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Thermogravimetric and microstructural studies on YBa2Cu3−xFexOy
SOLID STATE
Solid State Ionics 63-65 (1993) 866-871 North-Holland
IONICS Thermogravimetric and microstructural studies on YBa2Cu3_xFexOy E. G a r c i a a,c, M. V a l l e t - R e g i b,c, A. R e l i e r d a n d J.M. G o n z d l e z - C a l b e t .... 1 a Departamento Quimica lnorgdnica, Facultad de Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain b DepartamentodeQulmicalnorgrtnicayBioinorgdnica, FacultaddeFarmacia, UniversidadComplutensedeMadrid, 28040Madrid, Spain c lnstitutodeMagnetismoAplicado "Salvador Velayos'" Apdo. Correos 155, 28230-LasRozas, Madrid, Spain a Institute of Inorganic and Applied Chemistry, University of Hamburg, D-2000 Hamburg 13, Germany
Thermogravimetric and microstructural investigations on Fe-doped YBa2Cu307samples have been carried out, to study the accommodation of compositional variations as a function of the iron content. The results obtained allow us to conclude that the iron substitution is not isomorphous, the local anionic environment of the substituted positions being modified. The samples appear to be formed by disordered intergrowthsbetween YBa2Cu307and perovskite-typeblocks.
I. Introduction The iron doped YBa2Cu3_xFexOy system has been widely studied in order to specify the effect of coexistence of superconductivity and magnetism [ 1-6 ]. However, neither the exact structural positions of the Fe atoms, nor the role of oxygen stoichiometry have been elucidated. Detailed information on the local oxygen e n v i r o n m e n t of Fe atoms and on the changes of this e n v i r o n m e n t with oxygen reduction is necessary. In this sense, we have carried out thermogravimetric and microstructural investigations on YBaaCu3_xFexOy samples to study the thermal reducibility and the a c c o m m o d a t i o n of compositional variations as a function of the iron content.
2. Experimental Samples of various compositions of the YBa2Cu3_xFexOy system ( x = 0 . 1 , 0.2, 0.3, 0.45 and 0.6 ) were obtained by solid state reaction from stoichiometric a m o u n t s of Y203, BaCO3, ~-Fe203 a n d CuO. An intimate mixture of the reagents was heated in a l u m i n a crucibles at 950°C in air for 72 h. The
samples were then reground and annealed at 700°C in flowing oxygen for five hours, cooled to 400°C a n d kept at this temperature for 18 h. Finally, samples were cooled inside the furnace to room temperature in the oxygen flux. Powder X-ray diffraction ( X R D ) was carried out on a G u i n i e r camera with a graphite monochromator and using C u K u radiation (exposure time 90 min.) The thermogravimetric analysis ( T G A ) was performed on a Mettler TA2000 system when working under HE/Ar atmosphere ( 15% H2/85% Ar, flow rate 30 m ~ / m i n ) with a heating rate of 1 0 ° C / m i n from room temperature to 900°C. By using a Perkin-E1mer TGS-2 thermomicrobalance the samples were heated u n d e r Ar (flow rate 30 m ~ / m i n ) up to 850°C (heating rate: 1 0 ° C / m i n ) . Selected area electron diffraction (SAED) was carried out on a Jeol 2000FX electron microscope fitted with a double tilt goniometer stage ( + 45 ° ). High resolution electron microscopy ( H R E M ) was performed on a Jeol 4000EX electron microscope fitted with a double tilt goniometer stage ( + 25 ° ) by working at 400 kV.
t Author to whom all correspondenceshould be addressed. 0167-2738/93/$ 06.00 © 1993 ElsevierSciencePublishers B.V. All rights reserved.
E. Garcla et al. / Thermogravimetric and microstructural studies on YBa2Cu3_ xFexOy
3. Results and discussion
The thermogravimetric curves obtained under reducing atmosphere (15% H2/85% Ar) show three well differentiated steps at 450, 600 and 750°C for every composition. Fig. 1 shows the thermogravimetric curve of the x = 0 . 2 material. In spite of the evident reaction steps in the curves, there is no correspondence between these plateaus and the intermediate phases observed and characterized by X-ray diffraction studies. As observed in the corresponding XRD patterns, the products obtained at 450°C are YBa2Cu3Ox, Cu and BaCuO2. It is interesting to note the presence of Cu at an early stage in the reduction, suggesting a surface reaction before substantial reduction occurs in the bulk of the particle. By advancing the reduction, the YBa2Cu307 phase disappears at expense of the formation of Y203 and Y2Cu205 • In the proximity of 750°C, there is only Cu, BaO (which transforms into Ba(OH)2 at lower temperatures) and BaY204. The related phase Ba4Y207
seems also to be formed, according to the studies on the B a - Y - O system by Kwestroo et al. [7]. Thus, the TGA curve corresponding to the decomposition in H2 appears to progress through a series of solid state phases which might provide some insight into the identity of intermediate products. However, some of the intermediate steps involve hydroxide formation and subsequent thermal evolution of water [8]. Such reactions are generally reversible in nature and hence their kinetics will be a function of PH2 as well as temperature. In this case, flow rate, heating rate, particle size, porosity, etc., will all influence the course of the decomposition. Consequently, the establishment of a direct correlation between the observed reaction steps and the isolated products prove to be difficult. Moreover, the impossibility of identifying iron on the X-ray patterns of the final products (probably due to the small particle size of the ct-Fe formed) introduces additional difficulties on the determination of the oxygen content. On the other hand, all the samples of the YBa2-
RATE,
IO. O0 d a g / m l n
ImtB
l--r t_~ ,--. mann ILl
mall)
o.~
~du.®
2do.nn
~®
~nn
867
~nn
sdo.®
~o.®
sdo.nn
TEMPERATURE (C> Fig. I. TOA curve for the YBa2Cu2.sFeo.:Or material at 10°Clmin in 15% H2185%N:.
E. Garcla et al. / Thermogravimetric and microstructural studies on YBa2Cu3_xFexOy
868
Cu3_xFexOy system were heated under argon atmosphere. In the Y B a 2 C u 3 0 7 material, it is possible to eliminate one oxygen atom per unit cell, by heating the sample under inert atmosphere or in vacuum. If this O atom is eliminated from the Cu( 1 ) - O chains [9,10], it can be supposed that it will be independent of the substitution of copper by any other transition metal inside the C u ( 2 ) - O planes, but it will be affected by the Cu substitution inside the Cu( 1 ) O chains, because of the changes on the M - O bond strength. Fig. 2 shows the thermogravimetric curves obtained for the whole series when heating the samples from RT to 850°C under Ar atmosphere at a rate of 10°C/min. According to analogous results previously reported [2], it can be observed that the oxygen amount evolved depends on the amount of Fe doping in such a way that it decreases when increasing the iron content. This fact is consistent with the strengthening of the M - O bond that would occur in
the substitution of Cu by Fe in the chains as a consequence of a higher coordination. From the presented results, it follows that the Fe substitution is not isomorphous, and it takes place preferably in the (001) plane. In order to solve the questions about the accommodation of iron and oxygen "extra" atoms, a study by SAED and HREM has been performed. SAED patterns along the [001 ]c zone axis for the 0.1~
1G3. ~31]
x_
L'.J
X-
x:01
T E3
g7.5~
95. TEMPERATURE
(C)
Fig. 2. TGA curves are shown as a function o f x for the YBa2Cu3_xFexOy series when the samples are heated under Ar up to 850°C at a rate of 10°C/min.
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu3_xFexO r
869
Fig. 3. SAED pattern for the 0.1 ~
870
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu ~_xFexOy
Fig. 4. (a) SAED pattern for the 0.3
sponding SAED patterns along [ 100] can be interpreted as the superimposition o f two different sets o f reflections: - one set corresponding to the [ 100 ] zone axis o f the 1 : 2 : 3 phase; one set o f m a x i m a doubling the (001) and equivalent reflections o f the cubic perovskite subcell. -
As the high resolution micrograph (fig. 4b) reveals these crystals are formed by a disordered intergrowth m a d e up o f d o m a i n s o f the YBa2Cu307type and other regions where the spacing is twice the door of the perovskite subcell ( -~ 0.78 n m ) which in turn lead to the extra superstructure spots. Taking into account the m e n t i o n e d results and
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu3_ xFexOr
considering that iron is responsible for the observed defects, two possibilities can exist with respect to the c o m p o s i t i o n and stoichiometry o f the intergrowth phase: the YBaCuFeO5 material [12] (tetragonal s y m m e t r y P 4 m m , a = 0 . 3 8 7 n m and c = 0 . 7 6 n m ---2a¢) or the YFeO3 ferrite [13] ( o r t h o r h o m b i c symmetry, a c x / 2 × a c x / ~ × 2 a c ) . W h e n Ba is occupying the A positions, and all iron is found as Fe ( I I I ) , the monoclinic BaFeOz.so phase is obtained [ 14 ]; if there is some F e ( I V ) , different hexagonal phases adopting BaFeO3_y compositions (0.07 ~
4. Concluding remarks F r o m the a b o v e results it can be derived that the iron substitution is not isomorphous, varying with the local anionic e n v i r o n m e n t o f the substituted positions. The microstructural characterization reveals a clear "heterogeneity" in the d o p a n t distribution, and the d i s o r d e r observed along the c axis increases with increasing a m o u n t o f doping; for iron contents below x = 0.3, there are isolated defect planes which
871
become discrete blocks o f about 10-20 nm for compositions in the range o f 0.3
Acknowledgements We acknowledge the financial support of C.I.C.Y.T. ( S p a i n ) , through Research Projects M A T 90-0858C02-02 and MAT 91-0331, and M I D A S Program. E G G also thanks the Spanish Ministry o f Education for a supporting grant and the Swiss National Foundation, project n u m b e r 2.772-0.87. We are also grateful to Mr. A. G a r c i a for valuable technical assistance.
References [ 1] I. Felner, I. Nowik and Y. Yeshura, Phys. Rev. B 36 ( 1987 ) 3923. [2] J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz and S.A. Sunshine, Phys. Rev. B 37 (1988) 7458. [3]J. Jing, J. Bieg, H. Engelmann, Y. Hsia, U. Gonser, P. Gutlich and R. Jacobi, Solid State Commun. 66 (1988) 7271. [4] H.U. Krebs and O. Bemert, J. Less-Common Met. 150 (1989) 159. [5] I.S. Lyubutin, V.G. Terziev, E.M. Smirnovskaya and A.Ya. Saphiro, Physica C 169 (1990) 361. [6] H. Oesterreicher, M. Smith and D. Taylor, J. Magn. Magn. Mater. 104-107 (1992) 497. [7] W. Kwestroo, H.A.M. van Hal and C. Langeries, Mat. Res. Bull. 9 (1974) 1631. [ 8 ] P.K. Gallagher, D.W. Johnson, E.W. Vogel, G.K. Wertheim and F.J. Schnettler, J. Solid State Chem. 21 (1977) 277. [9]J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyen, B. Raveau, J.L. Soubeyroux, J.L. Tholence and R. Tournier, Europhys. Lett. 3 (1987) 1301. [ 10] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace and I.K. Schutler, Phys. Rev. B 36 (1987) 3608. [ 11 ] P. Bordet, J.L. Hodeau, P. Strobel, M. Marezio and A. Santoro, Solid State Commun. 66 (1988) 435. [ 12] L. Er-Rakho, C. Michel, P. Lacorre and B. Raveau, J. Solid State Chem. 73 (1988) 531. [13]M. Marezio, J.P. Remeika and P.D. Dernier, Acta Crystallogr. B 26 (1970) 2008. [ 14] M. Parras, L. Fournes, J.C. Grenier, M. Pouchard, M. Vallet, J.M. Gonz~ilez-Calbetand P. Hagenmuller, J. Solid State Chem. 88 (1990) 261. [ 15 ] M. Parras, M. Vallet-Regi, J.M. Gonz~ilez-Calbetand J.C. Grenier, J. Solid State Chem. 83 (1989) 121.
Solid State Ionics 63-65 (1993) 866-871 North-Holland
IONICS Thermogravimetric and microstructural studies on YBa2Cu3_xFexOy E. G a r c i a a,c, M. V a l l e t - R e g i b,c, A. R e l i e r d a n d J.M. G o n z d l e z - C a l b e t .... 1 a Departamento Quimica lnorgdnica, Facultad de Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain b DepartamentodeQulmicalnorgrtnicayBioinorgdnica, FacultaddeFarmacia, UniversidadComplutensedeMadrid, 28040Madrid, Spain c lnstitutodeMagnetismoAplicado "Salvador Velayos'" Apdo. Correos 155, 28230-LasRozas, Madrid, Spain a Institute of Inorganic and Applied Chemistry, University of Hamburg, D-2000 Hamburg 13, Germany
Thermogravimetric and microstructural investigations on Fe-doped YBa2Cu307samples have been carried out, to study the accommodation of compositional variations as a function of the iron content. The results obtained allow us to conclude that the iron substitution is not isomorphous, the local anionic environment of the substituted positions being modified. The samples appear to be formed by disordered intergrowthsbetween YBa2Cu307and perovskite-typeblocks.
I. Introduction The iron doped YBa2Cu3_xFexOy system has been widely studied in order to specify the effect of coexistence of superconductivity and magnetism [ 1-6 ]. However, neither the exact structural positions of the Fe atoms, nor the role of oxygen stoichiometry have been elucidated. Detailed information on the local oxygen e n v i r o n m e n t of Fe atoms and on the changes of this e n v i r o n m e n t with oxygen reduction is necessary. In this sense, we have carried out thermogravimetric and microstructural investigations on YBaaCu3_xFexOy samples to study the thermal reducibility and the a c c o m m o d a t i o n of compositional variations as a function of the iron content.
2. Experimental Samples of various compositions of the YBa2Cu3_xFexOy system ( x = 0 . 1 , 0.2, 0.3, 0.45 and 0.6 ) were obtained by solid state reaction from stoichiometric a m o u n t s of Y203, BaCO3, ~-Fe203 a n d CuO. An intimate mixture of the reagents was heated in a l u m i n a crucibles at 950°C in air for 72 h. The
samples were then reground and annealed at 700°C in flowing oxygen for five hours, cooled to 400°C a n d kept at this temperature for 18 h. Finally, samples were cooled inside the furnace to room temperature in the oxygen flux. Powder X-ray diffraction ( X R D ) was carried out on a G u i n i e r camera with a graphite monochromator and using C u K u radiation (exposure time 90 min.) The thermogravimetric analysis ( T G A ) was performed on a Mettler TA2000 system when working under HE/Ar atmosphere ( 15% H2/85% Ar, flow rate 30 m ~ / m i n ) with a heating rate of 1 0 ° C / m i n from room temperature to 900°C. By using a Perkin-E1mer TGS-2 thermomicrobalance the samples were heated u n d e r Ar (flow rate 30 m ~ / m i n ) up to 850°C (heating rate: 1 0 ° C / m i n ) . Selected area electron diffraction (SAED) was carried out on a Jeol 2000FX electron microscope fitted with a double tilt goniometer stage ( + 45 ° ). High resolution electron microscopy ( H R E M ) was performed on a Jeol 4000EX electron microscope fitted with a double tilt goniometer stage ( + 25 ° ) by working at 400 kV.
t Author to whom all correspondenceshould be addressed. 0167-2738/93/$ 06.00 © 1993 ElsevierSciencePublishers B.V. All rights reserved.
E. Garcla et al. / Thermogravimetric and microstructural studies on YBa2Cu3_ xFexOy
3. Results and discussion
The thermogravimetric curves obtained under reducing atmosphere (15% H2/85% Ar) show three well differentiated steps at 450, 600 and 750°C for every composition. Fig. 1 shows the thermogravimetric curve of the x = 0 . 2 material. In spite of the evident reaction steps in the curves, there is no correspondence between these plateaus and the intermediate phases observed and characterized by X-ray diffraction studies. As observed in the corresponding XRD patterns, the products obtained at 450°C are YBa2Cu3Ox, Cu and BaCuO2. It is interesting to note the presence of Cu at an early stage in the reduction, suggesting a surface reaction before substantial reduction occurs in the bulk of the particle. By advancing the reduction, the YBa2Cu307 phase disappears at expense of the formation of Y203 and Y2Cu205 • In the proximity of 750°C, there is only Cu, BaO (which transforms into Ba(OH)2 at lower temperatures) and BaY204. The related phase Ba4Y207
seems also to be formed, according to the studies on the B a - Y - O system by Kwestroo et al. [7]. Thus, the TGA curve corresponding to the decomposition in H2 appears to progress through a series of solid state phases which might provide some insight into the identity of intermediate products. However, some of the intermediate steps involve hydroxide formation and subsequent thermal evolution of water [8]. Such reactions are generally reversible in nature and hence their kinetics will be a function of PH2 as well as temperature. In this case, flow rate, heating rate, particle size, porosity, etc., will all influence the course of the decomposition. Consequently, the establishment of a direct correlation between the observed reaction steps and the isolated products prove to be difficult. Moreover, the impossibility of identifying iron on the X-ray patterns of the final products (probably due to the small particle size of the ct-Fe formed) introduces additional difficulties on the determination of the oxygen content. On the other hand, all the samples of the YBa2-
RATE,
IO. O0 d a g / m l n
ImtB
l--r t_~ ,--. mann ILl
mall)
o.~
~du.®
2do.nn
~®
~nn
867
~nn
sdo.®
~o.®
sdo.nn
TEMPERATURE (C> Fig. I. TOA curve for the YBa2Cu2.sFeo.:Or material at 10°Clmin in 15% H2185%N:.
E. Garcla et al. / Thermogravimetric and microstructural studies on YBa2Cu3_xFexOy
868
Cu3_xFexOy system were heated under argon atmosphere. In the Y B a 2 C u 3 0 7 material, it is possible to eliminate one oxygen atom per unit cell, by heating the sample under inert atmosphere or in vacuum. If this O atom is eliminated from the Cu( 1 ) - O chains [9,10], it can be supposed that it will be independent of the substitution of copper by any other transition metal inside the C u ( 2 ) - O planes, but it will be affected by the Cu substitution inside the Cu( 1 ) O chains, because of the changes on the M - O bond strength. Fig. 2 shows the thermogravimetric curves obtained for the whole series when heating the samples from RT to 850°C under Ar atmosphere at a rate of 10°C/min. According to analogous results previously reported [2], it can be observed that the oxygen amount evolved depends on the amount of Fe doping in such a way that it decreases when increasing the iron content. This fact is consistent with the strengthening of the M - O bond that would occur in
the substitution of Cu by Fe in the chains as a consequence of a higher coordination. From the presented results, it follows that the Fe substitution is not isomorphous, and it takes place preferably in the (001) plane. In order to solve the questions about the accommodation of iron and oxygen "extra" atoms, a study by SAED and HREM has been performed. SAED patterns along the [001 ]c zone axis for the 0.1~
1G3. ~31]
x_
L'.J
X-
x:01
T E3
g7.5~
95. TEMPERATURE
(C)
Fig. 2. TGA curves are shown as a function o f x for the YBa2Cu3_xFexOy series when the samples are heated under Ar up to 850°C at a rate of 10°C/min.
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu3_xFexO r
869
Fig. 3. SAED pattern for the 0.1 ~
870
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu ~_xFexOy
Fig. 4. (a) SAED pattern for the 0.3
sponding SAED patterns along [ 100] can be interpreted as the superimposition o f two different sets o f reflections: - one set corresponding to the [ 100 ] zone axis o f the 1 : 2 : 3 phase; one set o f m a x i m a doubling the (001) and equivalent reflections o f the cubic perovskite subcell. -
As the high resolution micrograph (fig. 4b) reveals these crystals are formed by a disordered intergrowth m a d e up o f d o m a i n s o f the YBa2Cu307type and other regions where the spacing is twice the door of the perovskite subcell ( -~ 0.78 n m ) which in turn lead to the extra superstructure spots. Taking into account the m e n t i o n e d results and
E. Garcia et al. / Thermogravimetric and microstructural studies on YBa2Cu3_ xFexOr
considering that iron is responsible for the observed defects, two possibilities can exist with respect to the c o m p o s i t i o n and stoichiometry o f the intergrowth phase: the YBaCuFeO5 material [12] (tetragonal s y m m e t r y P 4 m m , a = 0 . 3 8 7 n m and c = 0 . 7 6 n m ---2a¢) or the YFeO3 ferrite [13] ( o r t h o r h o m b i c symmetry, a c x / 2 × a c x / ~ × 2 a c ) . W h e n Ba is occupying the A positions, and all iron is found as Fe ( I I I ) , the monoclinic BaFeOz.so phase is obtained [ 14 ]; if there is some F e ( I V ) , different hexagonal phases adopting BaFeO3_y compositions (0.07 ~
4. Concluding remarks F r o m the a b o v e results it can be derived that the iron substitution is not isomorphous, varying with the local anionic e n v i r o n m e n t o f the substituted positions. The microstructural characterization reveals a clear "heterogeneity" in the d o p a n t distribution, and the d i s o r d e r observed along the c axis increases with increasing a m o u n t o f doping; for iron contents below x = 0.3, there are isolated defect planes which
871
become discrete blocks o f about 10-20 nm for compositions in the range o f 0.3
Acknowledgements We acknowledge the financial support of C.I.C.Y.T. ( S p a i n ) , through Research Projects M A T 90-0858C02-02 and MAT 91-0331, and M I D A S Program. E G G also thanks the Spanish Ministry o f Education for a supporting grant and the Swiss National Foundation, project n u m b e r 2.772-0.87. We are also grateful to Mr. A. G a r c i a for valuable technical assistance.
References [ 1] I. Felner, I. Nowik and Y. Yeshura, Phys. Rev. B 36 ( 1987 ) 3923. [2] J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz and S.A. Sunshine, Phys. Rev. B 37 (1988) 7458. [3]J. Jing, J. Bieg, H. Engelmann, Y. Hsia, U. Gonser, P. Gutlich and R. Jacobi, Solid State Commun. 66 (1988) 7271. [4] H.U. Krebs and O. Bemert, J. Less-Common Met. 150 (1989) 159. [5] I.S. Lyubutin, V.G. Terziev, E.M. Smirnovskaya and A.Ya. Saphiro, Physica C 169 (1990) 361. [6] H. Oesterreicher, M. Smith and D. Taylor, J. Magn. Magn. Mater. 104-107 (1992) 497. [7] W. Kwestroo, H.A.M. van Hal and C. Langeries, Mat. Res. Bull. 9 (1974) 1631. [ 8 ] P.K. Gallagher, D.W. Johnson, E.W. Vogel, G.K. Wertheim and F.J. Schnettler, J. Solid State Chem. 21 (1977) 277. [9]J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyen, B. Raveau, J.L. Soubeyroux, J.L. Tholence and R. Tournier, Europhys. Lett. 3 (1987) 1301. [ 10] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace and I.K. Schutler, Phys. Rev. B 36 (1987) 3608. [ 11 ] P. Bordet, J.L. Hodeau, P. Strobel, M. Marezio and A. Santoro, Solid State Commun. 66 (1988) 435. [ 12] L. Er-Rakho, C. Michel, P. Lacorre and B. Raveau, J. Solid State Chem. 73 (1988) 531. [13]M. Marezio, J.P. Remeika and P.D. Dernier, Acta Crystallogr. B 26 (1970) 2008. [ 14] M. Parras, L. Fournes, J.C. Grenier, M. Pouchard, M. Vallet, J.M. Gonz~ilez-Calbetand P. Hagenmuller, J. Solid State Chem. 88 (1990) 261. [ 15 ] M. Parras, M. Vallet-Regi, J.M. Gonz~ilez-Calbetand J.C. Grenier, J. Solid State Chem. 83 (1989) 121.