Journal of Science: Advanced Materials and Devices (2019) 584e590 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Role of Co dopants on the structural, optical and magnetic properties of lead-free ferroelectric Na0.5Bi0.5TiO3 materials D.D Dung a, *, N.B Doan b, c, N.Q Dung d, L.H Bac a, N.H Linh a, L.T.H Thanh a, D.V Thiet 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, Dai Co Viet Road, Ha Noi, Viet Nam CNRS, Institut N eel, F-38042, Grenoble, France Univ Grenoble Alpes, Institut N eel, F-38042, Grenoble, France d Department of Chemistry, Thai Nguyen University of Education, 20 Luong Ngoc Quyen Street, Thai Nguyen, Viet Nam e Center for Nano Science and Technology, Ha Noi National University of Education, 136 Xuan Thuy Road, Ha Noi, Viet Nam f School of Materials Science and Engineering, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam g School of Electronics and Telecommunications, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam b c a r t i c l e i n f o a b s t r a c t Article history: Received 21 March 2019 Received in revised form 12 August 2019 Accepted 23 August 2019 Available online 29 August 2019 Co-doped Na0.5Bi0.5TiO3 materials were fabricated by a sol-gel technique The structural distortion of Codoped 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 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 (http://creativecommons.org/licenses/by/4.0/) Keywords: Lead-free ferroelectric Multiferroics Na0.5Bi0.5TiO3 Ferromagnetism Sol-gel Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3; NBTO)-based materials 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 temperature in transition metal-doped perovskite ferroelectric materials provides a new approach for developing multiferroic materials for spintronics applications In fact, room temperature ferromagnetism was reported in various lead-free ferroelectric materials doped with transition metals [2e5] Wang et al reported that Fedoped NBTO exhibits room temperature ferromagnetism, which originates from an intrinsic phenomenon [2] Thanh et al suggested that a self-defected NBTO exhibits weak room temperature * Corresponding author E-mail address: dung.dangduc@hust.edu.vn (D.D Dung) Peer review under responsibility of Vietnam National University, Hanoi ferromagnetism [3] They also suggested that the ferromagnetic signal was enhanced by Cr replacement at the Ti site, and this enhancement was due to the promotion of oxygen vacancies [3] In addition, Thanh et al reported that the substitution of Mn cations in the Ti sites of NBTO changes its magnetic properties because of the compensation of diamagnetism (at low doping Mn concentration) and the compensation of paramagnetism/antiferromagnetism (at high doping Mn concentration) with ferromagnetism [4] By contrast, Co-doped NBTO synthesized by the hydrothermal technique was reported to exhibit ferromagnetism at room temperature owing to the formation of Co clusters [5] Recently, a theoretical study predicted that V-, Cr-, Fe-, and Co-doped NBTO materials are all half-metals and magnetic with 100% spin polarization [6] Despite these studies, the origin of the room temperature ferromagnetism in Na0.5Bi0.5TiO3 doped with transition metals has remained unclear To address this important issue, in the present work, Co impurities 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 https://doi.org/10.1016/j.jsamd.2019.08.007 2468-2179/© 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 (http://creativecommons.org/licenses/by/4.0/) D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 temperature ferromagnetism in Co-doped NBTO materials originates as an intrinsic phenomenon Experimental Na0.5Bi0.5Ti1-xCoxO3 (x ¼ 0%, 0.5%, 1%, 3%, 5%, 7%, and 9%; BNTxCo) samples were fabricated through the sol-gel method Stoichiometric 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 fabricated by using ground and calcined dry gels at 400 C for h and sintered at 900 C for h in air Sodium concentration was added in excess (around 40 mol.%) to compensate for losses during the gelling 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 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, respectively 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) magnetometer at K and a vibrating sample magnetometer (VSM) at room temperature Results and discussion The FE-SEM images of pure and Co-doped Na0.5Bi0.5TiO3 with different molar ratios are shown in Fig 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 sintering, 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 585 Fig 2(a) shows the XRD patterns of pure and Co-doped Na0.5Bi0.5TiO3 samples The peak position and relative peak intensity were indexed as rhombohedral structures [2e5] The impure phase could not be detected by the XRD method The role of the Co ions in the host lattices of NBTO is depicted in Fig (b), where the XRD patterns are magnified in 2q angle ranges of 46.0 e47.5 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 mol%, indicating that the lattice parameter 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 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, Co2ỵ cations (in VI coordination) have radii of 0.65 Å (in low spin states) and 0.735 Å (in high spin states), whereas Co3ỵ cations (in VI coordination) have radii of 0.545 Å (in low spin states) and 0.61 (in high spin states) [7] Co4ỵ cations are only stable at high spin states, with a radius of 0.53 , whereas Ti4ỵ cations have a radius of 0.605 [7] Therefore, the substitution of Ti4ỵ cations by Co2ỵ cations resulted in the expansion of the lattice constants of the host NBTO materials because the radii of the Co2ỵ cations in both spin states were larger than those of the Ti4ỵ cations; meanwhile, the presence of higher valence states of cobalt as Co3ỵ and Co4ỵ resulted in the reduction of lattice parameters as their radii were smaller than those of Ti4ỵ [7] The valence states of Co cations were complicated because of their dependence on the host environmental materials and fabrication method [8,9] Huan et al reported that Co2ỵ and Co3ỵ cations coexist in Na0.5Bi0.5TiO3e6%BaTiO3 single crystals, and their relative 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 Co3ỵ to Co2ỵ [9] Schimitt et al observed that the valence state of Co-doped NBTO changed from Co3ỵ to Co2ỵ at high sintering temperatures [10] and that Co cations occupied octahedral B-sites in a NBTO lattice, thereby increasing the number of Fig Surface morphology of (a) pure Na0.5Bi0.5TiO3 and Co-doped Na0.5Bi0.5TiO3 with different Co concentrations: b) mol.%, c) mol.%, d) mol.%, e) mol.%, and f) mol.% 586 D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 Fig (a) X-ray diffraction patterns of pure and Co-doped Na0.5Bi0.5TiO3 samples as a function of cobalt doping concentration, (b) a comparison of (003)/(201) diffraction peak positions for pure and Co-doped Na0.5Bi0.5TiO3 samples Fig (a) Raman spectra of the pure and Co-doped Na0.5Bi0.5TiO3 samples as a function of Co doping concentration, (b) and (c) magnification Raman spectra in the wavenumber range of 100e200 cmÀ1 and 150e450 cmÀ1 for pure and Co-doped Na0.5Bi0.5TiO3 samples with varying Co amounts, respectively oxygen 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 Co2ỵ cations with high states are substituted in Bi and Na sites because their radii (0.735 ) are comparable to those of Bi3ỵ and Naỵ cations (1.11 and 1.16 Å, respectively) [7] The substitution of Co2ỵ in these sites inuences the distortion of the lattice parameter [7] Our work showed that Co doping at low concentrations is increasingly stable at high valence states, and this stability 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 Codoped Na0.5Bi0.5TiO3 samplesat room temperature in a wave number range of 100 cmÀ1 - 1000 cmÀ1 All the samples exhibited broad 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 cmÀ1 to 200 cmÀ1, 200 cmÀ1 to 400 cmÀ1, and 400 cmÀ1 to 650 cmÀ1 Experimental and theoretical studies that predicted the vibration modes of NBTO materials reported that the lowest frequency modes ranging from 109 cmÀ1 to 187 cmÀ1 are dominated by Bi/NaeO vibration, the frequency modes ranging from 240 cmÀ1 to 401 cmÀ1 are dominated by TiO6 and TieO vibrations, and the higher frequencies modes ranging from 413 cmÀ1 to 826 cmÀ1 are primarily associated with oxygen octahedron vibrations/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 cmÀ1 D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 to 400 cmÀ1 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 vibration modes of Na0.5Bi0.5TiO3-based ceramic materials [3,4,7] Fig (c) shows the magnified Raman scattering spectra of pure and Co-doped Na0.5Bi0.5TiO3 samples in wavenumbers ranging from 100 cmÀ1 to 200 cmÀ1 The vibration range was related to the Bi/NaeO vibration The peak position did not shift clearly as compared to that of TieO/TiO6 vibration shown in Fig (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 Codoped 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 samples 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 Nidoped Bi0.5K0.5TiO3 or Cr- and Mn-doped Na0.5Bi0.5TiO3 materials) 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 bestfit 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 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 absorption 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 determining 587 the magnetic moment as a function of applied external magnetic field (M-H) at room temperature, as shown in Fig 5(a) An anti-Sshape was obtained from the pure NBTO due to both the ferromagnetic 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 cations 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 remanence magnetization (Mr) values of the pure and Co-doped NBTO materials were approximately 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 Codoped 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 Na0.5Bi0.5Ti0.99Co0.01O3 sample under an applied magnetic field of 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 K Unsaturation in the magnetization was observed in the MÀH 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 Codoped Bi0.5K0.5TiO3 materials or BiCoO3-modified Bi0.5K0.5TiO3 materials [18,19] Maximum magnetization (MS) was approximately 0.168 emu/g at 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 cations, because the valence state of Co is extremely complex in the lattice [20,21] The Co3ỵ (3d64so) valence states have two spin congurations, 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 Co3ỵ to Co2ỵ, 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 Co2ỵ (3d74so) Thus, the spin congurations of Co2ỵ are magnetic The spin congurations of Co2ỵ and Co3ỵ in the low-spin and highspin states are shown in Fig 5(c) and (d), respectively Our results suggested that both the valence states of Co2ỵ/3ỵ were present in Fig (a) UVeVis absorption spectra of Co-doped Na0.5Bi0.5TiO3 samples as a function of Co concentration, and (b) the (ahn)2 proposal with photon energy (hn) of the Na0.5Bi0.5TiO3 samples as a function of Co concentration The inset of Fig 4(b) shows the optical band gap Eg value of Na0.5Bi0.5TiO3 as a function of Co concentration 588 D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 Fig (a) The MeH curves of pure and Co-doped Na0.5Bi0.5TiO3 samples with different Co concentrations, (b) the MeT curve at kOe magnetic field for the Co-doped Na0.5Bi0.5TiO3 sample with mol% The inset of Fig 5(b) shows the MeH curve of the mol% Co-doped Na0.5Bi0.5TiO3 sample at K Low-spin and high-spin congurations of (c) Co2ỵ ions and (d) Co3ỵ ions in the octahedral site of Na0.5Bi0.5TiO3 NBTO 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 Co3ỵ ions were 0.545 and 0.61 for low-spin and high-spin configurations, respectively, and the radii of Co2ỵ ions were 0.65 and 0.745 for low-spin and highspin configurations, respectively [9] The XRD results indicated that the lattice parameters tend to shrink Thus, we suggested that the major states of Co3ỵ ions are of low-spin conguration because Co3ỵ ions have smaller radii than Ti4ỵ ions (0.545 and 0.605 Å, respectively) Therefore, the enhancement of the magnetic moment seemed to arise from the oxygen vacancies due to the noncompensation of charge between Co3ỵ and Ti4ỵ [3] During the transfer from the Co3ỵ to Co2ỵ state, the magnetic properties could be enhanced because of the presence of magnetic states in the high-spin configuration with S of 3/2 and low-spin configuration with S of 1/2 The results are possibly consistent with the expansion tendency of lattice parameter because Co2ỵ had the larger radius than Ti4ỵ in both the spin congurations However, at this moment, we not have direct evidence of exact percent contributions of Co2ỵ and Co3ỵ to the total magnetic moment of the samples Additional contributions of magnetic moment of oxygen vacancies and/or Ti3ỵ defects need to be further investigated To further understand the role of transition metals such as Codoped lead-free ferroelectric Na0.5Bi0.5TiO3 samples, we performed the first principles calculation on the electronic structures of the pure and Co-doped Na0.5Bi0.5TiO3 samples The crystal structure was prepared by using the VESTA package, as shown in Fig [22] All the density function theoretical (DFT) calculations were performed by PWScf code implemented in the QuantumEspresso package [23] The exchange correlation energy was Fig Rhombohedral supercell of   established from a rhombohedral primitive cell of BNT Purple, yellow, blue, red, and silver cycles represent Bi, Na, Ti, O, and doping Co in (Bi0.5Na0.5)(Ti0.9375Co0.0625)O3 (BNCT), respectively Ti1 and Ti2 denote Ti ions of first nearest neighbor and the second nearest neighbor octahedral TiO6 of transition metal doping position, viz Ti1st and Ti2nd O1st and O2nd are twofold coordinated bridge O between MÀ Ti1st and Ti1st e Ti2nd Crystal structure prepared using the VESTA package [22] carried out by the Generalized Gradient Approximation (GGA) method using PerdeweBurkeeErnzerhof (PBE) exchangecorrelation functions [24] Plane-wave basis set cutoffs for the smooth part of wave functions and the augmented density were 45 D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 589 Fig Calculated projected density of states (PDOS) of cations (Bi, Na, Ti, and doping Co) and anion O of (a) BNT and (b) BNCT compounds Energies are given in [eV] with respect to the Fermi energy (EF) Ry and 250 Ry, respectively The amount of computational resources used was reduced by simulating pristine NBTO with the primitive cell of R3c perovskite structure with Bi/Na cation in Wyckoff symmetric position 2a, Ti/M cation in 2a and O anion in 6a (Fig 6), and lattice constants from the experiments [25,26] A   supercell of the doped Bi0.5Na0.5Ti0.9375Co0.0625O3 (BNCT) compounds with a doping concentration of 6.25% was created by replacing one Ti atom with a Co atom for the formation of a BNCT unit cell The MonkhorstePack scheme is used to sample the Brillouin zone [27] The structures were fully relaxed with a mesh of   2, and the mesh of k-space was increased to   in the static and density of state (DOS) calculations Fig (a) and (b) show the results of the PDOS calculation for pure and Co-doped Na0.5Bi0.5TiO3 compounds, respectively Similar to NBTO, Co-doped NBTO materials showed the broad NaeO hybridization, which represents the strong covalent bond between Na and O cations As seen in Table 1, Ti cations gain more electrons by doping Co into pristine NBTO, and thus the band in PDOS became broadened In contrast to Na cations, Bi cations only show hybridization at energy range under À10 eV and near Fermi level This suggests that the interactions between Bi and O are ionic rather than covalent In the Co-doped NBTO materials, Bi cations lose electrons and become more positive, broadening the PDOS of Bi in the Co-doped Na0.5Bi0.5TiO3 materials In rhombohedral perovskites structures, cations at A sites, that is, Bi and Na, have covalent radii, and are quite similar to cations at B-sites, that is, Ti and doped Co Thus, the PDOS of Bi cations was affected by the B-site cations, and spin-polarization was slightly induced in the PDOS of Bi near the Fermi level in BNCT While NBTO materials was completely spin-unpolarized, the Co-doped NBTO materials showed small spin polarization near the Fermi level, mostly due to the contribution of the PDOS of Ti and Co The interactions between B-site cations and O anions in the BO6 octahedrons of perovskite structures are more ionic than covalent However, in these materials, the interaction between B-site cations and O anions seems to be increasingly complicated Fig provides a detailed view of the interaction between B-site cations and O anions In the Co-doped Na0.5Bi0.5TiO3 samples, the 3d electrons of Co are predominantly distributed in dxy orbitals, and are strongly localized in the energy range of À2 eVe0 eV However, these d-electrons establish the hybridization with the px orbital of the nearest O cation, that is, the O1st p-orbital It prompts the spin polarization of the first nearest O1st ions The doping of Co into the pristine NBTO enabled Ti1st and Ti2nd ions to gain electrons, as shown in Table 1, and increased the states located near the Fermi level Thus, Ti1st and Ti2nd cations became less positive, and the Ti1steO and Ti2ndeO bonds were more covalent than TieO bonds in pristine NBTO, as shown in Fig It induces spin polarization, mostly near the Fermi level Consequently, the electronic structure of Co-doped NBTO samples leads to the magnetic moments of Co, O1st, O2nd, Ti1st, and Ti2nd ions in Co-doped NBTO samples, which are presented in Table Co ion exhibited a small magnetic moment (0.12 mB), whereas the Ti1st and Ti2nd ions established relatively larger magnetic moments of 0.21 mB and 0.25 mB, respectively, which are also larger than those of Co cations Thus, we suggest that Co cations are stable in the low-spin configuration and that the Table Charge (±DQ) gain/loss by Bi, Na, O1st, O2nd, Ti1st, and Ti2nd due to doping of Co into the host BNTO compounds to form the BNCT compounds Plus/negative signs are represented for gaining/losing charge, respectively, and magnetic moment of Co, O1st, Ti1st, O2nd, and Ti2nd in BNCT Charge DQBi ðeÞ Magnetic moment À0.54 mCo ðmB Þ 0.12 DQNa ðeÞ 0.37 mO1st ðmB Þ 0.02 DQO1st ðeÞ 0.16 DQO2nd ðeÞ À0.03 mTi1st ðmB Þ 0.21 DQTi1st ðeÞ 0.13 mO2nd ðmB Þ 0.01 DQTi2nd ðeÞ 0.12 mTi2nd ðmB Þ 0.25 590 D.D Dung et al / Journal of Science: Advanced Materials and Devices (2019) 584e590 Fig Calculated PDOS of specific position sites in BNCT, i.e O1st, Ti1st, O2nd, Ti2nd, and Co Ti1st and Ti2nd denote Ti cations of the nearest and next nearest TiO6 octahedrons to doping Ni position O1st and O2nd denote the twofold coordinated O ion which are the bridge bonds between Ni and Ti1st, Ti2nd, respectively Energies are given in [eV] with respect to the Fermi level (EF) room-temperature ferromagnetism is possibly induced by the nearest Ti to Co cation through the charge transfer process Conclusion The pure and Co-doped NBTO samples were successfully fabricated through the sol-gel method The Co doping in NBTO resulted in the reduction of the optical band gap from 3.11 to 1.83 eV in mol% Co dopants Compensation between the diamagnetism and ferromagnetism was achieved at room temperature The maximum magnetic moments were around 0.64 mB/Co at K owing to the main interaction of the complex states of Co2ỵ/3ỵ through oxygen vacancies in the Co-doped NBTO materials We suggested that the ferromagnetism at room temperature in Co-doped NBTO is an intrinsic property The first principles calculation for the Co-doped NBTO samples suggested that the spin configuration of the Co cations is stable in low-spin states and this stability results in the observed low magnetization moment Meanwhile, the magnetic moment of the samples was enhanced because of the contribution of the magnetization of Ti-nearest cations through charge transfer These findings will advance the current understanding of the origin of the room temperature ferromagnetism in transition metaldoped lead-free ferroelectric compounds for multifunctional smart devices Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grand number 103.02-2015.89 References [1] N.D Quan, L.H Bac, D.V Thiet, N.V Hung, D.D Dung, Current development in lead-free Bi0.5(Na,K)0.5TiO3-based piezoelectric materials, Ann Mater Sci Eng 2014 (2014) 365391 [2] Y Wang, G Xu, L Yang, Z Ren, X Wei, W Weng, Room-temperature ferromagnetism in Fe-doped Na0.5Bi0.5TiO3 crystals, Mater Sci Poland 27 (2009) 471e476 [3] L.T.H Thanh, N.B Doan, N.Q Dung, L.V Cuong, L.H Bac, N.A Duc, P.Q Bao, D.D Dung, Origin of room temperature ferromagnetism in Cr-doped leadfree ferroelectric Bi0.5Na0.5TiO3 materials, J Electron Mater 46 (2017) 3367e3372 [4] L.T.H Thanh, N.B Doan, L.H Bac, D.V Thiet, S Cho, P.Q Bao, D.D Dung, Making room-temperature ferromagnetism in lead-free ferroelectric Bi0.5Na0.5TiO3 material, Mater Lett 186 (2017) 239e242 [5] Y Wang, G Xu, X Ji, Z Ren, W Weng, P Du, Room-temperature ferromagnetism of Co-doped Na0.5Bi0.5TiO3: diluted magnetic ferroelectrics, J Alloy Comp 475 (2009) L25eL30 [6] L Ju, T.S Xu, Y.J Zhang, L Sun, First-principles study of magnetism in transition metal doped Na0.5Bi0.5TiO3 system, Chin J Chem Phys 29 (2016) 462e466 [7] R.D Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr A 32 (1976) 751e767 [8] T Huang, P Zhang, L.P Xu, C Chen, J.Z Zhang, Z.G Hu, H.S Luo, J.H Chu, Electronic structures and abnormal phonon behaviors of cobalt-modified Na0.5Bi0.5TiO3-6%BaTiO3 single crystals, AIP Adv (2016) 105311 [9] H Hu, M Zhu, F Xie, N Lei, J Chen, Y Hou, H Yan, Effect of Co2O3 additive on structure and electrical properties of 85(Bi1/2Na1/2)TiO3-12(Bi1/2K1/2)TiO33BaTiO3 lead-free piezoceramics, J Am Ceram Soc 92 (2009) 2039e2045 [10] V Schmitt, F Raether, Effect of cobalt doping on the sintering mechanisms of the lead-free piezoceramic (Bi0.5Na0.5)TiO3, J Eur Ceram Soc 34 (2014) 15e21 [11] V Schmitt, T.E.M Staab, Co-doping of (Bi0.5Na0.5)TiO3: secondary phase formation and lattice site preference of Co, J Phys.: Condens Mater 24 (2012) 455901 [12] M.K Niranjan, T Karthik, S Asthana, J Pan, U.V Waghmare, Theoretical and experimental investigation of Raman modes, ferroelectric and dielectric properties of relaxor Na0.5Bi0.5TiO3, J Appl Phys 113 (2013) 194106 [13] J Anthoniappen, C.S Tu, P.Y Chen, S.C Chen, Y.U Idzerda, S.J Chiu, Raman spectra and structural stability in B-site manganese doped (Bi0.5Na0.5)0.925Ba0.075TiO3 relaxor, J Eur Ceram Soc 35 (2015) 3495e3506 [14] J Kreisel, A.M Glazer, G Jones, P.A Thomas, L Abello, G Lycazeau, An x-ray diffraction and Raman spectroscopy investigation of A-site substituted perovskite compounds: the (Na1-xKx)0.5Bi0.5TiO3 (0 x 1) solid solution, J Phys.: Condens Mater 12 (2000) 3267e3280 [15] D.D Dung, D.V Thiet, D Odkhuu, L.V Cuong, N.H Tuan, S Cho, Room-temperature ferromagnetism in Fe-doped wide band gap ferroelectric Bi0.5K0.5TiO3 nanocrystals, Mater Lett 156 (2015) 129e133 [16] D.V Thiet, D.D Cuong, L.H Bac, L.V Cuong, H.D Khoa, S Cho, N.H Tuan, D.D Dung, Room-temperature ferromagnetism in nickel-doped wide band gap ferroelectric Bi0.5K0.5TiO3 nanocrystals, Mater Trans 56 (2015) 1339e1343 [17] J Tauc, A Menth, States in the gap, J Non-Cryst Solids 8e10 (1972) 569e585 [18] L.V Cuong, N.H Tuan, D Odkhuu, D.V Thiet, N.H Dung, L.H Bac, D.D Dung, Observation of room-temperature ferromagnetism in Co-doped Bi0.5K0.5TiO3 materials, Appl Phys A 123 (2017) 563 [19] N.H Tuan, N.H Linh, D Odkhuu, N.N Trung, D.D Dung, Microstructural, optical, and magnetic properties of BiCoO3-modified Bi0.5K0.5TiO3, J Electron Mater 47 (2018) 3414e3420 [20] N Hollmann, M.W Haverkort, M Benomar, M Cwik, M Braden, T Lorenz, Evidence for a temperature-induced spin-state transition of Co3ỵ in La2xSrxCoO4, Phys Rev B 83 (2011) 174435 [21] S.K Hoffmann, J Goslar, S Lijewski, Electron paramagnetic resonance and electron spin echo studies of Co2ỵ coordination by nicotinamide adenine dinucleotide (NADỵ) in water solution, Appl Magn Reson 44 (2013) 817e826 [22] K Momma, F Izumi, VESTA for three-dimensional visualization of crystal, volumetric and morphology data, J Appl Crystallogr 44 (2011) 1272e1276 [23] P Giannozzi, et al., Quantum espresso: a modular and open-source software project for quantum simulations of materials, J Phys Condens Matter 21 (2009) 395502 [24] J.P Perdew, K Burke, M Ernzerhof, Generalized gradient approximation made simple, Phys Rev Lett 77 (1996) 3865 [25] G.O Jones, P.A Thomas, Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3, Acta Crystallogr Sect B Struct Sci 58 (2002) 168e178 [26] G.O Jones, J Kreisel, V Jennings, M.A Geday, P.A Thomas, Investigation of a peculiar relaxor ferroelectric: Na0.5Bi0.5TiO3, Ferroelectrics 270 (2002) 191e196 [27] H.J Monkhorst, J.D Pack, Special points for Brillouin-zone integrations, Phys Rev B 13 (1976) 5188 ... absorption spectra of Co- doped Na0. 5Bi0. 5TiO3 samples as a function of Co concentration, and (b) the (ahn)2 proposal with photon energy (hn) of the Na0. 5Bi0. 5TiO3 samples as a function of Co concentration... inset of Fig 5(b) shows the MeH curve of the mol% Co- doped Na0. 5Bi0. 5TiO3 sample at K Low-spin and high-spin congurations of (c) Co2 ỵ ions and (d) Co3 ỵ ions in the octahedral site of Na0. 5Bi0. 5TiO3. .. radius of 0.605 [7] Therefore, the substitution of Ti4ỵ cations by Co2 ỵ cations resulted in the expansion of the lattice constants of the host NBTO materials because the radii of the Co2 ỵ cations