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Superconductivity at 40 K in the 1212 system (Pb1−xVx)Sr2(Ca1−zYz)Cu2O7−δ
Physica C 217 (1993) 121-126 North-Holland
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Superconductivity at 40 K in the 1212 system (Pbl_xVx)Sr2 (Cal_zYz)Cu207_, W. Widder, M. Franz, L. B a u e r n f e i n d a n d H.F. Braun Physikalisches lnstitut, UniversitlitBayreuth, D-95440 Bayreuth, Germany Received 12 May 1993 Revised manuscript received 19 August 1993
Samples of nominal composition (Pbt_xVx)Sr2(Cat_zY,)Cu20~_6 (0.1
1. Introduction Superconductors with the "TI-I 212" and "TI1223" structure are of special interest due to the fact that the successive groups of CuO2 layers are closer together than in "2212" and "2223" compounds. This should lead to a reduction of the anisotropy, an improvement of flux pinning and therefore an increase of the critical current density. In he highly anisotropic high-temperature superconductors with interlayer distances larger than the appropriate coherence length, the flux lines decouple into independent and weakly pinned "pancakes" [ 1,2]. A lower degree of anisotropy is desirable since it might permit the flux lines to interact as a whole with the pinning sites. This has been observed, e.g., in T1based "1212" and "1223" compounds [3]. The great disadvantage of the Tl-based parent compounds is the toxicity of TI which may prevent large-scale production of this material. Thus, complete substitution of TI by other metals is of special interest. The already known substitutions which lead to superconducting compounds can be distinguished between compounds containing Hg, compounds containing Pb, and substitutions without Hg or Pb. The recent discovery of the first group [4] has led to compounds with the highest T¢'s known. Members of the second group are the (Pb, Ca) [5], (Pb,
Cd) [6], (Pb, Cu) [7-9], (Pb, In) [10], (Pb, Mg) I l l ] and (Pb, St) [12] substitutions. The third group can be subdivided in "copper-containing" substitutions [ 13-17 ] which are closely related to the YIBa2Cu3OT_~ structure and the "copper-free" substitutions (Bi,Cd) and (Ce,Cd) [18]. After investigation of the (Ph,Sb)-system [19] where we found no superconducting 1212 member, we now report results of investigations of the (Pb,V)- 1212 system. The influence of post-treatment in different atmospheres as well as the systematics of the lattice parameters and the occurrence of superconductivity will be discussed.
2. Experimental Three series of ceramic samples with nominal composition (Pb~ _xVx)Sr2 (Cat _zYz)Cu207_d were prepared (see fig. 1 ). Series 1 contained the samples with (x, z ) = (0.2, 0), (0.3, 0.2), (0.4, 0.4), (0.5, 0.6), (0.6, 0.8), (0.7, 1). In Series 2 the values for x a n d zwere given by (x, z) = (0.1, 0.2), (0.2, 0.4), (0.3, 0.6), (0.4, 0.8) and (0.5, 1). "Series" 3 consisted of a single sample with nominal composition (x, z ) = (0.2, 0.8). For the choice of nominal composition in these three series, we were guided by the formal valency of copper. All these samples were
0921-4534/93/$06.00 © 1993 ElsevierScience Publishers B.V. All fights reserved.
122
W. Widder et al. / Superconductivity in the 1212 system
>-
1
0 o
o
Hz, about 1 mA). DC magnetization was measured in a SQUID magnetometer.
O •
3. Renlts and d i ~ l e a []
N
•
0.5 v
•
A
o
• !
Fig.
I.
0
0.5
1
Pb
X
V
Nominal
compositions
of
the
samples
(Pbt_~V~)Sr2(Cat_,Y,)Cu20~_6. Series 1: full symbols; series 2: open symbols; "'series" 3: dotted symbol.
prepared via a solid-state reaction using a precursor route. High-purity powders (>99.9%) of SrCO3, CaCO3, Y,O3 and CuO were mixed, ground in an agate mortar and subsequently calcined in corundum crucibles at 970°C for 18 h in air. After addition of PbO and V205 in appropriate proportions, these mixtures were reground and pressed into pellets of 10 mm diameter and about 3 mm thickness, using a pressure of about 0.3 GPa. The pellets were sintered at 950°C for 16 h in air and subsequently quenched in air to room temperature. Samples were investigated in the as-prepared state and also after post-treatment at 500°C for 20 h in flowing oxygen or argon. All post-treated samples were furnacecooled at a rate of about 1°C/min. The phases occurring were investigated by X-ray powder diffraction on a Seifert XRD 3000 P diffractometer using Cu Kct radiation. SEM images were used to determine the morphology of the grains. The chemical compositions of individual, plate-like microcrystals were examined by energy dispersive X-ray spectrometry (EDX) from a Jeol JSM-840 A electron microscope operating at 10 kV. The surfaces of the samples for the SEM (EDX) measurements were sputtered with gold (carbon). The resistivity behaviour was measured by the four-probe AC method (20
For superconductivity to occur in hole-doped T11212 cuprates, the formal copper valency must be around +2.15 [20]. In a given cuprate-system, like in our case for (Pbm_xVx)Sr2(Cal_zYz)Cu207_~, this value can be obtained theoretically by variation of the metal composition as well as by varying the oxygen content. We used the formal copper valence as a guideline for the selection of the nominal compositions of our samples. The 1212 phase occured in all samples of the three series with the only exception of (x, z)--(0.2, 0). The amount of the 1212 phase decreased with increasing Ca content. Figure 2 shows the powder Xray pattern of the superconducting sample with nominal composition (Pbo.TVo.3)Sr2(Cao.4Yo.6) Cu2OT_s. The peaks of the 1212 phase were indexed using the space group P4/mmm and lattice constants a=3.822(1) A and c=11.818(5) A. The number in parentheses is the estimated error in units ofth e least significant digit. The amount of impurity phases (which could not be identified using the JCPDS data from 1991 ) was comparatively small. ¢q Q
e'-1
o qr--
0
C/) te-
03
t~
co ne
2O
i
=
40
60
Bragg Angle 2GB F] Fig. 2. The powder XRD pattern for the superconducting sample with nominal composition (Pbo.~Vo.DSr2(Cao.4Yo.s)Cu20~_. Impurity peaks are marked by ( 0 ) .
W. Widder et al. / Superconductivity in the 1212 system
The impurity peaks are marked by ( • ) . The lattice constants for the 1212 phase of the samples investigated chansed in a systematic way as is shown in fig. 3. The a-lattice parameter was nearly constant. For constant Pb:V ratio (constant x) the c-lattice parameter and the volume of the unit cell decreased with increasing Y content as was expected by the size of the ionic radii. For constant Ca:Y ratio (constant z) the c-lattice parameter and the volume of the unit cell were also decreasing with increasin8 V content. The only exception were the samples with (x, z)=(0.3, 0.2) and (x, z)=(O.l, 0.2) which might indicate that the compositions of these final members of the series were not inside the homogeneity range of the 1212 phase. As mentioned above, no 1212 phase was formed for the sample with (x, z) = (0.2, 0).
3.825 i
v
.~__
o 3.815
"
t
o
:
11.79
! o o
0
~7
I J
-- 173.0 .~_ > 172.0
0
" A
11.84
g
o
t
O3
0
0.2
0.6
1.0
Y-Concentration Z Fig. 3. The lattice constants a, ¢ and the unit-cell volume (Pb~_xVx)Sr2 (Cat_,Y=)Cu2OT_8 as a function o f l h e Y content. The different symbols correspond to those used in fig. 1. Estimated errors are + 0.001 ~, for • and +_0.005 )k for ¢.
123
SEM imases revealed plate-like microcrystals of about 0.5 ttm thickness and area of several ~tm2 and showed the porosity of the samples (see fig. 4). In order to test whether V is really incorporated into the 1212 phase, energy dispendve X-ray (EDX) spectrometry measurements were performed. The chemical composition of eleven individual, plate-like microcrystals was analysed for each of the samples with nominal compositions (Pbo.TVo.3)St2 (Cao.iYo.6)Cu2OT_6 and (Pbo.sVo.5)Sr2(Cao.4Yo.6)Cu2OT_8 which had been post-treated in argon. Care was taken to compare m i ~ s selected at random from different parts of the peget investigated. The pellet was broken and only microcrystals whose surfaces were parallel to the fracture surface have been investigated. Since the penetration depth of the primary electrons is a function of the acceleration voltage, the small thickness of the grains required low acceleration voltages in order to avoid that contributions from impurity phases below the 1212-phase grains would disturb the analyses. For pure vanadium, the (~(pZ)) curves, which give the number of Kct quanta generated per incoming electron as a function of depth in the specimen [21 ], show that at 10 kV (30 kV) acceleration voltage 99% of the emitted X-ray are produced within a surface layer of thickness 0.3 gtm (3 ttm) of the specimen. For this reason, we used an acceleration voltage of 10 kV for our investigations. The take-off angle was 40 ° . The characteristic lines for the analyses were Pb M, V K, Sr L, Ca K, Y L and Cu L. The counting time per measurement was about 1200 s. To test the influence of the low acceleration voltage (10 kV) on the accuracy of the analysis of the crystals, Y-123 and Bi-2212 single crystals were also investigated under the same conditions. At low acceleration voltages the correction programs produced systematic deviations from the stoichiometry of the crystals, in particular, the Sr and Ba content were severely overestimated. The analysis was, however, reproducible to within 1%--5% per element on the same crystal. Since the analysed composition does not represent the stoichiometry of the microcrystal we do not give detailed results. Nevertheless, the analyses of the microerystals could be used to test whether V is incorporated into the 1212-grains: The mean Sr-Ca-Y-Cu concentration was constant within the standard deviation for the 1212 micro-
124
W. Widder et al. / Superconductivity in the 1212 system
Fig. 4. SEM image from a part of the sample with nominal composition (Pbo.~Vo.5)Sr,(Cao.4Yo.e)Cu2OT_6.The surface has been sputtered with a few nm Au. crystals of both samples with nominal compositions ( Pbo.TVo.3) Sr2 (Cao.4Yo.6) Cu207-S and (Pbo.sVo.5)Sr2(Cao.4Yo.6)Cu2OT_a, while the P b - V concentration of the 1212 grains differed for both samples following the changes given by the nominal compositions (Pbo.TVo.3)and (Pbo.sVo.5). This is consistent with an incorporation of V into the 1212 phase and thus the described changes in the lattice constants should be a result of different (Pb, V) ratios. In addition, with the help of the EDX measurements we could observe in one measurement a V- and Sr-rich impurity phase, which contained traces of Cu and Ca. None of the as-prepared samples and of the samples post-treated in oxygen was superconducting. All these samples showed a semiconducting behaviour. The resistance behaviour of the samples posttreated at 500°C for 20 h in argon is shown in fig. 5. (Pbo.sVo.5) Sr2 (Cao.4Yo.6) Cu207_6 and (Pbo.7Vo.3)Sr2(Cao.4Yo.6)Cu2OT_6 became superconducting at around 25 K and 40 K, respectively, as measured by the onset of the resistance drop. Resistance curves for both samples are shown in fig. 6 on an enlarged scale. The samples used for the re-
sistance measurements were of similar size and we used similar configurations for the contacts. However, since their shape was irregular and, more importantly, the porosity of the samples and the amount of impurity phases varied as a function of x and z, no specific resistivities were calculated. Nevertheless, we expect that the measured resistances reflect the intrinsic resistivities, and can be compared within the series to within a factor of about two. The superconducting transition was confn'med by the SQUID magnetization measurements as is shown in fig. 7 for sample ( Pbo.7Vo.3 ) Srz (Cao.4Yo.6)Cu2OT_s. Both the field-cooled curve and the zero-field-cooled curve are displayed for a measuring field of B = 6 . 5 mT. The onset of the superconducting transition was observed at about 25 K which corresponds to a vanishing resistance. The strength of the diamagnetic signal at 5 K was comparatively weak and corresponded to a volume fraction of about one percent of an ideal superconductor. Due to the small magnitude of the diamagnetic signal it cannot be excluded that small amounts of (Pb, Ca)-1212, (Pb, Cu)-1212 or (Pb, Sr)-1212 phases are responsible for the superconductivity, al-
W. Widder et ol. / Superconductivity in the 1212 system
125
800
9
•
z=o.2
•,
voZ=°'4 -
,
'i
,,c
®
o e,
400
[ c
.2
•
Z=0.8
m "G ID
o
em
n-
ZFC
0 I
0
-0.005
I
1 O0
200
300
~'
E
" Z=0.2 0
(D 0 eQ
v
Z=0.4
n Z=0.6 400
0 0
(kA
o Z=0.8
@ nr. . . . .
0
1oo
200
300
Temperature [K]
Fig. 5. The temperature dependence of the electrical resistance for the (Pbl_~V~)Sr2(Cal_,Y=)Cu2Oy_ssamplesof series 1 (a) and series 2 (b) (same symbolsas in fig. 1).
1.6
2 8 =
• X=0.5 Z=0.6
X_Coo
~
o x=0.3
0.8
g ID
o ~
~
~1 iiiiiiiiiil[iJil[llllll
i [ i J [ ] 1 i ii
0 0
100
200
300
Temperature [K]
Fig.
6.
Resistance of the
(PbonVo.3)Sr2 (Cao.,Yo.6)Cu20~_a
60
Fig. 7. D C m a ~ e t i z a t i o n obtained in ZFC and FC measurements o f sample (Pb0.yV0.3)Sr2(Cao.,Yo.6)Cu,Oy_~
800
2
30 Temperature [K]
Temperature [K]
0
0
superconducting samples ( [] )
and
(Pbo.sVo.s)Sr2(Cao.4Yo.6)Cu2Oy-a( I ) (same symbolsas in fig. 1).
though we have not seen such phases in the EDX measurements. We point out that no need of an argon post-treatment to introduce superconductivity is reported in the literature for these phases. If the superconductivity observed is due to the (Pb, V)- 1212 phase the small magnitude of the diamagnetic signal could be explained by the small grain size of the material. SEM images revealed plate-like grains of about 0.3 p m thickness which is of the order of the expected penetration depth. The comparison with other Tl- and Pb-based 1212 compounds shows that we have not yet reached the optimum synthesis conditions. Small apparent superconducting volume fractions are not uncommon in some of the 1212-substitutions (see e.g. refs. [6] and [13]) and can usually be increased by varying the synthesis conditions [ 22 ]. For constant Ca: Y ratio the c-lattice parameter and the volume of the unit cell decreased with increasing V content. As Rouillon et al. [ 12 ] propose for the ( Pb, Sr )- 1212 compound, it seems likely that Pb has the valency+ 2 and possesses a lone pair. The lone pair could compensate oxygen vacancies in the SrO layers which are created by the argon post-treatment needed to introduce superconductivity. This model is consistent with the observed variation of the lattice parameters as a function of x and z and with the formal copper valency of about + 2.15 for reasonable values of oxygen-deficiency J ~ 0.4 and a formal vanadium valency of + 3.
126
W. Widder et al. / Superconductivity in the 1212 system
4. Conclusions
References
Superconductivity with Tc's up to 40 K was found in the (Pb~_xVx)Sr2(Cal_zYz)Cu2OT_6 system for the samples with nominal compositions (Pbo.vVo.3)Sr2(Cao.4Yo.6)Cu207_6 and (Pbo.sVo.s)Sr2(Cao.Ho.e)Cu2OT_6 after a thermal post-treatment in flowing argon. Thus the occurrence of superconductivity seems to be closely related to the oxygen stoichiometry. EDX measurements of individual microcrystals of these samples revealed that V is incorporated into the 1212 phase. The small diamagnetic signal could be explained by the small size of the (Pb, V)-1212 grains, which is of the order of the expected penetration depth. However, due to the small size of the diamagnetic signal it cannot be excluded that also a small number of (Pb, Ca), (Pb, Cu) or (Pb, Sr)-1212 grains exist in the samples, which are possibly responsible for the superconductivity, although we have not seen them in our EDX measurements. Only an enlargement of the diamagnetic signal could clarify this point. Therefore a study of the preparation conditions with the aim to increase the (Pb, V)-1212 grain size and the effective superconducting volume fraction is under way.
[ 1 ] D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richeson, TJ. Marks, J.H. Karts, J. Talvcchio and M. Eddy, Physica C 177 ( 1991 ) 431. [2] J.R. Clem, Phys. Rev. B 43 (1991) 7837. [3] R.S. Liu, D.N. Zhen8, J.W. Loram, K.A. Mirza, A.M. Campbell and P.P. Edwards, Appl. Phys. Lett. 60 (1992) 1019. [41A. Schillin& M. C,antoni, J.D. Guo and H.R. Ott, Nature (London) 363 (1993) 56. [ 5 ] T. Rouillon, A. Mai~an, M. Hervieu, C. Michel, D. Groult and B. Raveau, Physica C 171 (1990) 7. [61T.P. Beales, C. Dineen, W.G. Freeman~ S.R. Hall, M.R. Harrison, D.M. Jacobson and S.J. Zammattio, Supercond. Sci. Teclmol. 5 (1992) 47. [7] M.A. Subramanian, J. Gopalakrishnan, C.C. Torardi, P.L. Gai, E.D. Boyes, T.R. Askew, R.B. Flippen, W.E. Franeth and A.W. Slight, Physica C 157 (1989) 124. [81J.Y. Lee, J.S. Swinnea and H. Steinfmk, J. Mater. Res. 4 (1989) 763. [91 S. Koriyama, K. Sakuyama, T. Maeda, H. Yamauchi and S. Tanaka, Physica C 166 (1990) 413. [ 10] R.S. Liu, P.T. Wu, S.F. Wu, W.N. Wang and P.P. Edwards, Physica C 165 (1990) 111. [ 11 ] H.B. Liu, D.E. Morris and A.P.B. Sinhs; Physica C 204 ( 1993 ) 262. [ 12 ] T. Rouillon, J. Provost, M. Hervieu, D. Grouit, C. Michel and B. Raveau, Physica C 159 (1989) 201. [ 13 ] A. Ehmann~ S. Kemmler-Sack, S. L6~h, M. Schlichenmaier, W. Wischert, P. Zoller, T. Nisael and R.P. Htibener, Physica C 198 (1992) 1. [ 141 P.R. Slater and C. Greaves, Physica C 180 ( 1991 ) 299. [ 15 ] T. Den and T. Kobaya,~i, Physica C 196 (1992) 141. [ 16 ] Y. [VtiyA,nkj,H. Yamane, N. Kobayashi, T. Hira~ H. Nakata, K. Tomimoto and J. Akimitsu, Physica C 202 (1992) 162. [ 17 ] W.J. 7_,hu,J.J. Yue, Y.Z. Huan8 and Z.X. Zhao, Physica C 205 (1993) 118. [ 18] T.P. Beales, C. Dineen, S.R. Hall, M.R. Harrison and J.M. Parberry, Physica C 207 ( 1993 ) 1. [19] W. Widder, L. Bauernfeind, M. Franz and H.F. Braun, J. Alloys Compounds 195 (1993) 61. [20] H. Zhan8 and H. Sato, Phys. Rev. Left. 70 (1993) 1697. [211 J.I. Goldstein and H. Yakowitz, Practical Scanning Electron Microscopy (Plenum, New York, 1975). [221 A. Ebmann; T. Fries, S. Kemmler-Sack, S. L6~h, W. Paulus, C. Schulz, W. Wischert and P. Zoller, J. Alloys Compounds 195 (1993) 57.
Acknowledgements This work was partially supported by the Bayerische Forschungsstiftung (FORSUPRA) and one of us (WW) by the Deutsche Forschungsgemeinschaft. SEM and EDX analyses have been performed at BIMF (Bayreuther lnstitut fiir Makromolekfilforschung). We thank C. Drummer for her assistance in the EDX measurements, W. Ettig for his technical assistance, and H. Uniewski for the SQUID measurements.
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Superconductivity at 40 K in the 1212 system (Pbl_xVx)Sr2 (Cal_zYz)Cu207_, W. Widder, M. Franz, L. B a u e r n f e i n d a n d H.F. Braun Physikalisches lnstitut, UniversitlitBayreuth, D-95440 Bayreuth, Germany Received 12 May 1993 Revised manuscript received 19 August 1993
Samples of nominal composition (Pbt_xVx)Sr2(Cat_zY,)Cu20~_6 (0.1
1. Introduction Superconductors with the "TI-I 212" and "TI1223" structure are of special interest due to the fact that the successive groups of CuO2 layers are closer together than in "2212" and "2223" compounds. This should lead to a reduction of the anisotropy, an improvement of flux pinning and therefore an increase of the critical current density. In he highly anisotropic high-temperature superconductors with interlayer distances larger than the appropriate coherence length, the flux lines decouple into independent and weakly pinned "pancakes" [ 1,2]. A lower degree of anisotropy is desirable since it might permit the flux lines to interact as a whole with the pinning sites. This has been observed, e.g., in T1based "1212" and "1223" compounds [3]. The great disadvantage of the Tl-based parent compounds is the toxicity of TI which may prevent large-scale production of this material. Thus, complete substitution of TI by other metals is of special interest. The already known substitutions which lead to superconducting compounds can be distinguished between compounds containing Hg, compounds containing Pb, and substitutions without Hg or Pb. The recent discovery of the first group [4] has led to compounds with the highest T¢'s known. Members of the second group are the (Pb, Ca) [5], (Pb,
Cd) [6], (Pb, Cu) [7-9], (Pb, In) [10], (Pb, Mg) I l l ] and (Pb, St) [12] substitutions. The third group can be subdivided in "copper-containing" substitutions [ 13-17 ] which are closely related to the YIBa2Cu3OT_~ structure and the "copper-free" substitutions (Bi,Cd) and (Ce,Cd) [18]. After investigation of the (Ph,Sb)-system [19] where we found no superconducting 1212 member, we now report results of investigations of the (Pb,V)- 1212 system. The influence of post-treatment in different atmospheres as well as the systematics of the lattice parameters and the occurrence of superconductivity will be discussed.
2. Experimental Three series of ceramic samples with nominal composition (Pb~ _xVx)Sr2 (Cat _zYz)Cu207_d were prepared (see fig. 1 ). Series 1 contained the samples with (x, z ) = (0.2, 0), (0.3, 0.2), (0.4, 0.4), (0.5, 0.6), (0.6, 0.8), (0.7, 1). In Series 2 the values for x a n d zwere given by (x, z) = (0.1, 0.2), (0.2, 0.4), (0.3, 0.6), (0.4, 0.8) and (0.5, 1). "Series" 3 consisted of a single sample with nominal composition (x, z ) = (0.2, 0.8). For the choice of nominal composition in these three series, we were guided by the formal valency of copper. All these samples were
0921-4534/93/$06.00 © 1993 ElsevierScience Publishers B.V. All fights reserved.
122
W. Widder et al. / Superconductivity in the 1212 system
>-
1
0 o
o
Hz, about 1 mA). DC magnetization was measured in a SQUID magnetometer.
O •
3. Renlts and d i ~ l e a []
N
•
0.5 v
•
A
o
• !
Fig.
I.
0
0.5
1
Pb
X
V
Nominal
compositions
of
the
samples
(Pbt_~V~)Sr2(Cat_,Y,)Cu20~_6. Series 1: full symbols; series 2: open symbols; "'series" 3: dotted symbol.
prepared via a solid-state reaction using a precursor route. High-purity powders (>99.9%) of SrCO3, CaCO3, Y,O3 and CuO were mixed, ground in an agate mortar and subsequently calcined in corundum crucibles at 970°C for 18 h in air. After addition of PbO and V205 in appropriate proportions, these mixtures were reground and pressed into pellets of 10 mm diameter and about 3 mm thickness, using a pressure of about 0.3 GPa. The pellets were sintered at 950°C for 16 h in air and subsequently quenched in air to room temperature. Samples were investigated in the as-prepared state and also after post-treatment at 500°C for 20 h in flowing oxygen or argon. All post-treated samples were furnacecooled at a rate of about 1°C/min. The phases occurring were investigated by X-ray powder diffraction on a Seifert XRD 3000 P diffractometer using Cu Kct radiation. SEM images were used to determine the morphology of the grains. The chemical compositions of individual, plate-like microcrystals were examined by energy dispersive X-ray spectrometry (EDX) from a Jeol JSM-840 A electron microscope operating at 10 kV. The surfaces of the samples for the SEM (EDX) measurements were sputtered with gold (carbon). The resistivity behaviour was measured by the four-probe AC method (20
For superconductivity to occur in hole-doped T11212 cuprates, the formal copper valency must be around +2.15 [20]. In a given cuprate-system, like in our case for (Pbm_xVx)Sr2(Cal_zYz)Cu207_~, this value can be obtained theoretically by variation of the metal composition as well as by varying the oxygen content. We used the formal copper valence as a guideline for the selection of the nominal compositions of our samples. The 1212 phase occured in all samples of the three series with the only exception of (x, z)--(0.2, 0). The amount of the 1212 phase decreased with increasing Ca content. Figure 2 shows the powder Xray pattern of the superconducting sample with nominal composition (Pbo.TVo.3)Sr2(Cao.4Yo.6) Cu2OT_s. The peaks of the 1212 phase were indexed using the space group P4/mmm and lattice constants a=3.822(1) A and c=11.818(5) A. The number in parentheses is the estimated error in units ofth e least significant digit. The amount of impurity phases (which could not be identified using the JCPDS data from 1991 ) was comparatively small. ¢q Q
e'-1
o qr--
0
C/) te-
03
t~
co ne
2O
i
=
40
60
Bragg Angle 2GB F] Fig. 2. The powder XRD pattern for the superconducting sample with nominal composition (Pbo.~Vo.DSr2(Cao.4Yo.s)Cu20~_. Impurity peaks are marked by ( 0 ) .
W. Widder et al. / Superconductivity in the 1212 system
The impurity peaks are marked by ( • ) . The lattice constants for the 1212 phase of the samples investigated chansed in a systematic way as is shown in fig. 3. The a-lattice parameter was nearly constant. For constant Pb:V ratio (constant x) the c-lattice parameter and the volume of the unit cell decreased with increasing Y content as was expected by the size of the ionic radii. For constant Ca:Y ratio (constant z) the c-lattice parameter and the volume of the unit cell were also decreasing with increasin8 V content. The only exception were the samples with (x, z)=(0.3, 0.2) and (x, z)=(O.l, 0.2) which might indicate that the compositions of these final members of the series were not inside the homogeneity range of the 1212 phase. As mentioned above, no 1212 phase was formed for the sample with (x, z) = (0.2, 0).
3.825 i
v
.~__
o 3.815
"
t
o
:
11.79
! o o
0
~7
I J
-- 173.0 .~_ > 172.0
0
" A
11.84
g
o
t
O3
0
0.2
0.6
1.0
Y-Concentration Z Fig. 3. The lattice constants a, ¢ and the unit-cell volume (Pb~_xVx)Sr2 (Cat_,Y=)Cu2OT_8 as a function o f l h e Y content. The different symbols correspond to those used in fig. 1. Estimated errors are + 0.001 ~, for • and +_0.005 )k for ¢.
123
SEM imases revealed plate-like microcrystals of about 0.5 ttm thickness and area of several ~tm2 and showed the porosity of the samples (see fig. 4). In order to test whether V is really incorporated into the 1212 phase, energy dispendve X-ray (EDX) spectrometry measurements were performed. The chemical composition of eleven individual, plate-like microcrystals was analysed for each of the samples with nominal compositions (Pbo.TVo.3)St2 (Cao.iYo.6)Cu2OT_6 and (Pbo.sVo.5)Sr2(Cao.4Yo.6)Cu2OT_8 which had been post-treated in argon. Care was taken to compare m i ~ s selected at random from different parts of the peget investigated. The pellet was broken and only microcrystals whose surfaces were parallel to the fracture surface have been investigated. Since the penetration depth of the primary electrons is a function of the acceleration voltage, the small thickness of the grains required low acceleration voltages in order to avoid that contributions from impurity phases below the 1212-phase grains would disturb the analyses. For pure vanadium, the (~(pZ)) curves, which give the number of Kct quanta generated per incoming electron as a function of depth in the specimen [21 ], show that at 10 kV (30 kV) acceleration voltage 99% of the emitted X-ray are produced within a surface layer of thickness 0.3 gtm (3 ttm) of the specimen. For this reason, we used an acceleration voltage of 10 kV for our investigations. The take-off angle was 40 ° . The characteristic lines for the analyses were Pb M, V K, Sr L, Ca K, Y L and Cu L. The counting time per measurement was about 1200 s. To test the influence of the low acceleration voltage (10 kV) on the accuracy of the analysis of the crystals, Y-123 and Bi-2212 single crystals were also investigated under the same conditions. At low acceleration voltages the correction programs produced systematic deviations from the stoichiometry of the crystals, in particular, the Sr and Ba content were severely overestimated. The analysis was, however, reproducible to within 1%--5% per element on the same crystal. Since the analysed composition does not represent the stoichiometry of the microcrystal we do not give detailed results. Nevertheless, the analyses of the microerystals could be used to test whether V is incorporated into the 1212-grains: The mean Sr-Ca-Y-Cu concentration was constant within the standard deviation for the 1212 micro-
124
W. Widder et al. / Superconductivity in the 1212 system
Fig. 4. SEM image from a part of the sample with nominal composition (Pbo.~Vo.5)Sr,(Cao.4Yo.e)Cu2OT_6.The surface has been sputtered with a few nm Au. crystals of both samples with nominal compositions ( Pbo.TVo.3) Sr2 (Cao.4Yo.6) Cu207-S and (Pbo.sVo.5)Sr2(Cao.4Yo.6)Cu2OT_a, while the P b - V concentration of the 1212 grains differed for both samples following the changes given by the nominal compositions (Pbo.TVo.3)and (Pbo.sVo.5). This is consistent with an incorporation of V into the 1212 phase and thus the described changes in the lattice constants should be a result of different (Pb, V) ratios. In addition, with the help of the EDX measurements we could observe in one measurement a V- and Sr-rich impurity phase, which contained traces of Cu and Ca. None of the as-prepared samples and of the samples post-treated in oxygen was superconducting. All these samples showed a semiconducting behaviour. The resistance behaviour of the samples posttreated at 500°C for 20 h in argon is shown in fig. 5. (Pbo.sVo.5) Sr2 (Cao.4Yo.6) Cu207_6 and (Pbo.7Vo.3)Sr2(Cao.4Yo.6)Cu2OT_6 became superconducting at around 25 K and 40 K, respectively, as measured by the onset of the resistance drop. Resistance curves for both samples are shown in fig. 6 on an enlarged scale. The samples used for the re-
sistance measurements were of similar size and we used similar configurations for the contacts. However, since their shape was irregular and, more importantly, the porosity of the samples and the amount of impurity phases varied as a function of x and z, no specific resistivities were calculated. Nevertheless, we expect that the measured resistances reflect the intrinsic resistivities, and can be compared within the series to within a factor of about two. The superconducting transition was confn'med by the SQUID magnetization measurements as is shown in fig. 7 for sample ( Pbo.7Vo.3 ) Srz (Cao.4Yo.6)Cu2OT_s. Both the field-cooled curve and the zero-field-cooled curve are displayed for a measuring field of B = 6 . 5 mT. The onset of the superconducting transition was observed at about 25 K which corresponds to a vanishing resistance. The strength of the diamagnetic signal at 5 K was comparatively weak and corresponded to a volume fraction of about one percent of an ideal superconductor. Due to the small magnitude of the diamagnetic signal it cannot be excluded that small amounts of (Pb, Ca)-1212, (Pb, Cu)-1212 or (Pb, Sr)-1212 phases are responsible for the superconductivity, al-
W. Widder et ol. / Superconductivity in the 1212 system
125
800
9
•
z=o.2
•,
voZ=°'4 -
,
'i
,,c
®
o e,
400
[ c
.2
•
Z=0.8
m "G ID
o
em
n-
ZFC
0 I
0
-0.005
I
1 O0
200
300
~'
E
" Z=0.2 0
(D 0 eQ
v
Z=0.4
n Z=0.6 400
0 0
(kA
o Z=0.8
@ nr. . . . .
0
1oo
200
300
Temperature [K]
Fig. 5. The temperature dependence of the electrical resistance for the (Pbl_~V~)Sr2(Cal_,Y=)Cu2Oy_ssamplesof series 1 (a) and series 2 (b) (same symbolsas in fig. 1).
1.6
2 8 =
• X=0.5 Z=0.6
X_Coo
~
o x=0.3
0.8
g ID
o ~
~
~1 iiiiiiiiiil[iJil[llllll
i [ i J [ ] 1 i ii
0 0
100
200
300
Temperature [K]
Fig.
6.
Resistance of the
(PbonVo.3)Sr2 (Cao.,Yo.6)Cu20~_a
60
Fig. 7. D C m a ~ e t i z a t i o n obtained in ZFC and FC measurements o f sample (Pb0.yV0.3)Sr2(Cao.,Yo.6)Cu,Oy_~
800
2
30 Temperature [K]
Temperature [K]
0
0
superconducting samples ( [] )
and
(Pbo.sVo.s)Sr2(Cao.4Yo.6)Cu2Oy-a( I ) (same symbolsas in fig. 1).
though we have not seen such phases in the EDX measurements. We point out that no need of an argon post-treatment to introduce superconductivity is reported in the literature for these phases. If the superconductivity observed is due to the (Pb, V)- 1212 phase the small magnitude of the diamagnetic signal could be explained by the small grain size of the material. SEM images revealed plate-like grains of about 0.3 p m thickness which is of the order of the expected penetration depth. The comparison with other Tl- and Pb-based 1212 compounds shows that we have not yet reached the optimum synthesis conditions. Small apparent superconducting volume fractions are not uncommon in some of the 1212-substitutions (see e.g. refs. [6] and [13]) and can usually be increased by varying the synthesis conditions [ 22 ]. For constant Ca: Y ratio the c-lattice parameter and the volume of the unit cell decreased with increasing V content. As Rouillon et al. [ 12 ] propose for the ( Pb, Sr )- 1212 compound, it seems likely that Pb has the valency+ 2 and possesses a lone pair. The lone pair could compensate oxygen vacancies in the SrO layers which are created by the argon post-treatment needed to introduce superconductivity. This model is consistent with the observed variation of the lattice parameters as a function of x and z and with the formal copper valency of about + 2.15 for reasonable values of oxygen-deficiency J ~ 0.4 and a formal vanadium valency of + 3.
126
W. Widder et al. / Superconductivity in the 1212 system
4. Conclusions
References
Superconductivity with Tc's up to 40 K was found in the (Pb~_xVx)Sr2(Cal_zYz)Cu2OT_6 system for the samples with nominal compositions (Pbo.vVo.3)Sr2(Cao.4Yo.6)Cu207_6 and (Pbo.sVo.s)Sr2(Cao.Ho.e)Cu2OT_6 after a thermal post-treatment in flowing argon. Thus the occurrence of superconductivity seems to be closely related to the oxygen stoichiometry. EDX measurements of individual microcrystals of these samples revealed that V is incorporated into the 1212 phase. The small diamagnetic signal could be explained by the small size of the (Pb, V)-1212 grains, which is of the order of the expected penetration depth. However, due to the small size of the diamagnetic signal it cannot be excluded that also a small number of (Pb, Ca), (Pb, Cu) or (Pb, Sr)-1212 grains exist in the samples, which are possibly responsible for the superconductivity, although we have not seen them in our EDX measurements. Only an enlargement of the diamagnetic signal could clarify this point. Therefore a study of the preparation conditions with the aim to increase the (Pb, V)-1212 grain size and the effective superconducting volume fraction is under way.
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Acknowledgements This work was partially supported by the Bayerische Forschungsstiftung (FORSUPRA) and one of us (WW) by the Deutsche Forschungsgemeinschaft. SEM and EDX analyses have been performed at BIMF (Bayreuther lnstitut fiir Makromolekfilforschung). We thank C. Drummer for her assistance in the EDX measurements, W. Ettig for his technical assistance, and H. Uniewski for the SQUID measurements.