The Evolution of Microstructure and Magnetic Properties of the

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The Evolution of Microstructure and Magnetic Properties of the Bismuth Layer Compounds with Cobalt Ions Substitution Weipeng Wang,†,‡ Xi Shen,† Wei Wang,†,‡ Xiangxiang Guan,†,‡ Yuan Yao,† Yanguo Wang,† and Richeng Yu*,†,‡ †

Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: One of the core issues for the A/B site doping in the bismuth layer magnetoelectric materials is to find out the evolution of the magnetic structure, crystal structure and elemental distribution, and the coupling effects between spin and lattice with the increase of ion substitution. Here, we have conducted systematic structural and physical property studies on the series samples of Bi5Ti3Fe1−xCoxO15. This work presents that Bi5Ti3Fe1−xCoxO15 forms a single four layer perovskite-like structure for 0 ≤ x < 0.67, while a three layer perovskite-like structure block begins to arise for x ≥ 0.67. With different cobalt content, the sample demonstrates antiferromagnetism, spin state determined magnetism, or magnetic anisotropy determined magnetism. The weak ferromagnetism is considered to be induced by the larger displacement of Co3+ ions from the center of octahedra and the change of the spin state of Co3+ ions. It is also observed that Fe and Co elements are homogeneously substituted in the three layer structure block, accompanied by the rotation (and/or distortion) of BO6 octahedra. order ME coefficients of Bi5Ti3FeO15 to be 0.1 mV cm−1 Oe−1 and 1.37 × 10−5 mV cm−1 Oe−2, respectively.12 It is obvious that the compounds Bi4Bin−3Ti3Fen−3O3n+3 are of antiferromagnetic nature and weak ME coupling,13 which destroy the foundation of practical application. In order to tune the magnetic orderings structure and ME coupling, some research groups have been doing much effort to A/B site doping, for instance the half doping of Co or Cr ions in Bi5Ti3FeO15.14,15 It is generally recognized that the ME effect of this bismuth layer magnetoelectric materials originates from the strain induced by the displacement of magnetic ions from their regular octahedral site under an external magnetic field.13 However, are the ferroelectric properties from the B site ions displacement,16 the bismuth lone pair electrons, or the entire displacement of the perovskite blocks with respect to the Bi2O2 layers?17,18 For the occupation of different ions at the B site, the study curtain just begins recently. Hervoches et al. demonstrated that the occupation ratio of Ti4+ and Fe3+ is almost equal by the Mossbauer spectrum study on Bi5Ti3FeO15.19 In the recent paper by Yan et al., the atomic resolution STEM EDX-mapping images show that the Fe3+ ions of Bi10Ti3Fe6O30 are inclined to occupy the B site near the (Bi2O2)2+ layers, while the Ti4+ ions are to be far away from the (Bi2O2)2+

1. INTRODUCTION Magnetoelectric materials, which not only possess the orders of ferroelectricity and ferromagnetism but also exhibit the mutual coupling properties among these physical orders, have been considered as a captivating research field of the fundamental physics and potential device application in digital memories, magnetic sensors, and transducers for magnetoelectric energy conversion.1−5 In order to explore good practical magnetoelectric materials, the traditional bismuth layer ferroelectrics have been experiencing a renaissance for their high Curie temperature, large spontaneous polarization, and potential single-phase magnetoelectric characteristics above room temperature.6−9 These compounds are of atomic-scale superlattices built from regular intergrowths of fluorite-like (Bi2O2)2+ layers and perovskite-like (An−1BnO3n+1)2− layers (where n is the number of perovskite-like layers formed by corner-sharing BO6 octahedra; A and B are the 12-fold and 6-fold coordination sites of the perovskite slab, respectively).10 The magnetoelectric properties can be realized by the inset of ferromagnetic phase ABO3 into the isostructural Bi4Ti3O12 ferroelectric, forming Bi5Ti3FeO15 for instance. Meanwhile, the (Bi2O2)2+ layers play an important role in both space charge compensation and insulation, avoiding the disadvantage of large leakage current of the ABO3 ferromagnetic phase.11 The magnetoelectric properties of the bismuth layer compounds have been observed; for example, Srinivas et al. reported the first-order and second© XXXX American Chemical Society

Received: September 27, 2016

A

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry layers.20 On the other hand, it is known that the ions doping has a great effect on the bismuth layer materials, for example, the change of ferromagnetic Curie temperature, magnetization value, ferroelectric properties, and so on.21−24 How does the ion doping influence the properties? Is it through the inner strain, crystal structure, spin structure or the oxygen vacancy?25 The researchers have paid much attention to the improvement of physical properties and the effect of different element substitution, but there are few works on the microstructural modification of spin and crystal introduced by elemental substitution. The mutual effect between the spin structure and crystal structure caused by elemental substitution is still unclear for the bismuth layer materials. In this work, a series of Bi5Ti3Fe1−xCoxO15 (x = 0, 0.25, 0.33, 0.50, 0.67, 0.75, 1.00, respectively) polycrystals, in which the ratios of iron to cobalt are, respectively, 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, 0:1, were synthesized by the polyacrylamide gel method. Xray diffraction (XRD) and transmission electron microscopy (TEM) diffraction were carried out to examine the crystal structure of the compounds. The temperature dependence of magnetization and magnetization versus magnetic field for each sample were measured to study the magnetic property. Atomic scale scanning TEM (STEM) high-angle annular dark-field (HAADF), annular bright-field (ABF), and energy-dispersive X-ray (EDX) mapping techniques were applied to get the information on the microstructure and elemental distribution. The evolution of the elemental distribution, crystal structure and magnetic structure with increasing cobalt are investigated, and the coupling between spin and lattice is also discussed.

CS correctors for the condenser lens and objective lens. At the operating voltage of 200 kV, the available point resolution is better than 0.08 nm. The HAADF and ABF images were severally acquired at acceptance angles of 70−150 mrad and 10−20 mrad, respectively. The Fourier filter method was implemented to images to minimize the contrast noise and does not have any effect on the results of our measurements. The magnetization measurements were carried out on a Physical Property Measurement System (PPMS, Quantum Design).

3. RESULTS 3.1. XRD Analysis. In order to trace the evolution of the crystal structure of Bi5Ti3Fe1−xCoxO15 (x = 0, 0.25, 0.33, 0.50, 0.67, 0.75, 1, respectively) samples, room temperature XRD patterns were examined, and the results are shown in Figure 1. Here, the bar graphs in green and red at the bottom of Figure 1 represent the standard XRD patterns of Bi5Ti3FeO15 (space group: Cmc2 1 ) and Bi 4 Ti 3 O 12 (space group: Fmmm), respectively. It can be seen that the two sets of bar graphs are very similar to each other except only several slightly deviated peaks, denoted as black downward arrows, appear at around 2θ = 16.15°, 38.38°, 51.47° for Bi4Ti3O12 compared with 2θ = 17.11°, 39.45°, 51.96° for Bi5Ti3FeO15. Bi5Ti3Fe1−xCoxO15 (x = 0, 0.25, 0.33, 0.50) samples can be easily matched to the Bi5Ti3FeO15-like structure, while Bi5Ti3CoO15 (x = 1) is well matched to the Bi4Ti3O12-like structure. The difficulty is that which structure Bi5Ti3Fe1−xCoxO15 (x = 0.67, 0.75, respectively) samples are matched to. According to their crystal parameters, the lattice constant a of Bi5Ti3FeO15 is about 41 Å, much longer than 36 Å of Bi4Ti3O12. We carefully performed selected area electron diffraction (SAED) experiments to distinguish these two structures (seen in Figure S1 of the Supporting Information) and found that Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) has a Bi5Ti3FeO15-like structure, while Bi5Ti3Fe0.25Co0.75O15 (x = 0.75) contains the Bi5Ti3FeO15-like structure as majority and the Bi4Ti3O12 structure as minority. The bismuth layer structure Bi5Ti3Fe1−xCoxO15, constituted by regular layers of (Bi2O2)2+ and (An−1BnO3n+1)2−perovskite slabs, can be regarded as incorporating the magnetic units BiFeO3 and BiCoO3 into the ferroelectric blocks Bi4Ti3O12. Figure 1b,c shows tetragonal crystals BiFeO3 and BiCoO3, with the same space group P4mm at room temperature. From the previous papers, we can get that the CoO6 octahedral cage has a c/a ratio of 1.29 for BiCoO3, much larger than that of BiFeO3 (1.01).27−29 Concerning Bi5Ti3Fe1−xCoxO15 series samples, the more substitution of Co3+ ions into the compound, the more distortion of constituent BO6 cages may have and the larger inner stress Bi5Ti3Fe1−xCoxO15 has until the crystal symmetry is destroyed finally, and then is divided into Bi4Ti3O12 and other crystal phase(s). The relationship between the lattice parameter and the doped Co3+ content is further discussed in Figure 2 and Table S1 of the Supporting Information. 3.2. HAADF Image Analysis. Figure 2a,b shows the STEM images of Bi5Ti3CoO15 (x = 1.00) and Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) samples along the [011] projection, respectively. The atomic scale HAADF images clearly demonstrate that Bi5Ti3CoO15 (x = 1.00) is of a three layer perovskite-like structure similar to Bi4Ti3O12, while Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) is of a four layer perovskite-like structure similar to Bi5Ti3FeO15. Figure 2c,d shows two images of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) along the [001] projection with different structural defects. From Figure 2c, one can clearly see the intergrowth structure of three and four layers perovskite-like structure along

2. EXPERIMENTAL SECTION The Bi5Ti3Fe1−xCoxO15 (x = 0, 0.25, 0.33, 0.50, 0.67, 0.75, 1.00, respectively) nanoparticles were prepared by the polyacrylamide gel method.26 Then, the gel was calcined at 700 °C for 5 h in a tube furnace. The resulting nanopowders were compressed to a disc with a diameter of 10 mm and a thickness of about 1−2 mm under a pressure of 400 MPa. Afterward, the discs were calcined at 850 °C for 5 h. The inset of Figure 1 presents the morphology of the nanopowders and

Figure 1. (a) The XRD patterns of the Bi5Ti3Fe1−xCoxO15 disc sample at room temperature; the insets are the corresponding images of the intermediate nanopowders and the used disc sample. (b, c) The schematic diagrams of tetragonal crystal BiFeO3 and BiCoO3. disc. The calcined discs were used for XRD measurements and magnetic property characterizations. The specimens for STEM investigation were prepared by a standard procedure including mechanical grinding, dimpling, polishing, and final thinning by Arion milling with liquid nitrogen cooling. The XRD measurements were performed on a Philips X’Pert Pro diffractometer, and high-resolution STEM was carried out on a JEM-ARM200F microscope with double B

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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3.3. STEM-EDX Mapping Analysis. Figure 3 illustrates the atomic resolution STEM-EDX mapping images of Fe, Co, Ti

Figure 2. (a, b) The atomic resolution HAADF images of Bi5Ti3CoO15 (x = 1.00) and Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) along the [011] projection, and their corresponding structure schematic diagram indicated in the red rectangular boxes. (c, d) The HAADF images of two sections of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) sample along the [001] projection and the corresponding simplified structural model in which the white circles denote the Bi atomic columns location generated by the PPA software. (e) The schematic diagram of Bi4Ti3O12 and Bi5Ti3FeO15 along the [001] zone axis.

Figure 3. The atomic resolution STEM-EDX mapping. (a−c) The images of Fe (red), Co (cyan), Ti (blue) mapping overlaying on according HAADF image of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) sample along the [011] projection. (d) The mixed color map with Fe, Co, Ti overlaying on according HAADF image.

the [100] direction. We extracted the location of Bi atomic columns using the software of peak pair analysis (PPA).30 In this process, the PPA software determines the atom positions through the peak-finding method by detecting the local gray maxima in the real space. We finally denoted the Bi atom positions with the white circles on a gray background, as shown in Figure 2c. It again demonstrates that this region has an intergrowth of a four layer structure (corresponding to three Bi layers) and three layer structure (corresponding to two Bi layers) by the intervals of Bi2O2 layers highlighted in light violet. Moreover, we describe the intersection angle relationship of the four adjacent Bi atomic column locations in the Bi2O2 layers by regular orange trapezoids and inverted blue trapezoids. It is obvious that the trapezoids on the both sides of three Bi layers are of opposite types, while the trapezoids on the both sides of two Bi layers are of the same type. Schematic structures of Bi5Ti3FeO15 with three Bi layers and Bi4Ti3O12 with two Bi layers are presented in Figure 2e. One can see that the four layer structure (three Bi layers) has a trapezoids configuration and the trapezoids on the both sides of three Bi layers are opposite to each other, while the three layer structure (two Bi layers) has a rectangle component, which is different from the trapezoids in Figure 2c. It should be noted that the intersection angles in the Bi2O2 layers can be a sign of rotation of BO6 octahedra. The rectangle represents no rotation of octahedra, while the trapezoid represents rotated octahedra. In the intergrowth structure, the three Bi layers, namely, the four layer structure, can induce the rotation of the octahedra in the two Bi layers, namely, the three layer structure. Figure 2d shows another type of structural defect that the transformation from the three layer structure to the four layer structure along the direction parallel to the Bi2O2 layers. It is easy to find that the transformation is accomplished by the inset of additional Bi2O2 layers. We used PPA software to extract the locations of Bi atomic columns and show the result in Figure 2d, denoted with white circles on a gray background. It is obvious that the trapezoids on the both sides of three Bi layers are of the opposite type, while the trapezoids on the both sides of two Bi layers are of the same type. Additionally, the trapezoids in the transformation section are of the opposite type.

elements and their mixed overlaying on the corresponding HAADF image with four layer and three layer structures in the Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) sample along the [011] projection. From the images, one can see that the Fe, Co, Ti elements are homogeneously distributed in the perovskite layers between the two Bi2O2 layers. It should be noted that the Fe and Co elements also exist in the three layer structure, indicating that the three layer structure is Bi4Ti3−m−nFemConO12 rather than pure Bi4Ti3O12. On the basis of the above results, it is found that the ability of substitution of Fe, Co elements at the B site is closely related to the rotation of BO6 octahedra. To our knowledge, no substitution of Fe and/or Co elements has been realized in the three layer structure Bi4Ti3O12. 3.4. Magnetic Properties Analysis. To further understand the influence of the substitution of cobalt on the magnetic properties of Bi5Ti3Fe1−xCoxO15 samples, we carefully examined the temperature dependences of magnetization in zero field cooled (ZFC) and field cooled (FC) modes in the warming process under an applied DC magnetic field of 200 Oe. Figure 4a presents the ZFC-FC curves of Bi5Ti3FeO15 (x = 0) and BiTi3CoO15 (x = 1) samples. In the temperature range of 5−400 K, there is no obvious difference between the ZFC and FC curves for each sample. It can be seen from the 1/χ−T curves, shown in Figure 4b, that the magnetic susceptibilities of the two samples follow the Curie−Weiss law above 205 K for BiTi3FeO15 (x = 0) and above 16 K for BiTi3CoO15 (x = 1). With decreasing temperature, the reciprocal magnetic susceptibility exhibits deflexibility toward the temperature axis, implying that these two samples are of weak antiferromagnetism at low temperature. The AFM characteristics can be reconfirmed by the M−H plots at 5 and 300 K in Figure 4c,d, showing almost inclined lines. The AFM property of the Bi5Ti3FeO15 (x = 0) sample was also reported in previous literature.17 On the basis of the above XRD and STEM results, we consider that the weak AFM of Bi5Ti3CoO15 (x = 1) may originate from a second phase. It should be noted that the C

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) The ZFC-FC curves of Bi5Ti3FeO15 (x = 0) and Bi5Ti3CoO15 (x = 1) measured in 200 Oe. (b) The reciprocal magnetic susceptibility versus temperature 1/χ−T curves derived from the ZFC mode curves of (a). (c, d) The hysteresis loops of Bi5Ti3FeO15 (x = 0) and Bi5Ti3CoO15 (x = 1) at temperatures of 5 and 300 K.

Figure 5. (a) The ZFC-FC curves of Bi5Ti3Fe0.75Co0.25O15 (x = 0.25), Bi5Ti3Fe0.67Co0.33O15 (x = 0.33), and Bi5Ti3Fe0.50Co0.50O15 (x = 0.50) measured in 200 Oe. (b) The reciprocal magnetic susceptibility versus temperature 1/χ−T curves derived from the ZFC mode curves of (a). (c, d) The hysteresis loops of Bi5Ti3Fe0.75Co0.25O15 (x = 0.25), Bi5Ti3Fe0.67Co0.33O15 (x = 0.33), and Bi5Ti3Fe0.50Co0.50O15 (x = 0.50) at temperatures of 5 and 300 K.

= 0.33) show an obvious hysteresis behavior with a coercive field of 2680 and 3067 Oe and a remnant magnetization of 0.121 μB/magnetic-atom and 0.128 μB/magnetic-atom, respectively, implying the existence of FM interaction. At 300 K, the two samples also show hysteresis behavior, but reach their saturation magnetization under a very small magnetic field. There is a divergence between the ZFC and FC curves of these two samples at about 400 K. From the ZFC curves of the two samples, one can see that their magnetizations increase with increasing temperature from 160 to 400 K. On the basis of the

effective magnetic moment of Bi5Ti3CoO15 (x = 1) is 1 order of magnitude smaller than that of Bi5Ti3FeO15 (x = 0). From the XRD patterns, Bi5Ti3CoO15 (x = 1) has the most amount of the second phase among the series of Bi5Ti3Fe1−xCoxO15 samples, but it has so weak effective magnetic moment, so we can conclude that the contribution of the second phase in the series of samples to the magnetic moment can be negligible. The M−T and M−H measurements of Bi5Ti3Fe1−xCoxO15 (x = 0.25, 0.33, and 0.50) samples are presented in Figure 5. At 5 K, Bi5Ti3Fe0.75Co0.25O15 (x = 0.25) and Bi5Ti3Fe0.67Co0.33O15 (x D

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) The ZFC-FC curves of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) and Bi5Ti3Fe0.25Co0.75O15 (x = 0.75) measured in 200 Oe. (b) The reciprocal magnetic susceptibility versus temperature 1/χ−T curves derived from the ZFC mode curves of (a). (c, d) The hysteresis loops of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) and Bi5Ti3Fe0.25Co0.75O15 (x = 0.75) at temperatures of 5 and 300 K.

of Bi5Ti3FeO15. Different from Ti4+, Fe3+ with 3d0 and 3d5 configurations, respectively, the Co3+ with an intermediate spin state t2g5eg1 or a high spin state t2g4eg2 is a Jahn−Teller ion, which leads to a distortion of the CoO6 octahedron identified by the difference of Co−O bond lengths. Due to the different radii between Co3+ ion and Ti4+, Fe3+ ions, the substitution of Co ions could also cause the rotation of BO6 octahedra. Additionally, the displacement of Co3+ ions from the center of octahedra is much larger than that of Fe3+ ions in tetragonal crystal BiFeO3 and BiCoO3,28,29 and may play an important role in physical properties. It is very necessary to find out which effect has the most important role in inducing the change of crystal structure and magnetic properties through increasing the doping content of cobalt ions. An atomic scale ABF image of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) and partially enlarged details along the [011] projection are shown in Figure 7. The set of blue balls, green octahedra with green balls in the center, and small double red balls on the

ZFC curves and M−H plots, it can be concluded that this phenomenon is caused by the magnetic anisotropy. It is interesting that the blocking phase transition occurs in a so broad temperature interval. For Bi 5 Ti 3 Fe 0.50 Co 0.50 O 15 (x = 0.50), the magnetic susceptibility follows the Curie−Weiss law above 302 K in the 1/χ−T curve. The M−H curve at 5 K shows an obvious hysteresis behavior with a coercive field of 2196 Oe and a remnant magnetization of 0.063 μB/magnetic-atom, while the M−H curve at 300 K shows a typical paramagnetic behavior. There is also a divergence between the ZFC and FC curves at about 245 K and a broad peak near 231 K, caused by the magnetic anisotropy. In contrast with Bi5Ti3Fe0.75Co0.25O15 (x = 0.25) and Bi5Ti3Fe0.67Co0.33O15 (x = 0.33), the blocking phase transition occurs in a relatively narrower temperature interval, showing superparamagnetism-like behavior. These experimental results of BiTi3Fe0.50Co0.50O15 (x = 0.50) are different from the earlier reports.15 Figure 6 shows the ZFC-FC curves and M−H plots of Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) and Bi5Ti3Fe0.25Co0.75O15 (x = 0.75). It is obvious that both the ZFC and the FC curves show that the magnetization first decreases, then increases and increases again with increasing temperature, forming peaks at 31 K for BiTi3Fe0.33Co0.67O15 (x = 0.67) and 20 K for BiTi3Fe0.25Co0.75O15 (x = 0.75). All of these are different from the above other samples. These steep rise curves are caused by the spin state transition of Co ions from the low spin state to the intermediate spin state, similar to the LaCoO3 compound.31,32 At the low temperature, the ground state of Co3+ ions is nonmagnetic with a low spin state of t2g6eg0 (S = 0). With increasing temperature, the spin state transfers to an intermediate spin state t2g5eg1 (S = 1), demonstrated by the steep increase of magnetic susceptibility.

Figure 7. (a, b) The atomic resolution ABF images of the Bi5Ti3Fe0.33Co0.67O15 (x = 0.67) sample along the [011] projection and partial enlarged details. The sets of blue balls and green octahedral are of the typical structure of Bi5Ti3FeO15. The red balls linked by blue dashed lines and green balls surrounded by blue dashed lines are the sites of double oxygen and the B site which are obtained from the contrast of ABF images.

4. DISCUSSION The B site substitution of Co3+ ion with a 3d6 configuration will give rise to the changes of the structure and physical property E

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the National Key Research Program of China (Grant No. 2016YFA0300701) and the National Natural Science Foundation of China (Grant Nos. 11174336, 11374343).

corner are of the typical structure of Bi5Ti3FeO15. The green balls surrounded by blue dashed lines and the red balls linked by blue dashed lines are B site atoms and overlapped sites of double oxygen, judged by the image contrast of the ABF. From the comparison between areas 1 and 2 in Figure 7b, we can clearly observe that the Bi3+ sites (blue) and B (Ti4+, Fe3+, Co3+) sites (green) almost remain on the positions of the typical structure, and the double oxygen sites (red) also keep unchanged along the [100] direction but have an obvious holistic movement along the [011̅] direction, inducing a much larger displacement of B site atoms from the center of BO6 octahedra. According to the analyses above, we can draw a scenario as follows: With increasing cobalt content in Bi5Ti3Fe1−xCoxO15, the displacement of the B site atom from the center of BO6 octahedra increases, thus resulting in a decreased Fe−O−Fe bond angle. Such a weakening of the Fe− O−Fe AFM exchange interaction would induce weak ferromagnetism.33,34 Therefore, it is reasonable that the magnetization increases with increasing cobalt content from x = 0 to x = 0.33. With increasing cobalt content, the amount of Fe−O−Fe exchange interaction decreases, instead Fe−O−Co interaction as well as Co−O−Co coupling increases, resulting in a decrease of magnetic moment. As a result, as the cobalt substitution increases to x = 0.50, the magnetic moment begins to decrease. With further increasing cobalt content, the samples ultimately show cobalt ions dominated properties, as in the cases of x = 0.67 and x = 0.75 samples. Eventually, the results of the sample of x = 1.00 come from structural phase separation, namely, by the Bi4Ti3O12 and other impurities.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02347. Selected area electron diffractions of Bi5Ti3FeO15-like and Bi4Ti3O12-like samples; relationship between the cobalt doping content and the lattice parameters obtained from the XRD patterns (PDF)

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5. CONCLUSIONS Co3+ ions doping at the B site of the perovskite block in Bi5Ti3Fe1−xCoxO15 has strong effects on the lattice structure and magnetic properties. With different doping content, the sample is of four layer, three layer, or coexisting phases of the two perovskite-like structures. For the magnetic property, it shows antiferromagnetism, spin state of Co3+ determined, or magnetic anisotropy determined magnetisms. These changes are mainly caused by the different displacement of ions from the center of BO6 octahedra. Meanwhile, Bi5Ti3Fe1−xCoxO15 (x = 0.25 and 0.33) samples show room temperature ferromagnetism, which could be new candidates for practical application of magnetoelectric materials.

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*E-mail: [email protected]. ORCID

Richeng Yu: 0000-0002-8086-0910 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02347 Inorg. Chem. XXXX, XXX, XXX−XXX