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Lithium substituted superconducting Y1Ba2Cu4O8 formed in elevated oxygen pressure
PhysicaC 192 (1992) 103-!07 North-Holland
Lithium substituted superconducting Y iBa2Cu408 formed in elevated oxygen pressure M . R . C h a n d r a c h o o d ~, A.P.B. Sinha ~, D.E. Morris 2 a n d J.S. S u r n o w Morns Research Inc., 1918 University Ave., Berkeley, CA 94704, USA
Received 29 August 1990 Revised manuscriptreceived 10 January 1992
Partial substitution of lithium in YtBa:Cu4Os has been accomplishedby reacting the constituentoxides with LiOH-H,O at elevated oxygenpressure (200 bar). T, fallsgraduallyat a rate of approximately2 K/at.% of hthium added, up to the solubility hmit of ~ 0.13 Li/formula unit. When Li content is increased further T¢ remains nearly constantat 57-58 K and the volume fraction of the superconductingphase decreasesprogressively.
1. Introduction Studies on the effects of non-isovalent substitutions in the YiBa2Cu408 system are interesting primarily because of the fixed oxygen content of 1 : 2: 4 in contrast to 1 : 2: 3. As a result, doping in 1 : 2: 4 can vary the hole concentration in a controlled manner. Interesting results have been published on substitution of Ca in place of Y which raises Tc [ 1-3 ], La in place of Ba [4], co-doping of Ca at Y site along with La at Ba site which maintains T, and increases the solubility range in 1 : 2: 4 [ 5 ], and Fe at Cu sites [ 6 ]. In addition to these nonisovalent substitutions, isovalent substitutions of rare earths in place of Y [7] and of Sr in place of Ba [8] have also been reported. Lithium addition to 1:2:3 has been studied by several authors [9], and recent work shows that Li does indeed go into the structure and substitute at the Cu sites [10]. A similar substitution in 1:2-4 would appear to be particularly interesting because thc k^, . . . . . . . . . . . :^" ;" I" 2" A ,,,;,I,, be . . . . ,-,~a to go up wtth increasing lithium substitution in view of the fixed oxygen content ( 8 per formula unit ). This paper reports r,~sults obtained with starting compoPresent address: Dept. of Material $ctenceand Mineral Engtneermg. Umvers~tyof Cahfornta, Berkeley,CA 94720, USA. 2 To whom all correspondenceshould be addressed
sition of Y : B a : C u : L i = 1 : 2 : 4 : x with ( 0 < x < 1.0), heated at 930°C in 200 bar oxygen pressure•
2. Sample preparation Samples were prepared by sohd state reaction between Y203, BaO2, CuP and L i ( O H ) . H 2 0 at three different temperatures (900, 930 and 980°C) and 200 bar oxygen pressure. As mentioned above, the starting stoichiometry was Y: Ba: Cu: Li = 1 : 2: 4: x, Le. the Y : B a : C u ratios were maintained as in the parent 1:2:4 compound and the lithium was additional, so no site was given preference for accommodating lithium. Since the molecular weights of lithium compounds are small, the compositions were prepared by using two master mixtures, prepared from ( 1) Y : B a : C u : L i = 1:2:4:1 and (2) Y: Ba: Cu: Li = 1 : 2: 4: 0. The first batch had composition ( 1 ). The second batch consisted of ( 1 ) diluted with an equal quantity of (2). Each batch was successively diluted by adding ~he second mixture so that x was successively decreased to O_~: 0.25, 0.125, 0.062, 0.031 and 0.016. The compositions were well mixed in an agate mortar and pestle and pressed into 6 mm d i a × 2 mm thick tablets. They were fired for 15 h in 200 bar oxygen pressure in a commercial high pressure oxygen furnace [11 ]. Samples were prepared at three temperatures: 900, 930 and 980°C.
0921-4534/92/$05.00 © 1992 ElsevierSciencePubhshers B.V. All rightsreserved.
104
M.R. Chandrachoodet aL / Lithzum substttuted Y ~Ba2Cu40s
All the eight compositions were fired together, but each pellet was wrapped individually in a separate piece o f gold foil. The samples were cooled at approximately 10°C/rain in the pressurised oxygen atmosphere. After cooling, the tablets were reground and retired once more under the same conditions. Three separate sets o f samples were synthesized at each temperature. After synthesis, all samples were analysed by X-ray powder diffraction ( X R D ) and by S Q U I D magnetometry.
3. Phase fractions by X-ra~ powder diffraction analysis The tablets obtained after heating were powdered and spread over a 3 × 3 cm -~glass slide and examined on a Scintag PAD-V diffractometer using Cu Kct radiation and an intrinsic Ge detector with 200 eV energy resolution. The Kct2 contribution was removed using the available software, and the crystalline phases present were identified by comparing with standard patterns. The fractions of the different phases were estimated by comparing the intensities of selected non-overlapping lines. The Y~Ba,Cu4Os phase formed at 900 and 930°C at 200 bar. On the other hand, at 980 ~C Y2Ba4Cu7OI5 was formed at the same pressure. The samples treated at the lower temperatures obtained, m addition to the 1:2:4 phase, a small amount of the high pressure form o f BaCuO_~ [ 12 ] and some CuO. With careful mixing of the ingredients and careful control of the heating profile we found it possible to nearly eliminate the BaCuO~_impurity phase. On the other hand, CuO was always observed to be present and its peak intensity increased with increasing Li concentration. The proportion of unreacted CuO increased to -,-5 weight percent at x=O. 125 and x=0.25. This is taken as an indication that lithium may be substituting for copper. The distribution of Li over Cu chains sites versus plane sites has not yet been determined. At lithium concentration beyond x=0.25, the proportion of 1.2 4 which is formed decreases systematically and an umdentified phase appears. This phase, and the CuO as well, increases as x increases. A Lvpical X R D pattern is shown m fig. 1.
4. Diamagnetic susceptibility measuremeats The Meissner magnetic susceptibility ZM was measured over the temperature range o f 10-100 K using a Quantum Design SQUID magnetometer. The field was held constant at 10 Oe. Each sample was prepared by grinding the sintered pellet into fine powder, which was mixed with an equal weight of talc and pressed. The sample was first cooled down to 10 K, then the temperature was gradually increased and the diamagnetic moment was measured as a function of temperature. The susceptibility was calculated and plotted as a function of temperature, and the diamagnetic onset temperature was read from the susceptibility-temperature graph. Figure 2 shows plots o f XM as a function of temperature for three compositions (data for other compositions have been omitted for clarity). The diamagnetic susceptibility (diamagnetic fraction) of all samples was measured at 10 K. These values are plotted as a function of x in fig. 3. The fall in Xto K beyond x = 0.125 corresponds to the decreasing fraction of the superconducting 1:2:4 phase. A summary of the magnetic data obtained is pre',ented in table 1.
5. Solubility limit of Li in YxBa.,Cu,.Os The T, was observed to decrease w~th increasing x up to x = 0 . 1 2 5 ; beyond that point T¢ remains constant (fig. 4). This indicates that lithium goes into solution up to x~0.13. Thus the solubility limit of Li in 1 : 2: 4 is much smaller than it is in 1 : 2: 3 where x = 0 . 5 [9]. The solubility limit o f Li in 1:2:4 is of the same order as the limit for Ca or La in 1 : 2: 4, but is significantly higher than for Fe [6] which also substitutes in the Cu sites. It was expected that Li substitution would cause a shift in lattice parameters, and the solubihty limit would be indicated by a saturation in parameter values. However, because of the similar sxzes of Li ~+ and Cu ~'+, the observed changes are vet3 small ( see table 2 ), so the solubility hmit could not be determined by lattice parameters alone. We find that in general the solubility hmits for non~sovalent substitutions m 1.2:4 are lower than that m 1 : 2: 3. Th~s can be understood as a consequence
M.R. Chandrachood et al /Ltthtum substttuted YtBa2Cu40s
%-
105
,,r
8
J
r~
_= cs:~ ..... _tLJl.i.I.. 5
la
15
20
25
30
35
40
0,5
50
55
60
2 0 (deg.) Fig. I. X-ray diffraction pattern of a sample w~th composition x = 0.25 indicating the presence of 1.2 4 ( indexed peaks } along with small amounts of unreacted CuO {peaks marked CuO) and xmpm,ty phase {peaks marked* }.
0 001
114
T = I0 K;
H = I00e
0.000
-g
%
o
t~
~n
B k~
-{) 001
>
L 2° P~ r,,
"
=; . o 2 5 /
-0 002
:"
8
,
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(i ~
H I
[i (,
l) ,,4
j l)
n"
[ I t{'mll(.t)llllJllOn
/
-0 003
Frog. 3. The dtamagnetic volume fraction at 10 K as a functmn of the lithium concentration (x) m YBa.,Cu4Os+x Li.
X = 0.062.,~'-0.004
0
20
4o
60
8o
TFMPERATURE Ftg. 2. Suscepttbdnty as a funcuon of temperature for the composmons YBa,CuaOs+x L~, with x=0.062, 0.25, and 0.50.
of the fixed oxygen content in the 1:2:4 phase, which does not leave scope for the oxygen content to adjust ~tself to compensate for valency changes in the catmn sites. In table 3 we s u m m a n s e the solubility limits of various substituents in 1:2:4 at 930 or 950°C and P ( O 2 ) = 2 0 0 bar. One point of difference with respect to other nomsovalent substitutions in 1 : 2: 4 may be noted. For Ca -'+, La 3+ and Fe z÷ substitu-
tions, the 1 : 2: 4 structure gives way to a tetragonal (pseudocubic) l : 2: 3 structure and this defines the substitution limit in the l •2: 4 phase. This tetragonal l :2:3 transformation takes place at roughly x=0.25 in these cases. The preference for the system to go over to 1 : 2 : 3 is presumably due to the greater adaptibility in oxygen stoichiometry, of 1:2-3, while the preference for the tetragonal (pseudocubic) form of I 2: 3 is presumably due to the disorder in the chains mduced by fluctuations in the charge d~stribution in a highly substituted system. The pseudocubic unit cell ( c = 3 a ) will be associated with 90 ° twinning, again caused by fluctuations in the dopant concentration. With Li substitution, the conversion into te-
M.R. Chandrachoodet al. / Lithium substituted Y tBa 2Cu,Os
106
Table 1 Dependence of T¢ and diamagnetic fraction on lithmm concentration (x) in the staring mixture Y~Ba.,Cu4Os+x L1 (x) 0.000 0.016 0.031 0.062 0.125 0.25 0.5 1.0
T~oa~ (K)
Ternbl (K)
AT ° (K)
ZIoK d~ (4~ emu/cm ~)
80 77 72 64 58 58 57 57
66 60 59 54 49 50 51 51
14 17 13 l0 8 8 6 6
0.27 0.25 0.2 0.32 0.28 0.20 0.13 0.04
a~ The diamagnetic onset temperature. bl The mid point of the Meissner diamagnetization curve. o AT is the difference between T~o and T~, and indtcates the width of the transition. a~ The diamagnetic susceptibility at 10 K expressed in units (4ffPt emu/g), where perfect d i a m a g n e t i s m = - I (the density of 1: 2: 4, p, ~ 6.3). The superconducting volume fraction indicates the relative amount of the superconducting phase.
Table 2 Lattice constants oflithmm substituted 124. Sets of samples s~nthesized separately showed the same dependence on L~ content, but lattice constant differed by up to _+0.01
x
a(A)
b(A)
c(A)
v(A3)
0.0 0.0 i 6 0.031 0.062 0.125 0.250
3.837 3.838 3.838 3.830 3.827 3.818
3.867 3.869 3.868 3.869 3.869 3.868
27.23 27.21 27.20 27.20 27.20 27.18
404.0 404. I 403.8 403.1 402.7 401.4
Table 3 Solubility limits of various cations substituted in 1 : 2: 4 near 950°C at P ( O : ) = 200 bar Substituted compound
Solubility limit (x)
References
CaxY t - ~Ba2Cu408 YLa~Ba2_.,Cu4Os YBa2Cu4_ rFe ~Os YBa2Cu~_ ,Li~O~ YBa2Cu4_ ~L1~Os
0.1 0.2 0.025 0.5 0.13
[ 1-3 ] [4l [6 ] [9 ] (this work)
120,
6. Li site location LI-123
t.1-124
~ 40[ t
20 [
00
Fe-124
i 02
04 06 Corlccntr,tltorl of Stlb~tlttterll
08
10
Fig. 4. Effect of Li doping on the superconducting transition temperatures T~ of YtBa2Cu~_ ~LI,O~. [9], YBa2Cu4Os+x Li (this work), and comparison with Fe doping in 1'2.4 i.e. YIBa,,Cu4_ ~Fe,O, [6].
tragonal 1 : 2: 3 is not seen. Orthorhombic 1 : 2: 4 remains stable up to the solubihty limit. This may be connected with the possible formation ofLi~ +-Cu 3+ oxide clusters [9], which would leave the bole concentration unchanged.
Both the fall in Tc as well as 'he X-ray results (the appearence of C u P ) suggests that Li is substituting at the copper sites. This is consistent with the substitution of Li at Cu sites observed in 1 : 2: 3 [ 9 ] and also with the fact that ionic radius ofLP + ( r = 0 . 7 6 ~ in six fold coordination) is much closer to the radius of Cu 2+ than it is to that of Ba 2+ or ya+. We have not determined whether Li substitutes into the C u P chain layers or into the CuP2 planes or both. One might expect that Li substitution in the CuP chain layers would mainly change the b lattice parameter, since the C u - O bonds are in that direction in the chain layers. In contrast, Li substitution in the CuP2 layers might change both a and b. Thus comparison of the a and b changes caused by Li substitution might indicate the site into which Li goes. The values of lattice constants in the Li substituted 1 : 2: 4 samples, determined from the X-ray powder diffraction data, are given in table 2. A small decrease in the lattice parameters with increasing Li concentration is seen, but the decrease is too small to enable us to indicate which site Li enters. Rietveld refinement of neutron diffraction data should be able to identify the site(s)
M.R. Chandrachood et al. /Ltthtum substituted Y tBa,Cu,O~
occupied by the Li: the scattering cross-section of Li is small so Li occupation would be indicated by a substantial apparent vacancy fraction in a Cu site.
107
gradual lowering of T,.. Lithium appears to substitute at the Cu sites.
Acknowledgements 7. Variation of Tc within the solubility limit of Li in YBa2Cu4Os
The authors thank K. Takano, V.T. Shum, H. Bornemann and P.K. Narwankar for valuable assistance.
If Li~ + substitutes for Cu-"+, and if the oxygen content o f 1:2:4 remains constant at eight, one might expect the hole concentration and thereby the Tc to increase, as found when Ca is introduced in place of Y. Unfortunately, we find that with Li addition Tc decreases systematically. The rate of decrease in T~ is found to be about 2K/at.% of Li (fig. 1 ( c ) ) . (We note that this is smaller than the rate of decrease for Fe substitution at the Cu sites in 1:2:4 (6 K/at.%) [6] ). It is well-known that when substitution takes place at Cu sites, several other factors such as scattering and bond breaking are involved besides the change in hole concentration caused by doping. As a consequence, all substitutions in the CuO2 planes of cuprate superconductors have been found to decrease To. Thus it would be unreasonable to expect the only effect of Li substitution at the Cu site to be that of increasing the hole concentration to cause an increase of To. Furthermore, as mentioned above, Li ~+ may form bound clusters with the neighbouring Cu raised into 3 + state [9], and such localised clusters would not contribute mobile holes.
References
8. Conclusions Partial substitution of Li in 1 : 2: 4 has been shown to be possible. However, this substitution causes a
[ I ] T. Miyatak¢, S. Gotoh, N. Koshizuka aud S. Tanaka, Nature 41 (1989) 341: T. Mlyatake, M. Kosuge, N. Koshlzuka, H. Takahashi, N. Mon and S. Tanaka, Physica C 167 (1990) 297. [2] D.E. Morris, P. Narwankar, A.P.B. Sinha, K. Takano, B. Faym and V.T. Shum, Phys. Rev. B 41 (1990) 4118. D.E. Morns, P. Narwankar and A.P.B. Sinha, Physica C 169 (1990) 7. [3] R.G. Buckley, J.L. Tallon, D.M. Pooke and M.R. Presland, Physica C 165 (1990) 391. [4] D.E. Morns, P.K. Narwankar, A.P.B. Smha. K. Takano and V.T. Shum, Appl. Phys. Len. 57 ( 1990 ) 715. [5] P.K. Narwankar, D.E. Morris and A.P.B. Sinha, Physlca C 171 (1990) 305. [6 ] 13 E. Morns, A.P. Marathe and A.P.B. Smha, Phystca C 169 (1990) 386, S. Pradhan, D. McDamel, A Kdhng, W. Huff, P Boolchand and D.E. Farreil. Bull. Am Phys. Soc. 35 (i990) 208. [7] D.E. Morns, J.H. Ntckel. John Y T Wet. N.G Asrnar. J S Scott, U.M Scheven, C.T Huhgren, A G Markelz, J E Post, P.J. Heaney, D R Vablen and R M. Hazen, Ph~s Rex B 39 11989) 7347 [8] T Wada, T, Sakural, N SuzukI. S KorDama, H Yamautht and S. Tanaka, Phys. Rex,. B 41 (1990) 209. [9] M.V. Yan, W.W. Rhodes and P.K. Gallagher, J. Appl. Phys. 63 (1988) 821 J. Wang, F. Boterel, G. Desgardm, J.M. Haussonne and B Reveau, Ind. Ceram. (Pans) 832 (1988) 779 [10] L. Suchow, J.R. Adam and K.S. Sohn. J of Superconductivity 2 (1989) 485. [ 11 ] Morris Research Inc. [ 12 ] M.R. Chandrachood. D E Morns and A.P B. Smha. Ph3 slca C 171 (1990) 187.
Lithium substituted superconducting Y iBa2Cu408 formed in elevated oxygen pressure M . R . C h a n d r a c h o o d ~, A.P.B. Sinha ~, D.E. Morris 2 a n d J.S. S u r n o w Morns Research Inc., 1918 University Ave., Berkeley, CA 94704, USA
Received 29 August 1990 Revised manuscriptreceived 10 January 1992
Partial substitution of lithium in YtBa:Cu4Os has been accomplishedby reacting the constituentoxides with LiOH-H,O at elevated oxygenpressure (200 bar). T, fallsgraduallyat a rate of approximately2 K/at.% of hthium added, up to the solubility hmit of ~ 0.13 Li/formula unit. When Li content is increased further T¢ remains nearly constantat 57-58 K and the volume fraction of the superconductingphase decreasesprogressively.
1. Introduction Studies on the effects of non-isovalent substitutions in the YiBa2Cu408 system are interesting primarily because of the fixed oxygen content of 1 : 2: 4 in contrast to 1 : 2: 3. As a result, doping in 1 : 2: 4 can vary the hole concentration in a controlled manner. Interesting results have been published on substitution of Ca in place of Y which raises Tc [ 1-3 ], La in place of Ba [4], co-doping of Ca at Y site along with La at Ba site which maintains T, and increases the solubility range in 1 : 2: 4 [ 5 ], and Fe at Cu sites [ 6 ]. In addition to these nonisovalent substitutions, isovalent substitutions of rare earths in place of Y [7] and of Sr in place of Ba [8] have also been reported. Lithium addition to 1:2:3 has been studied by several authors [9], and recent work shows that Li does indeed go into the structure and substitute at the Cu sites [10]. A similar substitution in 1:2-4 would appear to be particularly interesting because thc k^, . . . . . . . . . . . :^" ;" I" 2" A ,,,;,I,, be . . . . ,-,~a to go up wtth increasing lithium substitution in view of the fixed oxygen content ( 8 per formula unit ). This paper reports r,~sults obtained with starting compoPresent address: Dept. of Material $ctenceand Mineral Engtneermg. Umvers~tyof Cahfornta, Berkeley,CA 94720, USA. 2 To whom all correspondenceshould be addressed
sition of Y : B a : C u : L i = 1 : 2 : 4 : x with ( 0 < x < 1.0), heated at 930°C in 200 bar oxygen pressure•
2. Sample preparation Samples were prepared by sohd state reaction between Y203, BaO2, CuP and L i ( O H ) . H 2 0 at three different temperatures (900, 930 and 980°C) and 200 bar oxygen pressure. As mentioned above, the starting stoichiometry was Y: Ba: Cu: Li = 1 : 2: 4: x, Le. the Y : B a : C u ratios were maintained as in the parent 1:2:4 compound and the lithium was additional, so no site was given preference for accommodating lithium. Since the molecular weights of lithium compounds are small, the compositions were prepared by using two master mixtures, prepared from ( 1) Y : B a : C u : L i = 1:2:4:1 and (2) Y: Ba: Cu: Li = 1 : 2: 4: 0. The first batch had composition ( 1 ). The second batch consisted of ( 1 ) diluted with an equal quantity of (2). Each batch was successively diluted by adding ~he second mixture so that x was successively decreased to O_~: 0.25, 0.125, 0.062, 0.031 and 0.016. The compositions were well mixed in an agate mortar and pestle and pressed into 6 mm d i a × 2 mm thick tablets. They were fired for 15 h in 200 bar oxygen pressure in a commercial high pressure oxygen furnace [11 ]. Samples were prepared at three temperatures: 900, 930 and 980°C.
0921-4534/92/$05.00 © 1992 ElsevierSciencePubhshers B.V. All rightsreserved.
104
M.R. Chandrachoodet aL / Lithzum substttuted Y ~Ba2Cu40s
All the eight compositions were fired together, but each pellet was wrapped individually in a separate piece o f gold foil. The samples were cooled at approximately 10°C/rain in the pressurised oxygen atmosphere. After cooling, the tablets were reground and retired once more under the same conditions. Three separate sets o f samples were synthesized at each temperature. After synthesis, all samples were analysed by X-ray powder diffraction ( X R D ) and by S Q U I D magnetometry.
3. Phase fractions by X-ra~ powder diffraction analysis The tablets obtained after heating were powdered and spread over a 3 × 3 cm -~glass slide and examined on a Scintag PAD-V diffractometer using Cu Kct radiation and an intrinsic Ge detector with 200 eV energy resolution. The Kct2 contribution was removed using the available software, and the crystalline phases present were identified by comparing with standard patterns. The fractions of the different phases were estimated by comparing the intensities of selected non-overlapping lines. The Y~Ba,Cu4Os phase formed at 900 and 930°C at 200 bar. On the other hand, at 980 ~C Y2Ba4Cu7OI5 was formed at the same pressure. The samples treated at the lower temperatures obtained, m addition to the 1:2:4 phase, a small amount of the high pressure form o f BaCuO_~ [ 12 ] and some CuO. With careful mixing of the ingredients and careful control of the heating profile we found it possible to nearly eliminate the BaCuO~_impurity phase. On the other hand, CuO was always observed to be present and its peak intensity increased with increasing Li concentration. The proportion of unreacted CuO increased to -,-5 weight percent at x=O. 125 and x=0.25. This is taken as an indication that lithium may be substituting for copper. The distribution of Li over Cu chains sites versus plane sites has not yet been determined. At lithium concentration beyond x=0.25, the proportion of 1.2 4 which is formed decreases systematically and an umdentified phase appears. This phase, and the CuO as well, increases as x increases. A Lvpical X R D pattern is shown m fig. 1.
4. Diamagnetic susceptibility measuremeats The Meissner magnetic susceptibility ZM was measured over the temperature range o f 10-100 K using a Quantum Design SQUID magnetometer. The field was held constant at 10 Oe. Each sample was prepared by grinding the sintered pellet into fine powder, which was mixed with an equal weight of talc and pressed. The sample was first cooled down to 10 K, then the temperature was gradually increased and the diamagnetic moment was measured as a function of temperature. The susceptibility was calculated and plotted as a function of temperature, and the diamagnetic onset temperature was read from the susceptibility-temperature graph. Figure 2 shows plots o f XM as a function of temperature for three compositions (data for other compositions have been omitted for clarity). The diamagnetic susceptibility (diamagnetic fraction) of all samples was measured at 10 K. These values are plotted as a function of x in fig. 3. The fall in Xto K beyond x = 0.125 corresponds to the decreasing fraction of the superconducting 1:2:4 phase. A summary of the magnetic data obtained is pre',ented in table 1.
5. Solubility limit of Li in YxBa.,Cu,.Os The T, was observed to decrease w~th increasing x up to x = 0 . 1 2 5 ; beyond that point T¢ remains constant (fig. 4). This indicates that lithium goes into solution up to x~0.13. Thus the solubility limit of Li in 1 : 2: 4 is much smaller than it is in 1 : 2: 3 where x = 0 . 5 [9]. The solubility limit o f Li in 1:2:4 is of the same order as the limit for Ca or La in 1 : 2: 4, but is significantly higher than for Fe [6] which also substitutes in the Cu sites. It was expected that Li substitution would cause a shift in lattice parameters, and the solubihty limit would be indicated by a saturation in parameter values. However, because of the similar sxzes of Li ~+ and Cu ~'+, the observed changes are vet3 small ( see table 2 ), so the solubility hmit could not be determined by lattice parameters alone. We find that in general the solubility hmits for non~sovalent substitutions m 1.2:4 are lower than that m 1 : 2: 3. Th~s can be understood as a consequence
M.R. Chandrachood et al /Ltthtum substttuted YtBa2Cu40s
%-
105
,,r
8
J
r~
_= cs:~ ..... _tLJl.i.I.. 5
la
15
20
25
30
35
40
0,5
50
55
60
2 0 (deg.) Fig. I. X-ray diffraction pattern of a sample w~th composition x = 0.25 indicating the presence of 1.2 4 ( indexed peaks } along with small amounts of unreacted CuO {peaks marked CuO) and xmpm,ty phase {peaks marked* }.
0 001
114
T = I0 K;
H = I00e
0.000
-g
%
o
t~
~n
B k~
-{) 001
>
L 2° P~ r,,
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=; . o 2 5 /
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:"
8
,
29
un {pl) i] I)
(i ~
H I
[i (,
l) ,,4
j l)
n"
[ I t{'mll(.t)llllJllOn
/
-0 003
Frog. 3. The dtamagnetic volume fraction at 10 K as a functmn of the lithium concentration (x) m YBa.,Cu4Os+x Li.
X = 0.062.,~'-0.004
0
20
4o
60
8o
TFMPERATURE Ftg. 2. Suscepttbdnty as a funcuon of temperature for the composmons YBa,CuaOs+x L~, with x=0.062, 0.25, and 0.50.
of the fixed oxygen content in the 1:2:4 phase, which does not leave scope for the oxygen content to adjust ~tself to compensate for valency changes in the catmn sites. In table 3 we s u m m a n s e the solubility limits of various substituents in 1:2:4 at 930 or 950°C and P ( O 2 ) = 2 0 0 bar. One point of difference with respect to other nomsovalent substitutions in 1 : 2: 4 may be noted. For Ca -'+, La 3+ and Fe z÷ substitu-
tions, the 1 : 2: 4 structure gives way to a tetragonal (pseudocubic) l : 2: 3 structure and this defines the substitution limit in the l •2: 4 phase. This tetragonal l :2:3 transformation takes place at roughly x=0.25 in these cases. The preference for the system to go over to 1 : 2 : 3 is presumably due to the greater adaptibility in oxygen stoichiometry, of 1:2-3, while the preference for the tetragonal (pseudocubic) form of I 2: 3 is presumably due to the disorder in the chains mduced by fluctuations in the charge d~stribution in a highly substituted system. The pseudocubic unit cell ( c = 3 a ) will be associated with 90 ° twinning, again caused by fluctuations in the dopant concentration. With Li substitution, the conversion into te-
M.R. Chandrachoodet al. / Lithium substituted Y tBa 2Cu,Os
106
Table 1 Dependence of T¢ and diamagnetic fraction on lithmm concentration (x) in the staring mixture Y~Ba.,Cu4Os+x L1 (x) 0.000 0.016 0.031 0.062 0.125 0.25 0.5 1.0
T~oa~ (K)
Ternbl (K)
AT ° (K)
ZIoK d~ (4~ emu/cm ~)
80 77 72 64 58 58 57 57
66 60 59 54 49 50 51 51
14 17 13 l0 8 8 6 6
0.27 0.25 0.2 0.32 0.28 0.20 0.13 0.04
a~ The diamagnetic onset temperature. bl The mid point of the Meissner diamagnetization curve. o AT is the difference between T~o and T~, and indtcates the width of the transition. a~ The diamagnetic susceptibility at 10 K expressed in units (4ffPt emu/g), where perfect d i a m a g n e t i s m = - I (the density of 1: 2: 4, p, ~ 6.3). The superconducting volume fraction indicates the relative amount of the superconducting phase.
Table 2 Lattice constants oflithmm substituted 124. Sets of samples s~nthesized separately showed the same dependence on L~ content, but lattice constant differed by up to _+0.01
x
a(A)
b(A)
c(A)
v(A3)
0.0 0.0 i 6 0.031 0.062 0.125 0.250
3.837 3.838 3.838 3.830 3.827 3.818
3.867 3.869 3.868 3.869 3.869 3.868
27.23 27.21 27.20 27.20 27.20 27.18
404.0 404. I 403.8 403.1 402.7 401.4
Table 3 Solubility limits of various cations substituted in 1 : 2: 4 near 950°C at P ( O : ) = 200 bar Substituted compound
Solubility limit (x)
References
CaxY t - ~Ba2Cu408 YLa~Ba2_.,Cu4Os YBa2Cu4_ rFe ~Os YBa2Cu~_ ,Li~O~ YBa2Cu4_ ~L1~Os
0.1 0.2 0.025 0.5 0.13
[ 1-3 ] [4l [6 ] [9 ] (this work)
120,
6. Li site location LI-123
t.1-124
~ 40[ t
20 [
00
Fe-124
i 02
04 06 Corlccntr,tltorl of Stlb~tlttterll
08
10
Fig. 4. Effect of Li doping on the superconducting transition temperatures T~ of YtBa2Cu~_ ~LI,O~. [9], YBa2Cu4Os+x Li (this work), and comparison with Fe doping in 1'2.4 i.e. YIBa,,Cu4_ ~Fe,O, [6].
tragonal 1 : 2: 3 is not seen. Orthorhombic 1 : 2: 4 remains stable up to the solubihty limit. This may be connected with the possible formation ofLi~ +-Cu 3+ oxide clusters [9], which would leave the bole concentration unchanged.
Both the fall in Tc as well as 'he X-ray results (the appearence of C u P ) suggests that Li is substituting at the copper sites. This is consistent with the substitution of Li at Cu sites observed in 1 : 2: 3 [ 9 ] and also with the fact that ionic radius ofLP + ( r = 0 . 7 6 ~ in six fold coordination) is much closer to the radius of Cu 2+ than it is to that of Ba 2+ or ya+. We have not determined whether Li substitutes into the C u P chain layers or into the CuP2 planes or both. One might expect that Li substitution in the CuP chain layers would mainly change the b lattice parameter, since the C u - O bonds are in that direction in the chain layers. In contrast, Li substitution in the CuP2 layers might change both a and b. Thus comparison of the a and b changes caused by Li substitution might indicate the site into which Li goes. The values of lattice constants in the Li substituted 1 : 2: 4 samples, determined from the X-ray powder diffraction data, are given in table 2. A small decrease in the lattice parameters with increasing Li concentration is seen, but the decrease is too small to enable us to indicate which site Li enters. Rietveld refinement of neutron diffraction data should be able to identify the site(s)
M.R. Chandrachood et al. /Ltthtum substituted Y tBa,Cu,O~
occupied by the Li: the scattering cross-section of Li is small so Li occupation would be indicated by a substantial apparent vacancy fraction in a Cu site.
107
gradual lowering of T,.. Lithium appears to substitute at the Cu sites.
Acknowledgements 7. Variation of Tc within the solubility limit of Li in YBa2Cu4Os
The authors thank K. Takano, V.T. Shum, H. Bornemann and P.K. Narwankar for valuable assistance.
If Li~ + substitutes for Cu-"+, and if the oxygen content o f 1:2:4 remains constant at eight, one might expect the hole concentration and thereby the Tc to increase, as found when Ca is introduced in place of Y. Unfortunately, we find that with Li addition Tc decreases systematically. The rate of decrease in T~ is found to be about 2K/at.% of Li (fig. 1 ( c ) ) . (We note that this is smaller than the rate of decrease for Fe substitution at the Cu sites in 1:2:4 (6 K/at.%) [6] ). It is well-known that when substitution takes place at Cu sites, several other factors such as scattering and bond breaking are involved besides the change in hole concentration caused by doping. As a consequence, all substitutions in the CuO2 planes of cuprate superconductors have been found to decrease To. Thus it would be unreasonable to expect the only effect of Li substitution at the Cu site to be that of increasing the hole concentration to cause an increase of To. Furthermore, as mentioned above, Li ~+ may form bound clusters with the neighbouring Cu raised into 3 + state [9], and such localised clusters would not contribute mobile holes.
References
8. Conclusions Partial substitution of Li in 1 : 2: 4 has been shown to be possible. However, this substitution causes a
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