Characterization of lead free 2212 phase of bismuth superconductor with Tc of 93 K

Solid State Communications, Vol. 83, No. 11, pp. 905-913, 1992. Printed in Great Britain.

0038-1098/92 $5.00 + .00 Pergamon Press Ltd

CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH S U P E R C O N D U C T O R WITH T c of 93 K S.C. Bhargava, A. Sequeira, J.S. Chakrabarty, H. Rajagopal and S.K. Sinha Bhabha Atomic Research Centre, Bombay, India and S.K. Malik Tata Institute of Fundamental Research, Colaba, Bombay, India (Received 1 December 1991 by D. Van Dyck)

The lead free 2212 phase of the bismuth cuprate superconductor with Tc of 93K has been synthesized. The oxygen content and the structural charateristics which maximize Tc are determined using neutron diffraction. The chemical composition and the homogeneity of the superconducting phase is determined using energy dispersive Xray analysis and scanning electron microscopy. It is shown that the subsequent handling of these oxides needs greater care to prevent detrimental effects on their superconductivity.

1. INTRODUCTION THE THREE phases of the bismuth cuprate superconductor, popularly known as the 2201, 2212 and 2223 phases corresponding to n = 1, 2, 3 in the series Bi2Sr2Can_lCunOy, are well known [1-4]. Even though Tc of the 2212 phase is not the highest, it is interesting for several reasons. Firstly, all the Cu ions in this oxide possess square pyramidal coordination of oxygen ions. Planes formed by such ions are known to be primarily responsible for superconductivity. Secondly, incommensurate superstructure modulations have been observed in this phase using X-ray diffraction [5,6], electron diffraction [7-9], powder [10] and single crystal neutron diffraction [11]. These modulations are explained with the insertion of extra oxygen in the Bi-O layers [7]. The bismuth sites are assumed to be fully occupied in these modulated planes. The relation of the extra oxygen ions which are related to these superstructure modulations and the superconductivity is, however, far from clear. The oxygen content of the oxide as well as Tc critically depends on the preparation procedure. It is thus important to be able to prepare qualitatively good and well characterized samples of this phase for basic studies of these planes. The preparation procedure is, however, not straightforward [12-18]. The samples obtained using t h e conventional solid state reaction method give the

transition temperatures (To) lower than 85K [19]. Frequently, the superconducting transition is accompanied by a tail in the resistivity vs temperature curve, which signifies non-homogeneity and non-stoichiometry. A small amount of high T c 2223 phase also generally appears. A Tc higher than 85 K was obtained [19-21] using wet procedures to obtain a mixture for calcination and reducing atmosphere during sintering. The addition of a specific amount of lead also improves Tc by about 5 K. We used a different procedure and obtained Tc (R = 0) of 93 K in a lead free sample [22]. The SC transition is sharp and without tails. Even though studies of the crystal structure [5-11] and other properties of the 2212 phase of the bismuth cuprate have been done in the past, the samples used in these studies were either single crystals or prepared using the solid state reaction method. The latter method, as mentioned above, does not yield a good superconductor [19], whereas preparation procedures of single crystal and polycrystalline material are not comparable. In view of the strong dependence of the oxygen content on the preparation procedure and its relevance to superconductivity, we have investigated our sample, which used different preparation procedure and possesses a Tc of 93K, using neutron diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDAX) to determine the structural characteristics and composition responsible for maximizing T c. Neutron diffraction

905

906

CHARACTERIZATION OF L E A D FREE 2212 PHASE OF BISMUTH

results show that the oxygen content, as well as the distance apical Cu-O in the square pyramid of copper oxygen ions, is of prime importance• It is generally assumed that grinding of these superconductors does not affect the superconducting behavior. In the present study we show through detailed investigation that the effects of handling procedures such as grinding, and subsequent heat treatments are detrimental to superconductivity. 2. EXPERIMENTAL Bi2Sr2CaCu208+ a is prepared using Bi203, SrCO3, CaCO3, and CuO with purities better than 99.9%. A mixture of the oxides in the atomic ratio Sr:Ca:Cu=2:1:2 was heated several times for 24h at 900°C with intermediate grindings. Bi203 was then added to get the atomic ratio Bi:Sr:Ca:Cu=2:2:l:2. The mixture was taken to melting point for a few seconds and then annealed in air at 850°C for 16h and air quenched. We denote this sample ( T c = 9 2 . 4 K , R = 0 ) by BSCC-O. The preparation procedure of BSCC-O resulted in a shining surface which differed from the interior of the pellet, which was charcoal black in color. Subsequently, one of the pellets of BSCC-O was heated in N 2 atmosphere at 750°C for 8h and slow cooled to the ambient temperature. The shine on the surface disappeared. The T, was found to be lower (90K) than that of BSCC-O. We denote this sample by BSCC-ON. When a pellet of this sample was reannealed in air 850°C for 16h and air quenched, Tc increased to 93K. We denote this sample as BSCC-ONO. The transition from BSCC-O to BSCC-ON is thus found to be reversible. To investigate the effects of subsequent handling procedures, a part of the pellet of B S C C - O N was crushed and split into three parts and pelletized. One of them was treated in nitrogen at 750°C for 16 h, the other part was treated in oxygen at 850°C for 16 h, and the third pellet was treated in air also at 850°C for 16 h. The air treated sample was quenched and the other two were slow cooled. Surprisingly, all the three pellets showed drastic deterioration in superconducting properties. To see if grinding alone has any effect on the superconducting properties, one of the pellets of BSCC-ONO was completely scraped to a powder form and examined with a.c. susceptibility and X-ray diffraction. The X-ray diffraction pattern and Tc were not affected by grinding but the superconducting volume fraction decreased drastically.

Vol. 83, No. I I

The neutron diffraction study of the two samples, BSCC-O and BSCC-ON, was carried out at the ambient temperature at the Dhruva reactor, using a neutron wavelength of 1.216A. The instrument collimations from the in-pile to the detector end were 0.7°-0.5°-0.7 ° . The sample was in the form of a cylindrical stack of pellets, each of thickness about i mm, and diameter about 10mm. The overall size of the cylindrical stack was approximately I0 mm high. The temperature dependence of the resistivity is obtained using the d.c. four probe method and silver contacts• 3. RESULTS A N D DISCUSSION The temperature dependence of the resistivity of the sample prepared in air (BSCC-O) is shown in Fig. 1. The transition temperature (R = 0) is 92.4 K. The width of the SC transition in the temperature dependence of the resistivity of the 2212 phase of the bismuth cuprate is generally found to be greater than 10K, excluding the tail part if present, even in absence of impurities or impurity phases• In our sample, the width is certainly less than 10K. The neutron diffraction pattern is shown in Fig. 2. It is analyzed by the Rietveld profile refinement method, using a modified version of the program DBW3.2 [23]. All possible positional, thermal (B) and occupancy (N) parameters were varied in addition to the cell parameters, half-width parameters, zero angle and scale factor. The refinement was carried out using the established space group Amaa [24, 25]. The structure converged to an R-value (Rp) of 4.43%. The fit of the experimental data with the calculated pattern is also shown in Fig. 2. The refined structure is shown in Fig. 3. The refined structural parameters 0.6 0.5 0 0 CO v

0.4

2"

0.3

1 02 0.1 0.0 80

90

10

100

Temperature

(K)

Fig. 1. Temperature dependences of the resistivities (of Bi2Sr2CaCu2Os+a. Symbols 1 and 2 refer to BSCC-O (To = 92.4) and BSCC-ON (Tc = 90K), respectively.

Vol. 83, No. 11 10

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CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH

907

superconductor. This has also been found in earlier neutron diffraction studies of the polycrystalline samples of this phase [25]. The bond distances are given in Table 2. The average cation valence (V) is obtained using the relation [26], V = Ejwj exp[(r 0 - rj)/So],

(1)

0.0

where r0 = 2.09(Bi), 1.967(Ca), 2.118(Sr), and 1.679 (Cu) and B0 =0.37. The symbol wj gives the I,,, ,J,,,,I,l,,llL,,ll,t,l,,lllLIllltTlll , , , , t , , , , l , ~ , ,I , , , ~ l , l l l occupancy factor of the site at rj. The summation 10 15 20 25 30 ~5 "0 t.5 50 55 60 65 70 TWO THETA includes anions indicated as neighbors in Table 2. Fig. 2. Room temperature neutron diffraction pattern The cation valence thus obtained are also given in of BizSrzCaCu2Os+d (BSCC-O) with Tc of 92.4K. Table 2. The observed Cu valence is close to the The calculated pattern is shown with solid lines along optimum value of 2.13 for the square pyramidal with the difference pattern at the bottom. coordination of Cu and Tc of 93 K, as proposed by listed in Table 1 are in good agreement with the deLeeuw et al. [27]. It is also interesting to point out the environment values reported earlier in other studies [25] of this of Cu ions which optimizes the Cu valence and system where the refinement was also carried out maximizes T c. The two pairs of anions which form using the space group Amaa to determine the average the basal square are at approximate distances of 1.88 structure. The anomalously large values for the and 1.99A. The apical oxygen is at a distance of thermal (B) parameters of the 0(2) as well as 2.26A, which is smaller than the values found in 0(3) oxygen ions reflect the atomic displacements other studies of this phase with lower Tc. These due to the inherent superstructural modulations distances, particularly the distances of the oxygen [5, 11] associated with the 2212 phase of the bismuth ions in the basal plane forming the square, as well as the occupancies of these sites, which dominantly % contribute to the Cu valence [equation (1)], need to be optimum to get optimum Cu valence and higher To. The neutron diffraction study reveals lower concentration of oxygen ions than found in other studies of samples with lower T c [25, 28]. This has a large influence on the Cu valence and is directly or indirectly responsible for the highest value of Tc obtained by us for this lead free 2212 phase of the bismuth superconductor. It may, however, be noted that the oxygen sites Oi,2 and O3 are not fully occupied, leading to 1,2 imperfections in some of the square pyramids in the superconducting CuO2 planes. These imperfections are apparently not harmful to T¢. On the contrary, the lower oxygen content along with the lower C u ",p, apical oxygen 0(3) distance appears to be responsible for the optimum Cu valence and higher T~. Bismuth sites are also not fully occupied. The effect of this on superstructure modulation may be significant, but could not be investigated in the present study. The temperature dependence of the resistivity of the pellet obtained after treatment of the above sample BSCC-O at 750°C in an N 2 atmosphere and subsequent slow cooling (BSCC-ON) is also shown in Fig. 1. Tc decreases from 92.4 to 90 K. The neutron taJ / diffraction pattern of this N2-treated sample is shown Fig. 3. Structure of Bi2Sr2CaCu2Os+ d (BSCC~O) as in Fig. 4. Structural changes that occur as a result obtained from neutron Rietveld refinement. of the treatment in nitrogen atmosphere are not

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CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH

Vol. 83, No. 11

Table 1. Results of Rietveld refinement of the neutron diffraction pattern of Bi2Sr2CaCu208+ d (BSCC-O and BSCC-ON). The numbers in parentheses are the esd's referred to the last significant digit. Number of atoms per formula unit is 2 × N. There are four formula units per unit cell. The neutron scattering amplitudes used are 8.53(Bi), 7.03o(Sr), 4.90(Ca), 7.70(Cu), and 5.80(0) fm. Space group = Amaa. BSCC-O: a = 5.365(4) A, b = 5.424(4) A, c = 30.74(1)4, d = 0.28, Rp = 4.43%, Rw? -- 6.49%, R e = 1.69%, RB -----1.41%. Tc = 92.4K. BSCC-ON." a = 5.362(4) ,4, b = 5.423(4)A, c = 30.6855(14),4, d = 0.48, Rp = 4.98%, R~, = 6.88%, R E = 1.88%, Tc = 90K, Rn = 1.89%, N represents fractional site occupancy

Sample

Atom

Site

x

y

z

B

N

BSCC-O BSCC-ON

8 Bi

(8•)

0 0

0.215(3) 0.206(2)

0.4486(3) 0.4498(3)

1.6(2) 1.0(2)

0.82(1) 0.85(2)

BSCC-O BSCC-ON

8 Sr

(8l)

0 0

0.727(2) 0.725(2)

0.3571(3) 0.3568(3)

1.1(3) 1.5(3)

0.92(2) 0.95(2)

BSCC-O BSCC-ON

4 Ca

(4f)

0 0

0.75 0.75

0.25 0.25

0.5 0.5

1.16(4) 1.16(4)

BSCC-O BSCC-ON

8 Cu

(8l)

0 0

0.238(2) 0.237(2)

0.3027(3) 0.3023(3)

1.4(2) 1.5(2)

1.0 1.0

BSCC-O BSCC-ON

8 O1:

(8g)

1/4 1/4

0.0 0.0

0.3108(5) 0.3118(5)

1.2(2) 1.2(2)

1.03(3) 1.00(3)

BSCC-O BSCC-ON

8 Or,2

(8g)

1/4 1/4

0.5 0.5

0.2914(6) 0.2917(6)

1.2(2) 1.2(2)

0.87(3) 0.91(3)

BSCC-O BSCC-ON

16 02

(16m)

0.073(6) 0.091(6)

0.580(5) 0.583(5)

0.4478(6) 0.4494(6)

16.1(11) 14.4(10)

0.72(3) 0.71(1)

BSCC-O BSCC-ON

80s

(8•)

0 0

0.209(5) 0.202(4)

0.3761(7) 0.3752(7)

8.0(9) 9.5(9)

0.83(3) 0.90(4)

appreciable even though, unexpectedly, the occupancy factors of the O1,2 and O3 sites slightly increase, as can be seen from Tables 1 and 2, leading to an increase in the total oxygen content from 8.28 to 8.48 per formula unit. This in turn increases the Cu valence marginally. The occupancy factors of the Bi and Sr sites also increase, unexpectedly.

To see if the N2-treated sample recovers when treated in air, we resintered the pellet of BSCC-ON in air for 16 h at 850°C followed by quenching in air. We label this sample as BSCC-ONO. Along with this, we also resintered the air-treated pellet of BSCC-O. The Tc of BSCC-ON increased from 90 to 93 K, and becomes equal to the value for the air-

Table 2. Interatomic distances (in A) in Bi2Sr2CaCu208+ d. The cation valence V is calculated using equation (1)

Bond

BSCC-O

BSCC-ON

Bond

BSCC-O

BSCC-ON

Bi-Os(x 1) -02(×2) -O2(x2) -02(×2) -O2(x2) -O2(x2) V

2.23(2) 2.02(3) 2.54(3) 3.27(3) 3.39(2) 3.47(3) 2.87

2.29(2) 2.10(3) 2.47(3) 3.37(4) 3.33(2) 3.41(3) 2.55

Sr-O3(xl) -Ol,t(x2) -02(×2) -O1,2(x2) -03(×2 ) -O3(×1 ) V

2.68(3) 2.45(1) 2.93(2) 2.72(2) 2.77(2) 2.87(3) 1.94

2.65(3) 2.44(1) 2.98(2) 2.70(2) 2.77(1) 2.89(3) 1.97

Ca-Ol,2(x4) -O~:(×4)

2.29(1) 2.67(1)

2,30(1) 2.69(1)

V

2.14

2.03

Cu-Ol,2(x 2) -Or,t(× 2) -O3(x 1) V

1.99(1) 1.88(1) 2.26(2) 2.15

1.98(1) 1.88(1) 2.25(2) 2.16

Vol. 83, No. 11

909

CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH

~0

Table 3. Chemical composition determined using the E D A X method

-~2~2

Atomic ratio

0.~

z Z -- O~

0.0 l0

~S

23

25

30

35 TWO

t.0

45

50

55

50

55

Elements

BSCC-ON

BSCC-ONO

Bismuth Strontium Calcium Copper

1.80 1.94 0.88 2.00

1.56 1.91 0.95 2.00

THETA

Fig. 4. Room temperature neutron diffraction pattern of Bi2Sr2CaCu2Os+d obtained after the N2-treatment described in the text (BSCC-ON, T c = 90 K). The calculated pattern is shown with solid lines along with the difference pattern at the bottom. treated resintered sample BSCC-O (Fig. 6). This shows the reversible nature of the effects of the N2 treatment. It may be pointed out that the lower occupancy factor of any cation site (Table 1) obtained using the neutron diffraction method could be due to partial occupancy of that site by cations with lower scattering amplitudes or vacancies at those sites. Thus, the low occupancy factors for the bismuth site in both samples implies that a fraction (~ 15%) of the bismuth sites are vacant or partially occupied by ions with lower scattering amplitudes. Similarly, the high occupancy factor of the Ca site implies that these sites are partially occupied by Sr and Bi. On the other hand, the occupancy factors of the anion sites give the correct occupancies of these sites. In view of this uncertainty in the occupancies o f the cation sites, we used energy dispersive X-ray microanalysis (EDAX) to find the chemical composition. The sample surface was scraped, cleaned and examined with a scanning electron microscope. The surface appeared to be quite homogeneous (Fig. 5). Several spots were chosen and examined with EDAX. A maximum of 10% variation was found in the concentration of cations over these spots. The average concentrations over these spots are given in Table 3. The chemical formula thus found, Bit.a0Srl.94Ca0.ssCu2Oa.2s, is in good agreement with the results of Lee et al. [29], Bil.91Srl.72Ca0.80Cu208, obtained using anomalous scattering synchrotron crystallographic studies. The lower bismuth content in our case is due to the higher temperature to which our sample was subjected than in the case of Lee et al. (900°C). A comparison of the EDAX results (Table 3) with the occupancy factors given in Table 1 shows that the bismuth sites are partially vacant. The Sr sites are occupied by Sr only. The Cu sites are fully Occupied. The larger occupancy

factor of the Ca sites found using neutron diffraction compared with the value expected on the basis of the EDAX results is due to the partial presence (1020%) of bismuth on these sites. These results are reasonable on the basis of chemical considerations. It is interesting to note that the EDAX results show that the increase in Tc is accompanied by a decrease in the concentration of bismuth and strontium. This is consistent with the increase in their concentrations which occurred when Tc decreased during the transition from BSCC-O to BSCC-ON (Table 1). The reason for this small compositional change is not obvious at this stage, though it appears to suggest that the final composition of the superconductor depends on the heat treatment. The rest of the constituents reside on the grain boundaries. The low occupancy of the bismuth sites appears to be due to the fact that the sample was taken to the melting point for a few seconds, and bismuth tends to evaporate rapidly above 822°C. Nevertheless, the Tc obtained is higher. The bismuth deficiency is not likely to occur in the sample prepared using the solid state reaction method. However, this procedure gives a lower To. To investigate the effects of grinding and subsequent heat treatments, we ground these N2treated pellets (BSCC-ON) after the neutron diffraction study and subsequently performed X-ray diffraction study. The X-ray pattern showed no change (Fig. 7). Three pellets were made from this powder. One of them was heated in air at 850°C for 16 h and quenched, the other was heated at 750°C in N2 for 8 h and slowly cooled. The third was heated in oxygen at 850°C and slowly cooled. The temperature dependences of the resistivities of the three pellets thus treated (Fig. 8) show drastic deterioration in superconducting properties. X-ray diffraction was carried out to investigate the cause of this deterioration (Fig. 9). The sample treated in oxygen shows the presence of impurity phases, presumably due to slow cooling. The sample treated in N2, however, shows a negligible amount of impurity phases in spite of

910

CHARACTERIZATION OF L E A D F R E E 2212 PHASE OF BISMUTH

Vol. 83, No. 1 I

(a)

(b)

Fig. 5. Scanning electron micrograph of (a) the N2-treated sample of Bi2Sr2CaCu208.46 (BSCC-ON), T~ = 90K, (b) Bi2Sr2CaCu2Os+ d obtained from B S C C - O N after scraping off the shining surfaces and reannealing at 850°C for 16h and air quenching to room temperature, T c = 93 K. stow cooling. The deterioration in the SC properties was, however, large in both cases. This suggests that the deterioration in the superconducting properties as a result of the grinding and the subsequent heat treatments is not due to significant changes in the structure or appearance of an impurity phase. To find out if the grinding alone can deteriorate

the superconducting behavior, we studied the powder obtained from BSCC-ONO using a.c. susceptibility (resistivity measurements are not possible on pellets made from the powder unless it is sintered at temperatures higher than 600°C). The diamagnetic shielding decreased drastically even by mild powdering (Fig. 11), whereas Tc is not affected. It thus appears that the grinding, however mild it may be,

Vol. 83, No. 11

CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH

911

(a)

05

0.4! O (D v

0.3

I

o

..•

o :

QI

~

ca

0:' bv

~_. . . . .

i 1

O1,

=oo

~2oo

J U ~' u w ~J ~ L~,~_-.._3'%; .% ',..~ .J~b..;

~ooo ~ £~0~-- ~-6~o6 4~'oo ~zoo 60.00 2

8

angte

O0

85

90

95

~00

105

Fig. 6. Temperature dependences of the resistivities of Bi2Sr2CaCu2Os+d (BSCC-O and BSCC-ON) obtained after scraping off the shining surfaces and reannealing at 850°C for 16h and air quenching to room temperature. Curves 1 and 2 correspond to BSCC-ON and BSCC-O, respectively. -

900

!

(b),,0o

(K)

Temper'aLure

900

70O

i

500~-

~

I! n I

Ioo,~

.,,

6,

'

~i -_

~/

I1 I

6

112-:i~o

I'

-

~_~-oi ,. !:-

I

" ~ i:-~.~,%,J¢ '.~ ';" ~.',r ~V \

~,~.' w-\/

":1

700

Jlk

'~ 5 0 0 -

.=i ,oc 13.0

~ i , 29.0

21.0

t

, i , 570

29.0

37.0

450

53.0

610

...I _ -

-

i

,

C)900

_ ~ ;

~l

:

#+'~

~- • 5.0O

210

-

o

!

300

130

20

Z"

i "

5.00

!

I

l , 450

t

,

t , 53.0

i

700

, 610

Z8

Fig. 7. X-ray diffraction pattern of the N2-treated Bi2Sr2CaCu2Os+d (BSCC-ON), after the material was well ground.

~

i

500

i

i'! 1

50¢

[

~1

~

I00 ,

1.2 I 500

150

2 IO

290

37.0

45.0

53.0

t

1

61.0

28

~. I'0 I CD 0.8 0 E ,~. 0 . 6 ~L 04 2

I 80 0

L

i7 50

~oo

diffraction d of which

of the three pellets of resistivities

are

shown

in

Fig. 8. (a) Heated in air at 850°C for 16h and quenched, (b) heated in oxygen at 850°C for 16 h and slow cooled, and (c) heated in N2 at 750°C for 8 h and slow cooled.

7.:

02

Fig. 9. X-ray Bi2SE2CaCu208+

~o

Temper'at.ure

~oo

~ 5 o . . . . ~-oo

(K)

Fig. 8. Temperature dependence of the resistivity of the three pellets which were obtained after grinding and retreating the N2-treated sample of BizSr2CaCu2Oa+a (BSCC-ON). The three pelleis were heated in air at 850°C for 16h and quenched (curve 1), oxygen at 850°C for 16h and slow cooled (curve 2), and N2 at 750°C for 8 h and sl0w cooled (curve 3), respectively.

starts the deterioration. It reduces the superconducting volume fraction significantly, though Tc is not affected by grinding alone. Heat treatments done subsequently, however, also decrease the Tc and deteriorate the sample rapidly. 4. CONCLUSIONS The preparation procedure followed in the present study results in a homogeneous sample with Tc of 93 K. The chemical composition is found to be

912

CHARACTERIZATION OF LEAD FREE 2212 PHASE OF BISMUTH

Vol. 83, No. 11

search, Bombay, for their help in obtaining some of the data presented in this work.

1.2 10[ CD 0 CO v

REFERENCES

0.8

1.

0.6 0.4

C~

1

2.

0.2

I 0.0

3.

.

so 1oo iso zoo 2so T e m p e r a t u r e (K)

o

300

4.

Fig. 10. Temperature dependence of the resistivity of Bi2Sr2CaCu2Os+d obtained after scraping off the shining surfaces of (BSCC-ON), reannealing a t 850°C for 16h, air quenching to room temperature, grinding and reannealing at 850°C for 19h and air quenching to room temperature. The deterioration in SC properties is remarkable.

5. 6. 7. 8.

°

(a)

.............

9. 10. Z

11. t"

12.

I

Y I 50

I 100

I 1,50

I 200

I 250

300

13.

TEMPERATURE (K)

Fig. 11. Effect of grinding of BSCC-ONO on the a.c. susceptibility. Bi2Sr2CaCu2Os+ d obtained after scraping off the shining surfaces of pellet of BSCC-ON, reannealing (without grinding) at 850°C for 16 h, air quenching to room temperature and (a) with grinding or (b) without grinding.

14. 15. 16. 17.

Bi1.s0Srl.94Ca0.ssCu200.28. The low oxygen content and the Cu valence close to 2.13 are the key factors which lead to the highest Tc for this phase of the lead free bismuth superconductor. The superconducting volume fraction starts decreasing as soon as pellets are powdered, even though the X R D pattern and Tc are not appreciably changed by mild powdering. Subsequent heat treatments, however, also decrease the T~ and further deteriorate the superconducting behavior. To avoid any deterioration of the superconducting behavior, it is important 'to avoid powdering.

20.

Acknowledgements - We are indebted to Drs C.V. Tomy, Ravi Kumar, T. Nagarajan and C.V. Purandare of Tata Institute of Fundmental Re-

23.

18. 19.

21.

22.

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