Article pubs.acs.org/JPCC
Room Temperature Ferromagnetism in Shuttle-like BaMoO4 Microcrystals Donglin Guo, Qi Yang, Hao Hua, and Chenguo Hu* Department of Applied Physics, Chongqing University, Chongqing 400044, PR China
ABSTRACT: Diluted magnetic semiconductors (DMSs), with Curie temperature at room temperature, are of technological and fundamental importance. Defect engineering has been an effective way to introduce magnetic moment in various nonmagnetic systems. Here, we report for the first time that BaMoO4 with oxygen vacancy shows ferromagnetic behavior. The first-principles calculations suggest that the oxygen vacancy is responsible for the ferromagnetism. When one oxygen vacancy is introduced, the 0 related occupation state of Mo 4d is t1↑ 2g eg, and a local magnetic moment of 1.0 μB is found. When two oxygen vacancies are 0 introducd, the related occupation state of Mo 4d is t2↑ 2g eg, and a local magnetic moment is up to 2.0 μB. Therefore, the magnetism results from the unpaired electrons on the d orbital, which show high-spin states. Our findings demonstrate that roomtemperature ferromagnetism can also be induced through defect engineering. ence charge, and spin charge density, we find that it is oxygen vacancy that leads to the room-temperature ferromagnetism.
1. INTRODUCTION Owing to development of spintronics, much attention has been focused on the diluted magnetic semiconductors (DMSs).1,2 Although doping semiconductors with transition metal is a route to obtain DMSs, this method has some intrinsic weaknesses, including cluster or secondary phases developing possibly in the doped materials.3−5 Using defect dopant is another effective way to achieve magnetism in semiconductors, such as BaNbO3, BaTiO3, Ba3V2O8.6−8 Barium molybdate (BaMoO4) with scheelite-type tetragonal structures where molybdenum atoms are arranged in tetrahedral coordination has been known as scintillating crystals and is used in solid-state lasers and optical fibers.9−14 Previous investigations have mainly focused on the luminescence properties in the intrinsic blue and green luminescence.15−19 Although a constant paramagnetic susceptibility (215 × 10−6 in cgs unit per mole) of perovskite structured BaMoO3 has been found in the temperature range of 2−300 K,20 there has not been a report on the ferromagnetism of tetragonal structured BaMoO4. In order to explore possible ferromagnetic properties of tetragonal structured BaMoO4, we have investigated these related properties experimentally and theoretically. In this work, we report room temperature ferromagnetism of shuttle-like BaMoO4. The microstructural, optical, and magnetic properties are investigated experimentally in detail. Through first-principles calculations concerning density of states, differ© XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Preparation of BaMoO4. A 20 mL NaOH aqueous solution with pH = 9 is prepared, and then 2 mmol Na2MoO4 and BaCl2 are added into the above aqueous solution. The mixture is kept at room temperature for 12 h. The final products are collected by washing with distilled water and absolute ethanol several times. 2.2. Characterization. The morphology, structure, composition, and optical properties are determined by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM-2100), X-ray diffractometer, Raman scattering (RS) spectrometer, X-ray photoelectron spectroscopy (XPS), and UV−vis−NIR spectrometer (Shimadzu UV 3600), respectively. 2.3. Calculation Details. We use VASP software with PAW potential21,22 to calculate the related properties. Plane waves with 550 eV in kinetic energy are used to expand the wave function of valence electrons (5s25p66s2 for Ba, 4p64d55s1 for Mo and 2s22p4 for O). The generalized gradient approximation (GGA)23 is chosen for the exchange and correlation function with a simple, Received: December 12, 2013
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Figure 1. XRD pattern (a), SEM image (b), TEM image of a single crystal (c), HRTEM image (d), and FFT (e) of BaMoO4.
Figure 2. Raman spectra (a) and XPS analysis: Ba element (b), Mo element (c), and O element (d).
nonempirical derivation (Perdew−Burke−Ernzerhof (PBE) gradient corrected functional) to simplify GGA. The residual
force and convergence threshold are converged to 0.01 eV/Å and 10−6 eV, respectively. B
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3. RESULTS AND DISCUSSION In Figure 1a, all the peaks match the tetragonal phase BaMoO4 (JCPDS card no. 29-0193). Figure 1b shows SEM images of the BaMoO4 microcrystals, in which many shuttle-like crystals are seen with 40−60 μm in length and 5−10 μm in width. Figure 1c,d shows TEM images of shuttle-like crystals of the BaMoO4, indicating that the crystal is single-crystalline and grows along the c-axis direction. Because of C64h symmetry in the scheelite structure of BaMoO4, the characteristic Raman vibration v2(B)g, v4(B)g, v4(E)g, v3(E)g, v3(B)g, and v1(A)g are reflected by the peaks centered at 325, 347, 360, 792, 837, and 891 cm−1, respectively,24 as are shown in Figure 2a, demonstrating the high quality of BaMoO4. Figure 2b,c shows XPS spectra of the prepared BaMoO4, in which the Ba 3d peaks locate at 780.5 and 794.7 eV (Figure 2b), while Mo 3d peaks locate at 231.6 and 235 eV (Figure 2c), and the results well match the previous literature.25 Through curve fitting, O 1s spectrum can be divided into two peaks located at 529.9 and 531.3 eV (Figure 2d). The peak of 529.7 eV is usually attributed to O2−, while the peak of 531.1 eV is associated with the oxygen deficiency in the sample.26,27 The valence of Ba, Mo, and O elements can be determined as +2, +6, and −2 from their characteristic peak location, and the surface atomic ratio of Ba/ Mo/O can be estimated from the peak area of Ba3d, V2p, and O1s curves in the XPS and sensitivity factors, according to which the surface atomic ratio of BaMoO4 is oxygen depleted (BaMoO3.58) in comparison with the standard molecular formula (BaMoO4), indicating that oxygen vacancies exist in the sample. In order to eliminate the possible existence of Fe impurity, the wide-scan XPS analysis is done. From Figure 3, the sample has no Fe impurity.
Figure 4. UV−visible reflection spectrum and Kubelka−Munk function (a) and UV−vis absorption spectrum (b) of BaMoO4.
Figure 3. Chemical analysis of BaMoO4 by XPS. Figure 5. Hysteresis loop of the BaMoO4 measured in room temperature.
The energy gap (Eg) of BaMoO4 can be determined by the optical spectrum analysis. The reflectance spectrum of BaMoO4 is illustrated in Figure 4a. The energy gap of the BaMoO4 can be estimated by the Kubelka−Munk function,28,29 from which the energy gap is obtained as Eg = 4.29 eV (Figure 4a). The UV−vis absorption spectrum of BaMoO4 is shown in Figure 4b, where the absorption intensity rises up abruptly at 4.31 eV, in accord with above energy gap. The magnetic properties of BaMoO4 are measured, and the related results are shown in Figure 5 with field sweeping from −15 000 to +15 000 Oe. The sample of BaMoO4 exhibits a welldefined hysteresis loop, which displays ferromagnetism with a saturation magnetization of ∼5.37 × 10−3 emu/g and a coercive
field of ∼272 Oe. The observed ferromagnetic properties should be the result of the point defects.30−35 In order to completely address the ferromagnetism of BaMoO4, possible defects are considered. Here, we consider three types of vacancy: barium vacancy, VBa, molybdenum vacancy, VMo, and oxygen vacancy, VO. The supercell calculated is shown in Figure 6a. In the supercell, the structure is composed of six primitive cells, containing 6 Ba atoms, 6 Mo atoms, and 24 O atoms. Among all possible defects in Table 1, only VO generates magnetism. As the energy of VO1 is lower than that of other defects, it suggests that the BaMoO4 with VO1 is more stable and can produce 1 μB magnetic moment. C
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Figure 6. Calculated structure (a) and total density of states (TDOS) without oxygen vacancy (b), with one oxygen vacancy (c), and two oxygen vacancies (d) of BaMoO4.
calculated results listed in Table 1, we find that the VO1−VO4 or VO1−VO4 does not generate magnetism. The magnetism induced by the defect is mainly controlled by carriers, such as electrons and holes. The possible result is that the electron and hole are recombined, leading to no magnetism. We use phonon calculation to study dynamical stability of the structure, and the phonon dispersion curves of BaMoO4 with pure, VO1 and VO1−VO4 are presented in Figure 7. The imaginary phonon modes of all curves are not found, thus confirming their dynamic stability. To check whether the magnetism would be affected by GGA+U approach, we calculate the magnetism using GGA+U approach. In GGA+U approach, the effective parameters, U and J, to include the Hubbard onsite Coulomb repulsion, are specified as 6.0 and 1.0 eV, respectively. By use of the GGA+U approach, the band gap of pure sample is calculated to be 4.28 eV (Figure 8), in accordance with experimental band gap, 4.29 eV (Figure 4), indicating that the relevant parameters we choose are appropriate. From Table 2, we find that GGA+U does not affect the magnetism results, and the possible reason is that the Mo atom (+6) of BaMoO4 does not possess d-orbital electron. To give a deeper insight into the electronic structure change after the introduction of oxygen vacancy, the decomposed projected density of states of (PDOS) of Mo 4d, spin charge density, difference charge density and occupation of the decomposed levels of Mo 4d are plotted in Figure 9. In pure BaMoO4, the Mo atom (+6) shows a d0 configuration with the unoccupied spin up and down channels of levels t2g and eg, indicating that the level of t2g and eg does not possess electron (Figure 9a, inset). In the difference charge density, the yellow/ blue color shows that charge increases/decreases. In Figure 9a, the Mo atom loses electrons, so the valence state is +6. However, the O atom obtains electrons, so the valence state is −2. The occupation state of the decomposed levels is t02ge0g (Figure 9a),
Table 1. Calculated Energy (eV) and Magnetism (μB) of BaNbO3 with Possible Defectsa
a
type of defect
energy (eV)
rel energy (eV)
magnetism (μB)
pure VO1 VO2 VO3 VO4 VBa VMo VO1−VO2 VO1−VO3 VO1−VO4 VO1−VBa VO1−VMo
−192.844 09 −182.894 892 −182.894 884 −182.894 858 −182.894 656 −188.066 87 −180.685 38 −172.419 577 −172.772 687 −172.773 115 −178.117 672 −170.736 182
0 9.949 198 9.949 206 9.949 232 9.949 434 4.777 22 12.158 71 20.424 513 20.071 403 20.070 975 14.726 418 22.107 908
0 1 1 1 1 0 0 2 2 2 0 0
The relative energy is referred to the pure sample.
The total electron density of states (TDOS) of BaMoO4 without and with VO1 is calculated, as are shown in Figure 6b,c. Without VO1, the total magnetic moment of the system is zero, which is evidenced by the symmetrical TDOS (Figure 6b). The oxygen vacancy (VO1) creates defect states which do not destroy the insulating behavior but reduce its band gap. The defect states in the band gap region are derived from dangling bonds created by the oxygen vacancy (VO1). Since the chemical formula of structure with one oxygen vacancy is BaMoO3.75, which does not well match the BaMoO3.58, we remove another oxygen atom. The results are listed in Table 1,; the energy of VO1−VO4 is lowest, so we choose the VO1−VO4 (BaMoO3.5) to match the laboratory results (BaMoO3.58). The total electron density of states (TDOS) of BaMoO4 with VO1−VO4 is shown in Figure 6d, indicating the existence of ferromagnetism (2.0 μB). Based on the VO1, the metal vacancy VBa and VMo considered, and the D
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Table 2. Calculated Energy (eV) and Magnetism (μB) of BaNbO3 with and without +U Correction type of defect
energy (eV)
magnetism (μB)
pure pure(+U) VO1 VO1(+U) VO1−VO4 VO1−VO4(+U)
−192.844 09 −168.765 122 −182.894 892 −162.835 351 −172.773 115 −148.644 223
0 0 1 1 2 2
Figure 7. Phonon dispersion curves of BaMoO4 with pure (a), VO1 (b), and VO1−VO4 (c).
Figure 9. Decomposed projected density of states of (PDOS) of Mo 4d, difference charge density, spin charge density, and occupation state of Mo 4d of BaMoO4 without oxygen vacancy (a), with one oxygen vacancy (b), and with two oxygen vacancies (c). Figure 8. Total density of states (TDOS) of BaMoO4 using the GGA+U approach.
electrons. Through Bader charge analysis, the Mo atom around oxygen vacancy obtains 0.85 e. The PDOS of Mo 4d (Figure 9b) is asymmetrical, indicating that magnetism generates. The spin charge density (Figure 9b inset) implies that both Mo atom and O atom produce spin polarization. Accordingly, the d orbital
and the PDOS of Mo 4d (Figure 9a) is symmetrical, implying no magnetism. From the difference charge density (Figure 9b, inset), it is found the Mo atom around oxygen vacancy obtains E
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(7) Qin, S. B.; Liu, D.; Zuo, Z. Y.; Sang, Y. H.; Zhang, X. L.; Zheng, F. F.; Liu, H.; Xu, X. G. UV-Irradiation-Enhanced Ferromagnetism in BaTiO3. J. Phys. Chem. Lett. 2010, 1, 238−241. (8) Guo, D. L.; Hu, C. G.; Xi, Y. UV-irradiation-Enhanced Ferromagnetism of Barium Vanadate (Ba3V2O8) Nanoflowers. J. Alloys Compd. 2013, 550, 389−394. (9) Pontes, F. M.; Maureram, M. A. M. A.; Souza, A. G.; Longo, E.; Leite, E. R.; Magnani, R.; Machado, M. A. C.; Pizani, P. S.; Varela, J. A. Preparation, Structure and Optical Characterization of BaWO4 and PbWO4 Thin Films Prepared by A Chemical Route. J. Eur. Ceram. Soc. 2003, 23, 3001−3007. (10) Ryu, J. H.; Yoon, J. W.; Lim, C. S.; Oh, W. C.; Shim, K. B. Microwave-Assisted Synthesis of CaMoO4 Nano-Powders by A Citrate Complex Method and Its Photoluminescence Property. J. Alloys Compd. 2005, 390, 245−249. (11) Wu, X.; Du, J.; Li, H.; Zhang, M.; Xi, B.; Fan, H.; Zhu, Y.; Qian, Y. Aqueous Mineralization Process to Synthesize Uniform Shuttle-Like BaMoO4Microcrystals at Room Temperature. J. Solid State Chem. 2007, 180, 3288−3295. (12) Errandonea, D.; Manjon, F. J. Pressure Effects on the Structure and Electronic Properties of ABX4 Scintillating Crystals. Prog. Mater. Sci. 2008, 53, 711−773. (13) Panchal, V.; Garg, N.; Chauhan, A. K.; Sangeeta, B.; Sharma, S. M. High Pressure Phase Transitions in BaWO4. Solid State Commun. 2004, 130, 203−208. (14) Luo, Z.; Li, H.; Shu, H.; Wang, K.; Xia, J.; Yan, Y. Sythesis of BaMoO4 Nestlike Nanostructures Under A New Growth Mechanism. Cryst. Growth Des. 2008, 8, 2275−2281. (15) Blasse, G.; Dirksen, G. J. Photoluminescence of Ba3W2O9: Confirmation of A Structure Principle. J. Solid State Chem. 1981, 36, 124−126. (16) Johnson, L. F.; Boyd, G. D.; Nassau, K.; Soden, R. R. Continuous Operation of A Solid-State Optical Maser. Phys. Rev. 1962, 126, 1406− 1409. (17) Grasser, R.; Pompe, W.; Scharmann, A. Defect Luminescence in Tungstates. J. Lumin. 1988, 40/41, 343−344. (18) Marques, A. P. A.; Melo, D. M. A.; Longo, E.; Paskocimas, C. A.; Pizani, P. S.; Leite, E. R. Photoluminescence Preperties of BaMoO4 Amorphous Thin Films. J. Solid State Chem. 2005, 178, 2346−2353. (19) Marques, A. P. A.; Melo, A. M. A.; Paskocimas, C. A.; Pizani, P. S.; Joya, M. R.; Leite, E. R.; Longo, E. Photoluminescence BaMoO4 Nanopowders Prepared by Complex Polymerization Method (CPM). J. Solid State Chem. 2005, 179, 671−678. (20) Bouchard, G. H.; Sienko, M. J. Magnetic Susceptibility of Barium Molybdate (IV) and Strontium Molybdate (IV) in the Range 2−300K. Inorg. Chem. 1968, 7, 441−443. (21) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using A Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (22) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Xia, C. T.; Fuenzalida, V. M.; Zarate, R. A. Electrochemical Preparation of Crystallized Ba1-xSrxMoO4 Solid-Solution Films at Room-Temperature. J. Alloys Compd. 2001, 316, 250−255. (25) Bi, J.; Xiao, D. Q.; Gao, D. J.; Yu, P.; Yu, G. L.; Zhang, W.; Zhu, J. G. BaMoO4 Thin Films Prepared by Electrochemical Method at Room Temperature. Cryst. Res. Technol. 2003, 38, 935−940. (26) Wang, J. P.; Wang, Z. Y.; Huang, B. B.; Ma, Y. D.; Liu, Y. Y.; Qin, X. Y.; Zhang, X. Y.; Dai, Y. Oxygen Vacancy Induced Band-Gap Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO. ACS Appl. Mater. Interfaces 2012, 4, 4024−4030. (27) Li, X. Y.; Wang, Y. L.; Liu, W. F.; Jiang, G. S.; Zhu, C. F. Study of Oxygen Vacancy Influence on the Lattice Parameter In ZnO Thin Flim. Mater. Lett. 2012, 85, 25−28. (28) Wesley, W. M.; Wendlandt, G. H. H. Reflectance Spectroscpy; Interscience Publishers, John Wiley: New York, 1966.
contains one electron due to the obtained electron (0.85 e) and the obtained electron occupies the low-lying t2g, so the related 0 occupation state is t1↑ 2g eg (Figure 9b) which contributes to the total magnetic moment of 1 μB. When the second oxygen vacancy is introduced, it is found that the Mo atom around oxygen vacancy acquires more electrons. From difference charge density (Figure 9c inset), we know that the Mo atom obtains more charges. Through Bader charge analysis, the Bader charge of Mo atom changes from 1.8 to 0.8, indicating that the Mo atom around oxygen vacancy obtains 1.85 e. The PDOS (Figure 9c) and spin charge density (Figure 9c inset) imply that the Mo atom produces local magnetic moment. The d orbital approximately obtains two electrons due to the obtained electron (1.85 e). Based on Hund’s rule, the second electron obtained is parallel to the electron on the Mo 4d orbital, so the occupation state of the 0 decomposed levels is t2↑ 2g eg (Figure 9c). Therefore, the magnetism is from the unpaired electrons on d orbital which show high-spin states.
4. CONCLUSIONS In summary, it is demonstrated that the BaMoO4 has ferromagnetism at room temperature both experimentally and theoretically. The origin of ferromagnetism is due to oxygen vacancy in the prepared BaMoO4. When one oxygen vacancy is 0 introduced, the related occupation state of Mo 4d is t1↑ 2g eg, which contributes to a local magnetic moment of 1.0 μB. When two oxygen vacancies are introduced, the related occupation state of 0 Mo 4d is t2↑ 2g eg, which contributes to a local magnetic moment of 2.0 μB. Therefore, the magnetic mechanism is unpaired electrons on d orbital caused from oxygen vacancies which show high-spin states.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 23 65678362. Fax: +86 23 65678362. E-mail: hucg@ cqu.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFCQ (cstc2012jjB0006), SRFDP (20110191110034, 20120191120039), and NSFC (11204388).
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REFERENCES
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