Synergistic effects of Ca2+ and high-valence Nb5+ co-doping on the structural, optical and magnetic properties of BiFeO3

The structural, optical and magnetic properties of BiFeO3, BiFe0.99Nb0.01O3 and Bi1−xCaxFe0.99Nb0.01O3 (BCFNO, 0 ≤ x ≤ 0.25) nanoparticles synthesized via sol–gel method are investigated. It has been found that a phase transition from the rhombohedral R3c structure (x ≤ 0.10) to the ideal cubic perovskite structure (x = 0.25) which can be attributed to Ca2+ doping. Increasing Ca2+ dopants results in the increase of oxygen vacancies. As doping amount x increase, the bandgap of BCFNO decreases and the valence band spectra indicates that it’s a p-type semiconductor, which indicates their favorable potential in photocatalytic applications. The remnant magnetization Mr of BCFNO reaches a maximum value (0.146 emu/g about 15 times compared with pure BFO) at x = 0.10. This enhancement of magnetic properties in BCFNO can be ascribed to the synergistic effect of A and B site ions co-doping. Higher valance Nb doping cause the size effect and the magnetic polarons bounded to the impurities by Ca ions.


Introduction
Multiferroics are those materials which posses ferroelectric (or antiferroelectric) properties in combination with ferromagnetic (or antiferromagnetic) properties in the same phase and have emerged as new appealing materials thanks to their enormous properties in the field of spintronic devices, dual storage devices, sensors, high density memory and multi-state electric devices [1][2][3]. Among the multiferroic materials, the room temperature phase of BiFeO 3 (BFO) possesses distorted rhombohedral perovskite structure as rhombohedral (R3c space group) with high ferroelectric Curie point (T & 830°C), high antiferromagnetic(AFM) Neel temperature (T & 370°C) and narrow bandgap (2.16 eV) [4,5]. It is highly investigated that BFO is characterized as a distorted perovskite rhombohedral structure (R3c space group) and a G-type antiferromagnetic (AFM) order modulated by a spatially spin structure along the [110]hex direction with a period of 62 nm [6]. Due to its small bandgap (2.18 eV), BiFeO 3 also exhibits significant degradation ability under visible light irradiation. In spite of various interesting features, BiFeO 3 exhibits several drawbacks like high leakage current, low magnetization, difficult to synthesize in single phase, etc., associated with it.
To overcome these shortcomings, many steps have already been taken including doping of divalent ions (Ca 2? , Sr 2? and Ba 2? ) and trivalent ions (La 3? , Nd 3? and Eu 3? ) at A-site [7][8][9] and high-valance ions (Nb 5? , Ti 4? ) at B site or co-doping [10][11][12][13][14] to greatly affect the ferroelectric, magnetic, optical and magnetoelectric properties of BiFeO 3 . Sandhaya Jangra et al. [15] have reported that remanent magnetization increased for Bi 0. 8 [16] observed structural transition from rhombohedral to monoclinic in La 3? doped BiFeO 3 (BLFO) and improved coercive field and remanent magnetization with Nb doping at B site of BLFO. All these research suggest that cosubstitution at A and B site suppresses the modulated spiral spin structure and enhances multiferroics properties of BFO. Concurrently, electrical neutrality of the heterovalent ion doped BiFeO 3 system has been preserved by inducing oxygen vacancies (defects) which may alter its electronic conductivity and magnetic properties [17][18][19]. Few reports are available on multiferroic properties of Ca and Nb-doped BiFeO 3 nanocrystalline samples. Therefore, it is very imperative for us to explore the effects of Ca and Nb doping on the structural, magnetic and optical properties of BFO. We have found that the remanent magnetization increases linearly with the x to the maximum for x = 0.01 and starts to decrease as more Nb is doped in BiFe 1-x Nb x O 3 (0 B x B 0.05) [20]. In the view of above, we believe that structural, optical properties and saturation magnetization may be enhanced simultaneously by Ca and Nb co-doping. The concentration of Nb was fixed as 1% at Fe sites since previously content has been confirmed to the strongest magnetic property for the Nb-doped BiFeO 3 [20]. The solution was dried at 120°C and the resultant was grinded into powders. The powders were calcined at 300°C for 1 h and heat preservation at 600°C for 2 h to remove NO x and hydrocarbons. The crystal structure of the X-ray diffraction patterns were examined by a Bruker D8 ADVANCE X-ray diffractometer with Cu K a radiation (k = 1.5418 Å , Ni filter) with the scanning rate of 0.6°/min in a step size of 0.02°. The microstructural properties of all specimens are measured by a field emission scanning electron microscopy (FE-SEM). The UV-Visible (UV-Vis) absorption spectra over a range of 350-800 nm wave length were measured by an Ocean Optical Fiber Spectrometer (USB4000). X-ray photoelectron spectra (XPS) were collected by a Kratos Axis Ultra DLD photoelectron spectrometer with Al Ka source (hm = 1486.6 eV, with power setting of 10 mA 9 12 kV) in a vacuum of 7 9 10 -7 Pa. Magnetic hysteresis loops were carried out on a vibrating sample magnetometer (VSM) in a physical property measurement system (PPMS, Quantum Design).

Structure analysis
Room temperature X-ray diffraction patterns of . This structural phase change is completely consistent with the change trend of the crystal structure of Ca-doped BFO alone [21], which shows that the main contribution of the change in the crystal structure of the Ca-Nb co-doped BCFNO sample comes from the replacement of Bi ions by Ca ions. With an increasing Ca concentration x, all diffraction peaks shift toward higher angle since the 12 coordination ionic radius of Ca 2? (134 pm) is slightly smaller than that of Bi 3? (139 pm) and the six coordination ionic radius of Nb 5? (64 pm) is slightly smaller than that of Fe 3? (64.5 pm).
To studied structural properties of Ca and Nb codoped BFO in detail, Rietveld refinement was adopted using the GSAS program. Patterns of refinements on XRD data were shown in Fig. 2. R3c space group was engaged in refining. As shown the remnant weight profile (R wp ) and the fitness factor v 2 B 2.564 in Fig. 2, it suggested that the single phase with R3c space group for 0 B x B 0.10 and with Pm3m phase group for x = 0.25, while the double phase for 0.10 \ x \ 0.25. The best fits were well consistent with XRD data and the phase transition in Ca-doped BFO [21]. The variation of lattice constant as a function of Ca concentration (x) of the BCFNO samples are shown in Fig. 3. With the increase of Ca content x, the lattice constant of BCFNO decrease which the overall trend was consistent with the Cadoped BCFO sample [21]. The obtained results confirm that 1% Nb 5? ions are the main cause of particle refinement, and the increased Ca 2? dopants have little effect on the particle size of the samples.
The structure and phase transition confirmed by XRD can also been characterized by the Raman peaks. Figure 5 represents the Raman spectra of BCFNO nanoparticles. For BFO, Raman active modes can be summarized using the following irreducible representation: 4A1 ? 9E [22,23]. The main 13 vibrating modes observed are consistent with the results in Ca-doped BFO [21]. It can be seen that the 13 Raman activity peaks of pure BFO decrease and disappear gradually as the Ca doping amount increases to 0.25 in BCFNO, which indicates that the structure change from R3c structure to Pm3m structure in the Ca-Nb co-doped sample without Raman activity peak. Compare Raman Scattering Spectra of BCFNO and BCFO Samples, the similar evolution trend of Raman peaks also proves the structural phase transition is mainly caused by Ca 2? doping [21]. Apparently, these Raman results are consistent with the XRD observations. The O1s XPS spectrum (see Fig. 6b) deconvoluted into two peaks, corresponding to the intrinsic O 2ions and the Vo [24]. As confirmed by the XPS patterns, the oxygen vacancies are increased with Ca concentration to maintain the electrical neutrality of the sample. In ferrite, the existence of abundant oxygen vacancy may lead to the uneven distribution of local charges and improvement of dielectric properties due to defect polarization [25]. Related reports show that Ca doping causes an increase of oxygen vacancies, which can effectively reduce the leakage current by an order of magnitude, making it possible for the Ca-Nb co-doped samples to have a certain degree of improvement in dielectric properties [26].

Optical properties
BiFeO 3 can be used as a narrow bandgap metal oxide semiconductor in applications such as photocatalytic materials. The room temperature UV-Vis absorption spectra of BCFNO samples are displayed in Fig. 7. The absorption band is comparatively broad in the wave length range of 480-560 nm, which indicates the high availability of visible light. The absorption edge shifts to higher wavelengths for the doped samples. The bandgap can be determined from Kubelka-Munk function [27]: (aht) n =A(ht -E g ). (aht) 2 as a function of ht is plotted in Fig. 7a, which provides the value of E g by the tangent at (aht) 2 = 0. The calculated optical bandgap E g decreases as x increases as shown in the inset (b) of Fig. 7. BFO has a maximum value of E g (2.06 eV), which gradually decreases down to 1.78 eV for the Bi 0.90 Ca 0.10 Fe 0.99-Nb 0.01 O 3 nanoparticles (see Fig. 7b). Two major reasons should be considered to make E g shorten clear in BCFNO. As XPS analyzed previously, on the one hand, the increased oxygen vacancy can act as a kind of defect induced energy levels below the conduction band (CB), which is closely associated with the shrinking of the bandgap. Besides, it is reported the increased of the Fe-O-Fe angle, which could suppress the FeO 6 -octahedral tilting, decreases the bandwidth of CB and VB of BFO [28,29], while the corresponding Fe-O-Fe angle increases to 180°during the structural phase transition from R3c to pm3m in BCFNO samples. Wang  with the above-reported ones. Thanks to the energy bandgap is located in the visible region, the as-prepared BiFeO 3 nanopowders can be used for photocatalysis in the decomposition of organic compounds.
Besides, XPS valence band spectra for Bi 1-x Ca x-Fe 0.99 Nb 0.01 O 3 (x = 0.00, 0.05, 0.10) samples are also given in Fig. 8. For Bi 0.90 Ca 0.10 Fe 0.99 Nb 0.01 O 3 sample, with the help of the optical bandgap E g = 1.94 eV calculated by the UV-Vis spectrum, we can determine the border between the Fermi energy (E F ) and valence band energy (VB) is about * 0.49 eV, suggesting that the E F is quite near to the VB. According to related reports, BFO is a p-type semiconductivity material [29,31,32], where the E F is near the VB, which means that the proportion of holes is higher than that of electrons. A schematic diagram (Fig. 8b) is introduced to better understand it. As the doping concentration x increases, the Ca and Nb co-doped samples gradually becomes a p-type semiconductor material, which might lead to a potential application in photocatalysis [33,34].

Magnetic properties
The M-H hysteresis loops for pure BFO and Bi 1-x-Ca x Fe 0.99 Nb 0.01 O 3 (x = 0.00,0.10,0.25) samples are shown in Fig. 9. All our samples present weak ferromagnetism at room temperature. To study the magnetic enhancement mechanism of the BCFNO sample more clearly, the contrast diagram according to the change of the M r of BFO, BCFO, and BCFNO   [35,36,37].
For the previous BCFO and BFNO series samples, in-depth discussion and analysis have been taken that the magnetic enhancement of pure Ca 2? doped and Nb 5? doped BFO samples are two different mechanisms [20,21]. The incorporation of the nonmagnetic Ca impurity ions increases oxygen vacancies, forming bound magnetic polarons at the defects, which enhance the magnetization at low Ca 2? doping content x [21]. The enhancement of the magnetic property of the Nb-doped BiFeO 3 samples is mainly attributed to the large ratio of surface to volume induced by the reduction of the particles size (size effect) [20]. Interestingly, the particle size of the Ca-Nb co-doped BCFNO sample is slightly smaller than that of BCFO due to the small amount of Nb introduced as discussed above. Under the same doping amount x, the magnetic properties of the Ca-Nb co-doped BFO samples are significantly better than that of BCFO. The M r of BCFNO at x = 0.10 reaches a maximum value (0.146 emu/g) which is about 15 times compared with pure BFO or larger than pure Nb-doped BFO. It is well known that the ratio between Fe 3? and Fe 2? might be modified by a chemical doping, which could enhance the magnetic property of BiFeO 3 . However, we do not observe the significant changes of proportion of Fe 3? and Fe 2? by XPS analysis. Evidently the valence fluctuation of Fe ions can not be the main reason for the magnetic improvement of our co-doped samples. Therefore, the superior magnetic properties in BCFNO can be ascribed to the synergistic effect of A and B site ions doping: (1) One of the important contributions is the decreasing of the grain size by appropriate Nb doping at Fe site. The particle sizes of proper fixed content (1%) Nb 5? doped samples decrease below the range of cycloid spin periodicity of BiFeO 3 (62 nm) and show a large ratio of surface to volume induced by the reduction of the particles size (size effect). Due to the decrease of grain size, the remanent magnetization of Bi 1-x Ca x Fe 0.99 Nb 0.01 O 3 sample is generally higher than that of the Bi 1-x Ca x-Fe 0.99 O 3 under the same Ca content. (2) Nonmagnetic Ca 2? dopants and the increased oxygen vacancies directly induce impurity levels within the forbidden band, and bound charge carriers to these impurity levels. The bound carriers polarize the localized magnetic cations (Fe 3? ) in their neighborhoods to form magnetic polarons (MPs) [21], which effectively interrupts the long range AF order modulated by a spatially spin structure [38,39] and the bounded magnetic polarons further enhances the magnetic properties of the co-doped BFO.
As discussed above, the enhancement of magnetization in BCFNO is possible due to the synergistic effect of nonmagnetic Ca 2? and Nb 5? ions doping, on the one hand, the size effect induced by Nb 5? ions, on the other hand, the magnetic polarons bounded to the impurities by Ca 2? ions.

Conclusions
In summary, BiFeO 3 , BiFe 0.99 Nb 0.01 O 3 and Bi 1-x Ca x-Fe 0.99 Nb 0.01 O 3 (BCFNO, 0 B x B 0.25) have been successfully synthesized using tartaric acid-based sol-gel route. The Ca substitution causes a structural transition from a rhombohedral R3c structure for x B 0.10 to a cubic Pm3m structure at x = 0.25 which is observable from the Rietveld analysis of the XRD data and Raman spectra. Through XPS measurement, it is found that the increase of oxygen vacancies in Ca and Nb co-doped samples, which makes contribution to the formation of magnetic polarons. The optical bandgap E g of the co-doped nanoparticles is apparently smaller than that of pure BiFeO 3 . Furthermore, the enhancement of the magnetization for Bi 0.90-Ca 0.10 Fe 0.99 Nb 0.01 O 3 are nearly 15 times of those for pure BiFeO 3 , which is due to the synergistic effect of the size effect and the formation of magnetic polarons.