- Email: [email protected]
Synthesis of a new series of rare-earth vanadates RBa2V3O11 (R = Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er, Tm)
Materials Letters 15 ( 1993) 343-346 North-Holland
Synthesis of a new series of rare-earth vanadates (R=Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er, Tm)
RBa2V301
I
X.L. Wang a, H. Wang a, S.X. Shang b, Z.P. Ai b, H.C. Chen a and M.H. Jiang a a Institute ofcrystal Materials, Shandong University, Jinan 250100, China h Experimental
Center, Shandong
University, Jinan 250100, China
Received 16 October 1992; in final form 12 November I992
A new series of rare-earth vanadates RBa2V,0,, (R=Y, La, Pr, Nd, Eu. Gd, Dy, Ho, Er, Tm) are first synthesized by the conventional solid-state reaction. The single phase can be obtained at a sintering temperature of IO10aC for most samples. The X-ray powder diffraction analysis has shown that the new compounds have an orthorhombic structure with cell parameters a= 7.60 to 7.88 A, h=7.58 to 7.85 8, and c=23.60 to 24.35 A for R=Y, Pr, Tm, Er, Dy, Ho compounds or a tetragonal structure with a=7.85 to 7.94 A and c=23.95 to 24.27 8, for R=La, Nd, Cd, Eu compounds. These cell parameters depend upon the rare-earth ionic radius. The RBa2V,0, I can be decomposed to RVO,, Ba3( V02)2 and V,O, by sintering temperature higher than 1010°C. The as-sintered samples are insulators with a resistivity of IO” R cm. Furthermore, if the as-sintered samples are annealed under H2 atmosphere, the RBa2V,0,, phase decomposes into RVO, and BaVO, despite the lower annealing temperature of 950°C.
1. Introduction Since the discovery of the copper oxides high-T, superconducting system, La-Ba-Cu-0, by Bednorz and Miiller [ 11, many other copper oxides high-T, superconducting systems such as Y-Ba-Cu-0 [ 21, Bi-Sr- (Ca)-Cu-0 [ 3,4 1, Tl-Ba-Ca-Cu-0 [ 5 1, Pb-Sr-Ca-Cu-0 [ 61 etc. have been found. It would be interesting to find a non-copper oxides high-T, superconducting system both for practice and for high-T, superconductivity theory. Ba-K-Bi-0 [ 71 and St-( Ln )-Nb-0 [ 81 were discovered to have T, of 30 or 12 K, respectively, and possible high-T, superconductivity with T, of 130 K in the doped vanadium-oxides-based Sr-V-O [9] system has been attracting study. The Sr,, ,V,O, (n = 1, 2, 3 ) system was synthesized by Naziki et al. [ lo]. Recently, Meng and Ren [ I I ] have reported the series of rare-earth vanadates LaSr,_,Ca,V,Og_y, which show low resistivity and antiferromagnetic properties. Up to now, although no superconductivity was observed or verified in this system, the vanadium oxide is still a candidate for the non-copper oxides superconducting system based on some important characteristics in common with high-T, cuprate HTSC: ( 1) the exisElsevier Science Publishers B.V.
tence of two-dimensional V02 sheets expected to be responsible for electronic conduction; (2) the expected existence of localized spin 1 / 2 arising from the nominally tetravalent vanadium ion; (3) the existence of metal-insulator transition. In this paper, a series of new compounds RBa2V30,, are reported for the first time. They are insulators.
2. Experimental Samples were prepared by the conventional solidstate reaction method. High-purity powders of R,03 (R=Y, La, Pr. Nd, Eu, Gd, Dy, Ho, Er, Tm ), BaCO, and V,05 were used as starting materials. The powders were weighed, mixed in the ratio R: Ba: V= 1 : 2: 3 and pressed into pellets and then sintered at 1010°C for 24 h in air. After characterization by XRD, TEM, EDAX and resistivity at room temperature, these as-sintered samples were annealed at 950°C for l-3 h in a H2 atmosphere. The XRD pattern was obtained with a Rikaku D/Max RA X-ray diffractometer using Cu Ka radiation. The compositions of crystal grains in these samples were analyzed by energy dispersive X-ray spectroscopy using 343
Volume 15, number
5,6
MATERIALS
LETTERS
a Philips EDAX model PV-9100 spectrometer. The electrical resistivity was measured using the standard four-probe method. The meter/de voltage source, 4 140B PA equipment made by HewlettPackard company, was used for high-resistivity measurements.
1993
(Ml), indicating that the c-axis orientation for this sample can be easily obtained by a high-pressure process before sintering. We can get the single l-2-3 phase at the same sintering temperature of 1010°C for 24 h except for the sample with R= Nd, in which there are Ba3(V0,)2, NdV04 and V205 phases as obtained from its XRD pattern or EDAX analysis. These phases must come from the decomposition of NdBa,V,O, which is very similar to that observed in LaBa,V,O, sintered at 1080°C. It may be concluded that the RBa2V30, can be decomposed if the sintering temperature is too high, according to the formula
3. Results and discussion Figs. la and lb show the XRD pattern of sample R=La sintered at 1010°C or 1080°C for 24 h, respectively. The latter sample was pressed at high pressure before sintering. One can see that the phase formation depends upon the sintering temperature. At a lower sintering temperature of 10 10 “C, a nearly single l-2-3 phase is formed, while at a high temperature of 108O”C, there are other phases (Ba3(V04),, VzOs and LaVO,) inside the sample which were determined by XRD or EDAX. The results indicate that the l-2-3 phase can be decomposed at a temperature higher than 10 10” C. The decomposition formula may be 3LaBazV,0r1 = 2Ba3( V04)* + 3LaV0, + VzOs. Furthermore, for the latter sample which was pressed under high pressure before sintering, its XRD diffraction peak of (00 1) is much higher and sharper compared with other
3RBa2V,01,
=2Ba,(VO,)*
+3RV04
+V205
.
So if one wants to get the single l-2-3 phase for the R=Nd sample the sintering temperature must be lower than 10 10°C in order to prevent the decomposition of the sample. The X-ray powder diffraction patterns for all the samples studied are similar to each other, indicating similar structure. Their diffraction peaks can be easily indexed by orthorhombic or tetragonal cell parameters. The cell parameters for samples with different R are shown in table 1. The indexed peaks for the sample LaBa2V,01, using tetragonal parameters a= 7.90 A and ~~24.11 A are shown in table 2. The lattice parameters for the compounds with different
28
(deg.)
Fig. I. XRD patterns of LaBazVsO,, (a) sintered at 1010°C for 24 h and (b) sintered before sintering. Peaks marked with 0 are from the Ba3(V04)2 phase.
344
January
at 1080°C
for 24 h and high-pressure
pressed
Volume 15,number 5,6
MATERIALSLETTERS
January I993
Table I Cell parameters of RBa2V,0jI
YO 24.3
Orthorhombic
R
Tetragonal
Eu
LO
0 NdQ
Q(A)
b(A)
c(A)
Y La
7.60
7.58
24.35
Pr
7.88
7.85
24.00
Nd EU Gd
DY Ho
7.80 7.80
7.78 7.79
23.85 23.88
Er
7.75
1.73
23.61
Tm
7.75
7.12
23.60
Table 2 Indexed XRD c= 24.276 8,
peaks
of LaBa,V30,,
Q(A)
c(A)
7.94
24.27
23.7
7.85 7.90 7.88
24.00 24.17 23.95
23.5
Ha 0
t
1
23.0 0.84
Gd 0
my 0
a=7.942
A and
795
Pro
0 Sr CTm
j 0.86
0.88
0.90
0.92 Ionic
using
0.94 radius
/
0.96
0.90
1.0
ELI Gd
La
0
0
0
hkl
d obr
I
00
4
6.075
6.069
00
b
4.046
4.046
3
20
0
3.971
3.972
4
11
5
3.688
3.673
5
10
6
3.589
3.605
6
00
7
3.480
3.468
7 8 9 IO II I2 13 14
02 01 00 it 22 II 03
4 7 8 7 0 8 3
I I 10
3.328 3.177 3.032 2.953 2.809 2.670 2.520 2.427
3.323 3.178 3.034 2.951 2.808 2.670 2.516 2.427
15 16
22 5 02 9 0011
2.240 2.217
2.431 2.231 2.207
17 18 19
40 0 22 9 01 13
1.984 1.953 1.819
1.985 1.945 I.818
20
3010
I .793
1.789
21 22
32 32
1.778 I.716
1.783 1.706
8 9
R are visualized in fig. 2. The cell parameters a or h range from 7.60 to 7.88 A or from 7.58 to 7.85 A, respectively, and c ranges from 23.60 to 24.35 A. The increases in a, h, and c are in reasonable agreement with the R ionic radius except for a and c for the
9Pr
Nd
d Cal
2
1
(8)
0
No.
o
7.6 5 yk? I
0.84
0.86
0.88
Fig. 2. Lattice parameters Open circles and closed respectively.
0.90
0.92
0.94
Ionic
radius
---
0.96
0.98
1.0
1.
t&1
for the compounds with different R. circles present u and h parameters,
R = Y sample, which has the maximum c value while its u has the minimum value compared with other samples. From the data reported in table 1, the orthorhombic unit cell observed for small cations seems to result from a distortion of the tetragonal cell determined in the case of large cations except for Y and Pr cations. The as-sintered samples are insulators with resistivity at room temperature higher than 10” 52 cm, and their colour close to white. After annealing at 950°C for l-3 h in a Hz atmosphere all these samples become black, meanwhile their resistivity at room temperature is much lower compared with that of the material sintered in air without H, annealing. The values of the very low resistivities range from 1 to 100 Q cm for different samples. We hope to keep the structure and the RBa,V,O,, composition unchanged, but the valence of vanadium is reduced to 345
Volume 15, number
5,6
MATERIALS
less than 5 + by the H2 annealing. UnfortunateIy, the X-ray powder diffraction patterns showed that the RBa*V,O, , was decomposed into BaVO, and RV03 after Hz annealing despite the low annealing temperature of 950°C. Thus, the black colour and the lower resistivity of l-l 00 Q cm for all the H,-annealed samples correspond to the colour and the low resistivity of BaVO, and RV03.
4. Conclusions
A series of new compounds RBa2V,0,, (R = Y, La, Pr, Nd, Eu, Cd, Dy, Ho, Er, Tm) are prepared. The single phase with l-2-3 composition was obtained. The structures are orthorhombic or tetragonal depending upon the R element. The cell parameters a range from 7.60 to 7.88 .& h from 7.58 to 7.85 8, and c from 23.60 to 24.35 .A for orthorhombic-structure samples (R =Y, Pr, Dy, Ho, Er, Tm), and a from 7.85 to 1.94 8, and c from 23.95 to 24.27 A for tetragonal-structure samples (R=La, Nd, Eu, Gd). The as-sintered samples are insulators with very high resistivity of 1O’* R cm. After annealing in a Hz atmosphere, they decomposed into BaV03 and RV03.
346
LETTERS
January
1993
Acknowledgement This work is supported Shandong University.
by Youth Science Fund of
References [ 1 ] G. Bednorz and K.A. Miiller, Z. Physik B 64 ( 1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Z. Wang and C.W. Chu, Phys. Rev. Letters 58 (1987) 908. [3] C. Michel, M. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [4] H. Maeda, T. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 ( 1988) L209. [ 51 Z.Z. Sheng and A.M. Hermann, Nature 332 ( 1988) 8 14. [6]R.J. Cava, B. Batlogg, J.J. Krajewski, L.W. Rupp, L.F. Schneemeyer, T. Siegrist, R.B. van Dover, P. Marsh, W.F. Peck Jr., P.K. Gallagher, S.H. Glarum, J.H. Ma~haiI, R.C. Farrow, J.V. Waszczak, R. Hull and P. Trevor, Nature 336 (1988)211. [ 71 R.J. Cava, B. Batlogg, J.J. Krajewski, R. Farrow, L.W. Rupp, A.E. White, K. Short, W.F. Feck and T. Kometani, Nature 332 (1988) 814. [S] J. Akimitsu, J. Amano. K. Tomimoto, T. Suzuki. M. Kogai, Y. Fukawa, H. Sawa and Y. Matsui, Physica C 185-189 (1991) 723. [ 91 S. Mats&a, S. Takeuchi, A. Soeta, T. Doi, K. Aihara and T. Kamo, Japan. J. Appl. Phys. 29 ( 1990) L1781. [lo] A. Nozaki, H. Yoshikawa, T. Wada, H. Yamauchi and S. Tanaka, Phys. Rev. B 43 (1991) 181. [ II] J. Meng and Y.F. Ren, Mater. Res. Bull. 26 ( 199 I ) 545.
Synthesis of a new series of rare-earth vanadates (R=Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er, Tm)
RBa2V301
I
X.L. Wang a, H. Wang a, S.X. Shang b, Z.P. Ai b, H.C. Chen a and M.H. Jiang a a Institute ofcrystal Materials, Shandong University, Jinan 250100, China h Experimental
Center, Shandong
University, Jinan 250100, China
Received 16 October 1992; in final form 12 November I992
A new series of rare-earth vanadates RBa2V,0,, (R=Y, La, Pr, Nd, Eu. Gd, Dy, Ho, Er, Tm) are first synthesized by the conventional solid-state reaction. The single phase can be obtained at a sintering temperature of IO10aC for most samples. The X-ray powder diffraction analysis has shown that the new compounds have an orthorhombic structure with cell parameters a= 7.60 to 7.88 A, h=7.58 to 7.85 8, and c=23.60 to 24.35 A for R=Y, Pr, Tm, Er, Dy, Ho compounds or a tetragonal structure with a=7.85 to 7.94 A and c=23.95 to 24.27 8, for R=La, Nd, Cd, Eu compounds. These cell parameters depend upon the rare-earth ionic radius. The RBa2V,0, I can be decomposed to RVO,, Ba3( V02)2 and V,O, by sintering temperature higher than 1010°C. The as-sintered samples are insulators with a resistivity of IO” R cm. Furthermore, if the as-sintered samples are annealed under H2 atmosphere, the RBa2V,0,, phase decomposes into RVO, and BaVO, despite the lower annealing temperature of 950°C.
1. Introduction Since the discovery of the copper oxides high-T, superconducting system, La-Ba-Cu-0, by Bednorz and Miiller [ 11, many other copper oxides high-T, superconducting systems such as Y-Ba-Cu-0 [ 21, Bi-Sr- (Ca)-Cu-0 [ 3,4 1, Tl-Ba-Ca-Cu-0 [ 5 1, Pb-Sr-Ca-Cu-0 [ 61 etc. have been found. It would be interesting to find a non-copper oxides high-T, superconducting system both for practice and for high-T, superconductivity theory. Ba-K-Bi-0 [ 71 and St-( Ln )-Nb-0 [ 81 were discovered to have T, of 30 or 12 K, respectively, and possible high-T, superconductivity with T, of 130 K in the doped vanadium-oxides-based Sr-V-O [9] system has been attracting study. The Sr,, ,V,O, (n = 1, 2, 3 ) system was synthesized by Naziki et al. [ lo]. Recently, Meng and Ren [ I I ] have reported the series of rare-earth vanadates LaSr,_,Ca,V,Og_y, which show low resistivity and antiferromagnetic properties. Up to now, although no superconductivity was observed or verified in this system, the vanadium oxide is still a candidate for the non-copper oxides superconducting system based on some important characteristics in common with high-T, cuprate HTSC: ( 1) the exisElsevier Science Publishers B.V.
tence of two-dimensional V02 sheets expected to be responsible for electronic conduction; (2) the expected existence of localized spin 1 / 2 arising from the nominally tetravalent vanadium ion; (3) the existence of metal-insulator transition. In this paper, a series of new compounds RBa2V30,, are reported for the first time. They are insulators.
2. Experimental Samples were prepared by the conventional solidstate reaction method. High-purity powders of R,03 (R=Y, La, Pr. Nd, Eu, Gd, Dy, Ho, Er, Tm ), BaCO, and V,05 were used as starting materials. The powders were weighed, mixed in the ratio R: Ba: V= 1 : 2: 3 and pressed into pellets and then sintered at 1010°C for 24 h in air. After characterization by XRD, TEM, EDAX and resistivity at room temperature, these as-sintered samples were annealed at 950°C for l-3 h in a H2 atmosphere. The XRD pattern was obtained with a Rikaku D/Max RA X-ray diffractometer using Cu Ka radiation. The compositions of crystal grains in these samples were analyzed by energy dispersive X-ray spectroscopy using 343
Volume 15, number
5,6
MATERIALS
LETTERS
a Philips EDAX model PV-9100 spectrometer. The electrical resistivity was measured using the standard four-probe method. The meter/de voltage source, 4 140B PA equipment made by HewlettPackard company, was used for high-resistivity measurements.
1993
(Ml), indicating that the c-axis orientation for this sample can be easily obtained by a high-pressure process before sintering. We can get the single l-2-3 phase at the same sintering temperature of 1010°C for 24 h except for the sample with R= Nd, in which there are Ba3(V0,)2, NdV04 and V205 phases as obtained from its XRD pattern or EDAX analysis. These phases must come from the decomposition of NdBa,V,O, which is very similar to that observed in LaBa,V,O, sintered at 1080°C. It may be concluded that the RBa2V30, can be decomposed if the sintering temperature is too high, according to the formula
3. Results and discussion Figs. la and lb show the XRD pattern of sample R=La sintered at 1010°C or 1080°C for 24 h, respectively. The latter sample was pressed at high pressure before sintering. One can see that the phase formation depends upon the sintering temperature. At a lower sintering temperature of 10 10 “C, a nearly single l-2-3 phase is formed, while at a high temperature of 108O”C, there are other phases (Ba3(V04),, VzOs and LaVO,) inside the sample which were determined by XRD or EDAX. The results indicate that the l-2-3 phase can be decomposed at a temperature higher than 10 10” C. The decomposition formula may be 3LaBazV,0r1 = 2Ba3( V04)* + 3LaV0, + VzOs. Furthermore, for the latter sample which was pressed under high pressure before sintering, its XRD diffraction peak of (00 1) is much higher and sharper compared with other
3RBa2V,01,
=2Ba,(VO,)*
+3RV04
+V205
.
So if one wants to get the single l-2-3 phase for the R=Nd sample the sintering temperature must be lower than 10 10°C in order to prevent the decomposition of the sample. The X-ray powder diffraction patterns for all the samples studied are similar to each other, indicating similar structure. Their diffraction peaks can be easily indexed by orthorhombic or tetragonal cell parameters. The cell parameters for samples with different R are shown in table 1. The indexed peaks for the sample LaBa2V,01, using tetragonal parameters a= 7.90 A and ~~24.11 A are shown in table 2. The lattice parameters for the compounds with different
28
(deg.)
Fig. I. XRD patterns of LaBazVsO,, (a) sintered at 1010°C for 24 h and (b) sintered before sintering. Peaks marked with 0 are from the Ba3(V04)2 phase.
344
January
at 1080°C
for 24 h and high-pressure
pressed
Volume 15,number 5,6
MATERIALSLETTERS
January I993
Table I Cell parameters of RBa2V,0jI
YO 24.3
Orthorhombic
R
Tetragonal
Eu
LO
0 NdQ
Q(A)
b(A)
c(A)
Y La
7.60
7.58
24.35
Pr
7.88
7.85
24.00
Nd EU Gd
DY Ho
7.80 7.80
7.78 7.79
23.85 23.88
Er
7.75
1.73
23.61
Tm
7.75
7.12
23.60
Table 2 Indexed XRD c= 24.276 8,
peaks
of LaBa,V30,,
Q(A)
c(A)
7.94
24.27
23.7
7.85 7.90 7.88
24.00 24.17 23.95
23.5
Ha 0
t
1
23.0 0.84
Gd 0
my 0
a=7.942
A and
795
Pro
0 Sr CTm
j 0.86
0.88
0.90
0.92 Ionic
using
0.94 radius
/
0.96
0.90
1.0
ELI Gd
La
0
0
0
hkl
d obr
I
00
4
6.075
6.069
00
b
4.046
4.046
3
20
0
3.971
3.972
4
11
5
3.688
3.673
5
10
6
3.589
3.605
6
00
7
3.480
3.468
7 8 9 IO II I2 13 14
02 01 00 it 22 II 03
4 7 8 7 0 8 3
I I 10
3.328 3.177 3.032 2.953 2.809 2.670 2.520 2.427
3.323 3.178 3.034 2.951 2.808 2.670 2.516 2.427
15 16
22 5 02 9 0011
2.240 2.217
2.431 2.231 2.207
17 18 19
40 0 22 9 01 13
1.984 1.953 1.819
1.985 1.945 I.818
20
3010
I .793
1.789
21 22
32 32
1.778 I.716
1.783 1.706
8 9
R are visualized in fig. 2. The cell parameters a or h range from 7.60 to 7.88 A or from 7.58 to 7.85 A, respectively, and c ranges from 23.60 to 24.35 A. The increases in a, h, and c are in reasonable agreement with the R ionic radius except for a and c for the
9Pr
Nd
d Cal
2
1
(8)
0
No.
o
7.6 5 yk? I
0.84
0.86
0.88
Fig. 2. Lattice parameters Open circles and closed respectively.
0.90
0.92
0.94
Ionic
radius
---
0.96
0.98
1.0
1.
t&1
for the compounds with different R. circles present u and h parameters,
R = Y sample, which has the maximum c value while its u has the minimum value compared with other samples. From the data reported in table 1, the orthorhombic unit cell observed for small cations seems to result from a distortion of the tetragonal cell determined in the case of large cations except for Y and Pr cations. The as-sintered samples are insulators with resistivity at room temperature higher than 10” 52 cm, and their colour close to white. After annealing at 950°C for l-3 h in a Hz atmosphere all these samples become black, meanwhile their resistivity at room temperature is much lower compared with that of the material sintered in air without H, annealing. The values of the very low resistivities range from 1 to 100 Q cm for different samples. We hope to keep the structure and the RBa,V,O,, composition unchanged, but the valence of vanadium is reduced to 345
Volume 15, number
5,6
MATERIALS
less than 5 + by the H2 annealing. UnfortunateIy, the X-ray powder diffraction patterns showed that the RBa*V,O, , was decomposed into BaVO, and RV03 after Hz annealing despite the low annealing temperature of 950°C. Thus, the black colour and the lower resistivity of l-l 00 Q cm for all the H,-annealed samples correspond to the colour and the low resistivity of BaVO, and RV03.
4. Conclusions
A series of new compounds RBa2V,0,, (R = Y, La, Pr, Nd, Eu, Cd, Dy, Ho, Er, Tm) are prepared. The single phase with l-2-3 composition was obtained. The structures are orthorhombic or tetragonal depending upon the R element. The cell parameters a range from 7.60 to 7.88 .& h from 7.58 to 7.85 8, and c from 23.60 to 24.35 .A for orthorhombic-structure samples (R =Y, Pr, Dy, Ho, Er, Tm), and a from 7.85 to 1.94 8, and c from 23.95 to 24.27 A for tetragonal-structure samples (R=La, Nd, Eu, Gd). The as-sintered samples are insulators with very high resistivity of 1O’* R cm. After annealing in a Hz atmosphere, they decomposed into BaV03 and RV03.
346
LETTERS
January
1993
Acknowledgement This work is supported Shandong University.
by Youth Science Fund of
References [ 1 ] G. Bednorz and K.A. Miiller, Z. Physik B 64 ( 1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Z. Wang and C.W. Chu, Phys. Rev. Letters 58 (1987) 908. [3] C. Michel, M. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [4] H. Maeda, T. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 ( 1988) L209. [ 51 Z.Z. Sheng and A.M. Hermann, Nature 332 ( 1988) 8 14. [6]R.J. Cava, B. Batlogg, J.J. Krajewski, L.W. Rupp, L.F. Schneemeyer, T. Siegrist, R.B. van Dover, P. Marsh, W.F. Peck Jr., P.K. Gallagher, S.H. Glarum, J.H. Ma~haiI, R.C. Farrow, J.V. Waszczak, R. Hull and P. Trevor, Nature 336 (1988)211. [ 71 R.J. Cava, B. Batlogg, J.J. Krajewski, R. Farrow, L.W. Rupp, A.E. White, K. Short, W.F. Feck and T. Kometani, Nature 332 (1988) 814. [S] J. Akimitsu, J. Amano. K. Tomimoto, T. Suzuki. M. Kogai, Y. Fukawa, H. Sawa and Y. Matsui, Physica C 185-189 (1991) 723. [ 91 S. Mats&a, S. Takeuchi, A. Soeta, T. Doi, K. Aihara and T. Kamo, Japan. J. Appl. Phys. 29 ( 1990) L1781. [lo] A. Nozaki, H. Yoshikawa, T. Wada, H. Yamauchi and S. Tanaka, Phys. Rev. B 43 (1991) 181. [ II] J. Meng and Y.F. Ren, Mater. Res. Bull. 26 ( 199 I ) 545.