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3)O3 perovskites by Raman spectroscopy
Materials Characterization 158 (2019) 109938
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Further ordering structural investigation of Ba((Co,Zn,Mg)1/3Nb2/3)O3 perovskites by Raman spectroscopy
T
Pian Pian Maa,∗, Xiang Ming Chenb a
Department of Materials Engineering & Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Zhejiang Sci-Tech University, Hangzhou, 310018, China b Laboratory of Dielectric Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Raman spectroscopy Dielectric properties Ordering structure
Correlations among the Raman vibration modes, ordering structures and dielectric properties have been established through further ordering structural investigation of Ba((Co0.6-x/2Zn0.4-x/2Mgx)1/3Nb2/3)O3 ceramics by Raman spectroscopy. Meanwhile an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba(B′1/3B″2/ 5+ -rich and 3)O3 perovskites has been gained. An approximate 1:1 ordered structure composed of alternate Nb Nb5+-dificient layers is detected by transmission electron microscopy observation, confirming that the four main Raman peaks of 1:2 ordered structure arise from the nanoscale regions of 1:1 ordering in B-site sublattice. As x increases, Ba-related and Nb-related phonons show growing lifetime with the increase of ordering degree and Qf values. Simultaneously, oxygen vibration along c axis (Eg(4) (O) mode) is favored, while the vibration in ab plane (Eg(3) (O) mode) weakens since 1:2 ordering forms along the c-axis of ordered structure. The lifetime of the oxygen-octahedron stretch A1g(4) (O) mode (∼788 cm−1) is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries.
1. Introduction
[2,3,10,11]. The ordering structure is commonly characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observation. These two techniques play complementary roles in characterizing the ordering features, as XRD reflects the ordering degree of the whole sample, while TEM provides additional details about the local ordering. However, ordering structural evalution surely has an impact on the lattice vibrations as well, which is directly related to the dielectric response. The knowledge of the lattice vibration modes is very helpful to understand the underlying physical mechanism of dielectric behaviors [12]. Far-infrared reflection (IR) and Raman spectrum are two main techniques to study the optical phonons [13,14]. However, peaks of IR spectrum for Ba(B′1/3B″2/3)O3 perovskites are always broad and seriously overlapped [15,16], the assignment of the structure-related phonon eigenvectors is unambiguous. The contributions of IR-active modes to the intrinsic dielectric response are usually determined with the help of the first-principle calculation [17–19]. Comparatively, Raman scattering spectrum exhibits clear and sharp peaks, and the number of Raman-active phonons is less than that of IR-active phonons, thus the Raman mode assignment is relatively easy to implement. Moreover, Raman spectrum is highly sensitive to the crystal structure change caused by different compositions or preparation conditions
Ba-based Ba(B′1/3B″2/3)O3 (B' = Co2+, Zn2+, Mg2+ etc., B'' = Ta5+ or Nb5+) complex perovskites have received considerable interest for base station technology because of their remarkable microwave dielectric properties [1]. The B-site ordering behavior is recognized to have a notably impact on the microwave dielectric properties, especially the Qf values [2–4]. For Ba(B′1/3B″2/3)O3 perovskite, there are three possible B-site cation arrangements, corresponding to three types of crystal structures. It is completely disorder when B-site cations distribute randomly, showing a cubic structure with Pm3¯m symmetry. When B-site cations show large difference in charge (Δq) and radius (Δr), they tend to exhibit a stoichiometrically 1:2 ordered sequence along the {111} direction [5,6]. This 1:2 ordered arrangement is the most common ordering state in Ba(B′1/3B″2/3)O3 perovskites [3,7], resulting in a hexagonal (P3¯m1) superstructure. The third arrangement is 1:1 ordered with Fm3¯m symmetry [5], which is usually observed in Pb-based perovskites [8,9], while metastable in Ba-based Ba(B′1/3B″2/3) O3 perovskites. It has been proven that hexagonal Ba(B′1/3B″2/3)O3 perovskites with 1:2 ordered structure exhibit much higher Qf values, which can be further tailored by optimizing the ordering-induced domain structures
∗
Corresponding author. E-mail address: [email protected] (P.P. Ma).
https://doi.org/10.1016/j.matchar.2019.109938 Received 24 May 2019; Received in revised form 16 September 2019; Accepted 16 September 2019 Available online 30 October 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
2. Materials and methods
Table 1 Cation ordering features and microwave dielectric properties of the as-sintered BCZMN ceramics [10]. x
ordering degree S
Domain Size
Qf (GHz)
εr
τf (ppm/oC)
0 0.1 0.2 0.3
0.536 0.849 0.893 0.901
– 20–30 nm 30–40 nm 40–60 nm
44 500 79 800 86 400 93 800
35.7 34.4 34.1 33.7
7.18 7.76 8.25 9.56
Ba((Co0.6-x/2Zn0.4-x/2Mgx)1/3Nb2/3)O3 (x = 0, 0.1, 0.2, 0.3) ceramics were prepared by the standard solid state action method from highpurity powders of BaCO3 (99.93%), Nb2O5 (99.99%), CoO (99.9%), ZnO (99.95%) and MgO (99.9%) as previously reported [10]. Powder samples for TEM observation were prepared by dispersing the suspended powders onto a holey carbon 200 mesh TEM grid. Ordering microstructure observation was carried out on an electron microscopy (Tecnai-F20, FEI Co., Hillsboro, Oregon) operating at 200 kV. Raman spectra measurement of the polished samples was performed at room temperature by a Raman spectrometer (HR-800 LabRAM, Jobin Yvon, Paris, France). The spectrometer was excited by Ar+ laser light (λ = 514.532 nm, 10 mW) with an acquisition time of 10 s. Raman spectra were recorded from 50 to 1000 cm−1, and the spectral resolution was about 1–2 cm−1. All Raman spectra in the present work were carefully processed by eliminating the temperature influence of the Raman intensity [20].
[20,21]. Therefore, Raman spectrum is a convenient and useful tool to detect the static and dynamic local ordering state of Ba(B′1/3B″2/3)O3 perovskites, the correlation between lattice vibration and dielectric response can be established. The Raman modes analysis for some Ba (B′1/3B″2/3)O3 systems have been conducted by some researches, the peak number, peak frequency and FWHM (full width at half maximum) of Raman spectrum have been analyzed [20–23]. However, opinions about the peak assignment are inconsistent, even contradictory for some ordering-related modes. The ordering degree and Qf value have been commonly used to analyze the Raman spectrum, while the detailed ordering microstructures have rarely been discussed together. Ba((Co, Zn, Mg)1/3Nb2/3)O3 (denoted as BCZMN) has been proved to be an important system of Ba(B′1/3B″2/3)O3 perovskites in our previous work [10,11]. The ordering features and the microwave dielectric properties of the as-sintered BCZMN ceramics are show in Table 1 [10]. In the present work, BCZMN is taken as representative of Ba(B′1/3B″2/3) O3 perovskites to evaluate further ordering structural evolution by Raman spectroscopy. Firstly, the origins of Raman active modes for 1:2 ordered perovskites are clearly revealed based on the ordering microstructure analysis. After successful assignment of all Raman peaks of BCZMN ceramics, the variation of Raman lattice vibrations with composition are revealed associating with Qf value as well as the ordering features (ordering degree and ordering microstructure). The present work not only establishes the correlations among the Raman vibration modes, ordering structures and dielectric properties, but also provides an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba (B′1/3B″2/3)O3 perovskites.
3. Results and discussion For Ba(B′1/3B″2/3)O3 complex perovskites, no Raman-active mode can be detected for disordered cubic structure with Pm3¯m symmetry. Raman-active modes are supposed to appear with the deviation from the ideal cubic symmetry by either chemical ordering or ion displacements. According to the group-theory analysis [24], the 1:1 ordered structure with Fm3¯m symmetry shows four Raman-active modes, given by Ref. [25]:
Γraman⋅(Fm3¯m) = A1g (O) + Eg (O) + F2g (Ba) + F2g (O)
(1)
While nine Raman-active modes are allowed for 1:2 ordered perovskites with P3¯m1 superstructure, the normal vibration modes are given by Ref. [26]:
Γraman⋅(P 3¯m1) = 4A1g + 5Eg = Eg (Ba) + A1g (Ba) + Eg (B″) + A1g (B″) + 3Eg (O) + 2A1g (O)
(2)
among them, Eg mode is doubly degenerate. The crystal structure of hexagonal perovskite with P3¯m1 symmetry is shown in Fig. 1a. There are fifteen atoms per unit cell, Raman active modes only arise from the vibration of Ba2 atoms (2d site), B″ atoms
Fig. 1. a) Crystal structure of 1:2 ordered Ba(B′1/3B″2/3)O3 perovskite with P3¯m1 symmetry; b) Schematic representations of the Raman vibrational modes, Ι and Π represent the doubly degenerate Eg submodes. 2
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
Fig. 2. a) HRTEM image of 1:2 ordered domains obtained from the highly ordered Ba((Co0.45Zn0.25Mg0.3)1/3Nb2/3)O3 sample, the inset is the SAED pattern along [11¯0]c zone axis; b) High magnification view of the designated area in a); c) Schematic diagram of the ordering transition evolution from disorder to 1:2 order.
the evolution process from disorder to 1:2 ordering. The ordering transition evolution is sketched in Fig. 2c. Oxygen octahedrons with different colors represent different B-site occupations. The B-site cations (B′ and Nb5+) distribute randomly for disordered structure, the corresponding oxygen octahedrons are marked in pink color. Firstly, due to the great driving force caused by Δq and Δr, the randomly distributed Bsite cations directional diffuse along the < 111 > c direction to form an approximate 1:1 ordered structure, which is composed of alternate Nb5+-rich (marked in purple color) and Nb5+-dificient layers (marked in orange color). Then, the metastable 1:1 ordering state further transforms into the steady state with a { … B′- Nb5+- Nb5+ … } repeat sequence. Oxygen octahedrons with full occupation of B′ and Nb5+ cations are marked in yellow and blue, respectively. The transition state is kinetically favored, and surely reduces the energy required for structural evalution. Above all, combined with microstructure observation, the existential form of the localized 1:1 ordering is revealed, and the origin of the four Raman main peaks for 1:2 ordered structure is further confirmed. In addition, the F2g (Ba) and F2g (O) modes allowed in Fm3¯m symmetry refer to the coupled vibrations of Ba and O in an octahedron [29]. The symmetry is lowered from 1:1 ordering (Fm3¯m) to the hexagonal (P3¯m1) superstructure, leading to the splitting of F2g mode [20]. Thus in P3¯m1 symmetry, F2g (Ba) mode (∼105 cm−1) and F2g (O) mode (∼ 380 cm−1) splits into A1g (Ba) and Eg (Ba) modes, A1g (O) and Eg (O) modes, respectively. Beyond these, three extra modes in 150–350 cm−1 frequency regions, which originate from the 1:2 ordered structure can be expected. From the above, a total of nine Raman-active modes for 1:2 ordered structure is clearly explained. In order to clarify the Raman parameters (peak frequency, intensity and FWHM), all Raman data are carefully fitted by Lorentz curves. Fig. 3 shows the experimental and fitting Raman spectra of the as-sintered BCZMN ceramics obtained from 50 cm−1 to 1000 cm−1. The
(2d site) and O1 atoms (6i site), while others make no contribution [21]. The nine Raman vibration modes for 1:2 ordered perovskite are sketched in Fig. 1b according to the Bilbao Crystallographic Center website [27]. Each submode of the degeneracy modes is sketched with different colors and marked with Ι and Π. The superscript for each mode is used to distinguish modes in the same symmetric category such as Eg(1), Eg(2). It has been found that the distribution of the four main peaks for 1:2 ordered perovskites (∼ 105 cm−1, ∼ 380 cm−1, ∼ 430 cm−1 and ∼ 790 cm−1) are consistent with the four peaks of 1:1 ordered perovskites. From this, literatures put forward that the four main peaks represent the spectra of the localized 1:1 ordering regions [20,21,28]. The diffuse 1 {111} superlattice spots indicating the 1:1 ordering have 2 been detected in 1:2 ordered Ba(Zn1/3Ta2/3)O3 samples [4]. However, ¯ for the Fm3m space symmetry, how should the B-site cations occupy in the stoichiometric ratio of 1:2? A detailed microstructure analysis from TEM observation is necessary to provide the existential form of the localized 1:1 ordering. The highly ordered Ba((Co0.45Zn0.25Mg0.3)1/3 Nb2/3)O3 sample is chosen for TEM observation due to the good accessibility to detect ordering areas. Fig. 2a shows the high-resolution transmission electron microscopy (HRTEM) images of 1:2 ordered domains, and the inset is the selected area electron diffraction (SAED) pattern along the given [11¯0]c zone axis. The marked area in high magnification is shown in Fig. 2b. Lattice with a periodicity of 0.71 nm refers to 1:2 ordered domain, as 1:2 ordering triples the {111}c interplaner spacing of cubic unit cell (0.71 nm ≈ 3*d(111)c). While the area with the modulation of 0.46 nm represents 1:1 ordering (0.46 nm ≈ 2*d(111)c). As shown in Fig. 2b, the localized 1:1 ordered structure is observed between two 1:2 ordered domains, showing the same ordering orientation with the adjacent 1:2 ordering. Thus, it can be assumed that the localized 1:1 ordering exists as a transition state in 3
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
While A1g(1) (Ba) mode represents Ba vibration along c-axis. The evolution of Raman shift and FWHM of the two peaks with composition are shown in Fig. 4b. A weak blue shift is found in Eg(1) (Ba) mode as Mg content increases, while weak red shift in A1g(1) (Ba) mode is observed. The phonon frequency f correlates with the force constant k and reduced mass m*, and can be expressed as [32]:
f=
1 2π
k m∗
(3)
On one hand, Mg shows a smaller polarizability than Co and Zn, the increasing of Mg content leads to a weaker covalent interaction, and the stiffness of oxygen octahedral decreases, resulting a smaller k value in equation; On the other hand, the lighter Mg substitution also decreases the m* value of atoms that participate in the vibration. Two opposite effects on the Raman frequency can be expected by Mg substitution, thus opposite variation rules are observed for Eg(1) (Ba) and A1g(1) (Ba). It can be speculated that the Ba vibration along c-axis (A1g(1) (Ba) mode) is mainly influenced by the stiffness of B′ oxygen octahedral, while the vibration in ab plane is affected by the reduced mass. It is said that the FWHM of Raman peak, rather than the peak frequency or the peak intensity is more representational to characterize the ordering features [33]. A smaller FWHM represents less interference between phonons and longer lifetime, corresponding to higher ordering of samples [30]. As shown in Fig. 4b, the FWHMs of the Eg(1) (Ba) and A1g(1) (Ba) modes decrease as x increases, the variation trend is consistent with that of ordering degrees and Qf values, as reported in our previous work [10]. Fig. 5 shows the analysis of Raman spectra in the frequency range of 150–650 cm−1. Peak 3, 4 and 5, which originates from 1:2 ordering can be observed clearly. However, the assignment of these three peaks is controversial. There are two main views: one attributes them as Eg (O), Eg (Nb), A1g (Nb), respectively [19,21,34]; the other one assigns as Eg (Nb), A1g (Nb), Eg (O) respectively [18,30,31]. Through comparison with other works and detailed analysis of the present data, the second view is considered to be more persuasive. A simple reason is that heavier atoms always show lower frequencies than lighter ones according to formula (3), so vibrations associated with O atoms show higher frequencies than others [30]. In addition, the FWHM is an important ordering parameter, the variation of FWHMs with composition may contribute to the assignment of the three peaks. As Fig. 5b shows, the FWHMs of peak 3 and peak 4 decrease sharply when x changes from 0 to 0.1, then steadily decrease as x continues to increase, the variation trend is completely opposite with that of Qf values. A same phenomenon has been observed in Ba(Mg1/3Ta2/3)O3–Ba(Mg1/3Nb2/3)O3 system [21], that is, samples with higher Qf values show narrow linewidths of Eg (B″) and A1g (B″) phonons. Since A1g (B″) and Eg (B″) are
Fig. 3. Experimental and fitting Raman spectra for the as-sintered BCZMN ceramics.
calculated results agree well with the experimental data. The following discussion will be conducted in three frequency regions: 50 - 150 cm−1, 150 - 650 cm−1 and 650 - 1000 cm−1 (marked with blue dotted lines), and x = 0.1 is taken as an example to show the detailed fitting data. Meanwhile, the first-principle calculation on the optical phonon frequencies and their contributions [18,19,30,31] will provide some guidance for the assignment of Raman-active modes. Fig. 4 shows the analysis of Raman spectra in the frequency range of 50–150 cm−1. As discussed above, two peaks in this range come from the splitting of F2g (Ba) mode, which refers to the vabration of Ba atoms against the oxygen octahedrons. Because Eg mode is expected to have a lower energy than A1g mode [20,28], peak 1 with smaller wavenumber is assigned as Eg(1) (Ba) mode and peak 2 as A1g(1) (Ba) mode. According to Fig. 1b, Eg(1) (Ba) mode is the vibration of Ba atoms in ab plane, and has two degenerated modes with vibrations in different directions.
Fig. 4. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 50–150 cm−1; b) Raman shifts and FWHMs of Eg(1)(Ba) and A1g(1)(Ba) as a function of x. 4
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
breath-vibration of oxygen octahedron in ab plane. It is said that each Raman vibrational mode is the combination of contributions from other modes in the same symmetry classes, and Eg(4)(O) mode has a negative contribution to Eg(3)(O) mode, while the contribution coefficient of Eg(3)(O) mode to Eg(4)(O) mode is also negative [30,31]. Thus, it can be considered the two vibrational modes are against with each other. The FWHMs of the two Eg (O) modes change in the opposite trends with composition(Fig. 5c), coincide with the theoretical result. Accordingly, it can also be concluded that as Mg content increases, the 1:2 ordering increase, the twisting vibration of oxygen octahedron along c axis (Eg(4)(O) mode) enhances, while the vibration in ab plane (Eg(3)(O) mode) weakens. It can be explained that the 1:2 ordering structural evolution happens along the c-axis of ordered structure, which is more favorable to the atoms vibration along c axis. The analysis of Raman spectra in the frequency range of 650–1000 cm−1 is shown in Fig. 6. Three peaks are obtained by Lorentz fitting (Fig. 6a). Peak 10 at ∼788 cm−1 is always observed in perovskite ceramics and assigned as A1g(4) (O) mode, which represents the stretching breath-vibration of oxygen octahedron along c-axis. The A1g(4) (O) mode can be treated as a probing means for different types of B-site ordering. Peak 9, as the shoulder of A1g(4) (O), has been reported by some researches, and is generally recognized as the indication of the possible short-range ordering [35–37]. According to Fu's explanation, peak 9 can also be regarded as A1g(4) (O) mode [38], and it must be correlated with the localized 1:1 ordering in the present system. The FWHM of 1:2 A1g(4) (O) mode (peak 10) and 1:1 A1g(4) (O) mode (peak 9) gradually decreases with composition (Fig. 6b), indicating growing lifetime of the phonons. Considering the unique ordering-induced microstructure of Ba(B′1/3B″2/3)O3 perovskites, the lifetime of the oxygenoctahedral stretching phonon is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries. The HRTEM images of these four compositions have been detailed analyzed in our previous work [10]. There are regions with different superlattice modulations exist at the domain boundaries for the non-substituted composition, while the domain boundaries are clear and natural in Mg-substituted ones. As x increases, the 1:2 ordered domain size gradually increase, accompanied with the reduction of complex domain boundaries, thus the interference between phonons is weakened, and smaller FWHM of 1:2 A1g(4) (O) mode (peak 10) is obtained. While for 1:1 A1g(4) (O) mode (peak 9), considering the localized distribution, the effect of the domain size can be neglected, while the transition interface is supposed to play a dominant role. As x increases, the gradually clear transition interface of 1:1 ordering domain contributes to the weakening of the interference between 1:1 ordering related phonons, leading to the decrease of FWHM for peak 9. Microdefects such as complex boundaries could also introduce active mode around the A1g(4) (O) mode [12,34]. Peak 11 is assigned as the defect activated mode (DAM). The above discussion about the domain boundaries also gives an explanation for the increase of the FWHM of peak 11 with composition. As x increases, the micro-defects reduces with the increase of 1:2 ordered domains, leading to the steady increase of Qf values. From the above, take x = 0.1 as an example, the Raman fitting parameters and attributions of Raman peaks are shown in Table 2. All Raman active modes are successfully assigned in the 1:2 ordered perovskites, based on the variation of both dielectric properties and ordering microstructure.
Fig. 5. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 150–650 cm−1; b) Qf values and FWHMs of Eg(2) (Nb), A1g(2) (Nb) and Eg(3) (O) as a function of x; c) FWHM variations of Eg(3) (O) and Eg(4) (O) (inset are the corresponding Raman vibration modes).
closely related to the 1:2 ordered structure, peak 3 and peak 4 are considered as Eg(2) (Nb) and A1g(2) (Nb) modes, which corresponding to the vibration of Nb atoms in ab plane and along c-axis, respectively. However, the FWHM of peak 5, which should be the Eg(3) (O) mode, gradually increases as x increases. It will be discussed together with peak 6 (∼ 378 cm−1), peak 7 (∼ 430 cm−1) and peak 8 (∼ 523 cm−1), because they all refer to the vibrational modes of O atoms. According to the first-principle calculations [30,31], peak 5, peak 6 and peak 8 are three types of Eg (O) modes (marked as Eg(3)(O), Eg(4)(O), Eg(5)(O)), referring to the oxygen octahedron twisting vibrations along different directions. While peak 7 is A1g(3) (O) mode, comes from the stretching
4. Conclusions Further ordering structural investigation of BCZMN ceramics have been systematically conducted by Raman spectroscopy, showing a correlation among the lattice vibration modes, ordering structure and dielectric properties. TEM observation confirms that the four main Raman peaks of 1:2 ordered structure arise from the localized 1:1 ordering, which is composed of alternate Nb5+-rich and Nb5+-dificient 5
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
Fig. 6. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 650–1000 cm−1; b) Qf values and FWHMs of two A1g(4) (O) modes with different ordering states and defected-active mode (DAM) as a function of x.
China (51702290), Public Welfare Technology Application Research Project of Zhejiang Province (LGG19E020005), Science Foundation of Zhejiang Sci-Tech University (ZSTU) under grant No. 16012171-Y and Zhejiang Top Priority Discipline of Textile Science and Engineering (2017YBZX05).
Table 2 Raman fitting parameters and attributions for the as-sintered BCZMN (x = 0.2) samples. f is the position and the intensities are in relative units. Peak No.
f (cm−1)
FWHM (cm−1)
Intensity (100%)
Attribution
1 2 3 4 5 6 7 8 9 10 11
102.0 104.6 171.7 263.3 293.7 377.7 429.9 523.3 732.2 788.5 824.1
3.7 2.8 4.8 15.6 17.9 6.9 10.4 45.9 25.6 28.1 15.0
9.7 35.9 1.1 0.5 2.2 24.1 5.5 2.3 3.0 100.0 2.7
Eg(1)(Ba) A1g(1)(Ba) Eg(2)(Nb) A1g(2)(Nb) Eg(3)(O) Eg(4)(O) A1g(3)(O) Eg(5)(O) A1g(4)(O)
a
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchar.2019.109938. References [1] I.M. Reaney, D. Iddles, Microwave dielectric ceramics for resonators and filters in mobile phone networks, J. Am. Ceram. Soc. 89 (2006) 2063–2072. [2] M.S. Fu, X.M. Chen, Y.W. Zeng, Effects of Mg-substitution on microstructures and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 perovskite ceramics, J. Am. Ceram. Soc. 93 (2010) 3787–3795. [3] P.K. Davies, J.Z. Tong, Effect of ordering-induced domain boundaries on low- loss Ba(Zn1/3Ta2/3)O3-BaZrO3 perovskite microwave dielectrics, J. Am. Ceram. Soc. 80 (1997) 1727–1740. [4] I.M. Reaney, I. Qazi, W.E. Lee, Order-disorder behavior in Ba(Zn1/3Ta2/3)O3, J. Appl. Phys. 88 (2000) 6708–6714. [5] P.K. Davies, H. Wu, A.Y. Borisevich, I.E. Molodetsky, L. Farber, Crystal chemistry of complex perovskites: new cation-ordered dielectric oxides, Annu. Rev. Mater. Res. 38 (2008) 369–401. [6] M.W. Lufaso, Crystal structure, modeling, and dielectric property relationships of 2:1 ordered Ba3MM'2O9 (M = Mg, Ni, Zn; M' = Nb, Ta) perovskites, Chem. Mater. 16 (2004) 2148–2156. [7] D.J. Barber, K.M. Moulding, J. Zhou, M.Q. Li, Structural order in Ba(Zn1/3 Ta2/3)O3, Ba(Zn1/3Nb2/3)O3 and Ba(Mg1/3Ta2/3)O3 microwave dielectric ceramics, J. Mater. Sci. 32 (1997) 1531–1544. [8] I.M. Reaney, J. Petzelt, V.V. Voitsekhovskii, F. Chu, N. Setter, B-site order and infrared reflectivity in A(B’B‘)O3 complex perovskite ceramics, J. Appl. Phys. 76 (1994) 2086. [9] H.B. Krause, J.M. Cowley, J. Wheatley, Short-range ordering in PbMg1/3Nb2/3O3, Acta Crystallogr. A35 (1979) 1015–1017. [10] P.P. Ma, L. Yi, X.Q. Liu, L. Li, X.M. Chen, Effects of Mg-Substitution on order/ disorder transition, microstructure and microwave dielectric characteristics of Ba ((Co0.6Zn0.4)1/3Nb2/3)O3 complex perovskite ceramics, J. Am. Ceram. Soc. 96 (2013) 1795–1800. [11] P.P. Ma, L. Yi, X.Q. Liu, L. Li, X.M. Chen, Effects of post-densification annealing upon microstructures and microwave dielectric characteristics in Ba((Co0.6-x/2Zn0.4x/2Mgx)1/3Nb2/3)O3 ceramics, J. Am. Ceram. Soc. 96 (2013) 3417–3424. [12] R.L. Moreira, F.M. Matinaga, A. Dias, Raman-spectroscopic evaluation of the longrange order in Ba(B'1/3B''2/3)O3 ceramics, Appl. Phys. Lett. 78 (2001) 428–430. [13] A. Dias, L.A. Khalam, M.T. Sebastian, M.M. Lage, F.M. Matinage, R.L. Moreira, Raman scattering and infrared spectroscopy of chemically substituted Sr2LnTaO6 (Ln = Lanthanides, Y and In) double perovskites, Chem. Mater. 20 (2008) 5253–5259. [14] I.N. Lin, C.T. Chia, H.L. Liu, H.F. Cheng, R. Freer, M. Barwick, F. Azough, Intrinsic dielectric and spectroscopic behavior of Perovskite Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/ 3)O3 microwave dielectric ceramics, J. Appl. Phys. 102 (2007) 044112. [15] S. Kamba, H. Hughes, D. Noujni, S. Surendran, R.C. Pullar, P. Samoukhina, J. Petzelt, R. Freer, N.M. Alford, D.M. Iddles, Relationship between microwave and lattice vibration properties in Ba(Zn1/3Nb2/3)O3-based microwave dielectric
DAMa
DAM: defect activated mode.
layers. As x increases, the FWHMs of Ba-related modes (Eg (1) (Ba) ∼ 102 cm−1, A1g(1) (Ba) ∼ 105 cm−1) and Nb-related modes (Eg (2) (Nb) ∼ 172 cm−1, A1g(2) (Nb) ∼ 263 cm−1) decrease with the increase of ordering degree and Qf values. Since the 1:2 ordering structural evolution happens along the c-axis of ordered structure, oxygen vibration along c axis (Eg(4) (O) mode ∼ 378 cm−1) is favored, while the vibration in ab plane (Eg(3) (O) mode ∼ 294 cm−1) weakens. The lifetime of the oxygen-octahedron stretch A1g(4) (O) mode (∼788 cm−1) is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries. The present work not only establishes the correlations among the Raman vibration modes, ordering structures and dielectric properties, but also provides an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba(B′1/3B″2/3)O3 perovskites. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing InterestCOI The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Science Foundation of 6
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phase-transition studies, Phase Transitions 76 (2003) 155–170. [28] I.G. Siny, R.S. Katiyar, A.S. Bhalla, Cation arrangement in the complex perovskites and vibrational spectra, J. Raman Spectrosc. 29 (1998) 385–390. [29] R. Tao, I.G. Siny, R.S. Katiyar, R. Guo, A.S. Bhalla, Temperature - dependent Raman studies of Ba(Mg1/3Ta2/3)O3, J. Raman Spectrosc. 27 (1996) 873–877. [30] C.H. Wang, X.P. Jing, L. Wang, J. Lu, XRD and Raman studies on the ordering/ disordering of Ba(Mg1/3Ta2/3)O3, J. Am. Ceram. Soc. 92 (2009) 1547–1551. [31] C.L. Diao, C.H. Wang, N.N. Luo, Z.M. Qi, T. Shao, Y.Y. Wang, J. Lu, Q.C. Wang, X.J. Kuang, L. Fang, F. Shi, X.P. Jing, First-principle calculation and assignment for vibrational spectra of Ba(Mg1/3Nb2/3)O3 microwave dielectric ceramic, J. Appl. Phys. 115 (2014) 114103. [32] F. Jiang, S. Kojima, C.L. Zhao, C.D. Feng, Chemical ordering in lanthanum - doped lead magnesium niobate relaxor ferroelectrics probed by A1g Raman mode, Appl. Phys. Lett. 79 (2001) 3938. [33] N. Setter, I. Laulicht, The observation of B-Site ordering by Raman scattering in A(B'B'')O3 perovskites, Appl. Spectrosc. 41 (1987) 526–528. [34] Y.D. Dai, G.H. Zhao, L.L. Guo, H.X. Liu, First-principles study of the difference in permittivity between Ba(Mg1/3Ta2/3)O3 and Ba(Mg1/3Nb2/3)O3, Solid State Commun. 149 (2009) 791–794. [35] C.T. Chia, P.J. Chang, M.Y. Chen, Oxygen-octahedral phonon properties of x BaTiO3 + (1−x) Ba(Mg1/3Ta2/3)O3 and x Ca(Sc1/2Nb1/2)O3 + (1−x) Ba (Sc1/2Nb1/2)O3 microwave ceramics, J. Appl. Phys. 101 (2007) 084115. [36] P.P. Ma, X.Q. Liu, F.Q. Zhang, J.J. Xing, X.M. Chen, Sr(Ga0.5Nb0.5)1-xTixO3 low-loss microwave dielectric ceramics with medium dielectric constant, J. Am. Ceram. Soc. 98 (2015) 2534–2540. [37] R. Ratheesh, M. Wohlecke, B. Berge, T. Wahlbrink, H. Haeuseler, E. Ruhl, R. Blachnik, P. Balan, N. Santha, M.T. Sebastian, Raman study of the ordering in Sr (B'0.5Nb0.5)O3 compounds, J. Appl. Phys. 88 (2000) 2813. [38] M.S. Fu, X.Q. Liu, X.M. Chen, Raman spectra analysis for Ca(B'1/3B''2/3)O3 -based complex perovskite ceramics, J. Appl. Phys. 104 (2008) 104108.
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Further ordering structural investigation of Ba((Co,Zn,Mg)1/3Nb2/3)O3 perovskites by Raman spectroscopy
T
Pian Pian Maa,∗, Xiang Ming Chenb a
Department of Materials Engineering & Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Zhejiang Sci-Tech University, Hangzhou, 310018, China b Laboratory of Dielectric Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Raman spectroscopy Dielectric properties Ordering structure
Correlations among the Raman vibration modes, ordering structures and dielectric properties have been established through further ordering structural investigation of Ba((Co0.6-x/2Zn0.4-x/2Mgx)1/3Nb2/3)O3 ceramics by Raman spectroscopy. Meanwhile an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba(B′1/3B″2/ 5+ -rich and 3)O3 perovskites has been gained. An approximate 1:1 ordered structure composed of alternate Nb Nb5+-dificient layers is detected by transmission electron microscopy observation, confirming that the four main Raman peaks of 1:2 ordered structure arise from the nanoscale regions of 1:1 ordering in B-site sublattice. As x increases, Ba-related and Nb-related phonons show growing lifetime with the increase of ordering degree and Qf values. Simultaneously, oxygen vibration along c axis (Eg(4) (O) mode) is favored, while the vibration in ab plane (Eg(3) (O) mode) weakens since 1:2 ordering forms along the c-axis of ordered structure. The lifetime of the oxygen-octahedron stretch A1g(4) (O) mode (∼788 cm−1) is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries.
1. Introduction
[2,3,10,11]. The ordering structure is commonly characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observation. These two techniques play complementary roles in characterizing the ordering features, as XRD reflects the ordering degree of the whole sample, while TEM provides additional details about the local ordering. However, ordering structural evalution surely has an impact on the lattice vibrations as well, which is directly related to the dielectric response. The knowledge of the lattice vibration modes is very helpful to understand the underlying physical mechanism of dielectric behaviors [12]. Far-infrared reflection (IR) and Raman spectrum are two main techniques to study the optical phonons [13,14]. However, peaks of IR spectrum for Ba(B′1/3B″2/3)O3 perovskites are always broad and seriously overlapped [15,16], the assignment of the structure-related phonon eigenvectors is unambiguous. The contributions of IR-active modes to the intrinsic dielectric response are usually determined with the help of the first-principle calculation [17–19]. Comparatively, Raman scattering spectrum exhibits clear and sharp peaks, and the number of Raman-active phonons is less than that of IR-active phonons, thus the Raman mode assignment is relatively easy to implement. Moreover, Raman spectrum is highly sensitive to the crystal structure change caused by different compositions or preparation conditions
Ba-based Ba(B′1/3B″2/3)O3 (B' = Co2+, Zn2+, Mg2+ etc., B'' = Ta5+ or Nb5+) complex perovskites have received considerable interest for base station technology because of their remarkable microwave dielectric properties [1]. The B-site ordering behavior is recognized to have a notably impact on the microwave dielectric properties, especially the Qf values [2–4]. For Ba(B′1/3B″2/3)O3 perovskite, there are three possible B-site cation arrangements, corresponding to three types of crystal structures. It is completely disorder when B-site cations distribute randomly, showing a cubic structure with Pm3¯m symmetry. When B-site cations show large difference in charge (Δq) and radius (Δr), they tend to exhibit a stoichiometrically 1:2 ordered sequence along the {111} direction [5,6]. This 1:2 ordered arrangement is the most common ordering state in Ba(B′1/3B″2/3)O3 perovskites [3,7], resulting in a hexagonal (P3¯m1) superstructure. The third arrangement is 1:1 ordered with Fm3¯m symmetry [5], which is usually observed in Pb-based perovskites [8,9], while metastable in Ba-based Ba(B′1/3B″2/3) O3 perovskites. It has been proven that hexagonal Ba(B′1/3B″2/3)O3 perovskites with 1:2 ordered structure exhibit much higher Qf values, which can be further tailored by optimizing the ordering-induced domain structures
∗
Corresponding author. E-mail address: [email protected] (P.P. Ma).
https://doi.org/10.1016/j.matchar.2019.109938 Received 24 May 2019; Received in revised form 16 September 2019; Accepted 16 September 2019 Available online 30 October 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 158 (2019) 109938
P.P. Ma and X.M. Chen
2. Materials and methods
Table 1 Cation ordering features and microwave dielectric properties of the as-sintered BCZMN ceramics [10]. x
ordering degree S
Domain Size
Qf (GHz)
εr
τf (ppm/oC)
0 0.1 0.2 0.3
0.536 0.849 0.893 0.901
– 20–30 nm 30–40 nm 40–60 nm
44 500 79 800 86 400 93 800
35.7 34.4 34.1 33.7
7.18 7.76 8.25 9.56
Ba((Co0.6-x/2Zn0.4-x/2Mgx)1/3Nb2/3)O3 (x = 0, 0.1, 0.2, 0.3) ceramics were prepared by the standard solid state action method from highpurity powders of BaCO3 (99.93%), Nb2O5 (99.99%), CoO (99.9%), ZnO (99.95%) and MgO (99.9%) as previously reported [10]. Powder samples for TEM observation were prepared by dispersing the suspended powders onto a holey carbon 200 mesh TEM grid. Ordering microstructure observation was carried out on an electron microscopy (Tecnai-F20, FEI Co., Hillsboro, Oregon) operating at 200 kV. Raman spectra measurement of the polished samples was performed at room temperature by a Raman spectrometer (HR-800 LabRAM, Jobin Yvon, Paris, France). The spectrometer was excited by Ar+ laser light (λ = 514.532 nm, 10 mW) with an acquisition time of 10 s. Raman spectra were recorded from 50 to 1000 cm−1, and the spectral resolution was about 1–2 cm−1. All Raman spectra in the present work were carefully processed by eliminating the temperature influence of the Raman intensity [20].
[20,21]. Therefore, Raman spectrum is a convenient and useful tool to detect the static and dynamic local ordering state of Ba(B′1/3B″2/3)O3 perovskites, the correlation between lattice vibration and dielectric response can be established. The Raman modes analysis for some Ba (B′1/3B″2/3)O3 systems have been conducted by some researches, the peak number, peak frequency and FWHM (full width at half maximum) of Raman spectrum have been analyzed [20–23]. However, opinions about the peak assignment are inconsistent, even contradictory for some ordering-related modes. The ordering degree and Qf value have been commonly used to analyze the Raman spectrum, while the detailed ordering microstructures have rarely been discussed together. Ba((Co, Zn, Mg)1/3Nb2/3)O3 (denoted as BCZMN) has been proved to be an important system of Ba(B′1/3B″2/3)O3 perovskites in our previous work [10,11]. The ordering features and the microwave dielectric properties of the as-sintered BCZMN ceramics are show in Table 1 [10]. In the present work, BCZMN is taken as representative of Ba(B′1/3B″2/3) O3 perovskites to evaluate further ordering structural evolution by Raman spectroscopy. Firstly, the origins of Raman active modes for 1:2 ordered perovskites are clearly revealed based on the ordering microstructure analysis. After successful assignment of all Raman peaks of BCZMN ceramics, the variation of Raman lattice vibrations with composition are revealed associating with Qf value as well as the ordering features (ordering degree and ordering microstructure). The present work not only establishes the correlations among the Raman vibration modes, ordering structures and dielectric properties, but also provides an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba (B′1/3B″2/3)O3 perovskites.
3. Results and discussion For Ba(B′1/3B″2/3)O3 complex perovskites, no Raman-active mode can be detected for disordered cubic structure with Pm3¯m symmetry. Raman-active modes are supposed to appear with the deviation from the ideal cubic symmetry by either chemical ordering or ion displacements. According to the group-theory analysis [24], the 1:1 ordered structure with Fm3¯m symmetry shows four Raman-active modes, given by Ref. [25]:
Γraman⋅(Fm3¯m) = A1g (O) + Eg (O) + F2g (Ba) + F2g (O)
(1)
While nine Raman-active modes are allowed for 1:2 ordered perovskites with P3¯m1 superstructure, the normal vibration modes are given by Ref. [26]:
Γraman⋅(P 3¯m1) = 4A1g + 5Eg = Eg (Ba) + A1g (Ba) + Eg (B″) + A1g (B″) + 3Eg (O) + 2A1g (O)
(2)
among them, Eg mode is doubly degenerate. The crystal structure of hexagonal perovskite with P3¯m1 symmetry is shown in Fig. 1a. There are fifteen atoms per unit cell, Raman active modes only arise from the vibration of Ba2 atoms (2d site), B″ atoms
Fig. 1. a) Crystal structure of 1:2 ordered Ba(B′1/3B″2/3)O3 perovskite with P3¯m1 symmetry; b) Schematic representations of the Raman vibrational modes, Ι and Π represent the doubly degenerate Eg submodes. 2
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Fig. 2. a) HRTEM image of 1:2 ordered domains obtained from the highly ordered Ba((Co0.45Zn0.25Mg0.3)1/3Nb2/3)O3 sample, the inset is the SAED pattern along [11¯0]c zone axis; b) High magnification view of the designated area in a); c) Schematic diagram of the ordering transition evolution from disorder to 1:2 order.
the evolution process from disorder to 1:2 ordering. The ordering transition evolution is sketched in Fig. 2c. Oxygen octahedrons with different colors represent different B-site occupations. The B-site cations (B′ and Nb5+) distribute randomly for disordered structure, the corresponding oxygen octahedrons are marked in pink color. Firstly, due to the great driving force caused by Δq and Δr, the randomly distributed Bsite cations directional diffuse along the < 111 > c direction to form an approximate 1:1 ordered structure, which is composed of alternate Nb5+-rich (marked in purple color) and Nb5+-dificient layers (marked in orange color). Then, the metastable 1:1 ordering state further transforms into the steady state with a { … B′- Nb5+- Nb5+ … } repeat sequence. Oxygen octahedrons with full occupation of B′ and Nb5+ cations are marked in yellow and blue, respectively. The transition state is kinetically favored, and surely reduces the energy required for structural evalution. Above all, combined with microstructure observation, the existential form of the localized 1:1 ordering is revealed, and the origin of the four Raman main peaks for 1:2 ordered structure is further confirmed. In addition, the F2g (Ba) and F2g (O) modes allowed in Fm3¯m symmetry refer to the coupled vibrations of Ba and O in an octahedron [29]. The symmetry is lowered from 1:1 ordering (Fm3¯m) to the hexagonal (P3¯m1) superstructure, leading to the splitting of F2g mode [20]. Thus in P3¯m1 symmetry, F2g (Ba) mode (∼105 cm−1) and F2g (O) mode (∼ 380 cm−1) splits into A1g (Ba) and Eg (Ba) modes, A1g (O) and Eg (O) modes, respectively. Beyond these, three extra modes in 150–350 cm−1 frequency regions, which originate from the 1:2 ordered structure can be expected. From the above, a total of nine Raman-active modes for 1:2 ordered structure is clearly explained. In order to clarify the Raman parameters (peak frequency, intensity and FWHM), all Raman data are carefully fitted by Lorentz curves. Fig. 3 shows the experimental and fitting Raman spectra of the as-sintered BCZMN ceramics obtained from 50 cm−1 to 1000 cm−1. The
(2d site) and O1 atoms (6i site), while others make no contribution [21]. The nine Raman vibration modes for 1:2 ordered perovskite are sketched in Fig. 1b according to the Bilbao Crystallographic Center website [27]. Each submode of the degeneracy modes is sketched with different colors and marked with Ι and Π. The superscript for each mode is used to distinguish modes in the same symmetric category such as Eg(1), Eg(2). It has been found that the distribution of the four main peaks for 1:2 ordered perovskites (∼ 105 cm−1, ∼ 380 cm−1, ∼ 430 cm−1 and ∼ 790 cm−1) are consistent with the four peaks of 1:1 ordered perovskites. From this, literatures put forward that the four main peaks represent the spectra of the localized 1:1 ordering regions [20,21,28]. The diffuse 1 {111} superlattice spots indicating the 1:1 ordering have 2 been detected in 1:2 ordered Ba(Zn1/3Ta2/3)O3 samples [4]. However, ¯ for the Fm3m space symmetry, how should the B-site cations occupy in the stoichiometric ratio of 1:2? A detailed microstructure analysis from TEM observation is necessary to provide the existential form of the localized 1:1 ordering. The highly ordered Ba((Co0.45Zn0.25Mg0.3)1/3 Nb2/3)O3 sample is chosen for TEM observation due to the good accessibility to detect ordering areas. Fig. 2a shows the high-resolution transmission electron microscopy (HRTEM) images of 1:2 ordered domains, and the inset is the selected area electron diffraction (SAED) pattern along the given [11¯0]c zone axis. The marked area in high magnification is shown in Fig. 2b. Lattice with a periodicity of 0.71 nm refers to 1:2 ordered domain, as 1:2 ordering triples the {111}c interplaner spacing of cubic unit cell (0.71 nm ≈ 3*d(111)c). While the area with the modulation of 0.46 nm represents 1:1 ordering (0.46 nm ≈ 2*d(111)c). As shown in Fig. 2b, the localized 1:1 ordered structure is observed between two 1:2 ordered domains, showing the same ordering orientation with the adjacent 1:2 ordering. Thus, it can be assumed that the localized 1:1 ordering exists as a transition state in 3
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While A1g(1) (Ba) mode represents Ba vibration along c-axis. The evolution of Raman shift and FWHM of the two peaks with composition are shown in Fig. 4b. A weak blue shift is found in Eg(1) (Ba) mode as Mg content increases, while weak red shift in A1g(1) (Ba) mode is observed. The phonon frequency f correlates with the force constant k and reduced mass m*, and can be expressed as [32]:
f=
1 2π
k m∗
(3)
On one hand, Mg shows a smaller polarizability than Co and Zn, the increasing of Mg content leads to a weaker covalent interaction, and the stiffness of oxygen octahedral decreases, resulting a smaller k value in equation; On the other hand, the lighter Mg substitution also decreases the m* value of atoms that participate in the vibration. Two opposite effects on the Raman frequency can be expected by Mg substitution, thus opposite variation rules are observed for Eg(1) (Ba) and A1g(1) (Ba). It can be speculated that the Ba vibration along c-axis (A1g(1) (Ba) mode) is mainly influenced by the stiffness of B′ oxygen octahedral, while the vibration in ab plane is affected by the reduced mass. It is said that the FWHM of Raman peak, rather than the peak frequency or the peak intensity is more representational to characterize the ordering features [33]. A smaller FWHM represents less interference between phonons and longer lifetime, corresponding to higher ordering of samples [30]. As shown in Fig. 4b, the FWHMs of the Eg(1) (Ba) and A1g(1) (Ba) modes decrease as x increases, the variation trend is consistent with that of ordering degrees and Qf values, as reported in our previous work [10]. Fig. 5 shows the analysis of Raman spectra in the frequency range of 150–650 cm−1. Peak 3, 4 and 5, which originates from 1:2 ordering can be observed clearly. However, the assignment of these three peaks is controversial. There are two main views: one attributes them as Eg (O), Eg (Nb), A1g (Nb), respectively [19,21,34]; the other one assigns as Eg (Nb), A1g (Nb), Eg (O) respectively [18,30,31]. Through comparison with other works and detailed analysis of the present data, the second view is considered to be more persuasive. A simple reason is that heavier atoms always show lower frequencies than lighter ones according to formula (3), so vibrations associated with O atoms show higher frequencies than others [30]. In addition, the FWHM is an important ordering parameter, the variation of FWHMs with composition may contribute to the assignment of the three peaks. As Fig. 5b shows, the FWHMs of peak 3 and peak 4 decrease sharply when x changes from 0 to 0.1, then steadily decrease as x continues to increase, the variation trend is completely opposite with that of Qf values. A same phenomenon has been observed in Ba(Mg1/3Ta2/3)O3–Ba(Mg1/3Nb2/3)O3 system [21], that is, samples with higher Qf values show narrow linewidths of Eg (B″) and A1g (B″) phonons. Since A1g (B″) and Eg (B″) are
Fig. 3. Experimental and fitting Raman spectra for the as-sintered BCZMN ceramics.
calculated results agree well with the experimental data. The following discussion will be conducted in three frequency regions: 50 - 150 cm−1, 150 - 650 cm−1 and 650 - 1000 cm−1 (marked with blue dotted lines), and x = 0.1 is taken as an example to show the detailed fitting data. Meanwhile, the first-principle calculation on the optical phonon frequencies and their contributions [18,19,30,31] will provide some guidance for the assignment of Raman-active modes. Fig. 4 shows the analysis of Raman spectra in the frequency range of 50–150 cm−1. As discussed above, two peaks in this range come from the splitting of F2g (Ba) mode, which refers to the vabration of Ba atoms against the oxygen octahedrons. Because Eg mode is expected to have a lower energy than A1g mode [20,28], peak 1 with smaller wavenumber is assigned as Eg(1) (Ba) mode and peak 2 as A1g(1) (Ba) mode. According to Fig. 1b, Eg(1) (Ba) mode is the vibration of Ba atoms in ab plane, and has two degenerated modes with vibrations in different directions.
Fig. 4. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 50–150 cm−1; b) Raman shifts and FWHMs of Eg(1)(Ba) and A1g(1)(Ba) as a function of x. 4
Materials Characterization 158 (2019) 109938
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breath-vibration of oxygen octahedron in ab plane. It is said that each Raman vibrational mode is the combination of contributions from other modes in the same symmetry classes, and Eg(4)(O) mode has a negative contribution to Eg(3)(O) mode, while the contribution coefficient of Eg(3)(O) mode to Eg(4)(O) mode is also negative [30,31]. Thus, it can be considered the two vibrational modes are against with each other. The FWHMs of the two Eg (O) modes change in the opposite trends with composition(Fig. 5c), coincide with the theoretical result. Accordingly, it can also be concluded that as Mg content increases, the 1:2 ordering increase, the twisting vibration of oxygen octahedron along c axis (Eg(4)(O) mode) enhances, while the vibration in ab plane (Eg(3)(O) mode) weakens. It can be explained that the 1:2 ordering structural evolution happens along the c-axis of ordered structure, which is more favorable to the atoms vibration along c axis. The analysis of Raman spectra in the frequency range of 650–1000 cm−1 is shown in Fig. 6. Three peaks are obtained by Lorentz fitting (Fig. 6a). Peak 10 at ∼788 cm−1 is always observed in perovskite ceramics and assigned as A1g(4) (O) mode, which represents the stretching breath-vibration of oxygen octahedron along c-axis. The A1g(4) (O) mode can be treated as a probing means for different types of B-site ordering. Peak 9, as the shoulder of A1g(4) (O), has been reported by some researches, and is generally recognized as the indication of the possible short-range ordering [35–37]. According to Fu's explanation, peak 9 can also be regarded as A1g(4) (O) mode [38], and it must be correlated with the localized 1:1 ordering in the present system. The FWHM of 1:2 A1g(4) (O) mode (peak 10) and 1:1 A1g(4) (O) mode (peak 9) gradually decreases with composition (Fig. 6b), indicating growing lifetime of the phonons. Considering the unique ordering-induced microstructure of Ba(B′1/3B″2/3)O3 perovskites, the lifetime of the oxygenoctahedral stretching phonon is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries. The HRTEM images of these four compositions have been detailed analyzed in our previous work [10]. There are regions with different superlattice modulations exist at the domain boundaries for the non-substituted composition, while the domain boundaries are clear and natural in Mg-substituted ones. As x increases, the 1:2 ordered domain size gradually increase, accompanied with the reduction of complex domain boundaries, thus the interference between phonons is weakened, and smaller FWHM of 1:2 A1g(4) (O) mode (peak 10) is obtained. While for 1:1 A1g(4) (O) mode (peak 9), considering the localized distribution, the effect of the domain size can be neglected, while the transition interface is supposed to play a dominant role. As x increases, the gradually clear transition interface of 1:1 ordering domain contributes to the weakening of the interference between 1:1 ordering related phonons, leading to the decrease of FWHM for peak 9. Microdefects such as complex boundaries could also introduce active mode around the A1g(4) (O) mode [12,34]. Peak 11 is assigned as the defect activated mode (DAM). The above discussion about the domain boundaries also gives an explanation for the increase of the FWHM of peak 11 with composition. As x increases, the micro-defects reduces with the increase of 1:2 ordered domains, leading to the steady increase of Qf values. From the above, take x = 0.1 as an example, the Raman fitting parameters and attributions of Raman peaks are shown in Table 2. All Raman active modes are successfully assigned in the 1:2 ordered perovskites, based on the variation of both dielectric properties and ordering microstructure.
Fig. 5. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 150–650 cm−1; b) Qf values and FWHMs of Eg(2) (Nb), A1g(2) (Nb) and Eg(3) (O) as a function of x; c) FWHM variations of Eg(3) (O) and Eg(4) (O) (inset are the corresponding Raman vibration modes).
closely related to the 1:2 ordered structure, peak 3 and peak 4 are considered as Eg(2) (Nb) and A1g(2) (Nb) modes, which corresponding to the vibration of Nb atoms in ab plane and along c-axis, respectively. However, the FWHM of peak 5, which should be the Eg(3) (O) mode, gradually increases as x increases. It will be discussed together with peak 6 (∼ 378 cm−1), peak 7 (∼ 430 cm−1) and peak 8 (∼ 523 cm−1), because they all refer to the vibrational modes of O atoms. According to the first-principle calculations [30,31], peak 5, peak 6 and peak 8 are three types of Eg (O) modes (marked as Eg(3)(O), Eg(4)(O), Eg(5)(O)), referring to the oxygen octahedron twisting vibrations along different directions. While peak 7 is A1g(3) (O) mode, comes from the stretching
4. Conclusions Further ordering structural investigation of BCZMN ceramics have been systematically conducted by Raman spectroscopy, showing a correlation among the lattice vibration modes, ordering structure and dielectric properties. TEM observation confirms that the four main Raman peaks of 1:2 ordered structure arise from the localized 1:1 ordering, which is composed of alternate Nb5+-rich and Nb5+-dificient 5
Materials Characterization 158 (2019) 109938
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Fig. 6. a) Experimental and fitting Raman spectra for the sample of x = 0.1 in the frequency range of 650–1000 cm−1; b) Qf values and FWHMs of two A1g(4) (O) modes with different ordering states and defected-active mode (DAM) as a function of x.
China (51702290), Public Welfare Technology Application Research Project of Zhejiang Province (LGG19E020005), Science Foundation of Zhejiang Sci-Tech University (ZSTU) under grant No. 16012171-Y and Zhejiang Top Priority Discipline of Textile Science and Engineering (2017YBZX05).
Table 2 Raman fitting parameters and attributions for the as-sintered BCZMN (x = 0.2) samples. f is the position and the intensities are in relative units. Peak No.
f (cm−1)
FWHM (cm−1)
Intensity (100%)
Attribution
1 2 3 4 5 6 7 8 9 10 11
102.0 104.6 171.7 263.3 293.7 377.7 429.9 523.3 732.2 788.5 824.1
3.7 2.8 4.8 15.6 17.9 6.9 10.4 45.9 25.6 28.1 15.0
9.7 35.9 1.1 0.5 2.2 24.1 5.5 2.3 3.0 100.0 2.7
Eg(1)(Ba) A1g(1)(Ba) Eg(2)(Nb) A1g(2)(Nb) Eg(3)(O) Eg(4)(O) A1g(3)(O) Eg(5)(O) A1g(4)(O)
a
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DAMa
DAM: defect activated mode.
layers. As x increases, the FWHMs of Ba-related modes (Eg (1) (Ba) ∼ 102 cm−1, A1g(1) (Ba) ∼ 105 cm−1) and Nb-related modes (Eg (2) (Nb) ∼ 172 cm−1, A1g(2) (Nb) ∼ 263 cm−1) decrease with the increase of ordering degree and Qf values. Since the 1:2 ordering structural evolution happens along the c-axis of ordered structure, oxygen vibration along c axis (Eg(4) (O) mode ∼ 378 cm−1) is favored, while the vibration in ab plane (Eg(3) (O) mode ∼ 294 cm−1) weakens. The lifetime of the oxygen-octahedron stretch A1g(4) (O) mode (∼788 cm−1) is supposed not only influenced by the ordering domain size, but also the micro-defects such as ordering domain boundaries. The present work not only establishes the correlations among the Raman vibration modes, ordering structures and dielectric properties, but also provides an in-depth understanding of Raman spectroscopy for 1:2 ordered Ba(B′1/3B″2/3)O3 perovskites. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing InterestCOI The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Science Foundation of 6
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