- Email: [email protected]
Effects of substituting divalent by monovalent ion on the physical properties of La0.7Ca0.3−xKxMnO3 compounds
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709 www.elsevier.com/locate/jmmm
Effects of substituting divalent by monovalent ion on the physical properties of La0.7Ca0.3xKxMnO3 compounds M. Bejara, H. Fekia, E. Dhahria,, M. Ellouzea, M. Ballib, E.K. Hlilb a
b
Laboratoire de Physique Applique´e, Faculte´ des Sciences de Sfax, B.P. 802, Sfax 3018, Tunisie Laboratoire de Cristallographie, CNRS, 25 avenue des Martyrs, B.P. 166, 38042 Grenoble-Cedex 9, France Available online 12 March 2007
Abstract We have prepared new perovskite manganites La0.7Ca0.3xKxMnO3 (0pxp0.15). These compounds are shown to undergo structural transition from orthorhombic to rhombohedral phase. When increasing x, the structural properties reveal the existence of a stronger disorder, for the orthorhombic structure, which leads to an increase of the Mn–O–Mn bond angle and a decrease of the Mn–O bond length. This behaviour contributes to the increase of the one-electron bandwidth W, which results in an increase of the Curie temperature TC. For the rhombohedral structure, the disorder is suppressed, so W and TC decrease. The Raman studies at 300 K indicate that the spectra of La0.7Ca0.3xKxMnO3 are modified when x increases. The strong x dependence of the phonon linewidth suggests that considerable lattice disorder is already present which confirms the structural transition observed by X-ray diffraction patterns. r 2007 Elsevier B.V. All rights reserved. Keywords: Manganite; Curie temperature; K doping; Disorder
1. Introduction
2. Preparation
The distorted perovskite manganites La1xAxMnO3 systems, where A is the divalent Ca, Sr or Ba, have recently attracted much attention because of their colossal magnetoresistance [1,2]. For the optimum doping range x0.3, manganites generally exhibit a ferromagnetic metallic state below the Curie temperature TC, which is governed by the double-exchange interaction [3]. Studies of La1xAxMnO3 with x close to 0.3 have shown the existence of a relationship between TC and the distortion of the perovskite structure as measured by the tolerance factor, t, or the mean size of cations at the A site /rAS. Some attention has been paid to the substitution of the divalent ion Ca2+ by another divalent ion such as Cd2+ for the 2þ 2þ La3þ 0:7 Ca0:3x Ax MnO3 compounds [4]. However, there are few results concerning the substitution of the divalent ion Ca2+ by a monovalent ion [5]. In this work, we present results concerning the structural properties and the evolution of TC of the La0.7Ca0.3MnO3 perovskite when Ca2+ is substituted progressively by K+.
The polycrystalline samples of La0.7Ca0.3xKxMnO3 were synthesized by thoroughly mixing La2O3, K2CO3, CaCO3 and MnO2 in stoichiometric proportions. The mixture was first heated at 1173 K for 3 days and then the powder was pressed into pellets forms under 4 tons/cm2 and sintered at 1673 K for 1 day in air with several periods of grinding and repelling. Finally, these pellets were rapidly quenched to room temperature.
Corresponding author. Tel.: +216 98373734; fax: +216 74274437.
E-mail address: [email protected] (E. Dhahri). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.067
3. Results and discussion Identification of the phase and structural analysis were carried out by X-ray diffraction with Mo radiation (l ¼ 0.709362 A˚) by step scanning (0.021). The diffraction patterns indicate that the synthesized compounds are single phase. For xp0.075 the samples crystallize in orthorhombic ¯ for structure (Pnma) and become rhombohedral (R3c) x ¼ 0.10 and 0.15. With increasing x content of potassium, the mean ionic radii /rAS increase, and an increase in the unit cell volume should be expected due to the bigger ionic
ARTICLE IN PRESS M. Bejar et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709
e708
2+ ˚ radius of K+ ion (/r+ one K S ¼ 1.55 A) than the Ca 2+ ˚ (/rCa S ¼ 1.18 A) [6]. However, we have found that there is first a decrease of that volume for the x ¼ 0.05 and 0.075 compositions than the volume increases again for x ¼ 0.10 and 0.15 (Fig. 1). These results prove that /rAS is not the only parameter that governs the structural properties of the La0.7Ca0.3xKxMnO3 manganites. This behaviour was observed also by Bhattacharya et al. [7] To explain these results, we have studied a further lattice effect whose influence is the random disorder of La3+, Ca2+ and K+ cations with different sizes distributed over the A sites in the perovskite structure. This effect is represented by the variance s2 of the A cation radii, defined by the relation [8]:
s2 ¼
n X
xi r2i hrA i2 .
i¼1
A-site cation disorder results mainly in random displacement of oxide ions from their average crystallographic positions. As a consequence, there are two competing mechanisms, as the mean ionic radii /rAS; and the cation radius distribution s2, have to be taken in account to explain the determined unit cell volume for each composition. The decrease of the unit cell volume for x ¼ 0.05 and 0.075 indicates that the structural disorder increases which is confirmed by the Rietveld refinement of Mn–O bond length calculated from the structural parameters, which decreases for x ¼ 0.05 and 0.075 (Fig. 2). This reduction is consistent with a decrease of the unit cell volume. The transition from orthorhombic structure to the rhombohedral structure, for x ¼ 0.10 and 0.15, reveals that the structural disorder decreases. The temperature dependence of the magnetization for La0.7Ca0.225K0.075MnO3 (x ¼ 0.075) sample at 0.1 T is
Fig. 2. Variation of the Mn–O–Mn bond-angle and the Mn–O bond length as a function of the ionic radii /rAS of La0.7Ca0.3xKxMnO3 samples.
Fig. 3. M(T) at m0H ¼ 0.1 T for La0.7Ca0.225K0.075MnO3 sample. The Curie temperature is deduced from the (dM/dT) curve.
Fig. 1. Variation of the unit cell parameter a and the unit cell volume V as a function of the ionic radii /rAS for La0.7Ca0.3xKxMnO3 samples.
shown in Fig. 3. The Curie temperature TC is defined as a maximum in (dM/dT) evolution. The variation of the ferromagnetic Curie temperature, TC, as a function of the mean ionic radii /rAS is illustrated in Fig. 4. This curve reveals that TC increases for /rAS values less than 1.235 A˚ (xp0.075) and decreases for /rAS values bigger than 1.24 A˚ (xX0.1). This behaviour was also observed in the Ln0.7A0.3MnO3 oxide manganites (with Ln ¼ rare earth and A ¼ alkaline earth), which shows an increasing trend up to 1.24 A˚ and a saturation or decrease thereafter [9]. Based on the model of double exchange, a higher TC corresponds to a bigger overlap between Mn(3d) and O(2p) orbitals resulting in an increase of the one-electron
ARTICLE IN PRESS M. Bejar et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709
Raman intensity (arb. units )
x=0.15
x=0.05
x=0
200
400
600 Wave number
Fig. 4. Variation of Curie temperature as a function of the ionic radii /rAS for La0.7Ca0.3xKxMnO3 samples.
e709
800
1000
(cm-1)
Fig. 5. Raman spectra of La0.7Ca0.3xKxMnO3 (x ¼ 0, 0.05, 0.15) at room temperature.
4. Conclusion bandwidth W given by [10] cos pg 2 , Wa ðd Mn2O Þ3:5 where g is the Mn–O–Mn bond angle and dMn–O the Mn–O bond length. As shown in Fig. 4, the evolution of W as a function of / rAS is remarkably similar with Curie temperature behaviour. At low values of /rAS (xp0.075), the increase of the Mn–O–Mn bond angle and the decrease of the Mn–O bond length both contribute to the rapid increase of W, which results in the increase of TC. On the other hand, at higher values of /rAS (xX0.1), the increase of the Mn–O bond length reduces the overlap between Mn(3d) and O(2p) orbitals and leads to the decrease of W, which results in the decrease of Curie temperature. The Raman spectroscopy is an efficient tool for the study of structural disorder, including the dynamical one. Therefore we have performed a Raman study for different concentrations (x ¼ 0; x ¼ 0.05; x ¼ 0.15) at room temperature in the range 20–1150 cm1. As shown in Fig. 5 the substitution of calcium for potassium is characterized by the following changes: (i) From x ¼ 0 to x ¼ 0.05 (orthorhombic phase) the band at 1088 cm1 shifts by 20 cm1 and the band at 715 cm1 shifts by 85 cm1 and shows a significantly broadening. (ii) From x ¼ 0 to x ¼ 0.15 (rhombohedral phase) the band at 1088 cm1 shifts by 58 cm1 and the band at 715 cm1 splits in two components at 638 and 680 cm1. These changes may be explained by the perovskite lattice distortion and the weakening of the force constant.
In summary, we have elaborated and studied the effect of the substitution of the divalent ion Ca2+ by a monovalent ion K+ on the structural and magnetic properties of the 2þ 2þ La3þ 0:7 Ca0:3x Ax MnO3 samples. We have found that these compounds crystallize in the orthorhombic structure for xp0.075 and become rhombohedral for x ¼ 0.1 and 0.15. The structural properties reveal that the presence of K ions in the structure leads to an increase of the disorder with increasing x contents for the orthorhombic phase. This increase is followed by an increase of the one-electron bandwidth W and the Curie temperature TC. For the rhombohedral phase this disorder is less well pronounced, so W and TC are reduced. Finally, the structural lattice changes are confirmed by the Raman study. References [1] F. Bridges, C.H. Booth, G.H. Kwei, J.J. Neumeir, G.A. Swatzky, Phys. Rev. B. 61 (2000) 9237. [2] A. Uruhibara, Y. Moritomo, T. Arima, A. Asamisu, G. Kido, Y. Tokura, Phys. Rev. B. 51 (1995) 14103. [3] C. Zener, Phys. Rev. 82 (1951) 403. [4] A. Pen˜a, J. Gutie´rrez, J.M. Barandiara´n, J.P. Chapman, M. Insausti, T. Rojo, J. Solid. State Chem. 174 (2003) 52. [5] Jirak, J. Hejtmanek, K. Knizek, M. Marysko, E. Pollert, M. Dlouha, S. Vratislav, R. Kuzel, M. Hervieu, J. Magn. Magn. Mater. 250 (2002) 275. [6] R.D. Shanon, Acta Crystallogr. Sec. A 32 (1976) 751. [7] S. Bhattacharya, S. Pal, R.K. Mukherjee, B.K. Chaudhuri, S. Neeleshwar, Y.Y. Chen, S. Mollah, H.D. Yang, J. Magn. Magn. Mater. 269 (2004) 359. [8] L.M. Rodriguez-Martinez, J.P. Attfield, Phy. Rev. B. 54 (1996) 15622. [9] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid. State. Chem. 120 (1995) 204. [10] M. Medarde, J. Mesot, P. Lacorre, S. Rosenkranz, P. Fischer, K. Gobrecht, Phys. Rev. B. 52 (1995) 9248.
Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709 www.elsevier.com/locate/jmmm
Effects of substituting divalent by monovalent ion on the physical properties of La0.7Ca0.3xKxMnO3 compounds M. Bejara, H. Fekia, E. Dhahria,, M. Ellouzea, M. Ballib, E.K. Hlilb a
b
Laboratoire de Physique Applique´e, Faculte´ des Sciences de Sfax, B.P. 802, Sfax 3018, Tunisie Laboratoire de Cristallographie, CNRS, 25 avenue des Martyrs, B.P. 166, 38042 Grenoble-Cedex 9, France Available online 12 March 2007
Abstract We have prepared new perovskite manganites La0.7Ca0.3xKxMnO3 (0pxp0.15). These compounds are shown to undergo structural transition from orthorhombic to rhombohedral phase. When increasing x, the structural properties reveal the existence of a stronger disorder, for the orthorhombic structure, which leads to an increase of the Mn–O–Mn bond angle and a decrease of the Mn–O bond length. This behaviour contributes to the increase of the one-electron bandwidth W, which results in an increase of the Curie temperature TC. For the rhombohedral structure, the disorder is suppressed, so W and TC decrease. The Raman studies at 300 K indicate that the spectra of La0.7Ca0.3xKxMnO3 are modified when x increases. The strong x dependence of the phonon linewidth suggests that considerable lattice disorder is already present which confirms the structural transition observed by X-ray diffraction patterns. r 2007 Elsevier B.V. All rights reserved. Keywords: Manganite; Curie temperature; K doping; Disorder
1. Introduction
2. Preparation
The distorted perovskite manganites La1xAxMnO3 systems, where A is the divalent Ca, Sr or Ba, have recently attracted much attention because of their colossal magnetoresistance [1,2]. For the optimum doping range x0.3, manganites generally exhibit a ferromagnetic metallic state below the Curie temperature TC, which is governed by the double-exchange interaction [3]. Studies of La1xAxMnO3 with x close to 0.3 have shown the existence of a relationship between TC and the distortion of the perovskite structure as measured by the tolerance factor, t, or the mean size of cations at the A site /rAS. Some attention has been paid to the substitution of the divalent ion Ca2+ by another divalent ion such as Cd2+ for the 2þ 2þ La3þ 0:7 Ca0:3x Ax MnO3 compounds [4]. However, there are few results concerning the substitution of the divalent ion Ca2+ by a monovalent ion [5]. In this work, we present results concerning the structural properties and the evolution of TC of the La0.7Ca0.3MnO3 perovskite when Ca2+ is substituted progressively by K+.
The polycrystalline samples of La0.7Ca0.3xKxMnO3 were synthesized by thoroughly mixing La2O3, K2CO3, CaCO3 and MnO2 in stoichiometric proportions. The mixture was first heated at 1173 K for 3 days and then the powder was pressed into pellets forms under 4 tons/cm2 and sintered at 1673 K for 1 day in air with several periods of grinding and repelling. Finally, these pellets were rapidly quenched to room temperature.
Corresponding author. Tel.: +216 98373734; fax: +216 74274437.
E-mail address: [email protected] (E. Dhahri). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.067
3. Results and discussion Identification of the phase and structural analysis were carried out by X-ray diffraction with Mo radiation (l ¼ 0.709362 A˚) by step scanning (0.021). The diffraction patterns indicate that the synthesized compounds are single phase. For xp0.075 the samples crystallize in orthorhombic ¯ for structure (Pnma) and become rhombohedral (R3c) x ¼ 0.10 and 0.15. With increasing x content of potassium, the mean ionic radii /rAS increase, and an increase in the unit cell volume should be expected due to the bigger ionic
ARTICLE IN PRESS M. Bejar et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709
e708
2+ ˚ radius of K+ ion (/r+ one K S ¼ 1.55 A) than the Ca 2+ ˚ (/rCa S ¼ 1.18 A) [6]. However, we have found that there is first a decrease of that volume for the x ¼ 0.05 and 0.075 compositions than the volume increases again for x ¼ 0.10 and 0.15 (Fig. 1). These results prove that /rAS is not the only parameter that governs the structural properties of the La0.7Ca0.3xKxMnO3 manganites. This behaviour was observed also by Bhattacharya et al. [7] To explain these results, we have studied a further lattice effect whose influence is the random disorder of La3+, Ca2+ and K+ cations with different sizes distributed over the A sites in the perovskite structure. This effect is represented by the variance s2 of the A cation radii, defined by the relation [8]:
s2 ¼
n X
xi r2i hrA i2 .
i¼1
A-site cation disorder results mainly in random displacement of oxide ions from their average crystallographic positions. As a consequence, there are two competing mechanisms, as the mean ionic radii /rAS; and the cation radius distribution s2, have to be taken in account to explain the determined unit cell volume for each composition. The decrease of the unit cell volume for x ¼ 0.05 and 0.075 indicates that the structural disorder increases which is confirmed by the Rietveld refinement of Mn–O bond length calculated from the structural parameters, which decreases for x ¼ 0.05 and 0.075 (Fig. 2). This reduction is consistent with a decrease of the unit cell volume. The transition from orthorhombic structure to the rhombohedral structure, for x ¼ 0.10 and 0.15, reveals that the structural disorder decreases. The temperature dependence of the magnetization for La0.7Ca0.225K0.075MnO3 (x ¼ 0.075) sample at 0.1 T is
Fig. 2. Variation of the Mn–O–Mn bond-angle and the Mn–O bond length as a function of the ionic radii /rAS of La0.7Ca0.3xKxMnO3 samples.
Fig. 3. M(T) at m0H ¼ 0.1 T for La0.7Ca0.225K0.075MnO3 sample. The Curie temperature is deduced from the (dM/dT) curve.
Fig. 1. Variation of the unit cell parameter a and the unit cell volume V as a function of the ionic radii /rAS for La0.7Ca0.3xKxMnO3 samples.
shown in Fig. 3. The Curie temperature TC is defined as a maximum in (dM/dT) evolution. The variation of the ferromagnetic Curie temperature, TC, as a function of the mean ionic radii /rAS is illustrated in Fig. 4. This curve reveals that TC increases for /rAS values less than 1.235 A˚ (xp0.075) and decreases for /rAS values bigger than 1.24 A˚ (xX0.1). This behaviour was also observed in the Ln0.7A0.3MnO3 oxide manganites (with Ln ¼ rare earth and A ¼ alkaline earth), which shows an increasing trend up to 1.24 A˚ and a saturation or decrease thereafter [9]. Based on the model of double exchange, a higher TC corresponds to a bigger overlap between Mn(3d) and O(2p) orbitals resulting in an increase of the one-electron
ARTICLE IN PRESS M. Bejar et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e707–e709
Raman intensity (arb. units )
x=0.15
x=0.05
x=0
200
400
600 Wave number
Fig. 4. Variation of Curie temperature as a function of the ionic radii /rAS for La0.7Ca0.3xKxMnO3 samples.
e709
800
1000
(cm-1)
Fig. 5. Raman spectra of La0.7Ca0.3xKxMnO3 (x ¼ 0, 0.05, 0.15) at room temperature.
4. Conclusion bandwidth W given by [10] cos pg 2 , Wa ðd Mn2O Þ3:5 where g is the Mn–O–Mn bond angle and dMn–O the Mn–O bond length. As shown in Fig. 4, the evolution of W as a function of / rAS is remarkably similar with Curie temperature behaviour. At low values of /rAS (xp0.075), the increase of the Mn–O–Mn bond angle and the decrease of the Mn–O bond length both contribute to the rapid increase of W, which results in the increase of TC. On the other hand, at higher values of /rAS (xX0.1), the increase of the Mn–O bond length reduces the overlap between Mn(3d) and O(2p) orbitals and leads to the decrease of W, which results in the decrease of Curie temperature. The Raman spectroscopy is an efficient tool for the study of structural disorder, including the dynamical one. Therefore we have performed a Raman study for different concentrations (x ¼ 0; x ¼ 0.05; x ¼ 0.15) at room temperature in the range 20–1150 cm1. As shown in Fig. 5 the substitution of calcium for potassium is characterized by the following changes: (i) From x ¼ 0 to x ¼ 0.05 (orthorhombic phase) the band at 1088 cm1 shifts by 20 cm1 and the band at 715 cm1 shifts by 85 cm1 and shows a significantly broadening. (ii) From x ¼ 0 to x ¼ 0.15 (rhombohedral phase) the band at 1088 cm1 shifts by 58 cm1 and the band at 715 cm1 splits in two components at 638 and 680 cm1. These changes may be explained by the perovskite lattice distortion and the weakening of the force constant.
In summary, we have elaborated and studied the effect of the substitution of the divalent ion Ca2+ by a monovalent ion K+ on the structural and magnetic properties of the 2þ 2þ La3þ 0:7 Ca0:3x Ax MnO3 samples. We have found that these compounds crystallize in the orthorhombic structure for xp0.075 and become rhombohedral for x ¼ 0.1 and 0.15. The structural properties reveal that the presence of K ions in the structure leads to an increase of the disorder with increasing x contents for the orthorhombic phase. This increase is followed by an increase of the one-electron bandwidth W and the Curie temperature TC. For the rhombohedral phase this disorder is less well pronounced, so W and TC are reduced. Finally, the structural lattice changes are confirmed by the Raman study. References [1] F. Bridges, C.H. Booth, G.H. Kwei, J.J. Neumeir, G.A. Swatzky, Phys. Rev. B. 61 (2000) 9237. [2] A. Uruhibara, Y. Moritomo, T. Arima, A. Asamisu, G. Kido, Y. Tokura, Phys. Rev. B. 51 (1995) 14103. [3] C. Zener, Phys. Rev. 82 (1951) 403. [4] A. Pen˜a, J. Gutie´rrez, J.M. Barandiara´n, J.P. Chapman, M. Insausti, T. Rojo, J. Solid. State Chem. 174 (2003) 52. [5] Jirak, J. Hejtmanek, K. Knizek, M. Marysko, E. Pollert, M. Dlouha, S. Vratislav, R. Kuzel, M. Hervieu, J. Magn. Magn. Mater. 250 (2002) 275. [6] R.D. Shanon, Acta Crystallogr. Sec. A 32 (1976) 751. [7] S. Bhattacharya, S. Pal, R.K. Mukherjee, B.K. Chaudhuri, S. Neeleshwar, Y.Y. Chen, S. Mollah, H.D. Yang, J. Magn. Magn. Mater. 269 (2004) 359. [8] L.M. Rodriguez-Martinez, J.P. Attfield, Phy. Rev. B. 54 (1996) 15622. [9] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid. State. Chem. 120 (1995) 204. [10] M. Medarde, J. Mesot, P. Lacorre, S. Rosenkranz, P. Fischer, K. Gobrecht, Phys. Rev. B. 52 (1995) 9248.