Structural Distortion, Spin-Phonon Coupling, Interband Electronic

Article pubs.acs.org/IC

Structural Distortion, Spin-Phonon Coupling, Interband Electronic Transition, and Enhanced Magnetization in Rare-Earth-Substituted Bismuth Ferrite Jie Wei,*,†,‡ Chunfang Wu,§ Yalong Liu,† Yaxin Guo,† Tiantian Yang,† Dawei Wang,† Zhuo Xu,† and Raphael Haumont‡ †

Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, P. R. China ‡ Laboratoire de Physico-Chimie de l’Etat Solide, ICMMO, CNRS-UMR 8182, Bâtiment 410 -Université Paris-Sud XI, 15 rue Georges Clémenceau 91405 Orsay Cedex, France § School of Photoelectric Engineering, Xi’an Technological University, Xi’an 710021, P. R. China ABSTRACT: Rare-earth ions (RE = La3+, Nd3+, and Er3+) substituted BiFeO3 (BFO) ceramics were synthesized by a conventional solid-state sintering procedure. X-ray diffraction patterns and Raman spectra confirm a rhombohedral R3c symmetry in all samples with significant distortion in FeO6 octahedron, as well as the occurrence of remarkable spin-phonon coupling and notable change in magnetic transition temperature induced by RE dopants. The enhanced magnetization was observed in all RE-doped BFO ceramics, unveiling that the spatial spin structure of BFO should be perturbed by RE dopants. Diffuse reflectance spectra show a conspicuous evolution of interband electronic transitions in RE-doped BFO ceramics. Especially, the two crystal-field d−d band transitions (6A1g→4T1 and 6A1g→4T2g) exhibit a linear red-shift behavior with the reduction in the cell volume, which is welllinked with a linear tendency of increased magnetization. On the basis of these investigations, a possible mechanism was proposed in this paper to demonstrate the correlation between the structural distortion, interband electronic transitions, and magnetic properties in RE-doped BFO ceramics.

I. INTRODUCTION Bismuth ferrite (BiFeO3, BFO), as a prototypical multiferroic oxide, has been intensely studied,1−6 since it can exhibit both ferroelectricity and anti-ferromagnetism above room temperature (the ferroelectric Curie temperature TC ≈ 1100 K and anti-ferromagnetic Néel temperature TN ≈ 643 K). At ambient condition, BFO crystallizes in a rhombohedrically distorted perovskite structure, which belong to the R3c space group allowing antiphase octahedral tilting and ionic displacements from the centrosymmetric positions about and along, respectively, a same [111] cubiclike direction.7 Over the years, it has attracted much attention in synthesis of BFO thin film,8−12 since a very large ferroelectric polarization (Pr ≈ 60 μC/cm2) was first achieved in a [001]-oriented BFO thin film epitaxially grown on SrRuO3/SrTiO3.1 Theoretical investigation has also addressed the extraordinarily large ferroelectric polarization of BFO system.2 In contrast to thin film, bulk BFO ceramic still exhibits very weak ferroelectric behavior, mainly due to its high electrical conductivity or appearance of secondary phases, which further hinders its practical application. What is worse, a cycloid-type spatial spin modulation superimposed on the G-type anti-ferromagnetic spin ordering of BFO, prevents the observation of an intrinsic and weak ferromagnetic moment driven by Dzyaloshinsky− © 2017 American Chemical Society

Moriya (DM) interactions occurring below the Néel temperature TN. Therefore, it is still difficult or more challenging to directly observe the room temperature magnetoelectric coupling in bulk BFO ceramic. Despite many possible attempts to overcome the aforementioned difficulties,13−18 substitution of rare-earth (RE) elements or other metal elements in A(Bi)- or B(Fe)-site of BFO compound has been proposed as a promising approach to suppress the formation of secondary phases,16 improve ferroelectric properties by reducing leakage current,19,20 and especially induce weak ferromagnetism by perturbing the spatially modulated cycloid spin structure.21,22 For the past few years, although numerous studies were conducted to investigate the effect of RE dopants (La, Gd, Ho, etc.)20,21,23−26 or metal dopants (Ba, Ti, Mn, etc.)22,27−30 on the multiferroic properties of BFO ceramic, the origin of the enhanced magnetization or weak ferromagnetism induced by dopants still remains controversial. In most cases,21−25,27 this enhanced magnetization has been thought of as an oversimplified contribution of partial collapse in the spatial spin magnetic structure without any further investigation of its origin. Even Received: April 17, 2017 Published: July 12, 2017 8964

DOI: 10.1021/acs.inorgchem.7b00914 Inorg. Chem. 2017, 56, 8964−8974

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Inorganic Chemistry more, some magnetic secondary phases such as Fe3O4, γ-Fe2O3, as a kind of byproduct during the high-temperature sintering or annealing procedure, are also speculated as the possible contribution to the enhanced magnetization in the doped BFO ceramics.26,29 More recently, much attention has been attracted to probe the complex and puzzling electronic band structure in BFO system and investigate its spin-charge-lattice coupling using optical spectroscopy.31−33 Especially, the interband electronic transition, for example, the two crystal-field transitions of 6 A1g→4T1 and 6A1g→4T2g, strongly depend on the Fe−O bond length, site symmetry, and Fe−O−Fe exchange interaction, which is also confirmed to be magnetically sensitive.32 In this case, crystalline structure and magnetic and optical properties of RE-doped BFO ceramics were investigated by X-ray diffraction (XRD), Raman scattering spectroscopy, magnetometer, and optical spectroscopy. The elaborate unit-cell parameters extracted by refining XRD patterns based on the Rietveld method manifest striking contraction in cell volume and significant distortions in FeO6 octahedron that may be associated with the Jahn−Teller or pseudo Jahn−Teller effect.34,35 Recent theoretical calculations suggested that the pseudo Jahn−Teller effect should play a key role in the origin of multiferroicity in some ABO3 perovskites (A = alkali ion, alkaline-earth ion, or f-filling rare-earth ion; B = magnetic transition-metal ion; O = oxygen ion) with a specific magnetic dn configuration of B ion (e.g., Fe3+(d5) in BiFeO3) .36−38 After proper fitting, the high-temperature Raman spectra reveal a remarkable spin-phonon coupling and notable changes in Neel temperature. On the basis of crystal field theory, a linear redshift behavior in interband electronic transitions, as well as the cell volume reduction, is appropriately correlated with the enhanced magnetization. All these investigations make it possible to propose a reasonable mechanism for good comprehension on correlation between the crystalline structural distortion, interband electronic transitions, and magnetic properties in RE-doped BFO ceramics.

III. RESULTS AND DISCUSSION A. Crystalline Structure. The typical XRD patterns of pure and rare-earth-doped BiFeO3 (RE-doped BFO) ceramics (Bi0.95La0.05FeO3 (BFLa05), Bi0.95Nd0.05FeO3 (BFNd05), and Bi0.95Er0.05FeO3 (BFEr05)) are shown in Figure 1. All the

Figure 1. (a) Rietveld refinements of the XRD patterns for pure and RE-doped BiFeO3 ceramics (the abbreviations of obs., cal. and diff. represent the observed data, calculated data, and difference between above data, respectively). Schematic representation of (b) the BiFeO3 unit cell shown in the pseudocubic setting, both the polarization and octahedral tilting along the same threefold axis of [111] cubiclike direction, and (c) its structural parameters, such as the FeO6 octahedral tilt angle (±ω) and the Fe−O−Fe bond angle (θ).

II. EXPERIMENTAL SECTION Pure BiFeO3 and Bi0.95RE0.05FeO3 ceramics (RE = La, Nd, Er) were prepared by a conventional solid-state sintering procedure. High-purity oxides of Bi2O3, Fe2O3, and La2O3 (Nd2O3, Er2O3) were used as the starting materials. After a series process of weighing, ball milling, and drying, the mixed powders were pressed into small cylindrical pellets with a diameter of ∼8 mm and thickness of ∼0.5 mm. Then, the samples were sintered at 820 °C for 3 h in air so as to obtain pure phase. Crystalline structure and phase purity of the samples were characterized by XRD on a Rigaku D/MAX-2400 X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å). Rietveld refinements were performed using XND analysis program. Room-temperature polarized Raman spectra were recorded in a backscattering geometry using a LABRAM Jobin−Yvon spectrometer (He−Ne laser, 632.8 nm). The high-temperature Raman measurements were performed by using a commercial LINKAM heating stage placed under the Raman microscope. The room-temperature ultraviolet−visible (UV−vis) diffuse reflectance spectra (DRS) were measured by using a PerkinElmer spectrometer (Lambda 950) equipped with a diffuse reflectance accessory. In this paper, a dilution technique that eliminates the regular part of the reemitted radiation was used, which was done by diluting a 10 wt % of BiFeO3 sample homogeneously in an inactive, nonabsorbing standard BaSO4 powder and then recording DRS. Magnetization−magnetic field-dependent (M−H) curves were obtained using a superconducting quantum interference device magnetometer (SQUID, Quantum Design).

diffraction peaks in the XRD patterns can be successfully indexed as a rhombohedral distortion perovskite structure with a space group of R3c (ICDD File Card No. 86−1518), which clearly indicates a pure phase for all sintered ceramics without any secondary phases. Therefore, RE-doped BFO ceramics (BFLa05, BFNd05, and BFEr05) have the same crystalline structure as that of the parent compound, suggesting that the ferroelectric features of doped ceramics should be close to that of BFO. Rietveld refinements for the XRD patterns were performed by using XND software (as shown in Figure 1). Furthermore, the variations of unit-cell parameters for pure and doped BFO samples are summarized in Table 1. The results and analysis reveal that both ah and ch parameters continuously decrease following the doping sequence of La, Nd, and Er, leading to the contraction in the unit cell volume (Vcell) because of the smaller ionic radii of La3+ (1.16 Å), Nd3+ (1.11 Å), and Er3+ (1.01 Å) than that of Bi3+ (1.17 Å). Contraction in the lattice should lead to the variation in the interatomic bond distances, bond angles, 8965


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Inorganic Chemistry Table 1. Unit Cell Parameters for BiFeO3 and Bi0.95RE0.05FeO3 (RE = La, Nd, and Er) Ceramics

local inhomogeneneous distortion

cell parameters samples

ah (Å)

ch (Å)

Vcell (Å3)

average Fe−O bond length (Å)

Fe−O−Fe angle, θ (deg)

FeO6 tilt angle, ω (deg)

(cc − ac)/cc

BiFeO3 Bi0.95La0.05FeO3 Bi0.95Nd0.05FeO3 Bi0.95Er0.05FeO3

5.5792(3) 5.5781(1) 5.5716(2) 5.5643(5)

13.8695(6) 13.8669(5) 13.8515(3) 13.8385(8)

373.88(2) 373.66(5) 372.38(1) 371.05(7)

2.0322(6) 2.0298(2) 2.0263(5) 2.0209(8)

157.76(3) 156.52(5) 155.75(2) 152.77(5)

12.6(2) 13.2(5) 13.6(3) 15.1(2)

1.46(6)% 1.46(7)% 1.47(2)% 1.50(9)%

ferromagnetic tilting distortions in some ABO3 perovskite crystals. In this case, the remarkable structural distortions induced by the RE dopants means the strong Jahn−Teller or pseudo Jahn−Teller effect. Therefore, the polarization is believed to present significant enhancement in these REdoped BFO compounds, as well as possibly occurred ferromagnetic ordering. The latter was confirmed by the magnetic measurement. B. Raman Scattering Spectra. Raman scattering spectroscopy has been well-known as a powerful technique for the investigation of even subtle structural distortions both within a space group (via band shifts) or due to a phase transition (via band splitting and/or soft modes, etc). At room temperature BFO belongs to rhombohedral R3c space group with two formulas in one primitive cell. With respect to the cubic Pm3̅m structure, the rhombohedral structure can be represented by an antiphase tilt of the adjacent FeO6 octahedron and a displacement of both Fe3+ and Bi3+ cations from their centrosymmetric position along the pseudocubic [111]pc direction. According to the group theory, the 10 atoms in the unit cell of its rhombohedral R3c (C3v) structure give rise to 13 Raman phonon modes in the zone center (k ≈ 0)40

and octahedral tilts or distortions, which further affect the multiferroic properties of BFO. As above-mentioned, BFO is a highly rhombohedrally distorted perovskite with space group R3c. This rhombohedral structure can be obtained by an antiphase rotation (tilt) of the adjacent FeO6 oxygen octahedra around the [111]c direction relative to the Pm3̅m cubic cell (a−a−a− in Glazer’s notations) and a displacement of Fe3+ and Bi3+ cations from their centrosymmetric positions along the same [111]c direction (illustrated in Figure 1b). BFO is one of the very few perovskites of ABO3 type, which exhibits both cation displacements that drive the ferroelectric property and octahedral tilts that play a key role regarding the magnetic property at ambient conditions. As a consequence, the complex competition or interplay between the two structure instabilities (tilts of FeO6 octahedra and polar cation displacements inside the FeO6 or BiO12 polyhedra) has significant impacts on the magnetoelectric coupling or multiferroic properties of BFO. Especially, the ab initio-based approach implied the occurrence of the complex interplay between the FeO6 octahedral tilt and magnetic ordering.5 Extracted from the refinements of the XRD patterns, the data of FeO6 octahedral tilt angle ω and Fe−O− Fe bond angle (where two Fe3+ are in the centers of neighboring octahedra) θ (schematically shown in Figure 1c) are summarized in Table 1. Apparently, a monotonic decrease of the average Fe−O−Fe angle from 157.76° in BFO to 152.77° in BFEr05 was observed, and also a similar tendency was observed in the decrease of the average Fe−O bond length. Correspondingly, there presents a monotonic increase of the FeO6 octahedral tilt angle from 12.6° in BFO to 15.1° in BFEr05. Obviously, RE substitution seems to buckle the FeO6 octahedra and undoubtedly will affect the magnetic properties of RE-doped BFO ceramics, since not only Fe ions are close to each other, derived from the decrease in the average Fe−O bond length or average Fe−O−Fe angle implying an enhancement in the oxygen-related superexchange interaction,16 but also there present intense changes in the FeO6 octahedral tilts or distortions playing a key role regarding the magnetic property.5 More interestingly, the rhombohedral distortion ((cc − ac)/ cc, cubiclike lattice parameters, ac = ah/√2, cc = ch/2√3) exhibits an anomalous increase of ∼3% in BFEr05 compared with pure BFO. In the literature,34,35 this structural distortion induced by dopants could mainly be ascribed to the Jahn− Teller distortion of FeO6 octahedron. Bersuker et al.36−38 theoretically demonstrated that the pseudo Jahn−Teller effect should be the origin of multiferroicity (coexistence of ferroelectricity and magnetism) in some ABO3 perovskites with a specific magnetic dn configuration of B ion (so-called as “pseudo Jahn−Teller coupling theory”). Garcia-Fernandez et al.39 also suggested that ferroelectric and octahedral distortions are favored by ferromagnetic ordering rather than anti-

ΓRaman,R3c = 4A1 + 9E

(1)

Figure 2 depicts the Raman scattering spectra of pure and RE-doped BFO ceramics at room temperature. To obtain the

Figure 2. Raman scattering spectra of pure and RE-doped BiFeO3 ceramics at room temperature.

exact peak position, each measured spectrum was properly fitted by decomposing the fitted curves into individual Lorentzian components. The corresponding fitting results are summarized in Table 2. As expected, all the Raman active modes (4A1+9E) associated with R3c structure of BFO 8966


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Inorganic Chemistry Table 2. Raman Modes of BiFeO3 and Bi0.95RE0.05FeO3 (RE = La, Nd, and Er) Ceramics Raman shift (cm−1) Raman modes

Bi0.95Er0.05FeO3

Bi0.95Nd0.05FeO3

Bi0.95La0.05FeO3

BiFeO3

A1-1 A1-2 A1-3 A1-4 E-1 E-2 E-3 E-4 E-5 E-6 E-7 E-8 E-9

142.9 ± 0.2 172.7 ± 0.5 231.4 ± 0.3 430.8 ± 0.3 72.5 ± 0.5 120.9 ± 0.4 262.7 ± 0.3 277.7 ± 0.5 345.1 ± 0.2 371.2 ± 0.4 474.6 ± 0.5 522.6 ± 0.3 619.8 ± 0.2

140.9 ± 0.3 172.9 ± 0.2 228.3 ± 0.3 430.3 ± 0.5 72.6 ± 0.2 119.3 ± 0.3 262.1 ± 0.5 275.3 ± 0.3 338.3 ± 0.5 370.1 ± 0.5 469.8 ± 0.3 522.1 ± 0.2 618.1 ± 0.3

140.1 ± 0.5 173.1 ± 0.3 230.1 ± 0.5 431.6 ± 0.3 72.8 ± 0.5 118.2 ± 0.5 261.8 ± 0.2 275.9 ± 0.2 345.7 ± 0.3 370.7 ± 0.3 470.2 ± 0.2 522.8 ± 0.4 618.7 ± 0.2

139.3 ± 0.3 173.3 ± 0.5 227.8 ± 0.4 431.2 ± 0.4 73.3 ± 0.3 117.3 ± 0.2 261.2 ± 0.4 276.8 ± 0.3 346.9 ± 0.5 370.3 ± 0.2 472.3 ± 0.4 522.3 ± 0.3 617.3 ± 0.5

Figure 3. (a) Elaborated fits of the four dominant Raman modes in the region of 50−250 cm−1 for pure and RE-doped BiFeO3 ceramics; (b) illustrations to display the A1 and E vibration normal modes.

first-principles calculation, Hermet et al.41 reported that Bi atoms only participated in low-frequency modes up to 167 cm−1 and that the motion of oxygen atoms dominated the modes above 262 cm−1, whereas Fe atoms were mainly involved in modes between 152 and 262 cm−1 as well with possible contribution to higher frequency modes. Therefore, the two characteristic modes such as A1-1 mode (∼140 cm−1) and E-1 mode at ∼73 cm−1 should be governed by Bi−O covalent bonds, which control the dielectric constant and the ferroelectric phase of BFO.41 The above Raman spectra results reveal that the crystal structure and Bi−O covalent bonds remain relatively stable in RE-doped BFO ceramics, since above two modes do not present any remarkable changes, although the line widths present slight broadening due to the A(Bi)-site ion disorder induced by the RE ions substitution. It suggests that RE dopants should not disturb the ferroelectric nature of BFO compound. Nevertheless, the other A1 modes (A1-2 ≈ 173 cm−1 and A1-3 ≈ 230 cm−1) associated with Fe atoms responsible for magnetism exhibited drastic changes accompanied by the

predicted by theory41 were observed in all the samples. Any other Raman mode derived from impure phases was not found in our samples, which further confirmed that RE dopants did not change the crystalline structure of parent BFO, coincided with the XRD results. A closer inspection of the Raman spectra shown in Figure 2 suggests that the four dominant Raman vibrations at lowfrequency region are E-1 mode near 73 cm−1, A1-1 mode near 140 cm−1, A1-2 mode near 173 cm−1, and A1-3 mode near 230 cm−1, respectively. The elaborated fits of Raman modes in the region of 50−250 cm−1 are shown in Figure 3a. The spectral features of RE-doped BFO ceramics present an apparent increase in the intensity of A1-3 mode (∼230 cm−1) and slight broadening for A1-1 or E-1 mode, which suggest apparent changes in phonon behavior. The slight broadening in E-1 and A1 Raman modes in RE-doped BFO ceramics should be attributed to the lattice anharmonicity and disorder produced by fluctuations in the Bi site occupancies. Figure 3b shows the illustration of A1-1, A1-2, A1-3, and E-1 vibration normal modes in BFO system. On the basis of the 8967


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increasing temperature were plotted to clearly display the phonon anomalies in the vicinity of TN, as shown in Figure 4d− f. The accurate phonon frequencies were evaluated by fitting the spectra using multiple peaks fitting with a Lorentzian line shape function. Likewise, the phonon modes softening behavior (such as the band position red shift and fwhm increase with increasing temperature) should be attributed to the anharmonic effect of the lattice. More interestingly, significant steplike anomalies in both band position and fwhm of two representative A1 modes were observed at the temperature of ∼640 K for BFLa05, ∼630 K for BFNd05, and ∼610 K for BFEr05 ceramic, respectively. Especially, any structural transition was not observed around this temperature in these ceramics, since no extra mode is induced through the entire temperature range. On the basis of earlier reports,42−44 this phonon anomaly behavior should be attributed to manifestation of the spin-phonon coupling, and it is believed to be linked to structural instability originated by gradual rotation of the FeO6 octahedron near TN. Therefore, these temperatures of 640, 630, and 610 K should be corresponding to the Neel temperatures for BFLa05, BFNd05, and BFEr05 ceramics, respectively. In short, the temperature-dependent Raman spectra further reveal that RE dopants did not disturb the multiferroic nature of BFO system, due to the strong spin-phonon interaction existing in these ceramics. Nevertheless, RE dopants indeed influenced the magnetic properties of BFO compound, since their magnetic ordering transition temperature intensely changed, which will be confirmed at the later magnetic measurement. C. Magnetic Properties. Since a G-type anti-ferromagnetic (AFM) spin ordering modulated by a cycloid-type spatial spin structure with a long periodicity of ∼62 nm exists in BFO, it prevents the observation of an intrinsic and weak ferromagnetic moment driven by Dzyaloshinsky−Moriya (DM) interactions below TN. However, it is believed that doping the crystalline structure is an efficient approach to perturb this spatial spin modulation and allow the appearance of a net magnetization through canting of the spins. Magnetization−magnetic field-dependent (M−H) curves of pure and RE-doped BFO ceramics measured at room temperature are shown in Figure 5. As expected, the results confirm that pure BFO ceramic acts as an intrinsic antiferromagnet with a collinear incommensurate anti-ferromagnetic structure because of its almost linear M−H curve (see the enlarged M−H curves in the inset of Figure 5) and a negligible remanent magnetization (Mr, shown in Table 4). However, an apparent weak ferromagnetism presents in the RE-doped BFO ceramics, since their M−H curves display nonlinear and apparent hysteretic character (shown in the inset of Figure 5), with remarkable enhancement of the remanent magnetization (Mr). For example, a remnant magnetization of Mr ≈ 0.012 emu/g that is 6 times more than that of pure BFO was observed in BFNd05 ceramic. As seen in Table 4, the remnant magnetization increased as the ions radii of dopants decreased. The largest Mmax of ∼1.423 emu/g (corresponding to 0.079 μB/ Fe) and Mr of ∼0.023 emu/g were observed in BFEr05 ceramic. Since BFO compound is unstable and possible decomposition during the high-temperature sintering or annealing procedure, some magnetic secondary phases such as Fe3O4 and γ-Fe2O3 are speculated as the possible contribution to the enhanced magnetization or weak ferromagnetism in RE-doped BFO ceramics. Nevertheless, both XRD results and Raman spectra of RE-doped BFO ceramics suggest that any secondary

introduction of RE dopants. The relative intensity of A1-2 mode slightly weakens, while that of A1-3 mode intensely strengthens. As shown in Table 3, compared with the A1-1 mode, the Table 3. Relative Intensity of Raman Modes for BiFeO3 and Bi0.95RE0.05FeO3 (RE = La, Nd, and Er) Ceramics samples relative intensity of Raman modes I(A1-2)/ I(A1-1) I (A1-3)/ I(A1-1)

BiFeO3

Bi0.95La0.05FeO3

Bi0.95Nd0.05FeO3

Bi0.95Er0.05FeO3

65.1%

53.8%

45.2%

43.5%

4.8%

5.7%

7.1%

12.9%

relative intensity of A1-2 mode gradually decreased from 65.1% for pure BFO to 43.5% for BFEr05 ceramic. On the contrary, the relative intensity of A1-3 mode intensely increased from 4.8% for pure BFO to 12.9% for BFEr05 ceramic. It is believed that the vibration mode (A1-3) near 230 cm−1 should correspond to the tilts or distortions of FeO6 octahedron.41 The intense changes in the relative intensity of A1-3 mode induced by RE dopants are well-consistent with the varieties in oxygen octahedral tilts revealed by the refinements of XRD patterns. These results further manifest that the magnetic properties of BFO are strongly affected by the introduction of RE dopants; consequently, the spin-phonon coupling should be strengthened, which is confirmed in the later temperaturedependent Raman spectra and magnetic measurement. More recently, the spin-phonon coupling in multiferroic BFO materials attracted much attention, since probing the correlation between structure and spin−phonon coupling could provide deep understanding of its magnetoelectric properties. Haumont et al.42 reported strong phonon anomaly in the Raman spectra of BFO crystal across the Neel temperature (TN), indicating the possible multiferroic character plays a key role in this anomaly and thus implies the occurrence of the spin-phonon coupling. Singh et al.43 and Rout et al.44 also observed phonon anomalies near TN owing to the spin− phonon coupling in BFO thin film and ceramic, respectively. To study the effect of RE dopants on the magnetic properties or magnetic ordering temperature, temperature-dependent Raman spectra analysis was performed on the RE-doped BFO ceramics. Figure 4 shows the temperature-dependent Raman spectra and temperature-dependent evolution of some spectral characteristics for RE-doped BFO ceramics. As shown in Figure 4a−c, conspicuous changes can be observed in the Raman spectra of all the samples with increasing temperature: (1) gradual reduction in intensity of all major peaks, (2) the high-frequency peaks severely broaden and merge into a broad peak, which later disappears completely as the temperature rises toward the Neel temperature TN, and (3) most peaks shift continually toward lower frequency at higher temperature. Red shift in band positions and broadening in the bandwidth can be explained by thermal broadening and thermal disorder that are so-called anharmonic effect of the lattice.42 However, the remarkable anomalies of phonon behaviors in the vicinity of TN (from 580 to 680 K) could not be simply attributed to the anharmonic effect. Furthermore, the band position and full width at half-maximum (fwhm) of two representative A1 modes (A1-1 ≈ 140 cm−1 and A1-2≈ 173 cm−1) as a function of 8968


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Figure 4. (a−c) Temperature-dependent Raman spectra for RE-doped BiFeO3 ceramics; (d−f) temperature-dependent evolution of some spectral characteristics to show the phonon anomaly behavior near the Neel temperature.

tilts or distortion. Especially, decrease in the Fe−O−Fe bond angle and average Fe−O bond length coincided with the contraction in cell volume made the Fe ions close to each other, and thus led to an enhancement in the oxygen-related superexchange interaction, as well as the intense changes in the FeO6 octahedral tilts or distortions playing a key role regarding the magnetic property. All above factors resulted in the enhanced magnetization or weak ferromagnetism presented in RE-doped BFO ceramics. D. Electronic Band Structure. More recently, it has attracted much attention to study the interplay between charge, structure, and physical properties in multiferroic BFO, since it will reveal rich physics in complex oxides. Especially, several physical properties have been reported to show anomalies in the vicinity of TN,42,45 manifesting the importance of spin-

phases or impurities have not been found in our samples. Moreover, the Neel temperatures of RE-doped BFO ceramics revealed by the temperature-dependent Raman spectra are less than that of BFO (TN ≈ 643 K). On the contrary, the magnetic ordering temperatures of these magnetic secondary phases such as Fe3O4 and γ-Fe2O3 are far beyond 700 K. Hence, the contribution to enhanced magnetization in RE-doped BFO ceramics derived from magnetic secondary phases could be excluded. Therefore, the enhancement in the magnetization of RE-doped BFO ceramics could be well-explained on the basis of the following two aspects, (i) RE dopants perturbed the spatial spin modulation, which led to the continuing collapse of the spiral spin structure; (ii) RE dopants induced striking change in the rhombohedral structure of BFO, such as the interatomic bond distances, bond angles, and FeO6 octahedral 8969


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Fe3+ in BFO is schematically depicted in Figure 6b. Note that these excitations are formally forbidden, because they change the total spin of Fe3+ from S = 5/2 to S = 3/2. However, spin− orbit coupling relaxes the spin selection rule and gives rise to these transitions.31−33 More specially, the first two crystal-field d−d transitions, namely, 6A1g→4T1 and 6A1g→4T2g, are the only ones detected below the gap absorption threshold in BiFeO3 or (RE)FeO3 (RE = rare earth).33,46 Their energy values decrease proportionally to the increase of the crystal-field splitting Δ, while the variety of their oscillator strength fd−d strongly depends on the activation mechanism, such as thermal activation by odd parity vibrations, pairwise mechanism between exchange coupled Fe3+ pairs, non-centrosymmetric static distortions, or a combination of these. Both these transition energies and oscillator strengths strongly depend on the Fe−O bond length, site symmetry, and Fe−O−Fe exchange interaction.46,47 Therefore, in this case, we focus on the evolution in these two crystal-field transitions, 6 A1g→4T1g and 6A1g→4T2g, induced by RE dopants and study the correlation between these evolutions and the physical properties of BFO. E. Optical Response. Optical spectroscopy is a widely used to probe electronic band structure in solids. It is possibly used to study magnetic excitations and spin ordering transitions in multiferroic materials in which charge and spin degrees of freedom are strongly coupled.31−33 In particular, the crystalfield electronic transitions of Fe3+ (3d5) are highly sensitive to slight distortions of coordination octahedron FeO6. Both the transition energy and oscillator strength strongly depend on the Fe−O bond length, site symmetry, and Fe−O−Fe exchange interaction.47 The foregoing XRD and Raman results implied impact changes in crystalline structure of BFO induced by RE dopants, such as contraction in volume cell leading to the variation in the interatomic bond distances, bond angles, and octahedral tilts or distortions. Especially, the distortions (tilts) in FeO6 octahedron easily detected by optical spectroscopy intensely affect the magnetic properties of BFO compounds. Therefore, the aim for using optical spectroscopy in this study is to investigate interplay between the evolutions in electronic structure and physical properties, special for magnetic property, as well as spin-charge-lattice coupling in BFO. Herein, DRS was used in this study to probe the electronic band structure and its evolution in RE-doped BFO ceramics. DRS of RE-doped BFO ceramics are well-analyzed on the basis of so-called Kubelka−Munk function F(R), which is expressed as48

Figure 5. M−H curves of pure and RE-doped BiFeO3 ceramics measured at room temperature.

Table 4. Magnetic Parameters of Pure and RE-Doped BiFeO3 Ceramics magnetic properties Mmax at 9 T samples

(emu/g)

(μB/Fe)

Mr (emu/g)

HC (kOe)

BiFeO3 Bi0.95La0.05FeO3 Bi0.95Nd0.05FeO3 Bi0.95Er0.05FeO3

0.629(1) 0.730(8) 0.850(1) 1.423(3)

0.035(3) 0.040(5) 0.047(1) 0.079(3)

0.002(1) 0.005(2) 0.012(3) 0.023(5)

0.23(3) 0.96(6) 1.33(5) 1.28(6)

charge-lattice coupling mechanism. To clearly understand these interplays or coupling mechanisms, it is necessary and more challenging to investigate the electronic band structure and ingap states of multiferroic BFO. As mentioned above, BFO crystallized in a rhombohedrically distorted perovskite structure; hence, there is a point group symmetry breaking from Oh to C3v as shown in Figure 6a. By

F(R ) = (1 − R )2 /2R

(2)

where R is the diffuse reflectance referred to the standard BaSO4. Diffuse reflectance spectra of pure and RE-doped BFO ceramics were depicted in Figure 7. As expected, all spectra consist of two weak and broad bands at ∼1.38 and ∼1.87 eV and an intense absorption edge above 2.1 eV. On the basis of previous reports,31−33,46 the two bands below 2.0 eV should be assigned to 6A1g→4T1g and 6A1g→4T2g crystal-field transitions of Fe3+ (3d5) in BFO system, and then the absorption edge above 2.1 eV should correspond to the band gap transition (“6A1g→(4Eg,4Ag)”), as seen in Figure 6b) involving mainly 6pBi levels and O2−→Fe3+ charge-transfer states in the conduction band.33 It should be mentioned that only first two crystal-field transitions could be observed in BFO system, because the third transition of “6A1g→(4Eg, 4Ag)” is covered by

Figure 6. (a) Schematic to describe the crystal-field effect lowering the symmetry from cubic octahedral Oh to rhombohedral C3v environment and the spin−orbit coupling stabilization gives rise to crystal-field spinforbidden d−d transitions in BiFeO3 system. (b) Representation of its electronic band structure. EC and EV indicate the expected bottom conduction and top valence band, respectively.

considering the C3v local symmetry of Fe3+ ions in BFO lattice and using the correlation group and subgroup analysis for the symmetry breaking from Oh to C3v, it is expected to have six d to d excitations for Fe3+ (3d5, high-spin configuration) ions between 0 and 3 eV. For example, these transition energies are expected to be around 1.36, 1.48, 1.79, 1.99, 2.56, and 3.3 eV, respectively. Corresponding electronic energy-level diagram of 8970


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Figure 7. DRS of pure and RE-doped BiFeO3 ceramics, focusing the two bands below 2.0 eV corresponding to 6A1g→4T1g and 6A1g→4T2g crystalfield transitions of Fe3+ (3d5), respectively.

the band gap.33,46 Fortunately, the 6A1g→(4Eg, 4Ag) transition energy does not depend on the crystal-field splitting Δ, which can be easily estimated from the Tanabe−Sugano diagrams 49. Some similar Fe3+ compounds suggested that E(4Eg,4Ag) varies from 2.3 eV (NdFeO3)50 to 2.8 eV (FeBO3).51 As seen in Figure 7, the absorption threshold in BFO system launching at 2.1 eV makes it impossible to observe this transition. To clearly obtain the transition energies of 6A1g→4T1g and 6 A1g→4T2g, each measured spectrum was properly fitted by decomposing the fitted curves into individual Gaussian components (as shown in Figure 7). The corresponding fitting results are summarized in Table 5. At first glance at Figure 7

leading to the enhanced magnetization in RE-doped BFO ceramics. For the sake of good comprehension at the interplay between the electronic structure and magnetic properties, we investigated the relation between the varieties of crystal-field transition energy, reduction in cell volume, and the remnant magnetization. The foregoing XRD results confirmed the reduction in cell volume as a consequence of the replacement of Bi by smaller RE ions (La3+, Nd3+, and Er3+) having an analogous effect to that of pressure and so-called “chemical pressure”.52 It is well-known that it is still fascinating and very challenging to study the multiferroic behavior and phase transitions as a function of high pressure or temperature in BFO. However, most studies focused on the temperature or pressure phase diagrams of BFO due to limitation in using experimental equipment at very high pressure or temperature.53−56 Smaller ions doping analogized to pressure effect open a new way and make it possible to investigate the physical properties as a function of high pressure. Herein a simple formula concerning the reduction in cell volume (VR) was used to depict the chemical pressure, expressed as follows.

Table 5. Transition Energies of 6A1g→4T1g and 6A1g→4T2g for Pure and RE-Doped BiFeO3 Ceramics transition energy (eV) sample BiFeO3 Bi0.95La0.05FeO3 Bi0.95Nd0.05FeO3 Bi0.95Er0.05FeO3

6

A1g→ T1g

1.28(5) 1.27(6) 1.24(9) 1.23(3)

4

1.44(4) 1.41(8) 1.40(6) 1.38(2)

6

A1g→4T2g

1.80(3) 1.80(1) 1.76(2) 1.73(8)

1.89(0) 1.89(1) 1.85(1) 1.82(6)

VR =

and Table 5, both the two transition energies shift to lower energy with the introduction of RE dopants; in other words, a red shift in the two d−d transition bands was found in all REdoped BFO ceramics. As mentioned above, the crystal-field electronic transitions of Fe3+ (3d5) are highly sensitive to slight distortions of coordination octahedron FeO6. Therefore, the red-shift behavior in these transition bands reveals remarkable changes of Fe3+ local structure in BFO system induced by RE dopants, coinciding with the XRD and Raman results and thus

V − V0 V0

(3)

The crystal-field d−d transition energies ( A1g→ T1g and A1g→4T2g) against the reduction in cell volume were plotted in Figure 8. It is clearly observed that the crystal-field transition energies of both 6A1g→4T1g and 6A1g→4T2g consisting of four fitting components shift toward lower energy with increase in cell volume reduction (or chemical pressure), namely, a redshift behavior. This red-shift behavior is well-coincided with the hydrostatic pressure-induced changes in the optical response 6

4

6

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chemical pressure induced by the volume reduction. From a macroscopic point of view, these microstructure changes ultimately resulted in the enhanced magnetization of BFO. To verify this conjecture, it is necessary to study the correlation between the variation in electronic transitions and magnetic properties. Likewise, a linear expression M r(VR ) = M r(0) − (dM r /dVR )VR

(5)

was also employed to linearly fit the data of the remnant magnetization Mr of RE-doped BFO samples, as shown by the dot lines in Figure 9. As expected, the remnant magnetization

Figure 8. Crystal-field d−d transition energies (6A1g→4T1g and 6 A1g→4T2g) as a function of the reduction in cell volume of pure and RE-doped BiFeO3 ceramics.

behavior of BFO in previous study.46 Nevertheless, it should be commendable that the crystal-field transition bands exhibit such a prominent red-shift behavior in this case, since any phase transition was not observed in RE-doped BFO ceramics. Furthermore, decrease in the crystal-field transition energy presents a linear behavior with the reduction in cell volume (see Figure 8). A linear expression E(VR ) = E(0) − (dE /dVR )VR

(4)

Figure 9. Remnant magnetization Mr as a function of the reduction in cell volume of pure and RE-doped BiFeO3 ceramics.

was used to linearly fit the data of the crystal-field d−d transition energy of RE-doped BFO samples, as shown as the dot lines in Figure 8. Estimated from these linear fittings, the E(VR) for the d−d transition energy (4T1g and 4T2g) decrease linearly with the volume reduction VR at a rate of −7.1, −7.3, −9.8, and −9.6 meV, respectively. Such a linear behavior is similar to the one that observed in variation for the d−d transition energy as a function of pressure.46 On the basis of the modified Tanabe−Sugamo diagram,46,49 such a red-shift behavior in the two crystal-field d−d transition bands should be attributed to increase in the e-t2 crystal-field splitting Δ, induced by the volume reduction due to the introduction of RE dopants in BFO. Moreover, the linear shift in the two d−d transition energies with the volume reduction is also consistent with the crystal compression (or crystal field theory).57−59 Briefly, the crystal-field transitions of Fe3+ (3d5) are highly sensitive to slight distortions of octahedral FeO6, and their transition energies strongly depend on the Fe−O bond length, site symmetry, and Fe−O−Fe exchange interaction. Remarkable red shift in the transition energies and linear behavior with the volume reduction reflect significant change in the local structure of Fe3+ in BFO system, which indeed affect the magnetic properties of BFO. Combined with the XRD and Raman results, the optical response in RE-doped BFO ceramics provides a good comprehension on the origin of the enhanced magnetization or weak ferromagnetism presented in RE-doped BFO ceramics. The replacement of Bi3+ by smaller RE ions (La3+, Nd3+, and Er3+) in BFO resulted in the cell volume reduction and thus led to remarkable changes in the local structure of Fe3+ in BFO, such as distortions in octahedron FeO6 and reduction in length of Fe−O bond, which could be clearly reflected by the linear red shift of the crystal-field transition energies with the

Mr increased continually with the volume reduction (VR) and also exhibited a linear behavior. This observation is wellcoincided with the optical response in RE-doped BFO ceramics, which also confirm our speculation.

IV. CONCLUSION Pure and RE-doped BFO ceramics were prepared by a conventional solid-state sintering procedure. Crystalline structure and magnetic and optical properties were wellinvestigated using XRD, Raman scattering spectroscopy, magnetometer, and optical spectroscopy, respectively. XRD and Raman spectra confirm a rhombohedral R3c symmetry in all samples and simultaneously reveal striking contraction in cell volume and significant distortions in FeO6 octahedron. Temperature-dependent Raman spectra suggest a remarkable spin-phonon coupling and notable changes in Neel temperature. Diffuse reflectance spectra clearly show a conspicuous evolution of interband electronic transitions in BFO system induced by RE dopants through observation of varieties in the transition energies of the two crystal-field d−d bands (6A1g→4T1 and 6A1g→4T2g). On the basis of crystal field theory, a linear red-shift behavior in interband electronic transitions accompanied by the cell volume reduction is wellcorrelated with the enhanced magnetization. According to these investigations, it could be concluded that the smaller RE ions (La3+, Nd3+, and Er3+) doped in BFO system resulted in the cell volume reduction and thus led to remarkable changes in the local structure of Fe3+ in BFO, which could be clearly reflected by the linear shift of the crystal-field transition energies with the chemical pressure induced by the volume reduction. Finally, these microstructure changes, for example, the significant 8972


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(13) Wang, Y. P.; Zhou, L.; Zhang, M. F.; Chen, X. Y.; Liu, J. M.; Liu, Z. G. Room-Temperature Saturated Ferroelectric Polarization in BiFeO3 Ceramics Synthesized by Rapid Liquid Phase Sintering. Appl. Phys. Lett. 2004, 84, 1731−1733. (14) Wang, N.; Cheng, J.; Pyatakov, A.; Zvezdin, A. K.; Li, J. F.; Cross, L. E.; Viehland, D. Multiferroic Properties of Modified BiFeO3PbTiO3-Based Ceramics: Random-Field Induced Release of Latent Magnetization and Polarization. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 104434. (15) Zhu, W. M.; Guo, H. Y.; Ye, Z. G. Structural and Magnetic Characterization of Multiferroic (BiFeO3)1−x(PbTiO3) x Solid Solutions. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 014401. (16) Wei, J.; Haumont, R.; Jarrier, R.; Berhtet, P.; Dkhil, B. Nonmagnetic Fe-Site Doping of BiFeO3 Multiferroic Ceramics. Appl. Phys. Lett. 2010, 96, 102509. (17) Dai, Z.; Akishige, Y. Electrical Properties of Multiferroic BiFeO3 Ceramics Synthesized by Spark Plasma Sintering. J. Phys. D: Appl. Phys. 2010, 43, 445403. (18) Xue, F.; Liang, L.; Gu, Y.; Takeuchi, I.; Kalinin, S. V.; Chen, L.Q. Composition and Pressure-Induced Ferroelectric to Antiferroelectric Phase Transitions in Sm-doped BiFeO3 System. Appl. Phys. Lett. 2015, 106, 012903. (19) Qi, X.; Dho, J.; Tomov, R.; Blamire, M. G.; MacManus-Driscoll, J. L. Greatly Reduced Leakage Current and Conduction Mechanism in Aliovalent-ion-doped BiFeO3. Appl. Phys. Lett. 2005, 86, 062903. (20) Yuan, G. L.; Or, S. W. Enhanced Piezoelectric and Pyroelectric Effects in Single-Phase Multiferroic Bi1−xNdxFeO3 (x = 0−0.15) Ceramics. Appl. Phys. Lett. 2006, 88, 062905. (21) Zhang, S.-T.; Zhang, Y.; Lu, M.-H.; Du, C.-L.; Chen, Y.-F.; Liu, Z.-G.; Zhu, Y.-Y.; Ming, N.-B.; Pan, X. Q. Substitution-Induced Phase Transition and Enhanced Multiferroic Properties of Bi1−xLaxFeO3 Ceramics. Appl. Phys. Lett. 2006, 88, 162901. (22) Khomchenko, V. A.; Kiselev, D. A.; Vieira, J. M.; Jian, L.; Kholkin, A. L.; Lopes, A. M. L.; Pogorelov, Y. G.; Araujo, J. P.; Maglione, M. Effect of Diamagnetic Ca, Sr, Pb, and Ba Substitution on the Crystal Structure and Multiferroic Properties of the BiFeO3 Perovskite. J. Appl. Phys. 2008, 103, 024105. (23) Lin, Y.-H.; Jiang, Q.; Wang, Y.; Nan, C.-W.; Chen, L.; Yu, J. Enhancement of Ferromagnetic Properties in BiFeO3 Polycrystalline Ceramic by La Doping. Appl. Phys. Lett. 2007, 90, 172507. (24) Khomchenko, V. A.; Kiselev, D. A.; Bdikin, I. K.; Shvartsman, V. V.; Borisov, P.; Kleemann, W.; Vieira, J. M.; Kholkin, A. L. Crystal Structure and Multiferroic Properties Of Gd-Substituted BiFeO3. Appl. Phys. Lett. 2008, 93, 262905. (25) Cheng, Z. X.; Li, A. H.; Wang, X. L.; Dou, S. X.; Ozawa, K.; Kimura, H.; Zhang, S. J.; Shrout, T. R. Structure, Ferroelectric Properties, and Magnetic Properties of The La-Doped Bismuth Ferrite. J. Appl. Phys. 2008, 103 (7), 07E507. (26) Jeon, N.; Rout, D.; Kim, I. W.; Kang, S.-J. L. Enhanced Multiferroic Properties of Single-Phase BiFeO3 Bulk Ceramics by Ho Doping. Appl. Phys. Lett. 2011, 98, 072901. (27) Wang, D. H.; Goh, W. C.; Ning, M.; Ong, C. K. Effect of Ba Doping on Magnetic, Ferroelectric, and Magnetoelectric Properties in Mutiferroic BiFeO3 at Room Temperature. Appl. Phys. Lett. 2006, 88, 212907. (28) Cheng, Z. X.; Wang, X. L.; Du, Y.; Dou, S. X. A Way to Enhance the Magnetic Moment of Multiferroic Bismuth Ferrite. J. Phys. D: Appl. Phys. 2010, 43 (24), 242001. (29) Basith, M. A.; Kurni, O.; Alam, M. S.; Sinha, B.; Ahmmad, L. B. Room Temperature Dielectric and Magnetic Properties of Gd and Ti Co-Doped BiFeO3 Ceramics. J. Appl. Phys. 2014, 115, 024102. (30) Bertinshaw, J.; Cortie, D. L.; Cheng, Z. X.; Avdeev, M.; Studer, A. J.; Klose, F.; Ulrich, C.; Wang, X. L. Spin-Cycloid Instability as The Origin of Weak Ferromagnetism in The Disordered Perovskite Bi0.8La0.2Fe0.5Mn0.5O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89 (14), 144422. (31) Ramirez, M. O.; Kumar, A.; Denev, S. A.; Podraza, N. J.; Xue, X. S.; Rai, R. C.; Chu, Y. H.; Seidel, J.; Martin, L. W.; Yang, S. Y.; Saiz, E.; Ihlefeld, J. F.; Lee, S.; Klug, J.; Cheong, S. W.; Bedzyk, M. J.; Auciello,

structural distortions and degeneration or pseudo-degeneration in the electronic states, manifest the strong Jahn−Teller or pseudo Jahn−Teller effect. On the basis of the pseudo Jahn− Teller coupling theory, it can be predicted that the strong magnetoelectric coupling should present in these RE-doped BFO compounds.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the National Science Foundation of China (Nos. 51272204 and 51502123) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20110201120003). J.W. wishes to thank the China Scholarship Council for funding his stay in France.

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REFERENCES

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Structural Distortion, Spin-Phonon Coupling, Interband Electronic

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