A-Site-Doping Enhanced B-Site Ordering and Correlated Magnetic

Article pubs.acs.org/JPCC

A‑Site-Doping Enhanced B‑Site Ordering and Correlated Magnetic Property in La2−xBixCoMnO6

Yijia Bai,†,§ Yanjie Xia,†,§ Hongping Li,†,§ Lin Han,†,§ Zhongchang Wang,*,‡ Xiaojie Wu,† Shuhui Lv,†,§ Xiaojuan Liu,*,† and Jian Meng*,† †

State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Graduate School, Chinese Academy of Sciences, Beijing 10049, People’s Republic of China S Supporting Information *

ABSTRACT: A series of Bi-doped La2−xBixCoMnO6 double perovskite oxides are synthesized, and the impact of doping on crystal structures and magnetic properties is investigated comprehensively. Xray photoelectron spectroscopy and Raman spectrum analyses reveal that ordering of Co and Mn ions at B-site is gradually improved with the rise of Bi concentration. Meanwhile, magnetic disordering is suppressed greatly by showing larger magnetic moments. Structurally, the Rietveld refinement shows that the bonds are elongated, while the bond angles are shrunken after doping, giving rise to lowered Curie temperature. We also observe a large negative zero-field-cooling magnetization, which is attributed to the formation of spin antiparallel or canted ferromagnetic domains and clusters that are separated by the antiphase boundaries. First-principles calculations confirm the enhanced Co−Mn ordering upon Bi doping by taking into account both the ordering and disordering configurations of La2CoMnO6, LaBiCoMnO6, and Bi2CoMnO6. Moreover, we find a spin-state transition in the antisite Co ions from high-spin (Co2+-t2g5eg2) to low-spin state (Co3+-t2g6eg0), which is consistent with the increased total magnetic moments by the Bi doping.

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INTRODUCTION Double perovskite oxides A2BB′O6 with rocksalt ordered structures of BO6 and B′O6 octahedra have captured a great deal of attention for their diverse electrical and magnetic properties as well as for their structural nature.1−3 For instance, La2MMnO6 (M = Co, Ni) with B-site ordering shows a high Curie temperature,2,4 which has also been predicted upon the 180° ferromagnetic superexchange (FM-SE) interaction in the Goodenough-Kanamori rules.1,5,6 In addition, these oxides also present a rare high-temperature ferromagnetic insulating feature owing to the structural ordering and the charge-ordered M2+-Mn4+ configuration.7,8 Large magnetic-field-induced changes in electric resistivity9 and dielectric properties10,11 close to room temperature have been reported as well. Particularly, a significant large magnetodielectric coupling of 8−20% has been found recently in a partially disordered La2NiMnO6 perovskite oxide over a wide temperature range of 150−300 K.12 These pioneering discoveries reveal an interplay between the degree of cation ordering and strength of magnetodielectric coupling, which shall open up a promising way to tune the spin, charge, and dielectric properties via a fine control of the degree of cation ordering. One of the most critical issues for the double perovskites to date is the lacking of a high degree of structural ordering, which can often result in weakened ferromagnetic interactions.7 For instance, partially B-site ordered La2CoMnO6 with O vacancies © 2012 American Chemical Society

and antisite defects is often observed in the centrosymmetric space group without remnant polarization. One promising way to realize multiferroic function is to dope the oxides with those ions possessing the 6s2 lone pair electrons (e.g., Bi3+, Pb2+), which enable a strong interaction to the coordinated O-2p electrons and induce noncentrosymmetric domains via local substantial lattice distortion. One convincing evidence of the impact of stereochemically active 6s electrons on the magnetoelectric coupling rests with the Bibased perovskites, which take on an antiferromagnetic (Néel temperature TN = 643 K) and ferroelectric (ferroelectric Curie temperature TCE = 1103 K) nature at room temperature for the BiFeO3,13 whereas a ferromagnetic (Curie temperature TC = 110 K) and ferroelectric (TCE = 760 K) nature under a low temperature for the BiMnO3.14,15 The promising magnetic and electric properties of Bi2CoMnO6 are stimulating an extensive investigation focused on exploiting its novel functionalities and applications. However, the difficulties for its widespread commercial use remain twofold. On one hand, an oxygen pressure as high as 6 GPa is required to retain Bi2CoMnO6 ordering as well as to stabilize its heavily distorted structure.16 On the other hand, the Received: March 22, 2012 Revised: July 17, 2012 Published: July 18, 2012 16841

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Figure 1. XRD spectra of La2−xBixCoMnO6 with (a) x = 0 and (b) x = 0.3. The green circle indicates the measured diffraction intensity, and the orange line shows the calculated one. The right graph show the magnified patterns including the impurity CoO. The bars represent angulars position of the possible Bragg reflections for the major phase (magenta) and the impurity phase (brown). The difference between measured and calculated intensity is given at the bottom (blue).

X-ray emission spectroscopy (EDS, Bruker QUANTAX). The valence was examined by XPS using the Thermo-Electron ESCALAB 250 spectrometer equipped with monochromatic Al X-ray source (1486.6 eV). The Raman spectra were obtained using the Jobin-Yvon Horiba LabRam-HR-800 micro-Raman spectrometer. We applied an incidence laser (17 mW He−Ne 632.8 nm with thermoelectric cooling charge couple device detector) with a small power of 0.12 mW to avoid local heating by passing through the D2 filter. The laser spot diameter was focused to ∼2 μm using a BX 41 microscope with a 50× objective (Olympus). The signal was collected using a focal length of 800 mm and a grating of 600 gr/mm in the backscattering geometry. The exposure time was set to 10 s × 3. The Raman shift was calibrated using the standard single crystal silicon plate at 520.8 cm−1. The quantum design superconducting quantum interference device based magnetic properties measurement system (SQUID-MPMS) was employed to investigate the temperature dependence of DC magnetization (M−T) under a zero-field-cooling (ZFC) and a field-cooling (FC) of 100 Oe and the effect of applied magnetic field on magnetic moments at 5 K (M−H). Computational Details. La2CoMnO6 crystallizes in two phases: low-temperature monoclinic phase with a space group of P21/n and high-temperature rhombohedral phase with a space group of R-3.20 Here, all calculations were based on the low-temperature P21/n phase, which included a four unit formula of La2CoMnO6. The model of LaBiCoMnO6 was constructed by replacing half of the La3+ in La2CoMnO6 with Bi3+, taking into account the order/disorder ratio of 0.5 and the full crystal structural optimization. The atomic model of Bi2CoMnO6 was established by the full substitution of Bi for La, followed by structural relaxation. The effect of the B-site disordering was considered by interchanging atomic sites of an

magnetic transition temperature of Bi2CoMnO6 is estimated to be 95 K,16,17 far from room temperature. Here, we substitute partially the A-site La3+ by Bi3+ in La2CoMnO6 under ambient pressure, aimed at improving structural ordering and meanwhile retaining room-temperature ferromagnetic character and obtaining magnetodielectric coupling effect. We further offer insight into the complex intrinsic interplay between structures and magnetodielectric properties.

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EXPERIMENTAL AND THEORETICAL SECTION Material Synthesis. The La2−xBixCoMnO6 oxides with compositions of x = 0, 0.1, 0.2, and 0.3 were synthesized from La 2 O 3 (99.99%), Bi(NO3) 3 ·5H 2 O (≥99.5%), Co(NO3) 2 ·4H 2 O (≥99.5%), and Mn(CH 3 COO) 2 ·5H 2 O (≥99.5%) using the modified Pechini method.7,18 La2O3 was precalcined at 950 °C for 12 h to ensure that the possible carbonate and water were eliminated completely. To remove the excess carbon, the as-prepared dried gel was ground and calcined at 600 °C for 12 h. The precursors were then pressed into pellets and sintered at 920 °C for 24 h in air, followed by twice repetition of grinding and sintering at 1100 °C for 20 h in oxygen atmosphere. All samples were cooled down slowly in oxygen flow at a rate of 10 °C/h to ensure stoichiometry of the prepared oxides. Material Characterization. Microstructures were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and Raman spectra. For XRD, a Rigaku D/Max 2500 diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 200 mA was used. The XRD patterns were obtained from 10° to 120° in a step of 0.02° with a counting time of 2 s per step, and the Rietveld refinement was performed using the GSAS-EXPGUI program.19 The composition and distribution of metal elements were monitored using the energy-dispersive 16842

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Figure 2. Changes in bond length for (a) Mn(Co)−O1 and (b) Mn(Co)−O2, and in bond angle for (c) Mn(Co)−O1−Mn(Co) and (d) Mn(Co)−O2−Mn(Co) as a function of the Bi concentration.

Mn(Co)−O2 bonds (Table S1), indicating that cations are ordered. It should be noted that it is difficult to fit the XRD patterns to the monoclinic P21/n space group (no. 14) due to the similarity in X-ray scattering factors between Mn and Co.29,30 Nevertheless, we notice that change in average bond lengths and angles may provide an alternative to investigate the impact of Bi-doping on the structural and magnetic properties, as will be addressed below. Another interesting feature is that the difference in the La(Bi)−O bonds turns more pronounced as the Bi concentration is increased (Table S1) particularly in between the La(Bi)−O1 and La(Bi)−O2 bonds. In addition, bond lengths and angles of Mn(Co)−O1 along the c axis suffer larger changes than those of Mn(Co)−O2 in ab plane (figure 2), which indicates that the Bi doping affects the electronic interactions between Mn(Co) cations and their adjacent O anions anisotropically. Since La3+ and Bi3+ ions are close in ionic radius (1.16 Å for La3+ and 1.17 Å for Bi3+ at a coordination number of eight),28 the difference in bond parameters can be attributed mainly to the enhanced orbital hybridization between the Bi 6s and its adjacent O 2p states. These states are closely linked to the Mn(Co) 3d orbital along c direction. Clearly, such orientation preferable hybridization is induced by the stereochemical effect of the 6s2 lone pair electrons. As a result, the bond lengths and angles via O1 are more sensitive to Bi doping than those via O2. Raman Investigation of B-Site Ordering in Bi-Doped Samples. It has been reported that the stretching vibration of the (Mn/Co)O6 octahedra can result in two Raman peaks at ∼645 and 490 cm−1.30−32 Theoretically, lattice dynamic calculations predict that the mode at ∼645 cm−1 is due to the stretching (“breathing”), while that at ∼490 cm−1 originates from mixed antistretching and bending.32 Figure 3 shows Raman spectra of La2−xBixCoMnO6 with x = 0, 0.1, and 0.3. The Raman peaks are broadened and asymmetric with the rise of Bi concentration, which is understood upon the following aspects:31 (i) the B-site Co and Mn ions in La2−xBixCoMnO6 are on the way to be ordered, (ii) the degree of Co−Mn ordering affects not only the Raman behavior of the Mn(Co)− O stretching vibrations but the oxidation states of B-site ions, which in turn influence the stretching frequency of Mn(Co)−O (mixture of Co 2+ and Co 3+ (Mn 3+ and Mn 4+ ) in

equal amount of Mn and Co ions. The corresponding disordered models of the La2CoMnO6, LaBiCoMnO6, and Bi2CoMnO6 were optimized as well. Calculations of electronic and magnetic properties were carried out using the full-potential linearized augmented plane wave (FPLAPW) plus the local orbital method (LO),21,22 as implemented in the WIEN2K package.23 The sphericalharmonic expansion of the potential was performed up to lmax = 10. The product of the minimum muffin-tin radius with the maximum k value in the expansion of plane waves was set to be 7.0 to ensure the accuracy of the used basis set. The spin− orbital coupling (SOC) was taken into account as a perturbation to original Hamiltonian in a scalar relativistic form, and the magnetization axis was chosen along [001] direction where the SOC was considered. To address the 3d electrons of Co and Mn correctly, we applied the plus Umethod24,25 with a U value of 3.0 eV.25 We also tested a range of U values from 1.0 to 6.0 eV, and found that the application of different U affects neither the electronic structures nor the magnetic moments of Co and Mn. Local-spin density approximation of Perdew and Wang (PW92)26 was used to describe exchange-correlation functional. Sampling of the Brillouin zone was conducted using the modified tetrahedron method27 with k points of 1000. The self-consistence was achieved when energy was converged to smaller than 10−5 Ry. All atoms were fully relaxed until the magnitude of force on each atom fell below 0.05 eVÅ−1.

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RESULTS AND DISCUSSION XRD Analysis. Figure 1 shows XRD patterns of La2−xBixCoMnO6 with x = 0 and 0.3, where the Rietveld refinements are performed. The refinement results are summarized in Tables S1 and S2 as well (Supporting Information). All samples are indexed as orthorhombic structure ((2)1/2ac × 2ac × (2)1/2ac, where ac is lattice constant of a perovskite oxide) within Pnma space group (no. 62). The samples are chemically pure in principle, although a small amount of CoO (less than 3 mol %) is detected with the rise of Bi concentration. Ideally, bond length is almost uniform between B-site ions and O in Co−Mn randomly distributed systems.28 However, in-depth analysis of the bond length and angle reveals a remarkable difference between the two 16843

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Figure 3. Raman spectra of La2−xBixCoMnO6 with x = 0, 0.1, and 0.3 recorded using a laser excitation line of 632.8 nm at 298 K. The inset highlights the spectra of x = 0.3 near 650 cm−1, where the Raman peak is fitted and resolved by two peaks of 640 cm−1 (blue dashed line) and 645 cm−1 (orange dashed line) respectively.

La2−xBixCoMnO6 perovskites), and (iii) the distinct stretching vibrations of the bonds between B-site ions and O have a similar frequency, posing a significant hurdle to resolving the contribution of MnO6 from CoO6 octahedra. As a consequence, the Raman peaks with both high and low frequencies are substantially broadened and the peak at ∼650 cm−1 is separated to doublet modes at the frequencies of 640 and 645 cm−1 for the x = 0.3 sample (inset of Figure 3), which agrees well with what was seen in the ordered La2CoMnO6.32 These clearly resolved stretching modes imply a symmetry lowering, which are believed to originate from the octahedral stretching vibration of Co−O and Mn−O bonds. It should be noted that as Bi content increases, the two Raman peaks at 500 cm−1 and 650 cm−1 are slightly shifted to lower wave numbers by ∼10 cm−1 (Figure 3). Recalling the elongated Mn(Co)−O bonds from 1.973 to 1.986 Å (Table S1), we attribute such a shift to the shrunken bonds and the reduced vibration mode energy.33,34 XPS Spectrum Analysis. To shed more light on how the Bi doping affects the chemical state of B-site ions, we present in Figure 4 XPS spectra of Mn-3s core level for the La2−xBixCoMnO6 with x values ranging from 0 to 0.3. It is known that the splitting distance of Mn-3s doublets is strongly affected by the charge of central Mn ions, thereby enabling a quantitative determination of Mn valence.35 The splitting value for the La2−xBixCoMnO6 with x = 0.1 is estimated to be 5.00 eV, whereas it is 4.84 eV for the x = 0.3 sample. Since the 3s doublet splitting value is 5.4 eV36 for Mn3+ and 4.5 eV37 for Mn4+, our results reveal unambiguously that the amount of Mn4+ is increased with the rise of Bi concentration. These provide further evidence that Mn4+−Co2+ ordering and Mn3+− Co3+ disordering coexist in the Bi-doped samples, yet the degree of Mn4+−Co2+ ordering is improved as the Bi concentration is increased, consistent with the aforementioned XRD and Raman analyses. Magnetic Properties. It has been reported that the B-site ordered La2CoMnO6 displays a unique Curie temperature of ∼245 K,9,16 whereas the disordered one presents a second magnetic transition at ∼145 K.7 However, such a magnetic anomaly cannot be detected on the FC and ZFC curves below 150 K (Figure 5a), indicating that the B-site Co and Mn ions in our samples are largely ordered. As the Bi concentration is increased, both the FC and ZFC magnetizations (MFC, MZFC)

Figure 4. XPS spectra and their peak fitting curves of the Mn-3s region for the La2−xBixCoMnO6 with x = 0, 0.1, 0.2, and 0.3.

increase evidently but differ remarkably below the MZFC maximum (recognized as spin freezing temperature Tf), showing a strong magnetic irreversible phenomenon, which is due to the spin cluster-glass behavior. Interestingly, we find a large negative MZFC, which is first seen in the La2CoMnO6 system, for the samples with x = 0 and 0.1. However, it is positive for the sample with x = 0.3. We attribute these to the misplaced B-site cation induced formation of the antiphase boundary. To gain further insight into the mechanism, we show in Figure 5b a schematic illustration of microscopic domain structure. From this figure, one can see a number of FM clusters and domains, where short-ranged Co2+−O−Mn4+ SE interaction takes place. The clusters have an antiferromagnetic coupling in an antiphase boundary. In particular, the spins divided by the antiphase boundary with a 360° domain wall7,38 are extremely difficult to align along the applied magnetic field even under a large H. In the ZFC mode, the field of 100 Oe is not sufficient to align all the frozen spins of clusters and domains along field direction. In this sense, as temperature is decreased, the spin antiparallel or canted state is stabilized, and the residual magnetization tends to be negative. Moreover, change in the MZFC−T curves suggests that the Bi doping can indeed render the Co−Mn sublattice more ordered by reducing greatly the amount of antiphase boundaries and antisite defects. The TC is decreased monotonically from 230 to 205 K as the Bi concentration is increased from 0 to 0.3 (inset of Figure 5a). The same trend is also observed for the paramagnetic Weiss temperature (Θp), as seen in Table 1. It seems intuitively adverse that a lower magnetic transition temperature is obtained in a more ordered system. However, this abnormal phenomenon can also be seen in La2−xBixNiMnO639 and La2−xBixFeMnO6,40 which is different from what was seen in La2CoMnO67 and La2−xBixCoMnO6.39 This may be ascribed to the fact that TC and Θp are more sensitive to the orbital overlapping geometry than to the degree of ordering in the matrix that is governed by the Co−Mn ordering. This agrees with the report by Asai et al.29 revealing that the bond parameters, especially the Mn−O−Co bond angles, are crucial to the TC. Here, the decrease in the TC can be mainly ascribed 16844

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to the reduced Mn(Co)−O−Mn(Co) bond angle from 160.2(4)° to 157.0(6)° and the elongated Mn(Co)−O bond length from 1.973(7) to 1.986(9) Å (Table S1), which may suppress effectively the Co2+−Mn4+ FM−SE interactions via the O2‑ by lowering the degree of orbital overlapping. A considerably large effective paramagnetic moment (Meff) is obtained in each sample by the Curie−Weiss (C−W) fitting, as listed in Table 1. The high Meff can also be seen in other perovskites such as La 2 NiMnO 6 , 10 La 2 CoMnO 6 , 4 and LaMnO3+δ.41 In order to gain deeper insight into the origin underlying the high Meff, we proposed a special paramagnetic (PM) state comprising superparamagnetic clusters and domains that is favorable near TC.8,41−43 Upon such a physical scenario, the FM interactions take place within the clusters and domains whereas the PM feature is seen in between them. In this respect, a wider temperature range (e.g., 300−700 K7) is needed to obtain a better C−W fitting. It is noteworthy that the contribution from the orbital moment of Co2+ (HS) should be taken into account when calculating the Meff.44 The saturated magnetic moment under 5 K (M5T) is enhanced only slightly upon the Bi substitution, as what was seen in Meff (Figure 5c). The largest M5T rests with the sample with x = 0.3, reaching a value of 5.77 μB/f.u. (Table 1), which is close to the ideal value of 6.0 μB/f.u. shown in the sample with a fully Co−Mn ordering. Dass and Goodenough7 have reported that the La2CoMnO6 oxides with different degree of structural ordering possess the same TC (226 K) yet different Meff and M5T values. This is consistent with our results demonstrating that magnetic moment is susceptible to be affected by the degree of B-site cation ordering. First-Principles Calculations on Atomic Structures. To shed more light on the physical impact of Bi doping on the Bsite ordering, we further calculate total energies of ordered (Figure 6a−c) and disordered La2CoMnO6, LaBiCoMnO6, and Bi2CoMnO6 (Figure 6d−f). The calculations predict that La2CoMnO6 with half of Co−Mn random distribution is energetically favored than 100% ordered oxide by saving 0.33 eV. In contrast, for the Bi2CoMnO6, its 100% ordered Co−Mn phase is energetically preferred than the case of half of Co−Mn random distribution by saving 2.76 eV. This implies that the Bidoping can indeed promote the B-site Co/Mn ordering, providing further support to the experimental results. First-Principles Calculations on Magnetic Properties. We further calculate the energy difference between the ferromagnetic (FM) and antiferromagnetic (AFM) configurations for the La2CoMnO6 and Bi2CoMnO6, using the GGA +U+SOC method and the optimized crystal structures. The energy difference is calculated to be −274 meV for La2CoMnO6 and −76 meV for Bi2CoMnO6, which is consistent with the experimental reports that the ferromagnetic Curie temperatures are 226 K for La2CoMnO67 whereas only 95 K for Bi2CoMnO6,17 validating our applied methodology. To investigate the effect of Co−Mn disordering on the magnetic property, we design a couple of likely magnetic configurations of LaBiCoMnO6 by considering the Co−O−Co, Mn−O−Mn and Co−O−Mn couplings in the disordered phase. Keeping the ferromagnetic coupling of Co−O−Mn, we further devise different spin orientations of Co−O−Co (FM or AFM) and Mn−O−Mn (FM or AFM), as shown in Figure 6d−6f. The energy calculations indicate first that the Co−O− Co favors an AFM coupling, while the Mn−O−Mn prefers a FM coupling. Such a FM coupling for the Mn−O−Mn is ascribed to the double exchange interactions of Mn4+−O−

Figure 5. (a) Temperature dependence of zero-field-cooling (ZFC) and field-cooling (FC) magnetization of La2−xBixCoMnO6 with x = 0, 0.1, and 0.3 measured under an applied field of 100 Oe. (b) Schematic diagram for the conceivable microstructure of magnetic domains within the La2−xBixCoMnO6 system. We assume the AFM coupling of Co2+−O−Co2+ and Mn4+−O−Mn4+ occurring at the antiphase boundary. (c) The M−H hysteresis loops of La2−xBixCoMnO6 with x = 0, 0.1, and 0.3 at T = 5 K. The inset magnifies the corresponding parts.

Table 1. Selected Magnetic Parameters of La2−xBixCoMnO6 with x = 0, 0.1, and 0.3a sample

TC (K)

M5T (μB/f.u.)

Meff (μB/f.u.)

Θp (K)

x=0 x = 0.1 x = 0.3

230 220 205

5.37 5.59 5.77

6.68 6.98 7.97

237 231 216

a

TC is the Curie temperature, M5T is the magnetic moment at 5 T, Meff is effective magnetic moment, and Θp is paramagnetic Weiss temperature.

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Interestingly, the magnetic moment for the 100% ordered Co is calculated to be 2.3 μB, while being reduced sharply by 0.4 μB for the antisite Co. Figure 7 shows the orbital-projected density of states (PDOS) for the Co. The t2g orbitals for the antisite Co (Co1) are occupied in both spin channels, while the e2g orbitals are empty. This unambiguously demonstrates that the Co1 is in low-spin (LS) state (Co3+-t2g6eg0). In contrast, the five spin-up orbitals are filled for the normally located Co (Co2), while only the dxy and dxz orbitals are occupied in the spin-down channels, implying that the Co2 takes on high-spin (HS) state (Co2+t2g5eg2). These provide further evidence that the antisite disordering can induce a state transition from Co2+(HS)− Mn4+(HS) to Co3+(LS)−Mn3+(HS), which is consistent with the above experimental observations. We therefore conclude that the increase in total magnetic moment via the Bi doping is not only due to the decreased number of antisite defects but also to the Co spin-state transition.

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CONCLUSIONS We demonstrate, through a combined experimental and theoretical study, that the A-site Bi doping can enhance the B-site cation ordering in La2CoMnO6 owing to the strong hybridization of the Bi-6s orbital with the 2p orbital of its neighboring O, resulting in a significant stabilization of chemical states of Co2+ and Mn4+. As a direct consequence of such a Bi doping, the bond parameters of B-site cations are modified, giving rise to a moderate decrease in TC. Moreover, we find that the spin-state transition of Co from the LS to HS state is one of the key factors to understanding the origin underneath the increase of total magnetic moment by the Bi doping. This combined study clarifies the mechanism whereby the Bi doping enhances B-site ordering in La2CoMnO6, which shall hold the promise to critically improve performances of multifunctional materials for a wide range of applications.

Figure 6. Representation of crystal structures of (a) La2CoMnO6, (b) LaBiCoMnO6, and (c) Bi2CoMnO6. The designed the magnetic configurations are shown respectively in (d), (e), and (f) for the three different spin configurations.

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ASSOCIATED CONTENT

S Supporting Information *

Additional structure information. This material is available free of charge via the Internet at http://pubs.acs.org.

Mn3+ rather than the Mn4+−O−Mn4+ or Mn3+−O−Mn3+ because there are some Mn3+ ions in our synthesized samples owing to the antisite disordering. The total magnetic moment of Co is calculated to be 5.5 μB using the GGA+U method, which increases by 0.2 μB when the spin−orbital coupling effect is considered. This value is close to our experimental value of 5.77 μB for the sample with x = 0.3.

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AUTHOR INFORMATION

Corresponding Author

*E- mail: [email protected] (X.L.); [email protected]. jp (Z.W.); [email protected] (J.M.). Telephone: +86-43185262415. Fax: +86-431-85698041.

Figure 7. Orbital-decomposed density of states for the Co ions in LaBiCoMnO6 at (a) antisite (Co1) and (b) normally placed site (Co2) calculated using the GGA+U+SOC method (U = 3 eV). Co1 shows the low-spin state, while the Co2 shows high-spin state. The vertical red line represents the Fermi level. 16846

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Notes

(31) Bull, C. L.; Mcmillan, P. F. J. Solid State Chem. 2004, 177, 2323−2328. (32) Iliev, M. N.; Abrashev, M. V.; Litvinchuk, A. P.; Hadjiev, V. G.; Guo, H.; Gupta, A. Phys. Rev. B 2007, 75, 104118. (33) Iliev, M. N.; Abrashev, M. V.; Lee, H. G.; Popov, V. N.; Sun, Y. Y.; Thomsen, C.; Meng, R. L.; Chu, C. W. Phys. Rev. B 1998, 57, 2872−2877. (34) Martín-Carrón, L.; de Andrés, A.; Martínez-Lope, M. J.; Casais, M. T.; Alonso, J. A. Phys. Rev. B 2002, 66, 174303. (35) Fujiwara, M.; Matsushita, T.; Ikeda, S. J. Electron Spectrosc. Relat. Phenom. 1995, 74, 201−206. (36) Carver, J. C.; Schweitzer, G. K.; Carlson, T. A. J. Chem. Phys. 1972, 57, 973. (37) DiCastro, V.; Polzonetti, G. J. Electron Spectrosc. Relat. Phemon. 1989, 48, 117−123. (38) Goodenough, J. B.; Dass, R. I. Int. J. Inorg. Mater. 2000, 2, 3−9. (39) Bridges, C. A.; Sefat, A. S.; Payzant, E. A.; Cranswick, L.; Paranthaman, M. P. J. Solid State Chem. 2011, 184, 830−842. (40) Ricciardo, R. A.; Hauser, A. J.; Yang, F. Y.; Kim, H.; Lu, W.; Woodward, P. M. Mater. Res. Bull. 2009, 44, 239−247. (41) Topfer, J.; Goodenough, J. B. J. Solid State Chem. 1997, 130, 117−128. (42) De, K.; Ray, R.; Panda, R. N.; Giri, S.; Nakamura, H.; Kohara, T. J. Magn. Magn. Mater. 2005, 288, 339−346. (43) Bhowmik, R. N.; Ranganathan, R. Phys. Rev. B 2006, 74, 214417. (44) Visvanathan, M.; Hsieh, H. H.; Lin, H. J.; Chen, C. T.; Anil Kumar, P. S. J. Phys. Chem. C 2011, 115, 4851−4855.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grants no. 20871023, 51002148, 21071141, and 20921002. Z.W. thanks support by a Grant-inAid for Young Scientists (A) (Grant no. 24686069) and a Challenging Exploratory Research (Grant no. 24656376).

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dx.doi.org/10.1021/jp302735x | J. Phys. Chem. C 2012, 116, 16841−16847