Effect of Nd³⁺ substitution for Bi³⁺ on the dielectric properties and conduction behavior of Aurivillius NdBi₄Ti₃FeO₁₅ ceramics

Abstract

NdBi₄Ti₃FeO₁₅ and Bi₅Ti₃FeO₁₅ ceramics were prepared by solid state reaction. The effect of Nd³⁺ on dielectric and electrical properties was investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies. The combination of impedance and electric modulus analysis enabled the distinguishing of two relaxation behaviors that were assigned to originate from grains and grain boundaries. Kinetic analysis of temperature-dependent dielectric data was performed to probe the defect-related conduction. The results demonstrated that the Nd³⁺ substitution for Bi³⁺ significantly suppressed the conductivity and the ferroelectric transition temperature was decreased to ∼250 °C in NdBi₄Ti₃FeO₁₅.

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1. Introduction

Bismuth layer-structured ferroelectrics known as Aurivillius phases are formulated as (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻, where n denotes the number of perovskite unit cell layers sandwiched by two (Bi₂O₂)²⁺ layers.^1,2 These Aurivillius phases have been extensively investigated for their applications in non-volatile random access memories^3,4 and piezoelectric devices,^5,6 for their fatigue-free behaviours, large spontaneous polarization and high Curie temperature (T_c) etc.^3,7,8 Recently, iron-containing Aurivillius compounds, (BiFeO₃)_m–Bi₄Ti₃O₁₂ are of great interest due to their potential multiferroicity in a single-phase.^9,10 These Aurivillius compounds could be useful multifunctional materials but their high leakage current put limitation on their usage for functional devices.^11,12 This is a common problem in Bi-based materials, especially for those containing transition metal ions with variable valences. It was reported that Bi-based and iron-contained ceramics usually suffered from high leakage current and low electrical resistivity owing to the effect of oxygen vacancies and ionic valence fluctuation.^13–17 Meanwhile, grains and grain boundaries could provide different contributions to electrical properties and dielectric relaxation of the ceramics.^18–21 The dielectric behaviours and conduction mechanisms of grains and grain boundaries is essentially important, which play a key role in the ceramic materials and directly related with defect behaviours especially oxygen vacancies and valence fluctuation of ions in the Aurivillius compounds.²² The doping of rare earth ions are capable to suppress the leakage current and improve electrical properties of Bi-based materials.^3,23 Huang et al.²⁴ reported that Nd³⁺ dopant improved the multiferroic and electrical properties of BiFeO₃ compound.²⁵ Kim²⁶ reported that the doping of Nd³⁺ can reduce the defect behaviours in Bi₄Ti₃O₁₂ compound by suppressing the bismuth and oxygen vacancies. Meanwhile, the rare-earth doping can greatly affect the ferroelectric distortion of bismuth-contained compounds due to weakening the Bi–O hybridization²⁷ and inducing ionic size effect.²⁸ Bi₅Ti₃FeO₁₅ (BTF) ceramics is a representative of iron-contained Aurivillius compounds,^25,29,30 which has been attracting extensive investigations on structural, magnetic and magnetodielectric behaviours.^31–33 In BTF materials, the Bi volatilization at A-sites³⁴ and disordered arrangements of Fe and Ti at B-sites^35,36 could cause formation of defects, such as oxygen vacancies and ionic valence fluctuation (Fe²⁺–Fe³⁺), resulting in electrical heterogeneities. To investigate the effect of the rare-earth doping on physical properties of Bi₅Ti₃FeO₁₅ compound, we substituted Nd³⁺ for Bi³⁺ in the present report. The dielectric and impedance spectrum analysis was carried out because it is a useful technique to investigate the effect of defect behaviours in the ceramics, distinguish the electrical contributions of grains, grain boundaries and electrode interfaces, and probe the phase transitions.

In this work, we demonstrated the effect of Nd³⁺ on dielectric and electrical properties of Aurivillius Bi₅Ti₃FeO₁₅ ceramics. The contribution of grains and grain boundaries to the electrical properties were evaluated by impedance and electric modulus analyses. Dielectric relaxation processes and conduction mechanisms were discussed in the light of kinetic analyses.

2. Experimental

NdBi₄Ti₃FeO₁₅ (NBTF) and Bi₅Ti₃FeO₁₅ (BTF) ceramics were synthesized by a conventional solid-state reaction route. Stoichiometric proportions of Nd₂O₃, Bi₂O₃, TiO₂, and Fe₂O₃ (purity > 99.9%) powders were mixed thoroughly in ethanol medium. The mixed powder was initially pre-calcined at 780 °C for 3 h in air. The calcined powder was pressed into pellets and sintered at 1100 °C for 2 h. X-ray diffraction (XRD, Cu-Kα, X'pert Pro, PANalytical B. V., Almelo, Netherlands) was used to identify the phase constituent of the sintered samples. The sintering process was repeated until the XRD patterns did not change between two sequential sintering cycles. The sintered pellets were polished to make their faces parallel and smooth, and coated with silver paint in order to carry out the electrical measurements. The coated pellets were heated at 620 °C for 15 minutes. Temperature dependent dielectric and impedance responses were measured in a temperature range from room temperature to 600 °C using Agilent 4294A impedance analyzer (40 Hz to 10 MHz) with a variable-temperature sample stage.

3. Results and discussion

3.1. Phase identification and dielectric characterization

The XRD pattern of BTF and NBTF ceramics at room temperature is shown in Fig. 1. All the diffraction peaks of the samples can be indexed in an orthorhombic structure (JCPDS no. 38-1257, for Bi₅Ti₃FeO₁₅), indicating the formation of 4-layered Aurivillius single-phase samples. The lattice parameters are calculated as a = 5.4329(1) Å, b = 5.4290(1) Å, c = 41.221(1) Å for NBTF, and a = 5.4676(1) Å, b = 5.4393(1) Å, c = 41.216(1) Å for BTF. It is observed that the peak position of NBTF sample slightly shifted to higher angle compared to that of BTF sample, which could be ascribed to the smaller ionic radius of Nd³⁺ (0.111 nm, CN = 8) than that of Bi³⁺ (0.117 nm, CN = 8). Armstrong and Newnham indicated that the (Bi₂O₂)²⁺ layer and the pseudo-perovskite units in the bismuth titanate are under tensile and compressive strain^28,37 because the (Bi₂O₂)²⁺ layers has an ideal a-parameter of ∼3.80 Å smaller than the perovskite units [Bi₂Ti₃O₁₀]. To derive the lattice a-parameter for BTF and NBTF ceramics the empirical formula can be used,

a = 1.33r_B + 0.60r_A + 2.36 Å

where r_B and r_A are the six-coordinate cation radius and eight-coordination cation radius, respectively taken from Shannon.³⁸ The calculated idealized a-parameter for an ideal [B₂A₃O₁₀] unit for BTF is 3.8965 and that for NBTF is to be 3.8785 Å. This observation indicates that Nd³⁺ may have replace Bi³⁺ in the perovskite units for NdBi₄Ti₃FeO₁₅ ceramics.^19,39 Therefore, the smaller ionic radii of doped Nd³⁺ will release a stress between the perovskite unit and the (Bi²O²)²⁺ layer, as a result structural distortion will be decreased of the Aurivillius structure. This can be observed from the characteristic doublet (200/020) of the orthorhombic symmetry of the Aurivillius structures as shown in the inset of Fig. 1. For BTF the two peaks (200) and (020) are distinctly split while for NBTF the splitting of the peaks are very slight which indicating that the structural distortion of NBTF is weakened compared to that of BTF. Consequently the substitution of Nd³⁺ for Bi³⁺ will decrease of structural distortion, which will reduce the unit cell volume, especially in the a–b plane and a small change in interlayer spacing (along c-axis) as observed in NBTF ceramics.

Fig. 1 XRD pattern of NBTF and BTF ceramics at room temperature. The inset shows the enlarged diffractions profile of (200) and (020) peaks of BTF and NBTF ceramics.

Fig. 2 shows the temperature dependence of real (ε′) and (ε′′) imaginary parts of complex permittivity of BTF and NBTF ceramics at various frequencies, respectively. With decreasing frequency and increasing temperature, notable increase in values of ε′ and ε′′ is observed, suggesting a strong dielectric dispersion due to the presence of thermally activated charges i.e. space charges, charged defects and related defects complexes.⁴⁰ It is observed that the dielectric dispersion of BTF sample is larger than that of NBTF sample as shown in Fig. 2. An additional dielectric peak in NBTF sample with respect to BTF sample is observed at ∼250 °C, which might correspond to a ferroelectric phase transition (details for identification of the ferroelectric transition are discussed in the section of ac conductivity). Li et al.⁴¹ studied the dielectric properties of BTF ceramics for phase transition and reported the T_c to be occurred at ∼748 °C,⁴² which is much higher than that of NBTF. While it could be reasonable considering the fact that the substitution of Nd³⁺ for Bi³⁺ in Bi₄Ti₃O₁₂ can weaken the structural distortion. This observation is consistent with that of ref. 43.

Fig. 2 Temperature dependent of real ε′ and imaginary ε′′ parts for BTF (a, b) and NBTF (c, d) at various frequencies.

Fig. 3(a)–(d) shows frequency (f) dependence of ε′ and ε′′ parts of complex permittivity of BTF and NBTF ceramics at various temperatures, respectively. The frequency dependence of ε′ shows two different regions, i.e. a strong dielectric dispersion at lower frequency and a plateau region at higher frequency. While the plateau region for BTF sample occurs at higher frequency than that for NBTF sample as shown in Fig. 3(a) and (c). The sharp decrease in ε′ at low frequency range is mainly due to the space charges, which contributes to the significant frequency dispersion and high permittivity values at low frequency.⁴⁴ The log(ε′′) − log(f) plots (Fig. 3(b) and (d)) show high values of ε′′ for BTF sample compared to NBTF sample. At lower frequency and higher temperature, there is a linear regime in the logε′′ − log(f) plots with slope ∼−1 for both the samples, indicating the dominance of dc (direct current) conduction at high temperature and low frequency in the ceramics. In the log(ε′′) − log(f) plots, broad dielectric peaks are observed, which shift to higher frequency with increasing temperature, indicating a typical characteristic of dielectric relaxations in the ceramics.

Fig. 3 Frequency dependent of real ε′ and imaginary ε′′ parts of (a, b) BTF and (c, d) NBTF ceramics at various temperatures.

3.2. Relaxation behavior and electrical properties of grains and grain boundaries

To get a clear understanding of the dielectric relaxation process, impedance and electric modulus analyses were carried out. The complex impedance −Z′′ vs. Z′ plots at various frequencies of BTF and NBTF samples are shown in Fig. 4(a) and (b). At low temperatures, the nearly-straight lines with large slope suggesting high insulating nature of the ceramics in both the samples. With increasing temperature, the curves steadily bend toward the Z′-axis to form distorted semicircular arcs. It is noted that the BTF sample impedance curves band toward the real Z-axis at more low temperature than NBTF ceramics. With further increasing temperature the radii of the corresponding semicircular arcs of both the samples decreases, signifying smaller values of resistance at higher temperatures. The Nyquist plots of BTF sample shows that the radii of semicircular arcs approaches to the origin at more lower temperature than that of NBTF sample, indicating that the NBTF sample is more resistive than BTF sample. The Bode-phase diagram in the inset of Fig. 4(a) and (b) shows a step-like change of phase angle vs. frequency as marked by arrow. It indicates that the distorted semicircular arcs may consist of multi-relaxations.

Fig. 4 (a) Nyquist diagram of BTF ceramics and (b) Nyquist diagram of NBTF ceramics at various temperatures. The inset shows Bode-phase diagram at various temperatures. (c) Nyquist diagram of NBTF ceramics at 500 °C. Inset shows the equivalent circuit and the corresponding parameters of elements. The solid line represents the fitted curves.

A simulation of complex impedance data is performed with an equivalent electric circuit to identify the existence of multi-relaxations in the NBTF ceramics. The equivalent circuit containing a series combination of parallel (CR) and (QR) elements, which gives a good fitting as shown in the inset of Fig. 4(c). R is the resistance, C is the capacitance and Q is the phase constant element (CPE). CPE is the modified capacitance of the NBTF ceramics having impedance Z^*_Q = [A(jω)ⁿ]⁻¹, where, A is pre-exponential factor and n (1 > n > 0) determines the departure from ideal Debye behaviour. Q behaves as perfect resistor for n = 0 and as ideal capacitor for n = 1. The simulation clearly shows existence of two relaxations in the NBTF ceramics, which could rise from two microstructural components. Here, the relaxation at lower frequency (Lf-Rel) corresponding to much larger resistance is attributed to the grain boundary contribution and the other at higher frequency (Hf-Rel) is to the grain contribution. The impedance data reveals that the distorted semicircular arcs consist of at least two relaxations. As known, complex impedance plots give emphasis on the largest resistance of the microstructural components, whereas the electric modulus spectra stress the smallest capacitance and can suppress the contribution of electrode polarization effects.^18,45 To get the complete understanding of the multi-relaxation behaviours only impedance analysis is insufficient, the combination of electric modulus and impedance analyses is of great importance to resolve multi-relaxation behaviours in the electroactive ceramic.

Fig. 5 shows frequency dependence of electric modulus M′′(f) at various temperatures. The complex electric modulus (M*) was derived from complex impedance (Z*) data in the form M* = jωC_oZ*, where ω = 2πf is the angular frequency and C_o is the vacuum capacitance of the measuring plates. With increasing temperature the peaks shift to higher frequency. The M′′(f) plots are well fitted with two Gaussian peaks, conforming the existence of two relaxation behaviours in the NBTF ceramics. The relaxation frequencies (f^r) of two relaxations at various temperatures are obtained and the relation ln(f^r) − 1/T is plotted as shown in the inset of Fig. 5. The relaxation activation energies (E^r_a) are obtained by the Arrhenius relation, (f^r = f₀ exp(−E^r_a/K_BT)), where f₀ is the pre-exponential factor. The calculated E^r_a is 1.10 eV for Hf-Rel, and 1.23 eV for Lf-Rel.

Fig. 5 Frequency dependence of modulus M′′ at various temperatures of NBTF ceramics. The solid lines are the fitted curves to Gaussian model. The inset shows the variation of relaxation frequency with temperature for relaxation 1 and relaxation 2.

Fig. 6 shows the plots of Z′′/Z′′_max and M′′/M′′_max as a function of frequency at 400 °C. The impedance Z′′/Z′′_max plot shows single relaxation peak corresponding to the low frequency peak of the two modulus M′′/M′′_max peaks. This is because the large difference of resistance values for the two microstructural components which results in one prominent peak of Z′′/Z′′_max emphasizing the more resistive grain boundary over grains.

Fig. 6 Variation of Z′′/Z′′_max and M′′/M′′_max with frequency of NBTF ceramics at 400 °C.

3.3. Electrical properties and kinetic analysis of relaxation–conduction behaviours

To investigate the correlation of relaxation–conduction behaviours and the effect of Nd³⁺ substitution for Bi³⁺ in BTF ceramics, ac conductivities (σ_ac) of BTF and NBTF samples are calculated with the relation, σ_ac(ω) = ωε_oε′′, where ω is angular frequency and ε_o is permittivity of free space. Fig. 7 shows the frequency dependent σ_ac at various temperatures. σ_ac is observed to increase with increasing temperature, but significantly decreased with Nd³⁺ substitution for Bi³⁺. A frequency independent plateau (region I) at lower frequency and higher temperature is observed in the log(σ_ac) − log(f) plots, which indicates the dominance of dc conduction, in consistence with the linear regime (slope ∼−1) of log(ε′′) vs. log(f) plots in Fig. 3(b and d). Plateau regions occur at higher temperatures in NBTF sample than those in BTF sample. Apparently, the substitution of Nd³⁺ for Bi³⁺ depresses the conductivity of the present material. It is known that at high temperatures charged defects such as ionized oxygen vacancies normally contribute to the conduction and relaxation behaviours in the oxide ceramics.^46,47 Bismuth volatilization at high sintering temperature (1100 °C) will inevitably result in the formation of oxygen vacancies in materials, which would play an important role in dielectric behaviours.^48,49 Nd³⁺ substitution for Bi³⁺ can suppress the volatilization of bismuth and decrease the oxygen vacancy concentration in ceramics, which may be the reason causing the significant decrease of conductivity in NBTF sample compared to BTF sample as shown in Fig. 7(a) and (b). The inset of Fig. 7 shows the plot ln(σ_dc) vs. 10³/T, where σ_dc is the dc conductivity obtained by extrapolating the frequency independent σ_ac to f = 0. The conduction activation energies (E^dc_a) is calculated by using the Arrhenius relation and the obtained values are 0.85 eV and 1.23 eV for BTF and NBTF sample, respectively. Comparing with the temperature dependent behaviour of relaxations in the inset of Fig. 5, the value of activation energy of conduction is similar to those of relaxations in NBTF, especially for that of grain boundaries, indicating the same origin of conduction and relaxation. In addition, the similar values of E^dc_a and E^r_a of grain boundaries manifest the dominant contribution of more resistive grain boundaries to conduction in NBTF ceramics. It was reported that the energies for diffusion of doubly ionized oxygen vacancies in SrBi₂Ta₂O₉ compound was ∼0.98 eV.^50,51 In various perovskite oxides, the conduction and dielectric relaxation at high temperature were ascribed to long/localized migration of doubly-ionized oxygen vacancies, and the activation energies were reported in an approximate range 0.87–1.4 eV.^50,52 In the present work, the activation energies for relaxation and conduction at high temperatures are in good agreement with those reported. Therefore, the conduction could be attributed to the long range movement of doubly-ionized oxygen vacancies,⁵³ and the relaxation is due to the short range hopping of doubly-ionized oxygen vacancies.^13,54

Fig. 7 Frequency dependence of σ_ac of (a) BTF and (b) NBTF ceramics at different temperatures. Inset shows ln(σ_dc) as a function of 1/T.

It is observed that the E^dc_a of the BTF sample is lower than NBTF sample. Li et al.⁵⁵ had studied the collective movement of oxygen vacancies and suggested that activation energy decreased with the increase of oxygen vacancies. In fact, the conduction activation energy of charged defects in a solid material should results from two different contributions, one is the diffusion energy of defects and the other is the formation energy of corresponding vacancies. A material containing low concentration of vacancies must show higher value of conduction activation energy than that with high concentration of vacancies. Hence, in our experimental results the increase of E^con_a in NBTF sample can be regarded as direct proof of the restraint of formation of oxygen vacancies by the substitution of Nd³⁺ for Bi³⁺.

Fig. 8 shows temperature dependence of ac conductivity at 100 Hz, which is explained by the Arrhenius law, σ = σ₀ exp(−E_A/K_BT). It is observed that conductivities of both the samples increase with increasing temperature, while show two temperature regions. At high temperature the E^ac_a are calculated to be ∼0.84 eV and ∼1.16 eV for BTF and NBTF samples, respectively. At low temperature the E^ac_a are ∼0.21 eV and ∼0.20 eV for BTF and NBTF samples, respectively. The ac conductivity at high temperature exhibits the similar activation energies with that of dc conductivity as shown in insets of Fig. 7. An additional peak is observed at ∼250 °C in the NBTF sample with respect to the BTF sample in the log(σ_ac) − 1/T plot of Fig. 8. It is believed to correspond to a ferroelectric transition.^56–58 In ferroelectric materials, free electrons are generally trapped at the domain boundaries to screen the spontaneous polarization. Upon ferroelectric transition, the trapped electrons will be released to increase the conductivity.⁵⁹

Fig. 8 Arrhenius plot of ac conductivity of the BTF and NBTF ceramics at 100 Hz.

In low temperature region, E^ac_a of BTF and NBTF samples show much less values than those at high temperature. Apparently the conduction mechanism is changed at low temperature with respect to the high temperature region. Generally, the conduction at low temperature is believed to cause from the electron hopping between defects.^40,50,59,60 At low temperature two kinds of electrons would strongly influence the conductivity of the materials besides other defect behaviours. One of the conduction electrons originates from the ionization of oxygen vacancies and the other from the valence fluctuations between Fe ions i.e.,

Fe³⁺ + e′ ⇔ Fe²⁺

The electron hopping under alternative field acting as the reorientation of dipoles would result in the relaxation behaviour at low temperature when the hopping cannot follow the alternative field.^60,61

4. Conclusions

Single-phase NdBi₄Ti₃FeO₁₅ and Bi₅Ti₄FeO₁₅ ceramics were synthesized by solid state reaction route and the effect of Nd³⁺ on dielectric and electrical properties was studied in a wide range of frequency and temperature. Two dielectric relaxation behaviours were observed in the present ceramics. We distinguished the contribution of grains and grain boundaries by combining impedance and electric modulus analyses. Electrical properties of grains and grain boundaries were evaluated and the resistance of grain boundaries was found much larger than grains. The ferroelectric phase transition was observed to take place at ∼250 °C. At high temperatures the conduction and relaxation mechanisms were ascribed to the movements of ionized oxygen vacancies in the present material. This work demonstrated the substitution of Nd³⁺ for Bi³⁺ effectively restrained the occurrence of defects, consequently improved the dielectric properties of the NBTF ceramics.

Acknowledgements

This work is supported by National Natural Science Foundation of China (No. 51372024, and 51072225).

Notes and references

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