Mat. Res. Bull . , Vol. 23, pp. 1139-1144, 1988. Printed in the USA. 0025-5408/88 $3.00 + .00 Copyr ight (c) 1988 Pergamon Press plc.

STRUCI'JRE AND SUPERCX)NIX]CTING PROPERTIES OF YBa2_xLnxCu307+6

(Ln = La,Nd or Gd) and (Ln,Ln')3_xBa3+xCU60|4+6

P. Somasundaram, K. S. Nanjundaswamy, A. M. Umarji and C. N. R. Rao Materials Research Laboratory

Indian Institute of Science, Bangalore-560 012, INDIA

(Received April 18, 1988; Communiacted by C.N.R. Rao)

ABSTRACT In YBa2_xLnxCu307± ~ (Ln = La, Nd or Gd), the structure becomes

tetragonal with increase in x and the Tcof the orthorhombic phase

decreases with x. Members of (Ln,Ln')3_xBa3+xCU6014+6 (Ln = Nd or Gd) are generally tetragonal and non superconducting. Ln3Ba3Cu6014+6 and LnBa2Cu307± 6 (Ln =Nd or Gd) prepared by low-temperature decomposition of nitrates are also tetragonal and nonsuperconducting. Random occupation of Ba in LnBa2Cu307± ~ or La3Ba3Cu6014+6 appears to favour the tetragonal structure. Furthermore oxygen is less labile when La

partly substituting Ba in these oxides.

Materials Index: Superconductors, oxides, rare-earths, barium, copper

Introduction

It is now well established that several cuprates of the general formula LnBa2Cu307_ ~ (Ln=Y,La and rare earths other than Pr,Ce and Tb) show high temperature superconductivity with Tc= 90K (1,2). Recent studies (3) have shown that La3_xBa3+xCu6014+~ is structurally related to YBa2Cu307_ 6 (123 structure) with the excess La occupying Ba sites. A preponderant occupation of O1 sites (in the Cu-O chains) relative to the 05 sites (along

*Communication No. 98 from Materials Research Laboratory.

1139

1140 P. SOMASUNDARAM, et al. Vol. 23, No. 8

the a-axis) is necessary for the structure of these oxides to be

orthorhombic. When the occupation of the O1 and the 05 sites is nearly equal

or when the 05 site occupation is higher, the structure tends to become tetragonal. It has been indeed found in this laboratory that members of the

La3_xBa3+xCU6014+6 system are almost always tetragonal and contain oxygen excess (4). It was of our intrest to examine how the substitution of the Ba

sites by La and other rare earths in YBa2Cu307_ 6 affects its structure and properties. We have therefore carried out a systematic investigation of the

YBa2_xLnxCu307± 6 (Ln= La, Nd and Gd) oxide system. We have also examined the effect of substitution of Y for La in La3_xBa3+xCU6014+6 system, besides

studying the properties of the Ln3_xBa3+xCu6014+6 oxide with Ln = Nd and Gd, prepared by low temperature decomposition of the mixture of nitrates. The

present studies have shown that progressive replacement of Ba by La or Nd in

YBa2Cu307_ 6 favours the tertragonal structure and lowers the superconducting T c . In the (Ln,Y)3_xBa3+xCU6014+6 (Ln= La, Nd and Gd) system, all the

members were semiconducting and tetragonal. Ln3_xBa3+xCu6014+6 (Ln=Nd and Gd) prepared by low-temperature decomposition were always tetragonal and

semiconducting. .Even the LnBa2Cu307± ~ with Ln= Nd or Gd prepared by low- temperature method gets stabilized in the tetragonal structure, showing that

a random occupation of the Ba sites by trivalent rare-earth ions in the 123 system has a detrimental effect on the superconductivity.

Experimental

Members of the YBa2_xLnxCu307± 6 (Ln= La, Nd and Gd and X= 0.0 to 1.0) and (Ln,Y)3_xBa3+xCU6014+6 (Ln= La, Nd and Gd) families were prepared by the standard ceramic technique using high-purity CuO, BaCO 3 and rare-earth oxide. Higher reaction temperatures {=1300K) were needed for compositions

rich in Lanthanum. Compositions of Ln3_xBa3+xCU6014+6 (Ln= Nd and Gd, and x = 0.0 and 1.0) were prepared by the decomposition of mixture of nitrates

obtained by dissolving the oxides in dilute HNO3, in an inert atmosphere at 770K. The mixture thus obtained was ground, pelletized and heated in an

oxygen atmosphere to 1100K for 48 hours to avoid formation of BaCO 3 at any stage. Additional grinding and heating was necessary to complete the reaction. The final heat treatment given to all samples (includig the ones

prepared by the ceramic route) involve slow cooling from 1100K to room

temperatu~ in an oxygen atmosphere.

X-ray powder diffractograms were recorded using CuK~ radiation. Oxygen stoichiometries were determined by TGA in flowing hydrogen. Electrical resistivity was measured by the DC four-probe method in the 20-300K range.

Results and Discussion

In figure I. we show the x-ray diffraction patterns of the

YBa2_xLaxCu307±6 system. The patterns clearly show a gradual changeover from the orthorhombic to the tetragonal structure with increase in x. The absence of 103 and 006 reflection distinguishes this tetragonal structure from the oxygen-deficient 123 tetragonal structure. The lattice parameters of the samples are listed in Table I. Oxygen stoichiometry as determined by

thermogravimetry shows that 6 is close to 0.0 (±0.05) when x is in the range 0.0 to 0.25. Only the oxides in this composition range show superconductivity with T c decreasing with increase in x (Fig • 2). Random substitution of Ba sites by La up to 16% seems to be sufficient to decrease the T c below 20K. The orthorhombic structure of YBa2-xLaxCu307±6 as

determined by the x-ray method continues up to x= 0.35, but the material is

Vol. 23, No. 8 SUPERCONDUCTORS 1141

FIG. 1

X-ray powder diffractogrammes of

YBa2_xLaxCU 307_+ @

50 45 40 35 Cu Ka 28 (d~g)

30

2.0

1,6

~1.2

E 0.8

0.4

Y (B%_ x LCix)CU3 O7. &

o x= 0 0 0

x = 0.125

® = 0.250

o = 0 .325

= O . 375

,D = 0 . 5 0

o =0 .75

v = 1.00

FIG. 2 R(T)/R(300 ) versus temperature plots

for YBa2_xLaxCU307± ~

0 100 200 300 T K

1142 P. SOMASUNDARAM, et al. Vol. 23, No. 8

metallic or shows a temperature independent resistivity when x~<0.35. Oxide with x>0.5 are semiconducting and tetragonal. The electrical resistivity data shown in Fig. 2 clearly brings out the sensitivity of the

superconducting properties to the Ba/La ratio.

The tetragonal structure of the members of the YBa2_xLaxCu307± ~ system with x >0.35 can arise from the exchange of La and Ba sites as well as

oxygen excess. The actual composition of x= 0.5 sample was

YBal.5La0.5Cu307. 2. If only the oxygen excess were responsible for making the structure tetragonal and destroying superconductivity we would expect a drastic change in the properties on heating the sample to a high

temperature and cooling it in an appropriate atmosphere; under such treatment the tetragonal structure should give way to the orthorhombic

structure which should show superconductivity. This however is not the case. Heating the x = 0.5 sample to 1150K and cooling it in a N 2 atmosphere did

not change the structure nor eliminate oxygen excess. Random occupation of Ba sites by La seems to stabilise oxygen in these oxides.

In the YBa2_xNdxCU307+6, only the x = 0.125 composition showed the orthorhombic structure (a i 3.836A, b= 3.889A, c = 11.655A) and superconductivity (T c onset =78K and zero resistance =72K). The x = 0.25 composition was tetragonal and semiconducting. The Nd-containing oxides tended to show small amounts of Y2BaCuO5 (211 phase) impurity. We could not

prepare pure monophasic YBa2_xGdxCU307± ~. Nominal compositions with x = 0.125 to 0.375 were orthorhombic with increasing amounts of Y2BaCuO5; all

these compositions were superconducting showing zero-resistance at 71K, 63K and 50K respectively for x = 0.125, 0.25 and 0.38. We had earlier

noticed such a behaviour of T c in the case YBa2Cu307 due to the presence of the 211 impurity (5).

Table I. The lattice parameters, electrical properties, and superconducting

transition temperatures in the oxide system YBa2_xLaxCu307± 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

x in Lattice Param. Properties Tc(K)

YBa 2_xLaxCu307± ~ a(A) b(A) c(A) (R 300KOhms) onset zero

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.00 3.83 3.89 11.68 Supercond. 100 92 0.125 3.83 3.89 11.68 '' 82 72 0.25 3.83 3.89 11.66 '' 58 25 0.325 3.83 3.87 11.61 '' 45 <20 0.35 3.84 3.87 11.61 Temp. ind. resist.

(0.022)

0.375 3.87 11.60 Temp. ind. resist. (0.038)

0.50 3.86 - 11.60 Semicond. - - (0.171)

0.75 3.86 - 11.58 Semicond. - - ( 1 . 8 7 0 )

1 . 0 0 3 . 8 5 - 1 1 . 5 5 S e m i c o n d . - -

( 7 . 2 3 0 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Vol. 23, No. 8 SUPERCONDUCTORS 1143

Table 2. Compositions and lattice parameters of the

(Ln,Ln')3_xBa3+xCU6014+6 compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A 6 in Lattice parameters

(A6)Cu6014+6 a(~) c(~)

La1.75YBa3.25 3.88 11.64

Lal.5YBa3. 5 3.87 11.61

La2.0Y0.5Ba3.5 3.88 11.64

La2YBa 3 3.89 11.66

Nd YBa 3.88 11.62

Nd2Ba ~a) 3.91 11.73

Nd3Ba 3(a) 3.91 I~.73 2 4

Gd Ba (a) 3.90 11.70 3 3

Gd Ba (a) 3.90 11.70 2 4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(a) Prepared by low temperature nitrate decomposition method.

semiconducting

We prepared several compositions of the (Ln,Y)3_xBa3+xCU6014+6 system (Table 2) and all these oxides were tetragonal and semiconducting. It may be noted that La3_xBa3+xCU6014+~ oxides generally tend to be tetragonal and exhibit superconductivity, if at all, at very low temperatures (4). All the (Y,Ln) compositions did not show superconductivity; they are semiconducting at all temperatures.

YNd2Ba3Cu6014+6 was a semiconductor and the YGd2Ba3Cu6014+6(nom) was multip6asic.

Nd3Ba3Cu6014+~ and Gd3Ba3Cu6014+6 prepared by the low-temperature nitrate decompositfon were tetragonal and semiconducting. While this is not surprising, the fact that Nd2Ba4Cu6014+~and Gd2Ba4Cu6014+ ~ prepared by this low-temperature route are also tetragonal and semiconducting is surprising. This tetragonal structure is due to the randomness of Ba by Nd or Gd. We come to this conclusion because heating these 123 oxides (prepared by the low temperature method) to higher temperatures (1250K for 48 hours and slow cooling in oxygen atmosphere) does not transform them to the orthorhombic structure. This would not be the case if only oxygen excess were involved. This observation seems to indicate the importance of cation distribution for orthorhombic distortion and superconductivity.

Conclusions A systematic study of the oxide systems, YBa 2 xLn. Cu307+ ~ and

Ln3_xBa3+xCU~014+~ shows the importance of orthorhombicity to superconauctlvlty in this class of copper oxides. For a given composition, different structures can be obtained depending upon the method of prepration. The presence of a trivalent lanthanide ion at the Ba site, stabalizes oxygen close to seven. This observation could be used to advantage to process these materials for large scale applications.

1144 P. SOMASUNDARAM, et al . Vol . 23, No. 8

Acknowledg~nt The authors thank the Department of Science & Technology, Government of

India and the University Grants Commission for supporting this research.

References

7. D.L. Nelson, M.S. whittingham and T.F.George (eds), "Chemistry of High- temperature superconductors, "ACS Symfosium series 351, 1987.

2. C.N.R. Rao, J. Solid State Chem., 73, (1988), March issue; also see C.N.R. Rao ed, "Chemistry of oxide superconductors", Blackwell, Oxford, 1988.

3.

4.

C.U. Segre, B. Dabrowski, D. G. Hinks, K. Zang, J. D. Jorgensen, M. A. Beno and I. K. Schuller, Nature,329, 227 (1987).

L. Gan~pathi, A.K. Ganguly, R.A. Mohan Ram and C.N.R. Rao, J.Solid State Chem. in print.

5. A.M. Umarji and K.S. Nanjundaswamy, Pramana - J.Physics 29, L611 (1987).