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Normal state resistivity of Bi2Sr2CaCu2O8 + x
Physica B 223&224 (1996) 574 576
ELSEVIER
Neutron diffraction study of superconducting RESr2Cu3-xMxOz (M = Re, W) A. Sequiera a, H. Rajagopal a, M. Murugesan b, M.S.R. Ra&, L.C. Gupta ¢'*, R. Pint&, M. Sharon b, R. Vijayaraghavan ~ Bhabha Atomic Research Center, Bombay 400 085, India bDepartment o f Chemistry, Indian Institute of Technology, Bombay 400 076, India Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India
Abstract Superconductivity has recently been reported in RESr2Cu2.85Reo.150~ (RE = Y, Tb, Dy, Ho, Er and Tm) and RESr2Cu2.aoWo.2oO~ (RE = Y, Dy, Ho and Er) with T~ ranging from 22 to 41 K. Room temperature powder neutron diffraction studies have been carried out for obtaining structural parameters and their relation with superconductivity in these materials. Reitveld refinements were carried out adopting the space groups P4/mmm (tetragonal). Occupancy of the rare earths is less than 1 indicating vacancy at the rare earth sites. Cu(2)-O distance in RESr2Cu3-xMxOz (M = Re and W) is smaller than that of REBa2Cu3OT-~ which is consistent with the smaller ionic radius of Sr 2 +. Our studies show that the M-cations (Re or W) occupy the Cu(1) chain site.
1. Introduction Despite the chemical similarity between the alkaline earth Ba- and Sr-ions, it has been well-established that the solid solubility limit of Sr in YBa2-xSr~Cu3OT-~ system is 0~
* Corresponding author.
systematically studied [4]. In this paper, we report and discuss the results of room temperature powder neutron diffraction.
2. Experimental Polycrystalline single phase ceramic samples were prepared from the individual oxides and carbonates (/> 99.9% purity) namely, Y203, Ho203, SrCO3, CuO, ReO3 and WOa. The preparation procedure has been described elsewhere [4]. Phase purity was checked by X-ray diffraction. Superconducting transition temperature Tc was measured by DC-electrical four probe method (see Table 1) and AC-magnetic susceptibility (X). Powder neutron diffractograms were recorded at room temperature at the Dhruva reactor. The neutron wavelength was 1.216 A and the instrument collimations were 0.7-0.5-0.5 (deg) from the in-pile to the detector end.
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved plt ~ o o g ! _ a , ~ 9 ~ o g ~ o o ! T R _ o
575
A. Sequiera et al./Physica B 223&224 (1996) 574-576
Table 1 Tc values and refined structural parameters of RESr2Cu2.ss Mo.15Oz (RE = Y and Ho; M = Re) and RESr2Cu2.soMo.2oO~ (RE=Y;M=W) Sample
Y, Re
Ho, Re
Y, W
T¢
41
41
34
Space group a c
P4/mmm 3.8001(7) 11.483(4)
P4/mmm 3.8017(8) 11.492(4)
P4/mmm 3.8016(9) 11.500(5)
Atomic parameters
Cu(1)/M(la)
(o,o,o) B
N
0.5 0.772(5)/ 0.136(5)
0.5 0.751(6)/ 0.133(6)
0.5 0.740(10)/ 0.178(10)
0.3533(4) 0.3(1) 1.00
0.3543(6) 0.8(1) 1.00
0.3564(6) 0.5(1) 1.00
0.1852(5) 2.2(1) 0.97(1)
0.1840(6) 1.9(2) 0.95(1)
0.1854(7) 1.3(2) 0.97(1)
0.5(2) 0.95(1)
0.6(2) 0.95(1)
1.2(3) 0.93(1)
2.7(5) 0.44(1)
3.4(7) 0.46(1)
5.2(8) 0.46(2)
0.3596 (51) 2.7(5) 0.12(1)
0.3704 (74) 3.4(7) 0.13(1)
0.3867 (75) 5.2(8) 0.13(1)
0.1612 (7) 1.4(3) 1.00
0.1647 (10) 2.6(4) 1.00
0.1639 (11) 2.6(6) 0.95(3)
0.3763(3) 1.3(1)
0.3759(4) 1.7(1)
0.3744(4) 0.9(1) 1.00
Cu(2) (2g) (0, 0, z) z
B
N Sr (2h) (3, 3, z) 2
B
N RE (ld) (3, 3, 3) B
N O(1) (2f) (0, 3, 0) B
N O(1A) (4j) (x, x, 0) X
B N 0(2) (2g) (0, 0, z) z
B
N 0(3) (4i) (3, 0, z) z
B
N Oxygen content
1.00
1.00
7.36
7.44
7.34
R-factors (Rs)
1.65%
1.94%
2.24%
Table 1 (Continued). Sample
Y, Re
Ho, Re
Y, W
Tc
41
41
34
Space group a c
P4/mmm 3.8001(7) 11.483(4)
P4/mmm 3.8017(8) 11.492(4)
P4/mmm 3.8016(9) 11.500(5)
2.372(3) 2.852(3) 2.256(8) 2.701(1) 2.903(4) 1.9001(3) 1.850(8) 1.93(2) 2.206(9) 1.918(1)
2.376(3) 2.843(5) 2.226(11) 2.697(1) 2.911(6) 1.9009(4) 1.893(11) 1.99(3) 2.179(13) 1.917(1)
2.387(3) 2.856(6) 2.217(11) 2.700(l) 2.887(7) 1.9008(5) 1.885(13) 2.08(3) 2.214(14) 1.912(1)
Bond distances
RE-O(3) St-O(1) Sr-O(1A) Sr-O(2) St-O(3) Cu(1)-O(1) Cu(1)/M-O(2) M-O(1A) Cu(2)-O(2) Cu(2)-O(3)
The patterns were analyzed by Reitveld refinement method using a modified version of the program D B W 3.2 [5]. The positional, thermal (B) and occupancy (N) parameters were varied in addition to a scale factor, zero angle and half width and background parameters. Gaussian peak shape functions were assumed. The occupancy and the thermal parameters were varied in separate cycles.
3. Results and discussion
Powder neutron diffraction data refinements were based on tetragonal (123) structure (space group P4/mmrn). Initial refinements carried out by anchoring the Sr-site occupancy at 100%, indicated full occupancies at the Cu(2) and O(3)-sites in Cu-O2 planes, but significantly lower occupancy at the Cu(1) sites in all the sampies, regardless of the scattering amplitudes of the metal substituent (M) being higher or lower than that of Cu. This was taken as an indication of the presence of vacancies at Cu(1)-sites, in addition to the M (Re or W) substituent. Consequently, the site occupancies of the Cu(1)-sites were refined by constraining the Cu/M ratio to the nominal value. In addition, the B-values of the O(1) oxygens, obtained after refining were anomalously large, indicating disorder. Subsequently this disorder was traced to the presence of significant fraction of oxygen atoms sitting at the new O(1A)-sites (x, x, 0) as shown in Fig. 1. Refinements incorporating O(1A)-sites yielded
576
A. Sequiera et al./ Physica B 223&224 (1996) 574 576
The occupancy of O(1A) site depends upon which of the Cu(1) sites is occupied by the M-atom. Due to the weighted average of all these positional disorders neutron diffraction studies have yielded a tetragonal structural symmetry in all these systems. The observed Cu(2)-O bond distances in RESrzCua-xMxO= are considerably smaller (observed also by Slater and Greaves I-7]) as compared to those in REBa2Cu3Ov-6 [8] and some of the values are listed in Table 1. The lower Tc values in RESr2Cu3-xMxOz could be attributed to the observed copper [Cu(II)-O] valence which is slightly higher than that normally observed in the 90 K superconducting YBa2Cu3OT-6 system.
(
...~o~ ~ 4. Conclusion
X--4_0ttAI ,-<"
\ 03"'~.3'-J.. ~"~ /"t'
Fig. 1. Structure of RESr2Cu2.ssReo.~50. obtained by powder neutron diffraction. It may be noted that the oxygens occupy new positions shown as O(1A). This is the first example in the family of 1-2-3 materials where oxygens occupy positions marked as O(1A).
very good fits with low R-values. The final parameters are listed in Table 1. We would like to stress here that these are the first high-To materials (of 1-2-3 type) wherein oxygen occupies the (x, x, 0) sites. The combined occupancy of Cu and M ions at the Cu(1)-sites is ~ 88%, indicating about 12% vacancies at Cu(1)-sites in all the 3 samples. Further, as the occupancy obtained for O(1A)-sites is nearly the same as that for metal atoms M (Re, W) indicating their occupancies are strongly correlated. Thus M-atoms have an octahedron of oxygen-atom formed with 40(1A)-oxygens and two 20(4)-oxygens. It is to be emphasized that these Ooctahedra of the M-atoms are rotated by 45 ° in the ab-plane. It is shown [3,6] that in YSr2Cu3-xM~O= (M = Fe, A1, etc.) compounds, the substituted metal atoms at Cu(1)-sites prefer an octahedral coordination.
In summary, novel structural features have been observed in the Sr-analog, RESr2Cu3-~MxO~ (where M = Re/W) by neutron diffraction studies. Although, 1-2-3 structure in RESr2Cu3_~M~O= could be stabilized and superconductivity induced by M (Re or W) atom doping at Cu site, due to the observed deviations in bond distances, lower Tc values are encountered in RESr2Cu3-xMxO= compounds as compared to that in REBa2Cu307 6. The metal atoms Re(W) occupy the Cu(1)-sites with octahedral coordination. The octahedra around M-atoms are rotated by 45 ° with respect to those around Cu(I)-sites. The presence of M-atom induces partial vacancy at the Cu(1)-sites. Excess oxygen atoms occupy O(1A)-sites. These are the only materials where these sites are reported to have been occupied.
Acknowledgements One of the authors (MM) would like to thank UGC, New Delhi for the financial support.
References [1] B.W. Veal et al., Appl. Phys. Lett. 51 (1987) 279. [-2] B. Okai, Jpn. J. Appl. Phys. 29 (1990) L2180. [3] T. Den and T. Kobayashi, Physica C 196 (1992) 141. [4] M. Murugesan et al., Submitted for publication. [5] D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 (1981) 149. [-6] S.A. Sunshine et al., Chem. Mater. 1 (1989) 331. [7] P.R. Slater and C. Greaves, Physica C 180 (1991) 299. [8] W.I.F. David et al., Nature 327 (1987) 310.
ELSEVIER
Neutron diffraction study of superconducting RESr2Cu3-xMxOz (M = Re, W) A. Sequiera a, H. Rajagopal a, M. Murugesan b, M.S.R. Ra&, L.C. Gupta ¢'*, R. Pint&, M. Sharon b, R. Vijayaraghavan ~ Bhabha Atomic Research Center, Bombay 400 085, India bDepartment o f Chemistry, Indian Institute of Technology, Bombay 400 076, India Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India
Abstract Superconductivity has recently been reported in RESr2Cu2.85Reo.150~ (RE = Y, Tb, Dy, Ho, Er and Tm) and RESr2Cu2.aoWo.2oO~ (RE = Y, Dy, Ho and Er) with T~ ranging from 22 to 41 K. Room temperature powder neutron diffraction studies have been carried out for obtaining structural parameters and their relation with superconductivity in these materials. Reitveld refinements were carried out adopting the space groups P4/mmm (tetragonal). Occupancy of the rare earths is less than 1 indicating vacancy at the rare earth sites. Cu(2)-O distance in RESr2Cu3-xMxOz (M = Re and W) is smaller than that of REBa2Cu3OT-~ which is consistent with the smaller ionic radius of Sr 2 +. Our studies show that the M-cations (Re or W) occupy the Cu(1) chain site.
1. Introduction Despite the chemical similarity between the alkaline earth Ba- and Sr-ions, it has been well-established that the solid solubility limit of Sr in YBa2-xSr~Cu3OT-~ system is 0~
* Corresponding author.
systematically studied [4]. In this paper, we report and discuss the results of room temperature powder neutron diffraction.
2. Experimental Polycrystalline single phase ceramic samples were prepared from the individual oxides and carbonates (/> 99.9% purity) namely, Y203, Ho203, SrCO3, CuO, ReO3 and WOa. The preparation procedure has been described elsewhere [4]. Phase purity was checked by X-ray diffraction. Superconducting transition temperature Tc was measured by DC-electrical four probe method (see Table 1) and AC-magnetic susceptibility (X). Powder neutron diffractograms were recorded at room temperature at the Dhruva reactor. The neutron wavelength was 1.216 A and the instrument collimations were 0.7-0.5-0.5 (deg) from the in-pile to the detector end.
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved plt ~ o o g ! _ a , ~ 9 ~ o g ~ o o ! T R _ o
575
A. Sequiera et al./Physica B 223&224 (1996) 574-576
Table 1 Tc values and refined structural parameters of RESr2Cu2.ss Mo.15Oz (RE = Y and Ho; M = Re) and RESr2Cu2.soMo.2oO~ (RE=Y;M=W) Sample
Y, Re
Ho, Re
Y, W
T¢
41
41
34
Space group a c
P4/mmm 3.8001(7) 11.483(4)
P4/mmm 3.8017(8) 11.492(4)
P4/mmm 3.8016(9) 11.500(5)
Atomic parameters
Cu(1)/M(la)
(o,o,o) B
N
0.5 0.772(5)/ 0.136(5)
0.5 0.751(6)/ 0.133(6)
0.5 0.740(10)/ 0.178(10)
0.3533(4) 0.3(1) 1.00
0.3543(6) 0.8(1) 1.00
0.3564(6) 0.5(1) 1.00
0.1852(5) 2.2(1) 0.97(1)
0.1840(6) 1.9(2) 0.95(1)
0.1854(7) 1.3(2) 0.97(1)
0.5(2) 0.95(1)
0.6(2) 0.95(1)
1.2(3) 0.93(1)
2.7(5) 0.44(1)
3.4(7) 0.46(1)
5.2(8) 0.46(2)
0.3596 (51) 2.7(5) 0.12(1)
0.3704 (74) 3.4(7) 0.13(1)
0.3867 (75) 5.2(8) 0.13(1)
0.1612 (7) 1.4(3) 1.00
0.1647 (10) 2.6(4) 1.00
0.1639 (11) 2.6(6) 0.95(3)
0.3763(3) 1.3(1)
0.3759(4) 1.7(1)
0.3744(4) 0.9(1) 1.00
Cu(2) (2g) (0, 0, z) z
B
N Sr (2h) (3, 3, z) 2
B
N RE (ld) (3, 3, 3) B
N O(1) (2f) (0, 3, 0) B
N O(1A) (4j) (x, x, 0) X
B N 0(2) (2g) (0, 0, z) z
B
N 0(3) (4i) (3, 0, z) z
B
N Oxygen content
1.00
1.00
7.36
7.44
7.34
R-factors (Rs)
1.65%
1.94%
2.24%
Table 1 (Continued). Sample
Y, Re
Ho, Re
Y, W
Tc
41
41
34
Space group a c
P4/mmm 3.8001(7) 11.483(4)
P4/mmm 3.8017(8) 11.492(4)
P4/mmm 3.8016(9) 11.500(5)
2.372(3) 2.852(3) 2.256(8) 2.701(1) 2.903(4) 1.9001(3) 1.850(8) 1.93(2) 2.206(9) 1.918(1)
2.376(3) 2.843(5) 2.226(11) 2.697(1) 2.911(6) 1.9009(4) 1.893(11) 1.99(3) 2.179(13) 1.917(1)
2.387(3) 2.856(6) 2.217(11) 2.700(l) 2.887(7) 1.9008(5) 1.885(13) 2.08(3) 2.214(14) 1.912(1)
Bond distances
RE-O(3) St-O(1) Sr-O(1A) Sr-O(2) St-O(3) Cu(1)-O(1) Cu(1)/M-O(2) M-O(1A) Cu(2)-O(2) Cu(2)-O(3)
The patterns were analyzed by Reitveld refinement method using a modified version of the program D B W 3.2 [5]. The positional, thermal (B) and occupancy (N) parameters were varied in addition to a scale factor, zero angle and half width and background parameters. Gaussian peak shape functions were assumed. The occupancy and the thermal parameters were varied in separate cycles.
3. Results and discussion
Powder neutron diffraction data refinements were based on tetragonal (123) structure (space group P4/mmrn). Initial refinements carried out by anchoring the Sr-site occupancy at 100%, indicated full occupancies at the Cu(2) and O(3)-sites in Cu-O2 planes, but significantly lower occupancy at the Cu(1) sites in all the sampies, regardless of the scattering amplitudes of the metal substituent (M) being higher or lower than that of Cu. This was taken as an indication of the presence of vacancies at Cu(1)-sites, in addition to the M (Re or W) substituent. Consequently, the site occupancies of the Cu(1)-sites were refined by constraining the Cu/M ratio to the nominal value. In addition, the B-values of the O(1) oxygens, obtained after refining were anomalously large, indicating disorder. Subsequently this disorder was traced to the presence of significant fraction of oxygen atoms sitting at the new O(1A)-sites (x, x, 0) as shown in Fig. 1. Refinements incorporating O(1A)-sites yielded
576
A. Sequiera et al./ Physica B 223&224 (1996) 574 576
The occupancy of O(1A) site depends upon which of the Cu(1) sites is occupied by the M-atom. Due to the weighted average of all these positional disorders neutron diffraction studies have yielded a tetragonal structural symmetry in all these systems. The observed Cu(2)-O bond distances in RESrzCua-xMxO= are considerably smaller (observed also by Slater and Greaves I-7]) as compared to those in REBa2Cu3Ov-6 [8] and some of the values are listed in Table 1. The lower Tc values in RESr2Cu3-xMxOz could be attributed to the observed copper [Cu(II)-O] valence which is slightly higher than that normally observed in the 90 K superconducting YBa2Cu3OT-6 system.
(
...~o~ ~ 4. Conclusion
X--4_0ttAI ,-<"
\ 03"'~.3'-J.. ~"~ /"t'
Fig. 1. Structure of RESr2Cu2.ssReo.~50. obtained by powder neutron diffraction. It may be noted that the oxygens occupy new positions shown as O(1A). This is the first example in the family of 1-2-3 materials where oxygens occupy positions marked as O(1A).
very good fits with low R-values. The final parameters are listed in Table 1. We would like to stress here that these are the first high-To materials (of 1-2-3 type) wherein oxygen occupies the (x, x, 0) sites. The combined occupancy of Cu and M ions at the Cu(1)-sites is ~ 88%, indicating about 12% vacancies at Cu(1)-sites in all the 3 samples. Further, as the occupancy obtained for O(1A)-sites is nearly the same as that for metal atoms M (Re, W) indicating their occupancies are strongly correlated. Thus M-atoms have an octahedron of oxygen-atom formed with 40(1A)-oxygens and two 20(4)-oxygens. It is to be emphasized that these Ooctahedra of the M-atoms are rotated by 45 ° in the ab-plane. It is shown [3,6] that in YSr2Cu3-xM~O= (M = Fe, A1, etc.) compounds, the substituted metal atoms at Cu(1)-sites prefer an octahedral coordination.
In summary, novel structural features have been observed in the Sr-analog, RESr2Cu3-~MxO~ (where M = Re/W) by neutron diffraction studies. Although, 1-2-3 structure in RESr2Cu3_~M~O= could be stabilized and superconductivity induced by M (Re or W) atom doping at Cu site, due to the observed deviations in bond distances, lower Tc values are encountered in RESr2Cu3-xMxO= compounds as compared to that in REBa2Cu307 6. The metal atoms Re(W) occupy the Cu(1)-sites with octahedral coordination. The octahedra around M-atoms are rotated by 45 ° with respect to those around Cu(I)-sites. The presence of M-atom induces partial vacancy at the Cu(1)-sites. Excess oxygen atoms occupy O(1A)-sites. These are the only materials where these sites are reported to have been occupied.
Acknowledgements One of the authors (MM) would like to thank UGC, New Delhi for the financial support.
References [1] B.W. Veal et al., Appl. Phys. Lett. 51 (1987) 279. [-2] B. Okai, Jpn. J. Appl. Phys. 29 (1990) L2180. [3] T. Den and T. Kobayashi, Physica C 196 (1992) 141. [4] M. Murugesan et al., Submitted for publication. [5] D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 (1981) 149. [-6] S.A. Sunshine et al., Chem. Mater. 1 (1989) 331. [7] P.R. Slater and C. Greaves, Physica C 180 (1991) 299. [8] W.I.F. David et al., Nature 327 (1987) 310.