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Effect of alpha irradiation on polycrystalline Bi-2223 superconductors
PHYSICA ELqFNIER
Physica C 228 (1994) 109-114
Effect of alpha irradiation on polycrystalline Bi-2223 superconductors S.K. Bandyopadhyay a,., p. Barat a, Pintu Sen a, Udayan De a, A. De b, P.K. Mukhopadhyay c, S.K. Kar c, C.K. Majumdar c " Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700 064, India b Saha Institute of Nuclear Physics, I/AF Bidhan Nagar, Calcutta 700 064, India c S.N. Bose National Centre for Basic Sciences, DB-I 7, Sector-l, Salt Lake, Calcutta 700 064, India
Received 17 February 1994; revised manuscript received 26 May 1994
Abstract
Polycrystalline samples of Bil.84Pbo.34Srt.91Ca2.o3Cu3.o601o+x ((Bi, Pb)-2223) with Tc(R=0) = 112 K have been irradiated by a particles of 40 MeV energy up to a maximum dose of 1.5 × 1016 ot/cm 2. Tc is observed to decrease with increasing dose. At the highest dose, semiconducting behavior is observed in resistivity measurements, and X-ray diffraction and scanning electron microscopy indicate partial amorphization. Tc has not been recovered by annealing. The possible mechanism of radiation damage in (Bi, Pb)-2223 is discussed; it appears to be different from the mechanism in Bi2Sr2CaCu208 +x (Bi-2212).
1. Introduction The transition temperature of the copper oxide based superconductors is dependent on the oxygen content which controls the carrier concentration [ 1 - 3 ] . The oxygen content changes with different physical or environmental conditions, like temperature and oxygen partial pressure employed during annealing, external pressure [4,5] and irradiation with charged particles [ 6 - 8 ] . Earlier [9] we observed an increase in Tc due to ct irradiation on polycrystalline samples of Bi2Sr2CaCu208 + ~ ( henceforth referred to as Bi-2212) with T~ = 65 K. In Bi-2212, Tc passes through a maximum as a function of oxygen concentration [ 10], and the sample with T~ = 65 K has excess oxygen compared with a sample with the highest T~ in this system. The increase of Tc was attributed to oxygen knock-out or * Corresponding author. 0921-4534 / 94 / $07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10921-4534 (94)00309-4
oxygen disordering. Tc returned to the original value of the unirradiated sample when the irradiated sample was annealed in flowing oxygen [9]. Charged particle irradiation causes oxygen disordering in the YBa2Cu307 _ system also, which has labile oxygen in the C u - O chain. The irradiation-induced preferential disordering of oxygen in the C u - O chain of the basal plane causes a structural transition from the orthorhombic to the tetragonal phase [ 11-13]. In the present work we investigated effects of irradiation by a particles on high-To polycrystalline Bil.snPbo.34Srl.91Ca2.o3Cu3.o601O+x (henceforth referred to as (Bi, Pb)-2223) having Tc = 112 K. Ruault and Gasgnier [ 14] have earlier reported that under 50 KeV helium irradiation, the B i - S r - C a - C u - O ceramic undergoes a transition for the orthorhombic to the perovskite phase (interpreted as a partial breakdown of the layered structure) and amorphization. We have
110
S.K. Bandyopadhyay et al. / Phvsica (" 228 (1994) 109 114
seen amorphization, but have no definite proof of a new phase.
2. Experimental details Polycrystalline samples of Pb-doped Bi-2223 were prepared starting from nitrates of Bi, Sr, Ca, Cu and Pb leading to a nominal composition of B i h g 4P b o . 3 4 S r l . 9 1 C a 2 . o 3 C u 3 o 6 O l o _ ~ x [15]. An aqueous solution of these nitrates was prepared and evaporated to dryness. The mixed nitrates were heated at 500°C for complete decomposition of the nitrates. The mass was then calcined at 800°C for 24 h, ground to form fine powder, pelletized, sintered twice at 855°C for 100 and 48 h and finally annealed at 855°C for 48 h and cooled slowly at a rate of l°C/min. X-ray diffraction (XRD) of the samples thus prepared were recorded in a Philips PW 1710 diffractometer and revealed all the reflections corresponding to the high-T~ phase. The resistivity of rectangular-bar-shaped samples, mounted on a Leybold 10-300 cryogenerator, was measured by the four-probe method using a Keithley 182 nanovoltmeter and a typical sample current of 1 mA. T~ was found to be 112 K; the sample to sample variation was within 0.5 K. Irradiation was done with a beam of 40 MeV o~ particles obtained from the Variable Energy Cyclotron in Calcutta. The beam current was 100 hA. Electrical insulation of the target holder from the beam tube was ensured by a perspex flange and teflon bush in the screws connecting the target holder and the beam line. The samples attached to the aluminium target holder were cooled by flowing compressed air (heat generation ~ 1 W). The polycrystalline samples were thick enough ( ~ 0.3 mm) to stop the beam. Hence they were irradiated from both sides to have radiation damage throughout the volume. To study the irradiation effect as a function of the dose, samples with different doses were prepared. The original circular pellet of 13 mm diameter was divided into several rectangular segments. In the beginning, all the segments were irradiated simultaneously. After a certain desired dose, one segment was reversed so that its other surface was exposed to the beam. When this other side got the same dose, this segment was removed. Meanwhile, the other segments acquired a cumulative dose on one side. The process was repeated until another segment got some
desired higher dose on both sides and was removed; and so on. The number of ~ particles deposited was obtained by measuring the charge collected on the target holder through a current digitizer and a scaler. The doses received by the samples were 1 X 10 ~5, 2 × 10 ~5 and 1.5 × 10 ~6 odcm 2. Some samples irradiated with doses I × 10 ~s and 2 × 1015 o d c m : were annealed in air at 780°C, followed by cooling at 50°C/h, to observe if any recovery of transition temperature occurred. Irradiated and annealed samples were characterised by XRD and their T~.s were determined through resistivity measurements. Scanning electron micrograph (SEM) studies on the surfaces of the samples were performed by a Hitachi $2300 scanning electron microscope. The oxygen contents of the irradiated and the unirradiated samples were determined by thermogravimetric analysis (TGA) in a mixture of 90% argon and 10% hydrogen, and did not differ appreciably.
3. Results and discussion
T~(R= 0)s of all the samples have decreased after irradiation with c~-particles. Table 1 gives T~ and resistivities (at 300 K) of the samples -unirradiated, irradiated and annealed after irradiation. Resistivity vs. temperature plots of the unirradiated and irradiated samples are shown in Figs. 1 and 2. The resistivity increases with the dose, and the sample at the highest dose ( 1.5 K 10 ~' c,/cm") shows both high resistivity and non-metallic behaviour with negative dp/dT m a certain temperature range (Fig. 2). Alekshashin et al. [ 15 ] have observed the vanishing of superconductivity and non-metallic behaviour (i.e. dp/dT
71:
p f r a i l cm)
Unirradiated Dose: t × 10 ~5 a / c m 2, annealed Dose: 2 × 10 ~5 et/cm ~, annealed
112 I 11 75 108 74
3. I 7.3 10.9 8.1 II .5
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
inant effect in (Bi, Pb)-2223, as the TGA indicates no appreciable change in the oxygen content. The differences in behaviour can be explained by using an idea discussed by Zhang and Sato [20]. The structural stability in these cuprates is governed by a tolerance factor (as in cubic perovskites) which is defined as
A
El0. ¢D I
2X1 01 5 c ~ / c m A~"
C-
aA-
~AAAAAA~
E
A
~>, 4
o
,~°°
AA . . . . . t:¢t~tl~O Ot~ OO
1x1015~/cm
o
•>
L
O' ' ' m ° a
_._..,..,,~ ~
2
o
111
~
t = ( A - O ) / [21/Z(B--O) ] . "~09 rY
y 0
~ 60
90
_, 120
unirrodioted '
~
i
i
i
150
180
210
240
270
Temperature (K)
300
Fig. 1. Resistivity vs. temperature of unirradiated and irradiated samples at doses of I × 10 ~5 and 2 × 1015 a / c m 2.
In YBa2Cu307_8 films irradiated with oxygen ions, Clark et al. [ 16] have observed a transition from metallic to non-metallic behaviour at a dose of 1.1 × 10 TM oxygen/cm 2 and extensive amorphization. Both these groups [ 16-18] have discussed in detail the reasons for this non-metallic behaviour. Since in our case, the alpha particles are completely stopped in the sample, the energy deposition is high, and this may cause partial amorphization. In the irradiated sample (Fig. 3(b), dose 2 X 1015 a/cm2), the intensity of the characteristic reflections have considerably decreased; note the change of scale in the Y-axis from Fig. 3 (a). But no extra peaks due to another phase can be definitely identified. In the sample annealed after irradiation (Fig. 3 ( c ) ) , the intensity of the characteristic lines have further decreased, but the lines are still identifiable above the background; there is no signature of an appreciable development of other phases. SEM pictures (Fig. 4) also support partial amorphization. Grains of size 2-3 microns observed in the unirradiated samples are no longer present in the irradiated samples at a dose of 2 X 1015 o t / c m 2. No columnar defects are observed. The work on et irradiation in Bi-2212 has been mentioned earlier [9]. We have also observed a decrease in Tc after a-irradiation of single crystals of Bi-2212 grown by the flux method [ 19]. Tc(R= 0 ) which was 80 K before irradiation, has come down to 77 K after irradiation with a dose of 2 × 1014 o t / c m 2 and to 74 K with a dose of 2 × 1015 ot/cm 2. This can be explained by oxygen knock-out from Bi-2212 [ 10]. On the other hand, oxygen knock-out does not appear to be the dom-
(1)
A - O and B - O are bond lengths of Bi-O in a rock-salt block and Cu-O in a perovskite block respectively. If the bond lengths are taken to be the sum of the ionic radii of the respective ions, i.e. r(B:+) =0.093 nm, r(o2-) =0.14 nm, qcu2+) =0.072 nm, t comes out to be 0.78 in Bi-2212, and is less than the value needed for structural stability (0.8 < t < 0.9). Since the Cu-O bond is rigid, the structural stability is attained by accommodating excess oxygen in the Bi-O layer, whereby the Bi-O bond distance increases to 0.26 nm and the tolerance factor comes within the proper range. This excess oxygen is loosely bound and hence vulnerable to knock-out by energetic et particles; this is why in Bi-2212, oxygen is forced out easily. On the other hand, in (Bi, Pb)-2223, the mismatch between the rock-salt Bi-O layer and the perovskite Cu-O layer is balanced by the substitution of Bi 3 + (r(m3 + ) = 0.093 nm) by pbE+(rpb2+)= --0.12 nm) with larger ionic radius. So, (Bi, Pb)-2223 cannot accommodate much excess oxygen and there is not much loosely bound oxygen to be knocked out. Rather, higher Z elements like Bi and Pb can be easily displaced, as their alphascattering cross-sections are larger than the alpha-scattering cross-section of oxygen. The displacements of E
200 1.5x1016
0
I 03
160
G/cm 2
o
E 120
E
o '~
80-
o
o
o
oo
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o
> 03 O3 ©
rY
40"
0 40
o o°
i 80
I
I
I
I
I
120
160
200
240
280
Temperature
320
(K)
Fig. 2. Resistivity vs. temperature of irradiated sample at a dose of 1.5 X 10t6 u / c m 2.
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
112 1200
(2_I ~-I
(a)
I~ I°° gEl!
I000
800
5
1
600 Z LLJ Z 400
z
i
200I 0
4=0
I 60
5nO
20 ( DEGREES )
250I
(b)
200I 150
Z t~ Z
50
0
i
!
I0
20
i/
,
20 (
DEGREES
4O
50
60
")
Fig. 3. X-Ray diffraction patterns: (a) Unirradiated sample. (b) Irradiated sample of dose 2 x 10 t5 a / c m 2. (c) Irradiated and annealed sample of dose 2 × 10~s ct/cm 2.
these high-Z elements and other inelastic processes which stop alpha particles cause local amorphization. There is little change in the Tc °,set in the samples at doses 1 X 1015 and 2 × 1015 c~/cm 2, because a low dose of cx particles destroys weak-linking between grains, leaving the intragrain superconductivity intact [21]. The destruction of weak-linking has broadened the transition width. At the highest dose (1.5 × 1016 O~/
cm2), the intragrain superconductivity has been affected and the sample shows a lower T~...... and Tc(R = 0) is 44 K (Fig. 2). In fact, the sample becomes non-superconducting after one thermal cycle (i.e. when the sample has been brought to room temperature and again cooled for the Tc measurement). The resistivity also shows a semiconducting behaviour, which is indic-
140
(c) 120
I00
.< 8o
~ 6o 7 i,i F~ 4O
20
0
I
1
I0
20
I
28 (
30
DEGREES
I
I
40
50
60
)
Fig. 3. Continued.
6 E'ea armoRed o m/c,,,
I
/i I
c-
~:
,-:
/l.J
-J
m
/1
___/
.
annealed I
Temperoture (K) ~
>,/
~=== ~
:r
......
~ "~l . ~ t1xlO15°('/cm2~: ol _--,#~i/, 60
90
120
,
I
l
150
180
210
,
I
240
270
Temperature (K)
.300
annealed Temperoturl(K)&&A~=~1 jw~
60
Fig. 4. Scanning electron micrographs. Magnification: 6000. (a) Unirradiated sample. (b) Irradiated sample of dose 2 × 10 ~5a / c m 2.
90
120
150
LA IU
u(/tim
I
I
I
I
180
210
240
270
Temperature (K)
300
Fig.5. Resistivity vs. temperature of irradiated and annealed samples at doses of 1 × 10~5 and 2X 10 ~5 et/cm 2. Inset shows a tail near Tc(R= 0) indicating a slight development of low-To phase.
114
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
ative o f localization caused by irradiation-induced disordering. The b e h a v i o u r o f the annealed samples is not fully understood. The onset of superconductivity has not changed m u c h (Fig. 5 ). But the transition to zero resistivity is arrested m i d w a y and the resistivity falls nearly linearly o v e r a temperature range from about 120 to 75 K, and then the transition to R = 0 occurs (see inset). Near the final transition to R = 0, there appears to be a small tail perhaps due to a low-To phase. H o w e v e r , there is no appreciable e v i d e n c e o f a low-To phasc as the X R D pattern does not s h o w the p r o m i n e n t lines of the low-T~ phase. Resistivity at r o o m temperature of the annealed samples is also higher than that of the irradiated and the unirradiated samples, though d p / d T of the annealed samples is higher than that of the irradiated samples. During annealing at 780°C, defects do not disappear; a m o r p h i z a t i o n increases and superconducting correlations are frustrated from d e v e l o p i n g and spreading out from grains. Only at a m u c h lower temperature, w h e n such correlations are m u c h stronger, they can o v e r c o m e the frustration by the defects.
4. Conclusion W h e n polycrystalline samples o f (Bi, P b ) - 2 2 2 3 are irradiated with 40 M e V c~ particles, Tc decreases and the resistivity increases m o n o t o n i c a l l y with the dose. T G A shows that there is no appreciable change in oxygen content. X R D s h o w s appreciable amorphization, which does not get annealed at high temperature. Hardly any low-T~ phase is produced. O x y g e n knockout has been assigned to be the d o m i n a n t effect of c~ irradiation in Bi-2212. In (Bi, P b ) - 2 2 2 3 , this is not so. D i s p l a c e m e n t of higher Z e l e m e n t s by energetic a particles and local a m o r p h i z a t i o n by stopping oL particles are the m a j o r effects. A n n e a l i n g effects are not tully understood.
Acknowledgements The authors w o u l d like to thank the H u m b o l d t Foundation, G e r m a n y , for the L e y b o l d 10-300 C r y o g e n e r ator and the D e p a r t m e n t of Science and T e c h n o l o g y ,
G o v e r n e m e n t of India, for linancial support in the proj e c t on superconductivity ( S B R - 3 9 ) . They w o u l d also thank S.N. Pillai, R a d i o m e t a l l u r g y Division, B A R C , B o m b a y for the T G A Work.
References J I ] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rictnan, S.M. Zaharak and D. Werder, Phys. Rev. B 36 (1987) 5719. [ 2 ] JD..lorgensen, Phys. Today 44 ( 1991 ) 34; A.W. Sleight, Phys. Today 44 ( 1991 ) 24. [ 31 P. Mandal, A. Poddar, B. Ghosh and P. Choudhury, Phys. l,~cv B43(1991) 13102. 141 M.G. Alexander, Phys. Rev. B 38 (1988) 9194. [ 5 ] J. Bock and E. Preisler, Solid State Conunun. 72 I 1989 ) 453. [ 6 ] J. Geerk, G. Linker, O. Mejen, C. Politis, F. Ratzel, R. Smilhlcy, B. Strehl and G.C. Ziong, Z. Phys. B 67 (1987) 507 171 H. Kupfer, I. Apfelstedt, W. Sahaver, R. Fluckiger, R. MeierHirmor, H. Wuhl and H. Schevrer, Z. Phys. B 69 (1987) 167. [ 8 ] N. Moser, A. Hofmann, P. Schule, R. Henes and H. Kromnuller, Z. Phys. B 71 (1988) 37. 191 S.K Bandyopadhyay, P. Barat, S.K. Kar, Udayan De, A. Poddar, P. Mandal, B. Ghosh and C.K. Majumdar, Solid State Commun. 82 (1992) 397. I 0 ] C. Allgeier and J.S. Schilling, Physica C 168 (1990) 499. 11 ] M.O. Ruanlt, H. Bernas and M. Gasgnier, Phil. Mag. B 60 (1989) 801. 121 M.O. Ruault, H. Bernas, J. Lesueur, L. Dumoulin, M. Nicolar, J.P. Burger, M. Gasgnier, H. Noel, P. Gougeon, M. Potel and J.C. Levet, Europhys. Lett. 7 (1988) 435. 131 M.O. Ruault, H. Bernas, M. Gasgnier, J.C. Lever, H. Noel, P. Gougeon and M.O. Potel, Revue Phys. Appl. 25 (1990) 49. 141 M.O. Ruault and M. Gasgnier, Mater. Sci. Eng. B 5 (1989) 57. 151U. Endo, S. Koyama and T. Kawai, Jpn. J. Appl. Phys 28 (1989) LI90. 161 B.A. Aleksashin, I.F. Berger, V. Verkhovskii, V.I. Voronin, B.M Goshchitskii, S.A. Davydov, A.E. Karkin, V.L. Kozhevivnikov, A.V. Mirmalshtein, K.M. Mikhalyov, V.D. Parkhomenko and S.M. Chesnitskii, Physica C 153-155 (1988) 339. 17] G.J. Clark, A.D. Marwick, F.K. Le Goues, R.B. Laibowitz, R. Koch and P.B. Madakson, Nucl. lnstrum. Methods B 32 (1988) 405. 181 A.D. Marwick, G.J. Clark, D.S. Yee, R.B. Laibowitz, G Coleman and J.J. Como, Phys. Rev. B 39 (1989) 9061. 19] S.K. Bandyopadhyay, P. Barat, P. Sen, S.K. Kar, A. De, B. Ghose, B.G. Ghose and C.K. Majumdar, Proc. Solid State Phys. Symp. Dept. of Atomic Energy, India, at S.V. University. Tirupati, 28 Dec. 1992-1 Jan. 1993, p. 283. 20] H. Zhang and H. Sato, Physica C 214 (1993) 265. 211 GJ. Clark, A.D. Marwick, R.H.Koch and R.B. Laibowitz. Appl. Phys. Len. 51 (1987) 139.
Physica C 228 (1994) 109-114
Effect of alpha irradiation on polycrystalline Bi-2223 superconductors S.K. Bandyopadhyay a,., p. Barat a, Pintu Sen a, Udayan De a, A. De b, P.K. Mukhopadhyay c, S.K. Kar c, C.K. Majumdar c " Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700 064, India b Saha Institute of Nuclear Physics, I/AF Bidhan Nagar, Calcutta 700 064, India c S.N. Bose National Centre for Basic Sciences, DB-I 7, Sector-l, Salt Lake, Calcutta 700 064, India
Received 17 February 1994; revised manuscript received 26 May 1994
Abstract
Polycrystalline samples of Bil.84Pbo.34Srt.91Ca2.o3Cu3.o601o+x ((Bi, Pb)-2223) with Tc(R=0) = 112 K have been irradiated by a particles of 40 MeV energy up to a maximum dose of 1.5 × 1016 ot/cm 2. Tc is observed to decrease with increasing dose. At the highest dose, semiconducting behavior is observed in resistivity measurements, and X-ray diffraction and scanning electron microscopy indicate partial amorphization. Tc has not been recovered by annealing. The possible mechanism of radiation damage in (Bi, Pb)-2223 is discussed; it appears to be different from the mechanism in Bi2Sr2CaCu208 +x (Bi-2212).
1. Introduction The transition temperature of the copper oxide based superconductors is dependent on the oxygen content which controls the carrier concentration [ 1 - 3 ] . The oxygen content changes with different physical or environmental conditions, like temperature and oxygen partial pressure employed during annealing, external pressure [4,5] and irradiation with charged particles [ 6 - 8 ] . Earlier [9] we observed an increase in Tc due to ct irradiation on polycrystalline samples of Bi2Sr2CaCu208 + ~ ( henceforth referred to as Bi-2212) with T~ = 65 K. In Bi-2212, Tc passes through a maximum as a function of oxygen concentration [ 10], and the sample with T~ = 65 K has excess oxygen compared with a sample with the highest T~ in this system. The increase of Tc was attributed to oxygen knock-out or * Corresponding author. 0921-4534 / 94 / $07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10921-4534 (94)00309-4
oxygen disordering. Tc returned to the original value of the unirradiated sample when the irradiated sample was annealed in flowing oxygen [9]. Charged particle irradiation causes oxygen disordering in the YBa2Cu307 _ system also, which has labile oxygen in the C u - O chain. The irradiation-induced preferential disordering of oxygen in the C u - O chain of the basal plane causes a structural transition from the orthorhombic to the tetragonal phase [ 11-13]. In the present work we investigated effects of irradiation by a particles on high-To polycrystalline Bil.snPbo.34Srl.91Ca2.o3Cu3.o601O+x (henceforth referred to as (Bi, Pb)-2223) having Tc = 112 K. Ruault and Gasgnier [ 14] have earlier reported that under 50 KeV helium irradiation, the B i - S r - C a - C u - O ceramic undergoes a transition for the orthorhombic to the perovskite phase (interpreted as a partial breakdown of the layered structure) and amorphization. We have
110
S.K. Bandyopadhyay et al. / Phvsica (" 228 (1994) 109 114
seen amorphization, but have no definite proof of a new phase.
2. Experimental details Polycrystalline samples of Pb-doped Bi-2223 were prepared starting from nitrates of Bi, Sr, Ca, Cu and Pb leading to a nominal composition of B i h g 4P b o . 3 4 S r l . 9 1 C a 2 . o 3 C u 3 o 6 O l o _ ~ x [15]. An aqueous solution of these nitrates was prepared and evaporated to dryness. The mixed nitrates were heated at 500°C for complete decomposition of the nitrates. The mass was then calcined at 800°C for 24 h, ground to form fine powder, pelletized, sintered twice at 855°C for 100 and 48 h and finally annealed at 855°C for 48 h and cooled slowly at a rate of l°C/min. X-ray diffraction (XRD) of the samples thus prepared were recorded in a Philips PW 1710 diffractometer and revealed all the reflections corresponding to the high-T~ phase. The resistivity of rectangular-bar-shaped samples, mounted on a Leybold 10-300 cryogenerator, was measured by the four-probe method using a Keithley 182 nanovoltmeter and a typical sample current of 1 mA. T~ was found to be 112 K; the sample to sample variation was within 0.5 K. Irradiation was done with a beam of 40 MeV o~ particles obtained from the Variable Energy Cyclotron in Calcutta. The beam current was 100 hA. Electrical insulation of the target holder from the beam tube was ensured by a perspex flange and teflon bush in the screws connecting the target holder and the beam line. The samples attached to the aluminium target holder were cooled by flowing compressed air (heat generation ~ 1 W). The polycrystalline samples were thick enough ( ~ 0.3 mm) to stop the beam. Hence they were irradiated from both sides to have radiation damage throughout the volume. To study the irradiation effect as a function of the dose, samples with different doses were prepared. The original circular pellet of 13 mm diameter was divided into several rectangular segments. In the beginning, all the segments were irradiated simultaneously. After a certain desired dose, one segment was reversed so that its other surface was exposed to the beam. When this other side got the same dose, this segment was removed. Meanwhile, the other segments acquired a cumulative dose on one side. The process was repeated until another segment got some
desired higher dose on both sides and was removed; and so on. The number of ~ particles deposited was obtained by measuring the charge collected on the target holder through a current digitizer and a scaler. The doses received by the samples were 1 X 10 ~5, 2 × 10 ~5 and 1.5 × 10 ~6 odcm 2. Some samples irradiated with doses I × 10 ~s and 2 × 1015 o d c m : were annealed in air at 780°C, followed by cooling at 50°C/h, to observe if any recovery of transition temperature occurred. Irradiated and annealed samples were characterised by XRD and their T~.s were determined through resistivity measurements. Scanning electron micrograph (SEM) studies on the surfaces of the samples were performed by a Hitachi $2300 scanning electron microscope. The oxygen contents of the irradiated and the unirradiated samples were determined by thermogravimetric analysis (TGA) in a mixture of 90% argon and 10% hydrogen, and did not differ appreciably.
3. Results and discussion
T~(R= 0)s of all the samples have decreased after irradiation with c~-particles. Table 1 gives T~ and resistivities (at 300 K) of the samples -unirradiated, irradiated and annealed after irradiation. Resistivity vs. temperature plots of the unirradiated and irradiated samples are shown in Figs. 1 and 2. The resistivity increases with the dose, and the sample at the highest dose ( 1.5 K 10 ~' c,/cm") shows both high resistivity and non-metallic behaviour with negative dp/dT m a certain temperature range (Fig. 2). Alekshashin et al. [ 15 ] have observed the vanishing of superconductivity and non-metallic behaviour (i.e. dp/dT
71:
p f r a i l cm)
Unirradiated Dose: t × 10 ~5 a / c m 2, annealed Dose: 2 × 10 ~5 et/cm ~, annealed
112 I 11 75 108 74
3. I 7.3 10.9 8.1 II .5
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
inant effect in (Bi, Pb)-2223, as the TGA indicates no appreciable change in the oxygen content. The differences in behaviour can be explained by using an idea discussed by Zhang and Sato [20]. The structural stability in these cuprates is governed by a tolerance factor (as in cubic perovskites) which is defined as
A
El0. ¢D I
2X1 01 5 c ~ / c m A~"
C-
aA-
~AAAAAA~
E
A
~>, 4
o
,~°°
AA . . . . . t:¢t~tl~O Ot~ OO
1x1015~/cm
o
•>
L
O' ' ' m ° a
_._..,..,,~ ~
2
o
111
~
t = ( A - O ) / [21/Z(B--O) ] . "~09 rY
y 0
~ 60
90
_, 120
unirrodioted '
~
i
i
i
150
180
210
240
270
Temperature (K)
300
Fig. 1. Resistivity vs. temperature of unirradiated and irradiated samples at doses of I × 10 ~5 and 2 × 1015 a / c m 2.
In YBa2Cu307_8 films irradiated with oxygen ions, Clark et al. [ 16] have observed a transition from metallic to non-metallic behaviour at a dose of 1.1 × 10 TM oxygen/cm 2 and extensive amorphization. Both these groups [ 16-18] have discussed in detail the reasons for this non-metallic behaviour. Since in our case, the alpha particles are completely stopped in the sample, the energy deposition is high, and this may cause partial amorphization. In the irradiated sample (Fig. 3(b), dose 2 X 1015 a/cm2), the intensity of the characteristic reflections have considerably decreased; note the change of scale in the Y-axis from Fig. 3 (a). But no extra peaks due to another phase can be definitely identified. In the sample annealed after irradiation (Fig. 3 ( c ) ) , the intensity of the characteristic lines have further decreased, but the lines are still identifiable above the background; there is no signature of an appreciable development of other phases. SEM pictures (Fig. 4) also support partial amorphization. Grains of size 2-3 microns observed in the unirradiated samples are no longer present in the irradiated samples at a dose of 2 X 1015 o t / c m 2. No columnar defects are observed. The work on et irradiation in Bi-2212 has been mentioned earlier [9]. We have also observed a decrease in Tc after a-irradiation of single crystals of Bi-2212 grown by the flux method [ 19]. Tc(R= 0 ) which was 80 K before irradiation, has come down to 77 K after irradiation with a dose of 2 × 1014 o t / c m 2 and to 74 K with a dose of 2 × 1015 ot/cm 2. This can be explained by oxygen knock-out from Bi-2212 [ 10]. On the other hand, oxygen knock-out does not appear to be the dom-
(1)
A - O and B - O are bond lengths of Bi-O in a rock-salt block and Cu-O in a perovskite block respectively. If the bond lengths are taken to be the sum of the ionic radii of the respective ions, i.e. r(B:+) =0.093 nm, r(o2-) =0.14 nm, qcu2+) =0.072 nm, t comes out to be 0.78 in Bi-2212, and is less than the value needed for structural stability (0.8 < t < 0.9). Since the Cu-O bond is rigid, the structural stability is attained by accommodating excess oxygen in the Bi-O layer, whereby the Bi-O bond distance increases to 0.26 nm and the tolerance factor comes within the proper range. This excess oxygen is loosely bound and hence vulnerable to knock-out by energetic et particles; this is why in Bi-2212, oxygen is forced out easily. On the other hand, in (Bi, Pb)-2223, the mismatch between the rock-salt Bi-O layer and the perovskite Cu-O layer is balanced by the substitution of Bi 3 + (r(m3 + ) = 0.093 nm) by pbE+(rpb2+)= --0.12 nm) with larger ionic radius. So, (Bi, Pb)-2223 cannot accommodate much excess oxygen and there is not much loosely bound oxygen to be knocked out. Rather, higher Z elements like Bi and Pb can be easily displaced, as their alphascattering cross-sections are larger than the alpha-scattering cross-section of oxygen. The displacements of E
200 1.5x1016
0
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160
G/cm 2
o
E 120
E
o '~
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o
o
o
oo
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o
> 03 O3 ©
rY
40"
0 40
o o°
i 80
I
I
I
I
I
120
160
200
240
280
Temperature
320
(K)
Fig. 2. Resistivity vs. temperature of irradiated sample at a dose of 1.5 X 10t6 u / c m 2.
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
112 1200
(2_I ~-I
(a)
I~ I°° gEl!
I000
800
5
1
600 Z LLJ Z 400
z
i
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4=0
I 60
5nO
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250I
(b)
200I 150
Z t~ Z
50
0
i
!
I0
20
i/
,
20 (
DEGREES
4O
50
60
")
Fig. 3. X-Ray diffraction patterns: (a) Unirradiated sample. (b) Irradiated sample of dose 2 x 10 t5 a / c m 2. (c) Irradiated and annealed sample of dose 2 × 10~s ct/cm 2.
these high-Z elements and other inelastic processes which stop alpha particles cause local amorphization. There is little change in the Tc °,set in the samples at doses 1 X 1015 and 2 × 1015 c~/cm 2, because a low dose of cx particles destroys weak-linking between grains, leaving the intragrain superconductivity intact [21]. The destruction of weak-linking has broadened the transition width. At the highest dose (1.5 × 1016 O~/
cm2), the intragrain superconductivity has been affected and the sample shows a lower T~...... and Tc(R = 0) is 44 K (Fig. 2). In fact, the sample becomes non-superconducting after one thermal cycle (i.e. when the sample has been brought to room temperature and again cooled for the Tc measurement). The resistivity also shows a semiconducting behaviour, which is indic-
140
(c) 120
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.< 8o
~ 6o 7 i,i F~ 4O
20
0
I
1
I0
20
I
28 (
30
DEGREES
I
I
40
50
60
)
Fig. 3. Continued.
6 E'ea armoRed o m/c,,,
I
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~:
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-J
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.
annealed I
Temperoture (K) ~
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90
120
,
I
l
150
180
210
,
I
240
270
Temperature (K)
.300
annealed Temperoturl(K)&&A~=~1 jw~
60
Fig. 4. Scanning electron micrographs. Magnification: 6000. (a) Unirradiated sample. (b) Irradiated sample of dose 2 × 10 ~5a / c m 2.
90
120
150
LA IU
u(/tim
I
I
I
I
180
210
240
270
Temperature (K)
300
Fig.5. Resistivity vs. temperature of irradiated and annealed samples at doses of 1 × 10~5 and 2X 10 ~5 et/cm 2. Inset shows a tail near Tc(R= 0) indicating a slight development of low-To phase.
114
S.K. Bandyopadhyay et al. / Physica C 228 (1994) 109-114
ative o f localization caused by irradiation-induced disordering. The b e h a v i o u r o f the annealed samples is not fully understood. The onset of superconductivity has not changed m u c h (Fig. 5 ). But the transition to zero resistivity is arrested m i d w a y and the resistivity falls nearly linearly o v e r a temperature range from about 120 to 75 K, and then the transition to R = 0 occurs (see inset). Near the final transition to R = 0, there appears to be a small tail perhaps due to a low-To phase. H o w e v e r , there is no appreciable e v i d e n c e o f a low-To phasc as the X R D pattern does not s h o w the p r o m i n e n t lines of the low-T~ phase. Resistivity at r o o m temperature of the annealed samples is also higher than that of the irradiated and the unirradiated samples, though d p / d T of the annealed samples is higher than that of the irradiated samples. During annealing at 780°C, defects do not disappear; a m o r p h i z a t i o n increases and superconducting correlations are frustrated from d e v e l o p i n g and spreading out from grains. Only at a m u c h lower temperature, w h e n such correlations are m u c h stronger, they can o v e r c o m e the frustration by the defects.
4. Conclusion W h e n polycrystalline samples o f (Bi, P b ) - 2 2 2 3 are irradiated with 40 M e V c~ particles, Tc decreases and the resistivity increases m o n o t o n i c a l l y with the dose. T G A shows that there is no appreciable change in oxygen content. X R D s h o w s appreciable amorphization, which does not get annealed at high temperature. Hardly any low-T~ phase is produced. O x y g e n knockout has been assigned to be the d o m i n a n t effect of c~ irradiation in Bi-2212. In (Bi, P b ) - 2 2 2 3 , this is not so. D i s p l a c e m e n t of higher Z e l e m e n t s by energetic a particles and local a m o r p h i z a t i o n by stopping oL particles are the m a j o r effects. A n n e a l i n g effects are not tully understood.
Acknowledgements The authors w o u l d like to thank the H u m b o l d t Foundation, G e r m a n y , for the L e y b o l d 10-300 C r y o g e n e r ator and the D e p a r t m e n t of Science and T e c h n o l o g y ,
G o v e r n e m e n t of India, for linancial support in the proj e c t on superconductivity ( S B R - 3 9 ) . They w o u l d also thank S.N. Pillai, R a d i o m e t a l l u r g y Division, B A R C , B o m b a y for the T G A Work.
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