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

Journal of Science: Advanced Materials and Devices 4 (2019) 584e590

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Journal of Science: Advanced Materials and Devices
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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, 1 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, 1 Dai Co Viet Road, Ha Noi, Viet Nam
g
School of Electronics and Telecommunications, Ha Noi University of Science and Technology, 1 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 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 ( />
Keywords:
Lead-free ferroelectric
Multiferroics
Na0.5Bi0.5TiO3
Ferromagnetism
Sol-gel

1. 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: (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

/>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

( />

D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590

temperature ferromagnetism in Co-doped NBTO materials originates as an intrinsic phenomenon.
2. 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 2 h and
sintered at 900  C for 3 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 5 K and a vibrating sample magnetometer (VSM) at
room temperature.
3. Results and discussion
The FE-SEM images of pure and Co-doped Na0.5Bi0.5TiO3 with
different molar 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 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. 3 (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 3 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 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 coordination) have radii of 0.65 Å (in low spin states) and 0.735 Å (in
high 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 fabrication method [8,9]. Huan et al. reported that Co2þ and Co3þ 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 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 octahedral B-sites in a NBTO lattice, thereby increasing the number of

Fig. 1. Surface morphology of (a) pure Na0.5Bi0.5TiO3 and Co-doped Na0.5Bi0.5TiO3 with different Co concentrations: b) 1 mol.%, c) 3 mol.%, d) 5 mol.%, e) 7 mol.%, and f) 9 mol.%.


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D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590

Fig. 2. (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. 3. (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
O2À [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 substitution of Co2þ in these sites influences 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 4 (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. 3 (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. 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 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 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 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 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 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 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 cations, 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 are magnetic.
The spin configurations of Co2þ and Co3þ 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 Co2þ/3þ were present in

Fig. 4. (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.


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D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590

Fig. 5. (a) The MeH curves of pure and Co-doped Na0.5Bi0.5TiO3 samples with different Co concentrations, (b) the MeT curve at 1 kOe magnetic field for the Co-doped Na0.5Bi0.5TiO3
sample with 1 mol%. The inset of Fig. 5(b) shows the MeH curve of the 1 mol% Co-doped Na0.5Bi0.5TiO3 sample at 5 K. Low-spin and high-spin configurations of (c) Co2þ ions and (d)
Co3þ 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 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 highspin 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 the magnetic moment
seemed to arise from the oxygen vacancies due to the noncompensation of charge between Co3þ and Ti4þ [3]. During the
transfer from the Co3þ to Co2þ 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 Co2þ had the larger radius
than Ti4þ in both the spin configurations. However, at this moment,
we do not have direct evidence of exact percent contributions of
Co2þ and Co3þ to the total magnetic moment of the samples.
Additional contributions of magnetic moment of oxygen vacancies
and/or Ti3þ 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. 6 [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. 6. Rhombohedral supercell of 2 Â 2 Â 2 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 4 (2019) 584e590

589

Fig. 7. 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 2 Bi/Na cation in
Wyckoff symmetric position 2a, 2 Ti/M cation in 2a and 6 O anion in
6a (Fig. 6), and lattice constants from the experiments [25,26]. A
2 Â 2 Â 2 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 Â 2 Â 2, and the mesh of k-space was increased to 4 Â 4 Â 4 in the
static and density of state (DOS) calculations.
Fig. 7 (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. 8 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. 8. 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 1. 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 1
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


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D.D. Dung et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 584e590

Fig. 8. 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.
4. 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
9 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 5 K owing to the
main interaction of the complex states of Co2þ/3þ 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
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