Physica C 153-155 (1988) 910-911 North-Holland, Amsterdam

PHYSICAL PROPERTIES OF THE SERIES OF OXIDES Y1-xPrxBa2Cu307 6 (0~x~l)

Ant6nio P. GONGALVES, Isabel C. SANTOS, Elsa B. LOPES, Rui T. HENRIQUES, Manuel ALMEIDA (*), Ondina FIGUEIREDO (**), Jorge M. ALVES, Margarida GODINHO (***)

(*) Dept. Quimica, LNETI, P-2686 Sacav~m Codex, Portugal (**) Centre de Cristalografia e Minerologia, IICT, P-1000 Lisboa, Portugal (***) Dept. Fisica, FCUL, P-1700 Lisboa, Portugal

The effects of substituting Pr for Y in the physical properties of Yl-xPrxBa2Cu3OT-6 are studied by X-ray diffraction, electrical resisitivity and thermopower. The orthorhombic distortion is decreased by increasing x while Tc is decreased. For x>0.55 there is a subtle structural change and the superconduc- tivity is definitely supressed for x>0.6. The transports results are indicative of strongly correlated carriers and their dependence with x is interpreted in therms of an increased band filling.

The substitution of most rare earths for Y in YBa2CusOT-6 almost do not af- fect the superconducting properties with exception of the tetravalent rare earths like Ce, Pr and Tb. The incorporation of Pr in the Y compound is known a steadily decrease Tc, and superconductivity is lost for x > 60% (i).

In order to get further insight into the role of the rare earth we have prepared the oxides Yl-xPrxBa2Cu3OT- 6 (0 < x <i) by solid state reaction of the appropriate amounts of Y203, Pr6Oll, BaC03 and CuO. First an intimate mixture of the ground materials was fired in air at 850C for 12 h and slow cooled in the furnace. Then the mixture was reground and sintered in air for 12 h at 850C and slowly cooled in the furnace and finally heat treated for ii h at 600C followed of cooling to room temperature at 30C/h in flowing oxygen.

X-ray diffraction patterns can be in- dexed to the orthorhombic structure of YBazCu307- 6(2) with a slight expansion of the unit cell (mainly ~ parameter) and a decrease of the orthorhombic dis- tortion towards a pseudo-tetragonal cell. It is worthwhile noticing the ap- parent reversibility of values for the cell parameters between x=0.5 and x=0.6 (Fig.l), in a similar way as noticed by (3) in YBa2Cu307- 6 with different oxygen contents.

Resistivity measurements show a continuum decrease of Tc for x up to 0.5 as previously reported (i). However, as it will be shown by thermopower results, from the activated regime of resistivity observed for x >0.6, it cannot be con- cluded that the material is a semicon- ductor (that is a material having a gap at the Fermi level), as suggested in ref 1 based only in conductivity data.

Thermopower was measured using a slow a.c. method (4,5) as decribed el- sewere ( 6 ) . As shown in Fig. 2 the sub- stituon of Pr for Y, in amounts as small as 5%, dastically affects the ther- mopower S, that increases and changes its temperature dependence, becoming semiconductor like (dS/dT<0) in spite of metalic resistivity (d~d~ 0). For x< 0.5 S drops to zero at a temperature in good agreement with the resistivity

iO

10

tO

!0

I0

79 77 70 68 48 46

-9~.- 2 e

Fig. i- X-ray diffraction patterns of Y(l-x)PrxBa2Cu307- 6 (x values are indicated).

drop. For x~0.6, in spite of the fact that resistivity is activated, the ther- mopower aproaches zero as T decreases, clearly demonstrating that conduction takes place in a continuum of states arround the Fermi level. In the case a of semiconductor S should in- crease aproximately linearly with I/T.

0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

A.P. Gongaloes et aL / Series o f oxides Y~_~Pr~Ba:Cu~O z ~ (0 <~ x <~ 1) 911

The present results cannot be explained within a single particle transport model, suggesting hopping transport in narrow bands.

The temperature dependence of. ther- mopower at high temperatures is weak, and the present results can be qualita- tively explained within the Hubbard model and the modified Heikes formula ( 7 , 8 ) :

S = -KB/e [in (i- p )/ p- in 2]

Where ~ is the number of carrier per site, in this case with transport by hopping between Cu 2+ and Cu 3+ atoms, the ratio of CuZ÷ to the total number of Cu sites ( p =(x+26 )/3. The incorporation of Pr, with a stable 4+ oxidation state, not possible in YS+, gives one extra electron to the conduction band, in- creasing O and decreasing the average oxidation state of Cu. The low tempera- ture deviations to this temperature in- dependent model can be explained within the framework of correlated electrons

150

125

100

75

Prx Y(1-x)Ba2 Cu3 0%8

,7 #

x = 0 9

/ ~ ° ~

o ° ~ o -

- ~ ~@°°oo 0,o 0o _x=O.5

~o

% o

o ' ~ . ' % ~ oX = 0.10

• ,. = v . v ~ - - . . ~ . . T ~ o ~ o o,~ o ~ q ~ o

. ~ ,~ '~ ~' " ~=000

I I 50 250

| l I

100 150 200 TEMPERATURE (K)

5O

> 25 _2 n-

W

o D_ o

6

L~

o L, IE

~ 2

. J o

Fig. 2- Absolute thermopower Y(1-x)PrxBa2Cus07-6

of

300

when kT< bandwidth(8) or in the case of intermediate on site Coulomb repulsions (9). Acording to this model the decrease of the oxygen content is expected to have a simillar effect as the increase of Pr content, and indeed very simillar results have been observed for the oxygen deficient sampleg(6).

150 , i i i i i i , ,

o

, ,~ !

c / 3 - l 3 . 9 0

=~ I 3 . 8 5 . m

so I I

3 . 0 0 - 4 I

0 I I I I I , I I I I

O 0,5 P R A S E O D Y M I U M F R A C T I O ~ t ( X )

Fig. 3- Phase diagram for Y(l-x)PrxBazCus07-6 .At left, Temperature of (o) 90 and 10% resistance drop, (x) zero thermopower. A right values of the cell parameters.

In conclusion our results are sumarized in Fig.3 showing " the phase diagram of the Pr-Y substitution, and their lattice parameters. The smooth decrease of the orthorhombic distortion and the decrease of Tc with increasing x are interrupted at x ~.55 here there is a subtle structural transition and superconductivity is lost.

This work was partially supported by JNICT under contract 87/640.

REFERENCES (I) L. Soderholm et al., Nature,

328 (1987) 604. (2) R.J. Cava et al., Phys Rev.

Lett. 58 (1987) 1676. (3) I-Wei Chen et al., Solid St. Comm.

83 (1987) 997. (4) P.M. Chaikin and J.F. Kwak,

Rev. Sci. Instrum. 46 (1975) 218.

(5) M. Almeida et al., Phys. Rev. B 30 (1984) 2839.

(6) I.C.Santos et al, this Conference. (7) J.F. Kwak and G. Beni,Phys.

Rev. B 13 (1976) 852. (8) P.M. Chaikin and G. Beni, Phys.

Rev. B 13 (1976) 647. (9) J. Ihle and T. Eifrig, Phys.

Stat. Solid 91 (1979) 135.