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Synthesis, structural and physical properties of ScMn2O4
Solid State Communications 153 (2013) 71–75
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Synthesis, structural and physical properties of ScMn2O4 L. Wang a, Y.G. Shi a, Z. Chen a, Y.B. Qin a, H.F. Tian a, C. Ma a, H.X. Yang a,n, A.A. Belik b a b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
abstract
Article history: Received 11 September 2012 Accepted 12 October 2012 by F. Peeters Available online 22 October 2012
A new inverse spinel ScMn2O4 has been synthesized and characterized by measurements of structural and physical properties. The crystal structure of ScMn2O4, similar with the spinel Mn3O4, is made up of Mn2 þ located at tetrahedral site and Mn3 þ /Sc3 þ ions randomly located at the octahedral site. Experimental results of magnetic susceptibility and heat capacity demonstrate that ScMn2O4 undergoes a ferrimagnetic phase transition at the temperature of about 58 K. Extensive analyses on the data obtained from structural refinement, electronic structural calculation and EELS spectra measurement suggests that substitution of Sc for Mn in MnO6 octahedron could greatly suppress Jahn–Teller distortions in comparison with what observed in Mn3O4. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. ScMn2O4 D. Jahn–Teller distortion D. Magnetic phase transition
1. Introduction The spinel oxides AB2O4, as a significant structural family consisting of numerous notable compounds, generally show remarkable physical properties, such as superconductivity [1], heavy fermion phenomena [2], charge ordering [3] and unusual magnetic behaviors [3,4]. Recently, the manganese spinel oxides are found to display certain vital physical properties; one of the most studied materials is the cubic LiMn2O4 which is a promising material for cathode in lithium batteries [5]. Other spinel manganites with divalent cations on the tetrahedral site, such as Mn, Zn, Cd, and Mg, often adopt a tetragonal structure arising from a strong Jahn–Teller (J–T) effect due to the presence of Mn3 þ ions that fill the octahedral sites of the oxygen-ion closed-packed arrangement [6]. Moreover, spinel Mn3O4 with Mn ions at both tetrahedral (A ¼Mn2 þ ) and octahedral (B¼ Mn3 þ ) sites show evident magnetoelastic and magnetodielectric coupling in connection with structural and complex magnetic phase transitions [7,8]. Of general formula AB2O4, it is common to refer to a normal spinel when all the A cations are in the tetrahedral sites and all the B cations are found in the octahedral ones. When all of the A ions are placed in the octahedral sites and correspondingly half of the B ions are on the tetrahedral sites, it is referred as an inverse spinel. In this paper, we will report on the structural and physical properties as observed in a new inverse spinel ScMn2O4, which has a tetragonal structure with space group I41/amd. Structural refinement demonstrates that the Sc3 þ ions are located at the octahedral 8d site, i.e. half of the Mn in Mn3O4 at the
n
Corresponding author. Tel.: þ86 10 82648001; fax: þ 86 10 82649483. E-mail address: [email protected] (H.X. Yang).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.017
octahedral 8d site is replaced by Sc in this new inverse spinel material. The electronic structure investigation and structural analysis demonstrate a large suppression of Jahn–Teller distortions of MnO6 octahedra as a result of the disordered doping of Sc in MnO6 octahedra. Experimental measurements of magnetic susceptibility and heat capacity clearly show a ferrimagnetic phase transition at around Tc ¼58 K. 2. Experimental Polycrystalline samples of ScMn2O4 were synthesized by a conventional solid-state reaction method. Sc2O3 (99.99%) and guaranteed reagent grade Mn2O3 powders were used in stoichiometric proportions as starting materials. The starting materials were fully mixed in an agate mortar and then pressed into pellets and sintered with an CO2 atmosphere at 1300 1C for 12 h; XRD data of ScMn2O4 were collected at room temperature on a Bruker D8 advance using Cu Ka radiation (2y range of 10–901, a step width of 0.011, and a counting time of 2 s/step). The XRD data were analyzed by the Rietveld method with RIETAN-2000 [9]. Coefficients for analytical approximation to atomic scattering factors for Mn, Sc, and O were taken from Ref. [10]. The pseudoVoigt function of Toraya was used as a profile function [11]. The background was represented by a 10th-order Legendre polynomial. Isotropic atomic displacement parameters, B, with the isotropic Debye–Waller factor represented as exp(( B sin2 y)/l2) were assigned to all the sites. Microstructure and chemical composition were analyzed on a Philips XL30 scanning electron microscope (SEM). Specimens for transmission electron microscopy (TEM) observations were prepared by gently crushing the polycrystalline materials into fine fragments, which were then
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L. Wang et al. / Solid State Communications 153 (2013) 71–75
supported by a copper grid coated with a thin carbon film; TEM observations and electron energy loss spectroscopy (EELS) analysis were carried out on a Tecnai F20 microscope equipped with a postcolumn Gatan imaging filter. The energy resolution in the EELS spectra is about 1.0 eV. Magnetization measurements between 5 and 300 K were carried out on a commercial superconductor quantum interference device magnetometer. The zerofield-cooling (ZFC) and field-cooling (FC) curves were obtained at applied fields of 100 Oe, 1000 Oe, 10 kOe, respectively. The heat capacity measurement was performed by PPMS commercial device (Quantum Design) in the temperature range 5–300 K and in magnetic fields B ¼0.
parameter. Final lattice parameters, R factors, fractional coordinates, B parameters, and selected bond lengths are listed in Table 1 and the crystal structure is illustrated as the inset of Fig. 1(a). The lattice parameters are determined to be a ¼ ˚ c ¼9.01212(6) A. ˚ 6.02389(4) A, In order to understand the microstructure features of the ScMn2O4 materials, we have performed a series of investigations by means of scanning electron microscopy (SEM) and selectedarea electron diffraction (SAED). Fig. 1(b) show a SEM image illustrating the typical morphological features of ScMn2O4 crystal grains. In Fig. 1(c), we present the energy dispersive x-ray microanalysis (EDX) spectrum taken on a piece of single crystal, which confirmed the presence of the Sc, Mn, and O elements. The composition of the materials is estimated as ScMn2O4.
3. Results and discussion We have successfully synthesized stoichiometric ScMn2O4 samples under the CO2 atmosphere. To determine the crystal structure, high quality powder XRD data were collected at room temperature, as shown in Fig. 1(a). ScMn2O4 is isostructural with spinel Mn3O4, and we used fractional coordinates of Mn3O4 with space group of I41/amd as the initial ones in the structure analysis of ScMn2O4. The spinel structure has one tetrahedral (4a) and one octahedral (8d) site. For Mn3O4, Mn ions are at both tetrahedral (A ¼Mn2 þ ) and octahedral (B¼Mn3 þ ) sites. For ScMn2O4, as expected Sc3 þ ions were found to locate at the octahedral 8d site, replacing half of the Mn at the octahedral 8d site. When Sc3 þ ions were placed at the 4a site, it resulted in the negative thermal
Table 1 Structure parameters of ScMn2O4 at 293 K.a Site
Wyckoff position
x
y
z
B (A˚ 2)
Mn1 Mn/Scb O
4a 8d 16h
0 0 0
0 0.25 0.2241(2)
0 0.625 0.38056(14)
0.68(3) 0.69(3) 0.82(4)
a ˚ Space group I41/amd (No. 141) at the origin choice 1; Z ¼4; a¼ 6.02389(4) A, ˚ and V ¼ 327.026(4) A˚ 3, Rwp ¼2.48%, Rp ¼ 1.43%, RB ¼ 4.64%, and c¼ 9.01212(6) A, RF ¼3.81%. The occupation (g) of all the sites is unity. b g(Mn) ¼ 0.5 and g(Sc)¼ 0.5. d(Mn1–O)¼ 2.0362(12) A˚ ( 4), d(Mn/Sc–O)¼ 2.0232(8) A˚ ( 4) and d(Mn/Sc–O) ¼2.2084(13) A˚ ( 2).
Fig. 1. (a) Powder x-ray diffraction pattern at room temperature for ScMn2O4. The solid line represents the intensities calculated using the Rietveld method. The bottom curves are the differences between the experimental and calculated intensities. The vertical lines indicate the Bragg peak positions of the target compound. The inset shows the crystal structure of the tetragonal compound ScMn2O4 derived from the cubic spinel through strong Jahn–Teller distortion. (b) A SEM image of ScMn2O4. (c) The EDX spectrum taken on a piece of single crystal.
L. Wang et al. / Solid State Communications 153 (2013) 71–75
Fig. 2(a)–(c) show the electron-diffraction patterns for the ScMn2O4 sample taken along the [001], [100] and [101] zone axis directions, respectively. All primary diffraction spots with strong intensity in these patterns can be well indexed by a tetragonal ˚ c¼9.01 A, ˚ and a unit cell with lattice parameters of a ¼6.02 A, space group of I41/amd, which is in good agreement with the XRD data. No additional superstructure spots were detected at room temperature. Fig. 3(a) shows experimental results of the zero-field cooled (ZFC) and field cooled (FC) temperature dependent dc magnetic susceptibility of the ScMn2O4 under applied fields of 100 Oe, 1000 Oe, and 10 kOe, respectively. The data exhibit a typical paramagnetic behavior at high temperature, and a ferrimagnetic transition at about 58 K. Actually, it can be clearly recognizable in Fig. 3(a) that the FC data begin to deviate from the ZFC data at the low measuring field and this feature can be smeared out as the magnetic field increases up to 10 kOe. In order to further study the magnetic behaviors in present system, we applied the Curie– Weiss law, w ¼C/(T y), to analyze the data obtained in an applied magnetic field of 0.1 kOe, as shown in Fig. 3(b). The values of the Curie constant C and the paramagnetic Curie temperature y could be obtained from the fitting line (straight line) using the hightemperature (200–300 K) data. In ScMn2O4, the manganese ions
73
are the only magnetic species; its effective atomic magnetic moment meff ¼5.38 mB/Mn evaluated from the experimental data is very close to the theoretical value 5.4 mB for the 1:1 mixture of Mn2 þ and Mn3 þ based on the spin contribution with all unpaired spins in a high-spin configuration. The negative value of y suggests that antiferromagnetic interactions play a key role for understanding the magnetic properties at low temperatures in this compound. Hysteresis loops measured at 5, 55 and 300 K are displayed in Fig. 3(c), which shows a large coercivity and remanence at 5 K, with the net saturation magnetization corresponding to a moment of 1.73 mB/f.u. The fundamental properties of low temperature magnetic phase transition is an important issue concerned in present study, we therefore performed a measurement of the heat capacity on a wellcharacterized polycrystalline ScMn2O4. Fig. 3(d) shows the temperature dependence of heat capacity C, which clearly demonstrates a ltype shape anomaly at about 58 K, suggesting the presence of a long-range ferrimagnetic order of the manganese spins at low temperature as discussed in above context. To further understand the structure character and physical properties, we performed electronic structures calculations for ScMn2O4 using the full-potential linearized augmented plane wave plus local orbitals (FP-LAPWþlo) method via WIEN2k code [12].
Fig. 2. Selected-area electron diffraction patterns of ScMn2O4 along the (a) [001], (b) [100] and (c) [101] zone-axis directions at room temperature, respectively.
Fig. 3. (a) The ZFC and FC magnetization of ScMn2O4 as a function of temperature at an applied magnetic field of 0.1 kOe, 1 kOe, 10 kOe. (b) Inverse magnetization of ScMn2O4 in a 0.1 kOe magnetic field. (c) Magnetic hysteresis loops of ScMn2O4 measured at 300 K, 55 K and 5 K. (d) The Cp of ScMn2O4 as a function of temperature.
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L. Wang et al. / Solid State Communications 153 (2013) 71–75
The generalized gradient approximation (GGA, PBE 96) [13] was used as the exchange–correlation functional. The muffin-tin radii were 1.97, 1.97, and 1.75 a.u. for Mn, Sc and O atoms, respectively, and the cutoff RmtKmax was set to 8.0 to determine the basis sets. A supercell was created to treat the replacement of Sc for Mn atoms on the octahedral sites. A collinear ferrimagnetic structure in which the direction of Mn magnetic moment is reversed at the octahedral and tetrahedral sites, similar to the magnetic structure used for Mn3O4, was used in our calculations [7]. It was found that the Mn ions is in the high-spin state, and the calculated net magnetic moment per unit cell is 1.00 mB in this ferrimagnetic state, which is smaller than the experimental one, 1.73 mB. This discrepancy may result from the real non-collinear magnetic structure configuration occurs in ScMn2O4, as discussed in Mn3O4. The projected density of states (DOS) towards different atomic orbitals is shown in Fig. 4. An insulating state with an energy gap of about 0.25 eV is obtained for this system. From the Mn-3d orbital projected DOS (Fig. 4(a) and (b)), the exchange energy is estimated to be about 3.8 eV for Mn2 þ O4 tetrahedron and 3.2 eV for Mn3 þ O6 octahedron, similar to that in t-Mn3O4 [7]. The crystal field splitting for Mn3 þ O6 octahedron is about 1.8 eV, and the eg states are further split into an occupied dz2 and an unoccupied d(x2 y2) state by about 0.8 eV due to the Jahn–Teller distortions, both of these values are slightly smaller than those in Mn3O4 (about 2.0 and 1.3 eV, respectively.) The decreased separation between dz2 and d(x2 y2) states in ScMn2O4 suggests the suppression of Jahn–Teller distortions. By comparing the refined crystal structure of ScMn2O4 with the tetragonal structure of tMn3O4 [14], it was found that the Sc doping in t-Mn3O4 leads to an increase in a and a decrease in c, and thus a slightly increase in the a/c ratio. Therefore, the apical Mn–O bond length is shortened while the planar Mn–O bond length is elongated, resulting in a
suppression of Jahn–Teller distortions in MnO6 octahedra for ScMn2O4 (Table 2). One effective approach to characterize the chemical bonding discussed above is the EELS spectrum in TEM. Therefore, we carried out the EELS analysis to further investigate the difference of the electronic structure and chemical bonding between ScMn2O4 and t-Mn3O4. Fig. 5 shows the O–K and Mn-L2,3 edges spectra taken from these two compounds. It is well known that, for the O–K edge spectrum of manganese oxides [15–17] which is associated with the unoccupied O-2p states (Fig. 4(d)), the preedge peak around 530 eV arises from the O-2p states hybridized with the Mn-3d states, the peaks about 10 eV above edge onset from the O-2p states hybridized with the Mn-4sp states, and the higher-energy peaks from multiple scattering of the excited electrons. Hence, according to the projected DOS (Fig. 4), it was found that peak A is only contributed by the unoccupied Mn3 þ -3d states just above the Fermi energy (EF), peak B by the mixture of the unoccupied Mn3 þ - and Mn2 þ -3d states located in between 1 and 3 eV above EF, and peak C by the unoccupied Sc3 þ -3d states located in between 3 and 8 eV above EF. In addition, peak D is due to the Mn- and Sc-4sp states, and peaks E and F due to multiple
Table 2 Distortions of MnO6 octahedra in ScMn2O4 and t-Mn3O4. a/c
ScMn2O4 t-Mn3O4a a
0.668 0.611
MnO6 bond length Apical
Planar
2.208 2.277
2.023 1.930
Jahn–Teller distortion (apical/planar)
1.09 1.18
Ref. [14].
Fig. 4. Main orbital projected partial density of states (DOS) in ScMn2O4. (a) 3d orbitals DOS of tetrahedral Mn. (b) 3d orbitals DOS of octahedral Mn. (c) 3d orbitals DOS of Sc. (d) 2p orbitals DOS of one of O sites. Fermi energy is located at 0 eV and denoted as dotted line.
L. Wang et al. / Solid State Communications 153 (2013) 71–75
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1 eV further confirms the different oxidation states in MnO6 octahedron (Mn3 þ ) and MnO4 tetrahedron (Mn2 þ ). As a result, the enhancement of the low-energy sub-peak in the experimental spectra for ScMn2O4 indicates the increased concentration of Mn2 þ ions, and supports our conclusion that it is Mn3 þ ion in MnO6 octahedron substituted by Sc3 þ ion. 4. Conclusion In summary, we have successfully synthesized polycrystal of a new manganese-based inverse spinel ScMn2O4, which shows a tetragonal structure at room temperature as a consequence of a strong Jahn–Teller (J–T) effect due to the presence of Mn3 þ ions at the octahedral sites. Compared with Mn3O4, large suppression of Jahn–Teller distortions of MnO6 octahedra as a result of the disordered doping of Sc in MnO6 octahedra was confirmed by structural refinement, and further supported by electronic structure calculation and EELS experimental results. The magnetic susceptibility and specific heat measurements indicate that this compound orders ferrimagnetically below Tc at about 58 K. As proposed previously, the orbital degree of freedom is essentially vital for the coupling between dielectric, magnetic properties and crystal structure in the spinel compounds MnT2O4 (T¼V, Cr, Mn) [18]. The tuning of orbital states upon suppression of J–T effect in ScMn2O4 may provide an important possibility to unveil the origin of the rich physical phenomena in the spinel compounds [7,8,19].
Acknowledgments
Fig. 5. EELS O–K and Mn-L2,3 edges of ScMn2O4 compared with Mn3O4. (a) O–K edge. Experimental and simulated O–K edge of ScMn2O4, and experimental O–K edge of Mn3O4; (b) Mn-L2,3 edges of ScMn2O4 and Mn3O4. The spectra of Mn3O4 is replotted from Ref. [16].
scattering. The simulated O–K edge, as shown in Fig. 5(a), reproduces well the features of the experimental spectrum. By comparing the EELS spectra from ScMn2O4 and t-Mn3O4, it was obvious that the differences are dominated by the disappearance of peak C due to the absence of Sc atoms and a visible enhancement of peak A due to more MnO6 octahedra in t-Mn3O4. In addition, the spectra weight transfer from peak A to B in ScMn2O4 is likely due to different chemical bonding of the MnO6 octahedron in these two compounds, as discussed above. Careful analysis indicates that the peak separation between peaks A and B in ScMn2O4 (2.8 eV) is smaller than that in t-Mn3O4 (3.1 eV). This fact further evidences the decreased crystal-field splitting of the MnO6 octahedron in ScMn2O4, in consistence with our theoretical calculations. Mn-L2,3 edges spectra (Fig. 5(b)), arising from the excitations from the Mn-2p to 3d orbitals, can directly provide rich information on energy levels of the Mn-3d orbitals. The two main peaks, denoted by L3 and L2 edges respectively, are due to the spinorbital splitting 2p states, 2p3/2 and 2p1/2. By calculating the intensity ratio I(L3)/I(L2) [16], it was found the average valence state of Mn ions in ScMn2O4 is lower than that in t-Mn3O4, in consistence with the replacement of Mn3 þ by Sc3 þ ions. Because of the coexistence two Mn sites, the splitting of the L3 peak by
This work was supported by National Basic Research Program of China 973 Program (Grant Nos. 2011CBA00101, 2011CB921703, 2010CB923002, 2012CB821404), the Natural Science Foundation of China (Grant Nos. 90922001, 10904166, 11274368, 51272277, 11074292, 11004229, 11190022), and Chinese Academy of Sciences. References [1] D.C. Johnston, H. Prakash, W.H. Zachariasen, R. Viswanathan, Mater. Res. Bull. 8 (1973) 777. [2] S. Kondo, D.C. Johnston, C.A. Swenson, F. Borsa, A.V. Mahajan, L.L. Miller, T. Gu, A.I. Kojima, G.M. Luke, Y.J. Uemura, O. Chmaissem, J.D. Jorgensen, Phys. Rev. Lett. 78 (1997) 3729. [3] N. Fujiwara, H. Yasuoka, Y. Ueda, Phys. Rev. B 57 (1998) 3539. [4] C. Urano, M. Nohara, S. Kondo, F. Sakai, H. Takagi, T. Shiraki, T. Okubo, Phys. Rev. Lett. 85 (2000) 1052. [5] G. Amatucci, J.-M. Tarascon, J. Electrochem. Soc. 149 (2002) K31. [6] P.G. Radaelli, N. J. Phys. 7 (2005) 53. [7] R. Tackett, G. Lawes, B. Melot, M. Grossman, E. Toberer, R. Seshadri, Phys. Rev. B 76 (2007) 024409. [8] M. Kim, X.M. Chen, Y.I. Joe, E. Fradkin, P. Abbamonte, S.L. Cooper, Phys. Rev. Lett. 104 (2010) 136402. [9] F. Izumi, T. Ikeda, Mater. Sci. Forum 321–324 (2000) 198. [10] A.J.C. Wilson, E. Prince (Eds.), International Tables for Crystallography, 2nd ed., vol. C, Kluwer, Dordrecht, 1999, pp. 572–574. [11] H. Toraya, J. Appl. Crystallogr. 23 (1990) 485. [12] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, in: K. Schwarz (Ed.), WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program for ¨ Wien, Austria, 2001. Calculating Crystal Properties, Technische Universitat [13] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [14] V. Baron, J. Gutzmer, H. Rundlof, R. Tellgren, Am. Mineral. 83 (1998) 786. [15] F.M.F. de Groot, M. Grioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H. Petersen, Phys. Rev. B 40 (1989) 5715. [16] H. Kurata, C. Colliex, Phys. Rev. B 48 (1993) 2102. [17] H. Kurata, E. Lefevre, C. Colliex, R. Brydson, Phys. Rev. B 47 (1993) 13763. [18] T. Suzuki, K. Adachi, T. Katsufuji, J. Phys.: Conf. Ser. 31 (2006) 235. [19] T. Suzuki, T. Katsufuji, Phys. Rev. B 77 (2008) 220402(R).
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Synthesis, structural and physical properties of ScMn2O4 L. Wang a, Y.G. Shi a, Z. Chen a, Y.B. Qin a, H.F. Tian a, C. Ma a, H.X. Yang a,n, A.A. Belik b a b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
abstract
Article history: Received 11 September 2012 Accepted 12 October 2012 by F. Peeters Available online 22 October 2012
A new inverse spinel ScMn2O4 has been synthesized and characterized by measurements of structural and physical properties. The crystal structure of ScMn2O4, similar with the spinel Mn3O4, is made up of Mn2 þ located at tetrahedral site and Mn3 þ /Sc3 þ ions randomly located at the octahedral site. Experimental results of magnetic susceptibility and heat capacity demonstrate that ScMn2O4 undergoes a ferrimagnetic phase transition at the temperature of about 58 K. Extensive analyses on the data obtained from structural refinement, electronic structural calculation and EELS spectra measurement suggests that substitution of Sc for Mn in MnO6 octahedron could greatly suppress Jahn–Teller distortions in comparison with what observed in Mn3O4. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. ScMn2O4 D. Jahn–Teller distortion D. Magnetic phase transition
1. Introduction The spinel oxides AB2O4, as a significant structural family consisting of numerous notable compounds, generally show remarkable physical properties, such as superconductivity [1], heavy fermion phenomena [2], charge ordering [3] and unusual magnetic behaviors [3,4]. Recently, the manganese spinel oxides are found to display certain vital physical properties; one of the most studied materials is the cubic LiMn2O4 which is a promising material for cathode in lithium batteries [5]. Other spinel manganites with divalent cations on the tetrahedral site, such as Mn, Zn, Cd, and Mg, often adopt a tetragonal structure arising from a strong Jahn–Teller (J–T) effect due to the presence of Mn3 þ ions that fill the octahedral sites of the oxygen-ion closed-packed arrangement [6]. Moreover, spinel Mn3O4 with Mn ions at both tetrahedral (A ¼Mn2 þ ) and octahedral (B¼ Mn3 þ ) sites show evident magnetoelastic and magnetodielectric coupling in connection with structural and complex magnetic phase transitions [7,8]. Of general formula AB2O4, it is common to refer to a normal spinel when all the A cations are in the tetrahedral sites and all the B cations are found in the octahedral ones. When all of the A ions are placed in the octahedral sites and correspondingly half of the B ions are on the tetrahedral sites, it is referred as an inverse spinel. In this paper, we will report on the structural and physical properties as observed in a new inverse spinel ScMn2O4, which has a tetragonal structure with space group I41/amd. Structural refinement demonstrates that the Sc3 þ ions are located at the octahedral 8d site, i.e. half of the Mn in Mn3O4 at the
n
Corresponding author. Tel.: þ86 10 82648001; fax: þ 86 10 82649483. E-mail address: [email protected] (H.X. Yang).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.017
octahedral 8d site is replaced by Sc in this new inverse spinel material. The electronic structure investigation and structural analysis demonstrate a large suppression of Jahn–Teller distortions of MnO6 octahedra as a result of the disordered doping of Sc in MnO6 octahedra. Experimental measurements of magnetic susceptibility and heat capacity clearly show a ferrimagnetic phase transition at around Tc ¼58 K. 2. Experimental Polycrystalline samples of ScMn2O4 were synthesized by a conventional solid-state reaction method. Sc2O3 (99.99%) and guaranteed reagent grade Mn2O3 powders were used in stoichiometric proportions as starting materials. The starting materials were fully mixed in an agate mortar and then pressed into pellets and sintered with an CO2 atmosphere at 1300 1C for 12 h; XRD data of ScMn2O4 were collected at room temperature on a Bruker D8 advance using Cu Ka radiation (2y range of 10–901, a step width of 0.011, and a counting time of 2 s/step). The XRD data were analyzed by the Rietveld method with RIETAN-2000 [9]. Coefficients for analytical approximation to atomic scattering factors for Mn, Sc, and O were taken from Ref. [10]. The pseudoVoigt function of Toraya was used as a profile function [11]. The background was represented by a 10th-order Legendre polynomial. Isotropic atomic displacement parameters, B, with the isotropic Debye–Waller factor represented as exp(( B sin2 y)/l2) were assigned to all the sites. Microstructure and chemical composition were analyzed on a Philips XL30 scanning electron microscope (SEM). Specimens for transmission electron microscopy (TEM) observations were prepared by gently crushing the polycrystalline materials into fine fragments, which were then
72
L. Wang et al. / Solid State Communications 153 (2013) 71–75
supported by a copper grid coated with a thin carbon film; TEM observations and electron energy loss spectroscopy (EELS) analysis were carried out on a Tecnai F20 microscope equipped with a postcolumn Gatan imaging filter. The energy resolution in the EELS spectra is about 1.0 eV. Magnetization measurements between 5 and 300 K were carried out on a commercial superconductor quantum interference device magnetometer. The zerofield-cooling (ZFC) and field-cooling (FC) curves were obtained at applied fields of 100 Oe, 1000 Oe, 10 kOe, respectively. The heat capacity measurement was performed by PPMS commercial device (Quantum Design) in the temperature range 5–300 K and in magnetic fields B ¼0.
parameter. Final lattice parameters, R factors, fractional coordinates, B parameters, and selected bond lengths are listed in Table 1 and the crystal structure is illustrated as the inset of Fig. 1(a). The lattice parameters are determined to be a ¼ ˚ c ¼9.01212(6) A. ˚ 6.02389(4) A, In order to understand the microstructure features of the ScMn2O4 materials, we have performed a series of investigations by means of scanning electron microscopy (SEM) and selectedarea electron diffraction (SAED). Fig. 1(b) show a SEM image illustrating the typical morphological features of ScMn2O4 crystal grains. In Fig. 1(c), we present the energy dispersive x-ray microanalysis (EDX) spectrum taken on a piece of single crystal, which confirmed the presence of the Sc, Mn, and O elements. The composition of the materials is estimated as ScMn2O4.
3. Results and discussion We have successfully synthesized stoichiometric ScMn2O4 samples under the CO2 atmosphere. To determine the crystal structure, high quality powder XRD data were collected at room temperature, as shown in Fig. 1(a). ScMn2O4 is isostructural with spinel Mn3O4, and we used fractional coordinates of Mn3O4 with space group of I41/amd as the initial ones in the structure analysis of ScMn2O4. The spinel structure has one tetrahedral (4a) and one octahedral (8d) site. For Mn3O4, Mn ions are at both tetrahedral (A ¼Mn2 þ ) and octahedral (B¼Mn3 þ ) sites. For ScMn2O4, as expected Sc3 þ ions were found to locate at the octahedral 8d site, replacing half of the Mn at the octahedral 8d site. When Sc3 þ ions were placed at the 4a site, it resulted in the negative thermal
Table 1 Structure parameters of ScMn2O4 at 293 K.a Site
Wyckoff position
x
y
z
B (A˚ 2)
Mn1 Mn/Scb O
4a 8d 16h
0 0 0
0 0.25 0.2241(2)
0 0.625 0.38056(14)
0.68(3) 0.69(3) 0.82(4)
a ˚ Space group I41/amd (No. 141) at the origin choice 1; Z ¼4; a¼ 6.02389(4) A, ˚ and V ¼ 327.026(4) A˚ 3, Rwp ¼2.48%, Rp ¼ 1.43%, RB ¼ 4.64%, and c¼ 9.01212(6) A, RF ¼3.81%. The occupation (g) of all the sites is unity. b g(Mn) ¼ 0.5 and g(Sc)¼ 0.5. d(Mn1–O)¼ 2.0362(12) A˚ ( 4), d(Mn/Sc–O)¼ 2.0232(8) A˚ ( 4) and d(Mn/Sc–O) ¼2.2084(13) A˚ ( 2).
Fig. 1. (a) Powder x-ray diffraction pattern at room temperature for ScMn2O4. The solid line represents the intensities calculated using the Rietveld method. The bottom curves are the differences between the experimental and calculated intensities. The vertical lines indicate the Bragg peak positions of the target compound. The inset shows the crystal structure of the tetragonal compound ScMn2O4 derived from the cubic spinel through strong Jahn–Teller distortion. (b) A SEM image of ScMn2O4. (c) The EDX spectrum taken on a piece of single crystal.
L. Wang et al. / Solid State Communications 153 (2013) 71–75
Fig. 2(a)–(c) show the electron-diffraction patterns for the ScMn2O4 sample taken along the [001], [100] and [101] zone axis directions, respectively. All primary diffraction spots with strong intensity in these patterns can be well indexed by a tetragonal ˚ c¼9.01 A, ˚ and a unit cell with lattice parameters of a ¼6.02 A, space group of I41/amd, which is in good agreement with the XRD data. No additional superstructure spots were detected at room temperature. Fig. 3(a) shows experimental results of the zero-field cooled (ZFC) and field cooled (FC) temperature dependent dc magnetic susceptibility of the ScMn2O4 under applied fields of 100 Oe, 1000 Oe, and 10 kOe, respectively. The data exhibit a typical paramagnetic behavior at high temperature, and a ferrimagnetic transition at about 58 K. Actually, it can be clearly recognizable in Fig. 3(a) that the FC data begin to deviate from the ZFC data at the low measuring field and this feature can be smeared out as the magnetic field increases up to 10 kOe. In order to further study the magnetic behaviors in present system, we applied the Curie– Weiss law, w ¼C/(T y), to analyze the data obtained in an applied magnetic field of 0.1 kOe, as shown in Fig. 3(b). The values of the Curie constant C and the paramagnetic Curie temperature y could be obtained from the fitting line (straight line) using the hightemperature (200–300 K) data. In ScMn2O4, the manganese ions
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are the only magnetic species; its effective atomic magnetic moment meff ¼5.38 mB/Mn evaluated from the experimental data is very close to the theoretical value 5.4 mB for the 1:1 mixture of Mn2 þ and Mn3 þ based on the spin contribution with all unpaired spins in a high-spin configuration. The negative value of y suggests that antiferromagnetic interactions play a key role for understanding the magnetic properties at low temperatures in this compound. Hysteresis loops measured at 5, 55 and 300 K are displayed in Fig. 3(c), which shows a large coercivity and remanence at 5 K, with the net saturation magnetization corresponding to a moment of 1.73 mB/f.u. The fundamental properties of low temperature magnetic phase transition is an important issue concerned in present study, we therefore performed a measurement of the heat capacity on a wellcharacterized polycrystalline ScMn2O4. Fig. 3(d) shows the temperature dependence of heat capacity C, which clearly demonstrates a ltype shape anomaly at about 58 K, suggesting the presence of a long-range ferrimagnetic order of the manganese spins at low temperature as discussed in above context. To further understand the structure character and physical properties, we performed electronic structures calculations for ScMn2O4 using the full-potential linearized augmented plane wave plus local orbitals (FP-LAPWþlo) method via WIEN2k code [12].
Fig. 2. Selected-area electron diffraction patterns of ScMn2O4 along the (a) [001], (b) [100] and (c) [101] zone-axis directions at room temperature, respectively.
Fig. 3. (a) The ZFC and FC magnetization of ScMn2O4 as a function of temperature at an applied magnetic field of 0.1 kOe, 1 kOe, 10 kOe. (b) Inverse magnetization of ScMn2O4 in a 0.1 kOe magnetic field. (c) Magnetic hysteresis loops of ScMn2O4 measured at 300 K, 55 K and 5 K. (d) The Cp of ScMn2O4 as a function of temperature.
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L. Wang et al. / Solid State Communications 153 (2013) 71–75
The generalized gradient approximation (GGA, PBE 96) [13] was used as the exchange–correlation functional. The muffin-tin radii were 1.97, 1.97, and 1.75 a.u. for Mn, Sc and O atoms, respectively, and the cutoff RmtKmax was set to 8.0 to determine the basis sets. A supercell was created to treat the replacement of Sc for Mn atoms on the octahedral sites. A collinear ferrimagnetic structure in which the direction of Mn magnetic moment is reversed at the octahedral and tetrahedral sites, similar to the magnetic structure used for Mn3O4, was used in our calculations [7]. It was found that the Mn ions is in the high-spin state, and the calculated net magnetic moment per unit cell is 1.00 mB in this ferrimagnetic state, which is smaller than the experimental one, 1.73 mB. This discrepancy may result from the real non-collinear magnetic structure configuration occurs in ScMn2O4, as discussed in Mn3O4. The projected density of states (DOS) towards different atomic orbitals is shown in Fig. 4. An insulating state with an energy gap of about 0.25 eV is obtained for this system. From the Mn-3d orbital projected DOS (Fig. 4(a) and (b)), the exchange energy is estimated to be about 3.8 eV for Mn2 þ O4 tetrahedron and 3.2 eV for Mn3 þ O6 octahedron, similar to that in t-Mn3O4 [7]. The crystal field splitting for Mn3 þ O6 octahedron is about 1.8 eV, and the eg states are further split into an occupied dz2 and an unoccupied d(x2 y2) state by about 0.8 eV due to the Jahn–Teller distortions, both of these values are slightly smaller than those in Mn3O4 (about 2.0 and 1.3 eV, respectively.) The decreased separation between dz2 and d(x2 y2) states in ScMn2O4 suggests the suppression of Jahn–Teller distortions. By comparing the refined crystal structure of ScMn2O4 with the tetragonal structure of tMn3O4 [14], it was found that the Sc doping in t-Mn3O4 leads to an increase in a and a decrease in c, and thus a slightly increase in the a/c ratio. Therefore, the apical Mn–O bond length is shortened while the planar Mn–O bond length is elongated, resulting in a
suppression of Jahn–Teller distortions in MnO6 octahedra for ScMn2O4 (Table 2). One effective approach to characterize the chemical bonding discussed above is the EELS spectrum in TEM. Therefore, we carried out the EELS analysis to further investigate the difference of the electronic structure and chemical bonding between ScMn2O4 and t-Mn3O4. Fig. 5 shows the O–K and Mn-L2,3 edges spectra taken from these two compounds. It is well known that, for the O–K edge spectrum of manganese oxides [15–17] which is associated with the unoccupied O-2p states (Fig. 4(d)), the preedge peak around 530 eV arises from the O-2p states hybridized with the Mn-3d states, the peaks about 10 eV above edge onset from the O-2p states hybridized with the Mn-4sp states, and the higher-energy peaks from multiple scattering of the excited electrons. Hence, according to the projected DOS (Fig. 4), it was found that peak A is only contributed by the unoccupied Mn3 þ -3d states just above the Fermi energy (EF), peak B by the mixture of the unoccupied Mn3 þ - and Mn2 þ -3d states located in between 1 and 3 eV above EF, and peak C by the unoccupied Sc3 þ -3d states located in between 3 and 8 eV above EF. In addition, peak D is due to the Mn- and Sc-4sp states, and peaks E and F due to multiple
Table 2 Distortions of MnO6 octahedra in ScMn2O4 and t-Mn3O4. a/c
ScMn2O4 t-Mn3O4a a
0.668 0.611
MnO6 bond length Apical
Planar
2.208 2.277
2.023 1.930
Jahn–Teller distortion (apical/planar)
1.09 1.18
Ref. [14].
Fig. 4. Main orbital projected partial density of states (DOS) in ScMn2O4. (a) 3d orbitals DOS of tetrahedral Mn. (b) 3d orbitals DOS of octahedral Mn. (c) 3d orbitals DOS of Sc. (d) 2p orbitals DOS of one of O sites. Fermi energy is located at 0 eV and denoted as dotted line.
L. Wang et al. / Solid State Communications 153 (2013) 71–75
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1 eV further confirms the different oxidation states in MnO6 octahedron (Mn3 þ ) and MnO4 tetrahedron (Mn2 þ ). As a result, the enhancement of the low-energy sub-peak in the experimental spectra for ScMn2O4 indicates the increased concentration of Mn2 þ ions, and supports our conclusion that it is Mn3 þ ion in MnO6 octahedron substituted by Sc3 þ ion. 4. Conclusion In summary, we have successfully synthesized polycrystal of a new manganese-based inverse spinel ScMn2O4, which shows a tetragonal structure at room temperature as a consequence of a strong Jahn–Teller (J–T) effect due to the presence of Mn3 þ ions at the octahedral sites. Compared with Mn3O4, large suppression of Jahn–Teller distortions of MnO6 octahedra as a result of the disordered doping of Sc in MnO6 octahedra was confirmed by structural refinement, and further supported by electronic structure calculation and EELS experimental results. The magnetic susceptibility and specific heat measurements indicate that this compound orders ferrimagnetically below Tc at about 58 K. As proposed previously, the orbital degree of freedom is essentially vital for the coupling between dielectric, magnetic properties and crystal structure in the spinel compounds MnT2O4 (T¼V, Cr, Mn) [18]. The tuning of orbital states upon suppression of J–T effect in ScMn2O4 may provide an important possibility to unveil the origin of the rich physical phenomena in the spinel compounds [7,8,19].
Acknowledgments
Fig. 5. EELS O–K and Mn-L2,3 edges of ScMn2O4 compared with Mn3O4. (a) O–K edge. Experimental and simulated O–K edge of ScMn2O4, and experimental O–K edge of Mn3O4; (b) Mn-L2,3 edges of ScMn2O4 and Mn3O4. The spectra of Mn3O4 is replotted from Ref. [16].
scattering. The simulated O–K edge, as shown in Fig. 5(a), reproduces well the features of the experimental spectrum. By comparing the EELS spectra from ScMn2O4 and t-Mn3O4, it was obvious that the differences are dominated by the disappearance of peak C due to the absence of Sc atoms and a visible enhancement of peak A due to more MnO6 octahedra in t-Mn3O4. In addition, the spectra weight transfer from peak A to B in ScMn2O4 is likely due to different chemical bonding of the MnO6 octahedron in these two compounds, as discussed above. Careful analysis indicates that the peak separation between peaks A and B in ScMn2O4 (2.8 eV) is smaller than that in t-Mn3O4 (3.1 eV). This fact further evidences the decreased crystal-field splitting of the MnO6 octahedron in ScMn2O4, in consistence with our theoretical calculations. Mn-L2,3 edges spectra (Fig. 5(b)), arising from the excitations from the Mn-2p to 3d orbitals, can directly provide rich information on energy levels of the Mn-3d orbitals. The two main peaks, denoted by L3 and L2 edges respectively, are due to the spinorbital splitting 2p states, 2p3/2 and 2p1/2. By calculating the intensity ratio I(L3)/I(L2) [16], it was found the average valence state of Mn ions in ScMn2O4 is lower than that in t-Mn3O4, in consistence with the replacement of Mn3 þ by Sc3 þ ions. Because of the coexistence two Mn sites, the splitting of the L3 peak by
This work was supported by National Basic Research Program of China 973 Program (Grant Nos. 2011CBA00101, 2011CB921703, 2010CB923002, 2012CB821404), the Natural Science Foundation of China (Grant Nos. 90922001, 10904166, 11274368, 51272277, 11074292, 11004229, 11190022), and Chinese Academy of Sciences. References [1] D.C. Johnston, H. Prakash, W.H. Zachariasen, R. Viswanathan, Mater. Res. Bull. 8 (1973) 777. [2] S. Kondo, D.C. Johnston, C.A. Swenson, F. Borsa, A.V. Mahajan, L.L. Miller, T. Gu, A.I. Kojima, G.M. Luke, Y.J. Uemura, O. Chmaissem, J.D. Jorgensen, Phys. Rev. Lett. 78 (1997) 3729. [3] N. Fujiwara, H. Yasuoka, Y. Ueda, Phys. Rev. B 57 (1998) 3539. [4] C. Urano, M. Nohara, S. Kondo, F. Sakai, H. Takagi, T. Shiraki, T. Okubo, Phys. Rev. Lett. 85 (2000) 1052. [5] G. Amatucci, J.-M. Tarascon, J. Electrochem. Soc. 149 (2002) K31. [6] P.G. Radaelli, N. J. Phys. 7 (2005) 53. [7] R. Tackett, G. Lawes, B. Melot, M. Grossman, E. Toberer, R. Seshadri, Phys. Rev. B 76 (2007) 024409. [8] M. Kim, X.M. Chen, Y.I. Joe, E. Fradkin, P. Abbamonte, S.L. Cooper, Phys. Rev. Lett. 104 (2010) 136402. [9] F. Izumi, T. Ikeda, Mater. Sci. Forum 321–324 (2000) 198. [10] A.J.C. Wilson, E. Prince (Eds.), International Tables for Crystallography, 2nd ed., vol. C, Kluwer, Dordrecht, 1999, pp. 572–574. [11] H. Toraya, J. Appl. Crystallogr. 23 (1990) 485. [12] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, in: K. Schwarz (Ed.), WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program for ¨ Wien, Austria, 2001. Calculating Crystal Properties, Technische Universitat [13] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [14] V. Baron, J. Gutzmer, H. Rundlof, R. Tellgren, Am. Mineral. 83 (1998) 786. [15] F.M.F. de Groot, M. Grioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H. Petersen, Phys. Rev. B 40 (1989) 5715. [16] H. Kurata, C. Colliex, Phys. Rev. B 48 (1993) 2102. [17] H. Kurata, E. Lefevre, C. Colliex, R. Brydson, Phys. Rev. B 47 (1993) 13763. [18] T. Suzuki, K. Adachi, T. Katsufuji, J. Phys.: Conf. Ser. 31 (2006) 235. [19] T. Suzuki, T. Katsufuji, Phys. Rev. B 77 (2008) 220402(R).