Physica C 411 (2004) 180–188

www.elsevier.com/locate/physc

Effect of low temperature short time annealing on oxygencontent and surface quality of Bi2Sr2CaCu2O8+ d single crystals

P. Kumar a, B. Kumar a,*, D.N. Kumar b, G.K. Chadha a

a Department of Physics and Astrophysics, University of Delhi, Delhi 110007, Indiab Department of Chemistry, University of Delhi, Delhi 110007, India

Received 22 March 2004; received in revised form 8 July 2004; accepted 8 July 2004

Available online 14 August 2004

Abstract

High temperature superconducting single crystals of 2212 phase of Bi-system have been grown by self-flux technique.

They are annealed in different gas atmospheres of O2, N2 and Ar to obtain overdoped and underdoped samples. Both

time as well as temperature of the annealing has been optimized to get different Tc-value in the range of 81–94 K. The

oxygen contents d for overdoped and underdoped crystals have been precisely determined by iodometric titration tech-

nique. The change in Tc and DTc have been discussed in terms of change in O-value in as-grown as well as in the opt-

imally annealed crystals due to in- and out-diffusion of oxygen. The effect of annealing on surface morphology and

defects features like growth steps, slip lines/bands and etch pits have also been studied by scanning electron microscopy.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Bi2Sr2CaCu2O8+d HTSC; Single crystal growth; Annealing; Oxygen content d; Iodometric titration; SEM

1. Introduction

Since the discovery of high Tc superconducting

oxide, attempts are being made to grow good qual-

ity single crystals of these systems and to improve

0921-4534/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.physc.2004.07.008

* Corresponding author. Tel.: +91 11 27662026; fax: 91 11

27667061.

E-mail addresses: bkumar@physics.du.ac.in, b3kumar69@

yahoo.co.in (B. Kumar).

the superconducting onset temperature and crystal

quality by the proper heat treatment [1,2]. For the

growth of Bi-2212 single crystals, various methods

have been employed viz. melt, TSFZ, TSSG etc. [3–

6]. Out of these, melt growth is most-convenient,

since large number of single crystals can be grownin one run having similar properties. However,

crystals obtained in the melt growth technique are

rather smaller in size, normally not more than 2

mm across [7]. Different methods viz. various heat

treatment, application of pressure, change of flux,

ed.

P. Kumar et al. / Physica C 411 (2004) 180–188 181

crucible quality etc. have been employed to im-

prove the quality of the crystals [8–10]. We have

tried various temperature profiles, charge to flux

ratio and shape of crucible to grow crystals of big-

ger size.The effect of annealing on HTSC is a subject of

investigation for long. In YBa2Cu3O7+d, a large

number of works have come up highlighting the

role of oxygen-content in it [11–13]. Transition

temperature Tc and transition width DTc were

found to depend critically on O-contents. A sys-

tematic study on the dependence of transition

temperature on the oxygen-content in Bi2Sr2Ca-Cu2O8+d single crystals has been carried out by

many workers [5,8,14]. In Bi2Sr2CaCu2O8+d

crystals, the superconducting properties are found

to vary with O-content. However in this case

the dependence is less sensitive. Investigation of

annealing effect on HTSC has been carried out in

air for higher temperatures (upto 750 �C) for

longer duration (upto few hours) to observechanges in Tc and DTc [8,15]. Effect of annealing

(temperature range 20–875 �C) on structural

changes was studied by Bdikin et al. [7]. A marked

change has been reported in the transition temper-

ature Tc and transition width DTc, which has been

explained in terms of in- and out-diffusion of oxy-

gen [8]. However, in most of the cases, the content

of O2 were expressed as �overdoped� and �under-doped� in a general way, the exact value of �d�has not been determined. We have annealed the

crystals in specific gas atmospheres O2, N2 and

Ar so that the aforesaid changes can be brought

about even at lower temperature and in lesser time.

We have also determined the precise value of O-

content in different overdoped and underdoped

crystals by iodometric titration technique.Although scanning electron microscopy (SEM)

studies on Bi-2212 crystals had been carried out

by many workers, it was mostly confined to show

the crystal size and the smoothness [10,15,16]. We

have observed features like growth steps, slip lines/

bands, etch pits, over growths etc., from which

supplementary information have been gained

regarding defect surface structure of the crystals.The effect of annealing on the aforementioned sur-

face features have also been analysed by scanning

electron microscopy (SEM).

2. Experimental

For the growth of Bi2Sr2CaCu2O8+d supercon-

ducting single crystals by self-flux technique, the

starting materials (Bi2O3, SrCO3, CuO, CaCO3

purity >99.99%, ALDRICH) were mixed in an

agate mortar and pestle for several hours, then

transferred into a high purity alumina cylindrical

crucible (purity >99.0%, CERAC) and subjected

to heat treatment in a box-type programmable

furnace (HERAEUS K-1252). The soaking tem-

perature was kept at 1000 �C and cooling rate

was maintained at (1 �C/h). The resistive measure-ments have been carried out manually by stand-

ard four-probe method using air-drying silver

paste and Kiethley setup (181 Nano-voltmeter,

195 A Digital mutimeter and 224 Programmable

current source). The as-grown crystals were an-

nealed in O2, N2 and Ar gas atmospheres in a

tubular furnace, which was designed and fabri-

cated by us. The crystal surface was examinedby scanning electron microscopy (SEM), employ-

ing a JEOL JSM-840 electron microscope. The

samples were mounted on copper stubs with car-

bon electro-conducting paint and a thin uniform

gold film was sputtered on them, using an ion

sputterer (JFC-1100).

The oxygen content has been determined for

as-grown and annealed crystals by standard iodo-metric titration technique. The superconducting

transition temperature Tc critically depends on the

degree of oxidation, i.e. on the average [Cu–O]+p

charge (or formal oxidation state [Cu]+2+p of Cop-

per). The principal variable controlling the value of

Tc has been found to be �p� on each [Cu–O]+p species

[17,18]. The formal copper valency is simply (2 + p).

The value of �p� has been determined by standardiodometric titration method, which consists of

two parts. In first part, 40mg (W1) of sample and

7 g of KI was dissolved in 30 ml of distilled water.

Then 3 g of KI and 10 ml of 2 N HCl were added

and the solution was thoroughly mixed by using

magnetic stirrer after adding few drops of starch

(solution is now black). The whole process was

performed in nitrogen atmosphere. All the Cu+

will precipitate as CuI. The quantity of iodine

liberated is proportional to the degree of oxidation,

in excess of Cu+, as follows:

182 P. Kumar et al. / Physica C 411 (2004) 180–188

½Cu–O�þp þ ð2þ pÞI� ! CuIþ 1=2ðp þ 1ÞI2The titration was performed with 0.01 N sodium

thiosulphate (Na2S2O3 Æ 5H2O). Let V1 volume of

sodium thiosulphate is required to titrate the solu-

tion, as follows:

2S2O3�3 þ I2 ! S4O

6�2 þ 2I

In the second part, 30 g (W2) of the sample was

dissolved into 10 ml of 1 N HCl. The solutionwas heated for 10–15 min, and then cooled and

10 g of KI was added to it. Let V2 is the volume

of sodium thiosulphate used in the titration. In this

case the degree of oxidation is then given by

p ¼ V 1=W 1

V 2=W 2

� 1 ð1Þ

The oxygen content has been calculated by using

the charge neutrality principle. The valanciesof the Bi, Sr, Ca, Cu and O are +3, +2, +2, 2+p

and �2, respectively, in Bi2Sr2CaCu2O8+d super-

conducting system. Hence we have calculated the

value of oxygen content by using following

formula:

2� 3ðBiþ3Þ þ 2� 2ðSrþ2Þ þ 1� 2ðCaþ2Þþ 2� ð2þ pÞ½Cuþ2þp� � 2ð8þ dÞ ¼ 0 ð2Þ

The iodometric titration have been carried out

three times for each case and the average value

of volumes V1 and V2, measured with the help of

microburatte (least count 0.01 ml), for as-grown

and annealed crystals have been given in Table 1.

The value of W1 and W2 has been kept as 40and 30 mg (measured accurately upto five decimal

places of gram) for all the cases. The smaller least

count of microburatte enables us to measure the

Table 1

Microburatte readings for used sodium-thiosulphate in the titration a

Volume of sodium-thiosulphate As-grown Oxygen-

V1 (ml) 9.52 9.87

V2 (ml) 8.93 9.03

p ¼ V 1=W 1

V 2=W 2� 1 �0.200±0.004a �0.180±

a W1 = 40 mg, W2 = 30 mg, least count = 0.01 mg; V1 = V2 = 10 m

value of �p� upto third decimal place as given in

the third row of the Table 1. Since the arithmetical

mean value at the third decimal place is, inciden-

tally �zero� in all the three cases, we have ignored

it for further calculation of �d�. Further it is notconvenient to carry the error value (0.004) at dif-

ferent places during discussion, we have expressed

the value of oxygen content upto second decimal

place without showing the error value in the fol-

lowing discussion.

3. Results and discussion

The as-grown crystals had shiny flat surfaces

measuring upto 5 mm across (mostly 3 · 2 mm2)

and thickness upto 0.3 mm. The observed sharp

reflections in the oscillation photographs con-

firmed the single crystalline nature of the crystals.

The transition temperature Tc was determined to

be 83 K for most of the crystals with transitionwidth DTc nearly 1.5 K. The lattice parameters

of the crystals were determined to be a = b =

5.412 A, c = 30.842 A by X-ray diffraction.

The as-grown crystals have been annealed at

different temperatures (150, 200, 250, 300, and

400 �C) and for different durations (5, 10, 15

and 20 min) in different gas atmospheres viz.

O2, N2, and Ar. It is found that annealing thecrystals at a temperature 200 �C for the duration

of 5 min is sufficient to bring about a change in

transition temperature for which the Tc-value re-

mains within liquid N2 temperature. Under these

conditions of annealing, samples were over-

doped/underdoped without losing its supercon-

ducting nature. If the duration of annealing is

increased beyond 30 min and temperature is in-creased beyond 400 �C, the sample�s surface is

highly corroded and the change in Tc is more

nd �p� values for different crystals

annealed Nitrogen-annealed Argon-annealed

8.86 9.14

8.52 8.91

0.004a �0.220±0.004a �0.230±0.004a

l, least count = 0.01 ml.

P. Kumar et al. / Physica C 411 (2004) 180–188 183

pronounced. As the annealing in O2-atmospheres

starts, the Tc value started decreasing from 83 K

and attains a value of 81 K in 5 min. For further

annealing, the Tc value keep on decreasing gradu-

ally and after nearly 20 min, the sample fails toshow superconducting transition in liquid nitro-

gen range. In the case of N2-annealing, the change

in Tc is rapid. In the first 4–5 min Tc increases

rapidly to 94 K, and then starts decreasing very

rapidly; in 9–10 min the sample does not show

Tc upto 77 K. The effect of longer annealing is

also manifested in increased roughness and cracks

on the surface in SEM studies. In the case ofargon-annealing no change in Tc value is observed

for annealing for 5 min. It is found that longer

duration of annealing (30 min or more) does re-

sult in decrease in Tc value. However, the degree

of surface roughening and cracking is very high.

Therefore, we have kept the time of annealing

as 5 min for all the cases to get overdoped and

underdoped samples so that the effect of atmos-phere may be assessed. It may be noted that in

most of the earlier annealing work on Bi-2212

Fig. 1. Resistance versus temperature curves for as-grown and anneal

annealed for 5 min at 200 �C in different gas atmospheres viz. oxygen

crystals, much higher temperatures (700–800 �C)and longer duration (few hours) have been em-

ployed to get the desired superconducting and

structural changes. In the present work, however,

much lower temperature and time have beenfound sufficient to bring about desired changes

(discussed later), thus saving both energy and

time. It has become possible because crystals have

been annealed in high purity specific gas atmos-

pheres of oxygen, nitrogen and argon, instead of

air.

The resistivity of the as-grown and annealed

crystals has been measured by four-probe tech-nique. The superconducting transition tempera-

ture Tc was determined to be 81, 94 and 83 K

for the crystals annealed in the O2, N2 and Ar

gas atmospheres with transition width DTc as 2,

4 and 3 K, respectively. Fig. 1 shows the resistance

verses temperature curves for the as-grown and an-

nealed crystals. The larger transition width DTc,

particularly in case of N2 annealed crystal indi-cates that the oxygen content is not completely

homogeneous. We have discussed that in the

ed crystals. Inset shows the variation near R! 0. Samples were

, nitrogen and argon.

184 P. Kumar et al. / Physica C 411 (2004) 180–188

case of N2-annealing, the changes in Tc (so in the

value of O-content) takes place rapidly. The faster

rate of out-diffusion leaves the sample more oxy-

gen-inhomogeneous. For better homogenization

of oxygen content, the annealing time should beincreased [19,20]. But for longer duration of

annealing, the problem of surface corrosion and

decaying Tc-value increases (particularly in Ar-

gas atmospheres). It is noteworthy that in the case

of argon annealing although DTc and surface qual-

ity was found to change markedly, the transition

temperature remained at 83 K after annealing at

200 �C for 5 min.The critical temperature Tc of the Bi2Sr2Ca-

Cu2O8+d superconducting single crystals are very

sensitive to the oxygen content, because the excess

oxygen, which is inserted into crystals by anneal-

ing (O2-gas atmospheres) distorts CuO2 layers;

the distortion of CuO2 layer behaves as pining cen-

ters in the crystal structure of the HTSC [21]. The

superconducting layers (Ca–Cu–O Blocks) areweakly coupled in the crystal structure of Bi-

2212 system [19,22]. The region with excess oxygen

is expected to act as the field induced-pining cen-

ters, where its Tc is lower than that of the sur-

rounding region due to the strong distortion of

the lattice. It is reported that the doping state of

Bi-2212 results in the change of the critical temper-

ature Tc from the overdoped region to the under-doped region reversibly by annealing in various

atmospheres.

The oxygen contents d for the as-grown and an-

nealed crystals were found to be negative and the

values are 0.20, 0.18, 0.22, and 0.23 (column �E�in Table 2), thus giving the values of O-content in

Bi 2Sr2CaCu2O8+d as 7.80, 7.82, 7.78, and 7.77 (col-

umn �F�) for as-grown and for the crystals annealedin O2, N2 and Ar gas atmospheres, respectively.

Table 2

Tc, DTc and d-values in Bi2Sr2CaCu2O8+d single crystals annealed at

Annealing

atmosphere

(A)

Tc (K)

(B)

Change in

Tc (K)

(C)

DTc (K)

(D)

d-values(E)

As-grown 83 – 1.5 �0.20

Oxygen 81 02# 2" �0.18

Nitrogen 94 11" 4" �0.22

Argon 83 0 3" �0.23

The maximum value for Tc (94 K) was obtained

in the case of N2-annealed crystals for which the

O-content was found to be 7.78 (marked as bold

in column �F�), which we treat as optimum value

of O-content in our Bi-2212 crystal. It suggestedthat the as-grown crystals were grown as over-

doped since the O-content (7.80) in them was high-

er than the optimum value (7.78). Hence annealing

such overdoped crystals in N2 resulted in �out-diffu-sion� of O2 from Cu–O plane. This explains the in-

crease of Tc-value, since in the process; O-value is

optimized during annealing in N2 atmosphere. It

may be added that for longer duration of annealingin N2, the Tc-value decreases. It may be visualized

on the basis that if the �out-diffusion� of oxygen

continues for long, it will make the sample increas-

ingly �underdoped�. This explains the observed

non-superconducting behavior of samples (above

77 K) annealed for 10 min or more in N2. On the

other hand, when the samples are annealed in O2,

the O-content is further increased to 7.82 due to�in-diffusion� of oxygen and the samples become

more overdoped, resulting in further decrease of

Tc to 81 K. It is also significant that while a change

of Tc equal to 11 K is effected in the case of out-dif-

fusion of oxygen by an amount 0.02, in the case of

the in-diffusion of O2 by the same amount (viz.

0.02), the change in Tc is only 2 K (columns C

and G). It suggests that the dependence of Tc-valueon O-content is more sensitive near its optimum

value. Hence, when the O-value is close to opti-

mum value, a change in O-content affects the

Tc-value in a big way e.g. a change in O2-value

from its optimum 0.22–0.20 has changed the

Tc-value by 11 K. On the other hand, the same

change in O-value viz. 0.2, (but from 0.20 to 0.18)

could change the Tc value by merely 2 K when dis not so close to its optimum value.

200 �C for 5 min in different gas atmospheres

O-content

(F)

Change in O-value

w.r.t as-grown

(G)

Change in O-value

w.r.t optimum value

(H)

7.80 – 0.02"7.82 0.02" 0.04"7.78 0.02# –

7.77 0.03# 0.01#

P. Kumar et al. / Physica C 411 (2004) 180–188 185

Further it may be noted that the similar condi-

tions of temperature and time have changed the

value of oxygen-content by same amount in differ-

ent gas atmospheres. In both the cases the change

in d-value is 0.02 (from 0.20 to 0.18 in O2-anneal-ing and from 0.20 to 0.22 in N2-annealing, column

�G�). We conclude that there is no effect of atmos-

pheres on the rate of in- or out-diffusion of oxy-

gen. It has been suggested [8] that the rate of

out-diffusion of oxygen from Cu–O plane is faster

than the in-diffusion thereby effecting different

amount of changes in O-value in the two cases.

However, it is not observed in the present study.It may be due to the fact that the oxygen diffusion,

both in- and out-, is very slow [23]. Hence due to

much reduced time of annealing in the present case

(viz. 5 min as against few hours), the different rates

of in- and out-diffusion could not generate a meas-

urable difference in the change in O-content in the

two cases.

Different growth and defect features on the sur-face of the crystals were examined by scanning

electron microscopy. Fig. 2 shows the entire sur-

face of a crystal. The features of two dimensional

layer growth, etch pits along parallel and intersect-

ing rows, pattern of slip-lines and bands are ob-

served in the study which are similar to those

Fig. 2. Scanning electron micrograph of an as

reported earlier [24]. A marked change in these

surface features has been observed due to anneal-

ing of crystals, confirming a close correlation be-

tween the changes in Cu–O layers with the

surface quality.In the as-grown crystals the reported slip lines/

bands are nearly parallel and sharp [24,25]. How-

ever in O-annealed crystals they are not strictly

parallel and also cracks are seen to be developed

on the surface of the crystals along the slip line

(Fig. 3). In the case of the N2-annealed crystals,

on the other hand, more smooth surfaces are ob-

served (Fig. 4). These changes in surface featureswell conform to the changes in the structure due

to oxygen in- and out-diffusion during annealing.

As we have discussed that as-grown crystals are

slightly overdoped, hence the in-diffusion of oxy-

gen creates more distortion in CuO2 layers, which

are manifested on the surface as increased rough-

ness and cracks as evident in Fig. 3.

In case of annealing in N2 atmosphere, how-ever, the extra oxygen is removed, thus relieving

the crystals from such distortion and, as a result,

the surface quality is also improved (Fig. 4; mag-

nification in Figs. 3 and 4 are kept same for bet-

ter comparison). In the case of annealing at

higher temperatures and for longer duration,

-grown Bi2Sr2CaCu2O8+d single crystal.

Fig. 3. Scanning electron micrograph of oxygen-annealed crystal. Cracks developed along slip-lines are visible which are not strictly

parallel.

Fig. 4. Scanning electron micrograph of a nitrogen-annealed crystal. The surface of a crystal has become smoother and the slip lines

are straighter.

186 P. Kumar et al. / Physica C 411 (2004) 180–188

cracks are seen to be developed on the surface of

the crystals. Such degradation of surface is more

prominent in the case of argon-annealed crystals.

Even the crystallites grown at the surface of the

crystals are cracked due to over annealing

(Fig. 5).

Fig. 5. Scanning electron micrograph of an over annealed crystal. Crystallites on the surface are seen to be fully cracked. Similar fate

of the whole crystal are observed in many cases irrespective of atmosphere but more prominently in argon-annealed crystal.

P. Kumar et al. / Physica C 411 (2004) 180–188 187

4. Conclusion

The conditions of annealing in different gas

atmospheres are optimized. The annealing tem-

perature of 200 �C and time 5 min are found to

be sufficient to bring about a change in Tc in

the range 81–94 K. The Tc and DTc are foundto be dependent upon O-content and its homoge-

neity. The rate of in- and out-oxygen diffusion is

found to be slow and independent of annealing

atmosphere. The optimum value of oxygen is

found to be 7.78 for which Tc is maximum (94

K). The oxygen and nitrogen atmosphere are

found to be very effective to bring about a change

in Tc while argon atmosphere proved to be inef-fective in smaller duration of annealing. The

change in O-content has marked effect on the

structure and the surface features. The excess of

O-value renders poor surface quality.

Acknowledgments

The financial assistance from the University

Grants Commission (UGC) under the research

project ‘‘Superconductivity R&D Programme’’ is

thankfully acknowledged. We are thankful to

Prof. G.C. Trigunayat and Prof. B.S. Garg for

necessary support and discussion.

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