Mat . R e s . B u l l . , Vol . 25, p p . 3 3 1 - 3 3 5 , 1990. P r i n t e d i n t h e U S A . 0 0 2 5 - 5 4 0 8 / 9 0 $ 3 . 0 0 + .00 C o p y r i g h t ( c ) 1990 P e r g a m o n P r e s s p l c .

SYNTHESIS AND CHARACTERIZATION OF SUPERCONDUCTING YbBa2Cu307_@ AND

Y1_xLUxBa2Cu307_~(0.0 < x < 0.75): LOWER LIMIT OF THE RARE-EARTH

+ ION RADIUS TOLERATED IN THE 123 CUPRATE SYSTEM

@ * ** P. Scmasundaram , A. Mohan Ram , A.M. Umarji @ and C.N.R. Rao

@ Materials Research Centre and

• Solid State and Structural Chemistry Unit

Indian Institute of Science, Bangalore 560 012, INDIA.

( R e c e i v e d O c t o b e r 17, 1989; C o m m u n i c a t e d b y C . N . R . Rao)

ABSTRACT

YbBa2Cu~O . .with a T (zero-resistance) of 89K has been prepared • J l - O , , C

wlthout contamlnat±on of Yb2BaCuO . Monophaslc LuBa2Cu307 - cannot 5 -6

be prepared Solid solutions of the type Y Lu Ba Cu O~ " I x x 2 3 i-o

(0.0 < x ~ 0.75) are, however, stable and superconducting (zero-

resistance T , 89K). It appears that the lower limit of the rare • c

earth ion radlus tolerated by the 123 cuprate structure is close to that of Yb 3+ (0.985 ~).

MATERIALS INDEX: YTTERBIUM, LUTETIUM, BARIUM, CUPRATES

Introduction

Since the discovery of high T superconductivity in YBa2Cu307 ~ , other superconducting 123 cuprates ~f rare-earths such as Nd, Sm, Eu, -Gd,

Dy, Ho, Er and Tm have been synthezised and characterized (1,2). The

cuprates of Ce, Pr and Tb are known to be non-superconducting while the 123

cuprate of Lu has not been reported. Surprisingly, all the reports in

the literature describing the structure and properties of YbBa2Cu307~

+ . . Communlcatlon number 130 frcm the Materials Research Centre

To whom correspondence should be addressed.

331

332 P. SOMASUNDARAM, et al. Vol. 25, No. 3

suggest the preparations to be multiphasic (3-6). To the best of our

knowledge, monophasic YbBa Cu O_ ~ has not been prepared hitherto. It was our interest to explore 2 w~e~h~r we can indeed prepare monophasic

YbBa Cu 0 and LuBa Cu O 6 In this ccmmunication, we report the 2 -6 2 3 7- "

success~u~ preparation and characterization of monophasic YbBa2Cu307_ ~ and of the solid solutions, Y1_xLUxBa2Cu307_6 (0.0 < x,< 0.75).

Experimental

Members of YbBa~Cu_ O. ~ (x = 0.0 and 0.2) and YI Lu Ba CuO - . ~ ~+x i-o ~ ~ -x x 2. 3 7-~

(0.0 < x < 1.0) raml±les were prepared by the stanaara ceramlc teennlque

using high purity CuO, BaCO 3 and rare-earth oxides. Reaction temperatures were around 1200 K. The flnal heat treatment given to all the samples involves slow cooling fran 1100 K to room temperature in an oxygen

atmosphere. X-ray powder diffractograms were recorded with CuK~ radiation• Thermogravimetry was carried out in the range 300-I000K in a flowing

hydrogen atmosphere. Electricl resistivity measurements were measured by the four-probe method in the 20-300 K range.

Results and Discussion Starting with various initial compositions, it was found that an

initial cQmposition exactly corresponding to YbBa2Cu O_ 6 always yielded a 3 y-

product with a small proportion of Yb2BaCuO 5 (21~) as impurity. This 211 phase is an insulator just as Y2BaCuO_. The 211 phase of Yb seems to be

b more stable than that of lighter rare-earths; apparently, the Ln2BaCuO

5 phase occurs only with heavy rare-earths (7). By starting with ccmposition~

excess in copper, it is possible to prevent the formation of the 211

impurity phase as shown earlier in the case of YBa2Cu3~_6(8). Accordingly,

starting with a cc~nposition corresponding to YbBa2Cu 3 207 ~, we could obtain a monophasic product poossessing the orthorhcmb~6 structure (a = 3.798A, b = 3.872A, c = 11.65A). The orthorhQmbic a and b parameters are

closer to each other than in YBa_Cu_O 7 . . The ccmpound so prepared shows a sharp resistive transition wi~h ~he-o~nset of superconductivity at 95 K and zero-resistance at 89 K (Fig. I). This is compared with transition shown by the biphasic product obtained by starting with a cc~position which had no copper excess in Fig. l; we see that in the latter case, the

transition occurs over a wide temperature range showing zero-resistance at ~60K.

All our efforts, including the use of copper excess in the starting

ccmpositions, to prepare LuBa2Cu~O7_ x resulted in multiphasic product,s This observation suggested that £her~ may be a lower limit of the Ln

• yb 3+ ionic radius to%erated by 123 cuprates Since YbBa Cu O with a • 2 37-6,

radius of 0.985 A could be prepared, we sought no examlne whether we could prepare solid solutions of the type Y~ Lu Ba Cu O ^ and if so, how much

- x 2 37- of Lu is tolerated in the solid solutions. A careful study of several compositions with 0.0 < x ~ 1.0 showed that monophasic solid solutions

could be perpared only up to x = 0.75. In Table I, we summarize the results of structural data and superconducting properties of the solid solutions. FrQm the lattice parameters of these cQmpositions, we see that the a, b and c parameters decrease with increase in x. The unit cell volume also

Vol. 25, No. 3 CUPRATES 333

10

O6

0.4

0;t

00 AO

.;Y o Yb Ba z Cu307

60~ i ~ A YbBo z Cu3zO 7

I I i I 0 140 180 220 260

TEMPERATURE (K)

I 300

Fig. I

Tern; erature variation of the

normalized resistance, R _ /R ...... <T; / J u u )

of YbBa~Cu~ O. ~ ; x Value repre- Z A+X / - O . ,

sents startlng composltlon.

.o i 1 .B I ~6

J 1.2!

°,o

08~ 06

0&

02

0.0

y1 Lu Ba2Cu30. I

x = 0.50 x 075 o

I00 lz, O 180-- 220-- 260 300 TEMPERATURE (K)

Fig. 2

Temperature variation of the

normalized resistance, R(T)/R(300 ),

of YI _xLUxBa2Cu307_6.

334 P. S O M A S U N D A R A M , e t a l . V o l . 25 , No . 3

decreases with increase in x as expected. The x = 0.5 and 0.75 campositions showed sharp transitions, with onset of superconductivity at 94 K and zero-resistance of ~ 89 K as shown in Fig.2.

TABLE I.

Structure and superconducting properties of Y1_xLUxBa2Cu307_ 6

Camp ositi on Ln 3+ Ionic (a) Lattice Parameters Vol.

radius(A) a(A) b(A) c(A) (~3)

T (K) c

onset zero

YBa2Cu307_ 6 1.019 3.822 3.894 11.683 173.9 95 90

Y0.5Lu0.5Ba2Cu307_6 0.998 3.820 3.886 11.668 173.0 94 89

Y0.25Lu0.75Ba2Cu307_6 0.987 3.806 3.878 11.659 172.1 94 88

Y0.1Lu0.9Ba2Cu307_6 (b) 0.981 multiphasic Semiconducting (c)

(b) LuBa2Cu307_ 6 0.977 multiphasic ''

(a) Weighted average value given for Y~ Lu l-X x

(b) These ccmpositions were multiphasic even when tried with copper excess of 7%

c) Shows resistivity drop at 90K due to the presence of superconducting impurity

It appears that the maximum percentage of Y that can be replaced by Lu in Y Lu. Ba Cu 0 is 75%. The weighted ionic radius of the rare-earth

x l-x 2 3 7- 6 o in Y~Lu075~2~-~-Ba Cu O~ 6 works out to be 0.986A which is close to the radius'2of ~'y~+(0.985A).This observation suggests that the radius of the rare-earth ion has same bearing on the stability of the 123 structure. In order to further establish the point, we tried to prepare Yb0 1Lu Ba2Cu30 ~ which would have an effecti~ rare-earth radius of

o 0.9 7-6 0.984 A, a valus less than the ionic radius of Yb- . The preparation was mul tiphasic.

Conclusions It has been possible to prepare monophasic YbBa Cu 07 - with a sharp

superconducting transition (zero-resistance at 89 K~ b 3 u%ing about 8% excess copper in the initial starting cc[nposition. Although LuBa_Cu_O~ ~can not be prepared, it has been possible to obtain superconduc~in4 's°olid solutions of the type YI xLUxBa2Cu307-6 up to x = 0.75 (zero-resistance

T = 89K); campositions-with x > 0.75 are multiphasic. This observation c.

Vol. 25, N o . 3 CUPRATES 335

suggests that there is a lower limit of the radius of the rare-earth which o

the 123 structure can tolerate; the ~miting value appears to 0.985 A corresponding to the ionic radius of Yb

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