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Synthesis and superconducting properties of (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7
-
~ ...
I-
ELSEVIER
PHYSICA© Physica C 245 (1995) 281-286
Synthesis and superconducting properties of (Pbo.serlo.s)Sr2( Yo.sCao.s) CU 2 0 7 J.R. Min a,*', J.K. Liang a,b, X.L. Chen lJ, C. Wang u, C. Dong X.R. Cheng a, ('.H. Rao a
c,
Y.O. Yu
a,
" Im'tiWleofPhysics, Chillese Academy ojScience,~. Beijing 100080, ellina "Ilttc'marionalCellter for Mil/eria/s Physics, Ac/.ulewia Sinko, ShellYOllg IJOO/5, Chilla C Natiollal Laboratory for Sllperconductil'ity,JllstiWle ofPhysics, Cllillese AClldemy ofSciences, Beijing }{l0080, Cbilla Received J.9 November 1994: revised manuscript received 6 February 1995
Abstract The effect of synthesis conditions on (PbU,5Cdu.s)Sr;l(Yn,SCan.s)CU20, has been investigated. For the (Pb,Cd).1212 phase, quenching is not necessary. Quenching the (Pb,Cd)·1212 phase exclusively suppresses its superconductivity, although quenching is favorable for the {Pb,Cu)-1212 phase in removing the extra oxygen of the (Pb,Cu)O layers. On the other hand, annealing the (Pb,Cd)-1212 phase in flowing nitrogen leads to a phase decomposition from Pb-1212 phase to Pb-3212 phase. In this paper, we report the optimal synthesis condition for the (Pb,Cd)-1212 phase by annealing the (Pb,Cd)-1212 phase at 970°C in O2 for 2 h.
1. Introduction
Since high-temperature superconductivity in Ph:!" Sr:!ACu30H+c'i (where A is a lanthanide or a mixture of Ln + Sr or Ca) was discovered in 1988 [1], a considerable effort has been devoted to this class of material. Following the discovery of the nonsuperconducting lead-based 1212 cuprate (Pb,Cu)Sr2(Y,Ca)Cu:!07 [2,3], numerous attempts have been made to induce or optimize the superconductivity in the lead-based 1212 phase. These include exploring reasonable synthesis conditions [4-9], finding the optimal YjCa ratio [10], and carrying out different chemical subslitutions [11-21]. Because of the great flexibility of lhe rock-salt layers in the lead-based
• Corresponding author.
1212 cuprates (Pb,M)Sr2(Y.Ca)Cu:!07 (M = Cu [410], Cd [12-17], Sr [18], Ca [19], Hg [20], Mg [21], etc.), the chemical substitutions are more effectiv~ and more interesting than other means in enhancing superr.;ondueting properties of lead-based 1212 cupratt$ 'and exploring the mechanism of high-tern,. perature superconductivity. In the (Pb,Cu)-1212, quenching is necessary, oth~ erwise there are extra oxygen atoms interstitially present in the (Pb,Cu)O layers at the site correspond.,. ing to the chain site in the lead-based 1212 phase! Oxygen in this position tends to trap holes within th~ rock-salt-type (Pb.Cu)O layers and thus to naturally limit the Tc value [22.23]. From a structural point of view, lead prefers a rock-salt structure environment. whereas copper prefers (\ perovskitc-structure environment, thus, this random arrangement leads to extra oxygen atoms in the (Pb.Cu)O layers [12,13].
0921·4534/95/$09.50 10 1995 Elsevier Science B.V. All rights reserved SSDI 0921·4534(9S)00111-5
l.R. Min C!I a/.j PIly:J'icaC 245 (1995) 28/-286
282
heat treatments 011 the above as-synthesized samples, which will be described in detail below. X-ray diffraction (XRD) analyses were performed using a M18AHF X-ray diffractometer by Japanese MAC Science Ce. with Cu KG: radiation (50 kV, 200 rnA). High-purity Si was added to the samples as an internal standal'rl to correct the 2 (J position of the diffraction pe~k;-;, ,1\ stamlard four-probe method was used to meaSlm~ the electrical resistance of the samples.
A possible solution to this problem is to replace Cu in the (Pb,Cu)O layers by other elements, which prefer a rock-salt structure. Now this has been proven by a few authors. For example, an enhancement of 1'c up to 92 K [12] and 70 K [15] by Cd doping in the system (Pb,Cd)Sr2(Y,Ca)Cu 20 7 has recently been reported. In the paper, we report the optimal synthesis condition of (Pbu.sCdo.s)Sr2(Yu,sCao.s)Cu207 by a conventional solid-state reaction method, and study the effect of different synthesis conditions inc1udir.g atmosphere, annealing temperature, annealing time, and quenching on the superconducting properties.
3. Results and discussion Fig. 1 shows the powder XRD pattern of the as-synthesized sample with nominal composition of (Pb(J,scaO,S)Sr2(Y(J,SCa(),S)Cu~07' The majority of the diffraction lines from the XRD pattern can be ina dexed with a tetragonal unit cell with a == 3.817 A, c = 11.966 A, and V = 174.34 A3• The impurity phases have been identified as SrCu0 2 , Sr2Cu03 and CdO. This result coincides with that reported by Beales et ai. [13]. The lattice parameters of (Pb u.5CU IJ,5)Sr2Y~U207' reported aby Beales et aI. P3], are a = 3.830 A, C == 11.836 A, V = 173.62 A4• This indicates that the Cd containing sample has a slightly shorter a value and a longer c value than the
2. Experimental The samples with nominal composition of (Pbo,sCdO,S)Sr2(YO,SCao,S)CuZ07 were prepared by mixing appropriate amounts of PbO,CdO,SrC0 3,Y20 J, CaC0 3,CuO. The mixture was ground in an agate mortar. The well mixed powders were calcined in air at 850°C for 15 h, and then furnace cooled to room temperature. The prereacted material was reground and pressed into pellets. The pellets were sintered at 910°C for 10 h in air. Hereafter, we call thcfe as-synthesized samples. We carried out a series of
40000 C')
...
,.
0
.-.... r.n
impurity phase
30000
......
0
~
U ......... 20000 ~
r>o
... 0
'"...
N
...
t'l
0
10000
It)
8~ N 0
N
...
0
0
10
20
30
40
50
60
29 Fig. I. X-ray powder diffraction pattern of the as-synthesized sample with nominal composition of(Pbo,sCdo,s)Sr2(YO,SCaU,S)Cu20,.
J.R. Mill CI al. / Physica C 245 (J995J 281-286
283
1.6
1.4
-~
1.2
E r.r:l
0.8
-e:.
1.0
0.6 0_4
0.2
0.0 0
200
150
100
50
260
300
Temper&turt(K) Fig. 2. The temperature dependence of the normalized resistance of samples A,
n, C, D. Samples A, C, D were unncalcd at 970°C for 2 h in
0z, in air, and in Nz• respectively, and then slowly cooled to room temperature; Sample B wns annealed at 970"C for 2 II in O2 and then
quenched to room temperature.
(Pb,Cu)-1212 sample. The change in the c and V suggests that the Jarger cadmium ions (0.95 A for CN = 6) indeed replace the smaller copper ions (0.73 A for eN = 6) within the (Pb,Cu)O layers in the Pb-1212 phase. The as-synthesized samples were annealed at 970°C for 2 h in oxygen (sample A), air (sample C), and nitrogen (sample D), respectively, and then
slowly cooled to room temperature. Sample B was annealed at 970°C for 2 h in oxygen, and then quenched to room temperature. Fig. 2 shows the temperature dependence of the normalized electrical resistance of these samples. It is found that both sample A and sample C exhibit metaHic behavior in their norma] state. The superconducting transition temperatures for sample A are ~.onscl = 76 K, 'le.mo
10000
• impurity phase
BODO ('j) ---P..
U
*
6000
-........-
......
4000 It
2000
if
0 10
20
40
30
50
60
2e Fig. 3.
X~ray
powder diftral,,;lion pallern of sample D, annealed at Q70°C in nitrogen for 2 h and then slowly cooled to room temperature.
284
J.R. Mill Cl al. / Pltysica C 245 (1995) 281-286
= 51 K. Sample C has lower Te's with Te,onset = 32 K, Tc:.zcro = 12 K. Sample B exhibits a semiconduct-
ing behavior ~\~;·d docs not supercomluct. Unlike the case of (Pb,Cu)-12.12, quenching suppresses the superconductivity of the (Pb,Cd)-1212 phase. This will be discussed below. The broad superconducting transition temperature suggests an inhomogeneous composition distribuUon, especiaHy the oxygen distribu·, Hun in the Pb..1212 phase. The XRD patterns of samples A, Band C are similar to th,~t of the as-synthesized sample except for a small peak shift. The refined lattice parameters for samp';;: A,s3mplc ~ ° Band samnle C are a = 3.~nO A, C = 11.957 A; d 0 ° a=-;:: 3.814 A, C = 11.947 A; and a = 3.815 A, C = o 11.964 A, respectively. This inrlic:ltes that sample A has a smaller (\: value nian sample B and sample C. The smaller a value can be attributed to an increase in the average copper oxh1ation stat~, whkh results in shorter Cu-v distances in the Cu0 2 sheets. Thus sample A has a higher carrier density than sample B, Co It is also the reason why sample A has higher Te's.
Sample D exhibits a semiconducting behavicr in its normal state and has high superconducting transition temperatures (Tc:,onsct = 83 K, Te.zcro = 58 K). But its powder diffraction pattern (see Fig. 3) shows that the sample mainly consists of Pb-3212 phase, whkh can be indexed with a tetragonal unit cell of a =
°
o
3.840 A, C = 15.845 A. The impurity phases in this sample have been identified as SreuO;? and SrY204' Pb-3212 phase has been reported to be a superconductor by Cava et a1. [1], who synthesized this phase in 1%0 2 + N;? Thus we think that the superconductivity of sample Dshould be attributed to the Pb-3212 phase, and that the Pb-1212 phase is unstable in a reducing atmm;phere at high temperatures where it undergoes a phase decomposition from the Pb-1212 phase to the Pb-3212 phase. Naqvi et al. have also anntaled (PbO.7SCUO.?s)Sr;?(YI_xCa)Cu207 at about 820<>C in nitrogen atmosphere, and observed superconducting transition temperatures of Te.onset = 82 K, Te;lCTO = 76 K [24]. They thought that the superconductivity arose from a Pb-1212 phase, though there s~:;·>ned to be Pb-3212 phase present from the characteristic diffraction line (001) at 20::: 5.60 in their XRD pattern" Therefore, it seems to us that superconducting (Pb,Cd)-1212 can only be obtain~d in an oxidizing atmosphere. For the (Pb,Cu)-1212 phase, the removal of extra oxygen is necessary to make the 1212 compound superconducting. In the (Pb,Cu)-1212 phase, extra oxygen atoms in the (Pb" Cu}O layers can introduce a random potential around them, which affects the electronic state of the CuO z planes and the (Pb,Cu)O layers. This random potential may have two effects. One is Anderson localization at the Cu0 2 planes, the
1.6 1.4
~
~
1.2
1.0
et
0.8
~
0.6
e:t
0.4 0.2 0.0 0
50
100
150
200
250
300
Temperature(K) Fig. 4. The temperature dependence of the normalized resistance of the s:lmples annealed in O2 for 2 h at 910°C (sample 1), 940°C (sample 2), 970°C (sample 3) and 990°C (sample 4), respectively, and then slowly cooled to room temperature.
J.R. Mill et al. / Pllysica C 245 (1995) 281-286
other is degration of charge transfer from the (Pb,Cu)O layers to the. Cta02 planes. Namely, if there is extra oxygen in the (Pb.Cu)O layer, the holes in that layer can be trapped by the random potential around the oxygen alom!:, Consequently, the supply of hole carriers to the Cul)2 plane is decreased for the slowly cooled samples ~~"!* extra oxygen [23]. But for the (Pb,Cd)-1212 system, there will not exist such extra oxygen in the (Pb,Cd)O layers. In the (Pb,Cd)-1212, cadmium has a closed shell dlO electronic configt~ration, for which a distorted octahedral coordination is typical. Generally, cations in the rock-salt-type layer assume a distorted octahedral coordination. Thus cadmium prefers to form a rocksalt structure like lead, and the random arrangement of oxygen atoms in the (Pb,Cd)O layers is suppressed. So, we can conclude that quenching is necessary for superconducting (Pb,Cu)-1212 phase, intmducing more hole carriers by~tl;le removal of extra oxygen atoms in the chain sHe, making the samples more metallic and thus enhances their superconductivity. For the (Pb,Cd)-1212 pJ1..1Se, quenching decreases the carrier density because of the formation of oxygen vacancies, and thus suppresses the superconductivity in (Pb,Cd)-1212, like, in the case of YBa2Cu307' Samples 1, 2, 3 and 4 were annealed in 02 for 2 h at 910°C, 940°C, 970°C, and 990°C, respectively, and then slowly cooled to room temperature. Fig. 4 shows the temperature dependence of the normalized electrical resistance of these samples. As the annealin/:.. temperature increases, the temp'~fature dependence of the normalized electrical rcs;(.:"):1AOce cfllanges from a semiconducting behavior to ~t -metallic one. Moreover, changing the armealing time from 2 h to 32 h hardly affected the superconductivity, so in order to prevent lead from volatilizing, we selected 2 . h as our annealing time. The Tc's for th(lse samples are listed in Table 1. It shows that both ~,onsel and Tc,zero obviously change with the annealing temperature. According to Parberry et al. [14], the annealing temperature affects the superconducting properties of (Pb,Cd)-1212 by means of the variations of the overall oxygen stoichiometry ~!)d of the composition in the rock-sa1t-typ~ (Pb,Cd)O layer. The changes in the composition in the rock~salt-type (Pb,Cd)O layer occur by either cation reordering or from volatility at high temperatures because PbD and CdO are both
285
.,~"'
A
970°C, 02' 21~ 970°C, 02' ~(I,~ ~ 970°C, air, 2 11 97DoC, N2, 2 h 91Doe, 2, 2 h 940°C, 2 ,2 h 970°C, 2 , 2 h 990°C, 02' 2 h
B
C D
°°°
1 2 3 (A)
4 a Sample
Te,onscl
Tc:.zero
76
51
32 83
12 58
50 76 53
36 51 36
...-.-~,-----~--
B was then quenched to room temperature.
volatile above 800°C. From above, we can conclude that the optimal synthesis condition for the compound (Pbo.sCdo.s)Sr2(Yo,sCao.s)Cuz07 is annealing the sample at 970°C in O2 for 2 h. In summary, we have studied the effect of synthesis conditions on (Pbo.sCdo.5)Sr2(YO.~ Ca O•S)Cu 20 7 , and can conclude the following. For (Pbo.sCdo.s)Sr2(Yo.sCaO.S)Cu207' the optimal synthesis condition is annealing at 970°C in O2 for 2 h. Quenching is not necessary for the (Pb,Cd)-1212 phase, which is favorable for the (Pb,Cu)-1212 phase to remove extra oxygen in the (Pb,Cu)O layers. Quencbing the (Pb,Cd)-1212 phase exclusively suppresses its superconductivity. Annealing the (Pb,Cd)-1212 phase in .aitrogen leads to a phase decomposition from Pb1212 phase to Pb-3212 phase, although annealing in nitrogen induces bigher superconducting transition temperatures (Tc,onset = 83 K, ~,zero = 58 K).
Af.~mowiedgement
This work has been supported by the National Center for Research and Development of Superconductivity of China.
References [1] RJ. ('.Alva, 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. Marshall, R.C. Farrow, J.V. Waszczak, P. Hull and P. Trevor, Nature (London) 336 (1988) 211.
286
J.R. Mill ci a/./ PhysicoC 245 (1995) 2HI-28(j·
[2] MA Subramanian, l Gopalakrishnan, C.C. Torardi, P.L. Oai, E,O. Boyes, T.R. Askew, P.B. Floppen, W.E. Farneth and A.W. Sleighl, Physica C 157 (1989) 124. [3] J.Y. Lee, L.S. Swinnea and H. Steinfink, J. Matcr. Res. 4 (1989) 763. [4] A. Dna and Y. Uchida, lpn. J. Apr!. Phys. 29 (1990) 1.586. [5] S. Adachi, H. Adachi, K. Setsunc and K. Wasa, Jpn. J. Appl. Phys. 30 (1990) L690. [6] T. Maeda, K. Sakuyama, S. Koriyama, H. Yamaur.hi and S. Tanaka, Phys. Rev. B 43 (991) 7861. [7] X.X. Tang, D.E. Maries and A.P.B. Sinha, Phys. Rev. B 43 (1991) 7936. [8] A. rj{lIignan, T. Rouillon, D. Groult, J. Provost, M. Hervieu, C. Michel, B. Raveall, R.S. Liu and P.P. Edwards, Physica C 177 (1991) 461. [9] S.F. IIu, D.A. Jefferson, R.S. Liu and P.P, Edwards, J. Solid, State. Chern. 96 (1992) 455. [10] R.S. Liu, S.F. Hu, l. Gamcson, P.P. Edwards, A. Maignan. T. Rouillon, D. Groult and B. Raveau, J. Solid. Stale. Chern. 93 (It)i)l) 276. [11] S. Adachi, H. Adachi, K. Selsune and K. Wasa, Jpn. lApp!. Phys. 30 (1991) LI 099. [12] T.P. Beales, C. Dinecn, W.G. Freeman, S.R. Hall, M.R. Harrison, D.M. Jacobson and 5.1. Zammauio, Supercond, Sci. Techno!. 5 (1992) 47.
[13] T.P. Bcales. W.G. Freeman. S.R. H::". M.R. Harrison and J.M. Parberry. Physica C 205 (I 993) 383. {J4] J.M. Parberry and T.P. Beales, Chin. J. Phys. 31 (1993) 1221, [IS] R.S. Liu, D. Groult, A. Maignan, S.F. Hu, D.A, Jeffson, B. Ruvcall, C. Michel. M. Hcrvicn and P.P. Edwards, Physica C 195 (1992) 35. {16] J.R. Min, lK. Liang. X.L. Chen, C. Wang, C. Dong and G.H. Rao, Physica C 229 (1994) 169. [17] lR. Min, J.K. Liang. X.L. Chen, C. Wang, C. Dong and G.H. Rao, Physica C 230 (}994) 389. [18J T. Rouillon, J. Provost, M. Hcrvieu. D. Groult, C. Michel and B. Raveau, Physica C 159 (t 989) 20 I. [J 9] T. Rouillon, A. Maignall, M. Hcrvieu, C. Michel, n. G.~ult and B. Raveau, Physica C 171 (t 990) 7. [20] S.F. Bu, DA Jefferson, R.S. Liu and P.P. Edwards, J. £olid. Stile. Chern. 103 (t 993) 280. [2J] H.B. Liu, D.E. Morris and K. Wasa, Physica C 204 (1993) 262. [22] T. Maeda, K. Sakuyama. F. Izumi, H. Yamauchi. H. Asano and S. Tanaka, Physica C 175 (1991) 393. [23] M. Kosuge. T. Maeda, K. Sakuyama, H. Yamauchi, N. Koshizuka and S. Tanaka, Physica C 182 (1991) 157, [24] S.H.H. Naqvi and l.W. Boyd, Physica C 213 (1993) 161.
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I-
ELSEVIER
PHYSICA© Physica C 245 (1995) 281-286
Synthesis and superconducting properties of (Pbo.serlo.s)Sr2( Yo.sCao.s) CU 2 0 7 J.R. Min a,*', J.K. Liang a,b, X.L. Chen lJ, C. Wang u, C. Dong X.R. Cheng a, ('.H. Rao a
c,
Y.O. Yu
a,
" Im'tiWleofPhysics, Chillese Academy ojScience,~. Beijing 100080, ellina "Ilttc'marionalCellter for Mil/eria/s Physics, Ac/.ulewia Sinko, ShellYOllg IJOO/5, Chilla C Natiollal Laboratory for Sllperconductil'ity,JllstiWle ofPhysics, Cllillese AClldemy ofSciences, Beijing }{l0080, Cbilla Received J.9 November 1994: revised manuscript received 6 February 1995
Abstract The effect of synthesis conditions on (PbU,5Cdu.s)Sr;l(Yn,SCan.s)CU20, has been investigated. For the (Pb,Cd).1212 phase, quenching is not necessary. Quenching the (Pb,Cd)·1212 phase exclusively suppresses its superconductivity, although quenching is favorable for the {Pb,Cu)-1212 phase in removing the extra oxygen of the (Pb,Cu)O layers. On the other hand, annealing the (Pb,Cd)-1212 phase in flowing nitrogen leads to a phase decomposition from Pb-1212 phase to Pb-3212 phase. In this paper, we report the optimal synthesis condition for the (Pb,Cd)-1212 phase by annealing the (Pb,Cd)-1212 phase at 970°C in O2 for 2 h.
1. Introduction
Since high-temperature superconductivity in Ph:!" Sr:!ACu30H+c'i (where A is a lanthanide or a mixture of Ln + Sr or Ca) was discovered in 1988 [1], a considerable effort has been devoted to this class of material. Following the discovery of the nonsuperconducting lead-based 1212 cuprate (Pb,Cu)Sr2(Y,Ca)Cu:!07 [2,3], numerous attempts have been made to induce or optimize the superconductivity in the lead-based 1212 phase. These include exploring reasonable synthesis conditions [4-9], finding the optimal YjCa ratio [10], and carrying out different chemical subslitutions [11-21]. Because of the great flexibility of lhe rock-salt layers in the lead-based
• Corresponding author.
1212 cuprates (Pb,M)Sr2(Y.Ca)Cu:!07 (M = Cu [410], Cd [12-17], Sr [18], Ca [19], Hg [20], Mg [21], etc.), the chemical substitutions are more effectiv~ and more interesting than other means in enhancing superr.;ondueting properties of lead-based 1212 cupratt$ 'and exploring the mechanism of high-tern,. perature superconductivity. In the (Pb,Cu)-1212, quenching is necessary, oth~ erwise there are extra oxygen atoms interstitially present in the (Pb,Cu)O layers at the site correspond.,. ing to the chain site in the lead-based 1212 phase! Oxygen in this position tends to trap holes within th~ rock-salt-type (Pb.Cu)O layers and thus to naturally limit the Tc value [22.23]. From a structural point of view, lead prefers a rock-salt structure environment. whereas copper prefers (\ perovskitc-structure environment, thus, this random arrangement leads to extra oxygen atoms in the (Pb.Cu)O layers [12,13].
0921·4534/95/$09.50 10 1995 Elsevier Science B.V. All rights reserved SSDI 0921·4534(9S)00111-5
l.R. Min C!I a/.j PIly:J'icaC 245 (1995) 28/-286
282
heat treatments 011 the above as-synthesized samples, which will be described in detail below. X-ray diffraction (XRD) analyses were performed using a M18AHF X-ray diffractometer by Japanese MAC Science Ce. with Cu KG: radiation (50 kV, 200 rnA). High-purity Si was added to the samples as an internal standal'rl to correct the 2 (J position of the diffraction pe~k;-;, ,1\ stamlard four-probe method was used to meaSlm~ the electrical resistance of the samples.
A possible solution to this problem is to replace Cu in the (Pb,Cu)O layers by other elements, which prefer a rock-salt structure. Now this has been proven by a few authors. For example, an enhancement of 1'c up to 92 K [12] and 70 K [15] by Cd doping in the system (Pb,Cd)Sr2(Y,Ca)Cu 20 7 has recently been reported. In the paper, we report the optimal synthesis condition of (Pbu.sCdo.s)Sr2(Yu,sCao.s)Cu207 by a conventional solid-state reaction method, and study the effect of different synthesis conditions inc1udir.g atmosphere, annealing temperature, annealing time, and quenching on the superconducting properties.
3. Results and discussion Fig. 1 shows the powder XRD pattern of the as-synthesized sample with nominal composition of (Pb(J,scaO,S)Sr2(Y(J,SCa(),S)Cu~07' The majority of the diffraction lines from the XRD pattern can be ina dexed with a tetragonal unit cell with a == 3.817 A, c = 11.966 A, and V = 174.34 A3• The impurity phases have been identified as SrCu0 2 , Sr2Cu03 and CdO. This result coincides with that reported by Beales et ai. [13]. The lattice parameters of (Pb u.5CU IJ,5)Sr2Y~U207' reported aby Beales et aI. P3], are a = 3.830 A, C == 11.836 A, V = 173.62 A4• This indicates that the Cd containing sample has a slightly shorter a value and a longer c value than the
2. Experimental The samples with nominal composition of (Pbo,sCdO,S)Sr2(YO,SCao,S)CuZ07 were prepared by mixing appropriate amounts of PbO,CdO,SrC0 3,Y20 J, CaC0 3,CuO. The mixture was ground in an agate mortar. The well mixed powders were calcined in air at 850°C for 15 h, and then furnace cooled to room temperature. The prereacted material was reground and pressed into pellets. The pellets were sintered at 910°C for 10 h in air. Hereafter, we call thcfe as-synthesized samples. We carried out a series of
40000 C')
...
,.
0
.-.... r.n
impurity phase
30000
......
0
~
U ......... 20000 ~
r>o
... 0
'"...
N
...
t'l
0
10000
It)
8~ N 0
N
...
0
0
10
20
30
40
50
60
29 Fig. I. X-ray powder diffraction pattern of the as-synthesized sample with nominal composition of(Pbo,sCdo,s)Sr2(YO,SCaU,S)Cu20,.
J.R. Mill CI al. / Physica C 245 (J995J 281-286
283
1.6
1.4
-~
1.2
E r.r:l
0.8
-e:.
1.0
0.6 0_4
0.2
0.0 0
200
150
100
50
260
300
Temper&turt(K) Fig. 2. The temperature dependence of the normalized resistance of samples A,
n, C, D. Samples A, C, D were unncalcd at 970°C for 2 h in
0z, in air, and in Nz• respectively, and then slowly cooled to room temperature; Sample B wns annealed at 970"C for 2 II in O2 and then
quenched to room temperature.
(Pb,Cu)-1212 sample. The change in the c and V suggests that the Jarger cadmium ions (0.95 A for CN = 6) indeed replace the smaller copper ions (0.73 A for eN = 6) within the (Pb,Cu)O layers in the Pb-1212 phase. The as-synthesized samples were annealed at 970°C for 2 h in oxygen (sample A), air (sample C), and nitrogen (sample D), respectively, and then
slowly cooled to room temperature. Sample B was annealed at 970°C for 2 h in oxygen, and then quenched to room temperature. Fig. 2 shows the temperature dependence of the normalized electrical resistance of these samples. It is found that both sample A and sample C exhibit metaHic behavior in their norma] state. The superconducting transition temperatures for sample A are ~.onscl = 76 K, 'le.mo
10000
• impurity phase
BODO ('j) ---P..
U
*
6000
-........-
......
4000 It
2000
if
0 10
20
40
30
50
60
2e Fig. 3.
X~ray
powder diftral,,;lion pallern of sample D, annealed at Q70°C in nitrogen for 2 h and then slowly cooled to room temperature.
284
J.R. Mill Cl al. / Pltysica C 245 (1995) 281-286
= 51 K. Sample C has lower Te's with Te,onset = 32 K, Tc:.zcro = 12 K. Sample B exhibits a semiconduct-
ing behavior ~\~;·d docs not supercomluct. Unlike the case of (Pb,Cu)-12.12, quenching suppresses the superconductivity of the (Pb,Cd)-1212 phase. This will be discussed below. The broad superconducting transition temperature suggests an inhomogeneous composition distribuUon, especiaHy the oxygen distribu·, Hun in the Pb..1212 phase. The XRD patterns of samples A, Band C are similar to th,~t of the as-synthesized sample except for a small peak shift. The refined lattice parameters for samp';;: A,s3mplc ~ ° Band samnle C are a = 3.~nO A, C = 11.957 A; d 0 ° a=-;:: 3.814 A, C = 11.947 A; and a = 3.815 A, C = o 11.964 A, respectively. This inrlic:ltes that sample A has a smaller (\: value nian sample B and sample C. The smaller a value can be attributed to an increase in the average copper oxh1ation stat~, whkh results in shorter Cu-v distances in the Cu0 2 sheets. Thus sample A has a higher carrier density than sample B, Co It is also the reason why sample A has higher Te's.
Sample D exhibits a semiconducting behavicr in its normal state and has high superconducting transition temperatures (Tc:,onsct = 83 K, Te.zcro = 58 K). But its powder diffraction pattern (see Fig. 3) shows that the sample mainly consists of Pb-3212 phase, whkh can be indexed with a tetragonal unit cell of a =
°
o
3.840 A, C = 15.845 A. The impurity phases in this sample have been identified as SreuO;? and SrY204' Pb-3212 phase has been reported to be a superconductor by Cava et a1. [1], who synthesized this phase in 1%0 2 + N;? Thus we think that the superconductivity of sample Dshould be attributed to the Pb-3212 phase, and that the Pb-1212 phase is unstable in a reducing atmm;phere at high temperatures where it undergoes a phase decomposition from the Pb-1212 phase to the Pb-3212 phase. Naqvi et al. have also anntaled (PbO.7SCUO.?s)Sr;?(YI_xCa)Cu207 at about 820<>C in nitrogen atmosphere, and observed superconducting transition temperatures of Te.onset = 82 K, Te;lCTO = 76 K [24]. They thought that the superconductivity arose from a Pb-1212 phase, though there s~:;·>ned to be Pb-3212 phase present from the characteristic diffraction line (001) at 20::: 5.60 in their XRD pattern" Therefore, it seems to us that superconducting (Pb,Cd)-1212 can only be obtain~d in an oxidizing atmosphere. For the (Pb,Cu)-1212 phase, the removal of extra oxygen is necessary to make the 1212 compound superconducting. In the (Pb,Cu)-1212 phase, extra oxygen atoms in the (Pb" Cu}O layers can introduce a random potential around them, which affects the electronic state of the CuO z planes and the (Pb,Cu)O layers. This random potential may have two effects. One is Anderson localization at the Cu0 2 planes, the
1.6 1.4
~
~
1.2
1.0
et
0.8
~
0.6
e:t
0.4 0.2 0.0 0
50
100
150
200
250
300
Temperature(K) Fig. 4. The temperature dependence of the normalized resistance of the s:lmples annealed in O2 for 2 h at 910°C (sample 1), 940°C (sample 2), 970°C (sample 3) and 990°C (sample 4), respectively, and then slowly cooled to room temperature.
J.R. Mill et al. / Pllysica C 245 (1995) 281-286
other is degration of charge transfer from the (Pb,Cu)O layers to the. Cta02 planes. Namely, if there is extra oxygen in the (Pb.Cu)O layer, the holes in that layer can be trapped by the random potential around the oxygen alom!:, Consequently, the supply of hole carriers to the Cul)2 plane is decreased for the slowly cooled samples ~~"!* extra oxygen [23]. But for the (Pb,Cd)-1212 system, there will not exist such extra oxygen in the (Pb,Cd)O layers. In the (Pb,Cd)-1212, cadmium has a closed shell dlO electronic configt~ration, for which a distorted octahedral coordination is typical. Generally, cations in the rock-salt-type layer assume a distorted octahedral coordination. Thus cadmium prefers to form a rocksalt structure like lead, and the random arrangement of oxygen atoms in the (Pb,Cd)O layers is suppressed. So, we can conclude that quenching is necessary for superconducting (Pb,Cu)-1212 phase, intmducing more hole carriers by~tl;le removal of extra oxygen atoms in the chain sHe, making the samples more metallic and thus enhances their superconductivity. For the (Pb,Cd)-1212 pJ1..1Se, quenching decreases the carrier density because of the formation of oxygen vacancies, and thus suppresses the superconductivity in (Pb,Cd)-1212, like, in the case of YBa2Cu307' Samples 1, 2, 3 and 4 were annealed in 02 for 2 h at 910°C, 940°C, 970°C, and 990°C, respectively, and then slowly cooled to room temperature. Fig. 4 shows the temperature dependence of the normalized electrical resistance of these samples. As the annealin/:.. temperature increases, the temp'~fature dependence of the normalized electrical rcs;(.:"):1AOce cfllanges from a semiconducting behavior to ~t -metallic one. Moreover, changing the armealing time from 2 h to 32 h hardly affected the superconductivity, so in order to prevent lead from volatilizing, we selected 2 . h as our annealing time. The Tc's for th(lse samples are listed in Table 1. It shows that both ~,onsel and Tc,zero obviously change with the annealing temperature. According to Parberry et al. [14], the annealing temperature affects the superconducting properties of (Pb,Cd)-1212 by means of the variations of the overall oxygen stoichiometry ~!)d of the composition in the rock-sa1t-typ~ (Pb,Cd)O layer. The changes in the composition in the rock~salt-type (Pb,Cd)O layer occur by either cation reordering or from volatility at high temperatures because PbD and CdO are both
285
.,~"'
A
970°C, 02' 21~ 970°C, 02' ~(I,~ ~ 970°C, air, 2 11 97DoC, N2, 2 h 91Doe, 2, 2 h 940°C, 2 ,2 h 970°C, 2 , 2 h 990°C, 02' 2 h
B
C D
°°°
1 2 3 (A)
4 a Sample
Te,onscl
Tc:.zero
76
51
32 83
12 58
50 76 53
36 51 36
...-.-~,-----~--
B was then quenched to room temperature.
volatile above 800°C. From above, we can conclude that the optimal synthesis condition for the compound (Pbo.sCdo.s)Sr2(Yo,sCao.s)Cuz07 is annealing the sample at 970°C in O2 for 2 h. In summary, we have studied the effect of synthesis conditions on (Pbo.sCdo.5)Sr2(YO.~ Ca O•S)Cu 20 7 , and can conclude the following. For (Pbo.sCdo.s)Sr2(Yo.sCaO.S)Cu207' the optimal synthesis condition is annealing at 970°C in O2 for 2 h. Quenching is not necessary for the (Pb,Cd)-1212 phase, which is favorable for the (Pb,Cu)-1212 phase to remove extra oxygen in the (Pb,Cu)O layers. Quencbing the (Pb,Cd)-1212 phase exclusively suppresses its superconductivity. Annealing the (Pb,Cd)-1212 phase in .aitrogen leads to a phase decomposition from Pb1212 phase to Pb-3212 phase, although annealing in nitrogen induces bigher superconducting transition temperatures (Tc,onset = 83 K, ~,zero = 58 K).
Af.~mowiedgement
This work has been supported by the National Center for Research and Development of Superconductivity of China.
References [1] RJ. ('.Alva, 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. Marshall, R.C. Farrow, J.V. Waszczak, P. Hull and P. Trevor, Nature (London) 336 (1988) 211.
286
J.R. Mill ci a/./ PhysicoC 245 (1995) 2HI-28(j·
[2] MA Subramanian, l Gopalakrishnan, C.C. Torardi, P.L. Oai, E,O. Boyes, T.R. Askew, P.B. Floppen, W.E. Farneth and A.W. Sleighl, Physica C 157 (1989) 124. [3] J.Y. Lee, L.S. Swinnea and H. Steinfink, J. Matcr. Res. 4 (1989) 763. [4] A. Dna and Y. Uchida, lpn. J. Apr!. Phys. 29 (1990) 1.586. [5] S. Adachi, H. Adachi, K. Setsunc and K. Wasa, Jpn. J. Appl. Phys. 30 (1990) L690. [6] T. Maeda, K. Sakuyama, S. Koriyama, H. Yamaur.hi and S. Tanaka, Phys. Rev. B 43 (991) 7861. [7] X.X. Tang, D.E. Maries and A.P.B. Sinha, Phys. Rev. B 43 (1991) 7936. [8] A. rj{lIignan, T. Rouillon, D. Groult, J. Provost, M. Hervieu, C. Michel, B. Raveall, R.S. Liu and P.P. Edwards, Physica C 177 (1991) 461. [9] S.F. IIu, D.A. Jefferson, R.S. Liu and P.P, Edwards, J. Solid, State. Chern. 96 (1992) 455. [10] R.S. Liu, S.F. Hu, l. Gamcson, P.P. Edwards, A. Maignan. T. Rouillon, D. Groult and B. Raveau, J. Solid. Stale. Chern. 93 (It)i)l) 276. [11] S. Adachi, H. Adachi, K. Selsune and K. Wasa, Jpn. lApp!. Phys. 30 (1991) LI 099. [12] T.P. Beales, C. Dinecn, W.G. Freeman, S.R. Hall, M.R. Harrison, D.M. Jacobson and 5.1. Zammauio, Supercond, Sci. Techno!. 5 (1992) 47.
[13] T.P. Bcales. W.G. Freeman. S.R. H::". M.R. Harrison and J.M. Parberry. Physica C 205 (I 993) 383. {J4] J.M. Parberry and T.P. Beales, Chin. J. Phys. 31 (1993) 1221, [IS] R.S. Liu, D. Groult, A. Maignan, S.F. Hu, D.A, Jeffson, B. Ruvcall, C. Michel. M. Hcrvicn and P.P. Edwards, Physica C 195 (1992) 35. {16] J.R. Min, lK. Liang. X.L. Chen, C. Wang, C. Dong and G.H. Rao, Physica C 229 (1994) 169. [17] lR. Min, J.K. Liang. X.L. Chen, C. Wang, C. Dong and G.H. Rao, Physica C 230 (}994) 389. [18J T. Rouillon, J. Provost, M. Hcrvieu. D. Groult, C. Michel and B. Raveau, Physica C 159 (t 989) 20 I. [J 9] T. Rouillon, A. Maignall, M. Hcrvieu, C. Michel, n. G.~ult and B. Raveau, Physica C 171 (t 990) 7. [20] S.F. Bu, DA Jefferson, R.S. Liu and P.P. Edwards, J. £olid. Stile. Chern. 103 (t 993) 280. [2J] H.B. Liu, D.E. Morris and K. Wasa, Physica C 204 (1993) 262. [22] T. Maeda, K. Sakuyama. F. Izumi, H. Yamauchi. H. Asano and S. Tanaka, Physica C 175 (1991) 393. [23] M. Kosuge. T. Maeda, K. Sakuyama, H. Yamauchi, N. Koshizuka and S. Tanaka, Physica C 182 (1991) 157, [24] S.H.H. Naqvi and l.W. Boyd, Physica C 213 (1993) 161.