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Solid State Sciences 14 (2012) 1458e1461

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Solid State Sciences

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Normal pressure synthesis of (Tl1�yCy)Ba2Ca3Cu4O12�d superconductor

Nawazish A. Khan a, Shakeel Ahmad a, A.A. Khurram b,*, Maqsood Ahmad c

aMaterials Science Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad, Pakistanb Experimental Physics Labs, National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistanc Physics Division PINSTECH, P.O Nilore, Islamabad, Pakistan

a r t i c l e i n f o

Article history:Received 18 December 2011Received in revised form9 June 2012Accepted 29 August 2012Available online 6 September 2012

Keywords:SuperconductorElectron microscopyElectrical properties

* Corresponding author. Tel.: þ92 51 2896085; fax:E-mail address: khuram_qau@yahoo.com (A.A. Kh

1293-2558/$ e see front matter � 2012 Elsevier Mashttp://dx.doi.org/10.1016/j.solidstatesciences.2012.08.0

a b s t r a c t

Single phase (Tl1�yCy)Ba2Ca3Cu4O12�d (Tl1�yCy-1234) (y ¼ 0, 0.25, 0.5 and 0.75) superconductor sampleshave been prepared by solid state reaction method. The FTIR absorption measurements have confirmedthe substitution of carbon at thallium site in the charge reservoir layer, (Tl1�yCy)Ba2O4�d. The electronmicrographs of these samples have shown that the carbon substitution has improved the grainmorphology of Tl0.75C0.25-1234 sample. The y ¼ 0.25 was found to be the optimum carbon concentrationto achieve higher superconducting transition temperature Tc[0] and improved grain morphology. Thesuperconducting transition temperature of Tl0.75C0.25-1234 sample has been increased to 100 K whereasa decrease in the superconducting transition temperature of Tl1�yCy-1234 (y ¼ 0.5 and 0.75) samples wasobserved. However, the magnitude of diamagnetism has been decreased in all the carbon substitutedsamples.

� 2012 Elsevier Masson SAS. All rights reserved.

1. Introduction

Thallium based high Tc superconductors which werediscovered in 1988 [1] are specified with the general formulaTlmA2Can�1CunO2nþmþ2; where m ¼ 1 or 2, A: Ba or Sr and n(number of CuO2 planes) ¼ 1,2,3,4,5. The superconductors withsingle TleO layer are the promising materials among the family ofthallium based cuprates due to their high irreversibility field andcritical current density [2e4]. The superconducting transitiontemperature Tc of Tl1A2Can�1CunO2n�mþ2 (Tl-12(n � 1)n) super-conductors is increased monotonically up to n¼ 3 and then there isa decreases for n > 3 [5]. In spite of their superior superconductingproperties the Tl1A2Can�1CunO2nþmþ2 superconductors suffer fromthe issues of phase purity. Various research groups have tried toprepare phase pure Tl-1223 and Tl-1234 superconductors by thepartial replacement of Tl by Bi, and Pb and partial replacement of Baby Sr in the charge reservoir layer [6e8]. Some of the groups havechosen the high pressure synthesis technique to attain phase pureTl-1223 and Tl-1234 superconductors [5,9,10]. The high pressuresynthesis technique is however, not suitable for mass production ofthese materials. In the present article we have prepared almostphase pure Tl1Ba2Ca3Cu4O12�d superconductor using the normalpressure synthesis method. We have also substituted carbon in the

þ92 51 2896084.urram).

son SAS. All rights reserved.23

charge reservoir layer Tl1�yCyBa2O4�d (y ¼ 0, 0.25, 0.5, 0.75) atthallium site. The objectives of the substitution of carbon in thecharge reservoir layer were to use its higher electronegativity(2.5 pauling’s) to control the flow of electrons from the chargereservoir to the CuO2 planes, which will help to optimize the holedensity in CuO2 planes. Moreover, the smaller size of carbon ascompared to thallium would decrease the thickness of theTl1�yCyBa2O4�d charge reservoir layer which can enhance thecoupling between the superconducting blocks (CaCuO2)n [11,12].

2. Experimental

The (Tl1�yCy)Ba2Ca3Cu4O12�d (y ¼ 0, 0.25, 0.5 and 0.75) super-conductor samples were prepared by two stage solid-state reactionmethod. At the first stage CyBa2Ca3Cu4O12�d (y ¼ 0, 0.25, 0.5 and0.75) precursor material was prepared by using Ba(NO3)2, Ca(N-O3)2,Cu2(CN)2 and carbon powder as starting compounds. Thesecompounds were thoroughly mixed in appropriate ratios ina quartz mortar and pestle for about 1 h and fired in a quartz boat at860 �C for 24 h in air, followed by furnace cooling to roomtemperature. At the second stage Tl2O3, was mixed with precursormaterial in appropriate ratio to get (Tl1�yCy)Ba2Ca3Cu4O12�d asa final composition. The pellets of thallium mixed precursormaterial were prepared under 3.8 tons/cm2 pressure. The pelletswere wrapped in a gold capsule and heat treated at 880 �C for10 min followed by quenching to room temperature after the heattreatment. The crystal structure of the material and its lattice

Fig. 1. X-ray diffraction pattern of Tl1�yCy-1234 (y ¼ 0, 0.25 and 0.5) superconductor.

Fig. 2. FTIR absorption spectra of Tl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and 0.75)superconductor.

N.A. Khan et al. / Solid State Sciences 14 (2012) 1458e1461 1459

parameters were determined by X-ray diffraction (XRD) measure-ments using STOE, Germany diffractometer. The AC-magneticsusceptibility measurements were performed using a lock-inamplifier at lock-in frequency of 270 Hz. The four-probe tech-nique was used to measure the resistivity of these samples, whichwere rectangular in shape (13 mm � 4 mm). The grain morphologyof the sample was studied from the scanning electron micrographs.The FTIR absorption spectra were taken using the spectrometerfrom Thermoelectron Corporation.

3. Results and discussion

The representative X-ray diffraction (XRD) pattern of (Tl1�yCy)Ba2Ca3Cu4O12�d (Tl1�yCy-1234) (y ¼ 0, 0.25 and 0.5) superconduc-tors are shown in Fig. 1. The diffraction lines were fitted very well tothe tetragonal structure with P4/mmm symmetry using the checkcell program. The XRD pattern shows almost phase pure Tl1�xCx-1234 sample with very small inclusion of Tl-1223 and someunknown impurity phases. The lattice parameters and volume ofthe unit cell calculated from the fitting of the XRD pattern ofTl1�yCy-1234 superconductors are shown in Table 1. It can be seen

Table 1Lattice parameters and volume of the unit cell of the tetragonal structure of Tl1�yCy-1234 (y ¼ 0, 0.25, and 0.5) superconductor.

y a-axis (�A) c-axis (�A) Volume of unit cell, Vc (�A3)

0 3.87 17.90 268.090.25 3.84 17.93 264.390.5 3.82 17.94 261.03

from this table that the volume of the unit cell has been decreasedas expected with the increase of the carbon substitution in thecharge reservoir layer (Tl1�yCy)Ba2O4�d. It was also observed thatthere was a decrease in the a-axis parameter, whereas the c-axislattice parameter was increased. Carbon has smaller ionic radius ascompared to that of thallium and since carbon is partially replacingthallium in the charge reservoir layer, therefore, there is a possi-bility that either a-axis length or c-axis would be decreased. Thecomplete replacement of thallium by carbon would have beenresulted in the decrease of both a and c-axis. The FTIR absorptionmeasurements of Tl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and 0.75) samplesare shown in Fig. 2. We have concentrated on the oxygen relatedvibration modes in the wavenumber range 400e650 cm�1. Inthallium based superconductors the absorption modes around500 cm�1 are related to the vibrations of apical oxygen atomsTl(1)eO(2)eCu(2). Whereas the absorption mode around 580 cm�1

is attributed to the vibrations of planar oxygen atoms [13,14]. In thecase of Tl1�yCy-1234 superconductor there are two apical oxygenabsorption modes appeared around 505 cm�1 and 536 cm�1

whereas the planar oxygen mode is appeared at 586 cm�1. It can beseen from these FTIR absorption spectra that the absorption mode

100 150 200 250 3000.00

0.08

0.16

0.24

0.00 0.25 0.50 0.75

0.0

0.1

0.2

0.3

0.4

0.5

94

95

96

97

98

99

100

T(0)K

(-cm

)

Carbon content

400ρ

ρ(Ω

−cm

)

T (K)

y=0y=0.25 y=0.5 y=0.75

ρ/2

Fig. 3. Resistivity measurements of Tl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and 0.75) super-conductor. Inset: room temperature resistivity, and Tc(0) versus carbon content inTl1�yCy-1234 superconductor.

75 80 85 90 95 100 105

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

χ(em

u/g

m.O

e)

T (K)

y=0y=0.25 y=0.5 y=0.75

Fig. 5. AC magnetic susceptibility measurements of Tl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and0.75) superconductor.

N.A. Khan et al. / Solid State Sciences 14 (2012) 1458e14611460

appeared at 505 cm�1 and 586 cm�1 are un-effected after thesubstitution of carbon, whereas the absorption mode at 536 cm�1

has been hardened with the increase of carbon in the chargereservoir layer. We can therefore, assign 536 cm�1 mode to theC(1)eO(2)eCu(2) vibrations and 505 cm�1 to the Tl(1)eO(2)eCu(2)vibrations. The C(1)eO(2)eCu(2) apical oxygen mode has beenappeared at higher wavenumber side due to lighter atoms attachedas compared toTl(1)eO(2)eCu(2)mode.We can infer from the XRDand FTIR absorptionmeasurements that carbon is being substitutedat thallium site in the charge reservoir layer. Moreover, from theFTIR absorption spectra in the higher wavenumber side (notincluded), we did not observe any CO2 related phononmode, whichruled out the formation of CO2. The resistivity measurements of

Fig. 4. Scanning electron micrographs of Tl1�yCy-12

Tl1�yCy-1234 (y¼ 0, 0.25, 0.5 and 0.75) superconductor samples areshown in Fig. 3. The superconducting transition temperature [Tc(0)]of Tl0.75C0.25-1234 sample has been increased along with a signifi-cant decrease in normal state resistivity. Whereas a slight decreasein Tc(0) along with an increase in the normal state resistivity of thesamples with x¼ 0.5 and 1.0 (inset Fig. 3) was observed. The Tc(0) ofTl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and 0.75) samples are 95, 100, 94, and94 K respectively. The observed superconducting transitiontemperatures of Tl1�yCy-1234 are lower than that of earlier studiesin which higher Tc,s were observed. In sintered high Tc supercon-ductors the superconducting transition temperature also dependson the synthesis route followed [6e10]. The superconductingtransition temperature greater than 120 K were obtained using thehigh pressure synthesis techniques. The electron micrographs of

34 (y ¼ 0, 0.25, 0.5 and 0.75) superconductor.

N.A. Khan et al. / Solid State Sciences 14 (2012) 1458e1461 1461

Tl1�yCy-1234 (y ¼ 0, 0.25, 0.5 and 0.75) samples are shown in Fig. 4.It can be seen from these images that in Tl-1234 superconductorssome of the grains are bar shaped while some are plate like.Moreover, the grains are not fully connected and few voids caneasily be seen. However, after the substitution of carbon (y ¼ 0.25)the grains have been diffused together giving enhanced connec-tivity, but with further increase of carbon (y ¼ 0.5 and 0.75) thegrain morphology have become poor as larger voids are visiblefrom those micrographs. There are two possibilities of the increasein the Tc(0) of Tl0.75C0.25-1234, one is the improved grain connec-tivity and the second one is the optimization of carrier concentra-tion in CuO2 planes after the substitution of carbon. On the otherhand one can see that Tc(0) of Tl1�yCy-1234 (y ¼ 0.5 and 0.75)samples has been decreased to that of y ¼ 0 sample, which showsthat although carbon optimizes the carrier concentration in CuO2planes but its higher concentration in the charge reservoir layerleads to poor grain morphology, which is a source of decrease in thesuperconducting transition temperature. In granular high Tcsamples the zero resistivity transition depends not on the carrierconcentration but also on the grain connectivity [15]. The ACmagnetic susceptibility measurements of Tl1�yCy-1234 (y ¼ 0, 0.25,0.5 and 0.75) samples are shown in Fig. 5. It is observed from thesemeasurements that the magnitude of diamagnetism has beensuppressed in all carbon substituted samples as compared to the Tl-1234 superconductor sample. The decrease in the magnitude ofdiamagnetism in Tl1�yCy-1234 is possibly due to the creation ofvoids and the decrease in the number of electrons in CuO2 planes,which would be responsible for the diamagnetism in thesesamples.

4. Conclusion

We have prepared Tl1�yCy-1234 superconductor samples atnormal pressure. The carbon substitution at thallium site in thecharge reservoir layer of Tl1�yCy-1234 superconductor has resultedin a decrease in the unit cell volume. The carbon substitution hasimproved the grain morphology as well as optimized the carrierconcentration in the CuO2 planes of Tl0.75C0.25-1234 supercon-ductor sample. Whereas, higher concentration of carbon in the

charge reservoir has decreased the transition temperature and aswell as resulted in poor grain morphology of Tl1�yCy-1234 (y ¼ 0.5and 0.75) samples. These results show that y ¼ 0.25 is the optimumconcentration of carbon to achieve phase pure Tl1�yCy-1234superconductor with higher Tc(0). Carbon in the charge reservoir ofTl1�yCy-1234 superconductor has the ability to optimize the carrierconcentration in CuO2 planes due to its higher electronegativityand multiple valence state but its higher concentration suppressesthe grain connectivity resulted in a decrease of the super-conducting transition temperature.

Acknowledgments

Following are acknowledged for their financial support1. Higher Education Commission of Pakistan through grant no.

20-1482-R&D/09-1472.2. International Centre for Theoretical Physics, Italy through

grant no. PRJ-27.

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