Influence of chemical substituents on the superconductivity of La1.8Sr0.2CuO4

Solid State Communications, Printed in Great Britain.

INFLUENCE A. K. Grover,

Vo1.63,No.l1,

OF CHEMICAL

SUBSTITUENTS

S.K. Dhar, P.L. Paulose,

Tata Institute

pp.1003-1007,

ON THE SUPERCONDUCTIVITY V. Nagarajan,

0038-1098/87 $3.00 Pergamon Journals

1987.

+ .OO Ltd.

OF La1 8SrO 2CuO4

.

E.V. Sampathkumaran

.

and R. Nagarajan

of Fundamental Research, Homi Bhabha Road, Colaba, Bombay 400 005, India.

(RECEIVED

May

12th,

1987by

P. Wachter

1

The influence of Ca in place of Sr and Eu and Yb in place of La on the superconductinq and normal state behaviour of La, ,Sr, ,CuO, has been investigated. Eu and Ca substituents depress zerd'fe&fanc? temperature (T ). A semiconductina(localization)-like behaviour . aualitativelv is observed for the'air-annealed scsling with cation c¢ration, Ca specimens, whereas Eu and Yb substitutions do not very much alter the normal state behaviour. The data are discussed in terms of possible size and charge mismatch effects.

The discovery of superconductivity in $he 30K multiphase La-Ba-Cu-0 system range in anjL subsequently in the 90K range in Y-Ba-Cu-0 system among the has generated tremendous interest The phase responsible fo? scientific community. in the La based oxide system superconductivity Ba CuO has the chemical composition3 La to the K,NiF,, structure4&&. 'Whire Fde belonqs eompaund is a -semiconductor, La CuO4 parent structure at crystallizi ,Zg in an orthorhombic partial replacement of La by Ba staoilises 300K, leads tid tetragonal phase yng the superconductivity (T = 30K) ’ . Cava et al the effectC of Sr substitution and studied observed a Tc of about 36K for, La various groups Suosequently, Ca CuO ( x= 0.1, reported their results on La thgrx shper$onductivity 0.15 and 0.2) and found occurs at lower temperatures compared to SrThere is no correlation containing compound. and the size of tne alkaline-earth between T In the present cation subEtituted at the La site. paper, we discuss our further results on the (x = calcium substituted samoles La, Ca CuO, 0.2 and 0.3) in orddrxtoxthrsw light 0.075, 0.1, on the role of the suostituents on the normal state resistivity as well as on superconductivity. We have also studied the influence of partial the La by Eu and Yb in replacement of namelv, suoerconductina strontium comoound. _ 'Yb Sr La 8_xEuxSr0
regular geometrically shaped pieces. While some pellet pieces were annealed in air for 6 hours at IIOOC (and then cooled to room temperature by removing them from the hot zone), some others were annealed in a continuous flow of 0 gas at 900 C for 15 hours (with a similar meth 2d of cooling the specimens to 3OOK). The x-ray diffraction patterns of all these samples, except of Yb, could be indexed on the basis of K NiF type tetragonal structure. The Yb samples sh 8w ad ditional lines, the intensity of which increases with Yb concentration. We attribute these extra lines to unreacted Yb20Z. DC electrical resistance (R) measurements w re carried out in the temperature interval 4.2 - 300K by a conventional four-probe method. DC maqnetization studies were carried out in the same temperature interval \s,field values upto 8 kOe bv Faradav method. Eu Mb?ssbauer effect studies>on Eu specimens were carried out at selected temperatures across T using SmF3 as a C’ source. The results of resistance measurements on the Ca substituted samples are shown in Fig.1. The superconducting onset temperature (T 1, mid-paint (T ) and zero-resistance temperatupes (T 1 are li&.ed in table 1. The resistivity value a? 300K in oxygen annealed La specimen ’ estimated to be 3r&lQ*1c;104* 25%); tiz corresponding value for the air-annealed specimen is also the same within the limits of the experimental error. The resistivity values at 300K show a gradual increase (upto a factor of 2) as Ca concentration increases from 0.075 to 0.30. Several interesting trends can be noted from figure 1. (i) The air-annealed specimen of x = 0.075 exhibits an almost behaviour metallic (monotonic decrease with temperature) till the onset of superconductivity. (ii) For higher values of x, a semiconductor like response (identified as the localization behaviour, following Ref.1) is o&served before the resistance rapidly drops to zero. This means that, above T there is a thermally activated process fog' the current carriers and the mobility of these carriers decreases with temperature. (iii) The annealing of

CHEMICAL

0

0

80

40

120

SUBSTITLIENTS ON THE SUPERCONDUCTIVITY

160

TEMPERATURE Fig.1.

200

240

280

320

(K)

Resistance (R) (normalised to 300K values) as a respective function of temperature (4.2 - 3OOK) Ca CuO ( x = for the specimens La 0.2 anii-6.37. tt+air0.075, 0.1, annealed; Opoxygen annealed.

Table

1

List of superconducting onset (T 1, midpoint (Tc) (T ), and zero resistance temper8tures ofmour specimens.

OF LaI *SrO *Cu04

.

.

La2~x~a~C%!!! (Air)

To(K)

T,(K)

Tc(K)

30

18

12

:*1': I% 0120 (Air) 0.30 (Air)

30 25 22 22

22 25 20 18

20 18 14 IO

La 1&x:UxSr~A~~l;04 0.1 (Air)

;z

28 29

'2;

0.1 0.2

38 24

30 15

25 9

36

29 32 32

23 26 28

26

20

(0 ) (A&)

Lal~8~x;b~Sr&04

behaviour. (iv) The semiconducting behaviour in the normal state is such that the rate of build up increases as of values R(T)/R(300K) For x = 0.2 and 0.:: concentration increases. and values > 3 R(T)/R(300K) attain onset the before respectively just

11

superconductivity. It is worth recalling' here that in the case of La Sr synthesized by us under id&8ic!&2~~~? tr%~~~~"t R(T)/R(300K) nas a value ~1 at the onset 0; superconductivity. It is pre3Tntly believed that the substitution of La by divalent a$aligF-earth ions in La CuO4 tends to increase Cu /Cu ratio (as reflec z ed in the var&a&ion of c/a ratioa) as we$i as oxygen VacanciesO". The propqbtion of cu ions seems to with correlate superconducting T whereas oxygen vacancies are perhaps influencifii the localization behaviour seen in the normal state. Our results render for the strong support to these ideas. Thus, specimens synthesized in air at llOOC(for 6 an enhanced temperature coefficient of hours), resistance with increasino Ca concentration mioht suggest an increase in -the oxygen vacancies (resultina in the decrease in the mobilitv of the Therefore the annealing'of x = charge carriers). 0.1 specimen in O2 .atmosphere should restore metallic behaviour similar to that of x = 0.075 and it is satisfying to note that this is experimentally the case (see Fig.1). It is important to note that the specimen thus treated with 0, has hioher T_ with respect to the airannealed specimen, ' implying theryb+y 2+a corresoondino increase in the ratio of, Cu /Cu . Incidentally: a peaking in tne Cu"/CuL+ ratio for x = 0.2 for khe Sr series has been reported by van Dover et al and Nguyen et al . Similar maximum for the Ca series also at x = presumably occurs 0.1 under our preparative conditions. The fact that the air-annealed La soecimen exhibits hiaher dearee of cue and lower T compared to tnat in La si'9' e tempts us t6 attribute these observa$i~~~Ot~ mismatch mechanism operative in ,$Qis class of (1.121) is compounds. The ionic size 3yf Sr 2jl.14i), whereas neggly the same as that of,ia substitution is is much smaller (0.99A). Ca Ca probably causing more local strain in the lattice and this presumably contributes to the enhanced localization as well as reduced superconducting T induced effects). We? (disorder and strain however, would like to mention that the substitution by Ca involves the variation of charge also (with respect to La) and it would be interestinq to understand the relative roles of these charge and size imbalances on the observed we considered it experimental behaviour. Hence, substituted to investigate the Eu necessary as Eu substitution is not expected to samples, The results of such cause any charge imbalance. investigations discussed below suggest that both the size mismatch and the charge mismatch are whereas imoortant for the localization behaviour, size mismatch dominates the depression of Tc. Fig.2 shows the plot of resistance versus temperature in La A zero resislaB,-aE~~~f&'I,C",0rt~2n,d0,: %"K 0.2). annealed x = 0.1 for both air and oxygen Increasing Eu concentration to x = 0.2 specimens. results in marked decline in T to 9K in air This trend is sohe$at similar to annealed state. by smaller that ob.j+ervedon replacement of Sr sized Ca . Interestingly enough, the air annealed specimens of Eu compounds exhibit a metallic like below 80K, the observed behaviour till 80K; in the resistance is somewhat similar to increase that observed by us in air annealed parent d

Compounds

Vol. 63, No.

d

Vol. 63,

No.

11

1005

CHEMICAL SUBSTITUENTS ON THE SUPERCONDUCTIVITYOF La, SSrO 2Cu04

.

0.02 P

0.6

0

z 2

-0.02-

z 0 -

-0.04-

.

I

I

I

La1.7 E"0.1Sr0.2Cu04 H=

11

Oe

FC

k

x

La,.7Eu,,

Si-2 CuO4

OxyAnn

A La,.6Eu,Z

Syz CuO4

Air Ann

o La,,7Eu.,

Sr2

Air Ann

Cu04

TEMPERATURE TEMPERATURE(K)

Fig.4.

(R) (normalised to Resistance respective 300K values as a function of temperature (4.2 - 3OOK) for the = specimens Cal.8_xEuxSr0.2 CLJD4 (X 0.1 and 0.2).

Fig.2.

similar under This implies that (1.14 A) by smaller sized further decrease the mobility of current carriers. Comparison of data in figs. 1 and 2 clearly justify the earlier statements that in addition to the 3$ze mismatch, charge mismatch between La the and the substituent is also important in enhancing tne localization behaviour. We have also performed detailed measurements annealed on viz. oxygen one sample, 3 shows current versus cue . Lv~~t~~~"'~nS~Pe~ter4Sti~~g'at 15K. T9e critical current density at 15K is &0.5A/cm . The low

0.5

I

I

I

I

(K)

DC magnetization as a function temperature ( 4.2 - 32K ) for La, 7Eu0 ,Sr o 2Cu04.

.

of

.

field magnetization data obtained in the same in specimen in a field of ~11 Oe are depicted Fig.4. A diamagnetic response is observed near or below the temperature region of rapid fall in the resistance value. The strength of the diamagnetic signal depends on the thermomagnetic history of the specimen. Cooling the specimen down to 4.2K in a finite field decreases the diamagnetic response. We also noticed a small time dependence conqjstent with the observations of Muller et al on cue . The continuous curves depicted in FL:4'8B~0*~orr&pond to data recorded at small temperature steps after a few minutes of waiting at each temperature. Fig. 5 displays the magnetic nysteresis curves obtained in the superconducting state at 5K in the above specimen. We have obtained such curves at a few other temperatures also (not shown in the figure). The values of the

I

Lo1.7Eu0.1Sr0.2CuD4 0.4 T = 15K 9 E 0.3 :

a

0.2-

5 9

0.1-

0

azz 0

0

I 10

I

20

30

40

50

60

1.61

CURRENT

Fig.3.

Current versus voltage characteristics at 15K for the oxygen annealed specimens of Lal.7Eu0.,Sr0.2CU04.

’ 0

’ 2

’

MAGNETIC

Fig.5.

’ 4

’

’ 6

’

’ 8

1

FIELD(kOe)

Magnetic hysteresis loop at 5K for the specimen Eu0~lLal~7Sr0~2Cu04.

1006

Vol. 63, No. 11

CHEMICAL SLJBSTITUENTS ON THE SUPERCONDUCTIVITYOF La1 ,$rO . .

lower critical field (H ) obtainable from such cu es match with those gztimated by Batlogg et al?' in some of the La Sr CuO specimens. In the discussi gnx of t#e results on Eu compounds, it has been stated that Eu ions are in trivalent state. As Eu ions can exist in divalent or mixed valent states, we considered it necessary to confirm the trivalent nature of Eu in the The compounds under study. paramagnetic susceptibility data (not shown here) in the normal state of these compounds show them to be van Vleck paramagnets, thereby confAsTing the trivalent Eu MBsbauer isomer state of Eu. In addition, shift Fig.6 for data in values (see

I or-

0.8t

8

0.6 t I

CK \

c

0.4

LL -I

A Lo,.~Yb.,St2 Cu04 o La,.7Yb.,Sr2 Cu04

0

40

80

120

160

200

240

280

320

TEMPERATURE(K) Fig.7.

Resistance (R) (normalised to their 300K values) as a respective function of temperature for the specimen La, S_xYbxSro ,$uO4.

obvious if we recall (see above) that the solubility of Yb in this Sr compound is very mucn difference limited, unlike Eu. This solubility between Eu and Yb is not a surprise in view of the absence of Yb2Cu04 phase in the K2Nii4 structure. I

-12

I

-8

I

-4

:Qf ,

I

I

I

0

4

8

12

of

We also unsuccessfully searched for the onset superconductivity down to 4.2K in several

VELOCITY(mm/s)

Fig.6.

151

Eu M%sbauer spectra at selected temperatures for the specimen La1 ,EuO ,Sro $uO4.

.

.

.

La Eu Sr specimen) fall in the range cue ty&?al"'af 't?iva?ent Eu materials. It is also apparent from our Massbauer effect studies that there is no change in s electron density at the Eu nucleus, as evidenced by the absygye of any Eu isomer anomalous temperature variation of shift across the superconducting phase transition. careful It will be interesting to perform sonance intensity measurements to see Eu Mossbauer effect can detect any possibly are modes which anomalous phonon responsible for the superconductivity in these specimens. EU to the effects3+caused3+ by Contrary by Yb cupto x suostitution, replacement of La = 0.2) does not seem to result in any significant changes in the different parameters characterizing the superconducting benaviour of La (see Fig.7 and table 1). At the first zi$ry.?%%! observations may look puzzling in view of the fact that the size mismatch effects as well as magnetic moment of Yb3+ ions (paramagnetic susceptioility data confirm the trivalent state of Yb in these should have resulted in tne reduction compounds) of T values oelow that of corresponding Eu compo6nds. The reason for this discrepancy becomes

flowing oxygen atmosphere. In the early part of February 87, one specimen at stoichiometry La Pbo ,Cu04 showed a drop in resistance to zero at"&out 65K during the first rapid cooling down of the specimen; however, the specimen acquired finite resistance at 4.2K. Curiously, no anomalous jump in resistance values could be obtained while warming up or during subsequent cool down or warm The observed behaviour could also not up cycles. the other specimens oe reproduced in any of in the same manner. synthesized It may be added specimen composition here that a at heated in air at 1100 C for 6 ~~?~~'~Yni3~'aii~~~~~nducting beheviour analogous to

continues to show semiconducting La SSrO $uO behaviour' do& to 4.2K. This suggests that the structure alone is not the determining factor for superconductivity in this class of compounds; but additional features like a particular mode of phonon may be more crucial. It is also important to note that Eu$uO4 itself crystallizes in the tetragonal structure, unlike La CuO (at 30CK) and it is tnerefore possible that o&y t)hose compounds on the verge of such structural instabilities are candidates for high Tc inore likely superconductivity. would like to acknowledge The authors for constant Vijayaraghavan Professor R. encouragement and support throughout all phases of this work.

Vol. 63, No. 11

CHEMICAL SUBSTITUENTS ON THE SUPERCONDUCTIVITYOF La1 SSrO 2Cu04

.

.

1007

References 1. 2.

3. 4.

5. 6.

7.

J.G. Bednorz and K.A. Muller, Z. Phys. B 2, 189 (1986) M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58, 908 (1987) S. Uchida, H. Takagi, K. KiGzawa and S. Tanaka, Jap. J. Appl. Phys. 26, Ll (1987) See for an excellent review of features pertinent to compounds with this structure, P. Ganguly and C.N.R. Rao, J. Solid State Chem, 53, 193 (1984) C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, 2.3. Huang and Y.Q. Wang, Phys.Rev.Lett., 58, 405 (1987) R.J. Cava, R.B. van Dover, B. Batlogg and E.A. Reitman, Phys. Rev. Lett., z, 408 (1987); R.B. va7 DovOr, R.J. Cava, B. Batlogg and E.A. Rietman, Phys. Rev. B 2, 5337 (1987). J.G. Bednorz, K.A. Muller and M. Takashige, Science, 236, 73 (1987); E.V. Sampathkumaran, P.L. Paulose, A.K. Grover, V. Nagarajan and S.K. Dhar, 3. Phys. F (In press); I.K.

8. 9.

Gopalakrishnan, A.M. Umarji, J.V. Yakhmi, L.C. Gupta, R.M. Iyer and R. Vijayaraghavan, Mat. Lett. (In press); C.N.R. Rao and P. Ganguly, Curr. Sci., 5&, 47 (1987); P. Ganguly, R.A. Mohan Ram, K. Sreedhar and C.N.R. Rao, Solid State Commun. (In press); M. Sato, S.H. Hosoya, S. Shamoto, M. Onoda. K. Imaeda and H. Inokuchi, Solid State Commun. 62, 85 (1987). ' N. Nguyen, J. Choisnet, M. Hervieu ard 0. Rayeau, J. Solid State Chem., 39, 120 (1981) D.U. Gubser, R.A. Heeic S.H. Lawrence, M.S. Osofsky, D.J. Schrodt, L.E. Toth and S.A. Wolf,

Phys. Rev. B 2,

5350 (1987)

10. C.N.R. Rao, P. Ganguly, K. Sreedhar, R.A. Mohan Ram and P.R. Sarode, Mat. Res. Bull. (In press) 11. K.A. Muller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett., 58, 1143 (1987) 12. 8. Batlogg, A.P. Rzirez, R.J. Cava, R.B. van Dover and E.A. Rietman, Phys. Rev. B 5, 5340 (1987)