- Email: [email protected]
Fluorination of high Tc superconductors YBa2Cu3Ox : Influence on the superconducting properties
Physica C 153-155 (1988) 934 935 North-Holland, Amsterdam
FLUORINATION OF HIGH Tc SUPERCONDUCTORS YBa2Cu30x : INFLUENCE ON THE SUPERCONDUCTING PROPERTIES
Christiane PERRIN*, Octavio PENA*, Marcel SERGENT*, Peter Sonne CHRISTENSEN**, Gilles FONTENEAU**, Jacques LUCAS** Universit6 de R e n n e s - l , *Laboratoire de Chimie Min6rale B, **Laboratoire de Chimie Min~rale D, Unit~ Associ6e au C.N.R.S. n°254, Avenue du G~n6ral Leclerc, 35042 Rennes C~dex (France) Sintered samples of high Tc superconductors YBa2Cu30x, w i t h d i f f e r e n t values of x (6 < x < 7) are f l u o r i n a t e d at 300 ° C by a s o l i d / g a s chemical r e a c t i o n , using NF3 (10 % d i l u t e d in N2). Influence of f l u o r i n e i n s e r t i o n on superconducting p r o p e r t i e s is studied by mutual indqctance techniques or magnetization (SQUID) measurements. A f t e r f l u o r i n a t i o n , superconducting p r o p e r t i e s o f "good" samples (x = 7 and Tc = 91 K) are degraded, while non superconducting samples show evidence of a diamagnetic t r a n s i t i o n .
1. INTRODUCTION The discovery of the high Tc superconductors in the Y-Ba-Cu-O system (1) i n i t i a t e d an i n t e n se research f o r higher Tc in r e l a t e d systems. Recent reports (2, 3) o f f l u o r i n e a d d i t i o n , via BaF2 or ion i m p l a n t a t i o n , in the 91 K superconductor YBa2Cu30~ i n d i c a t e a zeroresistance state at about 150 K, but no c l e a r evidence of f l u o r i n e i n s e r t i o n was given. Anot h e r technique used r e c e n t l y is a chemical r e a c t i o n using F2 gas flow at low temperature (4, 5). We describe here the r e s u l t s we have obtained under m i l d e r c o n d i t i o n s using NF3 gas ( i 0 % d i l u t e d in N2). 2. PREPARATION OF FLUORINATED COMPOUNDS YBa2Cu3OxFy 2.1. Vi~ BaF2 In order to t e s t the possible i n s e r t i o n of f l u o r i n e from BaF2, we have f o l l o w e d the same synthesis procedure described in r e f . 2. Some i m p u r i t i e s of BaF2 appear in the X-ray diagram and no evidence of s u p e r c o n d u c t i v i t y at 155 K was observed. However, f o r the sample YBa2Cu3OxFy (y = O) sintered t o g e t h e r with the sample's of the other compositions (y = i , 2, 3, 4) in the same Pt boat, an i n t e r e s t i n g r e s u l t is observed in the superconducting behav i o u r ( f i g u r e s 1 and 2 : X curves). The a.c. s u s c e p t i b i l i t y e x h i b i t s a very sharp t r a n s i t i o n (onset 91 K, about 0.2 K wide), w i t h o u t the small "step" on the upper p a r t ( f i g u r e 2 : A curve), always observed f o r a l l samples of YBa2Cu30x which we have measured and discussed in relation to granular effects in r e f . 6 and 7. 2.2. Via s o l i d / g a s r e a c t i o n S t a r t i n g p e l l e t s (150 mg, 0 = 3,5 mm, 1 = 4 mm) of orthorhombic YBa2Cu30~, w i t h x ~ 7 (sample A) and x = 6.7 (sample B) were synthesized as described elsewhere (8). Other p e l l e t s s i n t e r e d at 970 ° C were heated at 700 ° C under 0921-4534/88/$03.50 ©ElsevierSciencePublishers B.V. (North-Holland PhysicsPublishing Division)
dynamic vaccuum f o r several hours in order to obtain the t e t r a g o n a l phase with x ~ 6,5 (sample C), as reported (9). The three samples A, B and C were heated at d i f f e r e n t temperatures under NF3 (10 % d i l u t e d in N2) f l o w during one hour : above 340 ° C, a decomposition of the samples r e s u l t e d in the formation of various f l u o r i d e s . Another s e r i e s of experiments were performed by heating the three samples at 300 ° C during d i f f e r e n t times. I t r e s u l t e d in a progressive increase of the weight depending on the x value of the s t a r t i n g sample. The weight is s t a b i l i z e d in about 8 hours. Evidence of f l u o r i n e i n s e r t i o n in the s t a r t i n g m a t e r i a l s is given by a chemical t e s t using a l i z a r i n e , by the increase of the weight and by the m o d i f i c a t i o n s in the X-ray diagrams. A f t e r 8 hours, the increase of weight corresponds to the i n s e r t i o n of 0.4 f l u o r i n e atoms/ mole of YBa2Cu30x f o r A sample, 0.7 F and 1.2 F f o r B and C samples r e s p e c t i v e l y i f no loss of oxygen is considered. In f a c t , i t was shown t h a t the oxygen content decreases during the f l u o r i n a t i o n under F2 f l o w (4). A f t e r f l u o r i n a t i o n , A, B, C samples become I , 2, 3 samples respectively. After fluorination, the powder patterns of the three samples give no evidence of any impurities. The m o d i f i c a t i o n of the X-ray diagram is more important f o r the t e t r a g o n a l phase (C sample) where some d i f f r a c t i o n peaks become broad and some s p l i t t i n g less resolved. For A and B samples, a very s l i g h t m o d i f i c a t i o n in the i n t e n s i t y of some d i f f r a c t i o n peaks was observed. For comparison, i d e n t i c a l starting samples were annealed at 300 ° during one hour under N2 f l o w : no m o d i f i c a t i o n was observed n e i t h e r in the X-ray diagrams nor in the superconducting behaviour. Superconducting properties were studied using a standard mutual inductance bridge and
C. P errin et aL / Fluorination of high-T~ superconductors YBa 2Cu aO~
a SQUID magnetometer below 120 K. The d.c. s i gnals at zero f i e l d were normalized at low temperature by measuring the in-phase x ' a • c at 4.2 K. Figure La shows the z e r o - f i e l d magnetization of samples A, B and C before f l u o r i n a t i o n . A-sample (x = 7.0) superconducts at 91 K, while B-sample (x = 6.7) shows an onset of 70 K I&K = 25 K). C-sample presents no si-gnal above 2 K (x ~ 6.5). A ft e r f l u o r i n a t i o n (figure l b ) , superconductivity of sample A is greatly degraded (curve 1), although i t s onset remains unchanged. B-sample however, transforms into curve 2, with onset temperature at 91 K, but its amplitude is reduced to = 30 %. C-sam-ple d o e s not show any v a r i a t i o n at zero field, after f l u o r i n a t i o n , although a 50-m-field magnetization reveals a small diamagnetic deviation below 10 K. The a.c. s u s c e p t i b i l i t y (measured as described in r e f . 8 ) is shown in figure 2. Inductive t r a n s i t i o n s are much narrower than the magnet i z a t i o n of figure i (see curves A) probably due to the fact that the " z e r o - f i e l d " condition is not f u l l y respected (remnant f i e l d of the magnetometer's superconducting c o i l ) , thus enlarging the superconducting t r a n s i t i o n .
I
[
I
J
'
• °(~'-..z_t.
+
'
'
'
,
Zero fietd °° ,.++.---.,-.~.-°
(a) °
÷ .d • ~+~++÷o+++*,*
°
• +
>i--
,
._1
:
I
J
I
J
I
I
I-LLI
:..
0
<5 c~
•
• (~
..°°'""
" . .~_:.
(b)
.® ,
0
,
........... J L
20
,
I
I
I
L
I
40 60 80 TEMPERATURE(K)
I
100
FIGURE i Meissner e f f e c t f o r samples : (a) before and (b) a f t e r f l u o r i n a t i o n .
935
A
m
u~
G
BO
B2 '
'
8+
. . B.6 . . BB
TEMPERATURE
9o--
92
(K)
FIGURE 2 Inductive t r a n s i t i o n s (above l i q u i d nitrogen) f o r f l u o r i n a t e d ( i , 21 3) and non- f luor inat ed (A, X) samples.
Further annealings of three samples I, 2, 3 were performed under 02 or N2 flow at various temperatures, followed by a slow cooling. Above bOO° C X-ray diagram shows the presence of BaF2 correlated to the f l u o r i n e concentration of each sample. Under oxygen, superconductivity is restored but the amplitude is s l i g h t l y smaller due to the o f f - s t o i c h i o m e t r y r e s u l t i n g from the BaF2 formation. Under nitrogen flow, above 700 ° C, oxygen content seems to decrease, leading to wide inductive t r a n s i t i o n , with a small amplitude. All these results will be thoroughly discussed elsewhere (10). REFERENCES ( i ) J.G. Bednorz and K.A. M~ller, Z. Phys. B64 (1986) 189 (2) S.R. Ovshinsky et a l . , Phys. Rev. Lett. 58 (1987) 2579 (3) M. Xian-Ren et a l . , Solid State Comm. 64 (1987) 325 (4) P.K. Davies e t a ] . , Solid State Comm. 64 (1987) 1441 (5) H.H. Wang et a l . , Inorg. Chem. 27 (1988) 5 (6) A. Raboutou et a ] . , Europhys. Lett. 4 (1987) 1321 (7) P. Peyral, t h i s volume (8) A. Perrin et a l . , J. Phys. 49 (1988) 301 (9) P. Monod et a l . , J. Phys. 48 (1987) 1369 (10) C. Perrin et a l . , to be published.
FLUORINATION OF HIGH Tc SUPERCONDUCTORS YBa2Cu30x : INFLUENCE ON THE SUPERCONDUCTING PROPERTIES
Christiane PERRIN*, Octavio PENA*, Marcel SERGENT*, Peter Sonne CHRISTENSEN**, Gilles FONTENEAU**, Jacques LUCAS** Universit6 de R e n n e s - l , *Laboratoire de Chimie Min6rale B, **Laboratoire de Chimie Min~rale D, Unit~ Associ6e au C.N.R.S. n°254, Avenue du G~n6ral Leclerc, 35042 Rennes C~dex (France) Sintered samples of high Tc superconductors YBa2Cu30x, w i t h d i f f e r e n t values of x (6 < x < 7) are f l u o r i n a t e d at 300 ° C by a s o l i d / g a s chemical r e a c t i o n , using NF3 (10 % d i l u t e d in N2). Influence of f l u o r i n e i n s e r t i o n on superconducting p r o p e r t i e s is studied by mutual indqctance techniques or magnetization (SQUID) measurements. A f t e r f l u o r i n a t i o n , superconducting p r o p e r t i e s o f "good" samples (x = 7 and Tc = 91 K) are degraded, while non superconducting samples show evidence of a diamagnetic t r a n s i t i o n .
1. INTRODUCTION The discovery of the high Tc superconductors in the Y-Ba-Cu-O system (1) i n i t i a t e d an i n t e n se research f o r higher Tc in r e l a t e d systems. Recent reports (2, 3) o f f l u o r i n e a d d i t i o n , via BaF2 or ion i m p l a n t a t i o n , in the 91 K superconductor YBa2Cu30~ i n d i c a t e a zeroresistance state at about 150 K, but no c l e a r evidence of f l u o r i n e i n s e r t i o n was given. Anot h e r technique used r e c e n t l y is a chemical r e a c t i o n using F2 gas flow at low temperature (4, 5). We describe here the r e s u l t s we have obtained under m i l d e r c o n d i t i o n s using NF3 gas ( i 0 % d i l u t e d in N2). 2. PREPARATION OF FLUORINATED COMPOUNDS YBa2Cu3OxFy 2.1. Vi~ BaF2 In order to t e s t the possible i n s e r t i o n of f l u o r i n e from BaF2, we have f o l l o w e d the same synthesis procedure described in r e f . 2. Some i m p u r i t i e s of BaF2 appear in the X-ray diagram and no evidence of s u p e r c o n d u c t i v i t y at 155 K was observed. However, f o r the sample YBa2Cu3OxFy (y = O) sintered t o g e t h e r with the sample's of the other compositions (y = i , 2, 3, 4) in the same Pt boat, an i n t e r e s t i n g r e s u l t is observed in the superconducting behav i o u r ( f i g u r e s 1 and 2 : X curves). The a.c. s u s c e p t i b i l i t y e x h i b i t s a very sharp t r a n s i t i o n (onset 91 K, about 0.2 K wide), w i t h o u t the small "step" on the upper p a r t ( f i g u r e 2 : A curve), always observed f o r a l l samples of YBa2Cu30x which we have measured and discussed in relation to granular effects in r e f . 6 and 7. 2.2. Via s o l i d / g a s r e a c t i o n S t a r t i n g p e l l e t s (150 mg, 0 = 3,5 mm, 1 = 4 mm) of orthorhombic YBa2Cu30~, w i t h x ~ 7 (sample A) and x = 6.7 (sample B) were synthesized as described elsewhere (8). Other p e l l e t s s i n t e r e d at 970 ° C were heated at 700 ° C under 0921-4534/88/$03.50 ©ElsevierSciencePublishers B.V. (North-Holland PhysicsPublishing Division)
dynamic vaccuum f o r several hours in order to obtain the t e t r a g o n a l phase with x ~ 6,5 (sample C), as reported (9). The three samples A, B and C were heated at d i f f e r e n t temperatures under NF3 (10 % d i l u t e d in N2) f l o w during one hour : above 340 ° C, a decomposition of the samples r e s u l t e d in the formation of various f l u o r i d e s . Another s e r i e s of experiments were performed by heating the three samples at 300 ° C during d i f f e r e n t times. I t r e s u l t e d in a progressive increase of the weight depending on the x value of the s t a r t i n g sample. The weight is s t a b i l i z e d in about 8 hours. Evidence of f l u o r i n e i n s e r t i o n in the s t a r t i n g m a t e r i a l s is given by a chemical t e s t using a l i z a r i n e , by the increase of the weight and by the m o d i f i c a t i o n s in the X-ray diagrams. A f t e r 8 hours, the increase of weight corresponds to the i n s e r t i o n of 0.4 f l u o r i n e atoms/ mole of YBa2Cu30x f o r A sample, 0.7 F and 1.2 F f o r B and C samples r e s p e c t i v e l y i f no loss of oxygen is considered. In f a c t , i t was shown t h a t the oxygen content decreases during the f l u o r i n a t i o n under F2 f l o w (4). A f t e r f l u o r i n a t i o n , A, B, C samples become I , 2, 3 samples respectively. After fluorination, the powder patterns of the three samples give no evidence of any impurities. The m o d i f i c a t i o n of the X-ray diagram is more important f o r the t e t r a g o n a l phase (C sample) where some d i f f r a c t i o n peaks become broad and some s p l i t t i n g less resolved. For A and B samples, a very s l i g h t m o d i f i c a t i o n in the i n t e n s i t y of some d i f f r a c t i o n peaks was observed. For comparison, i d e n t i c a l starting samples were annealed at 300 ° during one hour under N2 f l o w : no m o d i f i c a t i o n was observed n e i t h e r in the X-ray diagrams nor in the superconducting behaviour. Superconducting properties were studied using a standard mutual inductance bridge and
C. P errin et aL / Fluorination of high-T~ superconductors YBa 2Cu aO~
a SQUID magnetometer below 120 K. The d.c. s i gnals at zero f i e l d were normalized at low temperature by measuring the in-phase x ' a • c at 4.2 K. Figure La shows the z e r o - f i e l d magnetization of samples A, B and C before f l u o r i n a t i o n . A-sample (x = 7.0) superconducts at 91 K, while B-sample (x = 6.7) shows an onset of 70 K I&K = 25 K). C-sample presents no si-gnal above 2 K (x ~ 6.5). A ft e r f l u o r i n a t i o n (figure l b ) , superconductivity of sample A is greatly degraded (curve 1), although i t s onset remains unchanged. B-sample however, transforms into curve 2, with onset temperature at 91 K, but its amplitude is reduced to = 30 %. C-sam-ple d o e s not show any v a r i a t i o n at zero field, after f l u o r i n a t i o n , although a 50-m-field magnetization reveals a small diamagnetic deviation below 10 K. The a.c. s u s c e p t i b i l i t y (measured as described in r e f . 8 ) is shown in figure 2. Inductive t r a n s i t i o n s are much narrower than the magnet i z a t i o n of figure i (see curves A) probably due to the fact that the " z e r o - f i e l d " condition is not f u l l y respected (remnant f i e l d of the magnetometer's superconducting c o i l ) , thus enlarging the superconducting t r a n s i t i o n .
I
[
I
J
'
• °(~'-..z_t.
+
'
'
'
,
Zero fietd °° ,.++.---.,-.~.-°
(a) °
÷ .d • ~+~++÷o+++*,*
°
• +
>i--
,
._1
:
I
J
I
J
I
I
I-LLI
:..
0
<5 c~
•
• (~
..°°'""
" . .~_:.
(b)
.® ,
0
,
........... J L
20
,
I
I
I
L
I
40 60 80 TEMPERATURE(K)
I
100
FIGURE i Meissner e f f e c t f o r samples : (a) before and (b) a f t e r f l u o r i n a t i o n .
935
A
m
u~
G
BO
B2 '
'
8+
. . B.6 . . BB
TEMPERATURE
9o--
92
(K)
FIGURE 2 Inductive t r a n s i t i o n s (above l i q u i d nitrogen) f o r f l u o r i n a t e d ( i , 21 3) and non- f luor inat ed (A, X) samples.
Further annealings of three samples I, 2, 3 were performed under 02 or N2 flow at various temperatures, followed by a slow cooling. Above bOO° C X-ray diagram shows the presence of BaF2 correlated to the f l u o r i n e concentration of each sample. Under oxygen, superconductivity is restored but the amplitude is s l i g h t l y smaller due to the o f f - s t o i c h i o m e t r y r e s u l t i n g from the BaF2 formation. Under nitrogen flow, above 700 ° C, oxygen content seems to decrease, leading to wide inductive t r a n s i t i o n , with a small amplitude. All these results will be thoroughly discussed elsewhere (10). REFERENCES ( i ) J.G. Bednorz and K.A. M~ller, Z. Phys. B64 (1986) 189 (2) S.R. Ovshinsky et a l . , Phys. Rev. Lett. 58 (1987) 2579 (3) M. Xian-Ren et a l . , Solid State Comm. 64 (1987) 325 (4) P.K. Davies e t a ] . , Solid State Comm. 64 (1987) 1441 (5) H.H. Wang et a l . , Inorg. Chem. 27 (1988) 5 (6) A. Raboutou et a ] . , Europhys. Lett. 4 (1987) 1321 (7) P. Peyral, t h i s volume (8) A. Perrin et a l . , J. Phys. 49 (1988) 301 (9) P. Monod et a l . , J. Phys. 48 (1987) 1369 (10) C. Perrin et a l . , to be published.