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Crystal structure and magnetism of the double perovskites A2FeReO6 (A=Ca, Sr, Ba)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e607–e608
Crystal structure and magnetism of the double perovskites A2FeReO6 (A=Ca, Sr, Ba) N. Autha,*, G. Jakoba, W. Westerburga, C. Ritterb, I. Bonnc, C. Felserc, W. Tremelc Institut fur . Physik, Johannes Gutenberg-Universitat . Mainz, Staudinger Weg 7, Mainz 55099, Germany b Institute Laue Langevin, 6, rue Jules Horowitz, Bo#ıte Postale 156, Grenoble-Cedex 9 38042, France c Institut fur . Anorganische Chemie und Analytische Chemie, Universitat . Mainz, Mainz 55099, Germany
a
Abstract We synthesized a series of double perovskites A2 FeReO6 (A=Ca, Sr, Ba) with Curie temperatures above roomtemperature. Neutron and X-ray diffraction analysis have been performed in order to determine the structural and (local) magnetic properties of these materials. While Ba2 FeReO6 stays cubic over the whole temperature range we examined, the Sr-compound shows a tetragonal distortion of the perovskite structure which does not completely vanish up to about 520 K far above TC : Ca2 FeReO6 has a monoclinic unit cell at high temperatures. Below 400 K a phase separation in two monoclinic phases with identical cell volume is observed in neutron scattering. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.q; 75.50.Gg; 75.25.þz Keywords: Double perovskite; Neutron diffraction; Neutron scattering; Ferrimagnetism
The discovery of large magnetoresistivity in Sr2 FeB0 O6 (B0 =Mo, Re) raised renewed interest in double perovskites [1]. The B, B’ ions in the double perovskite structure A2 BB0 O6 can form an ordered sublattice with rock-salt structure. In this case half metallic behavior is possible due to a strong Hund’s rule coupling and a double exchange like interaction between B and B0 sites. To match application requirements we focus on compounds with high Curie temperatures (TC ). In the series Ca2 FeReO6 ; Sr2 FeReO6 and Ba2 FeReO6 the TC decrease from 540, 400 to 340 K. Increasing Asite cation radii lead to tolerance factors of 0.89, 0.94, and 0.99, respectively. The samples were synthesized by sintering stoichiometric mixtures of the corresponding oxides and metals in evacuated quartz tubes at 950 C for several days. In previous neutron diffraction measurements we found the Ca-compound to order ferrimagnetically and to crystal*Corresponding author. Tel.: +49-6131-39-23313; fax: +496131-39-24076. E-mail address: [email protected] (N. Auth).
lize with a monoclinic distortion due to the small tolerance factor of the perovskite structure [2]. To investigate the crystal and spin structure of the Ba- and Sr-compounds neutron diffraction experiments (Fig. 1) were carried out at the Institute Laue Langevin (D2B). The data were refined by the Rietveld method using the program FULLPROF [3]. Ba2 FeReO6 stays cubic (Fm3% m) over the examined ( a(30 K)= temperature range (a(370 K)=8.0598(1) A, ( At 30 K the oxygen polyhedra enclosing 8.0331(1) A). the Fe (Re) ions are enlarged (shrinked) by 2.5% (oxygen position (0,0,0.2563)) compared to the symmetric situation (O at (0,0,0.25)). The magnetic moments are 3.96(11) mB and 0:39(12) mB at the Fe and Re positions, respectively. In contrast the Sr-compound could be fitted best with the tetragonal I4/m space group ( c=7.9063(2) A). ( The octahe(at 1.5 K: a=5.5425(1) A, dra with Fe and Re centres show an alternating rotation around the c-axis (a0 a0 c in Glazer’s notation [4]), which was not visible in previous X-ray investigations [5]. At T ¼ 1:5 K the rotation angle is 6.5(1) leading to a Fe–O–Re bond angle of 167 : As is visible in Fig. 2 the
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.484
ARTICLE IN PRESS N. Auth et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e607–e608
e608
5000 4000
intensity [a.u.]
Table 1 Atomic positions of Sr2 FeReO6 (I4/m) at 1.5 K. The structural (magnetic) Bragg-R-factor is 3.6% (4.4%).
Sr2 FeReO6 T = 452K
3000
Atom
Site
x
y
2000
Sr Fe Re O1 O2
4d 2a 2b 4e 8h
0 0 0 0 0.2161(7)
1 2
1 4
0 0 0 0.2718(8)
0
1000 0
z
1 2
mB 4.19(11) 0.22(16)
0.2576(8) 0
-1000 20
40
60
80 100 20 [˚]
120
140
160
Fe-O-Re bond angle [degree]
Fig. 1. Neutron diffraction pattern for Sr2 FeReO6 : The refinement includes also 2% magnetite impurity phase.
178 176 174 172 170 168 166 0
2
4
400
450
500
550
600
Temperature [K] Fig. 2. Fe–O–Re bond angle as function of temperature. The inset shows the octahedra rotation at 1.5 K.
magnitude of this distortion drops with rising temperature. While the difference of the octahedra volumes at Fe and Re positions vanishes above TC ; the rotational distortion is clearly present throughout the whole measurement range of up to 555 K (1.3 ). Thus, the ferromagnetic transition does not coincide with a symmetry change of the lattice. In the refinement of the magnetism a ferrimagnetic alignment of the Fe-, Re-sites is found. As a magnetic
form factor for Re is not available in literature we used that for Mo3þ [6]. Due to the similarity of the scattering lengths of Fe and Re for neutrons additional X-ray measurements were performed to verify Fe, Re positional order. Starting from a disordered arrangement the refinement process leads to a remaining disorder of about 4%. We note that the expected error in the X-ray refinements is bigger compared to the neutron case due to the strong absorption of the rhenium atoms. We investigated the crystallographic and magnetic structure in the series of double perovskites A2 FeReO6 (A=Ca, Sr, Ba). With increasing tolerance factor the symmetry changes from monoclinic P21 /n, to tetragonal I4/m, to cubic Fm3% m. While the size of the A site cation strongly influences the crystallographic structure we find for all compound a ferrimagnetic ordering on Fe and Re positions with small moments at the Re-site (Table 1).
References [1] K. Kobayashi, et al., Nature 395 (1998) 677; K. Kobayashi, et al., Phys. Rev. B 59 (1999) 11159. [2] W. Westerburg, et al., Solid State Commun. 122 (2002) 201. [3] J. Rodr!ıguez-Carvajal, Physica B 192 (1993) 55. [4] A.M. Glazer, Acta Crystallogr. B 28 (1972) 3384. [5] T. Alamelu, et al., J. Appl. Phys. 91 (2002) 8909. [6] M.K. Wilkinson, E.O. Wollan, H.R. Child, J.W. Cable, Phys. Rev. 121 (1961) 74.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e607–e608
Crystal structure and magnetism of the double perovskites A2FeReO6 (A=Ca, Sr, Ba) N. Autha,*, G. Jakoba, W. Westerburga, C. Ritterb, I. Bonnc, C. Felserc, W. Tremelc Institut fur . Physik, Johannes Gutenberg-Universitat . Mainz, Staudinger Weg 7, Mainz 55099, Germany b Institute Laue Langevin, 6, rue Jules Horowitz, Bo#ıte Postale 156, Grenoble-Cedex 9 38042, France c Institut fur . Anorganische Chemie und Analytische Chemie, Universitat . Mainz, Mainz 55099, Germany
a
Abstract We synthesized a series of double perovskites A2 FeReO6 (A=Ca, Sr, Ba) with Curie temperatures above roomtemperature. Neutron and X-ray diffraction analysis have been performed in order to determine the structural and (local) magnetic properties of these materials. While Ba2 FeReO6 stays cubic over the whole temperature range we examined, the Sr-compound shows a tetragonal distortion of the perovskite structure which does not completely vanish up to about 520 K far above TC : Ca2 FeReO6 has a monoclinic unit cell at high temperatures. Below 400 K a phase separation in two monoclinic phases with identical cell volume is observed in neutron scattering. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.q; 75.50.Gg; 75.25.þz Keywords: Double perovskite; Neutron diffraction; Neutron scattering; Ferrimagnetism
The discovery of large magnetoresistivity in Sr2 FeB0 O6 (B0 =Mo, Re) raised renewed interest in double perovskites [1]. The B, B’ ions in the double perovskite structure A2 BB0 O6 can form an ordered sublattice with rock-salt structure. In this case half metallic behavior is possible due to a strong Hund’s rule coupling and a double exchange like interaction between B and B0 sites. To match application requirements we focus on compounds with high Curie temperatures (TC ). In the series Ca2 FeReO6 ; Sr2 FeReO6 and Ba2 FeReO6 the TC decrease from 540, 400 to 340 K. Increasing Asite cation radii lead to tolerance factors of 0.89, 0.94, and 0.99, respectively. The samples were synthesized by sintering stoichiometric mixtures of the corresponding oxides and metals in evacuated quartz tubes at 950 C for several days. In previous neutron diffraction measurements we found the Ca-compound to order ferrimagnetically and to crystal*Corresponding author. Tel.: +49-6131-39-23313; fax: +496131-39-24076. E-mail address: [email protected] (N. Auth).
lize with a monoclinic distortion due to the small tolerance factor of the perovskite structure [2]. To investigate the crystal and spin structure of the Ba- and Sr-compounds neutron diffraction experiments (Fig. 1) were carried out at the Institute Laue Langevin (D2B). The data were refined by the Rietveld method using the program FULLPROF [3]. Ba2 FeReO6 stays cubic (Fm3% m) over the examined ( a(30 K)= temperature range (a(370 K)=8.0598(1) A, ( At 30 K the oxygen polyhedra enclosing 8.0331(1) A). the Fe (Re) ions are enlarged (shrinked) by 2.5% (oxygen position (0,0,0.2563)) compared to the symmetric situation (O at (0,0,0.25)). The magnetic moments are 3.96(11) mB and 0:39(12) mB at the Fe and Re positions, respectively. In contrast the Sr-compound could be fitted best with the tetragonal I4/m space group ( c=7.9063(2) A). ( The octahe(at 1.5 K: a=5.5425(1) A, dra with Fe and Re centres show an alternating rotation around the c-axis (a0 a0 c in Glazer’s notation [4]), which was not visible in previous X-ray investigations [5]. At T ¼ 1:5 K the rotation angle is 6.5(1) leading to a Fe–O–Re bond angle of 167 : As is visible in Fig. 2 the
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.484
ARTICLE IN PRESS N. Auth et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e607–e608
e608
5000 4000
intensity [a.u.]
Table 1 Atomic positions of Sr2 FeReO6 (I4/m) at 1.5 K. The structural (magnetic) Bragg-R-factor is 3.6% (4.4%).
Sr2 FeReO6 T = 452K
3000
Atom
Site
x
y
2000
Sr Fe Re O1 O2
4d 2a 2b 4e 8h
0 0 0 0 0.2161(7)
1 2
1 4
0 0 0 0.2718(8)
0
1000 0
z
1 2
mB 4.19(11) 0.22(16)
0.2576(8) 0
-1000 20
40
60
80 100 20 [˚]
120
140
160
Fe-O-Re bond angle [degree]
Fig. 1. Neutron diffraction pattern for Sr2 FeReO6 : The refinement includes also 2% magnetite impurity phase.
178 176 174 172 170 168 166 0
2
4
400
450
500
550
600
Temperature [K] Fig. 2. Fe–O–Re bond angle as function of temperature. The inset shows the octahedra rotation at 1.5 K.
magnitude of this distortion drops with rising temperature. While the difference of the octahedra volumes at Fe and Re positions vanishes above TC ; the rotational distortion is clearly present throughout the whole measurement range of up to 555 K (1.3 ). Thus, the ferromagnetic transition does not coincide with a symmetry change of the lattice. In the refinement of the magnetism a ferrimagnetic alignment of the Fe-, Re-sites is found. As a magnetic
form factor for Re is not available in literature we used that for Mo3þ [6]. Due to the similarity of the scattering lengths of Fe and Re for neutrons additional X-ray measurements were performed to verify Fe, Re positional order. Starting from a disordered arrangement the refinement process leads to a remaining disorder of about 4%. We note that the expected error in the X-ray refinements is bigger compared to the neutron case due to the strong absorption of the rhenium atoms. We investigated the crystallographic and magnetic structure in the series of double perovskites A2 FeReO6 (A=Ca, Sr, Ba). With increasing tolerance factor the symmetry changes from monoclinic P21 /n, to tetragonal I4/m, to cubic Fm3% m. While the size of the A site cation strongly influences the crystallographic structure we find for all compound a ferrimagnetic ordering on Fe and Re positions with small moments at the Re-site (Table 1).
References [1] K. Kobayashi, et al., Nature 395 (1998) 677; K. Kobayashi, et al., Phys. Rev. B 59 (1999) 11159. [2] W. Westerburg, et al., Solid State Commun. 122 (2002) 201. [3] J. Rodr!ıguez-Carvajal, Physica B 192 (1993) 55. [4] A.M. Glazer, Acta Crystallogr. B 28 (1972) 3384. [5] T. Alamelu, et al., J. Appl. Phys. 91 (2002) 8909. [6] M.K. Wilkinson, E.O. Wollan, H.R. Child, J.W. Cable, Phys. Rev. 121 (1961) 74.