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

Therefore, the substitution of Ti 4þ cations by Co 2þ cations resulted in the expansion of the lattice constants of the host NBTO materials because the radii of the Co 2 þ cations in bot[r]

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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 Dunga,*, N.B Doanb,c, N.Q Dungd, L.H Baca, N.H Linha, L.T.H Thanha, D.V Thieta, N.N Trunga, N.C Khange, T.V Trungf, N.V Ducg

aSchool of Engineering Physics, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam bCNRS, Institut Neel, F-38042, Grenoble, France

cUniv Grenoble Alpes, Institut Neel, F-38042, Grenoble, France

dDepartment of Chemistry, Thai Nguyen University of Education, 20 Luong Ngoc Quyen Street, Thai Nguyen, Viet Nam eCenter for Nano Science and Technology, Ha Noi National University of Education, 136 Xuan Thuy Road, Ha Noi, Viet Nam fSchool of Materials Science and Engineering, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam gSchool of Electronics and Telecommunications, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam

a 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

Ferromagnetism Sol-gel

a b s t r a c t

Co-doped Na0.5Bi0.5TiO3materials were fabricated by a sol-gel technique The structural distortion of

Co-doped Na0.5Bi0.5TiO3materials 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 ~0.64mB/Co at K

The origin of the room temperature ferromagnetism in the Co-doped Na0.5Bi0.5TiO3materials was also

investigated through theﬁrst-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/)

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 tem-perature in transition metal-doped perovskite ferroelectric mate-rials provides a new approach for developing multiferroic matemate-rials for spintronics applications In fact, room temperature ferromag-netism was reported in various lead-free ferroelectric materials doped with transition metals[2e5] Wang et al reported that Fe-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

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 concentra-tion) and the compensation of paramagnetism/antiferromagnetism (at high doping Mn concentration) with ferromagnetism[4] By contrast, Co-doped NBTO synthesized by the hydrothermal tech-nique 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 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)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.08.007

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cated by using ground and calcined dry gels at 400 C for h and sintered at 900C for h in air Sodium concentration was 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 Na0.5Bi0.5TiO3compounds was characterized by

energy dispersive X-ray (EDX) spectroscopy The surface morphology and symmetry of the crystalline structures of the samples were characterized byﬁeld 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 K and a vibrating sample magnemagne-tometer (VSM) at room temperature

3 Results and discussion

The FE-SEM images of pure and Co-doped Na0.5Bi0.5TiO3with

different molar ratios are shown inFig The particles of pure NBTO samples were cubic, with an average size of about 300 nm, as shown inFig 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

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, Co2ỵcations (in VI co-ordination) have radii of 0.65 Å (in low spin states) and 0.735 (in high spin states), whereas Co3ỵcations (in VI coordination) have radii of 0.545 Å (in low spin states) and 0.61 (in high spin states) [7] Co4ỵcations are only stable at high spin states, with a radius of 0.53 , whereas Ti4ỵcations 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 Co2ỵcations in both spin states were larger than those of the Ti4ỵcations; meanwhile, the presence of higher valence states of cobalt as Co3ỵand Co4ỵresulted 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 fabri-cation method[8,9] Huan et al reported that Co2ỵand Co3ỵcations coexist in Na0.5Bi0.5TiO3e6%BaTiO3single 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 Co3ỵ to Co2ỵ [9] Schimitt et al observed that the valence state of Co-doped NBTO changed from Co3ỵto Co2ỵat high sintering temperatures[10]and that Co cations occupied octahe-dral B-sites in a NBTO lattice, thereby increasing the number of

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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 Co2ỵcations with high states are substituted in Bi and Na sites because their radii (0.735 Å) are comparable to those of Bi3ỵand Naỵcations (1.11 and 1.16 , respectively)[7] The sub-stitution of Co2ỵin these sites inuences 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

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 cm1to 200 cm1, 200 cm1to 400 cm1, and 400 cm1to 650 cm1 Experimental and theoretical studies that predicted the vibration modes of NBTO materials reported that the lowest frequency modes ranging from 109 cm1to 187 cm1are dominated by Bi/NaeO vibration, the frequency modes ranging from 240 cm1to 401 cm1are dominated by TiO6and TieO

vi-brations, and the higher frequencies modes ranging from 413 cm1 to 826 cm1are 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

Fig (a) X-ray diffraction patterns of pure and Co-doped Na0.5Bi0.5TiO3samples as a function of cobalt doping concentration, (b) a comparison of (003)/(201) diffraction peak

positions for pure and Co-doped Na0.5Bi0.5TiO3samples

Fig (a) Raman spectra of the pure and Co-doped Na0.5Bi0.5TiO3samples as a function of Co doping concentration, (b) and (c) magniﬁcation Raman spectra in the wavenumber

range of 100e200 cm1and 150e450 cm1for pure and Co-doped Na

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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-doped 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)2was plotted as the function of phonon energy (hn), as shown inFig 4(b)[17] The optical band gap values were estimated through the extrapolation of the best-ﬁt line between (ahn)2and (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 ofFig 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 Na0.5Bi0.5TiO3materials was observed by determining

NBTO materials[3,4] The observed nonzero values of HCand Mrin

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-doped Na0.5Bi0.5TiO3was 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

-Ti0.99Co0.01O3sample under an applied magneticﬁeld of kOe The

inset ofFig 5(b) shows the M-H curve of Na0.5Bi0.5Ti0.99Co0.01O3in

magnetic ﬁelds of up to 70 kOe at 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 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 Co3ỵ (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 Co3ỵto 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 Co2ỵare magnetic The spin congurations of Co2ỵand Co3ỵin the low-spin and high-spin states are shown inFig 5(c) and (d), respectively Our results suggested that both the valence states of Co2ỵ/3ỵwere present in

Fig (a) UVeVis absorption spectra of Co-doped Na0.5Bi0.5TiO3samples as a function of Co concentration, and (b) the (ahn)2proposal with photon energy (hn) of the Na0.5Bi0.5TiO3

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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 Co3ỵions were 0.545 and 0.61 for low-spin and high-spin congurations, respectively, and the radii of Co2ỵions 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 Co3ỵ ions are of low-spin conguration because Co3ỵ ions have smaller radii than Ti4ỵ ions (0.545 and 0.605 , respectively) Therefore, the enhancement of the magnetic moment seemed to arise from the oxygen vacancies due to the non-compensation of charge between Co3ỵ and Ti4ỵ [3] During the transfer from the Co3ỵto Co2ỵstate, 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 Co2ỵhad the larger radius than Ti4ỵin both the spin configurations 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 Ti3ỵdefects need to be further investigated

To further understand the role of transition metals such as Co-doped lead-free ferroelectric Na0.5Bi0.5TiO3 samples, we

per-formed theﬁrst 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 Quantum-Espresso package [23] The exchange correlation energy was

carried out by the Generalized Gradient Approximation (GGA) method using PerdeweBurkeeErnzerhof (PBE) exchange-correlation functions [24] Plane-wave basis set cutoffs for the smooth part of wave functions and the augmented density were 45

Fig (a) The MeH curves of pure and Co-doped Na0.5Bi0.5TiO3samples with different Co concentrations, (b) the MeT curve at kOe magnetic ﬁeld for the Co-doped Na0.5Bi0.5TiO3

sample with mol% The inset ofFig 5(b) shows the MeH curve of the mol% Co-doped Na0.5Bi0.5TiO3sample at K Low-spin and high-spin congurations of (c) Co2ỵions and (d)

Co3ỵions in the octahedral site of Na0.5Bi0.5TiO3

Fig Rhombohedral supercell of 2 established from a rhombohedral prim-itive 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 Ti1and Ti2denote Ti

ions ofﬁrst nearest neighbor and the second nearest neighbor octahedral TiO6of

transition metal doping position, viz Ti1stand Ti2nd O1stand O2ndare twofold

coor-dinated bridge O between M Ti1stand Ti1ste Ti2nd Crystal structure prepared using

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Ry and 250 Ry, respectively The amount of computational re-sources 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 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 Bril-louin zone[27] The structures were fully relaxed with a mesh of 2 2, and the mesh of k-space was increased to 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 Na0.5Bi0.5TiO3compounds, respectively Similar

to NBTO, Co-doped NBTO materials showed the broad NaeO hy-bridization, which represents the strong covalent bond between Na and O cations As seen inTable 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 hybrid-ization at energy range under10 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

perov-skites 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 BO6octahedrons 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 8provides a detailed view of the interaction between B-site cations and O anions In the Co-doped Na0.5Bi0.5TiO3samples, the

3d electrons of Co are predominantly distributed in dxyorbitals, and

are strongly localized in the energy range of2 eVe0 eV However, these d-electrons establish the hybridization with the pxorbital of

the nearest O cation, that is, the O1stp-orbital It prompts the spin

polarization of theﬁrst nearest O1stions The doping of Co into the

pristine NBTO enabled Ti1st and Ti2nd ions to gain electrons, as

shown inTable 1, and increased the states located near the Fermi level Thus, Ti1stand 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 Ti2ndions in Co-doped NBTO samples, which

are presented inTable 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

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)

Table

Charge (±DQ) gain/loss by Bi, Na, O1st, O2nd, Ti1st, and Ti2nddue to doping of Co into the host BNTO compounds to form the BNCT compounds Plus/negative signs are

rep-resented for gaining/losing charge, respectively, and magnetic moment of Co, O1st, Ti1st, O2nd, and Ti2ndin BNCT

Charge DQBiðeÞ DQNaðeÞ DQO1stðeÞ DQO2ndðeÞ DQTi1stðeÞ DQTi2ndðeÞ

0.54 0.37 0.16 0.03 0.13 0.12

Magnetic moment mCoðmBÞ mO1stðmBÞ mTi1stðmBÞ mO2ndðmBÞ mTi2ndðmBÞ

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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 fabri-cated 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/3ỵthrough oxygen vacancies in the Co-doped NBTO materials We suggested that the ferromagnetism at room temperature in Co-doped NBTO is an intrinsic property Thefirst 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 Thesefindings will advance the current understanding of the origin of the room temperature ferromagnetism in transition metal-doped 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

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Rev B 13 (1976) 5188

Fig Calculated PDOS of speciﬁc position sites in BNCT, i.e O1st, Ti1st, O2nd, Ti2nd, and

Co Ti1stand Ti2nddenote Ti cations of the nearest and next nearest TiO6octahedrons to

doping Ni position O1stand O2nddenote the twofold coordinated O ion which are the

bridge bonds between Ni and Ti1st, Ti2nd, respectively Energies are given in [eV] with

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