Preparation, structure and peritectic transition of La1.5 − x2 SrxBa1.5 − x2 Cu3Oy superconductors

Materials Research Bulletin, Vol. 31, No. 11, pp. 1391-1397, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/96 $15.00 +.OO

PII SOO25-5408(96)00130-4

PREPARATION, STRUCTURE AND PERITECTIC TRANSITION Lal.~,,2Sr,Bal.s,nCus0, SUPERCONDUCTORS

OF

C.C. Yuan, H.-C. I. Kao and C.M. Wang

Department of Chemistry, Tamkang University, Tamsui, Taiwan 25 137 (Refereed) (Received May 14, 1996; Accepted May 2 1, 1996)

ABSTRACT

A series of single phase Lal,5_x,2Sr,Bal.s_x,zCu~O~ (0 I x < 0.80) compounds with a tetragonal triple-perovskite unit-cell have been prepared by solid-state reaction. With increasing x, unit-cell a axis, volume, density, and peritectic transition temperature (Tp) of La, .s_& rxBal.S_&u30y decrease monotonically. The peak position of T, measured in the 25 mL/min flowing O2 atmosphere and lO”C/min heating rate decreases from 1105 to 1080°C as x increases from 0 to 0.80. The oxygen atom loss in the peritectic transition is 0.38 f 0.05 0-atom.mole-‘. KEYWORDS: A. oxides, A. superconductors, C. differential scanning calorimetry (DSC), C. thermogravimetric analysis (TGA), C. X-ray diffraction INTRODUCTION

Recently (1) we prepared a series of single phase La,.~.,,&a,Bar s.~2Cu~0,, 0 I x I 0.75 (LCB) compounds with tetragonal triple-perovskite unit-cell as YBa#&O, (2). For x = 0, La, sBa,.sCujO, is not superconducting (3). By substituting Ca*’ for La3’ and Ba2’ in LalsBal &30,, the resulting LCB becomes superconducting and its T, is as high as 80 K (1). It is found that T, of LCB is dependent on the occupancy factors of Ca” and La3+ions in the unit-cell (4). If the occupancy factor of Ca” ions in the Y3+site is denoted as OFcti and that of La3+ions in Ba*‘site is denoted as OF La/~a,then T, is dependent on AOF (AOF = OFC~ - OFL~/BJ(4). From Rietveld analysis, it is found that Ca*’ ions prefer to enter the Y3+ site and the maximum allowed OF ca/~ is about 0.50 (4,5). When the divalent Ca” ion occupies the Y3+site, it would lower the positive charge concentration around the Y3+site, resulting in an increase in the hole concentration of the nearby Cu2-G layers. On the other 1391

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hand, the trivalent La3’ ion would increase the positive charge concentration around the Ba” site if it occupies the Ba” site, resulting in an decrease in the hole concentration of the Cu20 layers. Thus, the net increase of the hole concentration in the Cu2-0 layers is dependent on the AOF and the T, is dependent on the hole concentration in the Cti-0 layers. In the study of La-Ca-Ba-Cu-0 tetragonal triple-perovskite series, a solubility limit of 0.50 for Ca2+ ions is generally observed. For example, Goldschmidt et al. found x = 0.50 in the (Lal_,Ca,)(Bar 75_xLao.zs+x)Cu30, series (5). We found x = 0.54 in the La, &a,Bar 71_ xCu30y series (6) and x = 0.59 in the La, ,I_xCa,Bai,&u30, series (7). Since the maximum OFcti is about 0.50 and the maximum solubility of Ca’+ions in the La-Ca-Ba-Cu-0 series is also close to 0.50, it is clear that most of the Ca2+ ions are located in the Y3’site. From a Rietveld analysis of a La I ,&-,, 72Bal &u30y compound, we found that OFsrlv = 0.07 (8). In other words, less than 10% of Sr*’ ions is located in the Y3+ site. It is suggested that due to the size difference between Ca2’ and Sr2+ ion, the smaller Ca2+ ion prefers to occupy the smaller Y3+ site and the larger Sr2+ ion prefers to occupy the larger Bz?+ site in a tetragonal triple-perovskite unit-cell. Using Sr” ions to replace Ca” ions in the LCB series, the La, 5_tiSrxBar.~-x~ZCu30y(LSB) series is prepared. Their crystal structure and peritectic transition are studied in this report. EXPERIMENTAL All of the samples were prepared by solid-state reaction. The Lax01 was preheated at 1000°C for 6 h and kept in a desiccator prior to use. Appropriate amounts of La203, SrC03, BaCO3, and CuO were weighed and ground thoroughly with a Retsch Spectra Mill Type MS for 30 min. The mixed powder was calcined at 1000°C for 6 h in a box furnace. It was ground, pressed, and sintered at 950-1000°C for 6 h and annealed at 95O’C for 8 h in the flowing O2 atmosphere. Samples were then furnace cooled to room temperature. Purity of the samples was examined by both X-ray diffraction (XRD) and thermal analysis. If a sample contained impurity, it was resintered and reannealed until single phase material was obtained. If the eutectic transition is observed before the peritectic transition in the DSC or the DTA curve, the product contains some reaction intermediates. XRD patterns were obtained with a Mac Science M03X-HF Diffractometer equipped with a Cu &source and a Ni filter. Diffraction angles were calibrated with an internal Si standard. Unit-cell parameters were calculated with a Treor program. Thermal analysis was done on a Mac Science Thermal Analyzer System 001 equipped with TG-DTA 2020 and DSC 3320. All of the thermal analyses were performed under 02 atmosphere with a flow rate of 25 mL*min-’ and a heating rate of lO”C.min-‘. RESULTS

AND DISCUSSION

Solubility of Sr” and Ca*’ ions in the LSB and LCB series is in the range of 0 I x I 0.80 and 0 < x I 0.75 (I), respectively. Both series have about the same solubility for SrO or CaO substitution in the La1.sBal.sCu30, compound. The XRD patterns for LSB series are shown in Figure 1. For x = 1.OO, LaSrBaCu30,, no single phase material was obtained and impurities, such as BaCuOz and SrCuO*, were observed, as shown in Figure 2. All of the single phase LSB compounds have a tetragonal triple-perovskite structure. Unit-cell a axis, volume, and density are listed in Table 1. A monotonic decrease in the a axis is observed with increasing

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20

40

60

so

FIG. 1 XRD patterns of single phase LSB compounds.

20

10

60

29 FIG. 2 XRD pattern of LSB series with x = 1 .O sample.

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Unit-cell

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TABLE 1 a Axis, Volume, and Density of La, s_x&,Bal s_xIzCujO, Compounds X

rrlnm

Vlnm’

0

0.3910

0.050 0.10 0.30 0.50 0.70 0.80

0.3902 0.390 1 0.3893 0.3889 0.3880 0.3877

0.1790 0.1791 0.1785 0.1766 0.1757 0.1755 0.1753

T, PC

dcnsity/gml-3 6.673 6.646 6.643 6.613 6.547 6.446 6.413

1105 1105 1104 1101 1096 1089 1084

x. The unit-cell c axis is about 3 times greater than the a axis and the ratio of a.43 is in the range of 0.99-1.00. As a consequence, the number of the diffraction peaks found in the XRD patterns is greatly reduced, compared with that in the tetragonal YBa&u30, compound numbers are 0.127, (9). The radii for the La3’, Sr+‘, and B&’ ions with 10 coordination 0.136, and 0.152 nm, respectively (10). The difference in the radius between the La3’ and Sr+’ ions is smaller than that between the Ba*+ and Sr+* ions. Therefore, simultaneously replacing the La” and Ba” ions with Sr” ions results in a gradual decrease in unit-cell volume as x increases, as listed in Table 1. The unit-cell volumes for LSB and LCB series are shown in Figure 3. For smaller x (0 I x IO.5), both series show a nearly linear decrease in the volume with respect to x, following Vegard’s law. For larger x (0.5 I x I O.S), the volume changes slightly with respect to x, showing a saturation effect for further reducing the volume. This is probably what prevents more Sr*’ or Ca” ions from being substituted into the LSB or LCB series. Because an Sr*+ ion is larger than an Ca*’ ion, for a given x, the 0.180

0.176

0.174

I

I

I

0.0

0.2

I

I

0.4

0.6 X

FIG. 3 Volume vs. x for LSB and LCB series.

I

0.8

I

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6.5 6.4 6.3 6.2 6.1 0.0

0.2

0.4

0.6

0.8

X FIG. 4 Density vs. x for LSB and LCB series. volume of an LSB series is always larger than that of an LCB series, and for larger x, the difference in volume between these two series is also larger. The smallest unit-cell volume observed for single phase LSB was 0.1753 nm3 and that for LCB was 0.1743 nm3. Comparing the single phase tetragonal triple-perovskite unit-cell volume of LCB, LSB, and R,,29Cao,43Bal,zsCusO, (R = La to Dy, except Ce) series (ll), we found that Dy129Cao.43Bal.zsCu30~ had a volume as small as 0.1719 nm3. Dy, &ao.43Ba,.29Cu30, was as stable as R,.zsCao.oBa,,z~CusO~. This means that it may be possible to build a stable tetragonal triple-perovskite unit-cell with a volume of 0.1719 nm3. However, it is not possible to replace more La3’ and Ba*’ ions in the LSB series to more than x = 0.80, to further reduce the unit-cell volume. This is probably due to not being able to insert the Sr*’ ions into the Y3+site. Limitation of SrO in the LSB series is thus related to the positions of the cations in different perovskite centers. The density of LSB series is listed in Table 1. The typical density for inorganic solid-state materials is about 6 gcmm3. Plotting the density of LSB and LCB series (1) in Figure 4, we found the same trend for both series: the density decreases with increasing x. Because the atomic weight of Sr and Ca atoms is smaller than that of the substituted La and Ba atoms, the density decreases as the amount of Sr and Ca atoms increases. In addition, the atomic weight of Sr is about twice that of Ca, and the descending rate of density vs. x for the LCB series is twice that of the LSB series. The peritectic transition was studied by DSC and TG. DSC curves for LSB compounds are plotted in Figure 5. Before the peritectic transition (Tp), an extra endothermic peak is observed around 1040°C for the x = 1.00 sample. This peak is a eutectic point, which is the lowest melting point for a mixture. It is obvious that this sample contained impurity. The rest of the samples do not have a eutectic transition; they are single phase materials. The DSC curves for x I 0.80 samples have two endothermic peaks, which accompany weight

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FIG. 5 DSC curves of LSB series. losses in TG measurements. The temperature difference between the two endothermic peaks increases from 7 to 63°C. The first transition is the peritectic transition of LSB. With increasing x, the T, of LSB is gradually decreased from 1105 to 1080°C. In general, for a series of ionic compounds with the same structure, the one with the smaller volume has a higher melting point (12). It is interesting to find that in the high-T, system, this is not always true. For example, in the Rl.zsCao43Ba1.2&u30y series, replacing R with smaller rareearth cation, unit-cell volume decreases and melting (peritectic) point decreases (11). In the LSB and LCB series, the same trend is observed as in the RI &ao43Ba1&u30y system. This implies that the ionicity of these high-T,superconductors is not compatible to the highly ionic NaCl compound. The average weight loss (AW) in the peritectic transition is equivalent to 0.38 f 0.05 Oatom*mole-‘. The AW is not changed systematically with respect to the amount of Sr2+ ion substitution. It is noted that the whole LSB series has the same crystal structure, which is probably the key factor in determining AW of these high-T, superconductors. ACKNOWLEDGMENT This work is financially

supported by the National Science Council of R.O.C.

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