Physica C 220 (1994) 41-49 North-HoUand PHYSICA

Superconducting properties of _xPrx) Ba2Cu408 compounds Z h e n G u o a, N o b u y o s h i Y a m a d a a, K e n Ich i ro G o n d a i r a a, Takeo I r i a and K a y K o h n b • Department of Applied Physics and Chemistry, The university of Electro-Communications, Chofu-shi, Tokyo 182, Japan b Department of Physics, School of Science andEngineering, Waseda University, Shinjuku-ku, Tokyo 169, Japan

Received 12 October 1993

(Yt_xPrx)Ba2Cu4Os compounds with x=0, 0.I, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were synthesized using a high-pressure oxygen technique. The solubility limit x, in this compound system was found to be 0.7. In this composition range, (YI-~Prx)Ba2Cu4Oa compounds have the YBa2Cu4Oa (124) structure and the lattice parameters a, b and c increase with increasing Pr content x. The superconducting transition temperature T¢ decreases monotonically with increasing x, and the critical content xc, where supercon- ductivity disappears, has been estimated to be 0.8. There is an obvious effect of Pr substitution on superconductivity, although the effect is smaller in the 124 system than that in the 123 system. X-ray diffraction results suggest that Pr is almost 3+ in this system. A model for the Pr substitution is suggested to make holes immobile to suppress superconductivity as the result of the hybridization of O 2p~ with Pr 4f orbitals.

1. Introduction

Since the 80-K superconductor YBa2Cu408 (124) was first discovered as planar defects in the YBa2Cu3OT_6 (123) phase by Zandbergen et al. [ 1 ], many works have been done on this oxide com- pound [2 -5 ] . The crystal structure of this com- pound was studied by neutron [6 ] and X-ray dif- fraction [ 7 ], and it was found that the 124 compound has a layered perovskite structure in which double C u - O chains run along the b-direction between two BaO planes. It is reported that the 124 compound has a more stable oxygen content than the 123 com- pound based on TGA and it does not show any or- thorhombic-tetragonal structural phase transition [3]. Morris et al. [4] have synthesized RBa2Cu4Os compounds ( R = N d , Sm, Eu, Gd, Dy, Ho, Er and Tm) and found that the superconducting transition temperature and the orthorhombici ty decrease with increasing ionic radius of the rare-earth element.

Meanwhile, a tremendous amount of work has been done on YBa2Cu3Ov_a. All the RBa2Cu307_ a com- pounds, in which Y is replaced by rare-earth ele- ments R, have been found metallic and supercon- ducting with critical temperatures Tc~ 90 K [8,9] with the exception of Ce, Pr, Pm and Tb. The spec- imens of CeBaECU307_ a and TbBaECU307_a pre-

pared by the solid-state reaction technique [ 10 ] nei- ther exhibit the 123 structure nor show superconductivity. Pm is radioactively unstable and the compound with Pm has not been investigated. PrBa2Cu307_6 is unique in being a semiconductor - not a supe rconduc to r - with the 123 structure. The reason why superconductivity is suppressed by Pr has not been clearly understood as yet. The elucidation of this fact is an attractive point that may help us to understand the interplay between magnetism and su- perconductivity. Therefore, the (Y~_xPrx)Ba2Cu307_6 system has been studied and it has been found that the superconducting transi- tion temperature Tc decreases monotonically to zero as x is increased to 0.5 [ 11 ]. The suppression of su- perconductivity by Pr in (Y~ _xPrx)Ba2Cu307_ a has been argued on the basis of three main mechanisms. The first mechanism is that Pr is tetravalent and ex- tra electrons released fill the holes in the CuO2 plane [ 12 ]. The second is that the magnetic moment of Pr suppresses superconductivity through the Cooper- pair-breaking mechanism [ 11 ]. The third is that the strong hybridization of Pr 4f and O 2p orbitals pro- motes the localization of mobile holes in the CuOz plane and the reduction of the superconducting tran- sition temperature [ 13-15 ].

It is interesting to study the effect of Pr substitu-

0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0921-4534(93)E0972-4

42 Z. G u o et a]. / . S 'uperconduc l ing prol~erHe.s ~!i ~ } ~ , t ' r , }Ba2( lt40,

l ion for Y in YBa2Cu408 on superconductivity ' , be- cause the local env i ronment a round Y ( P r ) sites in YBa2Cu408 is s imilar to that in YBa2Cu307. ,~. From the viewpoint of oxygen deficiency, investigation of (Y~_.~Prx)Ba2Cu4Os is more preferable than that of (Yl_xPr,.)Ba2Cu3Ov_~s because in YBa2Cu4Os the oxygen content is stable until up to 8 0 0 : C without oxygen deficiency. These have mot iva ted our inves- t igation of (YI _,Pr~)Ba2Cu4Os oxide superconductors .

In this article, we report the results of the prepa- ration, crystal structure, electric resistivity, magnetic susceptibi l i ty Z and scanning electron microscopy (SEM) measurements , and discuss both the super- conduct ing and the normal state propert ies of the (Y, ,Pr,-)Ba2Cu408 system. A part of this article and X-ray photoelectron spectroscopy (XPS) results for Yo.4Pro.6Ba2Cu4Os have been reported in refs. [ 16 ] and [17] , respectively. The results obta ined are compared with those for (Y~ ~Pr~)Ba2Cu.~O7_,~. The influence of Pr on superconductivi ty is also discussed.

2. Experimental

Specimens of (Y~ ~Prx)Ba2Cu40 ~ with x = 0 , 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8 and 1.0 were pre- pared using a high-oxygen pressure technique. After grinding a proper mixture of start ing mater ia ls 3N Y203, 3N Pr60~ , 4N BaO2 and 4N CuO in N 2 al- mosphere, the powders were pressed into pellets and sintered under certain anneal ing tempera tures be- tween 900°C and 980°C for 24 h in oxygen at 38 atm. The samples were cooled down to room ten> perature within 2 h by furnace cooling. These pro- cesses were repeated several t imes when required from the examinat ion of the 124 phase by X-ray diffraction.

The powder X-ray diffract ion pat terns were re- corded for all samples of (YI _~Pr.~)Ba2Cu40~ on a R I G A K U dif f rac tometer with Cu Ku radiat ion at room temperature . Latt ice parameters have been re- fined by the Rietveld method [18] , and the results are summar ized in table 1 including five kinds of re- l iabil i ty factors [ 18 ].

Electric resist ivity p(T) was measured using a s tandard DC four-probe method in the tempera ture range 4 .2-300 K.

Magnetizat ion measurements were per lbrmed us- ing a vibrat ing sample magnetometer in the temper- ature range 6-300 K. The susceptibil i ty Z m the no>. real stale was measured as a function of temperature at 20 kOe. The Meissner effect was measured using a field-cooled method at 50 Oc.

The microstructures of (Y~ ,Pr , ) Ba2Cu4Os were character ized using a scanning electron microscope (SEM) with an acceleration voltage of 10 kV.

3. Results

The specimen ofYBae('u.~Os ( .v=0) was found to have the 124 structure without other phases from X- ray diffraction.

The specimens of (Y~ ,Pr,)Ba2Cu4Os with .x= 0.1-0.7 wcre also found to have the 124 structure with a trace of other phases (BaCuO2 and ( 'u()}. Figure 1 illustrates the observed X-ray diffraction data and the profile fit for "~'o 3Pro.TBaeCu40,,. Dots in the figure are the obser ' ,ed intensities, a solid linc overlying them shows the calculated intensities, and A)" shows differences between the observed inten- sities and the calculaled ones. The short vertical lines mark the posi t ions of possible Bragg reflections. It can be seen from the figure that the calculated pal- tern fits very well with the observed one except for a small trace of BaCuO2 and CuO. The presence of secondary phases would not be a serious problem in our case, since Pr ions are not included in these phases and the Pr content in the 124 phase is onlx likel~ Io be somewhat larger than the intended value. The Rz ( integrated intensity R-factor) of Pr content . v=0-0 .7 are all less than 5%.

The specimens with .v= 0.75 and x = 0.8 were tbund to be composed of (Y~ ,Pr,)Ba2CuaOs. PrBa()> BaCuO2 and CuO. The lattice parameters of ~ , . Pr)Ba2Cu4Os in the specimen with . \=0 .8 arc a = 3.8681 (4) ,A. /~=3.8918(4) .& and c = 2 7 . 2 5 0 ( 4 ) 3~, which arc approximate ly the same as those v~ilh x = 0 . 7 . Hence we concluded that the solubili ty limit _v~ of the Pr ion in (Y, ,Pr, )BaeCu40, is 0.7. In the specimen of x = 1, the 124 phase was not found. The specimen was a mixture of PrBaO> BaCuO2 and CuO as reported previously [ l 6 ].

The room temperature values of the lattice param- eters a. t~. c and or thorhombic i ty .S" defined b\

Z. Guo et al / Superconducting properties of (Yl_xPr~)Ba2Cu,Os

Table 1 Crystallographic parameters for Y~ _xPrxBa2Cu4Os

43

X

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Lattice parameters (A) a 3.8370(3) 3.8405(2) 3.8750(1) 3.8485(1) 3.8531(2) 3.8585(3) 3.8629(1) 3.8667(1) b 3.8678(2) 3.8710(2) 3.8754(1) 3.8786(2) 3.8819(1) 3.8854(1) 3.8891(1) 3.8918(3) c 27.203(2) 27.206(1) 27.214(1) 27.215(2) 27.227(1) 27.236(1) 27.248(2) 27.251(2)

Orthorhombicity S a) 0.0080 0.0079 0.0078 0.0077 0.0074 0.0069 0.0067 0.0064

Volume (A3) 403.7(1) 404.4(2) 405.5(1) 406.2(3) 407.2(4) 408.3(2) 409.3(3) 410.0(2)

Atomic positions Y(0.5, 0.5, 0)

Ba(0.5, 0.5, Z) Z 0.1346(2) 0.1349(3) 0.1352(2) 0.1349(3) 0.1352(3) 0.1352(2) 0.1356(2) 0.1360(3)

Cu(l) (0, o, z) z 0.2123(3) 0.2119(2) 0.2123(3) 0.2132(4) 0.2125(4) 0.2130(3) 0.2127(3) 0.2127(4)

Cu(2) (0, o, z) z 0.0627(3) 0.0619(1) 0.0611(2) 0.0623(1) 0.0624(1) 0.0631(3) 0.0628(3) 0.0638(1)

0(1)(0, O, Z) Z 0.142(2) 0.143(2) (I.143(1) 0,145(1) 0.144(4) 0.145(2) 0.144(2) 0.144(2)

0(2) (0.5, O, Z) Z 0.050(1) 0.050(2) 0.047(1) 0.052(2) 0.052(1) 0.051(1) 0.050(2) 0.059(3)

O(3) (0, 0.5, Z) Z 0.054(2) 0.052(2) 0.053(1) 0.053(2) 0.055(2) 0.055(1) 0.054(2) 0.054(2)

O(4)(0, 0.5, Z) Z 0.217(2) 0.218(2) ( 3 . 2 2 0 ( 2 ) 0.219(2) 0.221(2) 0.221(2) 0.219(2) 0.219(2)

R values (%) R1 3.15 4.30 2.48 3.02 3.34 3.25 2.36 3.21 RE 1.93 1.74 1.46 2.33 2.31 2.11 1.44 2.16 Rwl, 8.59 6.19 5.67 6.47 6.35 6.15 5.46 5.79 R1, 6.57 4.84 4.34 5.13 5.08 4.89 4.27 4.56 RE 4.27 4.30 3.54 4.39 4.46 3.77 3.60 3.75

a) Orthorhombicity S=2(b-a)/(a+b).

2 ( b - a ) / ( a + b ) for the (Yt_~Prx)Ba2Cu4Os sys- tem are plot ted in fig. 2 as a function of Pr content x. The parameters a, b and c increase with increasing Pr content x, al though the or thorhombic i ty de- creases with increasing x. The lat t ice parameters a, b, and c of x = 1 (PrBa2Cu4Os) were es t imated by ext rapola t ion to be a = 3 . 8 7 9 9 /k, b = 3 . 9 0 3 0 / ~ and c = 27.274/~ using the lat t ice parameters of the spec- imens with x below 0.7. Thus, the compound in the (Yl_xPr~)Ba2CuaO8 system remains o r thorhombic even at x = 1 contrary to the (Yl -xPrx )Ba2Cu307-a system. The dependence o f the lat t ice parameters in

(Y1-xPrx)Ba2Cu4Os is s imilar to that in RBa2Cu408 ( R = N d , Sm, Eu, Gd, Dy, Ho, Er, and T m ) [4] in the sense that the latt ice parameters increase and the or thorhombic i ty decreases with increasing ionic radius.

Electrical resist ivity vs. tempera ture curves for the series of (Yl_xPrx)Ba2CuaOs are shown in fig. 3 (da ta for x = 0 . 2 and 0.3 have not been shown for clar i ty) . The magni tude of resist ivity of all the com- pounds increases monotonical ly with increasing Pr content. The values of normal state electric resistiv- ity at room temperature are given in table 2. It should

44 Z. Guo et al. / Superconducting properties o! (Y ~_ ,PrOBa/"u40s

7

j

J!l

20 30 ' 413 5"0 60 7(} f~0 90 10~

2 f l (deg)

Fig. I. X-ray diffraction pattern of Yo.3Pro.TBa2Cu40 s. Dots are

the observed intensities, the solid line is the calculated one, and AYis the difference between the observed and the calculated in- tensities. Short vertical lines mark the positions of the possible Bragg reflections. ( + )BaCuOv ( (3)CuO.

be noted that the temperature coefficient of the re- sistivity of ( Y j _ x P r ~ ) B a p u 4 O s in the normal state is always posit ive and all specimens show metallic behavior (i.e., d R / d T > O for T> T~) in contrast to the 123 system. In the curve o f (Yo.sPro.s)Ba2Cu408, a small two-step feature is seen at temperatures above T~. This behavior is apparently due to the inhomo- geneous distribution o f Pr content.

The superconducting transition temperature (T,•) is defined as the 50% reduction point of resistivity at the transition. Plots of T~ vs. x are given in fig. 4. The sharpness of the transition is also indicated in the same figure by vertical bars denoting the 10-90% width (ATe). The transition temperature T~ de- creases monotonical ly and AT~ increases with in- creasing Pr content x. The critical content xc, at which superconductivity disappears, has been estimated to be 0.8 by extrapolation. This est imation is consistent with the values obtained for samples prepared by the citrate pyrolysis method [19] , the nitrate pyrolysis method [20] and the HIP technique [21] for (Y~_xPrx)Ba2Cu408, and is higher than that (0 .5) for (Yi_:~Pr~)Ba2Cu3OT_,~ [ 11 ].

The data o f the Meissner effect are shown in fig. 5. The values of the transition temperature deter- mined from the Meissner effect measurement are in good agreement with the transition temperatures from the resistivity vs. temperature curves.

The magnetic susceptibility z ( T ) data at 20 kOe for the (Y~_xPr~)Ba2Cu408 system measured at

o <

eO

E

c~

~E E ©

©

0

2 7 . 2 8 . . . . . r ~

2 7 . 2 6

2 7 . 2 4

2 7 . 2 2 ~ - / , t

2 7 . 2 0 '

3 .91

3 . 8 9 ° ~ b

3 . 8 7

3 . 8 5

3 . 8 3

0 . 0 0 9

0 . 0 0 8 ~

t

1 I

_ 1

._.A~-~ .o - j j I

2 ( b - a ) / ( a + b ) I

A & &

L~

A 0.0[)7

2~

A

0 . 0 0 6 - - ~ - ~ . . . . . . . . . . . . . . . . . . ~ J 0.o o.2 o .4 o .o 0 .8 i . o

Pr content x Fig. 2. La t t i c e p a r a m e t e r s a, b. c and o r t h o r h o m h i c i t y S d e f i n e d

by 2 ( h - a ) / ( a + h ) for the (Yj .~Pr~)Ba. .Cu408 s y s t e m as a

f u n c t i o n o f Pr c o n t e n t .v al r o o m t e m p e r a t u r e • So l id l ines are de-

r i v e d f r o m the d a t a us ing the least square m e t h o d .

2q

l'r= o

2 Pr 0 1

Pr=04 Pr=O S

0] IS; E I )r:0

.~, Pr:0.7 I

~ P

o5; .!

°o 50 i00 l.Sn 2do 2So ~,,

Temperature(K)

Fig. 3. Electric resistivity vs. temperature curves for (Y~ ,Pr, ) BazCu408.

Z. Guo et al / Superconducting properties of (Yt_xPrx)Ba2Cu40s

Table 2 Superconducting transition temperature and susceptibility data for the (Yt_~Pr~)Ba2Cu4Oa system

45

X

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

p (mfl cm) 0.88 1.03 1.09 1.18 1.28 1.36 1.56 2.22 T¢ (K) 80 75 68 61 58 49 39 23 AT(K) 2.0 3.0 4.0 4.5 5.0 6.0 6.5 12.0 P~er (/~) 3.53(25) 3.55(20) 2.88(19) 3.17(10) 2.95(6) 3.05(15) 3.09(4) 0 (K) 6_+6 3+2 4_+2 5+_ 1 7+3 9+ 1 13+_ 1 Xo (10-~ emu/g) 1.82(5) 0.45(3) 0.41(2) 0.44(3) 0.37(4) 0.97(2) 1.30(6) 1.48(4)

y

1 0 0

[] P r e s e n t d a t a ' • ( Y , P r ) 1 2 3 ( P e n g e t a l )

8 0 [ •

6 0 • • •

4 0

2 0

0 • 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8

Pr c o n t e n t x

1 . 0

Fig. 4. Pr content x dependence of the transition temperature (T¢) determined by the midpoint of transition from the resistivity data. The vertical bar indicates 10-90% resistivity transition width (ATe). The solid curve is calculated by the AG theory.

temperatures above T¢ can be explained by the fol- lowing equat ion consist ing of the sum of the tem- perature independent component Zo and the Cur i e - Weiss contr ibut ion, i.e.

Z=Zo + C / ( T - O ) , ( 1 )

where C, O and ;to are the Curie constant , the Curie tempera ture and the Paul i paramagnet i sm contri- but ion, respectively. Using the least square method, the values of C, O and Zo were obtained. Plots of in- verse magnet ic susceptibi l i ty (Z-X0) - I vs. T are shown in fig. 6. The magnet ic susceptibil i ty increases with increasing Pr content x. The values of effective magnetic moments are es t imated f rom C and are given in table 2.

The microstructures of (Y~_xPr~)BaECu4Os with x = 0 , 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 were ob- served using the SEM method. The grain sizes o f the

0.00

-0.05

~ -0 .10

o o ~ o o ~ a o n o ~ o o ~ n ~ ° m | . mm •

• n *

• * J

• uP D P r = 0 . . . . . . . . . . . . . . . . . . i , . ~ . . . " , c p do P r=O.1 0 0 0 0 0 o , , 0 0 , , o o o * * ~ a ~ • P r = 0 . 2

P r = 0 . 3 • * * '***" * * * * * * * * ** "***** • P r = 0 . 4 ~ m m ~ u om oun ~ oDo nmn~ o P r = 0 . 5

P r = 0 . 6 • P r = 0 . 7

8'o "0"150 20 40 6'0 100

Temperature(K)

Fig. 5. Temperature dependence of field-cooled magnetization for the (Yt_xPrx)Ba2Cu4Oa system at 50 Oe.

800 o

a P r = O . I eo • P r = 0 . 2 • P r = 0 . 3 n na

6 0 0 * P r = 0 . 4 o n • P r = 0 . S o P r = 0 . 6 a ***** • P r = 0 . 7 mn

n n 0 ,**

4 0 0 ~ . " a **** u muu i o

... , . ,- .... o ° , • ~. *** msn s . o * * ° u ~ su

• * • o .~ o rams s o o ~ 2 0 0 • . .~o.~•B.~% D D , . ,

a

0 0 100 2 0 0 3 0 0

T e m p e r a t u r e ( K )

F i g . 6 . T e m p e r a t u r e d e p e n d e n c e o f i n v e r s e m a g n e t i c s u s c e p t i b i l -

i t y f o r t h e ( Y ] _ x P r x ) B a 2 C u 4 O s a t 2 0 k O e .

46 Z. Guo eta/./Superconducting properties qf (}'~ ,Pr~)Ba:( <u40,

sample were found to become bigger and the pores between grains became smaller with increasing x. Figure 7 is the SEM micrograph showing the micro- structures of (Y~ _,Pr,.)Ba2Cu4Os for x=0.1 and 0.7. The pores seem to have disappeared for the speci- men with x=0.7. These features of the microstruc- ture can be understood from the tendency that the sample of higher Pr content requires a higher an-

(a)

nealing temperature in order to have good quality.

4. Discussion

The superconducting transition temperature in the ( YI- ,Pr,)Ba2CuaOs system is found to decrease with increasing Pr content. The critical content .v~ has been estimated to be 0.8 which is higher than 0.5 for ( Y I_,Pr, )Ba2Cu30, ,~. There is an obvious effect of Pr substitution on superconductivity in the 124 sys- tem, although the effect is smaller in the 124 system than that in the 123 system. As .v increases, the av- erage ionic radius at the Y/Pr site of the 124 system becomes larger, and T~. becomes lower. The transi- tion temperature 7~ of RBa2Cu4Os (R = Nd, Sin, Eu, Gd, Dy, Ho, Er and Tin) [4] decreases with in- creasing ionic radius of the rare-earth element while Tc in the 123 system does not depend on the ionic radius. This indicates that the increase of ionic ra- dius at the Y/Pr site might be a reason why 7~. de- creases in (Y~_ ,Pr,)Ba2Cu4Os with increasing .\. Figure 8 shows a relation lbr T~ vs. ionic radius of ions in the rare-earth site for both (YI_,PG)Ba2Cu~Os and RBa2Cu4Os systems [4]. We adopted the average radius of Pr 3+ and y3+ lot the ionic radius of the Y/Pr site. Although 7"< de- creases with increasing ionic radius in both cases, their dependences are different in magnitude and the rate of decrease in (Y~ ,Pr, ) Ba2Cu408 is obviously

(b)

Fig. 7. SEM micrographs showing the microstructure of (a) (Yo.gProj)Ba2Cu4Os and (b) (Yo.3Pro.7)Ba2Cu4Os. Small black regions are pores.

Z

N 0

V )

20 • ( Y ] t P r ' , l J ~ a 2 ( ' u . l t )~ i

i~ t3; i2( d ( 8 (MOlr lS t'l :fl ; i i

I I , i , - J , . , i . ~ . !

0 ~,~) I I ) l I 0 3 I d ) 5 I I I ~ I 0 0 I 1 I ! !

Ionic radiust/\ i

Fig. 8. T~ as a function of the ionic radius of ions in rare-earth sites for both (Y~_ ~.PG)Ba2Cu4Os and RBa2Cu4Os (Morris et al. ).

Z. Guo et al / Superconducting properties of (Y~_xPr~jBa2Cu40s 47

larger than that in RBa2Cu4Os. This suggests that the ionic radius cannot be a main reason to determine Tc. Therefore, other mechanisms must be taken into consideration in order to clarify the reason of T~ reduction.

It is possible to imagine that the origin of the effect Pr substitution on superconductivity is the same in both the 123 and the 124 systems when we consider the local structural similarity. Therefore, we exam- ine the three main mechanisms proposed for the Pr substitution effect in the 123 system. The first mech- anism depends on the presence of Pr 4+, the second is the Cooper-pair-breaking mechanism and the third is the strong hybridization of Pr 4f and O 2p orbitals. ( 1 ) Valence of Pr: The effective moment of Pr in the 124 system has an intermediate value between the free-ion effective moment of Pr 3 + ( 3.58/~a ) and that of Pr 4+ (2.54 ~ta) as listed in table 2. This suggests that a mixed valence state is plausible for the Pr ion in the (Y~_xPrx)BaECu4Os system as it is in the 123 system, in which the effective moment remains nearly constant (2.7/~a) [22]. That is, the Pr valence state in the (Y~_~Prx)Ba2Cu408 system may be greater than 3+ and the extra electron released from Pr 3+ may cancel hole carriers in the conducting CuO2 plane. However, it cannot be concluded that the Pr ion is in a mixed valence state only from magnetic measurements because the reduction of the Pr mo- ment can be either the result of a splitting of the free- ion ground term by the crystal field as reported in ref. [23] or the result of the hybridization between Pr 4f and O 2p orbitals.

Figure 9 shows the relationship between Pr con- tent x and the Y / P r - O distance derived from the data of the powder X-ray diffraction (table 1 ). Here the Y / P r - O distance is an average distance between Y/ P r -O(2 ) and Y / P r - O ( 3 ) , where 0 ( 2 ) and 0 ( 3 ) are two oxygen sites in the CuO2 layer• The solid line in the figure is the Y / P r - O distance that is expected for Pr 3+, and the broken line is the Y / P r - O distance expected for Pr 4+. The ionic radii of Y 3+, O 2-, Pr 3+ and Pr 4÷ are 1•019, 1.35, 1.126 and 0.96 A, respec- tively [25 ]. The Y / P r - O distances estimated from neutron diffraction data are also plotted together [24 ] for comparison• The Y / P - O distance increases with increasing Pr content x. All the estimated Y/ Pr -O distances are almost overlying the solid line. If Pr ions are in a 4+ state, the Y / P r - O distance would

2.60 / , , ,

] [] Present data f • Neutron data(Berastegui et al.)

~ 2 . 5 2 £ ~

2.4

0.0 0.2 0.4 0.6 0.8 1.0

Pr content x

Fig. 9. Pr content x dependence of the Y / P r - O distance for (YI-xPrx)Ba2Cu4Oa • The solid line is the Y/pr3+-O distance and the broken line is the Y/Pr4+-O distance. The data from the neu- tron diffraction are plotted together [ 20 ].

change as the broken line which is quite different from the observed Y / P r - O distance• Hence the x de- pendence of the lattice parameters requires that Pr ions in the (YL-xPrx)Ba2Cu408 system are nearly in a trivalent state•

It is possible to distinguish Pr 3+ from Pr 4+ or to estimate the degree of mixture of these valence states using XPS measurements• Our XPS result supports also that the Pr valence state in (Yo.aPro.6)Ba2Cu4Os is 3+ [17].

In the (Yl_xPrx)Ba2Cu307_6 system, X-ray-ab- sorption [ 14,26 ] data have led to the conclusion that Pr ions are essentially in the 3 + state• (2) Cooper-pair-breaking mechanism: The transi- tion temperature Tc vs. Pr content x curve based on the Arbrikosov and Gor 'kov (AG) theory [27] is shown in fig. 4, overlying the experimental data for the 124 system. The AG curve can be fitted to the experimental data in x~< 0.6, but it cannot be fitted to the value of x = 0.7. This result indicates that the Tc vs. x curve in (Yl-xPrx)BaECU408 does not sim- ply obey the AG theory. (3) Hybridization of Pr 4f and O 2p orbitals: In the 123 system, the magnitude of electric resistivity de- creases with increasing Pr content x. EELS mea- surements [ 15 ] have shown that the total hole con- centration remains unchanged and does not depend on Pr content, and the Hall measurement [28] has shown that the mobile hole concentration becomes

48 Z. Guo et al. / Superconducting properties (~f (t'~ ~Pr,j Ba 2('l~40,~

lower with increasing Pr content x. From these re- sults in the 123 system, it is suggested that the hole changes from the mobile hole to the localized one with increasing Pr content x.

In our 124 system, the magnitude of the resistivity increases monotonically with increasing Pr content x. Moreover, our preliminary XAS result [29] of the (Y~ _xPrx)Ba2Cu408 system shows that the total hole concentration is independent of the Pr content. Therefore, the hole localization must also be the main mechanism in the (Yj_xPrx)Ba2Cu408 system. In our XPS measurement [ 17 ], the XPS spectra o f P r 3d and 4d states of Yo.4Pro.6Ba2Cu4Os are close to those of Pr203 in which there is hybridization between the Pr 4f and O 2p orbitals [30]. Then, it is concluded that Pr is almost trivalent with a 4f 2 configuration and a strong hybridization of Pr 4f and O 2p~ orbit- als would be expected in (Y~_xPrx)Ba2Cu4Os. Therefore, we have to consider the relation between the hole localization and the hybridization of Pr 4f and O 2p orbitals. It is reported from XPS data [ 31 ] that the Gd3+ :4f 7 level lies about 6-10 eV below Ev

(Fermi energy), and the Nd3+ :4f 3 level is about 3- 5 eV below EF in the 123 system. By extrapolation the Pr 3+ :4f 2 level would be expected to be close to EF. When the difference of the energy between the Pr 4f and the O 2p~ level becomes small, hybridiza- tion of Pr 4f, O 2p and Cu 3d becomes strong. Here, a model is suggested as illustrated in fig. 10. The hy- bridization of Cu 3d and O 2p~ gives rise to broad

E F

C u 3 d ~ (

(a) (b)

Prat

Fig. 10. Schematic energy level diagram for Cu 3d, O2p, Y4fand Pr 4forbitals. Without (a) and with (b) Pr ions.

bonding (~) and antibonding (~*) bands. A part of this G*-bands would be occupied by mobile holes [32 ]. Doping of Pr might promote the hybridization of Pr 4f and O 2p~ which could give rise to a broad bonding band (n) and a antibonding band (n*). It is possible for YBa2Cu408 ( x = 0 ) that the <J*-bands are partly occupied and the n*-bands arc fully oc- cupied. When Pr is introduced, the character of the holes gradually changes from <~* bands to n*-bands. Because the overlapping integral of the n*-bands is smaller than that of the G*-bands, the mobility of the holes in the n*-bands would be smaller than those in the ~*-bands. Hence Pr substitution could transform the holes from the itinerant holes into the localized ones, and could suppress T~ and lower electric con- ductivity. The effect of the Pr ion is not to destroy mobile holes by hole filling, but rather to localize them.

Therefore hybridization among Pr4f, O 2p and Cu 3d in the CuO2 planes may play a key role for our ( Y I -xPr~.) Ba2Cu40~. Further experimental and the- oretical studies will be required to resolve these issues.

5. Summary

We successfully synthesized (YI _ ~Prx) Ba2Cu408 by high-oxygen pressure technique, When the Pr content x is less than 0.7, Pr can be completely so- lidified; when x > 0.7, Pr cannot be completely so- lidified in (YI - ,Pr ,) Ba2Cu4Os. The superconduct- ing and the normal state properties of ( Y j _, Prx) Ba2CuaO8 ( 0 ~< A ~< 0.7 ) superconductors have been studied. The transition temperature ( 7~ ) and the or thorhombic distortion were reduced by in- creasing Pr content x, The lattice parameters a, b and c for the (YI_,Pr~)Ba2Cu408 system increase with Pr content x. The system remains orthorhombic in the composit ion range 0 ~< x ~ 0.7. The magnetic sus- ceptibility measurement o f the (Y~ .~Prx)Ba2Cu4Os system has revealed that the effective magnetic mo- ment o f the Pr ion is intermediate between the free- ion effective moment o f Pr 3+ (3.58/~B) and that of Pr 4+ (2.54/~B). On the other hand, X-ray diffraction and XPS results suggest that Pr is almost 3 + state in this system. The dominant effect of the Pr sub- stitution is to push up the energy of n*-bands higher

z. Guo et al / Superconducting properties of (Y~_xPr~)Ba2Cu~Os 49

than tha t o f t~*-bands as the resul t o f the hybr id i -

za t ion o f O 2p~ wi th P r 4 f orbitals , and to put holes

in to n*-bands and to m a k e t h e m i m m o b i l e to sup-

press superconduc t iv i ty .

Acknowledgements

T h e au thors w o u l d like to thank K. Asai for his

e n c o u r a g e m e n t in the course o f the study. O n e o f the

au thors ( Z G ) w o u l d l ike to thank K. Oka, F. Shi-

basaki and F. M a t s u o k a for the i r suppor t in exper-

i m e n t and c o m p u t e r ca lcula t ion .

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