Superconductivity and valence states of Ce, Pr and Tb in bulk Lu1−xMxBa2Cu3O7−δ (M = Ce, Pr and Tb) systems

PlIIIII

Physica C 219 (1994) 183-190 North-Holland

Superconductivity and valence states, of Ce, Pr and Tb in bulk LUl _xMxBa2Cu307_, (M = Ce, Pr and Tb ) systems K.I. G n a n a s e k a r b, A.S. T a m h a n e a, R. Pinto ", R. Nagarajan ", M. Sharon b, L.C. G u p t a a and R. V i j a y a r a g h a v a n " • Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India b Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400076, India Received 28 September 1993

Bulk samples of composition Lut_xMxBa2Cu3OT_s (M=Ce, Pr and Tb) are examined with respect to the formation of 1-2-3 phase and the superconducting properties. Samples with M = P r and Tb form a superconducting 1-2-3 phase for values of x<0.6. We observe that in case of M-: Tb, the zero-resistance transition temperature, Tc¢0),is 85-86 K and independent ofx for 0.1 < x < 0.3. For M = P r , a maximum Tcco) of 86 K is observed for x=0.1. Tc¢o) decreases with increasing x for x>0.1 and the sample with x=0.6 is observed to be semiconducting down to 12 IC With M=Ce, samples of the composition Lu~_x_yCexCa~Ba2Cu3OT_8 form the superconducting 1-2-3 phase for x, y=0.1 and 0.15. Samples of the compositions Lul_x_~Pr~?,avBa2Cu3OT_8with x=0.1, 0.2, 0.3 and y--0.1, have lower Tc¢o)'Sthan samples of a corresponding Pr content without Ca. We infer from our studies that the Pr ions are in the 3 + valence state for 0.1
1. Introduction Superconducting properties of layered orthorhombic cuprates of the composition RBa2Cu307_3 (R = Y or rare-earth element) do not vary significantly with the replacement of one R element with another. Exceptions to this are the rare-earth elements Ce, Pr and Tb; none of them forms a superconducting 1-23 phase. All these three elements have the common feature that they are the only rare-earth elements which have both a + 3 and a relatively stable + 4 valence state. Attempts to prepare RBa2Cu307_~ compounds with R = C e and Tb by means of conventional solid-state reaction methods have produced mixed-phase materials which include BaCeO3 or BaTbO3, in which Ce and Tb are in the + 4 oxidation state [ 1,2]. In case of Pr, one encounters a rather different situation, namely, P r B a 2 C u 3 0 7 _ 6 does form in the 1-2-3 phase but is not superconducting. The compound is semiconducting and the Pr moments order magnetically at a much higher temperature in this system compared to that of other rare earths in the 1-2-3 phase, with N6el temperature

TN(Pr) = 17 K, whereas TN(R) for other rare earths R is less than 3 K [ 3 ]. Studies of the system Y l _ x M x B a 2 C u 3 O T _ 6 ( M = C e , Pr or Tb) in which the elements M have been partially substituted for Y, also show interesting differences. In the cases of Ce and Pr such partial substitution tends to reduce the superconducting transition temperature with their content increasing [ 4-6 ]. In contrast to this, the transition temperature of thin films of Yl-xTbxBa2Cu3OT-6 Was observed to be independent o f x in the range 0 < x < 0 . 5 [6]. Of these three elements, partial substitution of Pr has been studied most extensively. A variety of physical mechanisms has been proposed to account for the quenching of superconductivity due to partial Pr substitution in the Y: 1-2-3 compound. These fall into two broad catogories: hole filling/localization which is based on Pr being, at least partly, in the 4 + valence state, and magnetic interactions or hole-localization effects which are mediated through hybridization o f P r 4f electrons with the valence band states, Pr being in the 3 + valence state [ 7]. Partial substitution of Pr has been carried out not

0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0921-4534(93)E0877-4

184

K.L Gnanasekar et al. / Superconductivity in Lul_~.Mflla2Cu30z ( M r Ce, Pr, Tb)

only in the Y: 1-2-3 compound but also in almost all other superconducting R: 1-2-3 compounds. The resuits show that the rate of suppression of superconductivity as a function of the Pr concentration is related to the ionic radius of the host rare earth. It is lower for rare earths having a smaller ionic radius [ 8 ]. The elements Yb and Lu have small ionic radii and form the compound RBa2Cu3OT_6 in the 1-2-3 phase with increasing difficulty. The ionic radius of Yb is nearly critical and it is rather difficult to prepare YbBa2Cu3OT_s in the 1-2-3 single phase [ 9 ]. In case of Lu, the phase forms only when deposited as a thin film, where the substrate presumably helps in stabilizing the phase [ I0 ]. Bulk material of composition LuBa2Cu307_s is multiphasic [ 9,11 ]; however, partial substitution of elements such as Y or Sm for Lu, stabilizes it in the superconducting orthorhombic 1-2-3 phase [ 9,12 ]. We had earlier observed that the material with composition Lul_xPrxBa2Cu3OT_6 can be prepared in the superconducting 1-2-3 phase [13]. We have now carried out experiments to see if such a stabilization could be achieved with Ce and Tb, which also are ambivalent ions. In the trivalent state they have a larger ionic radius than Lu3+ but, unlike Pr, do not form an orthorhombic 1-2-3 phase. In our present studies we find that while Pr and Tb do stabilize the Lu: 1-2-3 phase, Ce does not. Our results on Pr substitution fit into the pattern of the ionicradius dependence found for other R: 1-2-3 compounds. The results for Tb substitution show that Tb stabiliTeS the material in the 1-2-3 phase in bulk form without reduction of the transition temperature for zero resistance, To(o). Compounds of the composition Lul_x_yR~ CayBa2Cu3OT_6 with R f C e and Pr were also prepared. In the ease of Ce-substituted material, addition of Ca stabilizes the 1-2-3 phase. In materials containing Pr, we observe a reduction of To(o) with the introduction of calcium. In this paper we report the crystallographic and superconducting properties of all these materials and discuss the implications with respect to the valence states of Ce, Pr and Tb in these compounds.

2. Experimental Bulk samples with the nominal composition of M=Ce, Pr and Tb, and Lul_x_yRxCayBazCu3OT_~,R f C e and Pr, have been prepared by solid-state reactions of high-purity Lu203, CeO2, Pr6Oll, Tb4OT, BaCO3, CaCO3 and CuO. Stoichiometric proportions of the appropriate constituents were mixed thoronghly and ground for about an hour. As the samples conteinin2 Tb were found to melt beyond 9050C, they were calcined in air with the following heat tratment: LUl_xMxBa2Cu3OT,

Room temp 5°c/mm, 890oC l°C/m~ 3h 8950C 1*C/ram 9000C 5*C/mmRoomtemp. 10h 10h Samples containing Ce and Pr were fired in air at 900°C for 12 h without any intermediate steps at lower temperatures. After the first heat treatment the samples were reground for about an hour, pelletized and sintered in air with the same heat treatment as before and the process was repeated twice. Finally, the samples were annealed in oxygen flowing at the rate of 30 ml per minute. Here again the samples containing Th were given a slightly different heat treatment as follows: Roomtemp 3*c/mm 6000C rC/mm 3h 450oc ~-c/~t. Room temp. 12h The other samples were annealed in oxygen at 900°C for 24 h, cooled to 4500C at the rate of 2°C per minute, annealed at this temperature for another 24 h and then cooled to room temperature in about 8 h. In our earlier work o n Lut_xPrxBa2Cu307_,l sampies, the firing temperature ranged between 910 °C and 935 °C and oxygen annealing was carried out at 650°C. Our present results show that the thermal treatment followed in this work yield better samples. The composition and homogeneity of the samples were checked by energy dispersive X-ray analysis and crystallographic phases were characterized by X-ray diffraction (XRD). Four-probe electrical resistivity

185

K~L Gnanasekaret al. / Superconductivity in Lu ~_~M~BazCu~O~(M= Ce, Pr, Tb)

and AC susceptibility measurements were carried out using a closed-cycle helium cryocooler over the temperature range 300 K to 12 K.

3. R e s u l t s

x • 0'15

®

I

>I-

~

Jo -

Table 1

Summary of data on Lu, _xMxBa2Cu3OT_d Lu~_x_:MxCarBa2Cu~07_~bulk samples M

x

y

T¢(o)

Ce

0.1

0.1

73

0.I 0.15

Pr

0.15 0.15 0.07 0.1 0.1

70 66 78 86 78

0.15 0.2 0.2 0.3 0.3 0.4 0.5 0.55

0.1

0.1 0.1

V~(A~)

J~ (meV)

,'5

5

~J

o

~

.

v.,

,~o

o

[I

--

8-

~'5

36

25

,o

5'~

20

Fig. 1. X - r a y diffraction patterns of (a) a L~.TsC~.tsC~.tBa2C-~30~_8 ~ple and (b) a L~.~C~.IsC~.IsI~2Cu3OT_asample. The asterisks indi~te the l~CeO~ peaks.

x.o.oT

X'O.15

X'OA

X=0.2

~= m

g

2 8 (DEGREE)

N 2 8 (DEGREE)

Fig. 2. X-ray diffraction patterns of some Lut _xPrxBa2Cu307_8 samples. Impurity peaks are marked with an asterisk. to be a superconductor with To(o) -- 60 K; lower than that observed in the three samples containing both Ce and Ca.

171.285

82.5 80 74 71 60.5

172.338 172.728

33 " ) 57 b) 38 ~) 54 b)

172.794

48 ~) 59b)

49 29 13

174.054 174.445 174.468

values for n~fx. b) j= values for n~= (x-- O.I ). ") ./a

(K)

and

x -0-15 y - O . 1,5

m[~.[=

3. I. Lut_xCe~BazCu~Ov_~ and L u ~_~_ yCe~CayBa zCu s 0 7_n systems

O f the three systems Lux_~M~Ba2Cu3OT_~ ( M f C e , Pr, T b ) studied here, only Ce did not form a stable 1-2-3 phase in the bulk form. We tried two compositions, x=O.1 and 0.15. For both compositions the samples were observed to be multiphasic and insulating. However, samples of the composition Lum_x_yCexCayBa2Cu3OT_a in which Ce and Ca partially substitute for Lu, did stabilize in the 1-2-3 phase, albeit with some impurity phases. Three samples of compositions x = 0.1, y = O. 1; x = O. 15, y = O. 1 and x = 0 . 1 5 , y = 0 . 1 5 were prepared and were observed to be superconductors with Tc(o)=73 K, 70 K and 66 K, respectively (table 1 ). The impurity phases observed in the X R D data of samples with x = 0.15 include a low-intensity BaCeOs line (fig. 1 ). We also prepared one Luo.9CaoaBa2CuaOT_a sample for comparison. This sample did form in the 1-2-3 structure with good phase purity, and was observed

,.o1

3.2. Lul_xPr,,Ba2Cu3Oz_6 and LU ~_ x_ yPrxCayBa 2Cu sO v_a systems

In the case of Pr, we prepared ten samples of composition Lul_xPrxBa2Cu307_6 with 0 . 0 7 < x < 0 . 6 5 and three samples of composition Lul_x_yerxfayBa2Cu307_6 with x - - 0 . l , 0.2, 0.3, and y=0.1. The X R D spectra of some of the samples is shown in fig. 2. While the sample with x = 0 . 0 7 does contain impurity phases, samples with x = 0 . 1 and

186

K.L Gnanasekaret al. / Superconductivity in Lul _xM~Ba~Cu~Or(M= Ce, Pr, Tb)

0.15 are essentially 1-2-3 phase; the impurity phases, marked with an asterisk in the figure, are reduced substantially. Samples with x > 0.2 are single-phase 1-2-3 compounds. The sample with x = 0.07, though not single phase, is observed to be superconducting with a T~(o)= 78 K. The highest T~(o), 86 K, was observed for the sample with x = 0 . 1 , the same as that for a pure LuBa2Cu3OT_6 thin film [ 10]. For higher values of x, the Teto) steadily decreases. In the sample with x=0.55, the resistance is semiconductor-like down to about 60 K, below which it decreases and the sample superconducts at Tc(o)= 13 K (fig. 3). The samples with x = 0 . 6 and 0.65 are semiconducting down to the temperature of 12 IC AC susceptibility data show that the samples are bulk superconductor. The data in f'w,. 4 are normalized to a sample weight of 0.5 g in each case. The signal of the sample with x=0.1 is about the same as that of YBa2Cu3O7_6. Lattice parameters of the 1-2-3 phase were deduced using a least-squares fit of the XRD data and the calculated unit-cell volumes are given in table 1. A noticeable feature of these values is that the cell volume of the samples increases with increasing Pr content and that there is a rather discontinuous increase in cell volume after x_-_0.1 and x ~ 0 . 3 . The first jump in the lattice cell volume is accompanied by a decrease of To(o) (for x < 0.1, T¢(o) increases with x). The second is associated with a sudden drop in T¢(o), by as much as 20 K.

1.2

1.0

0,8

.%

nf

0.4

0.2

I 2

x'O.I X " 0.15

3 4 5

X'0.2 x=0.3 X • 0.0.7

6

X =0.4

7

X=0.5

8

• " 0.55

201)

3OO

T E MPERATURE (K)

Fig.

3.

Resistance

Lu, _xPrxBa2Cu307 _~.

,,

,'~

v v

4

•

-

~1

-20

•

m

~a

~II

vv

~,~, o o -40

o,~

-BO

•

•

x=O,07

•

x- O.I

•

x ffi 0.15

m

x-02

•

~.o.s

.

x.o.4

•

x- o,5

v

x - O, 5 5

•

"o o, %0"

v W

o , oo=

mA

uv

-60--

o

o

aa

•

•

•

•

n

•

•

I 50

•

*

n

TEMPERATURE

I I00

t

150

(K)

FiE, 4. AC susceptibilitydata for Lul_xPr~a2CUaOT_6samples. The Z' s i g n a l for the x=0.1 sampleis of the same magnitude as that ofa Y 123 sample. Addition of calcium in this system decreases To(o) for x in the range 0.15 ~ x ~ 0.3. 3.3. L u l_ xTbFBa2CusO7_6 system

Although neither LuBa2Cu3OT_~ nor TbBa2Cu307_ 6 forms an orthorhombie 1-2-3 phase in bulk form, Lul_xTbxBa2Cu3OT_~ does stabilize in the orthorbombic 1-2-3 phase in the nominal compositions 0.1 < x < 0 . 6 . The XRD data show that in the compound with x--0.1, 0.2 some impurity phases are present (fig. 5). For the compound with x = 0 . 3 , along with other impurity peaks, a small peak of BaTbO3 (marked with an asterisk in fig. 5) shows up which becomes more prominent with increasing Tb content. Obviously the quantity of Tb stabilizing in the 1-2-3 phase in these compositions is less than the nominal amount. The samples with nominal compositions of 0.1
4. Discussion

0 IO 0

I i I I v • v • v V~mI~a~A~BaqI~I~AtlUOWVlIO~aVOIAt

0

(R/R~oo)-temperature plots for

The multiphase nature of the bulk compound of the composition LuBa2Cu3OT_~ is a consequence of the small ionic radius of the Lu 3+ ion [9,11 ]. Partial substitution of Lu by other rare-earth elements including Pr and Tb, stabilizes it in 1-2-3 phase. That

IEI. Gnanasekar et aL / Superconductivity in LuI_~M~Ba2Cu3Ov(M=Ce, Pr, Tb)

it stabilizes with partial substitution by Tb is a rather interesting result in view of the fact that Tb by itself does not stabilize in this structure but forms BaTbO3 in which it is in the 4 + valence state. This may be contrasted with the results for Ce, which fails to form a stable Lum_xCexBa2Cu307_a compound in the 1-23 phase. The difference between the two elements is presumably due to the fact that Ce has a more stable 4 + valence state than Tb. The superconducting transition temperatures of

x -0-I I-

L u l _ x T b x B a 2 C u 3 O T _ a f o r 0.1 < x ~ 0 . 6

,,4 x -0"2

Z ttl I.Z

-~ 6

o

15

x -0"3

0 o

25

35

o4

-

45

55

28

Fig, 5. X-ray diffraction patterns of Lus_xTbxBazCu3OT_ 8 s a m pies. The BaTbOs peak marked with an asterisk is absent in sam* pies with x=0.1 and 0.2, but becomes more prominent with increuing x for x > 0.3.

1.00

0.50 n,-

I

0.00

0

X-0.5

2 X,

0.3

3 X-

0.6

.......

,'-oo . . . . . . . .

2oo'. . . . . . . . .

3oo

Tempero'fure (K) Fig. 6. Resistance (R/R~oo)-temperature data for some Lu!_xTbx]BalCU3OT_asamples showing that Tc(o) is independent

ofx.

187

a r e 8 5 - 8 6 K,

just as in the case of partial substitution of Lu by Y and Sm, where Tcco)=87-90 K for 0 . 2 < x < 0 . 9 [9,12]. Moreover, the ionic radius of Tb 4+ is less than that of Lu3+. Thus the two observations, (1) formation of a stable 1-2-3 phase, and (2) transition temperature Tc¢o) the same as that of a pure Lu 1-2-3 film, indicate that Tb is essentially trivalent in this system. As mentioned earlier, the XRD data indicate that for x > 0 . 3 , T b incorporated in the 1-2-3 phase is less than the nominal composition. Hence we infer that the Tb ions are mainly trivalent in Lut _xTbx~a2Cu307_6 for 0.1 < x < 0 . 3 in bulk samples. Two important points must be mentioned here. ( 1) Though our bulk samples of LUl_xThxBa2Cu307_6 with x > 0.3 have BaTbO3 as impurity phase, thin films of the same compositions have a very good phase purity and superconduct at 86 K [14]. (2) It is well known that in the Yl-xTbxBa2Cu307-6 system the bulk samples are multiphasic for all values ofx. In the case of Lu~ _xtbxBa2CU3OT_6we observe bulk samples with good phase purity for x < 0.3, though Lu 1-2-3 phase does not form in bulk. It appears that the reduction of Tb to the 3+ valence state at the high temperature of formation of 1-2-3 phase is brought about by the lattice stabilization energy of the Lu 1-2-3 structure. This observation has relevance for the valence state of Pr in this structure, since their oxidation potentials (R 3+ to R 4+) are nearly the same [ 15 ]. Understanding the systematics of the T~¢o)'Sin Prdoped compounds is somewhat more involved. The highest T~¢0) of 86 K, the same as that for a singlephase Lu 1-2-3 film is observed for a x=0.1 bulk sample. A thin f'dm of the same composition on the other hand has a lower T~¢o), 83 K [14]. The dif-

188

K.L Gnanasekar et al. / Superconductivity in Lu~_~MxBa2Cu30r (M= Ce, Pr, Tb)

ference, though small, is not due to experimental error, since repeated measurements on another sample of the film, deposited in a separate run, yielded the same result. We ascribe the difference to the fact that Pr plays a more complex role in the bulk samples. The observed T~to) shows that for x = 0 . 1 Pr supports the 1-2-3 phase formation with no decrease of T~to ) and in this sense behaves like any other trivalent rareearth element. Thus we conclude that it is in the 3 + valence state at this concentration. At higher concentrations, i.e. at x>0.15, there is a progressive reduction in T~to ) of the superconducting phase and we cannot rule out Pr being partly in the 4 + state for x>~0.15. However, the ionic radius of Pr 4+ is smaller than that o f Lu 3+ and the abrupt rise in the cell volume at x=O. 15 and a monotonical increase for higher values of x (table 1 ) suggests that Pr continues to be mainly in the 3 + valence state. This inference, particularly for 0 . 1 5 < x < 0 . 3 , is further supported by our data on the transition temperatures of samples of the system Lul_x_yPrxCayBa2Cu3OT_6. Neumeier et al. [16] have proposed counteracting effects of generation and filling o f holes in the CuO2 sheets by Ca 2+ and Pr 4+ ions to explain their observation of an increase in T¢¢o) in some of the samples of the system Yl -x-yPrxCayBa2Cu3OT-6• Before we proceed further on our Pr-doped system we make a brief digression and discuss our results on the Lut_x_yCexCayBa2Cu3OT_6 materials. Unlike Pr and Tb, partial substitution of Ce fails to stabilize the Lu-Ba-Cu--O system in the 1-2-3 phase. The ionic radius of Ce 4+ is also smaller than that of Lu 3+ and the failure to sabilize the system in the 1-2-3 phase indicates that Ce is not reduced to the 3 + valence state to any significant degree in these compounds. However, when Ce is substituted along with Ca (ionic radius of Ca 2+ _-_.Pr3+ ), the 1-2-3 phase is stabilized albeit with the presence of some impurity phases. We observed that the Lul_x_yCexCa~Ba2Cu307_6 compounds with the nominal compositions x=O.1, y=0.1, x=0.15, y=0.1 and x=0.15, y=0.15 superconduct. The Tc(o) of the sample with x = 0.15 and y = 0.1 is higher than T¢
amount of Ce in the 1-2-3 phase for samples with x = 0 . 1 5 appears to be between 0.1 and 0.15. However, all these samples have a higher Tc
K.L Gnanasekar et al. / Superconductivity in Lu~_xMxBa2CusOz( M f Ce, Pr, Tb)

ln[ T ~ ( o ) ( x f O ) /Tc(o)(X) ]

=~(½ + ½~,To~o)(X) )-~/(½) ,

(1)

where • 7 ~= (2n/kB)n,N(Ev)J~,, ( g - 1 )2j(jq. 1 ) .

Here T¢¢o)(x=0) is the Tc¢o) of the LuBa2Cu3OT_6 thin f'tim, or T~¢o) of the Luo.9Pro.iBa2Cu307_ 6 bulk sample; ¥ is the derivative of the logarithmic gamma function, nt is the concentration of Pr ions, g and j are, respectively, the Land6 g factor and the total angular momentum of the Hund-rules ground state of Pr 3+. N ( E v ) is the density of states at the Fermi level which we take to be ---0.44 states/eV atom spin, following Neumeier et al. and Malik et al. [16,8] and which we assume to be constant for the three compounds. We obtain IJ~l values which increase with increasing x (table 1 ). Since for x=0.1 T~to) is the same as that for a LuBa2Cu3OT_6 thin film, we may postulate that Pr does not have a magnetic moment at this concentration (e.g. Pr in the R2CuO4 structure has a singlet ground state [ 20] ), but begins to have one only at higher concentration which is associated with a rather abrupt increase in the cell volume. On the basis of studies on CmBa2Cu307_6 Soderholm [21 ] has argued that the suppression of superconductivity by an f element has a magnetic origin and requires both a substantial local moment as well as extended f orbitals. The superconductivity of other Pr-containing high-T¢ oxides (e.g. Prl_xCe~CuO4) is then explained by the absence of a local moment on Pr [ 21 ]. We take this argument one step further in the context of our work, and suggest that in the LuPrBCO system the ground state of Pr 3+ may be changing from lJ r 4, M~= 0 ) to IJ = 4, M~+ 4 ) with the increase in Pr concentration and the concomitant increase in cell volume. In such a case, if we use n~= ( x - 0 . 1 ) instead o f x in eq. (1), we get IJo~l values which are reasonably consistent (table 1 ) and which support the conjecture that there is a change in the character of the ground state. The only essential assumption here is that the density of states at the Fermi level is the same for all three c o m positions ( x = 0.1, 0.2 and 0.3), which would be true if the Pr ions are trivalent. A similar calculation for compounds with a higher Pr concentration (x---0.4, 0.5, 0.55) with the same

189

assumptions yields a much higher value, }J~[ _-__77 meV, which suggests that some additional mechanism is operating at these concentrations. The reduction in metallicity observed in these samples (fig. 3) points to hole localization as the likely additional mechanism responsible for the higher values of d T ~ o ) / d x in this range, which in turn appear to be triggered by the rather discontinuous increase in the unit-cell volume between samples with x--0.3 and x=0.4.

5. Summary Bulk

samples

of

the

compositions Pr, Tb) and Lul_x_yRxCa~Ba2Cu3OT_~ ( R = C e , Pr) were prepared and examined with respect to phase formation and superconducting properties. Samples with M = Pr and Tb do form superconducting 1-2-3 phase for x ~ 0 . 6 , but those doped with Ce do not. When Ce was doped along with Ca, 1-2-3 phase did form and the samples were observed to be superconducting indicating that Ce is in a valence state close to 4 +. In case of Tb, Tc¢o) was independent of x and was 8586 K for nominal compositions 0 . 1 < x < 0 . 6 . For x>_0.3, some impurity peaks in the XRD data (corresponding to BaTbOa) suggest that all the Tb in the starting composition does not get accommodated in the matrix. This brings out the effect of the interplay of the matrix trying to stabilize the Tb ion in the 3 + state vis-a-vis BaTbOa trying to stabilize the Tb ion in the 4 + state. The resistivity and the XRD data indicate that Tb is mainly in the 3 + valence state in the Lul _xThxBa2Cu307_6 phase. For M = Pr a maximum in Tcto) was observed at x = 0.1 which was the same as that of a c-axis oriented single-phase LuBa2Cu307_6 film, and T~¢o)decreased for higher values ofx. Samples of Lul _x_yPrxCayBa2Cu307_6 with X=0.1, 0.2, 0.3 and y=0.1 have a lower T~(o) than samples of the corresponding Pr content without Ca, indicating a 3 + valence state for Pr in these compounds. The analysis of our data supports the conjecture that either the 4f orbitals of Pr hybridize with valence-band electrons only at concentrations higher than x=O.1 in this system, or that the crystal-field split ground state is nonmagnetic for x--- O. 1 and begins to become magnetic for x > O. 1.

Lul_xMxBa2Cu307_ ~ ( M = C e ,

190

K.L Gnanasekar et al. / Superconductivity in Lu~ _~tl~BazCu307 (Mffi Ce, IV, Tb)

Acknewled,,-ements The authors would like to thank S.P. Pal, C.P. D'Souza, D. Kumar and S.K. Pagdhar for help in experimental work. One of the authors (KIG) would like to thank the Council of Scientific and Industrial Research for fmancial support.

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