Role of Co dopants on the structural, optical and magnetic properties of lead-free ferroelectric Na0.5Bi0.5TiO3 materials

ti a N. Viet Str Center for Nano Science and Technology, Ha Noi National University of Education, 136 Xuan Thuy Road, Ha Noi, Viet Nam doped NBTO exhibits room temperature ferromagnetism, which originates from an intrinsic phenomenon [2]. Thanh et al. sug- gested that a self-defected NBTO exhibits weak room temperature the ferromagnetic e Ti site, and this n vacancies [3]. In tion of Mn cations perties because of ing Mn concentra- tiferromagnetism agnetism [4]. By ydrothermal tech- room temperature ntly, a theoretical all half-metals and magnetic with 100% spin polarization [6]. Despite these studies, the origin of the room temperature ferro- magnetism in Na0.5Bi0.5TiO3 doped with transition metals has remained unclear. To address this important issue, in the present work, Co impu- rities were introduced to host NBTO materials through the sol-gel method. Results demonstrated the reduction in the optical band gap of pure and Co-doped NBTO, and that the observed room * Corresponding author. E-mail address: dung.dangduc@hust.edu.vn (D.D. Dung). Contents lists availab Journal of Science: Advanc .e l Journal of Science: Advanced Materials and Devices 4 (2019) 584e590Peer review under responsibility of Vietnam National University, Hanoi.doped with transition metals [2e5]. Wang et al. reported that Fe- study predicted that V-, Cr-, Fe-, and Co-doped NBTO materials are1. Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3; NBTO)-basedmaterials have attracted attention as the most promising candidates to replace piezoelectric Pb(Zr,Ti)O3-based ceramic materials, which are prohibited due to their environmental and health concerns [1]. Understanding the origin of ferromagnetic ordering at room tem- perature in transition metal-doped perovskite ferroelectric mate- rials provides a new approach for developing multiferroic materials for spintronics applications. In fact, room temperature ferromag- netism was reported in various lead-free ferroelectric materials ferromagnetism [3]. They also suggested that signal was enhanced by Cr replacement at th enhancement was due to the promotion of oxyge addition, Thanh et al. reported that the substitu in the Ti sites of NBTO changes its magnetic pro the compensation of diamagnetism (at low dop tion) and the compensation of paramagnetism/an (at high doping Mn concentration) with ferrom contrast, Co-doped NBTO synthesized by the h niquewas reported to exhibit ferromagnetism at owing to the formation of Co clusters [5]. ReceSol-gela r t i c l e i n f o Article history: Received 21 March 2019 Received in revised form 12 August 2019 Accepted 23 August 2019 Available online 29 August 2019 Keywords: Lead-free ferroelectric Multiferroics Na0.5Bi0.5TiO3 Ferromagnetismhttps://doi.org/10.1016/j.jsamd.2019.08.007 2468-2179/© 2019 The Authors. Publishing services b ( b s t r a c t Co-doped Na0.5Bi0.5TiO3 materials were fabricated by a sol-gel technique. The structural distortion of Co- doped Na0.5Bi0.5TiO3 materials was due to the difference between the radii of Co dopants and Ti hosts. The optical band gap decreased from 3.11 to 1.83 eV because of the local state of the Co cation in the band structure. Room temperature ferromagnetism emerged as compensation of diamagnetic background and possibly intrinsic ferromagnetic signals. The magnetic moment was determined to be ~0.64 mB/Co at 5 K. The origin of the room temperature ferromagnetism in the Co-doped Na0.5Bi0.5TiO3 materials was also investigated through the first-principles calculation method. Our study provides physical insights into the complex magnetic nature of transition metal-doped ferroelectric perovskites and contributes to the integration of multifunctional materials into smart electronic devices. © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license ( of Electronics and Telecommunicationsf School of Materials Science and Engineering, Ha Noi University of Science and Technology, 1 Dai Co Viet Road, Ha Noi, Viet Nam g , Ha Noi University of Science and Technology, 1 Dai Co Viet Road, Ha Noi, Viet NamOriginal Article Role of Co dopants on the structural, op of lead-free ferroelectric Na0.5Bi0.5TiO3 m D.D. Dung a, *, N.B. Doan b, c, N.Q. Dung d, L.H. Bac a, N.N. Trung a, N.C. Khang e, T.V. Trung f, N.V. Duc g a School of Engineering Physics, Ha Noi University of Science and Technology, 1 Dai Co b CNRS, Institut N
eel, F-38042, Grenoble, France c Univ. Grenoble Alpes, Institut N
eel, F-38042, Grenoble, France d Department of Chemistry, Thai Nguyen University of Education, 20 Luong Ngoc Quyen e journal homepage: wwwy Elsevier B.V. on behalf of Vietnamcal and magnetic properties terials H. Linh a, L.T.H. Thanh a, D.V. Thiet a, Road, Ha Noi, Viet Nam eet, Thai Nguyen, Viet Nam le at ScienceDirect ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license temperature ferromagnetism in Co-doped NBTO materials origi- nates as an intrinsic phenomenon. 2. Experimental Na0.5Bi0.5Ti1-xCoxO3 (x ¼ 0%, 0.5%, 1%, 3%, 5%, 7%, and 9%; BNT- xCo) samples were fabricated through the sol-gel method. Stoi- chiometric amounts of sodium nitrate (NaNO3), bismuth nitrate (Bi(NO3)3.5H2O), and cobalt nitrate (Co(NO3)3.6H2O) were first dissolved in acetic acid. Hydrolysis was prevented by adding acetyl acetone before tetraisopropoxytitanium (IV; C12H28O4Ti) was added. The solutions were stirred until they became transparent and dried by heating under 100 C. Sample powders were fabri- cated by using ground and calcined dry gels at 400 C for 2 h and sintered at 900 C for 3 h in air. Sodium concentrationwas added in excess (around 40 mol.%) to compensate for losses during the gel- ling and sintering processes, which were confirmed by electron probe microanalysis (EPMA) [3,4]. The appearance of elements in pure and Co-doped Na0.5Bi0.5TiO3 compounds was characterized by energy dispersive X-ray (EDX) spectroscopy. The surface Fig. 2(a) shows the XRD patterns of pure and Co-doped Na0.5Bi0.5TiO3 samples. The peak position and relative peak in- tensity were indexed as rhombohedral structures [2e5]. The impure phase could not be detected by the XRDmethod. The role of the Co ions in the host lattices of NBTO is depicted in Fig. 3 (b), where the XRD patterns are magnified in 2q angle ranges of 46.0e47.5. The peak position of the Co-doped NBTO materials clearly shifted compared with pure NBTO materials. The distorted structure provided solid evidence of Co cation substitution in the host lattices. However, the shifted trend in the peak position was very complicated and depended on the amount of Co dopants. The peak positions shifted to higher diffraction 2q angles at Co dopant concentrations of up to 3 mol%, indicating that the lattice param- eter was compressed. However, the increased Co concentration resulted in the expansion of lattice constants because the peak position tended to shift to lower angles as the Co concentration increased up to 9 mol%. These results were possibly due to the difference between the radii of the Co cations and Ti hosts. The radii of the Co cations were strongly dependent on coordination and valence states. Based on Shannon's report, Co2þ cations (in VI co- ordination) have radii of 0.65 Å (in low spin states) and 0.735 Å (in D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590 5853. Results and discussion The FE-SEM images of pure and Co-doped Na0.5Bi0.5TiO3 with differentmolar ratios are shown in Fig.1. The particles of pure NBTO samples were cubic, with an average size of about 300 nm, as shown in Fig. 1(a). The particles of the pure NBTO were aggregated in big blocks. However, the Co-doped NBTO exhibited strong sin- tering, and the particles were hardly visible, as shown in Fig. 1(b)e(f). The Co dopant enhanced the diffusion of ions through the boundary and acted as a sintering aid.morphology and symmetry of the crystalline structures of the samples were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) method, respec- tively. The vibrational and rotational modes of the samples were characterized by Raman spectroscopy, whereas optical properties were studied by ultraviolet-visible (UV-Vis) spectroscopy. The magnetic properties of the samples were characterized by a superconducting quantum interference device (SQUID) magne- tometer at 5 K and a vibrating sample magnetometer (VSM) at room temperature.Fig. 1. Surface morphology of (a) pure Na0.5Bi0.5TiO3 and Co-doped Na0.5Bi0.5TiO3 with diffhigh spin states), whereas Co3þ cations (in VI coordination) have radii of 0.545 Å (in low spin states) and 0.61 Å (in high spin states) [7]. Co4þ cations are only stable at high spin states, with a radius of 0.53 Å, whereas Ti4þ cations have a radius of 0.605 Å [7]. Therefore, the substitution of Ti4þ cations by Co2þ cations resulted in the expansion of the lattice constants of the host NBTO materials because the radii of the Co2þ cations in both spin states were larger than those of the Ti4þ cations; meanwhile, the presence of higher valence states of cobalt as Co3þ and Co4þ resulted in the reduction of lattice parameters as their radii were smaller than those of Ti4þ [7]. The valence states of Co cations were complicated because of their dependence on the host environmental materials and fabri- cationmethod [8,9]. Huan et al. reported that Co2þ and Co3þ cations coexist in Na0.5Bi0.5TiO3e6%BaTiO3 single crystals, and their rela- tive amounts are strongly associated with the addition of Co [8]. Hu et al. also reported that the lattice parameter tended to decrease with the introduction of Co2O3 and increased again due to the reduction of Co3þ to Co2þ [9]. Schimitt et al. observed that the valence state of Co-doped NBTO changed from Co3þ to Co2þ at high sintering temperatures [10] and that Co cations occupied octahe- dral B-sites in a NBTO lattice, thereby increasing the number oferent Co concentrations: b) 1 mol.%, c) 3 mol.%, d) 5 mol.%, e) 7 mol.%, and f) 9 mol.%. cedD.D. Dung et al. / Journal of Science: Advan586oxygen vacancies [11]. The unbalanced charge of Co and Ti creates oxygen vacancies that affect the lattice parameters because the oxygen vacancies are smaller than the oxygen vacancies created by O2 [3]. Some Co2þ cations with high states are substituted in Bi and Na sites because their radii (0.735 Å) are comparable to those of Bi3þ and Naþ cations (1.11 and 1.16 Å, respectively) [7]. The sub- stitution of Co2þ in these sites influences the distortion of the lattice parameter [7]. Our work showed that Co doping at low concen- trations is increasingly stable at high valence states, and this sta- bility is reduced with the addition of Co cations. In other words, the XRD analysis provides solid evidence for Co substitution in the host NBTO lattice. Fig. 3(a) shows the Raman scattering spectra of pure and Co- doped Na0.5Bi0.5TiO3 samplesat room temperature in a wave number range of 100 cm1 - 1000 cm1. All the samples exhibited Fig. 2. (a) X-ray diffraction patterns of pure and Co-doped Na0.5Bi0.5TiO3 samples as a fun positions for pure and Co-doped Na0.5Bi0.5TiO3 samples. Fig. 3. (a) Raman spectra of the pure and Co-doped Na0.5Bi0.5TiO3 samples as a function of range of 100e200 cm1 and 150e450 cm1 for pure and Co-doped Na0.5Bi0.5TiO3 samplesMaterials and Devices 4 (2019) 584e590broad Raman bands due to the disordering distribution of Na and Bi ions located at the A-site and overlapping of multi-active Raman modes. Thus, each vibration band was hardly distinguishable, although the Raman spectra could be divided into three regions as follows: from 100 cm1 to 200 cm1, 200 cm1 to 400 cm1, and 400 cm1 to 650 cm1. Experimental and theoretical studies that predicted the vibration modes of NBTOmaterials reported that the lowest frequency modes ranging from 109 cm1 to 187 cm1 are dominated by Bi/NaeO vibration, the frequency modes ranging from 240 cm1 to 401 cm1 are dominated by TiO6 and TieO vi- brations, and the higher frequencies modes ranging from 413 cm1 to 826 cm1 are primarily associated with oxygen octahedron vi- brations/rotations [12e14]. The role of Co substitution at the Ti site on the lattice vibration of Na0.5Bi0.5TiO3 is shown in Fig. 3(b), where the Raman spectra in wavenumbers ranging from 150 cm1 ction of cobalt doping concentration, (b) a comparison of (003)/(201) diffraction peak Co doping concentration, (b) and (c) magnification Raman spectra in the wavenumber with varying Co amounts, respectively. to 400 cm1 are magnified. The peak positions shifted to lower frequencies as Co concentration increased. The change in the Raman spectra frequencies of the Co-doped NBTO at the TieO band provided solid evidence of Co substitution at the Ti site owing to the larger mass of Co (58.93 g/mol) as compared with that of Ti (47.90 g/mol). Our results were in agreement with recent studies that reported the effect of Co on the lattice vibrationmodes of Na0.5Bi0.5TiO3-based ceramic materials [3,4,7]. Fig. 3 (c) shows the magnified Raman scattering spectra of pure and Co-doped Na0.5Bi0.5TiO3 samples in wavenumbers ranging from 100 cm1 to 200 cm1. The vibration range was related to the Bi/NaeO vi- bration. The peak position did not shift clearly as compared to that of TieO/TiO6 vibration shown in Fig. 3 (b), indicating that the Co cations were not favored to substitute the (Bi,Na)-site compared with the Ti-site. In other words, the phonon Raman vibration modes and XRD provided evidence for the Co substitution in the octahedral site. Fig. 4(a) shows the optical absorbance spectra of pure and Co- the magnetic moment as a function of applied external magnetic field (M-H) at room temperature, as shown in Fig. 5(a). An anti-S- shape was obtained from the pure NBTO due to both the ferro- magnetic and diamagnetic contributions to the total magnetic signal of the sample. The diamagnetic behavior of pure NBTO was related to the empty state of Ti 3d cations [3,4]. The origin of the observed weak ferromagnetism in NBTO resulted from self-defect and/or promotion of surface effects [3,4]. By introducing Co cat- ions at the Ti-site, M-H curves tended to reverse the S-shape. This result provides solid evidence as an increasing ferromagnetic strength. The coercive field (HC) and remanencemagnetization (Mr) values of the pure and Co-doped NBTO materials were approxi- mately 150 Oe and 0.1 memu/g, respectively. The results are consistent with the recently reported values for Cr- and Mn-doped NBTO materials [3,4]. The observed nonzero values of HC and Mr in the pure and Co-doped NBTO samples provide solid evidence of the ferromagnetic ordering at room temperature. Unlike in the case of Wang et al. in which the room temperature ferromagnetism of Co- D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590 587doped Na0.5Bi0.5TiO3 samples at room temperature. A single absorbance peak was obtained from the pure NBTO, whereas two absorbance bands were obtained from the Co-doped NBTO sam- ples. These results showed that the band structure of NBTO was modified due to the substitution of Co ions at the Ti site. The multi-absorbance peaks obviously presented the multi-valence state of Co, which resulted in changes in the crystal structure. These results are consistent with the recent observation on transition metal-doped ferroelectric materials (e.g., Fe- and Ni- doped Bi0.5K0.5TiO3 or Cr- and Mn-doped Na0.5Bi0.5TiO3 mate- rials). The total density state of materials causes the appearance of the local state of a transition metal [3,4,15,16]. The optical band gap values of pure and Co-doped NBTO samples were estimated by Tauc method, by which (ahn)2 was plotted as the function of phonon energy (hn), as shown in Fig. 4(b) [17]. The optical band gap values were estimated through the extrapolation of the best- fit line between (ahn)2 and (hn) up to the point where the line crosses the energy axis. The optical band gap was around 3.11 and 1.83 eV in pure and 9 mol% Co-doped NBTO samples, respectively. The optical band gap values of the pure and Co-doped NBTO samples were plotted as a function of Co concentration and are shown in the inset of Fig. 4(b). The reduction in optical band gap energy and the appearance of multi-absorbance peaks in the ab- sorption spectra indicated the Co substitution in the host lattice, resulting in the change in the band structure. Furthermore, the influence of Co doping on the magnetic properties of Na0.5Bi0.5TiO3 materials was observed by determiningFig. 4. (a) UVeVis absorption spectra of Co-doped Na0.5Bi0.5TiO3 samples as a function of Co samples as a function of Co concentration. The inset of Fig. 4(b) shows the optical band gadoped Na0.5Bi0.5TiO3 was attributed to the formation of Co clusters [5], our results revealed a possible intrinsic ferromagnetism at room temperature in the Co-doped Na0.5Bi0.5TiO3. Fig. 5(b) shows the temperature dependence of magnetization of the Na0.5Bi0.5- Ti0.99Co0.01O3 sample under an applied magnetic field of 1 kOe. The inset of Fig. 5(b) shows the M-H curve of Na0.5Bi0.5Ti0.99Co0.01O3 in magnetic fields of up to 70 kOe at 5 K. Unsaturation in the magnetization was observed in the MH curves, suggesting the paramagnetic contribution of isolated Co cations that are randomly incorporated in the host lattice of NBTO [4]. The results are consistent with recent reports on the magnetic properties of Co- doped Bi0.5K0.5TiO3 materials or BiCoO3-modified Bi0.5K0.5TiO3 materials [18,19]. Maximum magnetization (MS) was approxi- mately 0.168 emu/g at 5 K and corresponded to 0.64mB/Co. The valence state of Co cations and the configuration of the spin states of Co play important roles in the magnetic interactions of Co cat- ions, because the valence state of Co is extremely complex in the lattice [20,21]. The Co3þ (3d64so) valence states have two spin configurations, namely, the nonmagnetic low-spin t2g[[Y[Y[Y] eg[] and the magnetic high-spin t2g[[Y[[]eg[[[] states. During the transition of valence state from Co3þ to Co2þ, the magnetic state may change. The reason is spin configuration changes due to the low-spin t2g[[Y[Y[Y]eg[[] and high-spin t2g[[Y[Y[]eg[[[] states of Co2þ (3d74so). Thus, the spin configurations of Co2þ aremagnetic. The spin configurations of Co2þ and Co3þ in the low-spin and high- spin states are shown in Fig. 5(c) and (d), respectively. Our results suggested that both the valence states of Co2þ/3þ were present in 2concentration, and (b) the (ahn) proposal with photon energy (hn) of the Na0.5Bi0.5TiO3 p Eg value of Na0.5Bi0.5TiO3 as a function of Co concentration. cedD.D. Dung et al. / Journal of Science: Advan588NBTO samples. The radius of Co was strongly dependent on the valence state and coordination number. In the octahedral site with six coordination numbers, the radii of Co3þ ions were 0.545 and 0.61 Å for low-spin and high-spin configurations, respectively, and the radii of Co2þ ions were 0.65 and 0.745 Å for low-spin and high- spin configurations, respectively [9]. The XRD results indicated that the lattice parameters tend to shrink. Thus, we suggested that the major states of Co3þ ions are of low-spin configuration because Co3þ ions have smaller radii than Ti4þ ions (0.545 and 0.605 Å, respectively). Therefore, the enhancement of themagnetic moment seemed to arise from the oxygen vacancies due to the non- compensation of charge between Co3þ and Ti4þ [3]. During the transfer from the Co3þ to Co2þ state, the magnetic prop