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Search for new superconducting oxides in SrNbO system
_--1
Jit!, ELSEVIER
PHYSICA ® Physica C 282-287 (1997) 727-728
Search for New Superconducting Oxides in Sr-Nb-O System Nianhua Peng and John T S Irvine School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, Scotland, UK In searching for possible new superconducting transition metal oxides without Cu ions, we have examined the phase diagram of the ternary SrO-NbO-~05 system. This leads to the establishment of the existence of a group of new perovskite related compounds with mixed Nb3+ and Nb4+ ions, i.e., Srl_xNb(),_y. The samples have been characterised by using powder x-ray diffraction, high resolution powder neutron diffraction, AC magnetic susceptibility, magnetisation, AC electrical resistance and thermal analysis. The observed superconducting phase transitions between 6 and 9 K have been attributed to residual Nb. 1. INTRODUCTION In contrast to the extensive studies on cuprate superconductors reported in the literature, there has been only a very limited coverage of recent studies on non-copper oxide superconductors with low Te values. Examples of such studies include superconducting phase transitions in the Li-Nb-0[11, in the Sr-La-Nb-<)I21, and in Ba_Nb-O[31. Previous studies on Sr-Nb-O system were mainly concentrated on materials containing Nb4+ and Nb5+ ions[41. The formation of ~ clusters has been reported in a series of studies in the Nb rich Sr-Nbo compounds[51. There is no report of a superconducting phase transition in the pure Sr-Nb-O system.
2. EXPERIMENTAL All samples were synthesised by high temperature solid state reaction in flowing Ar gas conditions at 1400 °C by mixing Sr4~09, Nb and ~05 powders. Sr4~01 was prepared by direct reaction of srC03 and ~05 powders in air at 1200 °C. The SrC03, Nb and ~05 were commerical 99.9% powders, The products have been characterised by using Cu Ka.1 radiation powder x-ray diffraction, AC electrical resistance, magnetisation, AC magnetic susceptibility, TGA and powder neutron diffraction techniques. 3. Srl_xNb03_y A primitive cubic perovskite phase can be formed for Srl_xNb03 (0.05 < x < 0.25). The powder x-ray diffraction patterns of samples of nominal composition Srl_xNb(), (x ~ 0.05) are characterised by strong lines which are in agreement with the primitive cubic perovskite structure but with 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00543-1
additional satellite lines. The detailed crystal structure is not known yet. In order to elucidate the possible contribution of oxygen vancancy ordering toward the formation of such satellite x-ray diffraction patterns, a group of model compounds with nominal compositions as SrnNbn03n_h where n = 2 to 7, have been synthesised. The overall x-ray diffraction profiles of all these SrnNbn03n-1 compounds are very similar. As n changed from 2 to 7, those strong lines associated with the primitive cubic cell are basically unchanged while satellite lines have been shifted consistently with change in n. The end compound Sr2~05 is of interest here, as the average valence state of Nb ions is 3+. By introducing Sr site vacancies, clean powder x-ray diffraction patterns without any satellite peaks have been observed for a group of SrO.8nNbn~n_1 samples. The refined lattice parameters (ap) from powder xray diffraction are listed in the table below. Oxidation of all these samples in flowing oxygen confirms their overall oxygen stoichiometries.
4. SUPERCONDUCTING PHASE TRANSITIONS All SrnNbn03n-1 and Sro8nNbn~n-1 samples are red in colour, but AC electrical resistances are low at room temperature and are basically temperature independent down to 2.6K. Exceptional cases have been recorded, for example, the resistance of Sr16~05 sample exhibits a small but distinct drop in magnitude at low temperature. This drop in resistance has been confirmed as due to a superconducting phase transition from magnetic measurements. Varied superconducting phase transition temperatures have been identified for a group of SrO.8nNbn03n-1 samples, as shown in the table. All T e values observed here are lower than
N. Pengo J.T.s. IrvinelPhysica C 282-287 (1997) 727-728
728
that observed in pure Nb metal. This suggests the possibility of slight oxidation of Nb residual elements after the formation of dominant SrnNbn03n-l phase. As a drop in Tc from 9.2 K to about 7 K has been observed in partially oxidised j Nb metal[6 before, though the mechanism of this oxidation and Tc drop is not very clear. It seems difficult to detect minor Nb residuals in the A-Nb-O system, as early work on Ba-Nb-O series also encountered possible Nb residual contamination. which failed to be revealed by x-ray diffraction[7( However, the presence of minor unreacted Nb residuals can be inferred from the thermogravimetric oxidation steps of the Sr1.6NlhOs sample, which is superconducting. Only one single oxidation step can be observed for the SrNb03 sample, which is not superconductor. Table Summary of Lattice parameters and Tc Composition SrNb03 Sr2NlhOs SruNlhOs Srl.~OS
Sr1.5NlhOs Srl.6NlhOS.2 Nb
TJK N/A 8.9 8.7 6.7 7.0 7.1 9.2
apiA 4.012(1) 4.007(1) 4.000(1) 4.000(1) 3.990(1) 3.995(1)
N/A
aNllA
N/A 3.2999 3.2996 3.3066 3.3052 3.3052 3.3066
Direct evidence for the existence of minor Nb residual left in the superconducting samples comes from neutron diffraction data collected at the ISIS of Rutherford-Appleton Laboratory, as shown in Fig. 1. In contrast to these, the neutron patterns for nonsuperconducting samples, e.g., SrNb03, are Nb-like impurity phase free. Refinement for the lattice parameters (aNb) of Nb-like impurity phase for all superconducting samples reveals an interesting aNb versus Tc correlation, as shown in the table. All compositions listed in this table are nominal ones except Nb. The Nb lattice parameter is taken from the JCPDS data. The samples with
from Nb-like impurity phases and Sr-Nb-O matrix prevent us from doing more detailed refinement on the distribution of oxygen impurities from the neutron data. No correlation is observed between T c and a p . 600
I~
f
H 0,
Nb
~'O---:-:,,:-:--,-""'-:-'~..L..-"----'''''''''4-.--J115 Tlm~ of Flight
(ms)
Fig. 1 Neutron diffraction pattern of Srl6NlhOs 5. CONCLUSIONS In summary, in searching for possible new superconducting oxide materials without copper ions, we have studied the srO-NbO-NlhOs system. A group of new phases has been identified with the general chemical composition as Sro8nNbn03n-l. The superconducting phase transitions observed in these samples have been associated with very weak Nblike metal impurities, which are believed to be the unreacted Nb residual after solid state reaction. ACKNOWLEDGEMENT We thank the EPSRC and the Nuffield Foundation for financial support. We also thank the EPSRC for provision of neutron diffraction facilities, and Dr. R. Smith, R K. B. Gover and A. J. Feighery for assistance in the collection of data. Thanks are due to Prof. R Cywinski, J. R. Stewart and A. D. Hillier for assistance in magnetic property measurements. REFERENCES [1] M. J. Geselbracht et aI, Nature, 345(1990)324 [2] J. Akimitsu et aI, JJAP, 30(1991)LlI55 [3] V. A. Gasparov et aI, JETP Lett., 60(1994)440 [4] e.g. K. lsawa et aI, Phys. Rev. B, 47(1993)2849 [5] J. Kohler et aI, Angew. Chern. Int. Ed. Engl., 31 (1991)1437 (6) J. Halbritter, J. Less-common Met., 139(1988) 133 [7] R. R Kresiser et aI, J. Sol. State. Chem., 1 (1970) 368
Jit!, ELSEVIER
PHYSICA ® Physica C 282-287 (1997) 727-728
Search for New Superconducting Oxides in Sr-Nb-O System Nianhua Peng and John T S Irvine School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, Scotland, UK In searching for possible new superconducting transition metal oxides without Cu ions, we have examined the phase diagram of the ternary SrO-NbO-~05 system. This leads to the establishment of the existence of a group of new perovskite related compounds with mixed Nb3+ and Nb4+ ions, i.e., Srl_xNb(),_y. The samples have been characterised by using powder x-ray diffraction, high resolution powder neutron diffraction, AC magnetic susceptibility, magnetisation, AC electrical resistance and thermal analysis. The observed superconducting phase transitions between 6 and 9 K have been attributed to residual Nb. 1. INTRODUCTION In contrast to the extensive studies on cuprate superconductors reported in the literature, there has been only a very limited coverage of recent studies on non-copper oxide superconductors with low Te values. Examples of such studies include superconducting phase transitions in the Li-Nb-0[11, in the Sr-La-Nb-<)I21, and in Ba_Nb-O[31. Previous studies on Sr-Nb-O system were mainly concentrated on materials containing Nb4+ and Nb5+ ions[41. The formation of ~ clusters has been reported in a series of studies in the Nb rich Sr-Nbo compounds[51. There is no report of a superconducting phase transition in the pure Sr-Nb-O system.
2. EXPERIMENTAL All samples were synthesised by high temperature solid state reaction in flowing Ar gas conditions at 1400 °C by mixing Sr4~09, Nb and ~05 powders. Sr4~01 was prepared by direct reaction of srC03 and ~05 powders in air at 1200 °C. The SrC03, Nb and ~05 were commerical 99.9% powders, The products have been characterised by using Cu Ka.1 radiation powder x-ray diffraction, AC electrical resistance, magnetisation, AC magnetic susceptibility, TGA and powder neutron diffraction techniques. 3. Srl_xNb03_y A primitive cubic perovskite phase can be formed for Srl_xNb03 (0.05 < x < 0.25). The powder x-ray diffraction patterns of samples of nominal composition Srl_xNb(), (x ~ 0.05) are characterised by strong lines which are in agreement with the primitive cubic perovskite structure but with 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00543-1
additional satellite lines. The detailed crystal structure is not known yet. In order to elucidate the possible contribution of oxygen vancancy ordering toward the formation of such satellite x-ray diffraction patterns, a group of model compounds with nominal compositions as SrnNbn03n_h where n = 2 to 7, have been synthesised. The overall x-ray diffraction profiles of all these SrnNbn03n-1 compounds are very similar. As n changed from 2 to 7, those strong lines associated with the primitive cubic cell are basically unchanged while satellite lines have been shifted consistently with change in n. The end compound Sr2~05 is of interest here, as the average valence state of Nb ions is 3+. By introducing Sr site vacancies, clean powder x-ray diffraction patterns without any satellite peaks have been observed for a group of SrO.8nNbn~n_1 samples. The refined lattice parameters (ap) from powder xray diffraction are listed in the table below. Oxidation of all these samples in flowing oxygen confirms their overall oxygen stoichiometries.
4. SUPERCONDUCTING PHASE TRANSITIONS All SrnNbn03n-1 and Sro8nNbn~n-1 samples are red in colour, but AC electrical resistances are low at room temperature and are basically temperature independent down to 2.6K. Exceptional cases have been recorded, for example, the resistance of Sr16~05 sample exhibits a small but distinct drop in magnitude at low temperature. This drop in resistance has been confirmed as due to a superconducting phase transition from magnetic measurements. Varied superconducting phase transition temperatures have been identified for a group of SrO.8nNbn03n-1 samples, as shown in the table. All T e values observed here are lower than
N. Pengo J.T.s. IrvinelPhysica C 282-287 (1997) 727-728
728
that observed in pure Nb metal. This suggests the possibility of slight oxidation of Nb residual elements after the formation of dominant SrnNbn03n-l phase. As a drop in Tc from 9.2 K to about 7 K has been observed in partially oxidised j Nb metal[6 before, though the mechanism of this oxidation and Tc drop is not very clear. It seems difficult to detect minor Nb residuals in the A-Nb-O system, as early work on Ba-Nb-O series also encountered possible Nb residual contamination. which failed to be revealed by x-ray diffraction[7( However, the presence of minor unreacted Nb residuals can be inferred from the thermogravimetric oxidation steps of the Sr1.6NlhOs sample, which is superconducting. Only one single oxidation step can be observed for the SrNb03 sample, which is not superconductor. Table Summary of Lattice parameters and Tc Composition SrNb03 Sr2NlhOs SruNlhOs Srl.~OS
Sr1.5NlhOs Srl.6NlhOS.2 Nb
TJK N/A 8.9 8.7 6.7 7.0 7.1 9.2
apiA 4.012(1) 4.007(1) 4.000(1) 4.000(1) 3.990(1) 3.995(1)
N/A
aNllA
N/A 3.2999 3.2996 3.3066 3.3052 3.3052 3.3066
Direct evidence for the existence of minor Nb residual left in the superconducting samples comes from neutron diffraction data collected at the ISIS of Rutherford-Appleton Laboratory, as shown in Fig. 1. In contrast to these, the neutron patterns for nonsuperconducting samples, e.g., SrNb03, are Nb-like impurity phase free. Refinement for the lattice parameters (aNb) of Nb-like impurity phase for all superconducting samples reveals an interesting aNb versus Tc correlation, as shown in the table. All compositions listed in this table are nominal ones except Nb. The Nb lattice parameter is taken from the JCPDS data. The samples with
from Nb-like impurity phases and Sr-Nb-O matrix prevent us from doing more detailed refinement on the distribution of oxygen impurities from the neutron data. No correlation is observed between T c and a p . 600
I~
f
H 0,
Nb
~'O---:-:,,:-:--,-""'-:-'~..L..-"----'''''''''4-.--J115 Tlm~ of Flight
(ms)
Fig. 1 Neutron diffraction pattern of Srl6NlhOs 5. CONCLUSIONS In summary, in searching for possible new superconducting oxide materials without copper ions, we have studied the srO-NbO-NlhOs system. A group of new phases has been identified with the general chemical composition as Sro8nNbn03n-l. The superconducting phase transitions observed in these samples have been associated with very weak Nblike metal impurities, which are believed to be the unreacted Nb residual after solid state reaction. ACKNOWLEDGEMENT We thank the EPSRC and the Nuffield Foundation for financial support. We also thank the EPSRC for provision of neutron diffraction facilities, and Dr. R. Smith, R K. B. Gover and A. J. Feighery for assistance in the collection of data. Thanks are due to Prof. R Cywinski, J. R. Stewart and A. D. Hillier for assistance in magnetic property measurements. REFERENCES [1] M. J. Geselbracht et aI, Nature, 345(1990)324 [2] J. Akimitsu et aI, JJAP, 30(1991)LlI55 [3] V. A. Gasparov et aI, JETP Lett., 60(1994)440 [4] e.g. K. lsawa et aI, Phys. Rev. B, 47(1993)2849 [5] J. Kohler et aI, Angew. Chern. Int. Ed. Engl., 31 (1991)1437 (6) J. Halbritter, J. Less-common Met., 139(1988) 133 [7] R. R Kresiser et aI, J. Sol. State. Chem., 1 (1970) 368