D. Triyono*,
S. N. Fitria
and
U. Hanifah
Department of Physics, Faculty of Mathematics and Natural Sciences (FMIPA), Universitas Indonesia, Depok 16424, Indonesia. E-mail: djoko.triyono@ui.ac.id
First published on 13th May 2020
Bulk-phase polycrystalline La1−xBixFeO3 (x = 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by citric sol–gel and sintering methods. The structural, morphological, and electrical properties of the resulting sol–gel solutions were investigated using various techniques. In an X-ray diffraction analysis, all samples crystallized in the orthorhombic structure with the Pbnm space group and showed an increase in lattice constant with increasing Bi content which was also confirmed by vibrational analysis. The sample surfaces and average grain sizes were examined by scanning electron microscopy. The grain distribution was non-uniform and the grain size increased with the increasing Bi content. The complex electrical conductivities and dielectric analyses of these materials were investigated as functions of frequency by impedance spectroscopy at various temperatures (75–200 °C). The frequency-dependent dielectric constant at each temperature increased with increasing Bi content. A Jonscher's power law analysis revealed that the AC and DC conductivities arose by completely different mechanisms. The temperature dependence and dielectric relaxation of the DC conductivity satisfied the Arrhenius law and decreased with increasing Bi content. The activation energy ranged from 0.20 to 0.45 eV and was similar in the conduction and relaxation mechanisms, indicating that both transport mechanisms were based on hopping phenomena. We believe that lowering the activation energy will help with the optimization of constituents as promising candidates in novel materials for future electrocatalysts.
Some studies have been done for Bi substituted in La site of LaFeO3.10–19 Bi-doped was reported causing noticeable improvement on the conductivity and electrochemical performance.10,11 In this, Bi-doped improved the electrochemical performance and reduced the interfacial polarization resistance from 1 to 0.1 Ω cm2 at 700 °C.10 Meng, et al.11 reported that Bi ion plays more importance role on electrochemical properties than microstructure and also can improves conductive properties in LaFeO3-based. Li et al.12 displayed that La1−xBixFeO3 powders (0 ≤ x ≤ 0.2) showed significantly effort to enhance the activity and effective approaching to optimize the crystal structure, enhance the surface oxygen vacancy, change the valence states of ions of the perovskite electrocatalysts, especially in La0.85Bi0.15FeO3 (the lowest ORR inset potential, the largest ORR kinetic current density, the lowest Tafel slope, and the optimal electron-transfer number). Ahmed et al.13 also reported that Bi substitution on La1−xBixFeO3 (0 ≤ x ≤ 0.2) caused the increasing of magnetization with Bi content and showed weak ferromagnetism influenced by both particle size and morphology at room temperature. Rangi et al.14 reported that Bi1−xLaxFeO3 with x = 0.5 showed the best value of magnetization due to mixed structural phase. Rusakov et al.15 reported that replace La3+ with Bi3+ showing better structural stability than modification with another rare-earth element. Chen et al.16 studied that the thermal stability of BiFeO3 was greatly enhanced by substitution with LaFeO3. Khalek et al.17 investigated the structural change from orthorhombic (Pbnm) to rhombohedral (R3c) phases for Bi1−xLaxFeO3 (x = 0.5 and 0.75), which is related to anomalous phenomenon dielectric constant at room temperature in the microwave region exhibited. Some theoretical studies about structural changes of Bi1−xLaxFeO3 with x = 0 to 1 have been investigated. By using first principle calculation, Kaczkowski18 reported the structural transition from rhombohedral to orthorhombic structure of Bi1−xLaxFeO3 was reached at x = 0.32. Another previous study by Arnold19 showed that lanthanum substitution lead the change to orthorhombic symmetry is coupled with a dramatic decrease in the lattice parameter, c. Previous report by Mao et al.20 displayed that single phase Eu and Sr- and Co-doped BiFeO3 is effective to enhance the multiferroics properties and minimize the leakage current for practical applications.
However, to our knowledge, La substitution with such tetravalent cations has not been widely explored. Therefore, a meticulous study on the factors influencing the structural changes and electrical mechanism of Bi-substitution on LaFeO3 is an interesting proposition. In this work, Bi ions were inserted into the La sites of LaFeO3 by a sol–gel method. The structural, morphological, and electrical characteristics of the La1−xBixFeO3 ceramics were investigated. The crystal structure and morphological characteristics were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The frequency and temperature dependences on the dielectric and electrical mechanisms in the La1−xBixFeO3 system were then studied by alternating current (ac) impedance analysis. The results are discussed below.
The lattice structure of the samples was determined by XRD (PANalytical X'pert Pro) with Cu-Kα radiation. The data were collected in the range of 20° to 2θ–90°, with a scanning step of 0.04°. The morphology of the prepared samples was analysed by SEM (QUANTA 650). The surface of all samples was gold-coated prior to surface observation. IS measurements were carried out in the temperature range of 75–200 °C while sweeping the frequency from 100 Hz to 1 MHz. The IS instrument was an RLC meter (FLUKE-PM 6303), and the circuit configuration was a single parallel resistance–capacitance configuration. The vibrational properties of the ceramics were measured using Raman scattering spectroscopy (THERMO SCIENTIFIC: DXR2 Raman Microscope) with a laser excitation wavelength of 532 nm.
The structural changes in term of lattice parameters, bond angle and bond length was obtained from refinement results. We refined the structural parameters of the samples using Fullprof 2k and VESTA software. Fig. 2 shows the Le-bail fitting as Rietveld refinement results of XRD pattern from the prepared La1−xBixFeO3 samples. We determine black dot as the experimental data, red lines as fitting results, and the green line as the difference between the experimental data and calculation. The difference pattern exhibited a good agreement with fairly accepted value χ2 as written in Table 2. The XRD patterns of all samples studied are in good agreement with LaFeO3-parent compound in the standard from JCPDS 96-1526451 data base. All samples exhibited the same crystalline perovskite structure, namely, the orthorhombic lattice belonging to the Pbnm space group, without any secondary phase. There are no additional peaks corresponding to the absence of secondary phase.
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Fig. 2 Rietveld refinement results of XRD patterns of La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5). |
The average crystallite size calculated using Debye–Scherrer formula:13
The final crystallographic parameters are summarised in Table 1. As the Bi content increased, the lattice parameters changed slightly but with a noticeable overall effect on the cell volume. The different ionic radii of the different cations probably contracted the lattice by distorting the FeO6 octahedra.6 However, the changes in the present case were minor because the ionic radii of La3+ (112 pm) and Bi3+ (117 pm) are similar, so the Bi-substitution at La sites only slightly increased the lattice parameters. This inference can be confirmed by observing the tolerance factor (an indicator of structural distortion) and the geometrical parameters.
Parameters | |||||
---|---|---|---|---|---|
a (Å) | b (Å) | c (Å) | Volume (Å3) | Crystallite size (nm) | |
x = 0.1 | 5.5499(8) | 5.5654(2) | 7.8536(8) | 242.58(4) | 256.49(3) |
x = 0.2 | 5.5525(4) | 5.5654(3) | 7.8548(0) | 242.73(1) | 259.02(3) |
x = 0.3 | 5.5516(4) | 5.5709(8) | 7.8587(0) | 243.05(4) | 273.18(8) |
x = 0.4 | 5.5536(8) | 5.5703(2) | 7.8605(4) | 243.17(2) | 396.47(1) |
x = 0.5 | 5.5636(3) | 5.5814(9) | 7.8844(9) | 244.84(0) | 525.61(2) |
The tolerance factor t is calculated as follows:19
Parameters | x = 0.1 | x = 0.2 | x = 0.3 | x = 0.4 | x = 0.5 |
---|---|---|---|---|---|
Atomic position | |||||
La/Bi | |||||
x | 0.9930 | 0.9930 | 0.9930 | 0.98996 | 0.99338 |
y | 0.0297 | 0.0297 | 0.0297 | 0.02491 | 0.02301 |
z | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 |
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Fe | |||||
x | 0 | 0 | 0 | 0 | 0 |
y | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 |
z | 0 | 0 | 0 | 0 | 0 |
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O1 | |||||
x | 0.719 | 0.719 | 0.719 | 0.719 | 0.719 |
y | 0.302 | 0.302 | 0.302 | 0.302 | 0.302 |
z | 0.029 | 0.029 | 0.029 | 0.029 | 0.029 |
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O2 | |||||
x | 0.08 | 0.08 | 0.08 | 0.08 | 0.08 |
y | 0.485 | 0.485 | 0.485 | 0.485 | 0.485 |
z | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 |
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Wickoff position | |||||
La/Bi | 4c | 4c | 4c | 4c | 4c |
Fe | 4b | 4b | 4b | 4b | 4b |
O1 | 4c | 4c | 4c | 4c | 4c |
O2 | 8d | 8d | 8d | 8d | 8d |
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Bond angle (°) | |||||
Fe–O2–Fe | 154.07(6) | 154.08(3) | 154.09(1) | 154.08(8) | 154.12(0) |
Fe–O1–Fe | 157.07(0) | 157.07(0) | 157.07(0) | 157.06(1) | 157.06(1) |
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Bond length (Å) | |||||
La/Bi–O1 (m) | 2.4500(7) | 2.4505(0) | 2.4516(7) | 2.4302(9) | 2.4401(0) |
La/Bi–O1 (l) | 2.6929(3) | 2.6934(5) | 2.6945(6) | 2.7124(3) | 2.7145(2) |
La/Bi–O2 (s) | 2.3828(6) | 2.3839(5) | 2.3835(9) | 2.3986(3) | 2.4078(1) |
La/Bi–O | 2.5086(2) | 2.5093(0) | 2.5099(4) | 2.5137(8) | 2.5208(1) |
Fe–O1 (l) | 2.0866(6) | 2.0869(8) | 2.0882(3) | 2.0883(4) | 2.0924(0) |
Fe–O1 (s) | 1.9231(1) | 1.9237(0) | 1.9241(4) | 1.9245(3) | 1.9281(5) |
Fe–O2 (m) | 2.0147(3) | 2.0150(4) | 2.0159(8) | 2.0165(0) | 2.0224(8) |
Fe–O | 2.0081(6) | 2.0085(7) | 2.0094(5) | 2.0097(9) | 2.0143(4) |
Tolerance factor | 0.8833 | 0.8834 | 0.8837 | 0.8844 | 0.8849 |
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R-factors | |||||
Rp | 5.60 | 5.89 | 6.56 | 8.36 | 6.06 |
Rwp | 7.50 | 7.59 | 8.52 | 12.5 | 8.17 |
Re | 6.10 | 6.57 | 7.40 | 7.36 | 6.76 |
χ2 | 1.51 | 1.33 | 1.33 | 2.89 | 1.46 |
Average tilt angle 〈φ〉 (°) | 14.915(1) | 14.915(2) | 14.912(4) | 14.912(4) | 14.906(1) |
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Fig. 3 (a) Raman scattering spectra of La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5) at room temperature and (b) the fitting results displayed for x = 0.1 using standard Lorentzian profile. |
x = 0.1 | x = 0.2 | x = 0.3 | x = 0.4 | x = 0.5 | Symmetry | |
---|---|---|---|---|---|---|
ω1 | 148.4 | 147.5 | 145.2 | 144.7 | 143.8 | Ag |
ω2 | 171.9 | 171.2 | 169.0 | 170.1 | 169.2 | Ag |
ω3 | 286.1 | 286.4 | 283.6 | 283.0 | 262.1 | Ag |
ω4 | 431.3 | 427.7 | 428.7 | 426.7 | 428.6 | Ag |
ω5 | 619.6 | 619.3 | 619.0 | 619.2 | 618.8 | B1g |
ω6 | 1125 | 1119 | 1115 | 1103 | 1103 | Second-order |
ω7 | 1297 | 1297 | 1283 | 1277 | 1277 | Second-order |
Following previous reports,24,25 the Raman phonon modes below 200 cm−1 are related to La/Bi-vibration with Ag symmetry. The Raman phonon modes between 250–350 cm−1 are associated with the tilting of octahedra corresponding to Ag symmetry. Finally, the Raman phonon modes between 400–500 cm−1 are assigned to the Jahn–Teller distortion with Ag symmetry, whereas the modes between 500–750 cm−1 corresponds to the symmetric stretching of FeO6 octahedra with B1g symmetry. Additional phonon modes between 700–1000 cm−1 are possibly related to Franck–Condon phonon modes.26 Another Raman phonon mode observed above 1000 cm−1 is related to second-order scattering.27
Now, we turn to explain the effect of Bi content on the phonon characteristics, i.e., Raman modes intensity, linewidth, and wavenumber. As shown in Fig. 3(a), the Raman modes intensity is reduced and linewidth becomes more broadens with increasing of Bi content indicating the increase in lattice disorder. In other words, Bi substitution changes the lattice constant, causing the change in Jahn–Teller distortion.28 This is consistent with the observation of an increase in the calculation tolerance factor by X-ray diffraction analysis. Besides, some specific Raman phonon modes are denoted in Fig. 3(b) and summarized in Table 3. The Raman phonon modes tend to shift to the lower wavenumber confirming the lattice constant and bond length increased with increasing Bi content which is consistent with X-ray diffraction analysis (Tables 1 and 2).
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Fig. 4 SEM images of annealed La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5) and the energy dispersive X-ray spectrum of the La0.5Bi0.5FeO3 ceramic. |
The impedance decreased with increasing temperature, suggesting increased movement of the charge carriers involved in the conduction mechanism.29 The increase or decrease of semicircular arcs can anticipate the dominant roles of different conduction mechanisms. At room temperature, the low-frequency semicircular arcs of the La1−xBixFeO3 ceramics were larger than the high-frequency arcs, implying that the grain boundary contribution dominated over the grain contribution. As the temperature increased, the emergence of semicircles in the high-frequency region implied an increasing grain effect; eventually, the grain contribution dominated over the grain boundary contribution.22 Additionally, the impedance decreased with increasing Bi content, indicating that the Bi substituents at La sites played an important role in the electrical conductivity of this La1−xBixFeO3 ceramics system.
Fig. 6 shows the real impedance versus frequency relationships (Bode plots) of the samples prepared at different temperatures. In the low-frequency region, the real impedance was higher at lower temperatures than at higher temperatures, suggesting an increase in the electrical conductivity with increasing temperature. The real impedance also decreased with increasing frequency. The real impedances merged in the high-frequency region, possibly because the barrier reduction at high temperature was compensated by the release of space charges.29,30
Fig. 7 plots the imaginary impedance versus frequency (Bode plots) of the samples prepared at different temperatures. Regardless of temperature, the imaginary impedance increased with frequency up to some maximum and then decreased at higher frequencies. However, the peak frequency increased with temperature and also broadened, indicating a thermal relaxation mechanism.29 This mechanism is plausibly caused by the immobile species at lower temperatures and by defects at higher temperatures.29,31
The shifting peaks toward the high-frequency region indicate a decrease in relaxation time as the temperature increased. The relaxation time τ satisfied the following Arrhenius law:29–31
The activation energy of the relaxation mechanism in each sample is tabulated in Table 4. The activation energy decreased with increasing Bi content, indicating increased hopping of the charge carrier concentration between the neighbouring lattice sites.32
x = 0.1 | x = 0.2 | x = 0.3 | x = 0.4 | x = 0.5 | |
---|---|---|---|---|---|
Ea (eV) | 0.43 | 0.41 | 0.35 | 0.25 | 0.23 |
τ0 (×10−10 s) | 2.36 | 0.13 | 0.36 | 3.40 | 6.22 |
To interpret the dielectric relaxation, the normalized spectra of the imaginary impedance in each sample are presented in the insets of Fig. 7. All normalized spectra were non-overlapping and scaled to the multiple master curve, indicating that the relaxation dynamics were temperature-dependent and localized.33 It was also noticed that all spectra shifted to higher frequencies with increasing temperature.
To determine the contributions of the grains and grain boundaries in the conduction process, Fig. 8 presents Bode plots of the phase angles of the samples prepared at different temperatures. Two peaks, one each in the high-and low-frequency regions, indicate a mixing of the grain and grain boundary contributions in the electrical transport mechanism.34
Fig. 9 plots the dielectric constants of the samples as functions of frequency at various temperatures. The dielectric constant changed with both frequency and temperature. The dielectric constant was relatively high (indicating dispersion) at a lower frequency, and decreased with increasing frequency. The dielectric constant at low frequency also strongly increased with increasing temperature. The dielectric behaviour was dominated by a polarisation process originating from the grain boundary contribution. In the low-frequency region, the charge carriers accumulated in the grain boundary and the hopping process required more energy than at higher frequencies, boosting the dielectric constant.35 Moreover, the dielectric constant was an increasing function of Bi content. Those dielectric constant value significantly increased compared with the pure-LaFeO3 compound.28,36
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Fig. 9 Frequency and temperature dependencies of the dielectric constant of the La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5). |
Fig. 10 shows the frequency dependence of the complex conductivity in the samples prepared at different temperatures. The complex conductivity spectra were divisible into two parts. The first part reflected the frequency-independent behaviour of the conductivity in the low-frequency region, the so-called DC conductivity. The second part was attributed to the AC conductivity, which is an increasing function of frequency. According to Koop's theory, the AC conductivity represents the dispersion in the high-frequency region, which is attributable to the highly conductive grains and the highly resistive grain boundaries.35 The increased grain conductivity at high frequencies is possibly caused by intensified hopping of the charge carrier mechanism. Meanwhile, the increased AC and DC conductivities at higher temperatures might be attributable to the increased tunnelling probability of charge carriers.35,37 Overall, the frequency dependence of the complex conductivity satisfied Jonscher's power law:37
σ(ω) = σDC + Aωs |
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Fig. 10 Frequency and temperature dependencies of the complex conductivities of the La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5). |
Fig. 11 shows the temperature dependences of the DC conductivities of the prepared samples. The DC conductivity increased with increasing temperature, confirming a thermally activated transport process in the conduction mechanism. As the conduction mechanism satisfies the Arrhenius law, the activation energy can be obtained from the Arrhenius equation as follows:34,37,38
The obtained activation energies ranged from 0.20 to 0.35 eV and decreased with increasing Bi content of the ceramic. The Bi substituents increased the lattice volume, so the specimen responsible for conduction were more easily released; that is, required lower energy for mobility. The DC conductivity also increased with increasing Bi content, confirming that the Bi substituents at the La sites strongly affected the conductivity of the La1−xBixFeO3 ceramic oxides. The obtained activation energies were similar to those of a dielectric relaxation analysis, indicating that the electrical transport arose from the hopping mechanism.39 The activation energies of all samples satisfied 0.20 < Ea < 1.0 eV, indicating a conduction mechanism dominated by p-type polaron hopping.6,8 Lowering the activation energy following by increasing in high conductivity makes our as-synthesize La1−xBixFeO3 a promising candidate for catalyst material applying in electrode surface for electrochemical reactions.20 With required lows activation energy as inhibitor, the kinetics energy could be increased, so that improve the large amount of electrochemical reaction occurring in the system. Consequently, this study could have the benefit of understanding and further development in electrochemical applications.
The increased conductivities at higher frequencies are attributable to the higher mobility of charge carriers at higher temperatures than at lower temperatures.35 The carrier mobility μ is calculated as follows:35,40
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Fig. 12 Temperature dependences of the charge carriers mobilities of the La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5). |
Finally, we consider the AC conductivity region. The possible mechanism of the AC conductivity can be deduced from the temperature variation of the frequency exponent s.40 Fig. 13 shows the temperature dependences of the parameter s for all samples. The value decreased with temperature, indicating a conduction mechanism dominated by correlated barrier-hopping.40,41 When s < 1, the hopping process jerks with translational motion, whereas s > 1 suggests a localised hopping that does not leave the neighbourhood.38 The ceramic with x = 0.1 exhibited a localised hopping process throughout the investigated temperature range. In the samples with higher Bi content, both hopping mechanisms were involved; accordingly, the Bi content played an important role in the hopping transformation in the La1−xBixFeO3 ceramics system. Over the investigated temperature range, the dominance of translational motion with a sudden hopping mechanism increased with increasing Bi content.
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Fig. 13 Temperature dependence of the frequency exponent s of the La1−xBixFeO3 ceramics (x = 0.1, 0.2, 0.3, 0.4, and 0.5). |
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