- Email: [email protected]
A neutron diffraction study of the structure of Bi1.6Pb0.4Ca1Sr2Cu2Oy
Physica C I73 North-Holland
( 199 1) 267-273
A neutron diffraction study of the structure of Bil .6Pb0.4CalSr2Cu20y A. Sequeira ‘, H. Rajagopal ‘, P.V.P.S.S. Sastry b, J.V. Yakhmi b and R.M. Iyer b Solid State Physics Division ‘, and Chemistry Division b, Bhabha Atomic Research Centre. Trombay. Bombay 400085, India Received
4 December
1990
Neutron profile refinement analysis conducted on Bi,.6Pb0.4Ca,Sr2Cu20y (Space Group Amaa) points to Pb-ions substituting at Bi-sites as Pb4+. A comparative study of air-annealed and vacuum-annealed samples of Bi,.,Pbo4Ca,Sr,Cu20, indicates a slight reduction in the cell parameters upon annealing in vacuum in line with the observation of a reduction in oxygen-content per formula unit (y) from 8.72 for the air-annealed sample to 8.39 for the vacuum-annealed one. We propose that the rather high value of y( = 8.72) for the air-annealed sample is due to the formation of dimerised oxygen-species in the bismuth layer. The significant tilting of Cu-0, pyramids observed for air-annealed Bi,,,Pba4Ca,SrzCuz0. is nearly eliminated upon vacuum-annealing treatment which is suggestive of a reduction in the hole-density as confirmed by a marginal drop in T,(R=O) from 93 to 89 K after vacuum-annealing treatment.
The substitution of Pb in Bi-Ca-Sr-Cu-0 system, and its role in stabilizing the 2223-phase (n = 3 ), with a superconducting transition temperature ( r,) of 110 K, has been the subject of a large number of publications recently [ l-41. Neutron profile refinement study of single-phase Bi,.,Pbo.,Ca2Sr,Cu30,,, by us has revealed that contrary to the general belief, Bidouble layers were devoid of Pb-ions [ 5 1. Detailed analysis of the structural features, in fact, suggested that Pb-ions substitute at Ca-sites, as Pb4+ in (Bi, Pb),Ca&,CuxO,. Incorporation of Pb is known to raise the T, of Bi,CaSr,Cu20s (n=2) phase, too, upto 93 K [ 6 1. It is, therefore, of interest to examine how the incorporation of Pb influences the Bi-2122 crystal structure, and, in particular, whether Pb actually substitutes at Bi-sites, as intended. In this paper, we report the findings of our structural studies on air-annealed and vacuum-annealed samples of Bi,.,Pb,,4Ca,Sr2Cuz0, using powder neutron diffraction measurements. The sample Bi, .6Pb0.4Cal Sr,CuzO, was prepared by the reaction of a precursor matrix CaSrzCuzOS with stoichiometric amounts of high purity B&O3 and Pb ( AC)*. The matrix was first prepared by the solid state reaction of pure CaCO,, SrC03 and CuO at 1235 K in air. Appropriate amounts of the matrix powder, Biz03 and Pb-acetate were mixed well and 0921-4534/91/$03.50
0 1991 - Elsevier Science Publishers
heated at 1180 K in air for 2-5 min till the reaction was complete which was indicated by the turning of the mass completely black. This product was cooled, powdered thoroughly, pelletised into 12 mm diax 12mm thick pellets and sintered in air at 1120 K for 24 h with several intermediate grindings and furnace-cooled to room-temperature. This air-annealed sample is hereafter referred to as samples # 1. The duration and temperature of sintering were carefully controlled because even a slightly higher sintering temperature ( >, 5 K) or longer duration of sintering was,observed to promote the growth of the Bi-2223 (n = 3 ) phase, in the presence of Pb. On the other hand, sintering at temperatures sufficiently lower than 1120 K tended to stabilize the low T,Bi2021 (n= 1) phase. In order to reduce the oxygen-content of the sample, one of the #l sample-pellets was heated under running vacuum ( low3 Torr) at 975 K for 3 h and slow-cooled to room-temperature at 5 K/min under vacuum. This vacuum annealed sample is referred to as sample #2. X-ray diffraction patterns recorded, using Cu Ku radiation, for both samples #l (fig. 1) and #2 confirmed that the sample was very nearly single-phase Bi-2 122 ( CE 30 A) with no evidence of impurity peaks due to Bi-2223 phase, Ca,PbO, (28s 17.8 ’ ) or any other unreacted oxides. A slight
B.V. (North-Holland)
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-Cu-0
Fig. I. Cu Ku X-ray diffraction
pattern
for sample # 1.
1 301 0 TEMPERATURE
Fig. 2. Normalised electrical resistance perature for samples #l and #2.
(K)
plots as a function
oftem-
impurity content due to the BizSr,CuOb (28= 2 1.9” ) cannot, however, be ruled out because of the presence of a very weak (002 ) peak due to this phase at 7.2”. The electrical resistance measurements using a standard DC four-probe method showed a singlesharp drop leading to zero-resistance at 93 K and 89 K for samples #l nd #2, respectively (fig. 2).
Neutron diffraction patterns were recorded at room temperature using the T-101 1 diffractometer at the 100 MW Dhruva reactor at Trombay. The samples were in the form of a cylindrical pellet ( z 6 g in weight, 12 mm diameter and 10 mm height) and the neutron wavelength was 1.2120 A. The instrument collimations were 0.7 o-0.5 ”-0.7 Ofrom the in-pile to the detector end. The patterns were analysed by the Rietveld profile refinement method using a modified version of the program DBW 3.2 [ 71. All possible positional, thermal (B) and occupancy (N) parameters, excepting the occupancy of Cu, 0( 1, 1) and 0 ( 1, 2), were varied in addition to the cell parameters, half width parameters, background parameters, zero-angle and the scale factor. Due to the strong correlations between the thermal and the occupancy parameters, they were varied in separate cycles. A Gaussian peak shape function was assumed. The observed and the calculated neutron diffraction patterns, as well as the difference patterns, for the samples #I and #2 are shown in fig. 3. The experimental resolution of neutron diffraction spectra recorded by us for a variety of high-T, cuprate superconductors has been found to be adequate to reproduce the structural parameters well within the expected standard deviations [ 5,8]. No perceptible degradation was noticed in the neutron diffraction patterns of the present samples over a period of one month indicating that the Bi,.6Pb0.4Ca,SrZCu20y sample is stable. The refined cell parameters (space group Amaa ), atomic coordinates, thermal parameters, occupancy parameters and the relevant R-values for the samples #l and #2 are given in table I. A look at this table reveals that the occupancies of the cation sites remain more or less unchanged, within the experimental error, upon annealing the sample in low P( 0,) (i.e. vacuum-annealing). The bond distances obtained for samples # 1 and #2 are given in table II. The crystal structure of these two samples is shown in fig. 4(a) and (b), respectively. An analysis of the derived structural parameters reveals the following interesting features. (i) The cell parameters of sample # 1 are in good agreement with the values reported in literature for Bi-2 122 compounds [9-l 11. However, there appears a small but significant reduction in the cell volume for sample #2, arising from a contraction of
A. Sequeira etal./A neutron diffraction
study of the structure of Bl-Pb-Ca-Sr-Cu-0
Fig. 3. The observed (dots) and calculated (curve) neutron diffraction patterns for air-annealed (#I ) and vacuum-annealed ples of Bi,.6Pb0.4Ca,SrZCu20g taken at room temperature. The difference pattern for each sample is shown at the bottom.
N 0.5% in a- and b-, and = 0.25% in c-parameter, perhaps due to the depletion of oxygen during the vacuum annealing treatment of sample #2. (ii) For both the samples #l and #2, the occupancy factors of the Bi-sites are close to 1.02, the value expected under the assumption of Pb-ions substituting at 20% of Bi-sites. This points to the occupancy of Pb-ions at Bi-sites in Bi,.bPb,&a,SrzCu20Ysince Pb is the only cation capable of scattering stronger than Bi, albeit marginally. This, however, is in contrast to our findings on Bi,.,Pbo_3Ca&-zCu30, where Pb-ions where shown to substitute at Ca- rather than at Bi-sites [ 5 1. The occupancy of Pb-ions at Bi-sites in Bi,.,Pbo,4Ca,Sr,Cu,0,, for both #l and #2 samples, is further substantiated by a much reduced value for the apical Bi-0( 3) bond-length, viz. 2.06 8, for #l and 1.97 A for #2, compared with values of 2.16 A
269
(#2) sam-
and 3.15 8, reported for Pb-free Bi2CalSrZCu208+6 [ 81 and Bi,.,Pbo.-JZazSrzCu30, [ 51 with Pb-substituted at Ca-sites, respectively. The short Bi-O(3) bond-length and the high estimated bond-strength for Bi-sites (viz. 3.7 and 3.1 for samples #l and #2, respectively) together, point to the substitution of Pb in 4+ state at Bi-sites. (iii) A 68% occupancy factor for O(2)-oxygens for sample #l is indicative of an extra high oxygencontent in the bismuth double-layer corresponding to - 1.36 oxygens per Bi-site, whereas a lower O( 2)occupancy ( - 60%) for sample #2 points to a reduction in the excess oxygen-content in Bi-layer consequent upon the vacuum annealing. The occupancy for all other oxygen-sites, however, is unaffected and remains full as in #l. The total oxygen-content per formula unit works out to be 8.72 and 8.39 for samples #l and #2, respectively.
Table I Positional (x, Y, z) and thermal (B) parameters for the two samples (#1 and #2) of Bi,,sPbo,Ca,Sr,CuzO, obtained from Rietveld refinement of neutron diffraction data. The neutron scattering amplitudes used for Bi, Sr, Ca, Cu and 0 are 8.53, 7.03, 4.90, 7.70 and 5.80 fm, respectively. Numbers in parentheses are esd’s referred to the last digit. ( 100 x N) represents percentage site occupancy. Parameter
Sample # 1
Sample #2
Space group
Amaa 5.347(3) 5.416(4) 30.67(l)
Amaa 5.324(4) 5.391(9) 30.60( 1)
a(A) b(A) c(A) Bi-site (81), (0, Y, z) Y z B N (assuming
b,,)
0.249(2) 0.4449(3) 3.5(2) 1.030(2)
0.240( 3) 0.4482(2) 1.9(2) 1.014( 10)
0.746(3) 0.3604(3) 0.8(3) 0.906( 14)
0.721(3) 0.3613(3) 1.2(3) 0.930( 16)
0.2 1.276( 18)
0.8(3) 1.312( 18)
0.287(2) 0.3003( 3) 2.3(3) 1.0
0.257(4) 0.3030(3) 1.9(2) 1.0
0.3140(5) 1.3(2) 1.0
0.3113(5) 2.1(2) I.0
0.2916(5) 1.3(2) 1.0
0.2944(5) 2.1(2) 1.0
Sr-site (81), (0, y, z) Y z B N (assuming,
b,,)
Ca-site (4f), (0, f, a) B N (assuming b,,) Cu-site (81), (0, Y, z) Y .? B N O(1, z B N
1)-site (8g),
0(1,2kslte
(a,O,z)
(8g), (i. f, z)
Z
B N O(2)-site*(16m),
(x.y,z)
X
Y z B’ N O(3)-site
0.144(5) 0.728(4) 0.4399( 5) 14.1(10) 0.680( 16)
0.090( 8) 0.645(4) 0.4456(6) 10.4(8) 0.602(11)
0.356(2) 0.3803(5) 2.2(4) 0.998(21)
0.251(6) 0.3838(4) 4.1(4) 0.990(22)
2.40 4.41 5.68 1.25
1.50 2.78 3.49 0.81
(8Q), (0, y, z)
Y ; N R-values R.,, (%) R, (%) R, (%) RB (%) Total oxygen/formula
8.72
-
8.39
* Refinements involving disordering of 0( 2) oxygens between general sites ( 16 m) and the average special sites (8 1) led to substantral reduction in the B-values but only marginal improvement in R-values. Sample #l, for instance, yielded values of R,=4.37% (R,,= 5.65%) with occupancy factors of 0.48 and 0.40, respectively at sites (0.170, 0.762, 0.4367) and (0, 0.649, 0.4458), and B= 11.7, while sample #2 yielded R,=2.72% (R,=3.43%) with occupancy factors of 0.45 and 0.30 at sites (0.106, 0.636, 0.4410) and (0,0.662, 0.4607) and B=7.5.
A. Sequeira
et al. 1 A neutron diffraction
study of the structure of Bi-Pb-Ca-Sr-Cu-0
211
Table II Interatomic distances (A) and site valences (V) in (Bi,.6Pb0.4) Ca,Sr2Cu20Y samples; V= 1 W,N,exp[ (rO-r,)/O.37] with parameters [ 141. r,=2.09 (Bi), 1.967 (Ca), 2.1 18 (Sr) and 1.679 (Cu). The numbers in parentheses are esd’s referred to the last digit. Bond Bi/Pb-O(3) -O(2)
a)
-O(2) -O(2) -O(2) -O(2) V(v.u.)
(usingr,forBi)
Sr-O(3) -O(l, -O(2) -O(3) -O(l, -O(3)
1)
(7-X) 2)
(2X) (2X) (1X)
(using r0 for Sr)
V (v.u.) Ca-0(
(1X) (2X)
1,2)
-O(l, V(v.u.) cu-O( -O(l, -O(3)
1)
(4X) (4X)
(usingr,forCa) 1,2) 1)
(2x1 (2X)
(1x1
V(v.u.) ‘) Occupancy
Sample # 1
Sample #2
2.07(2) 1.91(3) 2.71(3) 2.93(3) 3.45(3) 3.62(2)
1.97( 1) 2.27(4) 2.24(3) 3.24(3) 3.20(4) 3.34(2)
3.72
3.07
2.20(2) 2.39(2) 2.56(2) 2.80( 1) 2.83( 1) 3.36(2)
2.62(3) 2.53( 1) 2.65(2) 2.75(l) 2.72(2) 2.94( 1)
2.82
2.10
2.29( 1) 2.73( 1)
2.33( 1) 2.67(I)
2.17
2.10
1.79(I) 2.09( 1) 2.48(2)
1.89(l) 1.94(l) 2.47(2)
2.27
2.25
of 0( 2) is 68% in sample #l and 60% in sample #2
(iv) The occupancy factors for sample # 1 suggest the substitution of Ca-ions at nearly one-third of the Sr-sites and the substitution of Sr-ions at roughly twothirds of the Ca-sites. On the other hand, for sample #2, Ca-ions appear to occupy not more than onefourth of the Sr-sites whereas a higher occupancy factor for Ca-sites necessitates the presence of Sr-ions at nearly three-fourths of the Ca-sites. This appears somewhat unlikely and perhaps indicates the presence of a small amount of Pb-ions ( 5 10%) at Casites in sample #2. This may also explain a slightly lower occupancy at Bi-sites in this sample. (v) A large distortion is seen in the coordination polyhedra of Sr-ions, arising mainly from the distortion of the in-plane Sr-0 bonds, for the air-annealed sample #l. This distortion, however, gets practically eliminated in the case of sample #2, consequent upon depletion of oxygen-content after vacuum-annealing treatment. A similar trend is noticed
for the distortion in coordination polyhedra around Ca-ions, too, though to a lesser extent. (vi ) For sample # 1, of the four planar Cu-0 bonds, the two Cu-0 ( 1,2 ) bonds are much shorter ( - 1.79 A) than the other two Cu-0( 1, 1) bonds ( -2.09 A). Further, the Cu-O5 pyramid is tilted in such a way that the oxygen-ion 0 ( 1, 1) moves closer to Srions, whereas 0( 1, 2) moves closer to Ca-ion. This indicates that 0( 1, 2)-ions are more negative than 0( 1, 1 ), which could be due to the occurrence of fewer electron-holes at 0 ( 1,2 )- than at 0 ( 1, 1 )-sites. This could also be the cause for the shortening of CuO( 1, 2) bonds. On the other hand, the nearly equal bond-lengths for the four planar Cu-0 bonds for sample #2 and the consequent reduction in the tilting of its Cu-O5 pyramid is perhaps suggestive of an expected reduction in the hole-density upon vacuum annealing treatment. This is also supported by a marginal drop in the observed T,(R = 0).
272
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-CM-0
-b -b
Fig. 4. Crystal structure for samples #I and #2.
(vii) As discussed above, the oxygen-content per formula for sample #2 is 8.39, which is reasonably close to what could be accommodated in the Bi-double layer within the framework of the generally reported superstructure models [ 1O-l 3 1. However, the corresponding value of 8.72 for #I appears to be too large, and should be due to the formation of dimerised oxygen-species, as proposed earlier [ 8,9 1. The rather high temperature factors for 0( 2)- ion in both the samples is suggestive, in our opinion, of positional disorder, i.e. the 0( 2)-oxygens are disordered between the general positions, 0 (2A), and the special position, 0 (2B ), located on the mirror-plane, as shown in fig. 5 which depicts a single Bi-0 layer of sample # 1. Refinements in terms of disordering of O(2)-oxygens indeed lead to a significant drop of the temperature factors as indicated in the footnote of table I. The refined occupancy factors indicate that
the split 0(2A)-sites are occupied upto 48% whereas the unsplit O(2B) sites have an occupancy of -40%. As illustrated in fig. 5 each two oxygen-ions occupying the split 0(2A)-site form a triangle with the corresponding 0( 2B)-ion which is shifted significantly, along the bi-axis, from the average mid-position of the 0(2A)-sites. Interestingly, the depletion of oxygen-content of Bil.6Pb~.4Ca,Sr2Cuz0, consequent upon vacuum-annealing shifts the position of 0(2B)-ion closer to the midpoint of the two O(ZA)-positions, as observed for the sample #2. In conclusion, the present neutron study of Bi1,6Pb0,4Ca,Sr2Cu20y indicates that Pb substitutes at Bi-sites, as expected, and is likely to be in 4 + state. This is in contrast to our recent observation that Pb does not occupy Bi-sites in (Bi, Pb)-2223. The strikingly disparate behaviour of the two related Pb-incorporated homologues (n = 2 and n = 3 ) of Bi-Ca-
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-CM-0
Fig. 5. A section of the bismuth
layer showing disordered
Sr-Cu-0 system possibly holds the key to an understanding of the role of Pb in stabilizing the higher T, (n= 3) 110 K-phase. The present comparative study of air-annealed and vacuum-annealed samples of Bi,.6Pb0.4CalSr,Cuz0, re-affirms our earlier tindings that the usual method of preparation of samples (i.e. air-annealing) in Bi-2122 system yields extra high oxygen-content which is far greater than what could be accounted by the known superstructure models based on excess oxygen in Bi-layer.
References [ I ] S.A. Sunshine et al., Phys. Rev. B38 [2]B.W. Statt, 2. Wang, M.J.G. Lee, Camargo, J.F. Major and J.W. Rutter, 251. [3] U. Endo, S. Koyama and T. Kawai, (1989) L190.
( 1988) 893. J.V. Yakhmi, P.C. De Physica C 156 (1988) Jpn. J. Appl. Phys. 28
oxygen sites involving
short contacts
as explained
273
in the text.
[4] P.V.P.S.S. Sastry, J.V. Yakhmi and R.M. Iyer, Physica C 161 (1989) 656. [5] A. Sequeira, J.V. Yakhmi, R.M. Iyer, H. Rajagopal and P.V.P.S.S. Sastry, PhysicaC 167 (1990) 291. [ 61 J.L. Tallon, R.G. Buckley, P.W. Gilbred and M.R. Presland, Physica C 158 (1989) 247. [ 71 D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 ( 198 I ) 149. [8] A. Sequeira, H. Rajagopal and J.V. Yakhmi, 157 (1989) 515. (91 A. Sequeira, H. Rajagopal, R. Nagarajan and C.N.R. Rao, 159 (1989) 87. [lO]O.Eibl,PhysicaC 168 (1990) 215. [ 111 H.W. Zandbergen, W.A. Groen, A. Smith and G. Van Tendeloo, Physica C 168 ( 1990) 426. [ 121 A.I. Veskrovnyi, M. Dlouha, Z. Jirak, S. Vratislav and E. Pollert, Physica C 166 ( 1990) 79. [ 131 Y. Gao, P. Lee, P. Coppens, M.A. Subramanian and A.W. Sleight, Science 241 ( 1988) 954. [ 141 I.D. Brown and D. Altermatt, Acta Crystallogr. B41 ( 1985) 244.
( 199 1) 267-273
A neutron diffraction study of the structure of Bil .6Pb0.4CalSr2Cu20y A. Sequeira ‘, H. Rajagopal ‘, P.V.P.S.S. Sastry b, J.V. Yakhmi b and R.M. Iyer b Solid State Physics Division ‘, and Chemistry Division b, Bhabha Atomic Research Centre. Trombay. Bombay 400085, India Received
4 December
1990
Neutron profile refinement analysis conducted on Bi,.6Pb0.4Ca,Sr2Cu20y (Space Group Amaa) points to Pb-ions substituting at Bi-sites as Pb4+. A comparative study of air-annealed and vacuum-annealed samples of Bi,.,Pbo4Ca,Sr,Cu20, indicates a slight reduction in the cell parameters upon annealing in vacuum in line with the observation of a reduction in oxygen-content per formula unit (y) from 8.72 for the air-annealed sample to 8.39 for the vacuum-annealed one. We propose that the rather high value of y( = 8.72) for the air-annealed sample is due to the formation of dimerised oxygen-species in the bismuth layer. The significant tilting of Cu-0, pyramids observed for air-annealed Bi,,,Pba4Ca,SrzCuz0. is nearly eliminated upon vacuum-annealing treatment which is suggestive of a reduction in the hole-density as confirmed by a marginal drop in T,(R=O) from 93 to 89 K after vacuum-annealing treatment.
The substitution of Pb in Bi-Ca-Sr-Cu-0 system, and its role in stabilizing the 2223-phase (n = 3 ), with a superconducting transition temperature ( r,) of 110 K, has been the subject of a large number of publications recently [ l-41. Neutron profile refinement study of single-phase Bi,.,Pbo.,Ca2Sr,Cu30,,, by us has revealed that contrary to the general belief, Bidouble layers were devoid of Pb-ions [ 5 1. Detailed analysis of the structural features, in fact, suggested that Pb-ions substitute at Ca-sites, as Pb4+ in (Bi, Pb),Ca&,CuxO,. Incorporation of Pb is known to raise the T, of Bi,CaSr,Cu20s (n=2) phase, too, upto 93 K [ 6 1. It is, therefore, of interest to examine how the incorporation of Pb influences the Bi-2122 crystal structure, and, in particular, whether Pb actually substitutes at Bi-sites, as intended. In this paper, we report the findings of our structural studies on air-annealed and vacuum-annealed samples of Bi,.,Pb,,4Ca,Sr2Cuz0, using powder neutron diffraction measurements. The sample Bi, .6Pb0.4Cal Sr,CuzO, was prepared by the reaction of a precursor matrix CaSrzCuzOS with stoichiometric amounts of high purity B&O3 and Pb ( AC)*. The matrix was first prepared by the solid state reaction of pure CaCO,, SrC03 and CuO at 1235 K in air. Appropriate amounts of the matrix powder, Biz03 and Pb-acetate were mixed well and 0921-4534/91/$03.50
0 1991 - Elsevier Science Publishers
heated at 1180 K in air for 2-5 min till the reaction was complete which was indicated by the turning of the mass completely black. This product was cooled, powdered thoroughly, pelletised into 12 mm diax 12mm thick pellets and sintered in air at 1120 K for 24 h with several intermediate grindings and furnace-cooled to room-temperature. This air-annealed sample is hereafter referred to as samples # 1. The duration and temperature of sintering were carefully controlled because even a slightly higher sintering temperature ( >, 5 K) or longer duration of sintering was,observed to promote the growth of the Bi-2223 (n = 3 ) phase, in the presence of Pb. On the other hand, sintering at temperatures sufficiently lower than 1120 K tended to stabilize the low T,Bi2021 (n= 1) phase. In order to reduce the oxygen-content of the sample, one of the #l sample-pellets was heated under running vacuum ( low3 Torr) at 975 K for 3 h and slow-cooled to room-temperature at 5 K/min under vacuum. This vacuum annealed sample is referred to as sample #2. X-ray diffraction patterns recorded, using Cu Ku radiation, for both samples #l (fig. 1) and #2 confirmed that the sample was very nearly single-phase Bi-2 122 ( CE 30 A) with no evidence of impurity peaks due to Bi-2223 phase, Ca,PbO, (28s 17.8 ’ ) or any other unreacted oxides. A slight
B.V. (North-Holland)
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-Cu-0
Fig. I. Cu Ku X-ray diffraction
pattern
for sample # 1.
1 301 0 TEMPERATURE
Fig. 2. Normalised electrical resistance perature for samples #l and #2.
(K)
plots as a function
oftem-
impurity content due to the BizSr,CuOb (28= 2 1.9” ) cannot, however, be ruled out because of the presence of a very weak (002 ) peak due to this phase at 7.2”. The electrical resistance measurements using a standard DC four-probe method showed a singlesharp drop leading to zero-resistance at 93 K and 89 K for samples #l nd #2, respectively (fig. 2).
Neutron diffraction patterns were recorded at room temperature using the T-101 1 diffractometer at the 100 MW Dhruva reactor at Trombay. The samples were in the form of a cylindrical pellet ( z 6 g in weight, 12 mm diameter and 10 mm height) and the neutron wavelength was 1.2120 A. The instrument collimations were 0.7 o-0.5 ”-0.7 Ofrom the in-pile to the detector end. The patterns were analysed by the Rietveld profile refinement method using a modified version of the program DBW 3.2 [ 71. All possible positional, thermal (B) and occupancy (N) parameters, excepting the occupancy of Cu, 0( 1, 1) and 0 ( 1, 2), were varied in addition to the cell parameters, half width parameters, background parameters, zero-angle and the scale factor. Due to the strong correlations between the thermal and the occupancy parameters, they were varied in separate cycles. A Gaussian peak shape function was assumed. The observed and the calculated neutron diffraction patterns, as well as the difference patterns, for the samples #I and #2 are shown in fig. 3. The experimental resolution of neutron diffraction spectra recorded by us for a variety of high-T, cuprate superconductors has been found to be adequate to reproduce the structural parameters well within the expected standard deviations [ 5,8]. No perceptible degradation was noticed in the neutron diffraction patterns of the present samples over a period of one month indicating that the Bi,.6Pb0.4Ca,SrZCu20y sample is stable. The refined cell parameters (space group Amaa ), atomic coordinates, thermal parameters, occupancy parameters and the relevant R-values for the samples #l and #2 are given in table I. A look at this table reveals that the occupancies of the cation sites remain more or less unchanged, within the experimental error, upon annealing the sample in low P( 0,) (i.e. vacuum-annealing). The bond distances obtained for samples # 1 and #2 are given in table II. The crystal structure of these two samples is shown in fig. 4(a) and (b), respectively. An analysis of the derived structural parameters reveals the following interesting features. (i) The cell parameters of sample # 1 are in good agreement with the values reported in literature for Bi-2 122 compounds [9-l 11. However, there appears a small but significant reduction in the cell volume for sample #2, arising from a contraction of
A. Sequeira etal./A neutron diffraction
study of the structure of Bl-Pb-Ca-Sr-Cu-0
Fig. 3. The observed (dots) and calculated (curve) neutron diffraction patterns for air-annealed (#I ) and vacuum-annealed ples of Bi,.6Pb0.4Ca,SrZCu20g taken at room temperature. The difference pattern for each sample is shown at the bottom.
N 0.5% in a- and b-, and = 0.25% in c-parameter, perhaps due to the depletion of oxygen during the vacuum annealing treatment of sample #2. (ii) For both the samples #l and #2, the occupancy factors of the Bi-sites are close to 1.02, the value expected under the assumption of Pb-ions substituting at 20% of Bi-sites. This points to the occupancy of Pb-ions at Bi-sites in Bi,.bPb,&a,SrzCu20Ysince Pb is the only cation capable of scattering stronger than Bi, albeit marginally. This, however, is in contrast to our findings on Bi,.,Pbo_3Ca&-zCu30, where Pb-ions where shown to substitute at Ca- rather than at Bi-sites [ 5 1. The occupancy of Pb-ions at Bi-sites in Bi,.,Pbo,4Ca,Sr,Cu,0,, for both #l and #2 samples, is further substantiated by a much reduced value for the apical Bi-0( 3) bond-length, viz. 2.06 8, for #l and 1.97 A for #2, compared with values of 2.16 A
269
(#2) sam-
and 3.15 8, reported for Pb-free Bi2CalSrZCu208+6 [ 81 and Bi,.,Pbo.-JZazSrzCu30, [ 51 with Pb-substituted at Ca-sites, respectively. The short Bi-O(3) bond-length and the high estimated bond-strength for Bi-sites (viz. 3.7 and 3.1 for samples #l and #2, respectively) together, point to the substitution of Pb in 4+ state at Bi-sites. (iii) A 68% occupancy factor for O(2)-oxygens for sample #l is indicative of an extra high oxygencontent in the bismuth double-layer corresponding to - 1.36 oxygens per Bi-site, whereas a lower O( 2)occupancy ( - 60%) for sample #2 points to a reduction in the excess oxygen-content in Bi-layer consequent upon the vacuum annealing. The occupancy for all other oxygen-sites, however, is unaffected and remains full as in #l. The total oxygen-content per formula unit works out to be 8.72 and 8.39 for samples #l and #2, respectively.
Table I Positional (x, Y, z) and thermal (B) parameters for the two samples (#1 and #2) of Bi,,sPbo,Ca,Sr,CuzO, obtained from Rietveld refinement of neutron diffraction data. The neutron scattering amplitudes used for Bi, Sr, Ca, Cu and 0 are 8.53, 7.03, 4.90, 7.70 and 5.80 fm, respectively. Numbers in parentheses are esd’s referred to the last digit. ( 100 x N) represents percentage site occupancy. Parameter
Sample # 1
Sample #2
Space group
Amaa 5.347(3) 5.416(4) 30.67(l)
Amaa 5.324(4) 5.391(9) 30.60( 1)
a(A) b(A) c(A) Bi-site (81), (0, Y, z) Y z B N (assuming
b,,)
0.249(2) 0.4449(3) 3.5(2) 1.030(2)
0.240( 3) 0.4482(2) 1.9(2) 1.014( 10)
0.746(3) 0.3604(3) 0.8(3) 0.906( 14)
0.721(3) 0.3613(3) 1.2(3) 0.930( 16)
0.2 1.276( 18)
0.8(3) 1.312( 18)
0.287(2) 0.3003( 3) 2.3(3) 1.0
0.257(4) 0.3030(3) 1.9(2) 1.0
0.3140(5) 1.3(2) 1.0
0.3113(5) 2.1(2) I.0
0.2916(5) 1.3(2) 1.0
0.2944(5) 2.1(2) 1.0
Sr-site (81), (0, y, z) Y z B N (assuming,
b,,)
Ca-site (4f), (0, f, a) B N (assuming b,,) Cu-site (81), (0, Y, z) Y .? B N O(1, z B N
1)-site (8g),
0(1,2kslte
(a,O,z)
(8g), (i. f, z)
Z
B N O(2)-site*(16m),
(x.y,z)
X
Y z B’ N O(3)-site
0.144(5) 0.728(4) 0.4399( 5) 14.1(10) 0.680( 16)
0.090( 8) 0.645(4) 0.4456(6) 10.4(8) 0.602(11)
0.356(2) 0.3803(5) 2.2(4) 0.998(21)
0.251(6) 0.3838(4) 4.1(4) 0.990(22)
2.40 4.41 5.68 1.25
1.50 2.78 3.49 0.81
(8Q), (0, y, z)
Y ; N R-values R.,, (%) R, (%) R, (%) RB (%) Total oxygen/formula
8.72
-
8.39
* Refinements involving disordering of 0( 2) oxygens between general sites ( 16 m) and the average special sites (8 1) led to substantral reduction in the B-values but only marginal improvement in R-values. Sample #l, for instance, yielded values of R,=4.37% (R,,= 5.65%) with occupancy factors of 0.48 and 0.40, respectively at sites (0.170, 0.762, 0.4367) and (0, 0.649, 0.4458), and B= 11.7, while sample #2 yielded R,=2.72% (R,=3.43%) with occupancy factors of 0.45 and 0.30 at sites (0.106, 0.636, 0.4410) and (0,0.662, 0.4607) and B=7.5.
A. Sequeira
et al. 1 A neutron diffraction
study of the structure of Bi-Pb-Ca-Sr-Cu-0
211
Table II Interatomic distances (A) and site valences (V) in (Bi,.6Pb0.4) Ca,Sr2Cu20Y samples; V= 1 W,N,exp[ (rO-r,)/O.37] with parameters [ 141. r,=2.09 (Bi), 1.967 (Ca), 2.1 18 (Sr) and 1.679 (Cu). The numbers in parentheses are esd’s referred to the last digit. Bond Bi/Pb-O(3) -O(2)
a)
-O(2) -O(2) -O(2) -O(2) V(v.u.)
(usingr,forBi)
Sr-O(3) -O(l, -O(2) -O(3) -O(l, -O(3)
1)
(7-X) 2)
(2X) (2X) (1X)
(using r0 for Sr)
V (v.u.) Ca-0(
(1X) (2X)
1,2)
-O(l, V(v.u.) cu-O( -O(l, -O(3)
1)
(4X) (4X)
(usingr,forCa) 1,2) 1)
(2x1 (2X)
(1x1
V(v.u.) ‘) Occupancy
Sample # 1
Sample #2
2.07(2) 1.91(3) 2.71(3) 2.93(3) 3.45(3) 3.62(2)
1.97( 1) 2.27(4) 2.24(3) 3.24(3) 3.20(4) 3.34(2)
3.72
3.07
2.20(2) 2.39(2) 2.56(2) 2.80( 1) 2.83( 1) 3.36(2)
2.62(3) 2.53( 1) 2.65(2) 2.75(l) 2.72(2) 2.94( 1)
2.82
2.10
2.29( 1) 2.73( 1)
2.33( 1) 2.67(I)
2.17
2.10
1.79(I) 2.09( 1) 2.48(2)
1.89(l) 1.94(l) 2.47(2)
2.27
2.25
of 0( 2) is 68% in sample #l and 60% in sample #2
(iv) The occupancy factors for sample # 1 suggest the substitution of Ca-ions at nearly one-third of the Sr-sites and the substitution of Sr-ions at roughly twothirds of the Ca-sites. On the other hand, for sample #2, Ca-ions appear to occupy not more than onefourth of the Sr-sites whereas a higher occupancy factor for Ca-sites necessitates the presence of Sr-ions at nearly three-fourths of the Ca-sites. This appears somewhat unlikely and perhaps indicates the presence of a small amount of Pb-ions ( 5 10%) at Casites in sample #2. This may also explain a slightly lower occupancy at Bi-sites in this sample. (v) A large distortion is seen in the coordination polyhedra of Sr-ions, arising mainly from the distortion of the in-plane Sr-0 bonds, for the air-annealed sample #l. This distortion, however, gets practically eliminated in the case of sample #2, consequent upon depletion of oxygen-content after vacuum-annealing treatment. A similar trend is noticed
for the distortion in coordination polyhedra around Ca-ions, too, though to a lesser extent. (vi ) For sample # 1, of the four planar Cu-0 bonds, the two Cu-0 ( 1,2 ) bonds are much shorter ( - 1.79 A) than the other two Cu-0( 1, 1) bonds ( -2.09 A). Further, the Cu-O5 pyramid is tilted in such a way that the oxygen-ion 0 ( 1, 1) moves closer to Srions, whereas 0( 1, 2) moves closer to Ca-ion. This indicates that 0( 1, 2)-ions are more negative than 0( 1, 1 ), which could be due to the occurrence of fewer electron-holes at 0 ( 1,2 )- than at 0 ( 1, 1 )-sites. This could also be the cause for the shortening of CuO( 1, 2) bonds. On the other hand, the nearly equal bond-lengths for the four planar Cu-0 bonds for sample #2 and the consequent reduction in the tilting of its Cu-O5 pyramid is perhaps suggestive of an expected reduction in the hole-density upon vacuum annealing treatment. This is also supported by a marginal drop in the observed T,(R = 0).
272
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-CM-0
-b -b
Fig. 4. Crystal structure for samples #I and #2.
(vii) As discussed above, the oxygen-content per formula for sample #2 is 8.39, which is reasonably close to what could be accommodated in the Bi-double layer within the framework of the generally reported superstructure models [ 1O-l 3 1. However, the corresponding value of 8.72 for #I appears to be too large, and should be due to the formation of dimerised oxygen-species, as proposed earlier [ 8,9 1. The rather high temperature factors for 0( 2)- ion in both the samples is suggestive, in our opinion, of positional disorder, i.e. the 0( 2)-oxygens are disordered between the general positions, 0 (2A), and the special position, 0 (2B ), located on the mirror-plane, as shown in fig. 5 which depicts a single Bi-0 layer of sample # 1. Refinements in terms of disordering of O(2)-oxygens indeed lead to a significant drop of the temperature factors as indicated in the footnote of table I. The refined occupancy factors indicate that
the split 0(2A)-sites are occupied upto 48% whereas the unsplit O(2B) sites have an occupancy of -40%. As illustrated in fig. 5 each two oxygen-ions occupying the split 0(2A)-site form a triangle with the corresponding 0( 2B)-ion which is shifted significantly, along the bi-axis, from the average mid-position of the 0(2A)-sites. Interestingly, the depletion of oxygen-content of Bil.6Pb~.4Ca,Sr2Cuz0, consequent upon vacuum-annealing shifts the position of 0(2B)-ion closer to the midpoint of the two O(ZA)-positions, as observed for the sample #2. In conclusion, the present neutron study of Bi1,6Pb0,4Ca,Sr2Cu20y indicates that Pb substitutes at Bi-sites, as expected, and is likely to be in 4 + state. This is in contrast to our recent observation that Pb does not occupy Bi-sites in (Bi, Pb)-2223. The strikingly disparate behaviour of the two related Pb-incorporated homologues (n = 2 and n = 3 ) of Bi-Ca-
A. Sequeira et al. /A neutron diffraction study of the structure of Bi-Pb-Ca-Sr-CM-0
Fig. 5. A section of the bismuth
layer showing disordered
Sr-Cu-0 system possibly holds the key to an understanding of the role of Pb in stabilizing the higher T, (n= 3) 110 K-phase. The present comparative study of air-annealed and vacuum-annealed samples of Bi,.6Pb0.4CalSr,Cuz0, re-affirms our earlier tindings that the usual method of preparation of samples (i.e. air-annealing) in Bi-2122 system yields extra high oxygen-content which is far greater than what could be accounted by the known superstructure models based on excess oxygen in Bi-layer.
References [ I ] S.A. Sunshine et al., Phys. Rev. B38 [2]B.W. Statt, 2. Wang, M.J.G. Lee, Camargo, J.F. Major and J.W. Rutter, 251. [3] U. Endo, S. Koyama and T. Kawai, (1989) L190.
( 1988) 893. J.V. Yakhmi, P.C. De Physica C 156 (1988) Jpn. J. Appl. Phys. 28
oxygen sites involving
short contacts
as explained
273
in the text.
[4] P.V.P.S.S. Sastry, J.V. Yakhmi and R.M. Iyer, Physica C 161 (1989) 656. [5] A. Sequeira, J.V. Yakhmi, R.M. Iyer, H. Rajagopal and P.V.P.S.S. Sastry, PhysicaC 167 (1990) 291. [ 61 J.L. Tallon, R.G. Buckley, P.W. Gilbred and M.R. Presland, Physica C 158 (1989) 247. [ 71 D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 ( 198 I ) 149. [8] A. Sequeira, H. Rajagopal and J.V. Yakhmi, 157 (1989) 515. (91 A. Sequeira, H. Rajagopal, R. Nagarajan and C.N.R. Rao, 159 (1989) 87. [lO]O.Eibl,PhysicaC 168 (1990) 215. [ 111 H.W. Zandbergen, W.A. Groen, A. Smith and G. Van Tendeloo, Physica C 168 ( 1990) 426. [ 121 A.I. Veskrovnyi, M. Dlouha, Z. Jirak, S. Vratislav and E. Pollert, Physica C 166 ( 1990) 79. [ 131 Y. Gao, P. Lee, P. Coppens, M.A. Subramanian and A.W. Sleight, Science 241 ( 1988) 954. [ 141 I.D. Brown and D. Altermatt, Acta Crystallogr. B41 ( 1985) 244.