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Effects of Cr, Ta and Pb Substitutions on Phase Formation and Superconductivity of Tl1212 Ceramics

A.K. Yahya, M.I. Yusof, B. Musa, W.F. Abdullah, Z. Zamri and M.H. Jumali
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Effects of Cr, Ta and Pb substitutions for Sr/Tl on phase formation and superconductivity of Tl0.8 Bi0.2 Sr2-x CrxC a0.9Y0.1Cu2O7, Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 and Tl1-zPbzSr1.8Sb0.2CaCu2O7 ceramics were investigated. Powder X-ray diffraction patterns for the three series showed major 1212 phase with 1201 phase as the minor phase. Electrical (dc) resistance measurements showed that the elemental substitutions at Sr site for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7, (x = 0-0.3) and Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.3) series induced metal-insulator transition in normal state behavior accompanied by deterioration of superconductivity. On the other hand, Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0.1-0.6) series showed metallic normal state behaviors and enhancement in Tc zero from 26 K (z = 0.1) to a maximum of 80 K (z = 0.5). Results of structural investigation and phase formation using powder X-ray diffraction are reported and effects of Cr, Ta and Pb substitutions on superconductivity of Tl1212 are discussed in terms of ionic radius of substituting elements and the concept of average Cu valence.

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A.K. Yahya, M.I. Yusof, B. Musa, W.F. Abdullah, Z. Zamri and M.H. Jumali, 2008. Effects of Cr, Ta and Pb Substitutions on Phase Formation and Superconductivity of Tl1212 Ceramics. Journal of Applied Sciences, 8: 1007-1013.

DOI: 10.3923/jas.2008.1007.1013



Tl-based high temperature superconducting oxides with superconducting transition temperatures (Tc) around 100 K was discovered in the Tl-Ba-Ca-Cu-O system (Sheng and Hermann, 1988). Later, Ba-free Tl-Sr-Ca-Cu-O system was also discovered (Sheng et al., 1988). The superconducting phase in the Tl-Sr-Ca-Cu-O system such as TlSr2CaCu2O7 was reported to be superconducting at 70-80 K but was not easily synthesized in pure form (Martin et al., 1989). The difficulty was suggested to be due to lattice instability of the Tl1212 structure and its highly overdoped state with a calculated average Cu valence of 2.5+ (Sheng et al., 1991a-c). Later studies however, have shown that substituting an atomic site by a higher valence ion, for example, Ca2+ by RE3+ (RE = rare earth) can stabilize the 1212 phase and improve the superconducting behavior (Sheng et al., 1989). Tc of over 100 K has been produced by simultaneous substitution of Pb or Bi for Tl and RE for Ca in the (Tl, Pb) Sr2 (Ca, RE) Cu2O7 and (Tl, Bi) Sr2 (Ca, RE) Cu2O7 systems, respectively (Liu et al., 1989). Cr-substituted (Tl, Bi) Sr2 (Ca, Cr) Cu2O7 has shown a Tc onset of above 100 K (Abd-Shukor and Tiew, 1999). Such studies, among others, have shown that the optimum average Cu valence is between 2.20+ to 2.30+ for best superconducting behavior for Tl1212 (Abd-Shukor and Arulsamy, 2000; Hamid et al., 2004).

Besides the Ca site, the 1212 phase can also be stabilized by substitution at the Sr site. Partial substitution of (M = Sm, Eu, Dy, Ce and Nd) in Tl (Sr2-x Mx) CaCu2O7 was reported to stabilize 1212 phase and improve superconductivity (Lee and Wang, 1995; Lee and Huang, 1997; Abd-Shukor and Arulsamy, 2000). The superconducting properties of double substituted (Tl, Pb) Sr2-x Ybx CaCu2O7 was also reported to improve as the Tc value increases from 70 K (x = 0) to 105 K (x = 0.2) (Yahya et al., 2007). Triple substitution involving Ti at Sr-site in Tl0.8Pb0.2Sr2-xTixCa0.9Y0.1Cu2O7 series produces maximum Tc of 62 K at x = 0.2 (Yahya et al., 2003). The effects of Cr and V substitutions for Sr in the (Tl, Bi) Sr2 (Ca, Y) Cu2O7 ceramics have also shown improvement in the Tc and 1212:1201 phase ratio (Hamid et al., 2004). However, most of the previous works reported on the Tl1212 compound are limited to single and double substitution in the starting composition as the effects of simultaneous triple substitution are relatively more complex to analyze and formation of 1212 dominant samples are generally more difficult.

In this study, we report the effects of Cr, Ta and Pb substitutions at Sr and Tl sites on phase formation and superconductivity of 1212 dominant samples from Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7, Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 and Tl1-zPbzSr1.8Sb0.2CaCu2O7 starting compositions, respectively. Bi and Y were partially substituted for Tl and Ca, respectively, to stabilize and promote 1212 phase formation (Liu et al., 1989). The results are discussed according to ionic radius of substituting elements and the concept of average Cu valence. XRD diffraction analyses of the series are also presented.


All samples were prepared from high purity oxides (>=99.99%) of SrCO3, Cr2O3 (or Ta2O5), Y2O3, CaO and CuO (or Cu2O) using solid-state reaction and precursor methods. The powders in stoichiometry of Sr2-x CrxCa0.9Y0.1Cu2O7 (x = 0-0.3) (or Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.3)) were mixed and ground using a mortar and pestle. The compound Sr1.8Sb0.2CaCu2O7 was also prepared with the same method using oxides of SrCO3, Sb2O3, CaO and CuO (or Cu2O). The powders were then heated at 900°C for over 24 h with several intermittent grindings. Appropriate amounts of Tl2O3 and Bi2O3 were completely mixed to the precursor to form mixtures with nominal compositions Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 (x = 0-0.3), Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.3) and Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0-0.6). The powders were ground and pressed into a pellet 13 mm in diameter and 2 mm in thickness under a pressure of 4500-5000 kg cm-2. The pellets were placed in an alumina boat and heated at around 1000°C for 5 min in flowing O2 followed by furnace cooling to room temperature.

X-ray diffraction (XRD) analyses were performed using Bruker Model D8 Advance diffractometer with Cu-Kα source. The volume fractions of 1212 and 1201 phases were estimated from the intensities of major phase, minor phase peaks and other peaks observed, as used by Driessche et al. (1996) and Matsushita et al. (1994) namely, the Eq. 1 and 2 below;




I=Peak intensity of the present phase.

Lattice parameters of the 1212 unit cell were calculated from powder X-ray diffraction patterns by the method of least square fitting using a minimum of 15 indexed diffraction peaks. A standard four-point-probe method was used for electrical resistance (dc) measurements between 16 and 300 K. The electrical contacts to the sample were made by fine aluminum wires with a conductive silver paint; the applied current was 30 mA. The temperature was recorded using a calibrated TG-120P GaAlAs diode sensor located close to the sample. A CTI Cryogenics Closed Cycle Refrigerator Model 22 and Lake Shore Temperature Controller Model 340 were used for temperature-dependent measurements.


Powder X-ray diffraction patterns for Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7 (x = 0-0.3), Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0 - 0.3) and Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0.1-0.6) series revealed formation of dominant 1212 phase (>80 vol. %) with minor 1201 phase. The 1212 peaks for each of the observed patterns can be all indexed based on a tetragonal unit cell with space group, P4/mmm. Reflection peaks due to the 1201 phase are indicated in asterisks. X-ray diffraction (XRD) patterns for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 (x = 0 and 0.1), Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0 and 0.1) and Tl1-z PbzSr1.8Sb0.2CaCu2O7 (z = 0.2 and 0.3) are shown in the Fig. 1-3, respectively.

Table 1 shows slight decrease in 1212 volume percent decreasing with increasing value of x. The normalized resistance versus temperature curves of Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7 (x = 0-0.3) are shown in Fig. 4. Tl0.8Bi0.2Sr2Ca0.9Y0.1Cu2O7 showed metallic normal state behavior and superconducts with Tc onset and Tc zero of 72 and 52 K, respectively. Substitution of Cr (x = 0.1) for Sr caused the normal state behavior to change from metallic to semi-metallic accompanied by a decrease in Tc onset and Tc zero to 70 and 44 K, respectively. Further substitution of Cr (x = 0.2 and 0.3) produced insulating samples.

The 1212:1201 phase ratio and calculated 1212 lattice parameters are shown in Table 2. The volume fraction of 1212 phase showed little variation. Figure 5 shows the electrical resistance versus temperature curve for Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.4) series. Samples with y = 0 showed metallic-normal state behaviors with Tc zero of 63 K. Further substitution of Ta at y = 0.1 produced semiconductor-like normal state behavior accompanied by a drop in resistivity curve at around 90 K indicating a superconducting onset. For samples with y>0.1, insulating behavior was observed.

Table 1: Tc onset, Tc zero, 1212:1201 phase ratio and 1212 lattice parameters for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 series

Table 2: Tc onset, Tc zero, 212:1201 phase ratio and 1212 lattice parameters for Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 samples

Fig. 1: Powder X-ray diffraction patterns for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 (x = 0 and 0.1) showing major 1212 phase. Peaks due to 1201 phase are indicated by (*)

Fig. 2: Powder XRD patterns for Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0 and y = 0.1) showing dominant 1212 phase. Peaks due to 1201 phase are indicated by (*)

Fig. 3:
Powder X-ray diffraction patterns for Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0.2-0.3). Peaks due to 1201 phase are indicated by (*)

Table 3 shows the results of 1212:1201 phase ratio for Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0-0.6). The sample with z = 0 consists of the 1201 phase along with an unidentified phase. Substitution of Pb at z = 0.1 promotes formation of 1212 as the major phase alongside the 1201 minor phase. Further substitution of Pb at z = 0.2 caused an increase in the 1212:1201 phase ratio. For z>0.2 the 1212 phase gradually decreased.

Electrical (dc) resistance measurements on Tl1-z PbzSr1.8Sb0.2CaCu2O7 (z = 0.1-0.6) series showed metallic normal state behaviors and an increase in Tc zero from 26 K (z = 0.1) to a maximum of 80 K (z = 0.5) (Fig. 6). Further substitution of Pb (z = 0.6) caused Tc zero to decrease to 27 K. For comparison, Sb-free sample Tl0.5Pb0.5Sr2CaCu2O7 was prepared and four point probe measurement showed a lower Tc zero (62 K) and Tc onset (70 K) compared to Tl0.5Pb0.5Sr1.8Sb0.2CaCu2O7. This indicates that, in addition to Pb, substitution of Sb at the Sr-site affects the superconductivity of the material.

Table 3:
Tc onset, Tc zero, 1212:1201 phase ratio and 1212 lattice parameters for Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0-0.6) samples

Fig. 4:
Normalized resistance versus temperature curves for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 (x = 0 - 0.3)

Fig. 5:
Normalized resistance versus temperature curve for Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.3)

Fig. 6:
Normalized resistance versus temperature curve for Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0.1-0.6) series

The decrease in c-lattice with x in Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7 series and with y in Tl0.9Bi0.1Sr2-y TayCa0.9Y0.1Cu2O7 series may be due to the ionic radius effect. For Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 series, a smaller Cr ion (possible valence and ionic radius: Cr3+-(0.76 Å), Cr4+-(0.69 Å), Cr5+-(0.63 Å), Cr6+-(0.58 Å)) was substituted for a much larger Sr2+ ion (1.18 Å). Similarly, for Tl0.9Bi0.1Sr2-y TayCa0.9Y0.1Cu2O7 series, Ta ion which also has a smaller radius (possible valence and ionic radius: Ta3+-(0.86 Å), Ta4+-(0.82 Å), Ta5+-(0.78 Å)) could have produced the same effect. The decrease in c-lattice agrees well with previous reports (Sheng et al., 1991a-c; Liu et al., 1993). The increase in a-lattice with increasing Cr and Ta substitution for Tl0.8Bi0.2Sr2-xCrxCa0.9Y0.1Cu2O7 and Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 series may be explained in terms of the concept of average Cu valence (Yahya et al., 2007). Since a-lattice depends mainly on CuO plane, substitution of a higher valence Cr/Ta ion for Sr2+ decreases the average Cu valence and caused conversion from Cu3+ to larger Cu2+. Since a-lattice depends mainly on CuO plane, the conversion resulted in an increase of the Cu-O distance causing the a-lattice to expand. The same reasoning can be used to explain the a-lattice expansion observed for Tl1-zPbzSr1.8Sb0.2CaCu2O7 series where Pb4+ substitutes for Tl3+.

Our electrical (dc) resistance measurements results on the three series showed that only the Tl1-z Pbz Sr1.8 Sb0.2 CaCu2O7 series can be successfully optimized through elemental substitution. The Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7 (x = 0) and Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0) samples may carry an average Cu valence which is already lower than the optimum value for maximum Tc. Further substitution at the Sr site with Cr/Ta only caused further reduction of average Cu valence and resulted in a decrease in Tc. For the Tl1-zPbzSr1.8Sb0.2CaCu2O7 series, the best superconducting behavior was observed at z = 0.5 and as such, the ideal stoichiometry of the optimally doped sample is Tl0.5Pb0.5Sr1.8Sb0.2CaCu2O7. Based on charge neutrality requirement, simple valence calculations can be performed on optimally doped superconducting sample if ideal stoichiometry is assumed. Although the sample also consists of minor 1201 phase in addition to the dominant 1212 phase, it can be safely assumed that based on the observed high Tc onset value of 86 K (Table 3) and the fact that Tc onset for 1201 phase was reported to be below 50 K (Sheng et al., 1991a-c), the 1212 phase was solely responsible for the observed superconductivity of the sample. So assuming that only copper is multivalent and the valence states of Pb is 4+, as suggested by Haldar et al. (1988), Tl is 3+, as suggested by Suzuki et al. (1989) and Sb is 2+, the optimum Cu valence for Tl0.5Pb0.5Sr1.8Sb0.2CaCu2O7 is computed as 2.2+ which is within the suggested optimum average Cu valence range of between 2.2+ and 2.3+ reported for Tl1212 (Sheng et al., 1991a-c).


As a conclusion, 1212 phase dominant samples were successfully synthesized with from Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7 (x = 0-0.3), Tl0.9Bi0.1Sr2-yTayCa0.9Y0.1Cu2O7 (y = 0-0.3) and Tl1-zPbzSr1.8Sb0.2CaCu2O7 (z = 0.1-0.6) starting compositions. Electrical (dc) resistance measurements results showed that only Tc of Pb-substituted Tl1-zPbzSr1.8Sb0.2CaCu2O7 series can be successfully optimized to maximize Tc. Both Tl0.8Bi0.2Sr2-x CrxCa0.9Y0.1Cu2O7, (x = 0-0.3) and Tl0.9Bi0.1Sr2-y TayCa0.9Y0.1Cu2O7 (y = 0-0.3) series showed metal-insulator transition in their normal state behaviors accompanied by deterioration of Tc. Simple valence calculation based on charge neutrality requirement and the concept of average Cu valence on the optimized Tl0.5Pb0.5Sr1.8Sb0.2CaCu2O7 sample suggests the valences of Pb, Tl and Sb as 4+, 3+ and 2+, respectively.


A.K. Yahya would like to acknowledge Academy of Sciences Malaysia and MOSTI for the SAGA research grant.

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