Superconducting and electrical properties of Ca-substituted YBa2Cu3O6

PHYSICA ELSEVIER

PhysicaC252 (1995) 100-106

Superconducting and electrical properties of Ca-substituted YBa2Cu306 T. Watanabe, M. Fujiwara, N. Suzuki Department of Applied Electronics, Science Universityof Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan

Received 18 May 1995; revised manuscript received 23 June 1995

Abstract Superconducting properties of sintered Yl_xCaxBa2Cu306 (0 < x < 0.31) have been investigated. Measurements of resistivity, Hall effect and X-ray diffraction as a function of Ca content x were carded out. It was found that Yl-xCaxBa2Cu306 becomes superconducting above x > ~ 0.2, as had been reported previously, and T~ shows a maximum at x ~ 0.26 and declines at larger x. By increasing the Ca content, the carrier number in the CuO 2 plane increased suddenly in the range x > ~ 0.2, where samples were superconducting. A part of the carder introduced by Ca substitution, however, was localized at Cu sites forming CuO chains. About 40% of the doped carder was conductive in x < ~ 0.2 and ~ 80% was conductive in x > ~ 0.2 at room temperature and these results agree fairly well with the analysis for the results of the iodometric titration measurement. Finally, we pointed out that slight oxidation for these samples brought about the release of carder localization at low temperatures and a remarkable rise of To, indicating the crucial importance of the CuO chain or orthorhombicity for high Tc in YBa2Cu306+ z.

I. I n t r o d u c t i o n It is well known that the superconducting properties o f YBa2Cu306+ z ( Y B C O t + z) are greatly influenced by the planar hole concentration, which exhibits a strong dependence on the oxygen content z [1-4]. The change o f the oxygen content occurs at sites o f CuO chains which have a special characteristic for the superconductivity in this material. The mechanism o f the c a r d e r doping due to partial depletion o f oxygens from CuO chains, however, displays a rather complicated behaviour, and is still unclear. Several studies relating to Ca substitution for Y in YBCO6÷ z and its effect on superconducting properties have been performed to understand the relative

importance o f CuO a planes (Cu(2)) and CuO chains (Cu(1)) [5-9]. It is known that Ca 2+ ions, when incorporated into YBCO, are substituted preferentially for Y sites [7]. The substitution o f the Y site by Ca gives an extra hole carrier to the CuO 2 plane. Indeed, the compound Yl_xCaxBa2Cu306 (0 < x < 0.25) (CaxYBCO 6) is known to be superconducting for Ca concentration x > 0.2, antiferromanetic for 0 < x < 0.07, [10] as well as being tetragonal for all values o f x [11-13]. It was also shown that Y0.7Ca0.aBa2Cu3Ox is a superconductor for all values o f the oxygen content z ( ~ 6.85) and remains a superconductor with Tc ~ 34 K for z = 6.02 [7]. This phenomenon is different from that o f pure Y B C O z which generally possesses superconductivity above

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T. Watanabe et a l . / Physica C 252 (1995) 100-106

z ~ 6.4 [2,3]. Recently, it was shown that Y0.9Ca0.1Ba2Cu3Oz has superconductivity at z ~ 6.22 and the carder number of this material at 200 K, estimated from the Hall effect measurement, is slightly higher than that of pure YBCO z. By decreasing the oxygen content, the carrier number decreases in a similar manner to YBCO z for z > 6 . 7 and reaches a plateau at 6.5 < z < 6.7 [9]. These results indicate that extra planar hole carriers have an important role in addition to the carrier originating from the oxygen deficiency of the C u t chain. The role of the C u t chain, however, remains unknown. Fisher et al. suggested that the extra charge of the incorporated Ca ion is heavily screened by accompanying oxygen vacancies [6]. Indeed, it was shown theoretically and experimentally that the density of the planar hole carrier of pure YBCO z with partially depleted oxygen is strongly influenced in relation to the Cu and O at the C u t chain [2,3,14]. In this report, we have synthesized YI_~Ca xBa2Cu306 (CaxYBCO 6) (0 < x < 0.31) and Yl-xCaxBa2Cu306+z (CaxYBCO6+z; x = 0.23, z < 0.2) ceramics with the tetragonal structure, and have measured superconducting properties in relation to Ca content and oxygen content. The correlation of T~, Ca content and the planar hole concentration was investigated. The important role of the C u t chain in localization/delocalization for the doped carrier by the substitution of Ca was proposed.

2. E x p e r i m e n t a l

Ceramic samples of Ca~YBCOt+ z (0 < x < 0.31) oxides were prepared from an appropriate amount of high-purity powder of BaCO3(4N), Y203(4N), CuO(4N) and CaCO3(5N) by the standard solid-state reaction. This mixture was heated in air at 900°C for 20 h in an alumina crucible. This reacted powder was reground and reheated at 900°C for 20 h. In order to ensure homogeneous distribution of the substituted Ca in the material and good intergrain connection in the sintered sample, the reground powder was screened through a 53 Ixm mesh screen and the process of regrinding and the reheating was repeated four times. Finally, this powder was pressed into pellet form under a pressure of 20 t f / c m 2 and sintered in air at 960°C for 20 h. To obtain fully

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Fig. 1. Lattice parameters of a- and c-axis as a function of Ca content in Yl_xCaxBa2Cu3Ot, and YBa2Cu306 (Ref. [21]). Oxygen contents are also shown in the upper region. deoxygenated samples (Yl_xCaxBa2Cu306), the pellets were first annealed in Ar gas flow at 700°C for 20 ~ 40 h, so that the oxygen content was reduced down to z = 0.1-0.2. Further reduction of the oxygen content was made by heating the pellet with Zr metal in a sealed quartz tube at 430-450°C for 5 - 2 0 h, depending on the oxygen content of the starting pellets. By this procedure, almost fully deoxygenated samples (z ~ 0) could be obtained (see Fig. 1). The phase identification and lattice parameters were characterized by X-ray diffraction. From the X-ray diffraction pattern, it was confirmed that the crystal structure of samples of CaxYBCO6 (0 < x <0.31) prepared in this study was tetragonal as reported previously [11-13]. Final Y and Ca contents in samples were determined by inductively coupled plasma analysis (ICP) and confirmed to be almost consistent with the nominal Ca content. The oxygen content was determined by the iodometric titration technique. All samples, except for the x = 0.05 and 0.10 samples, were fully deoxygenated and the oxygen content was kept at six (z = 0) within our resolution (3z < 0.01). For the x = 0.05 and 0.10 samples, the removal of the oxygen is slightly incomplete and the oxygen content was 6.06 ( x = 0.05) and 6.05 (x = 0.10). In the region of small Ca content, however, such a slight excess of oxygen did not seriously affect the Hall coefficient because the

T. Watanabe et aL / Physica C 252 (1995) 100-106

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Hall coefficient of the sample with excess oxygen content z = 6.08 gave almost the same magnitude as that of the sample with z = 6.06. The electrical resistivity was measured using a conventional DC four-probe method down to 4.2 K. The Hall effect measurements were made in a field of 1.1 T at room temperature and at 77 K. For the measurements of the resistivity and the Hall effect, good electrodes were formed using a special solder and ultrasonic soldering iron (Asahi Glass Co., Cerasolzer #123).

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3. Results and discussion

o These series of samples were confirmed to be single-phase tetragonal structures, except for the x = 0.31 sample where small BaCuO z phases were included. The lattice constants at room temperature were plotted against the Ca content in Fig. 1, where the oxygen content was also shown. The observed change of the lattice parameters of a- and c-axis was

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very slight, and there was a tendency to decrease in the a-axis and to increase in the c-axis with increasing Ca content. The dispersion of values in the small Ca content region (x = 0.05, 0.10) is attributed to an oxygen content greater than six, because small oxygen doping has a strong effect on the lattice parameters, as shown later (see Fig. 5). The slight increase of c-axis with increasing Ca content can be interpreted by the replacing by Ca 2+ ( r = 0.99 ,~) of y3+ ( r = 0.93 A). The change of the lattice parameter of the c-axis by substituting Ca was extremely small compared to that with doping oxygen in the sample. This implies that the interlayer coupling or the electronic structure is slightly influenced by Ca substitution. In Fig. 2, the resistivity is shown in order to see the gross features of the successive change of the CaxYBCO6. As shown in the figure, the conduction of samples with Ca substituted for Y is semiconducting below x = 0.18, and the value of the resistivity decreases with increasing Ca content. Above x = 0.20, a superconducting transition was observed, although the conduction of samples indicates a weak semiconducting behaviour in the normal state. The increase of the resistivity is mainly caused by an increase in the localization of conductive carriers with decreasing temperature. As discussed later, the carrier concentration at 77 K de-

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duced from the Hall coefficient is smaller than that at room temperature (see Fig. 4). Tc's increase with increasing Ca content and indicate a peak around Ca content x ~ 0.26, and decrease at higher Ca content. These results are shown in Fig. 3. Previously, Casalta et al. reported that C a , Y B C O 6 is not a superconductor for x = 0.175 and is a superconductor for x = 0.2 and x = 0.225 with T~'s (midpoint of the transition) of about 15 and 30 K, respectively [10]. Both this trend and the magnitude of T~ are fairly consistent with our results. Further, at this time, we obtained new results of a maximum of T~ at x ~ 0.26. This superconducting phase diagram is similar to the case of La2_,SrxCuO 4 [16,17]. The carder concentration estimated from the relation 1/(qRn) (q :electronic charge, R H : Hall coefficient) is shown in Fig. 4 as a function of carrier number for a CuO 2 plane. Several interesting features are evident in this figure. The carrier numbers at room temperature first increase almost linearly with increasing Ca content in the range between x ~ 0.05 and x ~ 0.2, where samples were not superconductors, and increase suddenly at about x = 0.2 and again increase linearly in the range between x ~ 0.2 and x ~ 0.26, where samples became superconductors. This behaviour o f the carrier concentration is similar to the case of

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La2_xSrxCuO 4 [17], though the carrier number is very different from that of the La system. A sudden decrease of the carrier concentration was observed at x = 0.31, where the sample still showed superconductivity, though the Tc fell (see Fig. 3). The Hall number at 77 K also rapidly increases between x = 0.18 and 0.20, similar to that at room temperature. But the magnitude of the Hall number at 77 K is smaller than that at room temperature, as seen in Fig. 4. This indicates that more conductive holes are localized with decreasing temperature. The mechanism for this localization is not clear at present, but the localization may occur in the CuO 2 plane and may be suppressed by oxidation of the sample, as discussed later. If we assume that one Ca substituted for Y generates one hole carrier in the sample, the Hall number is considerably smaller than that estimated, which is shown with a dotted line in Fig. 4, in CaxYBCO6. While in the case of La2_xSrxfuO 4 the carder number is larger than that estimated in the same manner [17]. Such a small carder number of mobile holes in CaxYBCO 6 implies that a trap or localization occurs in a part of the carriers doped by Ca substitution. In that case, it is plausible that a carder localization occurs at Cu(1) sites. The existence of Cu(1) sites in the structure is characteristic in YBCO material and is clearly different from the La system. We can estimate that about 40% o f the carder doped by Ca substitution is conductive in 0 < x < 0.2 and x = 0.31, and about 80% is conductive at room temperature in 0.20 < x < 0.31 where the superconductivity occurs. The Hall number corresponding to these mobile holes is thought to be equal to the p value, when excess conductive carriers of the CuO 2 plane was expressed by [CuO] p [18]. It is plausible that the remaining carrier (which is not conductive) localizes at Cu(1) sites of CuO chain, that is, the valence of Cu is larger than + 1. From this perspective, we estimated the mean valence of Cu ions, where it is considered that the Hall number is equal to the mean valence of the Cu(2) ion of the CuO 2 plane and the valence of the Cu(1) ion of the CuO chain is deduced from the localized Hall number. The mean valence of the Cu ion in CaxYBCO 6 was calculated using {(2Cu(2)+ Cu(1))/3}. These estimated values of the mean valence of the Cu ion were compared with those which could be measured by iodometric titration, and are shown in Table 1. As

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T. Watanabe et aL / Physica C 252 (1995) 100-106

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shown in the Table, the estimated values are consistent with the experimental ones below x = 0.26. Considerable discrepancy between the two values was seen for the x = 0.31 sample. This may be caused by the structure defects which are related to the existence of the BaCuO 2 phase, possibly affecting the Hall effect. In this sense, the decreasing of the carrier concentration in the x = 0.31 sample is indistinct at the present stage. Next, we will mention the effect of oxidation on the superconducting properties. The change of lattice parameters of both a- and c-axis of the x = 0.23 sample is shown in Fig. 5. The dependence of these lattice parameters on the oxygen content is qualitatively the same as that on the Ca content, but the magnitude of change of the c-axis due to the oxidation is large compared with that due to the substitution of Ca, as shown in Figs. 1 and 5. This behaviour

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of the lattice parameters with the oxidation for Casubstituted samples is almost the same as for pure YBCO6+ z [6]. The temperature dependence of the resistivity, Te's deduced from the resistivity and the carrier concentration at room temperature are shown in Figs. 6 and 7. As shown in these figures, the Tc is increased drastically by oxidizing. This phenomenon indicates an important role of oxygen in the CuO chain for the superconducting characteristics of YBCO. The carrier concentration at room temperature, however, does not change drastically by increasing the oxygen in this range of oxygen content. But it is worth noting that, as shown in Fig. 6, the temperature dependence of the resistivity became

Table 1 Comparison for both values of the mean valence of Cu ion which were deduced from iodometric titration and the Hall coefficient measurements in Y1- xCaxBa2Cu306,respectively, x Ca content; Ca(2) valence: estimated value of Cu ion at CuO2 plane from the Hall number. Cu(1) valence: estimated value of the Cu ion at the CuO chain from the localized hole number,with the valence of Cu(1) assumed to be + 1 in YBCO6. Mean valence(l): calculated value from Ca(2) and Ca(l) valences (see text); mean valence(2): deduced value from iodometric titration x

Cu(2) valence

Cu(1) valence

Mean valence(1)

Mean valence(2)

0.20 0.23 0.25 0.26 0.31

2.08 2.09 2.10 2.11 2.07

1.04 1.05 1.05 1.04 1.17

1.74 1.74 1.75 1.75 1.77

1.73 1.73 1.74 1.74 1.74

T. Watanabe et al. / Physica C 252 (1995) I00-106 12

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larity to that of La2_xSrxCuO4 [16,17]. The conductive carriers in the C u t 2 plane increased remarkably in the region of Ca content where superconductivity occurs. A part of carrier doped by the substitution of Ca, however, was localized at Cu sites in the C u t chain. This is a special characteristic in YBCO and differs greatly from the La system. Slight oxidation for these samples brought about the release of carrier localization at low temperatures and a remarkable rise of T~. These results represent the unique characteristics and the crucial role of the C u t chain or orthorhombicity for the high T~ in YBCO.

Z Fig. 7. Variation of T~ and carrier concentration with oxidation in Y0.77 Ca0.23 Ba2 Cu 306 + z •

metallic with increasing oxygen content. This means that the carrier localization was suppressed at low temperatures due to the formation of C u t chains which maintain the necessary carrier concentration in the C u t 2 plane required for superconductivity. This may be the reason for the rise in T¢ in oxygen-incorporated CaxYBCO 6 superconductors. It may be doubtful that the small fraction of oxygen establishes ordered C u t chains. We, however, think that the ordered C u t chain exists on a local site and this degree of chain-order causes a transfer of carrier to the C u t 2 planes. In fact, an important chain ordering for the superconductivity in YBCO z was shown by several anthers [3,19,20]. Jorgensen et al. also proposed that the degree of the chain ordering or orthorhombicity, if viewed on a local scale, may have a large effect on the charge balance [3]. Our results also indicate that the C u t chain has a vitally important role for the high T~ in YBCO.

4. Conclusion

The charge-transfer model for superconductivity in Yl_xCaxBa2Cu306 (Ca,YBCO 6) and slightly oxygen-doped Y1 - xCax Ba2Cu 306 + z (Ca~YBCO6+ z) was tested. The phase diagram of Ca~YBCO 6 for superconductivity has the same feature as that previously found by Casalta et al. [10], and this time it was found that T~'s decrease in the heavily Ca-substituted region. This phase diagram has a great simi-

Acknowledgements

We wish to thank M. Ogino for assistance in the experimental work. We also wish to thank H. Ihara and K. Tokiwa (ETL) for assistance with the ICP experiment and discussions.

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