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Superstructure in Sr2YCu2FeO6+δ
Physica C 412–414 (2004) 115–119 www.elsevier.com/locate/physc
Superstructure in Sr2YCu2FeO6þd T. Mochiku a,*, Y. Nakano b, A. Hoshikawa c, S. Sato d, K. Oikawa T. Ishigaki d, T. Kamiyama c, K. Kadowaki b, K. Hirata a
c,1
,
a
d
Superconducting Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Institute of Materials Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8573, Japan c Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Ooho, Tsukuba, Ibaraki 305-0801, Japan Department of Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumoto-chou, Muroran, Hokkaido 050-8585, Japan Received 29 October 2003; accepted 19 January 2004 Available online 7 May 2004
Abstract Sr2 YCu2 FeO6þd exhibits superconductivity around 50 K, when it is properly annealed in N2 atmosphere and subsequently in O2 atmosphere under ambient and high pressure. From electron diffraction study streaks were observed on the N2 -annealed samples, which suggests existing some kind of atomic ordering. We have optimized the annealing condition for the study of the atomic ordering, and found a new phase with superstructure in the N2 -annealed sample. This indicates that the atomic ordering is strongly correlated with the N2 -annealing. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process. The in-situ study revealed that the N2 -annealing at 600 °C promoted the atomic ordering of Cu and Fe along the c-axis and that the ordering did not change at all during the following annealing process. We have also discovered beginning of the transformation occurred from tetragonal structure to orthorhombic superstructure at 750 °C in N2 and the superstructure remained until temperature returned to room temperature. These results indicate that the transformation to the superstructure is irreversible. In addition, the following O2 -annealing changed the crystal system to tetragonal with keeping the atomic ordering of Cu and Fe. This annealing supplies oxygen and changes the coordination around Fe, which might be related with the emergence of superconductivity. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Jt Keywords: Sr2 YCu2 FeO6 þ d ; Superstructure; Neutron powder diffraction
1. Introduction *
Corresponding author. Tel.: +81-29-851-3354x6630/8592000; fax: +81-29-859-2301. E-mail address: [email protected] (T. Mochiku). 1 Present address: Tokai Research Establishment, Japan Atomic Energy Research Institute, 2-4 Shirakata-Shirane, Tokai, Ibaraki 319-1195, Japan.
Recently, superconductivity has been discovered around 50 K in Sr2 YCu2 FeO6þd , annealed in an N2 atmosphere, and subsequently in a high pressure O2 atmosphere [1,2]. To understand what
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.01.042
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T. Mochiku et al. / Physica C 412–414 (2004) 115–119
condition is necessary for superconductivity, we have performed neutron powder diffraction to investigate the crystal structure of Sr2 YCu2 FeO6þd by comparing the crystal structures at each sintering process [2], since neutron diffraction can distinguish positions between Cu and Fe, compared with X-ray diffraction. From the neutron diffraction study it was found that the N2 -annealing causes atomic ordering of Cu and Fe, and that the O2 -annealing supplies carriers on the CuO2 sheets to exhibit superconductivity. Therefore, the superconducting sample consists of ordered stacking of sheets with a sequence (FeOd )o (SrO)c (CuO2 )o (Y)c (CuO2 )o (SrO)c according to a descriptive method proposed by Santoro et al. [3], where the subscripts o and c stand for whether cation on the each slice is at the origin or at the center of the sheet. From electron diffraction study the streaks were observed for the N2 -annealed Sr2 YCu2 FeO6þd , which suggested some kind of atomic ordering. After we have optimized the annealing condition for the study of atomic ordering, we have recently discovered a new phase with a superstructure in the N2 -annealed Sr2 YCu2 FeO6þd and analyzed its crystal structure using neutron powder diffraction [4]. The superstructure has the FeO4 tetrahedron instead of the oxygen-deficient FeO6 octahedron. The formation of the FeO4 tetrahedron needs not only the atomic ordering of Cu and Fe but also the oxygen ordering on the FeOd sheet. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process.
2. Experimental The samples were prepared by the solid-state reaction of stoichiometric mixture of SrCO3 , Y2 O3 , Fe2 O3 and CuO powders. The mixture was calcined at 900 °C for 24 h in air, ground and then pressed into pellets. The pellets were sintered at 1000 °C for 24 h in air. The as-synthesized sample was used for in-situ neutron powder diffraction study. To exhibit superconductivity the following process is needed: the as-synthesized sample was
subsequently annealed at 750 °C for 24 h in an N2 flow, at 300 °C for 24 h in an O2 flow, and finally at 350 °C for 24 h in high oxygen pressure of 195 atm. Neutron powder diffraction data for Sr2 YCu2 FeO6þd were taken with a time-of-flight (TOF) high-resolution and high-intensity diffractometer, Sirius, at the KENS pulsed spallation neutron source in the High Energy Accelerator Research Organization. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process using a high-temperature furnace with a gascontrol device [5]. The intensity data were collected by 90° detector bank at 600, 650, 700, 750, 800 °C and room temperature in N2 , and then without breaking the sample setting, at 300 °C and room temperature in O2 . The temperature condition was decided with analyzing the crystal structure. While the superstructure was observed at 750 °C at the annealing process to exhibit superconductivity [4], it was observed at 750 and 800 °C at the in-situ neutron powder diffraction measurement. The structure of the Sr2 YCu2 FeO6þd compound was refined using the Rietveld program RIETAN [6] on the basis of the tetragonal Ba2 YCu3 O6þd type structure model and the orthorhombic Sr2 YCu2 CoO7 -type superstructure model [7]. The crystal structures and the atomic coordinates are shown in Fig. 1 and Table 1, respectively. The coherent scattering lengths, b, used for the Rietveld refinement, were 7.02 fm (Sr), 7.75 fm (Y), 7.718 fm (Cu), 9.45 fm (Fe) and 5.805 fm (O). In the refinement, four sites were assigned for Cu and Fe: 1a site for Cu(1) and Fe(1), and 2g site for Cu(2) and Fe(1) in P4/mmm; 4b site for Cu(1) and Fe(1), and 8c site for Cu(2) and Fe(1) in Ima2. The occupation factors, g, of the Cu(1), Fe(1), Cu(2) and Fe(2) sites have the liner constraints: gCuð1Þ ¼ 2gFeð2Þ
ð1Þ
gFeð1Þ ¼ 1 2gFeð2Þ
ð2Þ
gCuð2Þ ¼ 1 gFeð2Þ
ð3Þ
Therefore, only gFeð2Þ was refined under the liner equality constraints, and the other g values were
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
117
Fig. 1. Crystal structure of Sr2 YCu2 FeO6þd with (a) tetragonal cell (P4/mmm) and (b) orthorhombic cell (Ima2). Table 1 The atomic coordinate of (Panel A) the tetragonal Ba2 YCu3 O6þd -type structure model and (Panel B) the orthorhombic Sr2 YCu2 CoO7 -type superstructure model Atom
Site
x
y
z
Panel A: Ba2 YCu3 O6þd -type structure (tetragonal, P4/mmm, Z ¼ 1) Sr 2h 1/2 1/2 z Y 1d 1/2 1/2 1/2 Cu(1), Fe(1) 1a 0 0 0 Cu(2), Fe(2) 2g 0 0 z O(1) 2f 0 1/2 0 O(2) 2g 0 0 z O(3) 4l 0 1/2 z Panel B: Sr2 YCu2 CoO7 -type Z ¼ 4) Sr 8c Y 4a Cu(1), Fe(1) 4b Cu(2), Fe(2) 8c O(1) 4b O(2) 8c O(3) 8c O(30 ) 8c
structure (orthorhombic, Ima2, x 0 1/4 x 1/4 x x x
y 0 y y y y y y
z 0 z z z z z z
calculated from gFeð2Þ . The occupation factor of the O(1) site, gOð1Þ was also refined and the oxygen content, 6 þ d, was calculated from gOð1Þ under the following equation: 6 þ d ¼ 4 þ 2gOð1Þ
ð4Þ
3. Results and discussion In Table 2 the final R factors, the lattice parameters and their deviation are listed. The R factors indicate that the structure model is supported by the good fit between the observed and calculated patterns. The R factors after returning to room temperature in N2 are deteriorated in comparison with that before 800 °C in N2 because the annealing time, which is almost equivalent to the measurement time of the neutron diffraction, is much shorter than that of the conventional
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T. Mochiku et al. / Physica C 412–414 (2004) 115–119
Table 2 R factors and lattice parameters during the annealing process Atmosphere
Temperature
N2 N2 N2 N2 N2 N2 O2 O2
600 650 700 750 800 RT 300 RT
°C °C °C °C °C °C
Rwp 3.78% 4.96% 4.46% 4.30% 4.42% 5.28% 7.67% 8.19%
S 2.16 2.77 2.29 2.38 2.40 3.14 4.60 4.51
a/nm 0.38576(1) 0.38620(2) 0.38658(1) 2.31528(11) 2.31781(11) 2.29291(10) 0.38326(2) 0.38186(1)
b/nm – – – 0.54799(4) 0.54992(3) 0.54533(3) – –
c/nm 1.15136(4) 1.15448(6) 1.15616(2) 0.54671(3) 0.54657(3) 0.54123(3) 1.14005(6) 1.13563(3)
Numbers in parentheses are estimated standard deviations of the last significant digit.
Fig. 2. Change of the lattice parameters, a, b and c, the occupation factors of Cu at the Cu(1) and Cu(2) site, gCuð1Þ and gCuð2Þ and the oxygen content, 6 þ d, along the annealing process. The lattice parameters of the orthorhombic cell were converted to those corresponding the tetragonal cell.
synthesis to exhibit superconductivity. Fig. 2 shows that the lattice and the typical structure
parameters are strongly correlative to the annealing process. As temperature increases in N2 , all lattice parameters increases and the crystal structure transforms from the tetragonal structure to the orthorhombic structure at 750 °C. Although the occupation factors gCuð1Þ and gCuð2Þ are refined, we obtain no occupancy of Cu for the Cu(1) site and full occupancy of Cu for the Cu(2) site at 600 °C within the standard deviations. These do not change at all during the following annealing process. This shows that the atomic ordering of Cu and Fe has been already almost completed at 600 °C in N2 . The oxygen content, 6 þ d, decreases at 650 °C in N2 from 7.44 to 6.98 and is almost constant at 7 until temperature returns to room temperature in N2 . This indicates that the oxygen coordination around Fe transforms from the FeO6 octahedrontype coordination to the FeO4 tetrahedron-type coordination. However, the arrangement of the FeO4 tetrahedron does not occur or is not finished between 650 and 700 °C, because the crystal structure has not been transformed to the orthorhombic structure yet. The arrangement of the FeO4 tetrahedron begins from 750 °C in N2 , and it causes the formation of orthorhombic superp p structure with the unit cell of 2a 2a 2c as compared with tetragonal structure with the unit cell of a a c. This is consistent with the electron diffraction study, which shows that the streaks due to the incomplete arrangement of the FeO4 tetrahedron were observed and the weak spots due to the complete arrangement of the FeO4 tetrahedron were observed [4]. The N2 -annealing promotes the atomic ordering of Cu and Fe by the formation of the FeO4 tetrahedron, and then the
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
temperature above 750 °C fixes the superstructure due to forming the arrangement of the FeO4 tetrahedron. The refinements reveal that the Fe and O(1) atoms in the FeO4 tetrahedron have extremely large thermal parameters. Those suggest that there is a possibility that two kinds of FeO4 tetrahedra which rotate clockwise and counterclockwise around the a-axis as seen in the structure at room temperature [4]. The superstructure remains until temperature returned to room temperature. These results indicate that the transformation to the superstructure is irreversible. After the process in N2 , the process in O2 causes the transformation from the superstructure to the tetragonal structure, which is similar to the structure between 600 and 650 °C in N2 . However, its structure is different from that of as-synthesized sample. Although the structure of the as-synthesized sample has mutual substitution of Cu and Fe [2], the structure during the process in O2 has the atomic ordering of Cu and Fe, because the occupation factors, gCuð1Þ and gCuð2Þ do not change compared to those of the superstructure under the process in N2 . The lattice parameters decrease and the oxygen content, 6 þ d, increases in O2 . Those indicate that the process in O2 supplies oxygen with returning the oxygen coordination around Fe to the FeO6 octahedron-type coordination and keeping the atomic ordering of Cu and Fe. Although oxygen can not be supplied to the FeO4 tetrahedron-type structure without the extra oxygen site, oxygen can be supplied to the FeO6 octahedron-type structure with deficient oxygen site. The increase of the oxygen content causes the increase of the valences of the Cu and Fe atoms. Therefore, the process in O2 after the process in N2
119
supplies enough amount of carrier onto the CuO2 sheet to exhibit superconductivity, where the formation of the FeO4 tetrahedron avoids the substitution of Fe for Cu on. 4. Conclusions To summarize, we have performed in-situ hightemperature neutron powder diffraction measurements of Sr2 YCu2 FeO6þd to understand the mechanism of the atomic ordering. The atomic ordering of Cu and Fe due to the formation of the FeO4 tetrahedron is indispensable for exhibiting superconductivity in Sr2 YCu2 FeO6þd . The formation of the superstructure due to the arrangement of the FeO4 tetrahedron is important to stabilize the atomic ordering of Cu and Fe because the transformation to the superstructure is irreversible.
References [1] J. Shimoyama, K. Otzschi, T. Hinouchi, K. Kishio, Physica C 341–348 (2000) 563. [2] T. Mochiku, Y. Mihara, Y. Hata, S. Kamisawa, M. Furuyama, J. Suzuki, K. Kadowaki, N. Metoki, H. Fujii, K. Hirata, J. Phys. Soc. Jpn. 71 (2002) 790. [3] A. Santoro, F. Beech, M. Marezio, R.J. Cava, Physica C 156 (1988) 693. [4] T. Mochiku, Y. Nakano, K. Oikawa, T. Kamiyama, H. Fujii, Y. Hata, J. Suzuki, I. Kakeya, K. Kadowaki, K. Hirata, Physica C 400 (2003) 43. [5] K. Oikawa, T. Kamiyama, S. Torii, T. Mochiku, Appl. Phys. A 74 (Suppl.) (2002) S267. [6] F. Izumi, T. Ikeda, Mater. Sci. Forum 321–324 (2002) 198. [7] Q. Huang, R.J. Cava, A. Santoro, J.J. Krajewski, W.F. Peck, Physica C 193 (1992) 196.
Superstructure in Sr2YCu2FeO6þd T. Mochiku a,*, Y. Nakano b, A. Hoshikawa c, S. Sato d, K. Oikawa T. Ishigaki d, T. Kamiyama c, K. Kadowaki b, K. Hirata a
c,1
,
a
d
Superconducting Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Institute of Materials Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8573, Japan c Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Ooho, Tsukuba, Ibaraki 305-0801, Japan Department of Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumoto-chou, Muroran, Hokkaido 050-8585, Japan Received 29 October 2003; accepted 19 January 2004 Available online 7 May 2004
Abstract Sr2 YCu2 FeO6þd exhibits superconductivity around 50 K, when it is properly annealed in N2 atmosphere and subsequently in O2 atmosphere under ambient and high pressure. From electron diffraction study streaks were observed on the N2 -annealed samples, which suggests existing some kind of atomic ordering. We have optimized the annealing condition for the study of the atomic ordering, and found a new phase with superstructure in the N2 -annealed sample. This indicates that the atomic ordering is strongly correlated with the N2 -annealing. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process. The in-situ study revealed that the N2 -annealing at 600 °C promoted the atomic ordering of Cu and Fe along the c-axis and that the ordering did not change at all during the following annealing process. We have also discovered beginning of the transformation occurred from tetragonal structure to orthorhombic superstructure at 750 °C in N2 and the superstructure remained until temperature returned to room temperature. These results indicate that the transformation to the superstructure is irreversible. In addition, the following O2 -annealing changed the crystal system to tetragonal with keeping the atomic ordering of Cu and Fe. This annealing supplies oxygen and changes the coordination around Fe, which might be related with the emergence of superconductivity. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Jt Keywords: Sr2 YCu2 FeO6 þ d ; Superstructure; Neutron powder diffraction
1. Introduction *
Corresponding author. Tel.: +81-29-851-3354x6630/8592000; fax: +81-29-859-2301. E-mail address: [email protected] (T. Mochiku). 1 Present address: Tokai Research Establishment, Japan Atomic Energy Research Institute, 2-4 Shirakata-Shirane, Tokai, Ibaraki 319-1195, Japan.
Recently, superconductivity has been discovered around 50 K in Sr2 YCu2 FeO6þd , annealed in an N2 atmosphere, and subsequently in a high pressure O2 atmosphere [1,2]. To understand what
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.01.042
116
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
condition is necessary for superconductivity, we have performed neutron powder diffraction to investigate the crystal structure of Sr2 YCu2 FeO6þd by comparing the crystal structures at each sintering process [2], since neutron diffraction can distinguish positions between Cu and Fe, compared with X-ray diffraction. From the neutron diffraction study it was found that the N2 -annealing causes atomic ordering of Cu and Fe, and that the O2 -annealing supplies carriers on the CuO2 sheets to exhibit superconductivity. Therefore, the superconducting sample consists of ordered stacking of sheets with a sequence (FeOd )o (SrO)c (CuO2 )o (Y)c (CuO2 )o (SrO)c according to a descriptive method proposed by Santoro et al. [3], where the subscripts o and c stand for whether cation on the each slice is at the origin or at the center of the sheet. From electron diffraction study the streaks were observed for the N2 -annealed Sr2 YCu2 FeO6þd , which suggested some kind of atomic ordering. After we have optimized the annealing condition for the study of atomic ordering, we have recently discovered a new phase with a superstructure in the N2 -annealed Sr2 YCu2 FeO6þd and analyzed its crystal structure using neutron powder diffraction [4]. The superstructure has the FeO4 tetrahedron instead of the oxygen-deficient FeO6 octahedron. The formation of the FeO4 tetrahedron needs not only the atomic ordering of Cu and Fe but also the oxygen ordering on the FeOd sheet. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process.
2. Experimental The samples were prepared by the solid-state reaction of stoichiometric mixture of SrCO3 , Y2 O3 , Fe2 O3 and CuO powders. The mixture was calcined at 900 °C for 24 h in air, ground and then pressed into pellets. The pellets were sintered at 1000 °C for 24 h in air. The as-synthesized sample was used for in-situ neutron powder diffraction study. To exhibit superconductivity the following process is needed: the as-synthesized sample was
subsequently annealed at 750 °C for 24 h in an N2 flow, at 300 °C for 24 h in an O2 flow, and finally at 350 °C for 24 h in high oxygen pressure of 195 atm. Neutron powder diffraction data for Sr2 YCu2 FeO6þd were taken with a time-of-flight (TOF) high-resolution and high-intensity diffractometer, Sirius, at the KENS pulsed spallation neutron source in the High Energy Accelerator Research Organization. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process using a high-temperature furnace with a gascontrol device [5]. The intensity data were collected by 90° detector bank at 600, 650, 700, 750, 800 °C and room temperature in N2 , and then without breaking the sample setting, at 300 °C and room temperature in O2 . The temperature condition was decided with analyzing the crystal structure. While the superstructure was observed at 750 °C at the annealing process to exhibit superconductivity [4], it was observed at 750 and 800 °C at the in-situ neutron powder diffraction measurement. The structure of the Sr2 YCu2 FeO6þd compound was refined using the Rietveld program RIETAN [6] on the basis of the tetragonal Ba2 YCu3 O6þd type structure model and the orthorhombic Sr2 YCu2 CoO7 -type superstructure model [7]. The crystal structures and the atomic coordinates are shown in Fig. 1 and Table 1, respectively. The coherent scattering lengths, b, used for the Rietveld refinement, were 7.02 fm (Sr), 7.75 fm (Y), 7.718 fm (Cu), 9.45 fm (Fe) and 5.805 fm (O). In the refinement, four sites were assigned for Cu and Fe: 1a site for Cu(1) and Fe(1), and 2g site for Cu(2) and Fe(1) in P4/mmm; 4b site for Cu(1) and Fe(1), and 8c site for Cu(2) and Fe(1) in Ima2. The occupation factors, g, of the Cu(1), Fe(1), Cu(2) and Fe(2) sites have the liner constraints: gCuð1Þ ¼ 2gFeð2Þ
ð1Þ
gFeð1Þ ¼ 1 2gFeð2Þ
ð2Þ
gCuð2Þ ¼ 1 gFeð2Þ
ð3Þ
Therefore, only gFeð2Þ was refined under the liner equality constraints, and the other g values were
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
117
Fig. 1. Crystal structure of Sr2 YCu2 FeO6þd with (a) tetragonal cell (P4/mmm) and (b) orthorhombic cell (Ima2). Table 1 The atomic coordinate of (Panel A) the tetragonal Ba2 YCu3 O6þd -type structure model and (Panel B) the orthorhombic Sr2 YCu2 CoO7 -type superstructure model Atom
Site
x
y
z
Panel A: Ba2 YCu3 O6þd -type structure (tetragonal, P4/mmm, Z ¼ 1) Sr 2h 1/2 1/2 z Y 1d 1/2 1/2 1/2 Cu(1), Fe(1) 1a 0 0 0 Cu(2), Fe(2) 2g 0 0 z O(1) 2f 0 1/2 0 O(2) 2g 0 0 z O(3) 4l 0 1/2 z Panel B: Sr2 YCu2 CoO7 -type Z ¼ 4) Sr 8c Y 4a Cu(1), Fe(1) 4b Cu(2), Fe(2) 8c O(1) 4b O(2) 8c O(3) 8c O(30 ) 8c
structure (orthorhombic, Ima2, x 0 1/4 x 1/4 x x x
y 0 y y y y y y
z 0 z z z z z z
calculated from gFeð2Þ . The occupation factor of the O(1) site, gOð1Þ was also refined and the oxygen content, 6 þ d, was calculated from gOð1Þ under the following equation: 6 þ d ¼ 4 þ 2gOð1Þ
ð4Þ
3. Results and discussion In Table 2 the final R factors, the lattice parameters and their deviation are listed. The R factors indicate that the structure model is supported by the good fit between the observed and calculated patterns. The R factors after returning to room temperature in N2 are deteriorated in comparison with that before 800 °C in N2 because the annealing time, which is almost equivalent to the measurement time of the neutron diffraction, is much shorter than that of the conventional
118
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
Table 2 R factors and lattice parameters during the annealing process Atmosphere
Temperature
N2 N2 N2 N2 N2 N2 O2 O2
600 650 700 750 800 RT 300 RT
°C °C °C °C °C °C
Rwp 3.78% 4.96% 4.46% 4.30% 4.42% 5.28% 7.67% 8.19%
S 2.16 2.77 2.29 2.38 2.40 3.14 4.60 4.51
a/nm 0.38576(1) 0.38620(2) 0.38658(1) 2.31528(11) 2.31781(11) 2.29291(10) 0.38326(2) 0.38186(1)
b/nm – – – 0.54799(4) 0.54992(3) 0.54533(3) – –
c/nm 1.15136(4) 1.15448(6) 1.15616(2) 0.54671(3) 0.54657(3) 0.54123(3) 1.14005(6) 1.13563(3)
Numbers in parentheses are estimated standard deviations of the last significant digit.
Fig. 2. Change of the lattice parameters, a, b and c, the occupation factors of Cu at the Cu(1) and Cu(2) site, gCuð1Þ and gCuð2Þ and the oxygen content, 6 þ d, along the annealing process. The lattice parameters of the orthorhombic cell were converted to those corresponding the tetragonal cell.
synthesis to exhibit superconductivity. Fig. 2 shows that the lattice and the typical structure
parameters are strongly correlative to the annealing process. As temperature increases in N2 , all lattice parameters increases and the crystal structure transforms from the tetragonal structure to the orthorhombic structure at 750 °C. Although the occupation factors gCuð1Þ and gCuð2Þ are refined, we obtain no occupancy of Cu for the Cu(1) site and full occupancy of Cu for the Cu(2) site at 600 °C within the standard deviations. These do not change at all during the following annealing process. This shows that the atomic ordering of Cu and Fe has been already almost completed at 600 °C in N2 . The oxygen content, 6 þ d, decreases at 650 °C in N2 from 7.44 to 6.98 and is almost constant at 7 until temperature returns to room temperature in N2 . This indicates that the oxygen coordination around Fe transforms from the FeO6 octahedrontype coordination to the FeO4 tetrahedron-type coordination. However, the arrangement of the FeO4 tetrahedron does not occur or is not finished between 650 and 700 °C, because the crystal structure has not been transformed to the orthorhombic structure yet. The arrangement of the FeO4 tetrahedron begins from 750 °C in N2 , and it causes the formation of orthorhombic superp p structure with the unit cell of 2a 2a 2c as compared with tetragonal structure with the unit cell of a a c. This is consistent with the electron diffraction study, which shows that the streaks due to the incomplete arrangement of the FeO4 tetrahedron were observed and the weak spots due to the complete arrangement of the FeO4 tetrahedron were observed [4]. The N2 -annealing promotes the atomic ordering of Cu and Fe by the formation of the FeO4 tetrahedron, and then the
T. Mochiku et al. / Physica C 412–414 (2004) 115–119
temperature above 750 °C fixes the superstructure due to forming the arrangement of the FeO4 tetrahedron. The refinements reveal that the Fe and O(1) atoms in the FeO4 tetrahedron have extremely large thermal parameters. Those suggest that there is a possibility that two kinds of FeO4 tetrahedra which rotate clockwise and counterclockwise around the a-axis as seen in the structure at room temperature [4]. The superstructure remains until temperature returned to room temperature. These results indicate that the transformation to the superstructure is irreversible. After the process in N2 , the process in O2 causes the transformation from the superstructure to the tetragonal structure, which is similar to the structure between 600 and 650 °C in N2 . However, its structure is different from that of as-synthesized sample. Although the structure of the as-synthesized sample has mutual substitution of Cu and Fe [2], the structure during the process in O2 has the atomic ordering of Cu and Fe, because the occupation factors, gCuð1Þ and gCuð2Þ do not change compared to those of the superstructure under the process in N2 . The lattice parameters decrease and the oxygen content, 6 þ d, increases in O2 . Those indicate that the process in O2 supplies oxygen with returning the oxygen coordination around Fe to the FeO6 octahedron-type coordination and keeping the atomic ordering of Cu and Fe. Although oxygen can not be supplied to the FeO4 tetrahedron-type structure without the extra oxygen site, oxygen can be supplied to the FeO6 octahedron-type structure with deficient oxygen site. The increase of the oxygen content causes the increase of the valences of the Cu and Fe atoms. Therefore, the process in O2 after the process in N2
119
supplies enough amount of carrier onto the CuO2 sheet to exhibit superconductivity, where the formation of the FeO4 tetrahedron avoids the substitution of Fe for Cu on. 4. Conclusions To summarize, we have performed in-situ hightemperature neutron powder diffraction measurements of Sr2 YCu2 FeO6þd to understand the mechanism of the atomic ordering. The atomic ordering of Cu and Fe due to the formation of the FeO4 tetrahedron is indispensable for exhibiting superconductivity in Sr2 YCu2 FeO6þd . The formation of the superstructure due to the arrangement of the FeO4 tetrahedron is important to stabilize the atomic ordering of Cu and Fe because the transformation to the superstructure is irreversible.
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