Ferroelectrics, breaking inversion symmetry due to intrinsic spontaneous polarization, has recently attracted much attention as a candidate class of materials for use in photovoltaic devices, and for the light absorption with other functional properties [1-6]. Electrons in ferroelectric semiconductors can be motivated upon the exposure to light with the photon energy greater than the band gap (Eg). Therefore, what challenge the ferroelectrics for photovoltaic application are mainly their wide band gaps (above 2.7 eV ), which allow the absorption of only 8%-20% of the solar spectrum. Quantities of efforts have been concentrated upon the optimization of ferroelectrics, to reach a lower band gap [6-14].
Most ferroelectrics are perovskite oxides with the general formula ABO3, characterized by long-term durability, non-toxicity and an easy tunability of the electronic structure. The excitations transfer across the band gap from oxygen (O) 2p states at the top of valence band to the transition-metal (B) d states at the bottom of the conduction band. Transition-metal cations play a decisive role in the ferroelectricity of perovskite oxides . Chemical doping in the A position or B position is an effective method to modify the crystal and electronic structure, reducing the optical band gap in some degree [6, 7, 9-12]. Among perovskite oxides, potassium niobate (KNbO3) is a typical one for its wide band gap with experimental value near 3.2 eV [16, 17]. And only a small number of studies on the bandgap-engineering strategy in KNbO3 have been detected. An enhancement of the optical property of KNbO3 was observed by Wang et al. with the doping introduction of Na at the K lattice site, which reduces the band gap to 3.09 eV . Shen et al. calculated the electronic structure and optical properties of KNbO3 doped Cu with first principles, reaching the band gap of 1.545 and 1.579 eV within local density approximation (LDA) and generalized gradient approximation (GGA) respectively . In particular, a family of single-phase synthesis made from solid-state reaction method: (KNbO3)1-x(BaNi1/2Nb1/2O3-δ)x (KBNNO) (x=0.1-0.5) were discussed. These syntheses exhibited a wide variation of direct bandgaps in the range of 1.1-3.8 eV . Soon, Zhou et al. explored KBNNO structure phase transition, and room-temperature ferromagnetism (paramagnetic Ni2+) for further study . Fe doped KNbO3 has attracted researchers' attention, because it can be a good promotion in the ferroelectric and magnetic properties [21-23]. While few studies in KNbO3-Fe emphasized on the optical properties in band gap engineering.
Here we make a systematic study of doping modification in KNb1-xFexO3-δ (x=0.01, 0.03, 0.05, 0.10, 0.15, 0.20, 0.25), not only to narrow band gap for photovoltaic devices and visible light photo-degradation but also to bring in magnetic Fe ions in Nb5+ lattice site for potential multiferroic applications.II. EXPERIMENTS
Polycrystalline KNb1-xFexO3-δ (x=0-0.25) were prepared by a traditional solid states reaction method. Stoichiometric amounts of K2CO3 (99%), Nb2O5 (99.9%), Fe2O3 (99%) were used as starting materials for the preparation of pure and doped KNbO3 samples. Considering that K+ will volatilize with the form of K2O when temperature is higher than 800 ℃, we made the chemical constitution of an 15% excess of K2CO3. The precursors were milled at 300 r/min for 12 h, using the agate jar with an alcohol moderate medium. The dried slurries were sintered at controlled atmosphere furnace with the heating rate of 10 ℃/min and finally calcined at 900 ℃ for 120 min. The crystal structure of the samples was detected by a Bruker D8 ADVANCE X-ray diffractometer (λ=1.5418 Å) with the scanning rate of 0.6°/min in a step size of 0.02°. Raman scattering spectra analysis was performed with a micro-Raman spectrometer (Renishaw inVia, λ=532 nm). The morphology characteristics of the samples were observed under a field emission scanning electron microscopy (Nova Nanosem430). X-ray photoelectron spectra (XPS) were gathered by a Kratos Axis Ultra DLD photoeletron spectrometer with A1 Kα source (hν=1486.6 eV, with the power setting of 10 mA×12 kV) in a vacuum of 7×10-7 Pa. The UV-Vis absorption spectra over a range of 300-1000 nm wavelength were tested by an ocean optical fiber spectrometer (USB4000) equipped with integration sphere. Magnetic hysteresis loops were measured on a vibrating sample magnetometer (VSM) in a physical property measurement system (PPMS, Quantum Design).III. RESULTS AND DISCUSSION
Room-temperature X-ray diffraction (XRD) patterns of KNb1-xFexO3-δ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) are shown in Fig. 1(a). All diffraction peaks of the samples are identified as those of the orthorhombic structure with a space group of Amm2, which is in good agreement with the standard powder diffraction file database (JCPDS No.32-0822). Moreover, it can be obviously observed no trace of any impurity phases. Although there is no obvious sign of phase transition observed from the XRD patterns, a gradually evolving lattice distortion is detected. From Fig. 1(b), it is clear that the peaks around 2θ=32° are slightly shifted to a lower angle with the increase of Fe doping, indicating the well incorporation of Fe ions into the KNbO3 matrix since the ionic radius of Fe3+ (r(Fe3+)=0.645 Å) is larger than that of Nb5+ (r(Nb5+)=0.64 Å).
To investigate the crystal structure of the Fe doped samples in detail, we executed the Rietveld refinement with GSAS program. Relative data of Amm2 phase were used as the datum curve. Figure 2(a) and Figure 2(b) present the best refinement results of the KNb1-xFexO3-δ with x=0 and 0.25, respectively. The Rietveld refinement factors Rwp for both two are far less than 0.08 and χ2 are approximately equal to 2.0, reveling another proof of the consistency with Amm2 phase. 3D spatial graph of each crystal texture in equilateral niobium-oxygen octahedron (NbO6) is intuitively emerged in the top right corner. For x=0 and x=0.25, the lattice parameter a is 3.976 and 3.969 Å, and the lattice parameter b is 5.694 and 5.712 Å, and c is 5.715 and 5.686 Å, respectively.
The surface microstructures of different Fe concentration in KNbO3 compositions are observed by field effect scanning electron microscopy (FESEM). As shown in Fig. 3, well developed particles with roughly cubic morphology can be observed. The average particle size of pure KNbO3 is about 500 nm-1 μm, while the average particle size of samples with x=0-0.03 gradually increases with the increasing of Fe concentration. And for further doping (x=0.03, 0.05, 0.10, 0.15, 0.20, 0.25), the average particle size among different samples basically keeps uniform with the value about 2.5 μm. As the same synthesis conditions were chosen for all the samples, the increase in the particle size of doping samples could be attributed to the addition of Fe ions.
Raman scattering spectra are also useful to analyze the structure and the vibrational properties of KNbO3 besides XRD analysis. There are 12 Raman active modes in the orthorhombic KNbO3 with the space group Amm2: 4A1+4B1+3B2+A2 . And the main narrow lines of transverse and longitudinal phonon modes examined in Fig. 4 are B1(TO), A1(TO), A1(LO), B1(TO), A1(TO) and A1(LO) observed at 192, 280, 294, 534, 597, and 833 cm-1, respectively, which match well with the previous reports [24, 25]. In the low to middle wavenumber region, those modes concluding two sharp modes B1(TO) at 192 cm-1 and A1(LO) at 294 cm-1 and a broad A1(TO) mode at 280 cm-1 assigned to BO6 bending vibrations, are identified as a sign for the occurrence of long-range polar order . As shown in Fig. 4, there are hardly any variation between pure KNbO3 and Fe doping in B sites samples, which may attribute to the nearby ionic radius and similar chemical environment between Fe ions and Nb ions, suggesting much more for the well incorporation of Fe ions into the KNbO3 matrix. Zhou et al.  ascribed the structure and vibrational changes of their synthesized samples (Ba2+ and Ni2+ introduced to the A sites and B sites of KNbO3, respectively) to the Ni2+ introduction at B sites, and the ionic radius of Ni2+(r(Ni2+)=0.69 Å) is similar to that of Nb5+ (r(Nb5+)=0.64 Å), too. Therefore, the doping in A sites may play the dominant role in the variation of structure phase and molecular vibration in doped KNbO3 synthesis system.
XPS studies were performed to identify the chemical state of KNb1-xFexO3-δ. Figure 5 shows the XPS spectra analysis of the Nb3d, Fe2p, and O1s core level by Lorentzian-Gaussian fitting for the samples, where the core level bonding energies were aligned with respect to C1s peak (284 eV). The bonding energy of Nb3d spectrum can be divided into two doublet peaks of 3d5/2 and 3d3/2, which are located at 207.2 and 210.0 eV respectively, corresponding to Nb2O5 [27, 28]. Obviously, no variation occurred in the valence states of Nb with the increase of Fe doping. The O1s peak can be fitted with two Gaussians curves, whose peaks positions are near 529.1 and 530.5 eV revealing in the Fig. 5(b). The lower bonding energy peak (green line) corresponds to the crystal O2- ions (O) and the higher bonding energy peak (blue line) can be attributed to the oxygen vacancy (VO) . To analyze the content variation of VO, areas of the two fitted peaks were compared. The concentration ratios of VO and O in x=0, 0.10, and 0.20 are 1:1.62, 1:1.03, and 1:0.46 respectively, revealing that the amount of oxygen vacancy increases effectively with the Fe dopant concentration. The reasonable interpretation for this phenomenon is the charge compensation, with the dope of lower valance state of ferric ion for higher valance state of Nb5+. Figure 5(c) shows the Fe2p3/2 core level spectra, where the fitted peak of Fe2+ (green line) is located at 709.6 eV and Fe3+ (blue line) is located at 711.6 eV . According to the fitting result, it can be observed that the ratio of Fe2+ to Fe3+ is decreased with doping (1:0.48 and 1:0.67 for 10% and 20% Fe content, respectively). The origin of Fe2+ might be ascribed to the increasing oxygen vacancies for charge balancing, whereas the decrease ratios of Fe2+/Fe3+ with doping can be owing to the charge compensation as explained for the increasing oxygen vacancies in Fig. 5(b).
Optical property in the variation of band gap in KNb1-xFexO3-δ (x=0-0.25) samples can be revealed with the room temperature UV-Vis absorption spectra as shown in Fig. 6(a). A broad absorption band in the range of 400-700 nm is observed clearly in the absorption spectra of the doping samples, which prove that the doping samples can absorb considerable amounts of visible light (400-760 nm), suggesting their potential application as visible-light photocatalyst.
The energy band gap values (Eg) were calculated according to the classical Tau's relation: ahν=C(hν-Eg)n, where a is the absorption coefficient, h is Planck constant, ν is frequency of the light and C is a constant . For the direct band gap of KNbO3, the parameter n should equal to 2 . The plots of (ahν)2 versus hν for the samples are displayed in Fig. 6(b), offering the values of Eg with the extrapolations of the linear region of these graphs to (ahν)2=0. And the plot of Eg against Fe concentration x is shown in Fig. 6(c), which provide a monotonic decline of Eg with the increasing x. The band gap (Eg) of pure KNbO3 at x=0 is estimated to be 3.18 eV, which agrees well with that reported in previous studies [16, 20, 32]. And the samples at x=0.15, 0.20 and 0.25 with the Eg of 1.61, 1.52, and 1.45 eV respectively, can absorb all the wavelengths of visible light. Therefore, to take full advantage of solar energy, these doping materials can afford significant potential applications to photocatalysts and photovoltaic devices, especially under the visible light. The inset shows in the top right of Fig. 6(c) is the schematic diagram of qualitative structure of electronic energy band in KNb1-xFexO3-δ. In KNbO3, the valence band is mainly composed of the O2p states and conduction band is composed of the Nb4d states . By doping Fe into KNbO3, the Fe2+ donor band and Fe3+ for both acceptor and donor band are introduced between the O2p states and the Nb4d states, which contribute to the narrowing of band gap . Furthermore, the increases of oxygen vacancies for charge compensation as analyzed in Fig. 5(b) bring in a new kind of broadening donor levels, which play another important part in lowering the band gap of doping samples .
The magnetic hysteresis (M-H) loops of KNb1-xFex O3-δ powders at room temperature are shown in Fig. 7. Compared with the pure KNbO3, Fe doped samples exhibits different level of room-temperature ferromagnetism. The inset of Fig. 7 displays the relationship between the remanent magnetization Mr and the Fe concentration x. It is clear that the magnetization in the samples with low-concentration dopant (x=0.01-0.10) increases monotonously firstly and reaches a local maximum at x=0.05, and then decrease at x=0.10. For the relatively high-concentration doping with x=0.20, the magnetism examined to increase again. According to the results of XRD and the Rietveld refinement all the samples maintain the same structure phase with Amm2, and the lattice distortion is pretty small. Therefore, lattice deformation for the main factor in magnetic variation of this KNbO3 doping system can be eliminated. In addition, from FESEM images in Fig. 3, the particle size is observed to increase monotonously within x=0.03 and then almost keep near for the higher doping samples, which is inconsistent with the common variation tendency in magnetic enhancement in previous reports (small size effect) . Coey et al.  suggested that bound magnetic polaron (BMP) mediated exchange is responsible for the ferromagnetism in Cu:ZnO films, which is one of the typical diluted magnetic semiconductors (DMS) replacing the nonmagnetic cations of the semiconductors with a small quantity of magnetic transition ions or rare earth ions. In this Fe doped system, samples with low-concentration dopant (x=0.01-0.10) can be well matched to the one model of diluted magnetic semiconductors. Plenty of bound polarons are introduced with the form of defects as oxygen vacancy in KNbO3 with a handful of Fe concentration. These bound polarons constrain the current carriers in the lattice and compel magnetic moment of the Fe ions around to turn to a same direction, which attribute to the form of ferromagnetic phase. However, bound magnetic polarons effect will recede with the increase of current carriers, which is ascribed to the narrower band gap with the increasing Fe doping as shown in Fig. 6. Meanwhile, the double-exchange interaction based on the interaction between itinerant electrons will be enhanced. Hence, the magnetism for the samples with low-concentration dopant (x=0.01-0.10) is strengthened and then weakened with the increasing of Fe doping. While, magnetism increase from x=0.10 to 0.20 may arise from the concentration of magnetic Fe ions.IV. CONCLUSION
In summary, the structure, optical and magnetic properties of ferroelectric KNb1-xFexO3-δ (x=0-0.25) was studied systematically. All samples exhibit an orthorhombic distorted perovskite structure with Amm2 space group without the onset of any secondary phase according to the XRD and the Rietveld refinement results. The XPS reveals the coexistence of Fe2+ and Fe3+ in the doped samples, which attribute to both the occurrence of new donor level and accepter level in the band gap. Additionally, the amount of oxygen vacancy increase effectively with the Fe dopant concentration from charge compensation, introducing a kind of donor level as well. Therefore, the band gaps are decreased with the increase of doping even narrowed to 1.45 eV, much smaller than the 3.18 eV band gap of pure KNbO3. All doped samples exhibited ferromagnetic ordering of spins and the remanent magnetization Mr for low-concentration dopant (x=0.01-0.10) is increased first and then decreased, which may stem from the effect from bound magnetic polarons (BMP). While, magnetic enhancement for x=0.10 to 0.20 may arise from the increased concentration of magnetic Fe ions.V. ACKNOWLEDGMENTS
The work was supported by the Fundamental Research Funds for the Central Universities, SCUT (No.2014ZZ0069) and the National Natural Science Foundation of China (No.21473211 and No.11304098).
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