Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

Applied Superconductivity Vol. 4, No. 718,pp. 327-335, 1996 0 1997 ElsevtcrScience Ltd

PII: SO964-1807(97)00011-2

Pergamon

Printed in Great Britain. All rights rewed 0964-1087/96$15.00 + 0.00

EFFECT OF Hf AND Hf-Ca SUBSTITUTION ON THE SUPERCONDUCTIVITY OF GdBa2Cu307_6 R. G. KULMRNI*,

U. S. JOSHI, K. M. PANSURIA, AMISH G. JOSHI, G. J. BALDA and D. G. KUBERKAR

Department of Physics, Saurashtra University, Rajkot-360 005, India (Received 25 November 1996)

Abstract-The structural and superconducting properties of (Gdt_,_,,Ca,,Hf,)BasCusOz samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd,,Hf~)Ba&O, low& the oxygen content and decreases r, which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gdo.ss_yC~Hfo,,,)Ba2Cu30, provides proper matching between the ionic radius and valence of Gd3+ (0.94 A) and the average ionic radius and valence of He’ (0.78 A) and Ca*+ (0.99 A). As the Ca content increases, the r, increases from 8 1K 0, = 0.05) to 86.5 K’@= 0.20, compensated oxide), closer to the value of 9 1 K for pure G~B~$ZU~O,_~ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R,_,Hf,)Ba&O, (R=Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T,. 0 1997 Elsevier Science Ltd

1. INTRODUCTION

The superconducting transition temperature T, of RBa,Cu307_g (R=Y, Er, Gd, etc.) materials depends on the mobile carrier concentration p or the effective copper valence in the CuOz plane. The oxygen content controls p in a non-linear fashion [ 1,2] because the carriers are distributed in both the plane and the Cu-0 chains. In addition to varying the oxygen content, cation doping [3, lo], e.g. at the R3+ = Y site, may also change p and affect many properties accordingly. Among all cation dopings, Ca substitution for Y in YI_xC&Ba2Cu307_q (YCaBCO) has been extensively examined by numerous workers [5,8] as the valence state of Ca + is lower than that of R3+ = Y3+, Ers+ and Gd3+* such a substitution will increase p due to hole doping by Ca. This behaviour ofp has also been’observed by us in Er,_,C~Ba&!u,O, (ErCaBCO) [l 11. The valence state of Hf4+ is higher than that of R3+ = Y3+, E? and Gd3+, and the ionic radius of Hf4+ (0.78 A) is about 0.10-o. 16 A smaller than that of Y3+ (0.89 A), Er3+ (0.88 A) and Gd3+ (0.94 A); such a substitution with Hf may decrease p due to hole filling by Hf. Our recent studies show that the effect of increasing the Hf concentration in (Y,_zHf,.)Ba,Cu307_g (YHfBCO) [ 121 and (Er1_~Hf,)Ba2Cu307_g (ErHfBCO) [13] lowers the mobile hole concentrationp and decreases T,. The average ionic radius and valence of Ca2+ (0.99 A) and HP+ (0.78 A) in equal proportions match those of Gd3+ (0.94A); a substitution with Hf and Ca will lead to a compensated oxide having T, closer to that of pure GdBa2Cu307_B (GdBCO). Recently, we have also examined the effect of Hf-Ca substitution in (Y,_,,Ca,,Hf,)Ba&u30z (YHfCaBCO) [12] and (Er1_,_,,Ca,,Hf,)Ba2Cu30z (ErHfCaBCO) [ 131, respectively, and have observed compensated oxides having T, closer to pristine 123 materials. Therefore, it is of interest to investigate the effect of Hf substitution for Gd in GdBCO on superconductivity and oxygen content. In order to understand the behaviour of Hf with respect to the GdBCO structure, the simultaneous substitution of Hf and Ca at Gd site has been undertaken. In this paper, we report X-ray diffiction, resistivity, AC susceptibility and oxygen content measurements on the series of compounds having the stoichiometric compositions (Gd,_,Hf,)Ba,Cu,O, (GdHfBCO) for x= 0.0-0.2 and (Gd,_,_,,Ca,,Hf,)Ba,Cu30z (GdHfCaBCO) for x = 0.15 and y = 0.0-0.4. The interrelationship between the superconducting transition + Author to whom all correspondence should be addressed.

327

R. G. KULKARNI et al.

328

temperature and the variation of the oxygen content is discussed in the context of the effective copper valence. Further, the role of magnetic moment carried by R =Y, Er and Gd ion on the T, and p in (Rl_xHfx)Ba&usO, and (~,s,,C~Hf,,,,)Ba2Cu30, will be explored.

2.

EXPERIMENTAL

A series of compounds having the compositions (Gd,_,Hf,)Ba,Cu,O, (x=0.0, 0.05, 0.10, 0.15, 0.20) and (Gd,_,,Ca,,Hf,)Ba,Cu,0, (x=O.15,y=O.O5, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40) were synthesized by a standard ceramic technique [ 141 under identical conditions. Stoichiometric quantities of fine powders of Gd203, BaCOs, CuO, HfOz and CaO (all 99.9% pure) were thoroughly mixed and heated in air at 940°C for 24 h in a platinum crucible. This reacted powder was reground and reheated at 940°C for 24 h to obtain a homogeneous single-phase sample. The black product was then pulverized and cold-pressed into pellets which were sintered in air at 940°C for 24 h. To obtain fully oxygenated samples, these pellets were annealed under oxygen flow at 450°C for 12 h followed by slow cooling at the rate of 1°C min- ’ until room temperature was reached. All the samples were characterized at room temperature by X-ray diffraction using CuK, radiation. The X-ray analysis revealed that all the samples were single phase with an impurity level less than 1%. The stoichiometric composition of the constituents in the sample was confirmed by energy-dispersive X-ray analysis using a JEOL scanning electron microscope. The oxygen content was determined by the iodometric method. Resistivity was measured as a function of temperature on regularly shaped samples using the standard four-probe method. The AC susceptibility was measured by an inductive technique with a frequency of 3 13 Hz at 50 mV.

3. RESULTS

3.1. (GdI_,Hf,)Ba2CuJ02

AND

DISCUSSION

and (Gdo.ss_yCa,Hfo.Is) Ba#+O,

An excellent agreement amongst X-ray diffraction patterns of GdHfBCO, GdHfCaBCO and GdBCO is an indication that the compounds GdHtBCO and GdHfCaBCO have the GdBCO structure. the observed X-ray diffraction peaks were modeled by modified Gaussian functions and the refined unit cell parameters, calculated using a standard least square program, are listed in Table 1. All four Hf-doped samples (x = 0.05-0.20) and all five of Hf-Ca doped samples (x = 0.15, y = 0.05-0.25) remain orthorhombic with distortion (b - a)/@ + a) very close to that of pure GdBCO except two Hf-Ca doped samples (x = 0.15, y = 0.30,0.40) which show decrease in orthorhombicity. Table 1 shows the behaviour of the lattice parameters a, b, and c as a function of x and y, respectively. The lattice parameters of (Gdt_,Hf,)Ga#.+O, display very small changes in a, b, and c with x indicating that the substitution of smaller HP+ for a larger Gd’+ is expected to change the c parameter which seems to be evident from Table 1. Similar structural behaviour has Table 1. Lattice parameters, orthorhombicity, oxgen content and effective Cu valence of (Gd,_,_,Ca,,Hf,)Ba,Cu,Oz Sample (X.Y) (0.0,O.O) (0.05,0.0)

(0.10,0.0) (0.15,O.O) (0.20,O.O) (0.15,0.05) (0.15,0.10) (0.15,0.15) (0.15,0.20) (0.15,0.25) (0.15,0.30) (0.15,0.40)

a 3.8378 (5) 3.8486 (5) 3.8326 (5) 3.8276 (5) 3.8285 (5) 3.8386 (5) 3.8545 (5) 3.8376 (5) 3.8272 (5) 3.8229 (5) 3.8444 (5) 3.8512 (5)

Lattice parameters b 3.8971(5) 3.8996 (5) 3.8961(5) 3.8993 (5) 3.8984 (5) 3.8972 (5) 3.9037 (5) 3.9008 (5) 3.8963 (5) 3.8800 (5) 3.8614(S) 3.8609 (5)

(A)

Orthorhombicity C

11.7007 (11) 11.7016(11) 11.7521(11) 11.7572(11) 11.7737 (11) 11.7300(11) 11.7389 (11) 11.7362 (11) 11.7575 (11) 11.7875 (11) 11.7684(11) 11.7464(11)

10%~

0.766 0.658 0.821 0.927 0.904 0.757 0.634 0.816 0.894 0.741 0.220 0.125

Oxygen content z

6.94 (2) 6.91(2) 6.88 (2) 6.84 (2) 6.79 (2) 6.85 (2) 6.89 (2) 6.93 (2) 6.96 (2) 7.01(2) 7.07 (2) 7.10(2)

Cu valence 2+P 2.293 2.258 2.221 2.180 2.130 2.203 2.245 2.286 2.354 2.373 2.430 2.483

The superconductivity

of GdBazCu,07_a

329

I

I

160

240

Temperature (K) Fig. 1. Resistance vs temperature for (Gd,,Hfx)Ba,Cu30z,

x= 0.0, 0.05, 0.10, 0.15 and 0.20.

been observed for (Y,_,Hf,)Ba,Cu,O, [ 121 and (Erl_,HfX)Ba&u30, [ 131 recently. In the case of Ca-doped series, the lattice parameters are approximately constant until y= 0.25 with nearly constant volume and for y> 0.25, lattice parameters show a slight decrease in volume [Table 11. This is consistent with the simple picture of an average ionic radius of HP+ (0.78 A) and Ca2+ (0.99 A) matching with Gd3+ (0.94 A). These structural considerations indicate that the nominal concentrations are close to the actual concentrations in the samples. Oxygen content of the single-phase Hf and Hf-Ca doped GdBCO samples was determined by an iodometric titration technique. The values are diplayed in Table 1. The effective Cu valence (2 +p) or the hole concentration @) per [Cu-0] unit was calculated from these data and is also included in Table 1. p and z denote the hole concentration per [Cu-G] unit and the oxygen

(a) 6

.

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.

00 oooooooo

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0

0

AA ~~~

++++;;_ AAAA

iv@@

W

aa l +++++ .t+++ AAAA laa

.(x=0.15, y=O.O5) +(x=0.15, y=O.lO) A (X=0.15,y=O.15) 0I(x=0.15, 200

y=O.20)1

300

Temperature (K)

,

00

VV OVV

0 65

as 'UK)

105

,O

Ov

a

lan

.a 'A~ oov .aflAA l aAA la&A

2~"

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a (x=0.15, y=O.30) 0 (x=0.15, y=O.40) A(XdO.IS, Y=0.25)

99

I

I

loo

200

I

300

Temperature (K) Fig. 2. Resistance vs temperature for (Gdo,s~_yC%Hf,,,S)B~Cu,O,,

y = 0.05-0.40.

R. G. KULKARNI et al.

330

Table 2. T, values obtained from measurements of resistivity and AC susceptibility for (Gd,,,C~Hf,)Ba,Cu,Oz. All r, values are in K Sample (X?Y) (0.00,0.00) (0.05,0.00) (0.10,0.00) (0.15,0.00)

(0.20,0.00) (0.15,0.05) (0.15,O.lO) (0.15,0.15) (0.15,0.20) (0.15,0.25) (0.15,0.30) (0.15,0.40)

Resistivity c 92.0 (1) 89.0(l) 87.0(l) 86.0(l) 84.0(l) 84.0(l) 85.0(l) 87.0(l) 88.0(l) 86.0(l) 79.5 (1) 76.0(l)

Tmid E

91.0(l) 87.0(l) 84.5 (I) 82.0 (1) 80.0 (1) 81.0(l) 83.0(l) 84.0 (1) 86.5 (1) 81.5(l) 76.5 (1) 72.0 (1)

AC susceptibility C=”

90.0(l) 86.0(l) 83.0(l) 81.0(l) 78.0(l) 79.0(l) 81.0(l) 82.0(l) 84.0(l) 80.0(l) 75.0(l) 70.0(l)

Tc tiAC)

90.0 (1) 85.0(l) 81.0(l) 79.0(l) 77.0 (1) 78.0 (1) 81.0(l) 84.0(l) 86.0 (1) 81.0(l) 75.0(l) 74.0 (1)

content, respectively. It is evident from Table 1 that both p and z decrease with increasing Hf content in Gd,,Hf,BqCu30Z for x = 0.0-0.2, indicating that HP+ replaces Gd3+, while both p and z increase with increasing Ca concentration in (Gd,,8S_YCa,,Hfo~,S)Ba2Cu3GZfor y = 0.05-0.4, demonstrating that Ca’+ replaces Gd3+ [Table 11. Resistivity results from our Hf and Hf-Ca doped samples are displayed in Figs 1 and 2. Table 2 shows the resistive superconducting transition temperatures: onset, mid point and zero resistance. The Hf ions suppress the superconducting effectively at an average rate of e 0.6 K per at % of Hf. The suppression in T,(E) by Hf substitution is compensated by the addition of holes by Ca doping at constant Hf concentration_ This is shown by observing the increase in T, with increasing y for u 3 0,. The temperature dependence of the AC susceptibility xAC for wO.8s-yC~~fo.l5Pa~ (Gd,_,Hf,)Ba,Cu,O, and (Gd,_,,Ca,,Hf,)Ba,Cu,O, are shown in Figs 3 and 4, respectively. The onset critical temperature determined from AC susceptibility, T, (xAC), is listed in Table 2. It is evident from Table 2 that Fid (R) and T&,& agree very well with each other. This shows the consistency of the measurements and the high quality of the samples. Figure 5 shows the dependence of T, and concentration (x or y) on the resistance, R, at room temperature for (a) Hf-doped samples and (b) Hf-Ca doped samples, respectively. Hf-doping in GdBCO results in an increase in resistance with decreasing T, [Fig. 5(a)] exhibiting a reduction in T, with increase in R and x. For increasing Ca doping in (G~,ss_~C~Hf,8,)Ba~Cu~0, decreases the resistance and increases T, for y=O.O5-0.20 [Fig. 5(b)] and thereafter the resistance shows slight increase for y > 0.20 but T, decreases with increasing y for y = 0.20-0.40. We have shown in Fig. 6(a) T, vs Hf concentration x and in Fig. 6(b) T, vs Ca concentration y for one Hf content (x = 0.15).

80 90 Temperature (K) Fig. 3. Temperature dependence of AC susceptibility for (Gdl_,Hf,)BazCu,Oz, and 0.20.

x = 0.0,0.05,O.10,0.15

The superconductivity

of GdBa2Cu307_a

+*

Ol-

331

*+

(a) -20 3 .Z Ei 4 -4On Yi d -x -6O-

. (X=0.15, y=o.O5) f

x(X=0.15,

y=o.lO)

Temperature (K) Fig. 4. Temperature dependence of AC susceptibility for (G4,ss_yC$Hfo,,s)BazCu30,,

y = 0.05-0.40.

The observed suppression of T, by Hf substitution in GdBCO is explained on the basis of the oxygen effect assuming it is real. The oxygen content z of a superconductor is directly related to the hole concentration p or the effective Cu valence 2 +p, which controls the superconductivity. The concentration of holes can be varied by varying the Hf doping concentration. It is evident from Fig. 6(a) that T, decreases from 91 K to 80 K as z decreases from 6.94 to 6.79 with corresponding increase of x from 0.0 to 0.2 [Table 21. The trend is clear: the hole concentration p decreases as the samples are doped with higher concentration of HP+ for Gd3+. The additional electrons contributed by Hf ions are expected to fill mobile holes in the CuO2 planes, reducing conduction and eliminating superconductivity. As a matter of fact, p decreases from 0.293 to 0.130 as x increases from 0.0 to 0.2, with a corresponding decrease in T, from 91 to 80K [Table 2; Fig. 6(a)]_ Present results agree with the T,-oxygen content behaviour observed in (Y1_XHf,)Ba.$u30z [ 121 and (Er,_,Hf,)Ba,Cu,O, [ 131 in which T, decreases with decreasing oxygen content from 6.83 to 6.65 and from 6.94 to 6.79, respectively. Moreover it is a well known fact that the T, decreases with decreasing oxygen content from 6.85 to 6.50 in orthorhombic YBCO [ 151. These results clearly establish that the effect of increasing Hf content in GdHtBCO is equivalent to that of decreasing oxygen content in GdBCO. The dependence of T, on Ca concentration in (GdesS,Ca,,Hfo.ts)Ba&u30z [Fig. 6(b)] is explained as follows. Both oxygen content and hole concentration increase as the samples are doped with higher concentrations of Ca. The T, of (Gdces_,,Ca,,Hfo,,5)Ba.$u30z increases with increasing y as p increases up to the optimum value 0.354 (u = 0.20, 7” = 86.5 K). The oxide around optimum value ofp is identified as compensated oxide with x = 0.15 at y = 0.20 displaying T, of 86.5 K. The T, of this oxide lies closer to that of pure GdBCO (T, = 91 K), as expected from ionic radius and valence considerations. Moreover, in this compensated oxide, the hole filling by Hf is completely balanced by hole doping from Ca. For further increase ofp from 0.354 to 0.483 @= 0.20-0.40), T, decreases from 86.5 K to 72 K due to excess hole doping by Ca. A similar behaviour (reduction in T,) due to hole over doping has been observed in YBaaCus0-I [ 15,161 and LaBa2Cu307_a [ 171.

R. G.

332 la)

KULKARNI

et al.

6.5 -

I 0.1

3.51 0

Concentration (xf

g5r

1577 R(mOhm)

I 0.1

2.51 0

I 0.3

I 0.2

I 0.4

1 0.5

Concentration (y) 90Q

2 cp

3 70 803

,

y 4

5

6

7

R(mOhm)

Fig. 5. Dependence of I”, and concentration (x or y) on the resistance for (a) (Gdl_,Hf,)Ba.#Zu,Oz, (x = 0.0-0.20) and (b) (Gd,,s~_yC$Hfo,15)Ba$u~Oz, y = 0.0-0.40.

3.2. Comparative study of Hf and Hf-Ca doped samples of (R1_,,Ca,HfJBa#u~Oz

(R = I: Ec Gd)

In order to understand the effect of Hf and Hf-Ca doping in nonmagnetic Y and magnetic Er and Gd ions, a comparative study of superconducting properties has been carried out. Figure 7(a, b) shows T, vs Hf concentration (x) and hole concentration p vs Hf content (x) for RHfBCO with R=Y, Er, Gd. The observed suppression of T, by Hf substitution in YBCO, ErBCO and GdBCO is explained on the basis of oxygen effect assuming it is real. It is evident from Fig. 7(a) that Tc decreases from 91 K to lower values as n increases from 0.00 to 0.20. The comparison of T; vs x data between YHfBCO, ErHfBCO and GdHfBCO suggest that the reduction in T, is much faster for nonmagnetic Y than magnetic Er and Gd. It is also interesting to note that the hole concentration decreases much faster for nonmagnetic Y than magnetic Er and

The superconductivity

7&I

333

I 0.2

I

0

(b)

of GdBa&u@_6

0.1

90 r

Q

I 0.1

I 0.2

I 0.3

I 0.4

Concentration (x) Fig. 6. (a) I’, vs Hf concentration x in (Gd,,Hf,)Ba,Cu,O, concentration y for (GQ,,,_~C$Hf,,l,)Ba,Cu,O,

(x = 0.0-0.20) and (b) T, vs Ca (x = 0.15, y = 0.0-0,40).

Gd. Further, for magnetic Er and Gd the decrease in T, and hole concentration is much faster for lesser magnetic Er compared to more magnetic Gd. These observations indicate that magnetic moment of Er and Gd plays an important role in suppression of superconductivity and T,. Perhaps an additional mechanism is going on which needs more exploration. We have shown in Fig. 8(a) and (b), T, vs Ca concentration at constant Hf content (x = 0.15) and (a)

gsr

2 5

I-

YHfBCO x ErHfECO A GdHfBCO

Q

-/5_

(b) 0.4 r

x ErHtBCO A GdHfBCO 701

0

I

I

0.1

0.2

Concentration (x) Fig. 7. (a) Comparison of T, vs Hf concentration (x) for (R,_,Hf,)Ba&u,O, (R =Y, Er and Gd, X= 0.0-0.20) and (b) Comparison of r, vs hole concentrationp for (RI,HfX)Ba#&Oz (R =Y, Er and Gd; x = 0.0-0.20).

R. G. KULKARNIet al.

334

Q YHfCaBO

0 (b)

I

I

I

I

0.1

0.2

0.3

0.4

I 0.5

,

@YHfCaBO x ErHfCaBO A GdHfCaBO 0

I

I

I

I

I

0.1

0.2

0.3

0.4

I 0.5

Concentration (y) Fig. 8. (a) Comparison of T, vs Ca concentration (v) for (R,,,Ca,,Hf,)Ba&u,O, (x = 0.15, y = 0.050.40; R =Y, Er and Gd) and @) Comparison of F, vs hole concentrationp for (R1_,_,,Ca,,HfX)Ba+&Oz (x=0.15, y=O.OS-0.40; R=Y, Er and Gd).

vs Ca concentration (v) for YHf CaBCO, ErHf CaBCO and GdHfCaBCO, respectively. It is evident from Fig. 8(a) that T, vs y data suggest that the maximum in T, occurs at y = 0.20 for YHf CaBCO and GdHf CaBCO where as for ErHf CaBCO it occurs at y = 0.25. On the other hand p vs y data does not yield any additional information about the effect of magnetic moment carried by R-ion. p

4.

CONCLUSIONS

In conclusion, the observed lowering of T, with increasing x in (Gdl_,HfX)BazCu30, provides convincing evidence that the filling of holes by Hf4+ reduces the hole concentration and suppresses superconductivity. This suppression can be compensated by appropriate hole doping with Ca. This has been shown by the fact that Ca doping in (Gd,,,s5_,,Ca,,Hfo~,S)Ba~Cu30z with y = 0.05-0.20, T, increases from 81 K 0, = 0.05) to 86.5 K 0, = 0.25), closer to the value of 91 K for pure GdBCO (x =y = 0.0). A comparative study of Hf and Hf-Ca doped samples of (R1_,_,,C~HfX)Ba,Cu30z (R =Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T, in (R1_,HfX)Ba,Cu30z where as it reflects some noticeable effect on (R1_,_,,Ca,,Hf,)Ba&u30,. Acknowledgements-This research was financially supported by the Department of Atomic Energy (BRNS), India. The authors are thankml to M. V. Subbarao and Nikesh A. Shah for assistance in experimental work.

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