Formation of Tl-1212 Phase in CR Substituted (Tl1-xCrx) Sr2CaCu2O7 Superconductor

Superconducting samples of (Tl 1-xCr x) Sr 2CaCu 2O7 for x = 0.0-0.7 have been prepared by a solid stat e reaction technique at normal pressure using high pu rity elements. The powder X-ray diffraction pattern s have been studied for samples. All samples showed t he Tl-1212 as the major phase and Tl-1201 as the minor phase. The superconducting transition temperature Tc has been determined from the electrical resistivity measurements. The data of Tc, for (Tl 1-xCr x) Sr 2CaCu 2O7 show enhancement in its value from 104 K to112 K as x (Cr-content) increases from 0.7 to 0.1. Our results shows the highest onset tempera ture for x = 0.2-0.3 and the lowest temperature for x = 0.7.


INTRODUCTION
The Tl-based cuprate high temperature superconductor continues to be an interesting family of material because it can form many phases. It is an ideal system to study the role of metal oxide and copper oxide layers in high temperature superconductivity. The studies of high temperature materials have been necessary to develop conductors for practical and also the studies of the phase formation can be useful to find out the crucial role of the oxygen partial pressure during the sintering process. The TlBaCaCuO (Tl/Ba) and TlSrCaCuO (Tl/Sr) make up the Tl-based cuprate superconductor. The Tl/Ba system can be written as Tl 2 Ba 2 Ca n-1 Cu n O z with n = 1-4 and TlBa 2 Ca n-1 Cu n O z with n = 1-5. The Tl/Sr system can be written as TlSr 2 Ca n-1 Cu n O z with n = 1-3 (Lee, 2011). The three phases in the Tl/Sr system are TlSr 2 CuO 5 (Tl/Sr-1201), TlSr 2 CaCu 2 O 7 ,(Tl/Sr-1212), TlSr 2 Ca 2 Cu 3 O 10 (Tl/Sr-1223). But still not too many reports on Tl-1234 phase. There are some elements for raising the transition temperature the Tl-1212 phase. Substitution of rare earth cations into the C a2+ sites (Eder and Gritzner, 2005;Yip and Abd-Shukor, 2009) and Sr + sites (Eder and Gritzner, 2005;Lee and Huang, 1997) and partial replacement of Tl 3+ by either Pb 4+ (Subramanian et al., 1988), Bi 3+ (Li and Greenblatt, 1988) or Cr 3+ (Sheng et al., 1989;Li et al., 1995;Abd-Shukor and Jaafar, 1999) may assist the formation of Tl-1212 phase. Critical temperature around 100 K of the Bi-substituted sample in Tl-1212 (i.e., (Tl,Bi)Sr 2 CaCu 2 O 7 ) has been reported (Abd-Shukor and Tee, 1998), Pb-doped (Cu 0.5-x Pb x Tl 0.5 ) Ba 2 Ca 2 Cu 3 O 10-δ superconductor was reported with a Tc of 115 K (Nawazish and Ullah, 2010). The effect of rare earth elements was found to favor the formation of the Tl-1212 phase (Eder and Gritzner, 2005).
And several researches were studied on the effect of partial substitution of Ca with R (R = Y and most rareearths) in TlSr 2 C a Cu 2 O 7 (Kondo et al., 1991). Also transition temperature of TlSr 2 CaCu 2 O 7 phase was found to increase up to 110 K when Cr was substituted at different metal sites (Sheng et al., 1991). Therefore Tl-1234 superconducting phase was prepared by (Ihara et al., 1997) under the high pressure of 3.5 GPa at 850-9500°C for 2 h in a gold capsule (Iyo et al., 2001). But sintering technique in the high-pressure is not a suitable practical application.

PI
Later on (Nawazish and Mumtaz, 2007) were prepared Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4-y Zn y O 12-δ by using two step of solid state reaction method with ZnO, Ba(No 3 )2, Ca(No 3 ) 2 and Cu(CN) as starting compound. Recently, was studied effect of Mg nano-oxide addition on superconductivity of Cu 0.25 Tl 0.75 Ba 2 Ca 3 Cu 4 O 12-δ by using a single step solid state reaction method (Mohammed, 2012). The samples in this study were synthesized under normal pressure with Cr substitution at the Tl site.
It is interesting to studied the effect of Cr on the superconductivity of (Tl 1-x Cr x ) Sr 2 Ca 3 Cu 4 O 11 phase. Also Cr was substituted to Tl-1234 due to never any researcher was studied on (Tl 1-x Cr x ) Sr 2 Ca 3 Cu 4 O 11 phase. Although in this research was attempted to prepare a Tl-1234 phase from base materials, but the final samples are shown a Tl-1212 phase.
In this study superconductivity of samples by using a solid state reaction method was studied for x = 0.0-0.7.

Materials
Samples with nominal starting composition of (Tl 1- x Cr x ) Sr 2 Ca 3 Cu 4 O 11 with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 were mixed by the solid state reaction method using metal oxides powders. Appropriate amounts of high purity (>99.99%) CuO nano size, Sr 2 O 3 and CaO were mixed completely using an agate mortal to obtain a homogeneous mixture. The precursor powders were heated at 900°C for 24 h with several intermittent grindings. Appropriate amounts of Tl 2 O 3 and Cr 2 O 3 were then added to the precursor powders, completely mixed and then pressed into pellets form of 1.3 cm diameter and 0.2 cm thickness. These pellets were then wrapped in a silver foil to reduce possible volatilization of Tl. The wrapped materials were first heated at 870°C in the flowing oxygen for 1 hour, followed by furnace cooling to room temperature. In order to compensate thallium loss during heating, excess 10% of Tl 2 O 3 were added.

Characterization Methods and Instrumentation
The powder X-ray diffraction method using a Bruker D8 Advance diffractometer with Cukα source was used to identify the resultant phases. The volume fraction of superconducting phases was estimated by assuming that the amount of the phases are proportional to the strongest diffraction line of each phase, i.e., the (110) reflection of the 1212 and the (200) reflection of the 1201 phase.
The electrical resistance versus temperature measurements was carried out using the four-point method with silver paste contacts in conjunction with closed cycle refrigerator from CTI Cryogenic (Model 22) and a temperature controlled from Lake Shore (Model 330). A constant current source between 1 and 100mA was used throughout the measurements. The Tc onset is defined as the temperature where there is a sudden drop in the resistance.

RESULTS AND DISCUSSION
The room temperature Powder X-Ray Diffraction (PXRD) patterns for (Tl 1-x Cr x ) Sr 2 CaCu 2 O 7 superconducting samples (x = 0.0-0.7) are shown in Fig.  1 and 2. The diffraction lines of the patterns are well indexed by a tetragonal unit cell of Tl-1212 with space group P4/mmm. It is interesting to note that although for all samples were used from base materials to prepare Tl-1234 but formation of PXRD patterns shows the Tl-1212 as a major phase and Tl-1201 as a minor phase. It is due to Sr 2 O 3 in the precursor powder. For improving the formation of Tl-1234 in the (Tl 1-x Cr x ) Sr 2 Ca 3 Cu 4 O 11 samples should substituted Sr 2 O 3 by other elements such as BaO 2 (Mohammed et al., 2010).
The amounts of Tl-1212 phase are assumed to be proportional to the intensity of their strongest diffraction lines the (103, 110 and 111 reflection of the 1212 phase and the 200 reflection of the 1201 phase) contained in the samples.
The comparison between the PDXD patterns determined that intensity of samples was decreased with increasing of x. In addition the strongest diffraction lines of Tl-1212 phase in the PXRD were appeared sharply for x = 0.2-0.3.
The electrical resistance versus temperature curves of (Tl 1-x Cr x ) Sr 2 CaCu 2 O 7 samples for x = 0.0-0.3 are shown in Fig. 3 and for x = 0.4-0.7 are shown in Fig. 4. The highest Tc (onset and zero resistance) are determine for x = 0.2-0.3 at (112 K and 93 K) respectively and then Tc onset are decreased to 103 K for x = 0.7.
As shown in Fig. 3    The Tc (onset and zero resistance) of (Tl 1-x Cr x ) Sr 2 CaCu 2 O 7 samples are shown in Table 1.

CONCLUSION
In this present study, we have synthesized (Tl 1-x Cr x ) Sr 2 CaCu 2 O 7 phase from the starting oxide elements by a solid state reaction technique at normal pressure and studied the effect of Cr substitution on it. Also it was successfully to find out a high Tc for all samples. The superconducting transition temperature enhanced as x increased from 0.1-0.3 and then decreased when x increased from 0.4-0.7. The PXRD patterns indicated a Tl-1212 as a major phase and Tl-1201 as a minor phase. To improve the formation of (Tl 1-x Cr x ) Sr 2 CaCu 2 O 7 phase have to substitute Sr 2 O 3 by other elements such as BaO 2 to can obtain a (Tl 1-x Cr x ) Sr 2 Ca 3 Cu 4 O 11 phase.

ACKNOWLEDGMENT
We would like to thanks University Kebangsaan Malaysia, for provide all facility and materials for this research. This research was funded by Ministry of high educate under ERGS/1/2001/STG/ukm/01/25. The research wishes to thank Dr. Kamyar Shameli for his help and his support during this study.