structural and physical characterization of bi2ca1-xlnxsr2cu2o8+δ (ln = dyory) glasses

Physica C 209 (1993) 393-399 North-Holland

Structural and physical characterization of Bi,Ca, _xLn,Sr2Cu208+6 (Ln = Dy or Y ) glasses

P. Somasundaram and A.M. Umarji Materials Research Centre. Indian institute of Science, Bangalore 560 012, India

Received 19 August 1992 Revised manuscript received 25 January 1993

Glasses of BirCa, _Ln>rzCuzOs+d prepared by the melt quenching of oxides have been studied for their physical and structural properties. With increasing rare earth concentration, it is found that the crystallinity increases in the as-prepared glass when observed by X-ray and electron diffraction. Multiple crystallization temperatures are observed in the differential scanning calorimetry studies, indicating co-existence of different crystalline phases. The capacitance measurements show a large anomaly at the glass crystallization temperature. The magnetic studies on the as-prepared glasses containing no magnetic rare earth ions show a sizeable paramagnetic contribution which is supported by the electron spin resonance studies. This is attributed to the localized 3d electrons of copper 2+ ions, which follow a Curie-Weiss behaviour up to the glass crystallization temperature, beyond which the material irreversibly transforms into a Pauli paramagnet and shows superconductivity at low temperatures.

1. Introduction

The superconducting properties of the high-temperature ceramic superconductors are sensitive to method of preparation, chemical composition, mi- crostructure and oxygen stoichiometry etc., which are being actively investigated [ 11. Such studies logi- cally follow the initial discovery of the phenomenon in a new material. We have been studying such effects in the superconducting bismuthates repre- sented by the general formula Bi,Ca,_ 1 SrZCu,04+2n, where n can be 1, 2 or 3. The effects of chemical modifications on the superconducting properties of the crystalline BizCaSrzCuzOs+d brought about by rare earth substitution at the Ca site are now well es- tablished [2-41. It is shown that almost all the rare earths fully substitute Ca, and that 10% to 20% rare earth substitution improves the superconducting properties by optimising the charge carrier concentration [ 51. The actual concentration, rather than the magnetic nature of the rare earth, seems to in- fluence the superconductivity in the BizCa, _-xLnxSrzCu208+6 system.

Preparing the Bi-Ca-Sr-Cu-0 samples by the glass recrystallization method produces highly dense ma-

terials with improved superconducting properties. This is because the compositional inhomogeneity and grain boundary effect, which plays an important role in ceramic superconductors, are minimised in the samples prepared by the glass crystallization method. The mechanism of recrystallization from the glassy phase is the subject of many investigations [ 6,7]. The dielectric and ferroelectric-like properties of the as- prepared glassy phase are interesting because of the presence of micro-crystalline regions [ 81. In this work we report our results on the structural and physical characterization of Bi,Cal_,Ln,Sr,CuzOs+s glasses prepared by melt quenching. The X-ray diffraction and high-resolution electron microscopy studies on the as-prepared glasses show the co-existence of glassy and crystalline phases throughout the composition range studied. The physical properties, like the dielectric constant, and magnetic studies show irreversible effects at the glass crystallization temperatures. The magnetic susceptibility and the electron spin resonance studies indicate that the copper ions are in the localized paramagnetic state in the as-prepared glass and become Pauli paramagnetic in the crystallized materials.

0921-4534/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

394 P. Somasundaram, A.M. Umarji / Bi2Ca,_,Ln~r2Cu,08+,glasses

2. Experimental 3. Results and discussion

The samples were prepared by calcining appro- priate mixtures of high-purity (at least 99.9%) Bi203, CaC03, Ln,O,, St-CO3 and CuO at 800°C for 10 h. Glasses were prepared by melting the mixture above 1100” C in a platinum crucible and quenching between two polished copper plates. The temperature of melting varied between 1100°C and 12OO”C, de- pending upon the rare earth concentration. The de- tails of the exact compositions studied and their melting conditions are mentioned in table 1 .The X- ray diffractograms were recorded using Co Ka ra- diation on a Philips diffractometer. The differential scanning calorimetry experiments were done on a Perkins Elmer DSC in the temperature range of 300 to 1000 K. The high-temperature capacitance measurements were done on samples using pressure con- tacts on gold-coated surfaces of 0.125 cm* area up to 850 K using a HP LCR bridge model 4274A at 1 kHz frequency. The ESR measurements were done on a Varian spectrometer operating in the X-band at a frequency of 9.1 GHz.The DC susceptibility experiments were done using a George Associates Lewis coil force magnetometer in the temperature range of 10 to 850 K with a measuring field of either lo-20 Oe or 5 kOe. High-resolution electron microscopy (HREM) was performed on a JEOL 200CX micro- scope having a resolution of 1.5 A.

The X-ray diffraction patterns of the as-prepared glasses of the type Bi2Ca,_,Dy,Sr2Cu208+s are shown in fig. 1. The x=0 composition is almost completely amorphous - whereas the crystallinity increases with the Dy content in the other samples. We could identify n= 1 and 2 phases of the BiZCan_,Sr2Cun04+2n system in these X-ray patterns.The electron diffraction pattern of the composition x= 0.1 indicates diffused rings confirming the amorphous nature as inferred from the X-ray diffraction studies. However, the HREM results of this sample shows the presence of microcrystallites of 20- 50 A (fig. 2). The electron diffraction patterns for the x=0.75 composition show (figs. 3(a) and (b)) d spacings of 12 and 15 8, corresponding to the (0 0 2) reflections of the n = 1 and 2 phases, respectively. This increase in crystallinity is consistent with

the X-ray diffraction results. The electron diffraction also shows 46 type modulation along the b direction for the x=0.75 composition (fig. 3(c)).

In table 1 the quenching temperatures of the melts, T, and TX values along with the DC resistances of the glasses studied are given. A few representative DSC curves are shown in fig. 4. The DSC results show multiple crystallization temperatures (TX) which are preceded by a small endothermic glass transition temperature ( T,), marked by an arrow in fig. 4. The glass transition temperature, though not well de- fined, increases from 670 K for the x=0 sample to a maximum of 684 K for x=0.25, before decreasing to 66 1 K for the fully Dy-substituted sample. The re-

Table 1

The preparation conditions and electrical and thermal characteristics of B&Ca, -xDyxSr2Cu20s+d glasses

xin

Bi&a, _,Dy,-

SrKNs+d

Heating

temp. (“C)

DC resistance (kQ )

(a) ” (b) b,

=, TX TX--=,

(K) (K) (K)

0.0 1100 39.4 4.3 670 730 60

0.1 1100 8.2 0.004 682 750 68

0.25 1150 89 0.012 683 749 66

0.5 1150 550 0.290 684 748 64

0.75 1150 7 0.190 674 728 54

1.0 1200 22 0.180 661 714 53

‘) Data for as-prepared glass.

b, Data for sample heated to 830°C for 30 min.

P. Somasundaram, A.M. Umarji / BizCa,_,Ln~r~Cu~~+,glasses 395

14.2 23.4 32.6 hi.@ 51.0

28

Fig. 1. The X-ray diffraction patterns of the as-prepared Bi,Ca,_-xDy$3rZCu208+6glasses.

sults are consistent with the X-ray results, which show the existence of a considerable amount of the crystalline phase after the crystallization (fig. 5). The corresponding decrease in the exothermicity of the glass crystallization peaks with the increasing Dy concentration is understandable as there is already a considerable amount of crystalline phases existing in these as-prepared glasses. The magnitude of TX- Tg, a measure of stability of the glassy phase, also decreases with the increase in the Dy content, indicating that the glassy phase is becoming less stable.

Analysis of the X-ray patterns of the samples taken after crystallization for 30 min above TX are shown

I 4 _: 5nm ‘6 ;

/j b\_

Fig. 2. The high-resolution electron microscopy of the Bi2Cao.sDy,,,SrzCu20s+~ glass. (Inset: electron diffraction confirming the amorphous nature of the sample.)

in fig. 5. The majority of the peaks could be indexed on the basis of the n= 1 or 2 phases. For low Dy concentrations (x= 0, 0.1 and 0.25), the at= 2 phase dominates, whereas the n= 1 phase dominates for higher Dy concentrations.

The room temperature two-probe DC resistance values are tabulated in table 1 for the-as prepared samples and for the samples after heat treatment for 30 min at 830°C (above TX). For the x=0 sample the change in resistance is minimal. This does not correspond to the bulk crystallization of the superconducting n = 2 phase within the duration of the heat treatment. Earlier workers had noted that, for almost 100% crystallization of the Bi2CaSr2Cuz0s+6 glass, annealing for up to 60 h or longer at 870’ C is needed [ 8 1. The corresponding change in the resistivity of the x=0.1 sample was from 8.2 kL? to 4 Q. This shows the hastening of the crystallization brought

396 P. Somasundaram, A.M. Umarji / Bi2Ca,_,LnSr2Cu,08+, glasses

a b

Fig. 3. Selected area diffracton pattern for the glass of composition BizC&.zSDy0.,,Sr2Cu 2 0 a+6 showing the existence of the (a) n = 1 and

(b) n=2 members of the bismuth series. (c) Selected area diffraction pattern showing 4b type modulation corresponding to the n=2

phase obtained for BizCao.25Dya,,SrzCu208+gglaSSeS.

about by the substitution of 10% Dy. Further addition of Dy does produce a highly crystalline phase, but the resistivity changes from kiloohms to about 200 Q only. This could be either due to the precip- itation of primarily n= 1 phase or the crystalline BizDySr2Cu208+6 phase (x= 1) which is a semiconductor.

The capacitance measurements (figs. 6 (a) and (b) ) show an anomaly at TX in the form of a sharp upturn. The behaviour beyond TX appears to be a complex behaviour determined by the electrical nature of the phase and probably the formation kinet- ics during measurement. These things are difficult to standardise in the measuring set-up where the sample temperature is being continuously swept during the measurement. Further study has to be carried out in this direction. Different behaviour is expected de-

pending upon whether the precipitated phase is a metal or a semiconductor.

The magnetic susceptibility studies on the polycrystalline samples prepared by the ceramic method show that the Bi2DySr2CuzOs+s compound orders antiferromagnetically at low temperature owing to the ordering of the Dy moments. The x=0 sample in the crystalline phase does not show the expected behaviour of Cu2+ ( d9 system). This has been shown by both the susceptibility and the ESR studies [9] where the Cu-Cu relaxation in the metallic phase is attributed to the absence of the ESR signal. We ob- serve for the as-prepared glassy phase of the x=0.0 composition a substantial magnetic susceptibility of 0.7 x 1 Oe6 emu/g which follows a Curie-Weiss type behaviour up to TX (fig. 7). The low temperature measurement of the same sample up to 10 K did not

P. Somasundaram. A.M. Umarji / Bi*Ca,_Ln~r~Cu,O,+dgl~SeS 397

I I I I 1 500 600 700 600 900

T(W)

Fig. 4. The DSC plots of the Bi&a, _xDy,$rzCu208+d glasses for x=0,0.25 and 0.75 compositions.

indicate any ordering. This is attributed to the localized 3d electrons of Cu*+ ions which irreversibly transform, upon crystallization, into a Pauli para-

magnetic phase. This effect could not be observed for Dy-containing samples as the contribution to the total susceptibility was dominated by the Dy ions. We confirmed this type of magnetic behaviour in the glasses of two other samples where Bi and Ca were partially substituted by Sb and Y, respectively, but having no magnetic ions. All these measurements were carried out in a measuring field of 5 kOe. As seen from fig. 7, the qualitative features are main- tained but the magnitudes of susceptibilities are smaller. This could be explained as a correspond- ingly reduced amount of the glassy phase existing in the sample and hence a smaller number of localised copper ions contributing to the susceptibility. The curve no. 4 in fig. 7 is actually an overlap of cooling data for all the compounds and is typical of a Pauli paramagnetic and metallic material. The samples after the high-temperature measurement showed superconductivity at low temperatures, as measured by the low-field ( lo-20 Oe) DC susceptibility. The x= 1 sample in the glassy state did not show any ordering down to 10 K, the limit of our measurement, unlike the crystalline phase.

The 1 /x versus Tplots for x= 0, 0.1 and 1 .O follow a linear behaviour in the temperature range lo-700 K for the as-prepared glasses. The slopes correspond to ,L& values of 0.90, 0.956 and 8.86 &, for the Dy

13.8 23.6 33.4 43.2 53.0

2’3(dog)

Fig. 5. The X-ray diffraction patterns of the glasses heated at T, for 30 min.

concentrations of x= 0, 0.1 and 1 .O, respectively. In the x=0 sample where there are no Dy ions this effective magnetic moment, entirely attributed to the Cu ions in the formula, works out to be 0.45 ,& per copper ion. For x=0.1 and 1.0, the effective moment corresponds to 9.56 and 8.86 PLg per Dy atom, which is close to the Dy3+ moment of 10.56 pn. The contribution due to Cu in the glassy phase is ne- glected in these two cases as the amount of copper in the glassy state is anyway less.

The low-temperature, low-field magnetic susceptibility measurements of the recrystallized Pauli par-

398 P. Somasundaram. A.M. Umarji / Bi2Ca,_,Ln~r2Cu,08+, glasses

17

13

9

5

I 1

1 0.32

iz - ! 1 0.26

8 so24- .Z

i 0.20 -

: 0.16 -

0.12 -

Bi2CaI-xDYx Sr,Cu,O,

x

0 l

1.0 0 0.5 A

0 08 t I

0,04 I I I ‘1

300 400 500 600 7(

T(K)

4, Fig. 6. Variation of the capacitance as a function of temperature

for various Bi&a, _,Dy,SrzCuzOa+a glasses.

amagnetic material do show the expected superconducting transition (fig. 8). These samples were crystallized at 850°C for 6 h and quenched to room temperature. The three samples x=0, 0.1 and 0.25 showed onset of superconductivity at 84, 82 and 78 K, respectively. The x=0.5 to 1.0 samples did not show an onset of superconductivity. It was not pos- sible to confirm the onset of superconductivity by the resistivity method because of the difficulty in making proper ohmic contact. However, the Meiss-

0.8- 1. Bi2CaSr2Cu208

0.7 - Z.Bi,,, Sb,,,CaSr,Cu,O( 3. B12Y sr2 cu,o,

0.6 - 4. Cooling curve

7; 0.5-

L & 0.4-

; g 0.3-

n 0.2 -

0.1 -

0.0 200 400 600 800

Temperature(K)-+

Fig. 7. Variation of the high-field magnetic susceptibility as a

function of the temperature for the Bi2CaSrzCu20a+s and substi-

tuted glasses.

0.5) 1

-4.51 I 1 I / I I 10 30 50 70 90 110 130

Temperature(K)

Fig. 8. The low-fieldI-rplots of the recrystallized samples show-

ing the onset of superconductivity.

ner fraction increases from x= 0 to x= 0.1 before decreasing for x=0.25. Here it may be mentioned that

the results on the BizCa, _,Ln,SrzCuzOs+d system prepared by the ceramic method showed optimisation of T, for x= 0.10 to 0.2 for the various rare earths [ 4,5 1. This effect was attributed to the optimisation of charge carrier concentration as measured by the thermoelectric effect. However, in the glass recrystallized samples there is no indication of enhancement of T, but definitely the Meissner fraction goes up. The small amount of Dy addition enhances the Meissner fraction, but with little reduction in T,, by accelerating the formation of the n=2 phase. This effect is similar to the effect of addition of Pb in place of Bi in the Biz_XPbXCaSrzCuzOs+a system [ lo]. The role of Pb is more towards enhancement in the formation of the n = 2 phase, rather than improving the

P. Somasundaram. A.M. Umarji / Bi2Ca,_xLnJr2Cu208+8 glasses 399

T, by solid solution effect. The corresponding ESR signal due to the unpaired

spins (originating from Cu’+ or Dy3+ ) in the sys-

tem BizCal _XDy,SrzCuzOs+s shows supporting evi- dence. The as-prepared glasses showed a considerable CL?+ signal for BiZCaSrzCuzOs+d, whereas the polycrystalline form is characterised by the absence of Cu*+ signals, as mentioned earlier [ 9 1. The rel- ative signal strengths for the x=0. I, 0.25 and 0.75 samples, as compared to the x= 0 sample, are 150%, 50% and 16%, respectively. Here the peaking of the signal strength occurs for the Dy concentration of 0.1 and may be indirectly related to the optimisation of the superconducting properties for the 10% to 20% rare earth substitution. After annealing at 830’ C for 30 min the strengths for all the signals were less than l-2%. Even this signal is not expected in the fully crystallised material and hence it shows that crystallization is not complete within the given heat treatment. Another interesting fact of our ESR ex- periment is that the Cu*+ signal is symmetrical for all the compounds except Bi2YSr2Cu20s+s. In this composition a signal corresponding to large anisot- ropy was observed (fig. 9 ) .

In conclusion, we have shown that the glasses of composition Bi2Cal_,Dy,Sr2Cu208+d prepared by

I I I

3.0 3.2 3.4 3.6

Ii (K Gauss) +

Fig. 9. The ESR spectrum of Bi2YSr2Cu20s+d.

the melt quenching method have finely dispersed microcrystalline phases the percentage of which increases with the rare earth content. The T, and TX are a function of composition and the physical properties like magnetic susceptibility, dielectric constant,

electrical resistivity and ESR signal show irreversible changes at TX. We have also shown that the Cu2+ ions in the glassy state are in the localized magnetic state. The glass crystallisation method produces dense materials, and substitution of small amounts of Dy for Ca accelerates the crystallization rate of the n= 2 phase and hence produces a sample with a higher Meissner fraction, though with a small reduction in

T,.

Acknowledgements

We wish to acknowledge C.N.R. Rao, FRS, for helpful discussions and encouragement during this work. We thank G.N. Subbanna for electron microscopy work.One of the authors (PS) acknowl- edges financial assistance from CSIR, Govt. of India.

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[4] J.M. Tarascon, P. Barboux, G.W. Hull, R. Ramesh, L.H. Greene, M. Girond, M.S. Hegde and W.R. McKinnon, Phys. Rev.B(1989)4316.

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[ 61 T. Komatsu, R. Sato, H. Meguro, K. Matusita and T. Yamashita, J. Mater. Sci. 26 ( 199 1) 683.

[ 7 ] Haixiang Zhang Mary, W. Colby and John D. Mackenzie, J. Non-Crystalline Solids 127 ( 199 1) 143.

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structural and physical characterization of bi2ca1-xlnxsr2cu2o8+δ (ln = dyory) glasses

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