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Cerium substitution in the 2212 bismuth cuprate : Bi2Sr2Ca1−xCexCu2O8+δ
Physica C 235-240 (1994) 945-946
PHYSICfi
North-Holland
Cerium substitution in the 2212 bismuth cuprate : B i 2 S r 2 C a l . x C e x C u 2 O S + ~ 5 F. Jordan a, O. Pefia a, R. Horylf b a C,S,I.M, - URA 1495. Universit6 de Rennes I. 35042 Rennes Cedex. FRANCE b Institute for Low Temperature and Structure Research. 50-950 Wroclaw. POLAND Magnetic and superconducting properties of Bi2Sr2CalCu208~ (2212) have been studied after partial substitution of calcium by cerium. Synthesis conditions show that a very small cerium doping (less than 10 %) is enough to increase the domain of formation of the 2212 phase by at least 30°C. A single-phase domain exists for x <_0.3 as revealed by X-ray diffraction, EDS and lattice parameters. Tc increases with slight substitutions and reaches a maximum at x ~ 0.02, before droping to almost zero at the limit of solubility. The superconducting volume decreases smoothly with increasing x(Ce). Cerium is non-magnetic and does not suppress superconductivity. Results are explained by a variation of the number of holes due to the substitution Ca2+ ~+ Ce4+. Trivalent rare-earths can substitute calcium in the bismuth cuprates without visible changes of the structure. However, important modifications in the hole carder concentration take place due to the additional electrons and changes in the oxygen content and on the oxidation state of copper and bismuth [1]. A significant increase of Tc occurs at low substitutions, followed by its fast decrease and a metalinsulator transition at x > 0.5. Several works have described :,he effects of rare-earth replacing calcium in the 2212 bismuth compounds, but no much emphasis has been given to their magnetic properties [2,3]. Cerium constitutes a special case since its oxidation state (3+ or 4+) will impose the magnetic phenomena (4f 1 or 4f 0 configuration) and the number of electrons added when replacing Ca2+. Synthesis of Bi2Sr2Cal.xCexCu208+fi was done from oxides and carbonates. The First step of synthesis took place at about 750-800 °C for 14 h, in order to avoid the melting of Bi203, which would result in its inhomogeneous diatribution in the sample. The powders were ground and pressed, after mixing them with a few drops of acetone to increase its weight density and to allow an easy extraction from the die. Each pellet was sintered for one hour at 840 ~'C <_ T _ 920 °C. From the intensities of the (113) and (115) lines of the X-ray diagrams, the proportion of the 2212 phase was determined (fig. 1). It is seen that at low doping, the temperature of synthesis can be increased by about 30 °C, which facilitates the formation of single-phase samples. At high substitutions (x _:20.5), multiphased samples composed of 22!2, 2201 and CeO2 were always oh0921-,1534/94/5()7.00 © 1994- Iitscvicr SS"DI I)921-4534(94)()1036-6
Science
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All rights
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Fig. 1. Percentage of the 2212 phase for x(Ce) = 0 and 0.1, after ~ synthesis. mined. Once the optimum temperature was found, the time was increased to 63 h, Single-phase samples were thus oi?.aiaed (fig. 2), with the c-parameter varying linearly from 30,83(1) to 30.57(1) A, while the a-parameter goes from 5.410(2) to 5.438(2) A, for 0 _< x < 0.3. EDS analysis showed a homogeneous distribution of Ce in the sample and in each g~ain. We can conclude that Ce replaces the calcium Site in the 2212 structure although i~s solubility is rather small, restricted to the region 0,0 <_ x <_ 0.3. The superconducting transitions were measured by Xac methods. For x _<0.1 the amplitude of the X' component does not change with the cerium content, and is equal to 100%. Figure 3 shows the Tc °riser for : (a) samples obtained after long-time sintering followed by slow (furnace) cooling ; (b) same samples but further annealed at 460 °C for 2 h under rest r~.cd.
E Jordan et al./Physica C 235-240 (1994) 945-946
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Fig. 2. X-ray diffraction for x(Ce) = 0.1 after 63 h at 880 °C. Peak marked "S" is due to the modulation. vacuum. A few comments can be made : firstly, a slight maximum of Tc is observed for x - 0.02 in the as-grown series ; this maximum levels off after vacuum annealing. Secondly, Tc increases when samples are annealed under vacuum (and decreases later when they are re-annealed in air at 460 °C). This procedure is reversible and reproducible, as confirmed by subsequent annealings in a second series of samples. At higher concentrations (x = 0.2, 0.3), a small decrease of the amplitude was observed with Tc's below 30 K ; when annealed under vacuum, the samples ~,,~,re non-superconducting, indicating ~,at the metal-insulator transition takes place at lowrx x when oxygen is removed. These resclts can be explained on the basis of the hole r' -~sityimposed by both the average valence of copper and the cerium substitution. Several authors have shown a strong correlation of Tc with the hole concentration in the bismuth cuprates [4,5], in spite of the negligible weight variation due to the oxygen uptake [6]. It is well established by now that the 2212 phase (at x = 0) is overdoped with hole carders and it attains a maximum Tc for about 0.2 holes/Cu (Vcu - 2.17). In the underdoped regime, the system turns into insulator when the hole concentration drops down to 0.07 (Vcu - 2.07). Similar conclusions were found when Ca 2+ was substituted by RE 3+, Tc reaching a maximum for x ~ 0.2 [7]. In our case, the number of additional electrons due to the substitution is: larger. If Lhe overall oxygen index is set equal to: 6 + 0.5x + Vcu = 8.17 for trivalent substituents (assuming an. excess of ~ 0 i7 oxygen/formula) then, for tetravalent cerium, it becomes : 6 + x + VCu = 8.17. Applying a copper valence of VCu = 2.07, it flows out that in our case. Tc varies more rapidly and the optimum Tc value is
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x (Ce) Fig. 3. Tc°nset as a function of the Ce content after: a) furnace cooling ; b) vacuum annealing at 460 °C. obtained at smaller x, compared to the case of trivalent lanthanides. Annealings under air or vacuum modify the ovelall oxygen content and change the hole carrier concentration ; as a result, the Tc-vs-x curces are shifted along the transversal axis [21. The magnetic susceptibility and magnetization measurements conf'mned that the electronic configuration of cerium in this material is 4f05d26s2, that is, the tetravalent non-magnetic state. The superconducting volume (dM/dH slope at H -o 0) decreases smoothly with x, indicating that paramagnetism appears when the number of hole carriers decreases. In conclusion, ceritlm substitutes calcium in a narrow domain of solubility (x _'2: 0.3) wi~out suppressing the superconducting state. Due to its tetravalent configuration, the concentration of holes in the Cu-O planes varies very rapidly and a maximum of Tc is attained at very low cerium doping (x ~ 0.02-0.05). R E F E R E N C E S
1. B. Jayaram, P.C. Lanchester, M.T. Weller. "~hysica C 160 (1989) 17. 2. K. Koyama, S. Kanno, S. Noguchi. Jpn. J. Appl. Phys. 29 (1990) L-53 3. E. Laxmi Narsaiah, U.V. Subba Rao, O. Pefia, C. Perrin. Solid State Comm. 83 (1992) 689. 4. C. Namgung, J. Irvine, J. Binks, E. Lachowski, A. WeSL Supercond. Sci. Tech. 2 (1989) 181. 5. T. Takabatake, W. Ye, S. Orimo, H. Kanawaka, H: Fujii etal. Physica C 157 (1989) 263. 6. M.A. Diaia, O. Pefia, C. Pcrrin, M. Sergent. Solid Share Commun. 73 (1990) 715. 7. W.A. Groen, D.M. de Leeuw, L.F. Feiner. Physica C 165 (1990) 55.
PHYSICfi
North-Holland
Cerium substitution in the 2212 bismuth cuprate : B i 2 S r 2 C a l . x C e x C u 2 O S + ~ 5 F. Jordan a, O. Pefia a, R. Horylf b a C,S,I.M, - URA 1495. Universit6 de Rennes I. 35042 Rennes Cedex. FRANCE b Institute for Low Temperature and Structure Research. 50-950 Wroclaw. POLAND Magnetic and superconducting properties of Bi2Sr2CalCu208~ (2212) have been studied after partial substitution of calcium by cerium. Synthesis conditions show that a very small cerium doping (less than 10 %) is enough to increase the domain of formation of the 2212 phase by at least 30°C. A single-phase domain exists for x <_0.3 as revealed by X-ray diffraction, EDS and lattice parameters. Tc increases with slight substitutions and reaches a maximum at x ~ 0.02, before droping to almost zero at the limit of solubility. The superconducting volume decreases smoothly with increasing x(Ce). Cerium is non-magnetic and does not suppress superconductivity. Results are explained by a variation of the number of holes due to the substitution Ca2+ ~+ Ce4+. Trivalent rare-earths can substitute calcium in the bismuth cuprates without visible changes of the structure. However, important modifications in the hole carder concentration take place due to the additional electrons and changes in the oxygen content and on the oxidation state of copper and bismuth [1]. A significant increase of Tc occurs at low substitutions, followed by its fast decrease and a metalinsulator transition at x > 0.5. Several works have described :,he effects of rare-earth replacing calcium in the 2212 bismuth compounds, but no much emphasis has been given to their magnetic properties [2,3]. Cerium constitutes a special case since its oxidation state (3+ or 4+) will impose the magnetic phenomena (4f 1 or 4f 0 configuration) and the number of electrons added when replacing Ca2+. Synthesis of Bi2Sr2Cal.xCexCu208+fi was done from oxides and carbonates. The First step of synthesis took place at about 750-800 °C for 14 h, in order to avoid the melting of Bi203, which would result in its inhomogeneous diatribution in the sample. The powders were ground and pressed, after mixing them with a few drops of acetone to increase its weight density and to allow an easy extraction from the die. Each pellet was sintered for one hour at 840 ~'C <_ T _ 920 °C. From the intensities of the (113) and (115) lines of the X-ray diagrams, the proportion of the 2212 phase was determined (fig. 1). It is seen that at low doping, the temperature of synthesis can be increased by about 30 °C, which facilitates the formation of single-phase samples. At high substitutions (x _:20.5), multiphased samples composed of 22!2, 2201 and CeO2 were always oh0921-,1534/94/5()7.00 © 1994- Iitscvicr SS"DI I)921-4534(94)()1036-6
Science
ILV.
All rights
× =03 v ¢"4
~ -
B0
7.,
¢,.q
6O ct, ¢-o &O
7.
/'
(b
j /
/
/
./
/ /,"
IX.
0
>...,O~OL ooo
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Fig. 1. Percentage of the 2212 phase for x(Ce) = 0 and 0.1, after ~ synthesis. mined. Once the optimum temperature was found, the time was increased to 63 h, Single-phase samples were thus oi?.aiaed (fig. 2), with the c-parameter varying linearly from 30,83(1) to 30.57(1) A, while the a-parameter goes from 5.410(2) to 5.438(2) A, for 0 _< x < 0.3. EDS analysis showed a homogeneous distribution of Ce in the sample and in each g~ain. We can conclude that Ce replaces the calcium Site in the 2212 structure although i~s solubility is rather small, restricted to the region 0,0 <_ x <_ 0.3. The superconducting transitions were measured by Xac methods. For x _<0.1 the amplitude of the X' component does not change with the cerium content, and is equal to 100%. Figure 3 shows the Tc °riser for : (a) samples obtained after long-time sintering followed by slow (furnace) cooling ; (b) same samples but further annealed at 460 °C for 2 h under rest r~.cd.
E Jordan et al./Physica C 235-240 (1994) 945-946
946
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.
.
.
.
.
.
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.
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.
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|
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Fig. 2. X-ray diffraction for x(Ce) = 0.1 after 63 h at 880 °C. Peak marked "S" is due to the modulation. vacuum. A few comments can be made : firstly, a slight maximum of Tc is observed for x - 0.02 in the as-grown series ; this maximum levels off after vacuum annealing. Secondly, Tc increases when samples are annealed under vacuum (and decreases later when they are re-annealed in air at 460 °C). This procedure is reversible and reproducible, as confirmed by subsequent annealings in a second series of samples. At higher concentrations (x = 0.2, 0.3), a small decrease of the amplitude was observed with Tc's below 30 K ; when annealed under vacuum, the samples ~,,~,re non-superconducting, indicating ~,at the metal-insulator transition takes place at lowrx x when oxygen is removed. These resclts can be explained on the basis of the hole r' -~sityimposed by both the average valence of copper and the cerium substitution. Several authors have shown a strong correlation of Tc with the hole concentration in the bismuth cuprates [4,5], in spite of the negligible weight variation due to the oxygen uptake [6]. It is well established by now that the 2212 phase (at x = 0) is overdoped with hole carders and it attains a maximum Tc for about 0.2 holes/Cu (Vcu - 2.17). In the underdoped regime, the system turns into insulator when the hole concentration drops down to 0.07 (Vcu - 2.07). Similar conclusions were found when Ca 2+ was substituted by RE 3+, Tc reaching a maximum for x ~ 0.2 [7]. In our case, the number of additional electrons due to the substitution is: larger. If Lhe overall oxygen index is set equal to: 6 + 0.5x + Vcu = 8.17 for trivalent substituents (assuming an. excess of ~ 0 i7 oxygen/formula) then, for tetravalent cerium, it becomes : 6 + x + VCu = 8.17. Applying a copper valence of VCu = 2.07, it flows out that in our case. Tc varies more rapidly and the optimum Tc value is
5
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.
.
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.
.
.
.
.
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x (Ce) Fig. 3. Tc°nset as a function of the Ce content after: a) furnace cooling ; b) vacuum annealing at 460 °C. obtained at smaller x, compared to the case of trivalent lanthanides. Annealings under air or vacuum modify the ovelall oxygen content and change the hole carrier concentration ; as a result, the Tc-vs-x curces are shifted along the transversal axis [21. The magnetic susceptibility and magnetization measurements conf'mned that the electronic configuration of cerium in this material is 4f05d26s2, that is, the tetravalent non-magnetic state. The superconducting volume (dM/dH slope at H -o 0) decreases smoothly with x, indicating that paramagnetism appears when the number of hole carriers decreases. In conclusion, ceritlm substitutes calcium in a narrow domain of solubility (x _'2: 0.3) wi~out suppressing the superconducting state. Due to its tetravalent configuration, the concentration of holes in the Cu-O planes varies very rapidly and a maximum of Tc is attained at very low cerium doping (x ~ 0.02-0.05). R E F E R E N C E S
1. B. Jayaram, P.C. Lanchester, M.T. Weller. "~hysica C 160 (1989) 17. 2. K. Koyama, S. Kanno, S. Noguchi. Jpn. J. Appl. Phys. 29 (1990) L-53 3. E. Laxmi Narsaiah, U.V. Subba Rao, O. Pefia, C. Perrin. Solid State Comm. 83 (1992) 689. 4. C. Namgung, J. Irvine, J. Binks, E. Lachowski, A. WeSL Supercond. Sci. Tech. 2 (1989) 181. 5. T. Takabatake, W. Ye, S. Orimo, H. Kanawaka, H: Fujii etal. Physica C 157 (1989) 263. 6. M.A. Diaia, O. Pefia, C. Pcrrin, M. Sergent. Solid Share Commun. 73 (1990) 715. 7. W.A. Groen, D.M. de Leeuw, L.F. Feiner. Physica C 165 (1990) 55.