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Crystal structure and magnetic properties of the double perovskite (Sr2−xCax)FeMoO6 (0⩽x⩽1.0)
Journal of Magnetism and Magnetic Materials 239 (2002) 164–166
Crystal structure and magnetic properties of the double perovskite (Sr2xCax)FeMoO6 (0pxp1:0) R.S. Liua,*, T.S. Chana, S.F. Hub, J.G. Linc, C.Y. Huangd a
Department of Chemistry, National Taiwan University, Taipei, Taiwan b National Nano Device Laboratory, Hsinchu, Taiwan c Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan d Institute of Materials Manufacturing, Chinese Culture University, Taipei, Taiwan
Abstract We have investigated the crystal structure and magnetic properties of the new series of ordered double perovskite oxides (Sr2xCax)FeMoO6 (0pxp1:0). A monotonous decrease of the lattice constants (a and c) has been found with increasing x: Such an effect may also give rise to a distorted structure with Fe–O–Mo angle changing from 1801 for x ¼ 0 to 1621 for x ¼ 1:0: The magnetization increases from x ¼ 0 to 0:5 and then slightly decreases from x ¼ 0:5 to 1.0. Such complex magnetic behavior is strongly correlated to the chemical size effect (corresponding to the internal pressure) in the title compounds. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Double perovskite; Magnetically ordered materials
1. Introduction The discovery of metallic ferromagnetism with large low-field room temperature magnetoresistance in Sr2FeMoO6 by Kobayashi et al. [1] has stimulated research on double perovskite oxides. The magnetic structure of Sr2FeMoO6 was attributed to an ordered arrangement of parallel Fe3+ (3d5, S ¼ 5=2) magnetic moments, antiferromagnetically coupled with Mo5+ (4d1, s ¼ 12) spins. Since Fe3+ is in the high-spin state, its d orbitals are split into spin-up and spin-down states. The empty spin down states (p*–b) of Fe3+ degenerate to the one electron occupied spin-down states (p*–b) of Mo5+ which may lead to the formation of a narrow band. The electrons in this band have their spins antiparallel to the localized spins in the spin-up states (s*–a and p*–a) of Fe3+[2]. A ferromagnetic half-metallic state is observed in this ordered perovskite with localized up-spins of Fe3+ and itinerant down-spin electron of Mo5+[3].
*Corresponding author. Tel.: +886-2-2369-0152 ext. 148; fax: +886-2-2363-6359. E-mail address: [email protected] (R.S. Liu).
Many reports [3–6] are based on the results of the single substituent of A (A=Ba, Sr and Ca) in A2FeMoO6. Here, we report the variation of crystal structure and magnetic properties of the double perovskite system by chemical substitution of the smaller Ca2+ ions into the bigger Sr2+ sites (applying the internal anisotropic pressure) resulting in the system (Sr2xCax)FeMoO6 (0pxp1:0).
2. Experimental procedures Powders of the (Sr2xCax)FeMoO6 system with (0pxp1:0) were prepared by solid-state reaction. High purity starting materials were calcined in air at 8001C for 12 h. The samples were then sintered in a 5% H2/N2 gas mixture at 10001C for 26 h. The resulting mixtures were ground and pressed into pellets (15 mm in diameter and 3 mm in thickness). The pellets were then sintered at 10001C for 12 h in a 5% H2/N2 gas mixture. X-ray diffraction (XRD) measurements were carried out on a SCINTAG (X1) diffractometer (Cu Ka radiation, ( at 40 kV and 30 mA. The program GSAS l ¼ 1:5406 A) [7] was used for the Rietveld refinement in order to
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 4 1 - 8
165
R.S. Liu et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 164–166
obtain information on the crystal structures of (Sr2xCax)FeMoO6. Mgnetization measurements were performed on a SQUID magnetometer (quantum design) in the temperature range from 0 to 400 K. Electron diffraction (ED) and high resolution transmission electron microscopy (HRTEM) were carried out using a JEOL 4000EX electron microscope operated at 400 kV.
3. Results and discussion From the XRD measurements, it can be found that the samples of the series (Sr2xCax)FeMoO6 (0pxp1:0) are of single phase and can be indexed on the basis of a tetragonal unit cell with a space group of I4/mmm. The initial structural model to be refined with the Rietveld profile analysis is based on the model of Sr2FeMoO6 [3] (as shown in the inset of Fig. 1) by assuming that the Ca2+ ions substituted into the Sr2+ sites. Fig. 1 displays
the observed X-ray powder diffraction data at 300 K, along with the calculated diffraction patterns, of (Sr2xCax)FeMoO6 with x ¼ 0:5: All the observed peaks satisfied the reflection conditions of the space group I4/ mmm, h þ k þ l ¼ even; h k 0, h þ k ¼ even; and 0 0 l, l=even. Refinement with this model yields a satisfactory fit to the overall profile with the reliability factors of Rp ¼ 7:97%, Rwp ¼ 11:89% and w2 ¼ 1:47: The structural parameters and the refined results from X-ray powder diffraction data at 300 K for the composition with x ¼ 0:5 are tabulated in Table 1. The occupancies for the (Sr, Ca, O) sites were kept constant in our data refinements. Refinement of occupancy at Fe and Mo sites indicates that the ordering of Fe and Mo atoms is 84% and 75%, respectively. Moreover, the Fe– O–Mo angle is reduced from 1801 for x ¼ 0 to 1621 for x ¼ 1:0 which indicates a distorted structure in the Carich sample. The lattice parameters (a and c) and cell volume (inset of Fig. 2) decrease with increasing Ca content as shown in Fig. 2, which is due to the chemical
7.94 7.92
a 5.56
7.90
5.52
Cell volume(Å3)
7.88 5.54
246
7.86
244
c
242
7.84
240
7.82
238
0.0
5.50
Lattice constant(Å)
Lattice constant(Å)
5.58
0.5
x
0.0
1.0
7.80 0.5
1.0
x
Fig. 1. Observed, calculated, and difference X-ray powder diffraction patterns of (Sr2xCax)FeMoO6 (x ¼ 0:5) at 300 K. Reflection positions are marked for the ideal phase.
Fig. 2. Lattice constants (a and c) as a function of x in (Sr2xCax)FeMoO6 (x ¼ 021:0). The cell volume as a function of x is shown in the inset.
Table 1 Structural parameters and refined results from XRD data at 300 K of (Sr2xCax)FeMoO6 (x ¼ 0:5) (Sr2xCax)FeMoO6 (x ¼ 0:5) Atom x
y
Sr1 0.5 0 Ca2 0.5 0 Fe(3) 0 0 Mo(4) 0 0 Mo(5) 0.5 0.5 Fe(6) 0.5 0.5 O(7) 0.2472 0.2472 O(8) 0 0 Space group: I4/mmm (tetragonal) Cell parameters ( b ¼ 5:5564ð4Þ A ( c ¼ 7:8824ð5Þ A ( a ¼ 5:5564ð4Þ A (3 Cell volume: 243.35(3) A
z 0.25 0.25 0 0 0 0 0.2025 0.2528(3)
(2 102 Uiso/A 2.99(5) 2.99(5) 3.03(7) 3.03(7) 2.13(5) 2.13(5) 2.40(1) 3.01 Reliability factors Rp ¼ 8:21% Rwp ¼ 11:99% w2 ¼ 1:50
Fraction 0.750 0.250 0.838(9) 0.172(9) 0.749(1) 0.241(1) 1 1
R.S. Liu et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 164–166
T=5K
x = 0.5
3
x=1 2
B
x=0 1
3.6
180
3.5
175
3.4
170
3.3
165
3.2 0.0
0.5 x
0 0
1
2
3
o
160 3.1
Fe-O -M o angle ( )
M(Bper formula unit)
4
M ( / formulaunit)
166
1.0
4
5
H (Telsa)
Fig. 3. (a) ED pattern and (b) HRTEM lattice image along the [0 1 1] direction of (Sr2xCax)FeMoO6 (x ¼ 0) sample.
( into the substitution of the smaller Ca2+ ions (1.34 A) ( sites [8]. bigger Sr2+ ionic (1.44 A) Fig. 3(a) shows the typical ED pattern while Fig. 3(b) shows the HRTEM lattice image recorded along the [0 1 1] zone-axis direction of the x ¼ 0 composition in the series of (Sr2xCax)FeMoO6. The cell symmetry as obtained by ED was identified by the observation of the reflection with the limiting conditions h k l; h þ k þ l ¼ 2n; with the I centering of the unit cell which is consistent with the XRD refinement data. Moreover, there is no superlattice modulation along the a direction. The lattice image from the HRTEM shows a tetragonal ( The Ca-doped samples in this cell with a ¼ b ¼ 5:57 A. study by ED and HRTEM show similar results. In Fig. 4, we show the field dependence of magnetization at 5 K of (Sr2xCax)FeMoO6 (x ¼ 0; 0.5 and 1.0). The magnetization at 5 T/formula unit of Sr2FeMoO6 is B3.2 mB at 5 K, which is the same as that reported by Tomioka et al. [3]. This satuation magnetization is less than the predicted value of 4.0 mB/formula unit. Goodenough and Dass [9] have suggested the existence of the antiphase boundary with antiferromagnetic coupling across the boundary due to the disordering of Fe and Mo, which may cause a decrease in magnetization as compared to the predicted value. The XRD refinement indicates B10% disordering between Fe and Mo in our samples. The smaller size of Ca2+ distorts the structure of (Sr2xCax)FeMoO6. Our results show that in the tetragonal structure from x ¼ 0 to x ¼ 1:0; the Fe–O– Mo angle decreases from 1801 to 1621 linearly. We propose that the complex variation of magnetization with Ca-doping in (Sr2xCax)FeMoO6 (0pxp1:0) may be strongly correlated to the reduction of bond angle of Fe–O–Mo. A detailed study of this aspect is still underway.
Fig. 4. Field dependence of magnetization at 5 K of (Sr2xCax)FeMoO6 (x ¼ 0; 0.5 and 1.0). The composition parameter x dependence of magnetization and Fe–O–Mo angle is shown in the inset.
In summary, we have studied the chemical size effect on the structure and magnetic properties of (Sr2xCax) FeMoO6 (0pxp1:0). Across the system, the materials are of single phase with the lattice parameters (a and c) and cell volume decreasing with increasing Ca content. When synthesized under identical conditions, the samples show the same tetragonal cell with a space group of I4/mmm. Moreover, the magnetization increases from x ¼ 0 to 0:5 and then slightly decreases from x ¼ 0:5 to 1.0 which may be controlled by the chemical pressure via Ca-doping in Sr2FeMoO6.
Acknowledgements We thank C.H. Shen and R. Gundakaram for valuable discussions. This research has been financially supported by the National Science Council of the Republic of China under Grant Numbers NSC 892113-M-002-059 and NSC 89-2112-M-002-039.
References [1] K.-I. Kobayashi, et al., Nature 395 (1998) 677. [2] A.W. Sleight, J.F. Weither, J. Phys. Chem. Solids 33 (1972) 679. [3] Y. Tomioka, et al., Phys. Rev. B 61 (2000) 422. [4] A. Maignan, et al., J. Solid State Chem. 14 (1999) 224. [5] J.A. Alonoso, et al., Chem. Mater. 12 (2000) 161. [6] T. Yamamoto, et al., J. Mater. Chem. 10 (2000) 2342. [7] A.C. Larson, R.B Von Dreele, Generalized Structure Analysis System, Los Alamos National Laboratory, Los Alamos, NM, 1994. [8] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [9] J.B. Goodenough, et al., Int. J. Inorg. Mater. 2 (2000) 3.
Crystal structure and magnetic properties of the double perovskite (Sr2xCax)FeMoO6 (0pxp1:0) R.S. Liua,*, T.S. Chana, S.F. Hub, J.G. Linc, C.Y. Huangd a
Department of Chemistry, National Taiwan University, Taipei, Taiwan b National Nano Device Laboratory, Hsinchu, Taiwan c Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan d Institute of Materials Manufacturing, Chinese Culture University, Taipei, Taiwan
Abstract We have investigated the crystal structure and magnetic properties of the new series of ordered double perovskite oxides (Sr2xCax)FeMoO6 (0pxp1:0). A monotonous decrease of the lattice constants (a and c) has been found with increasing x: Such an effect may also give rise to a distorted structure with Fe–O–Mo angle changing from 1801 for x ¼ 0 to 1621 for x ¼ 1:0: The magnetization increases from x ¼ 0 to 0:5 and then slightly decreases from x ¼ 0:5 to 1.0. Such complex magnetic behavior is strongly correlated to the chemical size effect (corresponding to the internal pressure) in the title compounds. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Double perovskite; Magnetically ordered materials
1. Introduction The discovery of metallic ferromagnetism with large low-field room temperature magnetoresistance in Sr2FeMoO6 by Kobayashi et al. [1] has stimulated research on double perovskite oxides. The magnetic structure of Sr2FeMoO6 was attributed to an ordered arrangement of parallel Fe3+ (3d5, S ¼ 5=2) magnetic moments, antiferromagnetically coupled with Mo5+ (4d1, s ¼ 12) spins. Since Fe3+ is in the high-spin state, its d orbitals are split into spin-up and spin-down states. The empty spin down states (p*–b) of Fe3+ degenerate to the one electron occupied spin-down states (p*–b) of Mo5+ which may lead to the formation of a narrow band. The electrons in this band have their spins antiparallel to the localized spins in the spin-up states (s*–a and p*–a) of Fe3+[2]. A ferromagnetic half-metallic state is observed in this ordered perovskite with localized up-spins of Fe3+ and itinerant down-spin electron of Mo5+[3].
*Corresponding author. Tel.: +886-2-2369-0152 ext. 148; fax: +886-2-2363-6359. E-mail address: [email protected] (R.S. Liu).
Many reports [3–6] are based on the results of the single substituent of A (A=Ba, Sr and Ca) in A2FeMoO6. Here, we report the variation of crystal structure and magnetic properties of the double perovskite system by chemical substitution of the smaller Ca2+ ions into the bigger Sr2+ sites (applying the internal anisotropic pressure) resulting in the system (Sr2xCax)FeMoO6 (0pxp1:0).
2. Experimental procedures Powders of the (Sr2xCax)FeMoO6 system with (0pxp1:0) were prepared by solid-state reaction. High purity starting materials were calcined in air at 8001C for 12 h. The samples were then sintered in a 5% H2/N2 gas mixture at 10001C for 26 h. The resulting mixtures were ground and pressed into pellets (15 mm in diameter and 3 mm in thickness). The pellets were then sintered at 10001C for 12 h in a 5% H2/N2 gas mixture. X-ray diffraction (XRD) measurements were carried out on a SCINTAG (X1) diffractometer (Cu Ka radiation, ( at 40 kV and 30 mA. The program GSAS l ¼ 1:5406 A) [7] was used for the Rietveld refinement in order to
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 4 1 - 8
165
R.S. Liu et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 164–166
obtain information on the crystal structures of (Sr2xCax)FeMoO6. Mgnetization measurements were performed on a SQUID magnetometer (quantum design) in the temperature range from 0 to 400 K. Electron diffraction (ED) and high resolution transmission electron microscopy (HRTEM) were carried out using a JEOL 4000EX electron microscope operated at 400 kV.
3. Results and discussion From the XRD measurements, it can be found that the samples of the series (Sr2xCax)FeMoO6 (0pxp1:0) are of single phase and can be indexed on the basis of a tetragonal unit cell with a space group of I4/mmm. The initial structural model to be refined with the Rietveld profile analysis is based on the model of Sr2FeMoO6 [3] (as shown in the inset of Fig. 1) by assuming that the Ca2+ ions substituted into the Sr2+ sites. Fig. 1 displays
the observed X-ray powder diffraction data at 300 K, along with the calculated diffraction patterns, of (Sr2xCax)FeMoO6 with x ¼ 0:5: All the observed peaks satisfied the reflection conditions of the space group I4/ mmm, h þ k þ l ¼ even; h k 0, h þ k ¼ even; and 0 0 l, l=even. Refinement with this model yields a satisfactory fit to the overall profile with the reliability factors of Rp ¼ 7:97%, Rwp ¼ 11:89% and w2 ¼ 1:47: The structural parameters and the refined results from X-ray powder diffraction data at 300 K for the composition with x ¼ 0:5 are tabulated in Table 1. The occupancies for the (Sr, Ca, O) sites were kept constant in our data refinements. Refinement of occupancy at Fe and Mo sites indicates that the ordering of Fe and Mo atoms is 84% and 75%, respectively. Moreover, the Fe– O–Mo angle is reduced from 1801 for x ¼ 0 to 1621 for x ¼ 1:0 which indicates a distorted structure in the Carich sample. The lattice parameters (a and c) and cell volume (inset of Fig. 2) decrease with increasing Ca content as shown in Fig. 2, which is due to the chemical
7.94 7.92
a 5.56
7.90
5.52
Cell volume(Å3)
7.88 5.54
246
7.86
244
c
242
7.84
240
7.82
238
0.0
5.50
Lattice constant(Å)
Lattice constant(Å)
5.58
0.5
x
0.0
1.0
7.80 0.5
1.0
x
Fig. 1. Observed, calculated, and difference X-ray powder diffraction patterns of (Sr2xCax)FeMoO6 (x ¼ 0:5) at 300 K. Reflection positions are marked for the ideal phase.
Fig. 2. Lattice constants (a and c) as a function of x in (Sr2xCax)FeMoO6 (x ¼ 021:0). The cell volume as a function of x is shown in the inset.
Table 1 Structural parameters and refined results from XRD data at 300 K of (Sr2xCax)FeMoO6 (x ¼ 0:5) (Sr2xCax)FeMoO6 (x ¼ 0:5) Atom x
y
Sr1 0.5 0 Ca2 0.5 0 Fe(3) 0 0 Mo(4) 0 0 Mo(5) 0.5 0.5 Fe(6) 0.5 0.5 O(7) 0.2472 0.2472 O(8) 0 0 Space group: I4/mmm (tetragonal) Cell parameters ( b ¼ 5:5564ð4Þ A ( c ¼ 7:8824ð5Þ A ( a ¼ 5:5564ð4Þ A (3 Cell volume: 243.35(3) A
z 0.25 0.25 0 0 0 0 0.2025 0.2528(3)
(2 102 Uiso/A 2.99(5) 2.99(5) 3.03(7) 3.03(7) 2.13(5) 2.13(5) 2.40(1) 3.01 Reliability factors Rp ¼ 8:21% Rwp ¼ 11:99% w2 ¼ 1:50
Fraction 0.750 0.250 0.838(9) 0.172(9) 0.749(1) 0.241(1) 1 1
R.S. Liu et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 164–166
T=5K
x = 0.5
3
x=1 2
B
x=0 1
3.6
180
3.5
175
3.4
170
3.3
165
3.2 0.0
0.5 x
0 0
1
2
3
o
160 3.1
Fe-O -M o angle ( )
M(Bper formula unit)
4
M ( / formulaunit)
166
1.0
4
5
H (Telsa)
Fig. 3. (a) ED pattern and (b) HRTEM lattice image along the [0 1 1] direction of (Sr2xCax)FeMoO6 (x ¼ 0) sample.
( into the substitution of the smaller Ca2+ ions (1.34 A) ( sites [8]. bigger Sr2+ ionic (1.44 A) Fig. 3(a) shows the typical ED pattern while Fig. 3(b) shows the HRTEM lattice image recorded along the [0 1 1] zone-axis direction of the x ¼ 0 composition in the series of (Sr2xCax)FeMoO6. The cell symmetry as obtained by ED was identified by the observation of the reflection with the limiting conditions h k l; h þ k þ l ¼ 2n; with the I centering of the unit cell which is consistent with the XRD refinement data. Moreover, there is no superlattice modulation along the a direction. The lattice image from the HRTEM shows a tetragonal ( The Ca-doped samples in this cell with a ¼ b ¼ 5:57 A. study by ED and HRTEM show similar results. In Fig. 4, we show the field dependence of magnetization at 5 K of (Sr2xCax)FeMoO6 (x ¼ 0; 0.5 and 1.0). The magnetization at 5 T/formula unit of Sr2FeMoO6 is B3.2 mB at 5 K, which is the same as that reported by Tomioka et al. [3]. This satuation magnetization is less than the predicted value of 4.0 mB/formula unit. Goodenough and Dass [9] have suggested the existence of the antiphase boundary with antiferromagnetic coupling across the boundary due to the disordering of Fe and Mo, which may cause a decrease in magnetization as compared to the predicted value. The XRD refinement indicates B10% disordering between Fe and Mo in our samples. The smaller size of Ca2+ distorts the structure of (Sr2xCax)FeMoO6. Our results show that in the tetragonal structure from x ¼ 0 to x ¼ 1:0; the Fe–O– Mo angle decreases from 1801 to 1621 linearly. We propose that the complex variation of magnetization with Ca-doping in (Sr2xCax)FeMoO6 (0pxp1:0) may be strongly correlated to the reduction of bond angle of Fe–O–Mo. A detailed study of this aspect is still underway.
Fig. 4. Field dependence of magnetization at 5 K of (Sr2xCax)FeMoO6 (x ¼ 0; 0.5 and 1.0). The composition parameter x dependence of magnetization and Fe–O–Mo angle is shown in the inset.
In summary, we have studied the chemical size effect on the structure and magnetic properties of (Sr2xCax) FeMoO6 (0pxp1:0). Across the system, the materials are of single phase with the lattice parameters (a and c) and cell volume decreasing with increasing Ca content. When synthesized under identical conditions, the samples show the same tetragonal cell with a space group of I4/mmm. Moreover, the magnetization increases from x ¼ 0 to 0:5 and then slightly decreases from x ¼ 0:5 to 1.0 which may be controlled by the chemical pressure via Ca-doping in Sr2FeMoO6.
Acknowledgements We thank C.H. Shen and R. Gundakaram for valuable discussions. This research has been financially supported by the National Science Council of the Republic of China under Grant Numbers NSC 892113-M-002-059 and NSC 89-2112-M-002-039.
References [1] K.-I. Kobayashi, et al., Nature 395 (1998) 677. [2] A.W. Sleight, J.F. Weither, J. Phys. Chem. Solids 33 (1972) 679. [3] Y. Tomioka, et al., Phys. Rev. B 61 (2000) 422. [4] A. Maignan, et al., J. Solid State Chem. 14 (1999) 224. [5] J.A. Alonoso, et al., Chem. Mater. 12 (2000) 161. [6] T. Yamamoto, et al., J. Mater. Chem. 10 (2000) 2342. [7] A.C. Larson, R.B Von Dreele, Generalized Structure Analysis System, Los Alamos National Laboratory, Los Alamos, NM, 1994. [8] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [9] J.B. Goodenough, et al., Int. J. Inorg. Mater. 2 (2000) 3.