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Ceramic processing, dopant-site occupancies and superconducting properties of YBa2(Cu1−xFex)3Oz
Physica C 217 (1993) 175-181 North-Holland
Ceramic processing, dopant-site occupancies and superconducting properties of Y-Ba2(Cut _xFex)3 O z R.G. Kulkarni, D.G. K u b e r k a r and G.J. Baldha Department of Physics, Saurashtra University, Rajkot-360005, India G.K. Bichile Department of Physics, Marathawada University, Aurangabad 431004, India Received30 August 1993
We providea recipeto processoxygen-richFe-dopodYBa2Cu3OT_6samples, whichdisplayunusual Fe-dopantsite occupancies and superconductingproperties.Specifically,we observesubstantial occupationof a chain-associatedquasi-octahedralsite (site E). Room-temperatureagingof such samplesat ambient conditionsleadsto the extinctionof site E and a concomitantgrowthof other chain sites, and an increase in To. The aged samples display the usual Fe-dopant site distribution and T¢'sreported by numerousgroupsfor this superconductor. 1. Introduction The potential of microscopic methods in characterizing metal-doped high-To superconductors is only now becoming apparent as dopant-site occupancies can be tuned by ceramic processing. Indeed an understanding of processing in controlling the physical properties of oxide superconductors is at present in its early stages. Although more than several hundreds of publications have emerged on the titled oxide superconductor, it was only recently that Boolchand et al. [ 1 ] succeeded in observing the elusive octahedrally coordinated Fe-dopant chain site in the overoxygenated YBa2Cu30: (Y-123) phase. We have summarized the site assignments currently accepted for the Fe-dopant in 1-2-3 in table 1 taken from ref. [2 ]. The octahedrally coordinated Fe-chain site (site E) possesses a vanishingly small quadrupole splitting, yielding a singlet in STFe M6ssbauer spectroscopy. This particular site is of special significance. It is the only singlet site observed in M6ssbauer spectra of Fe-doped 1-2-3 samples. All other sites usually display quadrupole doublets. The octahedrally coordinated Fe-chain site (E) is chemically metastable, transforming to the well known chain (A, C, D) sites upon room-temperature aging. This trans-
formation is significant because it provides some confidence in the proposed Fe-chain site assignments [2 ]. We have now succeeded in developing a ceramic processing method to observe the elusive site E in the Fe-doped 1-2-3 samples. Details of this ceramic processing are provided in this work. Furthermore, we confirm the metastable character of this site in Y123 and the conclusions of Boolchand et at. [ 1,2] on its microscopic character and other site assignments.
2. Experimental 2.1. Ceramic processing
The samples of YBa2(Cut_xFex)30: with 0 < x < 0.04 were prepared using Y203, BaCO3, CuO, FeaO3 and Fe-metal enriched in 5~Fe as precursors, in a three-step process. The precursors were mixed in the desired cation concentration ½[Y203 ] + 2BaCO3 + 3 [ ( 1 - x ) C u O +x(Fe203 +Fe) ],
(1)
ground and pelletized (under a pressure of 3
0921-4534/93/$06.00 © 1993ElsevierSciencePublishers B.V. All rights reserved.
176
R. G. Kulkarni et al. / Processing of YBaz(Cu j_ ,,Fe~)jO,
Table 1 Summary of Fe-dolmmt sites observed in Y~Ba2Cu30~ and YtBa2Cu4Os superconductors. The room-temperature isomer shift (3) and quadrupole splitting (zt) of various sites are listed. Shifts are quoted relative to a-Fe at 300 K. The table is taken from ref. [2 ]
Site
zt (ram/s)
di(mm/s)
A
1.97(5)
0.05(5)
B
0.40(5)
+0.20(5)
B'
0.78(5)
+0.28(5)
C
1.10(5)
0.03(5)
D
1.60(5)
Assignment chain-site, square planar,
Fe'÷ (s,=~)
E
<0.2
-10(5) +0.28(3)
plane site, quasi-octahedral, Fe 3+ (S,= {) plane site, square pyramidal, Fe s÷ (S,=~) chain-site, square pyramidal with O ( 5 )-site occupation, Fe 3+ (S,=~) defect chain-site, quasitetrahedral, twin-plane associated chain-site, with pairs of O (4), O ( 1 ) and O (5) nearest-neighbors occupied, Fe 3+ (S, = ~ )
tons em-2). In the first step, these pellets were sealed in an evacuated quartz ampule and reacted at 950°C for 24 h. In the second step, the pellets were reground and heated in an air ambient at 950°C for 24 h using a platinum crucible. In the third step, the samples were ground, pelletized and heated at 950°C for 24 h in a flowing oxygen ambient. The sample temperature was next slowly lowered to 450°C and held there for 12 h, before cooling further, at 1 ° C m i n -~ to attain room temperature.
M6ssbauer-effect measurements were performed on samples obtained by crushing the pellets to a fine powder just prior to the measurements. Such sampies are denoted as "fresh" samples. Ten months after storing the fresh samples (absorbers) at ambient conditions, these were studied again. Such powdered samples stored at room temperature for ten months are labelled as "aged" samples.
2.2. Characterization
3. I. X-ray diffraction and iodometry
X-ray diffraction ( X R D ) patterns of all samples were taken at room temperature using a Siemens Xray diffractometer and Cu Ka radiation. Lattice parameters and crystallographic phase of the samples were obtained from X R D data. The oxygen content of the samples was determined by iodometric titration. DC susceptibility measurements were carried out to determine Tc of the samples using an EG and G PAR model 4500 vibrating sample magnetometer (VSM). STFe Mgssbauer effect measurements were performed at room temperature (300 K) with a conventional constant-acceleration spectrometer using a 57Co: Pd source.
3. Results
X-ray diffraction measurements show that sampies at x=0.01, 0.02 and 0.03 are orthorhombic while the sample at x = 0 . 0 4 is tetragonal. Aging did not exhibit any significant changes in the X R D scans. The oxygen contents determined by iodometric titration are shown in fig. 1 (a) We note that the oxygen content of the fresh samples is significantly higher than that of the aged samples, and furthermore the difference in oxygen content between the fresh and the aged ones increases with Fe-doping concentration.
3.2. Superconductivity The superconducting transition temperatures, Tc's, determined from DC susceptibility as a function of
R.G. Kulkarni et al. I Processing of YBaz(Cuj_xFe~)~O, I
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z< 99 55
i 0
, I
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1 3
3 at.% Fe
I 4
t
t
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I
I
x (at. % of Fe) FiB. 1. (a) Oxygen content "z" and (b) Tc of YBai(Cu;_~FexhO, samples as a function o f x in the fresh and aged states. The open circles refer to fresh samples while the Idled circles refer to aged
*~
iO0
g
ones.
99.5 I--4
the Fe concentration x, are displayed in fig. 1 (b). A noteworthy fact is that the T¢'s of our fresh samples differ significantly from those of the aged samples. T¢'s of our fresh samples decrease at an average rate of 8 K per at.% of Fe as shown in fig. 1 (b). Tc's of our aged samples, on the other hand, decrease more slowly with x. These latter T~'s are closer to those reported by other groups [2,3].
3.3. M6ssbauer spectroscopy Room-temperature M~ssbauer spectra of YBa2(Cul _xFex)30, samples are displayed in figs. 2 and 3. A perusal of the observed lineshapes clearly shows a dramatic change between the fresh and aged samples. We tried to computer fit the M6ssbauer spectra of fresh samples in a number of ways. Physically acceptable best fits were obtained only in terms of five quadrupole doublets A, B,C D and E (fig. 2). The M6ssbauer parameters of each site are listed in table 2. The most striking feature in the lineshape of
r-r- 99 l
lot; Z 0
I
~
)--.4
{f} 99.5 Lt3 t--4
Z u3 Z rr 99 I---
tt
VELOCITY (mm/s) Fig. 2. MO.bauer spectra of flesh YBa2(Cu~_~Fe.hO, samples taken at 300 K at indicated Fe-dopant concentration.
the fresh sample is the appearance of a broad single line denoted as site "E" possessing a small quadrupole splitting of ~0.19 m m / s and an isomer-shift
R.G. Kulkarniet al. / Processingof YBaz(Cu~_~Fe~)jO:
178
"E
lation (In~I) and a concomitant growth in the population of the large qua_drupole-split sites A, C, D with a small reduction in the site-B population. Typical M6ssbauer parameters of aged sample are listed in table 3. The observed spectra of the aged samples resemble those of the usual MiSssbauer spectral-line shapes reported by numerous groups [3-5] in Fedoped Y-123 samples. Our aging results of the x = 0 . 0 3 sample are in good agreement with the ones reported by Boolchand et al. [1,2 ], who employed a three-step sintering procedure to prepare their Fedoped 1-2-3 samples. In their samples Boolchand et al. [ 1 ] could observe about a 15% site-E intensity ratio (IE/I) as compared to a 29% site-E intensity ratio Iv./l in the present x = 0 . 0 3 sample (fig. 4 ( a ) ) .
100
98.5
Q tO0
4. D i s c u s s i o n
gg
The principal result of the present work is the population of a new site (site E) in highly oxidized Fedoped Y-123 samples. In this section we will comment on the underlying assignment of site E based on the present experimental results.
Q I
f
I
4. I. Site-E assignment
VELOCITY
(mm/s)
Fig. 3. Room-temperature spectra of aged YBa2(Cul_~Fe~)~O: samples at x=0.03 and 0.04. These lineshapes differ qualitatively from those observed in the correspondingfresh samples in fig. 2. Table 2 Room-temperature M~sbauer parameters deduced from the spectrum of fig- 2 (fresh sample with x=0.04) Site "n"
4 (mm/s)
~') (mm/s)
7I.( ~ )
A B C D E
1.96(2) 0.41(2) 1.01(2) 1.60(2) 0.19(2)
+0.04(2) +0.19(2) -0.02(2) -0.10(2) +0.27(2)
20(1) 29(2) 20(1) 5(1) 26(2)
") Relative to ct-Feat 300 IC c~=0.27 m m / s relative to a-Fe at 300 K. The lineshapes of the aged samples, shown in fig. 3, display a complete extinction of the site-E popu-
Figure 4 ( a ) displays the Fe-dopant site distribution in our fresh Y-123 samples. A careful examination of these results reveals two important trends. The site-E intensity ratio (IE/I) appears to complement the site-C and -D intensity ratios (Ic/l, ID/I), i.e. an increase in IE/I derives from a decrease in ( Ic + ID ) / I leaving the sum (Iv. + Ic + It>)/I independent of x. These observations suggest that sites E, C and D are genetically related, a proposal advanced Table 3 Room-temperature M6ssbauer parameters deduced from the spectrum of fig- 3 (aged sample with x=0.04) Site "n'"
d (mm/s)
6 ") ( m m / s )
/~ (%) 1
A B C D
1.96(2) 0.36(2) 1.09(2) 1.50(2)
+0.10(2) +0.23(2) -0.07(2) -0.12(2)
40(2) 16(2) 38(2) 6(1)
"~ Relative to a-Fe at 300 K.
R.G. Kulkarni et aLI Processing of YBa2(Cul_,~Fe~)jOz
~r-----'r~ o [a)
~
/
\
30--
--
20-
1
x (at.% o f Fe)
,~ISO
4[(b4at.% )
Fe
Aged ^
12, OFig. 4. (a) Fe-dopant site intensity ratios ln/l (n = A, B, C, D and E) in YBa2(Cu~_~Fex)~O,samples as a function ofx deduced from the spectra of figs. 2 and 3. Compare the Fe-dopant site intensity ratios I./I for a sample at x= 0.04 in the fresh and aged states. Note the extinction of site E and the growth of sites A and C upon aging. earlier [ 1,2]. This observation suggests that these sites (E, C and D) are all chain-sites, and that a redistribution of oxygen in the chains in the different samples leads to a conversion of site E into sites C and D and vice-versa. Specifically, site E may represent an octahedraUy coordinated Fe-chain site formed with occupation of three pairs of oxygen sites along the a-, b- and c-axis. This would require occupation of the normally vacant O (5) sites along the x-axis, in the immediate vicinity of the Fe dopant. Removal of one O(5 ) sites will lead to a square pyramidal coordination of Fe in the chains, a configuration proposed earlier [ 1,2 ] for the site C. The site D is thought to be the counterpart of the site C that is formed at twin planes in Y-123 [6].
179
The complementary roles of E and C and D sites is also supported, at least partially, by our results on aged samples. Figure 4 ( b ) displays the underlying site-population changes for a sample at x = 0.04 upon aging. Parallel results are found at other Fe concentrations. The principal comment is that the extinction of site E intensity ( A / , : / I = - 0 . 2 7 ) partially compensates the growth in population of sites-C and -D integrated intensities, i.e. ( A / c + A / D ) / I = +0.18 with the rest appearing as growth of the site A.
4.2. Does Fe migrate from chains to planes in oxidized samples? An interesting observation of the present work is the complementary role of site-A and site-B populations. We note from rigA(a) that there is almost a one-to-one correspondence between the growth and extinction of these sites, their sum remaining constant at (IA+IB)/I= +0.5. The previous observation could imply one of two scenarios. It could mean that both sites A and B are chain sites. This would appear reasonable for site A but not for site B. Several previous experiments suggest site B to be a plane-associated site [2]. We therefore do not consider this possibility any further. The second scenario is that this conversion represents a plane-to-chain migration of the dopant. Fe normally prefers to equilibrate in an octahedrally coordinated Fe 3+ high-spin configuration with oxygen nearest neighbors, as for example the site B. In the fresh samples, the complementary role of sites A and B (fig. 4(a) ) then suggests that Fe migrates between the CuO2 planes and chains in Y-123. In this context it is important to point out that Baggio-Saitovitch et al. [ 7 ] have shown that mobile oxygen in the chains will have a tendency to be soaked by Cu at lower temperatures and by Fe at higher temperatures. Thus differences in site distributions at different x between the fresh samples probably reflect different thermal histories of processing rather than intrinsic dopant-concentration-induced effects. The effect of room-temperature aging on the Aand B-site populations surprised us. Indeed, given that sites A and B represent chain and plane sites these results can only be understood if one requires Fe and Cu to coherently exchange as additional oxygen in the planes leaves the sample. The underlying
180
R.G. Kulkarni et al. / Processingof YBa2(Cul-xFe.)jOz
conversion is shown schematically in fig. 5. 4.3. Tc and aging of Fe-doped Y-123 The observed increase in Tc (fig. 1 ( b ) ) between the aged and fresh samples may arise due to a reduced oxygen content in the aged sample. It has been popular to correlate T~ with the hole carrier concentration, p, in the planes for Y-123. Figure 6 shows a plot of Tc versus the hole carrier concentration (deO
(
cu(l)
b
aging
rived from the oxygen content) for our fresh and aged samples shown as data points. The continuous curve in fig. 6 represents results reported previously [ 8-10 ] for pristine Y-123 [8]. We note that Tc displays a broad maximum at p = p m ~ - 0 . 2 8 - 0 . 3 0 . The data points corresponding to our fresh samples lie to the right ofpm~ in fig. 6, and display a sharp reduction in Tc with increasing carrier concentration. The data points corresponding to the aged samples lie to the left ofpm~ in fig. 6, i.e., in the under-hole-doped region. In fig. 6, we are struck by the significant deviation in T~ between our aged samples and those reported on pristine Y-123. In calculating the hole carrier concentration in the Fe-doped sample, we took the dopant to be in the 3 + state, thus effectively reducing the carrier concentration for the sample with a larger Fe concentration. It is possible that the presence of Fe in the planes probably freezes some of the carriers in the CuO2 planes, effectively reducing T~ much more than expected.
5. Conclusions
Cu(l)
Fe
Fig. 5. Schematicillustration of the conversionof a site B into a site A upon aging. 100
I
I
I
o Fresh • Aged 90
/ /
80
-
•
% 't
;
_ \
'%
I
7C
--
I
\
--
We provide a recipe on processing Fe-doped Yo123 samples that display an octahedrally coordinated chain site in M6ssbauer spectroscopy. In fresh samples, possessing a large oxygen content, Tc's are found to be decreased probably because of an excess hole doping. Our M6ssbauer-effect results in the fresh samples at various x, clearly show that sites E, C, and D are genetically related; these represent chain sites. Upon room-temperature aging, there is evidence to suggest that the dopant migrates from the planes (site B) to the chains (site A). Such a migration may be induced by the Fe-associated oxygen from the planes leaving the sample due to aging.
/ / 0
/
6C
/
5G
I 0.1
0
I 0.2
I 0.3
I 0.4
Carrier Concentration p Fig. 6. Tc vs. carrier concentration,p, for the present fresh and aged samples shown as data points. The broken line represents resultsreportedpreviously [8-10 ] for pristineYBa,Cu3OT_~The data point with an asterisk representsthe result on a pristine Y123 sample.
Acknowledgements
The authors are thankful to P. Boolchand, University of Cincinnati, USA, for providing VSM and the M6ssbauer-effect characterization of our samples. Part of this work was supported by a grant from the Department of Atomic Energy (DAE), India.
R. G. Kulkarni et at. ~Processing o f YBa 2(Cu ~_ ~,Fex)30 ,
References [ 1 ] P. Boolchand, D. McDaniel, C. Blue, Y. Wu, R. Enzweiler, K. Elgaid and R. Burrows, Hyperfme Interactions 68 ( 1991 ) 15. [2] P. Boolchand and D. McDaniel, Hyperfine Interactions 72 (1992) 125. [3] S. Katsuyama, Y. Ueda and K. Kosuge, Physica C 165 (1990) 404. [4] C. Blue, K. Elgaid, I. Zitkovsky, P. Booichand, D. McDaniel, W.C.H. Joiner, J. Oostens and W. Huff, Phys. Rev. B 37 (1988) 5905. [5] P. Boolchand and D. McDaniel, in: Studies on High-To Superconductors, ed. A.V. Narlikar, Vol. 4 (Nova, Commack, 1990) p. 143.
181
[6] P. Boolchand, C. Blue, IC Elgaid, W. Huff, A. Kilinc, D. McDaniel, P. Biswas, D. 7_.hou and J. Oostens, Hyperfine Interactions 62 (1990) 73. [7] E. Basgio-Saitovitch, R.B. ScorzeUi, I. Souza Azevedo, F.J. Litterst and H. Miclditz, Physica C 215 ( 1993 ) 77. [8] M.W. Shafer, T. Penney and B.L. Olson, Solid State lonics 39 (1990) 63. [ 9 ] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogs, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419. [ 10] W.R. McKinnon, M.L. Post, L.S. Selwyn, G. Pleizier, J.M. Tarascon, P. Barboux, L.H. Greene and G.W. Hull, Phys. Rev. B 38 (1988) 6543.
Ceramic processing, dopant-site occupancies and superconducting properties of Y-Ba2(Cut _xFex)3 O z R.G. Kulkarni, D.G. K u b e r k a r and G.J. Baldha Department of Physics, Saurashtra University, Rajkot-360005, India G.K. Bichile Department of Physics, Marathawada University, Aurangabad 431004, India Received30 August 1993
We providea recipeto processoxygen-richFe-dopodYBa2Cu3OT_6samples, whichdisplayunusual Fe-dopantsite occupancies and superconductingproperties.Specifically,we observesubstantial occupationof a chain-associatedquasi-octahedralsite (site E). Room-temperatureagingof such samplesat ambient conditionsleadsto the extinctionof site E and a concomitantgrowthof other chain sites, and an increase in To. The aged samples display the usual Fe-dopant site distribution and T¢'sreported by numerousgroupsfor this superconductor. 1. Introduction The potential of microscopic methods in characterizing metal-doped high-To superconductors is only now becoming apparent as dopant-site occupancies can be tuned by ceramic processing. Indeed an understanding of processing in controlling the physical properties of oxide superconductors is at present in its early stages. Although more than several hundreds of publications have emerged on the titled oxide superconductor, it was only recently that Boolchand et al. [ 1 ] succeeded in observing the elusive octahedrally coordinated Fe-dopant chain site in the overoxygenated YBa2Cu30: (Y-123) phase. We have summarized the site assignments currently accepted for the Fe-dopant in 1-2-3 in table 1 taken from ref. [2 ]. The octahedrally coordinated Fe-chain site (site E) possesses a vanishingly small quadrupole splitting, yielding a singlet in STFe M6ssbauer spectroscopy. This particular site is of special significance. It is the only singlet site observed in M6ssbauer spectra of Fe-doped 1-2-3 samples. All other sites usually display quadrupole doublets. The octahedrally coordinated Fe-chain site (E) is chemically metastable, transforming to the well known chain (A, C, D) sites upon room-temperature aging. This trans-
formation is significant because it provides some confidence in the proposed Fe-chain site assignments [2 ]. We have now succeeded in developing a ceramic processing method to observe the elusive site E in the Fe-doped 1-2-3 samples. Details of this ceramic processing are provided in this work. Furthermore, we confirm the metastable character of this site in Y123 and the conclusions of Boolchand et at. [ 1,2] on its microscopic character and other site assignments.
2. Experimental 2.1. Ceramic processing
The samples of YBa2(Cut_xFex)30: with 0 < x < 0.04 were prepared using Y203, BaCO3, CuO, FeaO3 and Fe-metal enriched in 5~Fe as precursors, in a three-step process. The precursors were mixed in the desired cation concentration ½[Y203 ] + 2BaCO3 + 3 [ ( 1 - x ) C u O +x(Fe203 +Fe) ],
(1)
ground and pelletized (under a pressure of 3
0921-4534/93/$06.00 © 1993ElsevierSciencePublishers B.V. All rights reserved.
176
R. G. Kulkarni et al. / Processing of YBaz(Cu j_ ,,Fe~)jO,
Table 1 Summary of Fe-dolmmt sites observed in Y~Ba2Cu30~ and YtBa2Cu4Os superconductors. The room-temperature isomer shift (3) and quadrupole splitting (zt) of various sites are listed. Shifts are quoted relative to a-Fe at 300 K. The table is taken from ref. [2 ]
Site
zt (ram/s)
di(mm/s)
A
1.97(5)
0.05(5)
B
0.40(5)
+0.20(5)
B'
0.78(5)
+0.28(5)
C
1.10(5)
0.03(5)
D
1.60(5)
Assignment chain-site, square planar,
Fe'÷ (s,=~)
E
<0.2
-10(5) +0.28(3)
plane site, quasi-octahedral, Fe 3+ (S,= {) plane site, square pyramidal, Fe s÷ (S,=~) chain-site, square pyramidal with O ( 5 )-site occupation, Fe 3+ (S,=~) defect chain-site, quasitetrahedral, twin-plane associated chain-site, with pairs of O (4), O ( 1 ) and O (5) nearest-neighbors occupied, Fe 3+ (S, = ~ )
tons em-2). In the first step, these pellets were sealed in an evacuated quartz ampule and reacted at 950°C for 24 h. In the second step, the pellets were reground and heated in an air ambient at 950°C for 24 h using a platinum crucible. In the third step, the samples were ground, pelletized and heated at 950°C for 24 h in a flowing oxygen ambient. The sample temperature was next slowly lowered to 450°C and held there for 12 h, before cooling further, at 1 ° C m i n -~ to attain room temperature.
M6ssbauer-effect measurements were performed on samples obtained by crushing the pellets to a fine powder just prior to the measurements. Such sampies are denoted as "fresh" samples. Ten months after storing the fresh samples (absorbers) at ambient conditions, these were studied again. Such powdered samples stored at room temperature for ten months are labelled as "aged" samples.
2.2. Characterization
3. I. X-ray diffraction and iodometry
X-ray diffraction ( X R D ) patterns of all samples were taken at room temperature using a Siemens Xray diffractometer and Cu Ka radiation. Lattice parameters and crystallographic phase of the samples were obtained from X R D data. The oxygen content of the samples was determined by iodometric titration. DC susceptibility measurements were carried out to determine Tc of the samples using an EG and G PAR model 4500 vibrating sample magnetometer (VSM). STFe Mgssbauer effect measurements were performed at room temperature (300 K) with a conventional constant-acceleration spectrometer using a 57Co: Pd source.
3. Results
X-ray diffraction measurements show that sampies at x=0.01, 0.02 and 0.03 are orthorhombic while the sample at x = 0 . 0 4 is tetragonal. Aging did not exhibit any significant changes in the X R D scans. The oxygen contents determined by iodometric titration are shown in fig. 1 (a) We note that the oxygen content of the fresh samples is significantly higher than that of the aged samples, and furthermore the difference in oxygen content between the fresh and the aged ones increases with Fe-doping concentration.
3.2. Superconductivity The superconducting transition temperatures, Tc's, determined from DC susceptibility as a function of
R.G. Kulkarni et al. I Processing of YBaz(Cuj_xFe~)~O, I
7.2-
I
I
I
177
I
YBa2(Cux.,Fe~)30,
N
t00
Fresh
Z C) U3 9 9 2
7.c
H
x
O3
© ~9-
I
95
I
Z
(a)
I
i
I
I --
f
1
i
,
99
40t.?l.
Ire
CE F-
!
i
o
iO0 o
Z
0
1"4
03 99.5 O3 resh
z< 99 55
i 0
, I
I 2
1 3
3 at.% Fe
I 4
t
t
I
I
I
I
x (at. % of Fe) FiB. 1. (a) Oxygen content "z" and (b) Tc of YBai(Cu;_~FexhO, samples as a function o f x in the fresh and aged states. The open circles refer to fresh samples while the Idled circles refer to aged
*~
iO0
g
ones.
99.5 I--4
the Fe concentration x, are displayed in fig. 1 (b). A noteworthy fact is that the T¢'s of our fresh samples differ significantly from those of the aged samples. T¢'s of our fresh samples decrease at an average rate of 8 K per at.% of Fe as shown in fig. 1 (b). Tc's of our aged samples, on the other hand, decrease more slowly with x. These latter T~'s are closer to those reported by other groups [2,3].
3.3. M6ssbauer spectroscopy Room-temperature M~ssbauer spectra of YBa2(Cul _xFex)30, samples are displayed in figs. 2 and 3. A perusal of the observed lineshapes clearly shows a dramatic change between the fresh and aged samples. We tried to computer fit the M6ssbauer spectra of fresh samples in a number of ways. Physically acceptable best fits were obtained only in terms of five quadrupole doublets A, B,C D and E (fig. 2). The M6ssbauer parameters of each site are listed in table 2. The most striking feature in the lineshape of
r-r- 99 l
lot; Z 0
I
~
)--.4
{f} 99.5 Lt3 t--4
Z u3 Z rr 99 I---
tt
VELOCITY (mm/s) Fig. 2. MO.bauer spectra of flesh YBa2(Cu~_~Fe.hO, samples taken at 300 K at indicated Fe-dopant concentration.
the fresh sample is the appearance of a broad single line denoted as site "E" possessing a small quadrupole splitting of ~0.19 m m / s and an isomer-shift
R.G. Kulkarniet al. / Processingof YBaz(Cu~_~Fe~)jO:
178
"E
lation (In~I) and a concomitant growth in the population of the large qua_drupole-split sites A, C, D with a small reduction in the site-B population. Typical M6ssbauer parameters of aged sample are listed in table 3. The observed spectra of the aged samples resemble those of the usual MiSssbauer spectral-line shapes reported by numerous groups [3-5] in Fedoped Y-123 samples. Our aging results of the x = 0 . 0 3 sample are in good agreement with the ones reported by Boolchand et al. [1,2 ], who employed a three-step sintering procedure to prepare their Fedoped 1-2-3 samples. In their samples Boolchand et al. [ 1 ] could observe about a 15% site-E intensity ratio (IE/I) as compared to a 29% site-E intensity ratio Iv./l in the present x = 0 . 0 3 sample (fig. 4 ( a ) ) .
100
98.5
Q tO0
4. D i s c u s s i o n
gg
The principal result of the present work is the population of a new site (site E) in highly oxidized Fedoped Y-123 samples. In this section we will comment on the underlying assignment of site E based on the present experimental results.
Q I
f
I
4. I. Site-E assignment
VELOCITY
(mm/s)
Fig. 3. Room-temperature spectra of aged YBa2(Cul_~Fe~)~O: samples at x=0.03 and 0.04. These lineshapes differ qualitatively from those observed in the correspondingfresh samples in fig. 2. Table 2 Room-temperature M~sbauer parameters deduced from the spectrum of fig- 2 (fresh sample with x=0.04) Site "n"
4 (mm/s)
~') (mm/s)
7I.( ~ )
A B C D E
1.96(2) 0.41(2) 1.01(2) 1.60(2) 0.19(2)
+0.04(2) +0.19(2) -0.02(2) -0.10(2) +0.27(2)
20(1) 29(2) 20(1) 5(1) 26(2)
") Relative to ct-Feat 300 IC c~=0.27 m m / s relative to a-Fe at 300 K. The lineshapes of the aged samples, shown in fig. 3, display a complete extinction of the site-E popu-
Figure 4 ( a ) displays the Fe-dopant site distribution in our fresh Y-123 samples. A careful examination of these results reveals two important trends. The site-E intensity ratio (IE/I) appears to complement the site-C and -D intensity ratios (Ic/l, ID/I), i.e. an increase in IE/I derives from a decrease in ( Ic + ID ) / I leaving the sum (Iv. + Ic + It>)/I independent of x. These observations suggest that sites E, C and D are genetically related, a proposal advanced Table 3 Room-temperature M6ssbauer parameters deduced from the spectrum of fig- 3 (aged sample with x=0.04) Site "n'"
d (mm/s)
6 ") ( m m / s )
/~ (%) 1
A B C D
1.96(2) 0.36(2) 1.09(2) 1.50(2)
+0.10(2) +0.23(2) -0.07(2) -0.12(2)
40(2) 16(2) 38(2) 6(1)
"~ Relative to a-Fe at 300 K.
R.G. Kulkarni et aLI Processing of YBa2(Cul_,~Fe~)jOz
~r-----'r~ o [a)
~
/
\
30--
--
20-
1
x (at.% o f Fe)
,~ISO
4[(b4at.% )
Fe
Aged ^
12, OFig. 4. (a) Fe-dopant site intensity ratios ln/l (n = A, B, C, D and E) in YBa2(Cu~_~Fex)~O,samples as a function ofx deduced from the spectra of figs. 2 and 3. Compare the Fe-dopant site intensity ratios I./I for a sample at x= 0.04 in the fresh and aged states. Note the extinction of site E and the growth of sites A and C upon aging. earlier [ 1,2]. This observation suggests that these sites (E, C and D) are all chain-sites, and that a redistribution of oxygen in the chains in the different samples leads to a conversion of site E into sites C and D and vice-versa. Specifically, site E may represent an octahedraUy coordinated Fe-chain site formed with occupation of three pairs of oxygen sites along the a-, b- and c-axis. This would require occupation of the normally vacant O (5) sites along the x-axis, in the immediate vicinity of the Fe dopant. Removal of one O(5 ) sites will lead to a square pyramidal coordination of Fe in the chains, a configuration proposed earlier [ 1,2 ] for the site C. The site D is thought to be the counterpart of the site C that is formed at twin planes in Y-123 [6].
179
The complementary roles of E and C and D sites is also supported, at least partially, by our results on aged samples. Figure 4 ( b ) displays the underlying site-population changes for a sample at x = 0.04 upon aging. Parallel results are found at other Fe concentrations. The principal comment is that the extinction of site E intensity ( A / , : / I = - 0 . 2 7 ) partially compensates the growth in population of sites-C and -D integrated intensities, i.e. ( A / c + A / D ) / I = +0.18 with the rest appearing as growth of the site A.
4.2. Does Fe migrate from chains to planes in oxidized samples? An interesting observation of the present work is the complementary role of site-A and site-B populations. We note from rigA(a) that there is almost a one-to-one correspondence between the growth and extinction of these sites, their sum remaining constant at (IA+IB)/I= +0.5. The previous observation could imply one of two scenarios. It could mean that both sites A and B are chain sites. This would appear reasonable for site A but not for site B. Several previous experiments suggest site B to be a plane-associated site [2]. We therefore do not consider this possibility any further. The second scenario is that this conversion represents a plane-to-chain migration of the dopant. Fe normally prefers to equilibrate in an octahedrally coordinated Fe 3+ high-spin configuration with oxygen nearest neighbors, as for example the site B. In the fresh samples, the complementary role of sites A and B (fig. 4(a) ) then suggests that Fe migrates between the CuO2 planes and chains in Y-123. In this context it is important to point out that Baggio-Saitovitch et al. [ 7 ] have shown that mobile oxygen in the chains will have a tendency to be soaked by Cu at lower temperatures and by Fe at higher temperatures. Thus differences in site distributions at different x between the fresh samples probably reflect different thermal histories of processing rather than intrinsic dopant-concentration-induced effects. The effect of room-temperature aging on the Aand B-site populations surprised us. Indeed, given that sites A and B represent chain and plane sites these results can only be understood if one requires Fe and Cu to coherently exchange as additional oxygen in the planes leaves the sample. The underlying
180
R.G. Kulkarni et al. / Processingof YBa2(Cul-xFe.)jOz
conversion is shown schematically in fig. 5. 4.3. Tc and aging of Fe-doped Y-123 The observed increase in Tc (fig. 1 ( b ) ) between the aged and fresh samples may arise due to a reduced oxygen content in the aged sample. It has been popular to correlate T~ with the hole carrier concentration, p, in the planes for Y-123. Figure 6 shows a plot of Tc versus the hole carrier concentration (deO
(
cu(l)
b
aging
rived from the oxygen content) for our fresh and aged samples shown as data points. The continuous curve in fig. 6 represents results reported previously [ 8-10 ] for pristine Y-123 [8]. We note that Tc displays a broad maximum at p = p m ~ - 0 . 2 8 - 0 . 3 0 . The data points corresponding to our fresh samples lie to the right ofpm~ in fig. 6, and display a sharp reduction in Tc with increasing carrier concentration. The data points corresponding to the aged samples lie to the left ofpm~ in fig. 6, i.e., in the under-hole-doped region. In fig. 6, we are struck by the significant deviation in T~ between our aged samples and those reported on pristine Y-123. In calculating the hole carrier concentration in the Fe-doped sample, we took the dopant to be in the 3 + state, thus effectively reducing the carrier concentration for the sample with a larger Fe concentration. It is possible that the presence of Fe in the planes probably freezes some of the carriers in the CuO2 planes, effectively reducing T~ much more than expected.
5. Conclusions
Cu(l)
Fe
Fig. 5. Schematicillustration of the conversionof a site B into a site A upon aging. 100
I
I
I
o Fresh • Aged 90
/ /
80
-
•
% 't
;
_ \
'%
I
7C
--
I
\
--
We provide a recipe on processing Fe-doped Yo123 samples that display an octahedrally coordinated chain site in M6ssbauer spectroscopy. In fresh samples, possessing a large oxygen content, Tc's are found to be decreased probably because of an excess hole doping. Our M6ssbauer-effect results in the fresh samples at various x, clearly show that sites E, C, and D are genetically related; these represent chain sites. Upon room-temperature aging, there is evidence to suggest that the dopant migrates from the planes (site B) to the chains (site A). Such a migration may be induced by the Fe-associated oxygen from the planes leaving the sample due to aging.
/ / 0
/
6C
/
5G
I 0.1
0
I 0.2
I 0.3
I 0.4
Carrier Concentration p Fig. 6. Tc vs. carrier concentration,p, for the present fresh and aged samples shown as data points. The broken line represents resultsreportedpreviously [8-10 ] for pristineYBa,Cu3OT_~The data point with an asterisk representsthe result on a pristine Y123 sample.
Acknowledgements
The authors are thankful to P. Boolchand, University of Cincinnati, USA, for providing VSM and the M6ssbauer-effect characterization of our samples. Part of this work was supported by a grant from the Department of Atomic Energy (DAE), India.
R. G. Kulkarni et at. ~Processing o f YBa 2(Cu ~_ ~,Fex)30 ,
References [ 1 ] P. Boolchand, D. McDaniel, C. Blue, Y. Wu, R. Enzweiler, K. Elgaid and R. Burrows, Hyperfme Interactions 68 ( 1991 ) 15. [2] P. Boolchand and D. McDaniel, Hyperfine Interactions 72 (1992) 125. [3] S. Katsuyama, Y. Ueda and K. Kosuge, Physica C 165 (1990) 404. [4] C. Blue, K. Elgaid, I. Zitkovsky, P. Booichand, D. McDaniel, W.C.H. Joiner, J. Oostens and W. Huff, Phys. Rev. B 37 (1988) 5905. [5] P. Boolchand and D. McDaniel, in: Studies on High-To Superconductors, ed. A.V. Narlikar, Vol. 4 (Nova, Commack, 1990) p. 143.
181
[6] P. Boolchand, C. Blue, IC Elgaid, W. Huff, A. Kilinc, D. McDaniel, P. Biswas, D. 7_.hou and J. Oostens, Hyperfine Interactions 62 (1990) 73. [7] E. Basgio-Saitovitch, R.B. ScorzeUi, I. Souza Azevedo, F.J. Litterst and H. Miclditz, Physica C 215 ( 1993 ) 77. [8] M.W. Shafer, T. Penney and B.L. Olson, Solid State lonics 39 (1990) 63. [ 9 ] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogs, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419. [ 10] W.R. McKinnon, M.L. Post, L.S. Selwyn, G. Pleizier, J.M. Tarascon, P. Barboux, L.H. Greene and G.W. Hull, Phys. Rev. B 38 (1988) 6543.