Cr)1Sr2CuOy

ELSEVIER

Physica B 223&224 (1996) 580-583

X-ray structure, electrical resistivity, Hall effect and thermoelectric behavior of (Hg/Cr)lSr 2 CuOr B. Bandyopadhyay, J.B. Mandal, A. Poddar, P. Choudhury, B. Ghosh* Saha Institute of Nuclear Physics, I /AF Bidhannagar, Calcutta 700064, India

Abstract X-ray structural analysis of Hgo.7Cro.3Sr2fuO r (nominal composition) was done by Rietveld technique. The actual composition of the system and the position of the ions obtained from the analysis will be presented. The c-axis of this system is 0.9 A less than that of HgBa2CuO4 + 6. The Tc value obtained from resistivity measurement is ~ 51 K. The Hall angle cot On (p/BRH) shows a T 2 temperature dependence. Thermoelectric power is positive and large, indicating that the system is in the underdoped region.

1. Introduction

The results are discussed on the basis of the existing knowledge in this field.

Putlin et al. [1] first reported superconductivity in the HgBa2Cu206+6 (Hg-1201) system with T c ~ 9 4 K . Since then a number of new mercury-based superconductors HgBa2Ca2Cu306+6 (T c = 123 K) and HgBazCa2Cu308 + ~ (To = 133 K) have been synthesized [2, 31. The Ba-based systems are unstable in air, whereas the systems made by substitution of Ba by Sr and Hg by higher valence cations are stable in air. Different kinds of Sr-based Hg-1201 superconductors, e.g. (Hg, Cr)Sr2CuO r [4],

T~ = 58 K,

(Hg0.5, Bi0.5)Sr2-x LaxCuO5-6 [5], (Hgo.85, Moo.15)Sr/CuO4+6 I-6],

T~ = 27 K,

Tc = 78 K,

have been reported. We have prepared (Hg0.TCro.3) Sr2CuO4+6 (starting composition) sample and studied the structural property, electrical resistivity, Hall effect and thermoelectric power as a function of temperature.

* Corresponding author.

2. Experimental The samples were synthesized in a three-step process. First Sr(NO3)2 and Cu(NO3)2" 3H20 were dissolved in water in a stoichiometeric ratio of 2:1 and then heated until the system was free from N O / g a s . In the second step the precursor was made by mixing appropriate amounts of Cr203 with (Sr/CuO3 + 6) and fired at 900°C for 24 h. Finally HgO was added to the precursor, the mixture was pressed into pellets and sealed in an evacuated quartz tube. The sintering of the material was done at 880°C for 8 h and then quenched to room temperature. The samples were annealed at 300°C in 02 atmosphere. The powder X-ray diffraction pattern of the sample shows that Hg-1201 is the main phase with a few weak impurity lines of SrCuO2. The amount of SrCuO2 present in the sample was estimated using Rietveld refinement program (DBWS-9411). The oxygen content of the sample was changed by annealing it in H + Ar atmosphere for 2-6 h. The sample decomposes when the annealing temperature in above 350°C.

0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S092 1 - 4 5 2 6 ( 9 6 ) 0 0 1 80-9

B. Bandyopadhyay et al. /Physica B 223&224 (1996) 580-583 3. Results and discussions For Rietveld refinement we have used a tetragonal symmetry of space group P 4 / m m m and considered HgBa2CuO,÷~ structure as the starting model. First the positional parameters of the metallic atoms and oxygens were refined. In the second step, the B factors were considered. The reliability factors of the present refinement were better than those reported by Chmaissem et al. [7] and the refined values are given in Table 1. The weight percentage of SrCuO2 has been estimated to be 10%. Observed, calculated and difference diffraction patterns are shown in Fig. 1. The temperature dependence of resistivities for (Hg, Cr)-1201 (a) annealed at 300°C in oxygen atmosphere and (b) at 350°C in H + Ar atmosphere for 6 h are shown in Fig. 2. The Tc's are almost the same (51 K). In the HgBa2CuO4+~ system samples with different 3 are obtained by heating the as-synthesized samples at different temperatures and oxygen pressures for an appropriate period of time. Xiong et al. [8] found a parabolic variation of Tc with b over the entire range of superconductivity. However, in (Hg, Cr-1201) Tc remains the same. O n comparing the bond distance of the two samples, it is found that metal oxygen distances are less in (Hg, Cr)-1201. Therefore, one can argue that in HgBa2CuO,+~ it is easy to remove oxygen and hence one can change the carrier concentration and To, whereas in the Hgo.7Cro.3Sr2CuO r system it is difficult to change the oxygen content and this may be the reason why T~ remains the same in H + Ar annealed samples. The p - T curve is linear in the oxygen annealed sample in the temperature range 62-215 K, whereas above 215 K

581

it deviates from linearity and shows a saturation trend. Two groups I-9, 10] reported similar resistivity behavior in the HgBa2CaCu206+o system. They have explained the saturation phenomenon as due to the insulating phase forming at the grain boundaries or to the loss of mercury due to long time annealing. In (Hg, Cr)-1201 we have annealed the sample at 300°C for 1 h only and so the mercury loss is not the main reason for saturation. Recently we have prepared some new compounds of (Hg, A) Sr2_xLaxCuO r and found p is proportional to T in the high temperature region and to T 1"3 in the low temperature region. However, the impurity content in this system (SrCuO2) is not very much different from (Hg, Cr)-1201. Therefore, further studies are needed to understand the saturation effect in Hg-based systems. The T~ value of the Ba-based Hg-1201 system is 94 K, whereas in the Sr-based system this is 51 K. The c value of Hgo.TCro.aSr2CuO4+a is 8.7 A, whereas that for HgBa2CuO,+~ is 9.5 A. Kawabata and Nakanishi [11] from image processed photographs of high Tc systems found that the smaller the spacing d between C u O (or other elements) sheets along the c-axis, the higher the To. They obtained a linear relation between Tc and d:

Tc = -- 158.02d + 611.40.

(1)

H a r s h m a n and Mills [12] have argued that superconductivity may originate from a Coulomb mediated coupling between carriers on neighboring layers and the transition temperature should scale according to the relation

KBTc oc (ed) 1,

(2)

where e is the average dielectric constant and d is the interplaner spacing. If we put the d values of

Table 1 Crystallographic data for Hgo.7Cro.3Sr2CuO,+,~; Positional, thermal and occupancy parameters Atom

Position

x

y

z

Biso (A2)

Occupancy

Hg Cr Sr Cu O (1) O (2) O (3)

la la 2h lb 2e 2g lc

0 0 0.5 0 0 0 0.359 (10)

0 0 0.5 0 0.5 0 0.359 (10)

0 0 0.3009 (4) 0.5 0.5 0.2008 (30) 0

2.81 (11) 2.81 (11) 0.77 (10) 1.08 (19) 2.0 2.0 2.0

0.58 (1) 0.42 (1) 0.91 (1) 1 0.97 (4) 0.84 (3) 0.19 (2)

Selected interatomic distances (,~) Hg-O(2) ( x 2) 1.751 Hg-O(3) ( x 4) 1.929

Cu-O(1) ( x 4) Cu-O(2) ( x 2)

1.921 2.611

Sr-O(1) ( × 4) Sr-O(2) ( x 4) Sr-O(3) ( × 4)

Refined cell parameters: a = 3.842(2) A, c = 8.724(4) A, R9 = 6.42%, Rwp = 8.59%, R,~p = 2.53%.

2.59 2.854 2.741

582

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20 (degree) Fig. 1. Observed (...), calculated ( - - ) and difference X-ray powder diffraction profiles of Hgo.TCro.3Sr2CuO4+6 and SrCuO2. The short vertical lines represent the Bragg reflection positions for Hgo.TCro.3Sr2CuO,+~ (top) and SrCuO2 (bottom).

HgBa2CuO4+a and Hgo.TCro.3Sr2CuO~ systems in Eq. (1), T¢ should be ~ 4 K for the former sample and 83 K for the latter sample. F r o m Eq. (2) it is expected that T¢ of the Sr-based system should be higher than the Ba-based system. But in reality T~ of HgBa2CuO4+a is 94 K and that of Hgo.TCroo.3Sr2CuO~ is 51 K. This indicates that the interlayer distance (coupling) does not seem to be important for the determination of T~. A similar argument was put forward from pressure experiments on Y2Ba4CuTO15.32 by Van Eenige et al. [13] and by Tristan Jover et al. 1-14] in the T1-2223 system. The Hall constant (Ru) for the sample is positive and its variation with temperature is shown in Fig. 3. We have fitted the p/Rn result with a~ T + b~ and a2 T 2 + b2 and found that the mean square deviation is minimum for T 2 dependence. The parameters a2 and b2 for our sample are 0.0745 and 415, respectively. One may compare these values with those reported by Harris et al. [9] for Hg-1212 polycrystalline samples where a2 = 0.0891 (B), 0.186 (A) and b2 = 2020 (A), 337 (B). It has long been

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Temperature (K) Fig. 2. Resistivity as a function of temperature for Hgo.TCro.3Sr2CuO4+a: (a) annealed at 300°C in oxygen for 1 h; (b) annealed at 350°C in H + Ar for 6 h.

B. Bandyopadhyay et al. /Physica B 223&224 (1996) 580-583

The thermoelectric power S - T curve for the (Hg, Cr)1201 system is shown in Fig. 4. At 77 K S is 10 laVK -1, then increases up to 200 K and reaches a maximum value 25 laV K - * and then decreases in the region 200-300 K. At room temperature $3oo = 23 ~tV K - 1. High Saoo suggests that the sample is in the underdoped region. Obertelli et al. [15], Keshari et al. ['16] and Tallon et al. [17] have shown a universal correspondence of $3oo with carrier concentration p of the system. Using the $3oo value of our sample we found p~0.1/Cu-ion. It appears that the doping state of this sample is below the optimum level (Pop).If we assume that the Pov for Hg-1201 is ~0.16-0.17/Cu-ion [18], then the maximum Tc expected for this system is ~ 72 K. However, changing of the dopant concentration and the annealing condition does not improve To. Further studies about how to increase the p value in this system are necessary.

CA

(.3 E

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t.)

% 6' X

4

I

100

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3OO

200

T(K)

583

Fig. 3. Variation of Hall constant with temperature for Hgo.vCro.3Sr2CuO,+6.

Acknowledgements 30

The authors would like to thank Professor A.N. Das for helpful discussions and S.N. Dutta and A. Pal for technical help. One of the authors (BB) acknowledges CSIR Fellowship during the work.

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References

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Fig. 4. TEP as a function of temperature for Hgo.7Cro.aSr2 Cu04 + ~. known that as the amount of impurity in the system increases, b2 increases (that is the purer the sample, the less is b2). The ratio b2/a2 for our sample is 5570 and this value is in between the values 10 860 and 3780 reported by Harris et al. [9] for their samples A and B. A small value of the ratio is an indication of little disorder for electronic transport.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

S.N. Putlin et al., Nature 362 (1993) 226. Q. Huang et al., Physica C 218 (1993) 356. A. Schilling et al., Nature 363 (1993) 56. J. Shimoyama et al., Physica C 224 (1994) 1. D. PeUoquin et al., Physica C 214 (1993) 87. K.K. Singh et al., Physica C 231 (1994) 9. O. Chmaissem et al., Physica C 242 (1995) 17. Q. Xiong et al., Phys. Rev. B 50 (1994) 10346. J.M. Harris et al., Phys. Rev. B 50 (1994) 3246. R.L. Meng et al., Physica C 214 (1993) 307. C. Kawabata and T. Nakanishi, J. Phys. Soc. Japan 59 (1990) 3835. D.R. Harshman and A.P. Mills, Phys. Rev. B 45 (1992) 10684. E.N. Van Eenige et al., Europhys. Lett. 20 (1992) 41. D. Tristan Jover et al., Physica C 218 (1993) 24. S.D. Obertelli et al., Phys. Rev. B 46 (1992) 14928. S. Keshri et al. Phys. Rev. B 47 (1993) 9048. J.L. Tallon et al., Phys. Rev. B 51 (1995) 12911. L. Gao et al., Phys. Rev. B 50 (1994) 4260.