h o m b i c m a t e r i a l are very much s i m i l a r to each other. The s a m p l e s B and C were two-phased. T h e i r X - r a y s p e c t r a showed the p r e s e n c e
Table i. The approximate values of a c t i v a t i o n e n e r g y E A and r e l a x a t i o n time T O of the r e l a x a t i o n p r o c e s s e s in y t t r i u m c e r a m i c samples.
material
process no. 1
EA 0.07-0.1
2
.
log[ 0
3
0.Ii-0.14
4
0 • 13-0.15
E^ (eV)
< -13
.
1-2-3
1-2-30
1-2 -4
(eV)
0.07
.
-13
2-3T
log~ 0
0.2
iogT0
(ev) -13
.
0.16
EA
T
-12.5
I -13.5
-
-
0.ii
-12
-
-
|
MECHANICAL ENERGY DISSIPATION PHENOMENA
Vol. 83, No. 10 of
both
1-2-4
and
1-2-3
orthorhombic
materials. The remaining material was mainly CuO (the m i c r o r e g i o n s of c o p p e r oxide were detected by e l e c t r o n m i c r o probe). We estimated the c o n t e n t of t h e 1-2-4 phase to be a p p r o x i m a t e l y 70% in sample B and 40% in sample C. The t r a n s i tion t e m p e r a t u r e s of s a m p l e s B and C w e r e 79.5K and 81K, respectively. The h i g h e r critical temperatures and b r o a d e r t r a n s i tions (two-stepped t r a n s i t i o n in s a m p l e s C) were obtained in s a m p l e s c o n t a i n i n g s ignifi c a n t amount of the 1-2-3 and CuO phases. It should be t a k e n into c o n s i d e r ation that the s u p e r c o n d u c t i n g t r a n s i t i o n of the 1-2-3 material b r o a d e n s itself and shifts towards lower t e m p e r a t u r e s as CuO contamination increases ° . The results obtained for the t w o - p h a s e m a t e r i a l p l o t ted tog e t h e r with the 1-2-4 c u r v e (A) are shown in figure 2. The i n t e r n a l f r i c t i o n maxima seen in spectra B and C are considerably larger than t h o s e in the s p e c tra of pure 1-2-4 ceramics. Two-phase materia l inevitably c o n t a i n s m o r e d e f e c t s and is more d i s o r d e r e d t h a n the p u r e o n e . These d e f e c t p o s s i b l y e n h a n c e the i n t e r nal fr i c t i o n maxima. An a d d i t i o n a l p e a k (ib) at t h e temperature of a b o u t 50K appears in the t w o - p h a s e samples. It grows s i g n i f i c a n t l y and s h i f t s t o w a r d s lower t e m p e r a t u r e while the a m o u n t of the 1-2-3 and CuO compounds increases. In order to study the i n t e r n a l f r i c tion p r o c e s s e s in r e l a t i o n to the s t a t e of Cu-O chains the m e a s u r e m e n t s of i n t e r nal f r i c t i o n after a r g o n a n n e a l i n g w e r e carried out. The p r o c e s s of a n n e a l i n g was monitored by X-ray d i f f r a c t o g r a p h y . A gradual reduction in the 1-2-4 phase content along with g r o w i n g of b o t h 1-2-3 t e t r a g o n a l m a t e r i a l and c o p p e r o x i d e was detected. The A r - a n n e a l e d s a m p l e s AI, A2 and A3 c o n t a i n i n c r e a s i n g a m o u n t s of the
O.oo2o
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o .oo 18
y'\l
00016
.~_ 0.0014 00012
\,
i 00010
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0,0008 0,0006'
0.00040.0002 -
/I
\
-- A
/,'
Temperoture (K)
Figure 2. Internal friction versus t e m p e r a t u r e in t w o - p h a s e s a m p l e s (B and C) p l o t t e d t o g e t h e r w i t h the s p e c t r a of pure 1-2-4 s u p e r c o n d u c t o r (sample A). The B and C s p e c t r a were taken with the vibration frequency 212Hz, 200Hz respectively.
0 oo25
795
2
.....................................................................................
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...........................
.......................................................................................
ooo,: ........................ f':,_, .......................................................................... 2o
40
6o
80 i00 120 140 Temperoture (K)
160
180
20o
Figure 3. Internal friction versus t e m p e r a t u r e in o u t g a s s e d s a m p l e s (AI, A2 and A3). The s p e c t r a w e r e t a k e n w i t h the vibration frequency 330Hz, 350Hz and 510Hz r e s p e c t i v e l y .
1-2-3 tetragonal phase. The internal fr~ ~tion c u r v e s are p l o t t e d in F i g u r e 3. T h e d o m i n a n t f e a t u r e of the curves is the a p p e a r a n c e and g r a d u a l g r o w i n g of m a x i m u m 2, c h a r a c t e r i s t i c of the 1-2-3 t e t r a g o n a l m a t e r i a l . The a n a l y s i s of o b t a i n e d data in t e r m s of a s i n g l e - t i m e Debye r e l a x a t i o n 2j was p e r f o r m e d . A l t h o u g h it c a n n o t be i m m e d i a t e l y d e r i v e d from the figure, the a n a l y s i s s h o w e d t h a t the heat t r e a t m e n t did not a f f e c t p r o c e s s 3, as its parameters and the m a g n i t u d e do not change significantly. P r o c e s s e s 1 and 4 decreased their strength considerably, w h i l e peak 2 i n c r e a s e d w i t h a g r o w i n g a m o u n t of the 1-2-3 t e t r a g o n a l phase. The 1-2-4 s a m p l e s are softer than 1-2-3 (similar to Bi - b a s e d cuprates). It is i m p o s s i b l e to d e t e r m i n e the exact v a l u e of Y o u n g ' s m o d u l u s on the b a s i s of our m e a s u r e m e n t s . However, we can o b s e r v e its e v o l u t i o n w i t h t e m p e r a t u r e . The temp e r a t u r e d e p e n d e n c i e s of Young's m o d u l u s of the 1-2-4 and 1-2-3 s u p e r c o n d u c t o r s are s i m i l a r and d o e s not reveal any anomalous features T M . Discussion. Maximum 1 is v i s i b l e in both the 1-2-3 and 1-2-4 o r t h o r h o m b i c materials. As r e p o r t e d b e f o r e s, p e a k 1 d i s a p p e a r s in the 1-2-3 iron d o p e d s a m p l e s (with the iron c o n t e n t e q u a l to 1.75%). The subs t i t u t i o n of Fe for Cu r e d u c e s the conc e n t r a t i o n of m o b i l e h o l e s m, thus influe n c i n g the e l e c t r o n i c s t r u c t u r e of the s u p e r c o n d u c t o r . A l s o the removal of oxygen a t o m s from b a s a l p l a n e s results in: a decrease in m o b i l e h o l e s c o n c e n t r a t i o n and d i s a p p e a r a n c e of m a x i m u m I. Taking the a b o v e into c o n s i d e r a t i o n , it seems p o s s i b l e that an e l e c t r o n i c process may be r e s p o n s i b l e for p r o c e s s i. A small
796
MECHANICAL ENERGY DISSIPATION PHENOMENA
activation energy would be c o n s i s t e n t with this hypothesis. Maximum ib is v e r y small in the spectra of pure 1-2-4 ceramics. It is much bigger in the s p e c t r a of s a m p l e s with a considerable a m o u n t of the 1-2-3 phase and copper oxide (fig. 2). T h e r e fore we believe that this peak is not intrinsic to the 1-2-4 structure. It may be r e l a t e d to the defects a s s o c i a t e d with the pr e s e n c e of CuO ~. Maximum 3 is p r e s e n t in all the samples independently of t h e i r s u p e r c o n ducting properties. Cannelli et al. n proposed to relate this p h e n o m e n o n to stress induced jumps of the i s o l a t e d 0 atoms in the oxygen depleted regions which retain o r t h o r h o m b i c symmetry. The a nalys i s of the data o b t a i n e d for s a m p l e s anneal e d in argon showed that the state of copper - oxygen chains did not a f f e c t this process significantly. K a u f m a n n et al. 14 p e r f o r m e d systematic o u t g a s s i n g of the 1-2-3 material and found the peak i n s e n s i t i v e to oxygen content. We b e l i e v e that the insensitivity of this r e l a x a t i o n process to the state of c h a i n s might suggest that its origin s h o u l d not be looked for in the basal plane. The small activation energy of peak 3 seems to exclude an atom or a p o i n t d e f e c t m i g r a tion from one lattice p o s i t i o n to another 2°2j. However, a m o t i o n over a small fraction of the lattice d i s t a n c e m i g h t be c o n s i s t e n t with the a c t i v a t i o n e n e r g y of 0.13eV. In the last 25 years several cases of low t e m p e r a t u r e r e l a x a t i o n that are due to the "off s y m m e t r y " defects have been found ~820. A m o n g others, it was also o b s e r v e d in the p e r o v s k i t e structure. This kind of r e l a x a t i o n o c c u r s when the s y m m e t r i c c o n f i g u r a t i o n of the d e f e c t is u n s t a b l e and becomes s t a b l e only when it goes off symmetry. In this case, the stable defect s y m m e t r y is lower than that of the symmetric configuration. As a result, there will be more than one stable o r i e n t a t i o n of the d e f e c t for every s y m m e t r i c one. This allows for r e l a x a t i o n t h r o u g h atom or v a c a n c y m o v e m e n t s w h i c h are small fractions of a t o m i c d i s t a n c e s 2t~. We b e l i e v e that it may be the case in the c o p p e r oxide c e r a m i c s as well. G. Van T e n d e l o o et al. showed that in q u e n c h e d 1-2-3 crystals, part of Ba atoms substitute for Y atoms 22. A s i m i l a r d e f e c t was also r e p o r t e d in the 2-4-7 m a t e r i a l 15. This d i s o r d e r is frozen in the s a m p l e s as a r e s u l t of quenching, t h e r e f o r e it appears in quite a large quantity. It m i g h t be e x p e c t e d that this s u b s t i t u t i o n also takes place, but to s m a l l e r extent, in the y t t r i u m c o m p o u n d s c o o l e d slowly. As a r e s u l t of this imperfection, a lattice d i s t o r t i o n occurs. The b a r i u m o c c u p i e d cubes tend to u n d e r g o an a n i s o t r o p i c expansion, whereas the yttrium cubes b e c o m e flattened. Also the o x y g e n vacancies adapt their c o n f i g u r a t i o n to the arrangement of h e a v y atoms n. All this d i s o r d e r might c r e a t e an "off s y m m e t r y " c o n f i g u r a t i o n w h i c h can give rise to a
Vol. 83, No. 10
low t e m p e r a t u r e relaxation. The insensit i v i t y of m a x i m u m 3 to heat t r e a t m e n t w o u l d be c o n s i s t e n t with this hypothesis, because m i g r a t i o n of h e a v y atoms occurs in t e m p e r a t u r e h i g h e r than 660 ° C 22. Furt h e r e x p e r i m e n t s w h i c h will e i t h e r confirm or e l i m i n a t e the above are in progress. P r o c e s s 4 is v i s i b l e in the spectra of both 1-2-3 and 1-2-4 orthorhombic phases. This r e l a x a t i o n e f f e c t has been i n t e r p r e t e d as being due to the hops of o x y g e n a t o m s b e t w e e n two m i n i m a of energy in d i s t o r t e d Cu-O-Cu c h a i n s 12"13"I°. Such m i n i m a e x i s t in the o r t h o r h o m b i c struct u r e of the 1-2-3 m a t e r i a l II'15. The association of this p r o c e s s with the chain o x y g e n a t o m s is s u p p o r t e d by results of the a n n e a l i n g experiments. The m a x i m u m was s t r o n g l y a f f e c t e d by heat treatment. However, the n e u t r o n d i f f r a c t i o n studies show that the zig-zag arrangement of c o p p e r - o x y g e n chains, found in the 1-2-3 s t r u c t u r e d o e s not exist in the 1-2-4 s t r u c t u r e ~5"16'~7. In this case, one would e x p e c t the m a x i m u m to vanish. No signif i c a n t c h a n g e in the peak h e i g h t was observed. If the p r e s e n c e of zig-zag dist o r t i o n of c o p p e r - o x y g e n c h a i n s in the 1-2-4 m a t e r i a l is finally excluded, the above interpretation will have to be reconsidered. M a x i m u m 2 is c h a r a c t e r i s t i c for the tetragonal 1-2-3 specimens. It can be seen in the p u r e 1-2-3 t e t r a g o n a l sample (E) and in s a m p l e s AI, A2, A3 w h i c h cont a i n a s u b s t a n t i a l amount of t e t r a g o n a l m a t e r i a l . It was i n t e r p r e t e d by C a n n e l l i et al. 23 as b e i n g due to the s t r e s s - i n d u c e d d i f f u s i o n of r e s i d u a l o x y g e n atoms in the basal Cu-O plane. However, there is still some d o u b t about this h y p o t h e s i s s i n c e the a c t i v a t i o n energy c h a r a c t e r i s tic for this process is r a t h e r small (0.11eVil-23). We t h i n k that an "off symmetry" m e c h a n i s m is w o r t h considering. The r e l a x a t i o n p a r a m e t e r s of all the phenomena in the 1-2-4 c o m p o u n d d i f f e r from t h o s e c h a r a c t e r i s t i c for the 1-2-3 material. We b e l i e v e that t h e s e v a r i a t i o n s m i g h t be c a u s e d by the d i f f e r e n c e in the unit cell p a r a m e t e r s of these c o m p o u n d s . The change in the a c t i v a t i o n e n e r g y of m a x i m u m 4 could be also c a u s e d by the p o s s i b l e change in the c h a i n s c o n f i g u r a t i o n , p r o v i d e d that this p r o c e s s is r e l a t e d to the chain atoms relaxation. Conclusions. The i n t e r n a l f r i c t i o n m a x i m a o b s e r ved in the t e m p e r a t u r e range from 30K to 100K are of r e l a x a t i o n nature. We b e l i e v e that m a n y of them are c o n n e c t e d with s t r u c t u r e d e f e c t s of the 1-2-4 s u p e r c o n ductor. T h e y s h o u l d be d e t e c t e d by the u l t r a s o u n d a t t e n u a t i o n m e t h o d s at h i g h e r t e m p e r a t u r e s (for the f r e q u e n c y of 10MHz, the p e a k s s h o u l d a p p e a r at t e m p e r a t u r e s a b o u t IIOK, 150K and 230K). The internal friction phenomena found in the 1-2-4 phase are c l o s e l y
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MECHANICAL ENERGY DISSIPATION PHENOMENA
related to those observed in the 1-2-3 species. This is not s u r p r i s i n g in v i e w of the s i m i l a r i t y of t h e i r s t r u c t u r e s . On the basis of e x p e r i m e n t a l r e s u l t s we believe that the p r e v i o u s l y a c c e p t e d interpretation of the low t e m p e r a t u r e r e l a x a t i o n processes need to be revised. We p r o p o s e that the "off s y m m e t r y " d e f e c t m e c h a n i s m m i g h t be r e s p o n s i b l e for s o m e of the low temperature p h e n o m e n a . The anelastic e f f e c t m e a s u r e m e n t s c o n f i r m e d the thesis that the p h a s e t r a n sition into the s u p e r c o n d u c t i n g state influences neither the i n t e r n a l f r i c t i o n level nor the slope of Y o u n g ' s m o d u l u s change w i t h i n their a c c u r a c y 10 °5 and 2 x 10 .5' respectively. The m e a s u r e m e n t s h a v e not r e v e a l e d any other p h a s e t r a n s i t i o n in the t e m p e r a t u r e r a n g e from 20K to 200K, either. Further investigations w hich should give some m o r e i n f o r m a t i o n about the relaxation effects in both Y - B a - C u - O structures are in p r o g r e s s . A c k n o w l e d g e m e n t s - The a b o v e r e s e a r c h was partly s u p p o r t e d by K o m i t e t B a d a n N a u k o wych, P o l a n d (Grant no. 2 0317 91 01). We w ould like to thank dr. W. S a d o w s k i (Dep a r t e m e n t de Physique de la M a t i e r e C o n densee, U n i v e r s i t e de Geneve) for his ass istan c e in the X-ray d i f f r a c t i o n work.
I0.
ii.
12.
13.
14.
15. 16.
17. References. 18. i.
J. Karpinski, E. Kaldis, S. R u s i e c k i & B. Bucher,
2.
R.J. Cava, O.J. Krajewski, W.F. Peck Jr, B. B a t l o g g & L. R u p p Jr, Physica C 159 (1989) 372. S. Jin, H.M. Bryan, P.G. G a l l a g h e r , T.H. Tiefel, R.J. Cava, R.A. F a s t n a c h t & G.W. K a m m l o t t , P h y s i c a C 165 (1990) 415. G. Triscone, T. Graf, A. Junod, D. Sanchez, O. Bruner, D. C a t t a n i & J. Mulier, P h y s i c a C 168 (1990) 40. H. Ledbetter, Elastic Waves and Ultrasonic Nondestructive E v a l u a t i o n S.K. Datta, E l s e v i e r Sci. Publishers B.V. (North Holland) , 1990. Y. Mi, R. Schaller, H. Berger, W. B e n o i t & S. Satish, Physica C 172 (1991) 407. E. Rodriguez, J. L u z u r i a g a , M. Nunez R e g u e r i o & C. Fainstein, S o l i d State Commun. 77 (1991) 777. B. Kusz, R.J. B a r c z y n s k i , M. Gazda, L. Murawski, O. Gzowski, I. D a v o l i & S. Stizza, Solid State C o m m u n . 74 (1990) 595. D.P. Almond, M.W. Long & G.A. Saunders, J.Phys. : Condens. M a t t e r 2 (1990) 4667.
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O. Gzowski, I. Davoli, S. Stizza, B. Kusz, R.J. B a r c z y n s k i , M. G a z d a & L. M u r a w s k i , Rev. of S o l i d S t a t e Sci. 5 ( 1 9 9 1 ) 4 3 . M. Francois, A. Junod, K. Yvon, A.W. Hewat, J.J. Capponi, P. Strobel, M. M a r e z i o & P. Fisher, S o l i d State Commun. 66 (1988) 1117. G. Cannelli, R. C a n t e l l i , F. C o r d e r o , M. Ferreti & L. Verdini, Phys. Rev. B 42 (1990) 7925. G. Cannelli, R. C a n t e l l i , F. C o r d e r o & F. T r e q u a t t r i n i , IV National Conference on High Transition Temperature Superconductivity SATT 4, Parma, 11-13 F e b r u a r y 1991. H.J. Kaufmann, P.P. P a l - V a l , V.D. Natsik, J. F i c k e r t & V.V. Demirskij, VI European Conference on I n t e r n a l F r i c t i o n & Ultrasonic Attenuation in Solids, ECIFUAS-6, Cracow, Poland, 5-9 S e p t e m b e r 1991. K. Yvon, M. Francois, Z. Phys. B - C o n d e n s e d M a t t e r 76 (1989) 413. P. Marsh, R.M. Fleming, M.L. Mandich, A.M. DeSantolo, J. Kwo, M. Hong & L.J. M a r t i n e z Miranda, N a t u r e 334 (1988) 141. P. Lightfoot, Shiyou Pei, J.D. Jorgensen, Y. Y a m a d a , T. M a t s u m o t o , F. Izumi & Y. Kodama, Acta Cryst. C47 (1991). A.N. Cormack, C.R.A. C a t l o w & A.S. Nowick, J. Phys. Chem. Solids 50 (1989) 177. D.D. Sarma, P. Sen, R. cimino, C . Carbone , W . Gudat , E.V. S a m p a t h k u m a r a n & I. Das Solid S t a t e Commun. 77 (1991) 381. A.S. Nowick, The P r o c e e d i n g s of the 9th I n t e r n a t i o n a l C o n f e r e n c e on Internal Friction and Ultrasonic Attenuation ICIFUAS 9, Beijing, China 1989. A.S. Nowick, B.S. Berry, A n e l a s t i c Relaxation in C r y s t a l l i n e Solids, A c a d e m i c P r e s s (1972). G. Van Tendeloo, H.W. Zandbergen, T. O k a b e & S. A m m e n l i n c k x , Solid State Commun. 63 (1987) 969. G. Cannelli, R. C a n t e l l i , F. Cordero, M. F e r r e t t i & F. T r e q u a t t r i n i , Solid State Commun. 77 (1991) 429. S. Hoen, L.C. Bourne, C h o o n Kim & A. Zettl, Phys. Rev. B 38 (1988) 11949. He Yousheng, Xiang Jiong, Wang Hsin, Zhang Jingcang, Chang Fanggao & He Aisheng, The Proceedings of the 9th International Conference on Internal Friction and Ultrasonic Attenuation ICIFUAS 9, Beijing, C h i n a 1989.
) Solid State Communications, Vol. 83, No. 10, pp. 793-797, 1992. Printed in Great Britain.
SOME ASPECTS OF MECHANICAL SUPERCONDUCTORS.
ENERGY DISSIPATION
PHENOMENA
IN
YTTRIUM
M . G a z d a , B.Kusz, R . J . B a r c z y ~ s k i and L . M u r a w s k i D e p a r t m e n t of P h y s i c s and M a t h e m a t i c s T e c h n i c a l U n i v e r s i t y of G d a ~ s k , P o l a n d O . G z o w s k i , I . D a v o l i and S . S t i z z a D i p a r t i m e n t o di M a t e m a t i c a e F i s i c a U n i v e r s i t a di C a m e r i n o , I t a l y ( R e c e i v e d 21 J u n e
1991 by M.
Tosi)
The r e s u l t s of m e a s u r e m e n t s of i n t e r n a l f r i c t i o n a n d Y o u n g ' s modulus in yttrium 1-2-4 superconducting ceramics are reported. The internal friction maxima observed in the t e m p e r a t u r e r a n g e f r o m 30 K to i00 K are of r e l a x a t i o n n a t u r e . T h e a p p r o x i m a t e v a l u e s of a c t i v a t i o n e n e r g y a n d r e l a x a t i o n t i m e of t h e s e p h e n o m e n a are given. T h e a n e l a s t i c p r o p e r t i e s of the 1-2-4 and 1-2-3 superconductors are compared. An a l t e r n a t i v e m e c h a n i s m for one of t h e r e l a x a t i o n p r o c e s s e s is proposed.
Introduction.
p o s s i b l e c h a n g e s in the m e c h a n i c a l energy s p e c t r a w h i c h may be induced by the second Cu-O chain. These m i g h t give some clues as to the origin of r e l a x a t i o n e f f e c t s p r e s e n t in the y t t r i u m s u p e r c o n ductors.
The YBa2Cu40 ~ s u p e r c o n d u c t o r was first s y n t h e s i z e d by K a r p i n s k i et al. in 1988 I. It r e q u i r e d a p p l i c a t i o n of h i g h p r e s s u r e t e c h n i q u e to obtain a single p h a s e b u l k superconductor. Recently, several p a p e r s d e s c r i b i n g the s y n t h e s i s m e t h o d w h i c h r e q u i r e d oxygen p r e s s u r e of the a r d e r of one a t m o s p h e r e have been p u b l i s h e d 2'3. Consequently, the studies of the p r o p e r ties of this m a t e r i a l b e c a m e m u c h m o r e a c c e s s i b l e to m a n y laboratories. The crystal s t r u c t u r e of the 1-2-4 c o m p o u n d is in many r e s p e c t s s i m i l a r to that of the 1-2-3 material. The m a i n d i f f e r e n c e is a d o u b l e Cu-O c h a i n w h i c h is p r e s e n t only in the 1-2-4 phase. Each oxygen atom in these chains is b o u n d to three copper atoms. It r e s u l t s in the 1-2-4 s u p e r c o n d u c t o r h a v i n g stable o x y g e n s t o i c h i o m e t r y . The 1-2-4 unit cell exh i b i t s smaller o r t h o r h o m b i c i t y t h a n t h a t found in the 1-2-3 s t r u c t u r e (0.8% in the 1-2-4 phase and 1.8% in the 1-2-3 phase) 4' M a n y p a p e r s on the a n e l a s t i c effects in the 1-2-3 s u p e r c o n d u c t o r s have b e e n ~ u b l i s h e d in the last four y e a r s e.g. 5'6'''~. A number of r e l a x a t i o n p h e n o m e n a have been o b s e r v e d in o r t h o r h o m b i c and t e t r a g o n a l materials. Several m e c h a n i s m s w h i c h m i g h t be r e s p o n s i b l e for t h e s e p h e n o m e n a have b e e n proposed. T h e s e inc l u d e a v a c a n c y and oxygen atom jumps 7,n, the S n o e k - t y p e p r o c e s s 23 and the d i s t o r t e d Cu-O c h a i n r e l a x a t i o n 12.13.As two r e l x a t i o n p h e n o m e n a h a v e b e e n a s s o c i a t e d w i t h the r e l a x a t i o n of c h a i n atoms it s e e m e d int e r e s t i n g to us to study the p r o p e r t i e s of 1-2-4 c o m p o u n d and to o b s e r v e the
Experimental. The synthesis procedure of the YBa2Cu40 ~ s a m p l e s was based on the m e t h o d p u b l i s h e d by S . J i n et al. 3. A s t o i c h i o m e t ric 1-2-3 p o w d e r was m i x e d w i t h an app r o p r i a t e a m o u n t of CuO d i s s o l v e d in a small v o l u m e of 60% HNO 3 (i.e. the m i n i m u m v o l u m e r e q u i r e d to d i s s o l v e the CuO). T h e n the m i x t u r e was dried, g r o u n d and p r e s s e d into pellets. Three sets of 1-2-4 samples, f u r t h e r r e f e r r e d to as A, B and C, were p r e p a r e d . The s i n t e r i n g was carried out at 830°C in flowing o x y g e n at a p r e s s u r e of one atmosphere. The g r i n d i n g and s i n t e r i n g of the B and C samples was r e p e a t e d t w i c e w h i l e sample A was proc e s s e d t h r e e times. The total time of s i n t e r i n g was 70h, ll0h and 190h, respectively. A p a r t of samples A was a n n e a l e d at 660 ° C in a r g o n atmosphere. The time of a n n e a l i n g was lh (sample AI), 2h (A2) and 10h (A3). The s y n t h e s i s p r o c e d u r e of 1-2-3 s a m p l e s (orthorhombic - D and tet r a g o n a l - E) has been p u b l i s h e d elsew h e r e ~. The X - r a y tests were p e r f o r m e d with a Philips X-ray diffractometer. The m e a s u r e m e n t s of a n e l a s t i c effects w e r e c o n d u c t e d by the v i b r a t i n g reed technique. The m e a s u r e m e n t frequency was in the r a n g e of 95Hz - 660Hz. The s a m p l e s had a shape of a thin plate (30mm
793
MECHANICAL ENERGY DISSIPATION PHENOMENA
794
x 5mm x 0.4mm). The a m p l i t u d e of o s c i l l a tions was about 10#m w h i c h c o r r e s p o n d s to a the strain amplitude of 5 x 10 .6. The rate of temperature increase was 0.6 K/min. The decrement of the a m p l i t u d e d ecay was calculated by the least s q u a r e s method, fitting to a few h u n d r e d s u b s e quent amplitudes, simultaneously, the s h i e l d i n g effect was m e a s u r e d in o r d e r to d e t e r m i n e the transition t e m p e r a t u r e of the studied samples.
0 0012
Vol. 83, No. 10
2
f\
O00iO
~__%k --_
I-2-30
000o8
,
0
Results. The experimental r e s u l t s are presented in figures I, 2, 3 and t a b l e i. Several internal friction m a x i m a can be seen in all the spectra. The n u m b e r s i n d i c a t i n g the particular p e a k s are consistent with those u s u a l l y used for describing the 1-2-3 internal friction maxima s'~J2. The frequencies at w h i c h the spectra were obtained are shown in the figures. All the p h e n o m e n a o b s e r v e d in the 1-2-4 material in the t e m p e r a t u r e range studied here proved to be t h e r m a l l y a c t i v a t e d relaxation processes. The narrow frequency range available in our e x p e r i m e n t s did not allow us to d e t e r m i n e the e x a c t values of their activation e n e r g y and relaxation time. The q u a n t i ties p r e s e n t e d in table I, show the r a n g e of t he i r p o s s i b l e values. A l s o s h o w n in table i, for comparison, are the r e l a x a tion p a r a m e t e r s of the i n t e r n a l f r i c t i o n m a x i m a found in the i-2-3 m a t e r i a l ~J°. All the d i f f r a c t i o n p e a k s d e t e c t e d in the X-ray spectra of s a m p l e s A c o u l d be indexed on the basis of the k n o w n o r t h o r h o m b i c unit cell of YBa2Cu40~ 16, thus c o n f i r m i n g the s i n g l e - p h a s e n a t u r e of the
0002 ........ ~ : ~ " 2"0
.~
40
I -
~3
80 ,60 120 Ii0 160 i80 200 Temperature (K)
Figure I. Temperature dependence of internal friction in the 1-2-4, 1-2-3 orthorhombic and 1-2-3 tetragonal m a t e r i a l . The spectra were t a k e n w i t h the v i b r a t i o n f r e q u e n c y of 246Hz, 212Hz and 230Hz, r e s p e c t i v e l y .
sample. Its t r a n s i t i o n t e m p e r a t u r e was 76K. S a m p l e D was a s i n g l e - p h a s e 1-2-3 o r t h o r h o m b i c c e r a m i c s with the t r a n s i t i o n t e m p e r a t u r e of 90 K. Fig.l p r e s e n t s the r e s u l t s of internal f r i c t i o n m e a s u r e m e n t s v e r s u s t e m p e r a t u r e o b t a i n e d for the pure 1-2-4 (A), 1-2-3 o r t h o r h o m b i c (D) and 1-2-3 t e t r a g o n a l (E) ceramics. As one can see, the s p e c t r a of 1-2-4 and 1-2-3 ort h o r h o m b i c m a t e r i a l are very much s i m i l a r to each other. The s a m p l e s B and C were two-phased. T h e i r X - r a y s p e c t r a showed the p r e s e n c e
Table i. The approximate values of a c t i v a t i o n e n e r g y E A and r e l a x a t i o n time T O of the r e l a x a t i o n p r o c e s s e s in y t t r i u m c e r a m i c samples.
material
process no. 1
EA 0.07-0.1
2
.
log[ 0
3
0.Ii-0.14
4
0 • 13-0.15
E^ (eV)
< -13
.
1-2-3
1-2-30
1-2 -4
(eV)
0.07
.
-13
2-3T
log~ 0
0.2
iogT0
(ev) -13
.
0.16
EA
T
-12.5
I -13.5
-
-
0.ii
-12
-
-
|
MECHANICAL ENERGY DISSIPATION PHENOMENA
Vol. 83, No. 10 of
both
1-2-4
and
1-2-3
orthorhombic
materials. The remaining material was mainly CuO (the m i c r o r e g i o n s of c o p p e r oxide were detected by e l e c t r o n m i c r o probe). We estimated the c o n t e n t of t h e 1-2-4 phase to be a p p r o x i m a t e l y 70% in sample B and 40% in sample C. The t r a n s i tion t e m p e r a t u r e s of s a m p l e s B and C w e r e 79.5K and 81K, respectively. The h i g h e r critical temperatures and b r o a d e r t r a n s i tions (two-stepped t r a n s i t i o n in s a m p l e s C) were obtained in s a m p l e s c o n t a i n i n g s ignifi c a n t amount of the 1-2-3 and CuO phases. It should be t a k e n into c o n s i d e r ation that the s u p e r c o n d u c t i n g t r a n s i t i o n of the 1-2-3 material b r o a d e n s itself and shifts towards lower t e m p e r a t u r e s as CuO contamination increases ° . The results obtained for the t w o - p h a s e m a t e r i a l p l o t ted tog e t h e r with the 1-2-4 c u r v e (A) are shown in figure 2. The i n t e r n a l f r i c t i o n maxima seen in spectra B and C are considerably larger than t h o s e in the s p e c tra of pure 1-2-4 ceramics. Two-phase materia l inevitably c o n t a i n s m o r e d e f e c t s and is more d i s o r d e r e d t h a n the p u r e o n e . These d e f e c t p o s s i b l y e n h a n c e the i n t e r nal fr i c t i o n maxima. An a d d i t i o n a l p e a k (ib) at t h e temperature of a b o u t 50K appears in the t w o - p h a s e samples. It grows s i g n i f i c a n t l y and s h i f t s t o w a r d s lower t e m p e r a t u r e while the a m o u n t of the 1-2-3 and CuO compounds increases. In order to study the i n t e r n a l f r i c tion p r o c e s s e s in r e l a t i o n to the s t a t e of Cu-O chains the m e a s u r e m e n t s of i n t e r nal f r i c t i o n after a r g o n a n n e a l i n g w e r e carried out. The p r o c e s s of a n n e a l i n g was monitored by X-ray d i f f r a c t o g r a p h y . A gradual reduction in the 1-2-4 phase content along with g r o w i n g of b o t h 1-2-3 t e t r a g o n a l m a t e r i a l and c o p p e r o x i d e was detected. The A r - a n n e a l e d s a m p l e s AI, A2 and A3 c o n t a i n i n c r e a s i n g a m o u n t s of the
O.oo2o
5
o .oo 18
y'\l
00016
.~_ 0.0014 00012
\,
i 00010
A.., ,,,,J<-/
0,0008 0,0006'
0.00040.0002 -
/I
\
-- A
/,'
Temperoture (K)
Figure 2. Internal friction versus t e m p e r a t u r e in t w o - p h a s e s a m p l e s (B and C) p l o t t e d t o g e t h e r w i t h the s p e c t r a of pure 1-2-4 s u p e r c o n d u c t o r (sample A). The B and C s p e c t r a were taken with the vibration frequency 212Hz, 200Hz respectively.
0 oo25
795
2
.....................................................................................
ooo o 1................................. t..."i .................: •°ooo,J
...........................
.......................................................................................
ooo,: ........................ f':,_, .......................................................................... 2o
40
6o
80 i00 120 140 Temperoture (K)
160
180
20o
Figure 3. Internal friction versus t e m p e r a t u r e in o u t g a s s e d s a m p l e s (AI, A2 and A3). The s p e c t r a w e r e t a k e n w i t h the vibration frequency 330Hz, 350Hz and 510Hz r e s p e c t i v e l y .
1-2-3 tetragonal phase. The internal fr~ ~tion c u r v e s are p l o t t e d in F i g u r e 3. T h e d o m i n a n t f e a t u r e of the curves is the a p p e a r a n c e and g r a d u a l g r o w i n g of m a x i m u m 2, c h a r a c t e r i s t i c of the 1-2-3 t e t r a g o n a l m a t e r i a l . The a n a l y s i s of o b t a i n e d data in t e r m s of a s i n g l e - t i m e Debye r e l a x a t i o n 2j was p e r f o r m e d . A l t h o u g h it c a n n o t be i m m e d i a t e l y d e r i v e d from the figure, the a n a l y s i s s h o w e d t h a t the heat t r e a t m e n t did not a f f e c t p r o c e s s 3, as its parameters and the m a g n i t u d e do not change significantly. P r o c e s s e s 1 and 4 decreased their strength considerably, w h i l e peak 2 i n c r e a s e d w i t h a g r o w i n g a m o u n t of the 1-2-3 t e t r a g o n a l phase. The 1-2-4 s a m p l e s are softer than 1-2-3 (similar to Bi - b a s e d cuprates). It is i m p o s s i b l e to d e t e r m i n e the exact v a l u e of Y o u n g ' s m o d u l u s on the b a s i s of our m e a s u r e m e n t s . However, we can o b s e r v e its e v o l u t i o n w i t h t e m p e r a t u r e . The temp e r a t u r e d e p e n d e n c i e s of Young's m o d u l u s of the 1-2-4 and 1-2-3 s u p e r c o n d u c t o r s are s i m i l a r and d o e s not reveal any anomalous features T M . Discussion. Maximum 1 is v i s i b l e in both the 1-2-3 and 1-2-4 o r t h o r h o m b i c materials. As r e p o r t e d b e f o r e s, p e a k 1 d i s a p p e a r s in the 1-2-3 iron d o p e d s a m p l e s (with the iron c o n t e n t e q u a l to 1.75%). The subs t i t u t i o n of Fe for Cu r e d u c e s the conc e n t r a t i o n of m o b i l e h o l e s m, thus influe n c i n g the e l e c t r o n i c s t r u c t u r e of the s u p e r c o n d u c t o r . A l s o the removal of oxygen a t o m s from b a s a l p l a n e s results in: a decrease in m o b i l e h o l e s c o n c e n t r a t i o n and d i s a p p e a r a n c e of m a x i m u m I. Taking the a b o v e into c o n s i d e r a t i o n , it seems p o s s i b l e that an e l e c t r o n i c process may be r e s p o n s i b l e for p r o c e s s i. A small
796
MECHANICAL ENERGY DISSIPATION PHENOMENA
activation energy would be c o n s i s t e n t with this hypothesis. Maximum ib is v e r y small in the spectra of pure 1-2-4 ceramics. It is much bigger in the s p e c t r a of s a m p l e s with a considerable a m o u n t of the 1-2-3 phase and copper oxide (fig. 2). T h e r e fore we believe that this peak is not intrinsic to the 1-2-4 structure. It may be r e l a t e d to the defects a s s o c i a t e d with the pr e s e n c e of CuO ~. Maximum 3 is p r e s e n t in all the samples independently of t h e i r s u p e r c o n ducting properties. Cannelli et al. n proposed to relate this p h e n o m e n o n to stress induced jumps of the i s o l a t e d 0 atoms in the oxygen depleted regions which retain o r t h o r h o m b i c symmetry. The a nalys i s of the data o b t a i n e d for s a m p l e s anneal e d in argon showed that the state of copper - oxygen chains did not a f f e c t this process significantly. K a u f m a n n et al. 14 p e r f o r m e d systematic o u t g a s s i n g of the 1-2-3 material and found the peak i n s e n s i t i v e to oxygen content. We b e l i e v e that the insensitivity of this r e l a x a t i o n process to the state of c h a i n s might suggest that its origin s h o u l d not be looked for in the basal plane. The small activation energy of peak 3 seems to exclude an atom or a p o i n t d e f e c t m i g r a tion from one lattice p o s i t i o n to another 2°2j. However, a m o t i o n over a small fraction of the lattice d i s t a n c e m i g h t be c o n s i s t e n t with the a c t i v a t i o n e n e r g y of 0.13eV. In the last 25 years several cases of low t e m p e r a t u r e r e l a x a t i o n that are due to the "off s y m m e t r y " defects have been found ~820. A m o n g others, it was also o b s e r v e d in the p e r o v s k i t e structure. This kind of r e l a x a t i o n o c c u r s when the s y m m e t r i c c o n f i g u r a t i o n of the d e f e c t is u n s t a b l e and becomes s t a b l e only when it goes off symmetry. In this case, the stable defect s y m m e t r y is lower than that of the symmetric configuration. As a result, there will be more than one stable o r i e n t a t i o n of the d e f e c t for every s y m m e t r i c one. This allows for r e l a x a t i o n t h r o u g h atom or v a c a n c y m o v e m e n t s w h i c h are small fractions of a t o m i c d i s t a n c e s 2t~. We b e l i e v e that it may be the case in the c o p p e r oxide c e r a m i c s as well. G. Van T e n d e l o o et al. showed that in q u e n c h e d 1-2-3 crystals, part of Ba atoms substitute for Y atoms 22. A s i m i l a r d e f e c t was also r e p o r t e d in the 2-4-7 m a t e r i a l 15. This d i s o r d e r is frozen in the s a m p l e s as a r e s u l t of quenching, t h e r e f o r e it appears in quite a large quantity. It m i g h t be e x p e c t e d that this s u b s t i t u t i o n also takes place, but to s m a l l e r extent, in the y t t r i u m c o m p o u n d s c o o l e d slowly. As a r e s u l t of this imperfection, a lattice d i s t o r t i o n occurs. The b a r i u m o c c u p i e d cubes tend to u n d e r g o an a n i s o t r o p i c expansion, whereas the yttrium cubes b e c o m e flattened. Also the o x y g e n vacancies adapt their c o n f i g u r a t i o n to the arrangement of h e a v y atoms n. All this d i s o r d e r might c r e a t e an "off s y m m e t r y " c o n f i g u r a t i o n w h i c h can give rise to a
Vol. 83, No. 10
low t e m p e r a t u r e relaxation. The insensit i v i t y of m a x i m u m 3 to heat t r e a t m e n t w o u l d be c o n s i s t e n t with this hypothesis, because m i g r a t i o n of h e a v y atoms occurs in t e m p e r a t u r e h i g h e r than 660 ° C 22. Furt h e r e x p e r i m e n t s w h i c h will e i t h e r confirm or e l i m i n a t e the above are in progress. P r o c e s s 4 is v i s i b l e in the spectra of both 1-2-3 and 1-2-4 orthorhombic phases. This r e l a x a t i o n e f f e c t has been i n t e r p r e t e d as being due to the hops of o x y g e n a t o m s b e t w e e n two m i n i m a of energy in d i s t o r t e d Cu-O-Cu c h a i n s 12"13"I°. Such m i n i m a e x i s t in the o r t h o r h o m b i c struct u r e of the 1-2-3 m a t e r i a l II'15. The association of this p r o c e s s with the chain o x y g e n a t o m s is s u p p o r t e d by results of the a n n e a l i n g experiments. The m a x i m u m was s t r o n g l y a f f e c t e d by heat treatment. However, the n e u t r o n d i f f r a c t i o n studies show that the zig-zag arrangement of c o p p e r - o x y g e n chains, found in the 1-2-3 s t r u c t u r e d o e s not exist in the 1-2-4 s t r u c t u r e ~5"16'~7. In this case, one would e x p e c t the m a x i m u m to vanish. No signif i c a n t c h a n g e in the peak h e i g h t was observed. If the p r e s e n c e of zig-zag dist o r t i o n of c o p p e r - o x y g e n c h a i n s in the 1-2-4 m a t e r i a l is finally excluded, the above interpretation will have to be reconsidered. M a x i m u m 2 is c h a r a c t e r i s t i c for the tetragonal 1-2-3 specimens. It can be seen in the p u r e 1-2-3 t e t r a g o n a l sample (E) and in s a m p l e s AI, A2, A3 w h i c h cont a i n a s u b s t a n t i a l amount of t e t r a g o n a l m a t e r i a l . It was i n t e r p r e t e d by C a n n e l l i et al. 23 as b e i n g due to the s t r e s s - i n d u c e d d i f f u s i o n of r e s i d u a l o x y g e n atoms in the basal Cu-O plane. However, there is still some d o u b t about this h y p o t h e s i s s i n c e the a c t i v a t i o n energy c h a r a c t e r i s tic for this process is r a t h e r small (0.11eVil-23). We t h i n k that an "off symmetry" m e c h a n i s m is w o r t h considering. The r e l a x a t i o n p a r a m e t e r s of all the phenomena in the 1-2-4 c o m p o u n d d i f f e r from t h o s e c h a r a c t e r i s t i c for the 1-2-3 material. We b e l i e v e that t h e s e v a r i a t i o n s m i g h t be c a u s e d by the d i f f e r e n c e in the unit cell p a r a m e t e r s of these c o m p o u n d s . The change in the a c t i v a t i o n e n e r g y of m a x i m u m 4 could be also c a u s e d by the p o s s i b l e change in the c h a i n s c o n f i g u r a t i o n , p r o v i d e d that this p r o c e s s is r e l a t e d to the chain atoms relaxation. Conclusions. The i n t e r n a l f r i c t i o n m a x i m a o b s e r ved in the t e m p e r a t u r e range from 30K to 100K are of r e l a x a t i o n nature. We b e l i e v e that m a n y of them are c o n n e c t e d with s t r u c t u r e d e f e c t s of the 1-2-4 s u p e r c o n ductor. T h e y s h o u l d be d e t e c t e d by the u l t r a s o u n d a t t e n u a t i o n m e t h o d s at h i g h e r t e m p e r a t u r e s (for the f r e q u e n c y of 10MHz, the p e a k s s h o u l d a p p e a r at t e m p e r a t u r e s a b o u t IIOK, 150K and 230K). The internal friction phenomena found in the 1-2-4 phase are c l o s e l y
Vol. 83, No. 10
MECHANICAL ENERGY DISSIPATION PHENOMENA
related to those observed in the 1-2-3 species. This is not s u r p r i s i n g in v i e w of the s i m i l a r i t y of t h e i r s t r u c t u r e s . On the basis of e x p e r i m e n t a l r e s u l t s we believe that the p r e v i o u s l y a c c e p t e d interpretation of the low t e m p e r a t u r e r e l a x a t i o n processes need to be revised. We p r o p o s e that the "off s y m m e t r y " d e f e c t m e c h a n i s m m i g h t be r e s p o n s i b l e for s o m e of the low temperature p h e n o m e n a . The anelastic e f f e c t m e a s u r e m e n t s c o n f i r m e d the thesis that the p h a s e t r a n sition into the s u p e r c o n d u c t i n g state influences neither the i n t e r n a l f r i c t i o n level nor the slope of Y o u n g ' s m o d u l u s change w i t h i n their a c c u r a c y 10 °5 and 2 x 10 .5' respectively. The m e a s u r e m e n t s h a v e not r e v e a l e d any other p h a s e t r a n s i t i o n in the t e m p e r a t u r e r a n g e from 20K to 200K, either. Further investigations w hich should give some m o r e i n f o r m a t i o n about the relaxation effects in both Y - B a - C u - O structures are in p r o g r e s s . A c k n o w l e d g e m e n t s - The a b o v e r e s e a r c h was partly s u p p o r t e d by K o m i t e t B a d a n N a u k o wych, P o l a n d (Grant no. 2 0317 91 01). We w ould like to thank dr. W. S a d o w s k i (Dep a r t e m e n t de Physique de la M a t i e r e C o n densee, U n i v e r s i t e de Geneve) for his ass istan c e in the X-ray d i f f r a c t i o n work.
I0.
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14.
15. 16.
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