Dielectric characteristics of Fe-doped LaTiO_3+δ and visible light modulation

Abstract

Fe-doped La_1−xFe_xTiO_3+δ (x = 0.05, 0.1, and 0.3, 0.4) ceramic samples are prepared via a traditional solid-state reaction route. The low temperature (77 K ≤ T ≤ 360 K) dielectric properties of the samples have been systematically investigated in the frequency range from 100 Hz to 1 MHz. It is clearly observed that extraordinarily high low-frequency dielectric constants appear at room temperature in La_0.6Fe_0.4TiO_3+δ, which is ∼1000 times larger than that of La_0.95Fe_0.05TiO_3+δ. Interestingly, the dielectric characteristics of the samples have obvious light dependence at room temperature within the measured frequency range. The results have demonstrated that visible light improves the dielectric properties of the ceramics by means of I–V and complex impedance analysis.

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Introduction

Transition-metal perovskite oxides have long attracted the interest of theoretical and experimental researchers, because they exhibit various exotic properties, such as ferroelectrics, ferromagnetism, superconductivity, and colossal magnetoresistance effects.^1–4 Lanthanum titanate is an important rare-earth transition-metal perovskite compound. Known as a layered perovskite rare-earth titanate compound with many physical phenomena, LaTiO_3+δ (LTO) has attracted considerable attention recently both in science and engineering.^5–9 In recent years, the LTO system has been studied extensively in various fields, such as piezoelectrics, electro-optics, and photocatalysis.^10–13

Perovskite materials are doped on the A- and B-sites with other cations in order to tailor the material properties. Lanthanum titanate oxide materials offer a large range of electronic properties depending on the doped materials and concentration.^14–19 Several authors have investigated the A-site doped LaTiO₃ with different stoichiometric compositions. Vilquin et al. investigated the electronic properties of Sr doping LaTiO₃ thin films in the range between 0 and 50% Sr-doping contents, and revealed that the electron carrier density gradually decreased as increasing Sr-doping level and the Sr-doped films showed metallic conduction suffering electron–electron scattering.¹⁵ Wang et al. reported a novel glucose electrochemical sensor based on perovskite LaTiO₃–Ag0.2.²⁰ Bradha et al. studied the conductivity properties of Sc-doped LaTiO₃, and observed that different Sc-doped contents resulting in different properties.¹⁴ In our earlier work, we reported the dielectric constants of Cu-doped ceramic composites La_1−xCu_xTiO_3+δ, which improved the dielectric constants remarkably with the doped Cu concentrations.¹⁷

Fe is an important doping element commonly used to improve the performance of materials.^21–26 Coey et al. confirmed that Fe-doped SnO₂ being a transparent ferromagnet with an exceptionally large net moment per order ferric ion.²² Yu et al. prepared Fe-doped TiO₂ nanorods by an impregnating–calcination method, and found that Fe-doping greatly enhance the visible-light photocatalytic activity of mesoporous TiO₂ nanorods.²³ Karmakar et al. have investigated the nature of magnetism in the Fe-doped ZnO system, both experimentally and theoretically.²⁵ Staebler studied the multiple storage and erasure of fixed holograms in Fe-doped LiNbO₃.²⁷ Heikman investigated the Fe-doped semi-insulating GaN by metalorganic chemical vapour deposition, he found that the Fe concentration in the film showed a linear dependence on the precursor partial pressure, and it was insensitive to growth temperature, pressure, and ammonial partial pressure. He also observed the memory effects.²⁸

The band gap energy of LaTiO_3+δ was ∼3.8 eV, which made it impossible to absorb any visible light (>400 nm).²¹ But the band gap energy of the Fe-doped LaTiO_3+δ was ∼2.6 eV, which indicated that the Fe-doped LaTiO_3+δ compound has an advantage of absorbing visible light, since the band gap energy is smaller than 3.0 eV (λ > 400 nm).^29–34 Kim et al. studied the photocatalytic hydrogen production of Fe-doped La₂Ti₂O₇ under visible light irradiation.³⁵ The dielectric properties of the Fe-doped LaTiO_3+δ compound may be changed under visible light irradiation owing to the excited electron–hole pairs. Up to now, the dielectric properties of Fe-doped LaTiO_3+δ have been rarely reported. In this paper, we performed detailed investigations on the low-frequency (10² to 10⁶ Hz) dielectric properties of A-site Fe-doped LTO under (and out of) visible light irradiation.

Experimental

The ceramic La_1−xFe_xTiO_3+δ (x = 0.05, 0.1, 0.3, 0.4) samples were prepared by traditional solid state reaction process using high purity (99.99%) starting powders of La₂O₃, Fe₂O₃, and TiO₂. First, the stoichiometric amounts of starting powders were thoroughly mixed and grinding using an agate mortar for about 1 hour. And then they were fired at 1250 °C in air for 10 hours. The grinding and sintering processes repeated for 3 times. After that the resultant material was reground, and then pressed into pellets of 8.0 mm in diameter and 1.5–2.0 mm in thickness under a pressure of 10 MPa. Finally, the pellets were sintered at 1300 °C in air for another 10 hours, and then cooled down to room temperature with furnace.

The crystal structure, chemical compositions, and chemical states of the as-prepared pellets were characterized by X-ray diffraction (XRD) at room temperature on D/max2500 (Rigaku, Japan) with Cu-Kα radiation and X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Scientific). The microstructures of the bulks were examined using scanning electron microscope (SEM, model: S-4800, Hitachi Co., Tokyo, Japan). For dielectric characterization, pellets were printed with silver paste on both sides. The temperature dependent dielectric properties (capacitance and dielectric loss), the frequency dependent dielectric properties, and complex impedance were obtained with a precision impedance analyzer 6500B (Wayne Kerr corp.). Data were obtained in the frequency range from 100 Hz to 1 MHz and the temperature range from 77 K to 360 K.^36–41 In order to make the light to the ceramic bulk, thin Au electrodes were deposited on the bulks by magnetron sputtering through a metal mask, which diameter is 6 mm. The current–voltage (I–V) characteristics were measured by applying a pulse dc voltage across the pellets with an interval of 5 s between the two sequent pulses. To investigate the photo-response of the La_1−xFe_xTiO_3+δ samples in different resistance states, an ordinary low voltage halogen lamp (12 V, 100 W) with the wavelength of about 400–780 nm was used as the light source.^42,43

Results and discussion

Sample characterization

Fig. 1 displays X-ray diffraction patterns of as-prepared samples at room temperature. La–Fe–Ti–O bulks exhibit well-developed crystallization, and all of the four Fe-doped samples are nearly single phases which are almost the same as the parent LaTiO_3+δ sample. However, the intensities of the peaks change gradually with the increase of concentration of doped iron. It is clearly seen that the (002) peak intensities of samples become weak gently, but (021) peak intensities of samples become strong bit by bit with the increase of concentration of doped iron. The ions radius of La³⁺ is 115 pm,⁴⁴ and the ions radius of Fe³⁺ is 64 pm.⁴⁵ Due to different ion radius, when the Fe³⁺ replacement La³⁺, the lattice structure of ABO₃ can be distortion by size mismatch of the A-site cations, which may be the cause of peak intensity variations.

Fig. 1 The XRD patterns for La_1−xFe_xTiO_3+δ (x = 0, 0.05, 0.1, 0.3, 0.4) at room temperature.

The X-ray photoelectron spectroscope (XPS) analysis is also performed to further explore the surface chemical compositions of the Fe-doped LaTiO_3+δ samples. High-resolution XPS spectra are shown in Fig. 2. Fig. 2(a)–(d) represent La 3d state, Fe 2p state, Ti 2p state and O 1s state of the perovskites, respectively. The core and the satellite peaks of La 3d_5/2 and La 3d_3/2 show a clear doublet structure specific for oxides (with peak around ∼834.21 eV for La 3d_5/2 and ∼851.11 eV for La 3d_3/2). The satellite peaks are around ∼838.34 eV and ∼855.29 eV. The energy difference between core and satellite peaks is about 4 eV. The presence of satellite peak La 3d is due to the monopole excitation arising from a change in the screening of the valence electrons upon the removal of a core electron. From the data it is clear that the valence states of La ions correspond to La³⁺.⁴⁶ Similar phenomenon was reported earlier by Miao el al for La³⁺.⁴⁷

Fig. 2 The XPS spectra of the surface of sintered pellets La_1−xFe_xTiO_3+δ (x = 0, 0.05, 0.1, 0.3, 0.4), depicting the La 3d, Fe 2p, Ti 2p, and O 1s peaks.

As shown in Fig. 2(b), it is very easy to find that the peaks at 711.2 eV and 725.1 eV of the oxidized Fe³⁺ species respectively, which are assigned to the Fe 2p_3/2 and Fe 2p_1/2 orbits. Yamashita et al. reported the similar peaks of oxidized Fe³⁺.⁴⁸

Fig. 2(c) shows Ti 2p and valence photoemission spectra. The Ti 2p spectrum displays on state with a Ti 2p_3/2 binding energy of 457.74 eV, which is assigned to Ti³⁺ (about 457.5 eV (ref. 49)). The Ti⁴⁺ sate would otherwise have appeared at 1.5 eV higher binding energy, i.e., at about 485.9 eV.⁴⁹

The peak of Ti²⁺ could not be found in XPS spectra. We don't find peaks of TiO₂ and Fe₂O₃ in XRD spectra too. As shown in Fig. 2(c), the valence states of Ti ions remain unchanged. Therefore, we believe that the Fe³⁺ irons replacement La³⁺ ions, which should belong to A-site doping route. Fig. 2(d) shows the O 1s spectrum. Its binding energy is about ∼529.24 eV.

The typical scanning electron micrographs are illustrated in Fig. 3. The ceramic samples are granular- or square-shaped grains, and several micrometers in size. It is obvious that the grains size becomes bigger with the increase of concentration of doped iron. It can be seen that the grains of La_0.6Fe_0.4TiO_3+δ are the biggest one among the four samples. Although these samples are prepared using the same pressure and sintering time, it is clearly observed that the compactness of grains gets better and holes gradually disappear with the increase of iron concentration. Because of lower melting point of alloy, the doping in LaTiO₃ can obviously decrease the sintering temperature of the ceramic, so the decrease of the sintering temperature may lead to the growing of grain size with increasing doped Fe.

Fig. 3 The typical scanning electron micrographs of the La_1−xFe_xTiO_3+δ (x = 0.05, 0.1, 0.3, 0.4) samples.

Dielectric properties

Dielectric constant is characteristic of the material insulating capability. The temperature dependence of the dielectric permittivity ε* = ε′ − iε′′ at various frequencies is shown in Fig. 4 for the as-prepared Fe-doped bulks (in which, ε* is the complex dielectric, ε′ is the real part of the complex dielectric, ε′′ is the imaginary part of the ε*). Fig. 4(a), (c), (e) and (g) show the temperature dependence of dielectric constant ε′ for the four Fe-doped bulks. It can be seen that all the dielectric constants ε′ increase monotonously with the increase of concentration of doped iron. It is clearly observed that extraordinarily high low-frequency dielectric constants (nearly 10⁵) appear at room temperature in La_0.6Fe_0.4TiO_3+δ sample, while that of La_0.95Fe_0.05TiO_3+δ, La_0.9Fe_0.1TiO_3+δ, La_0.7Fe_0.3TiO_3+δ samples are no more than 10², 10³, 10⁴, respectively.

Fig. 4 The temperature dependence of the dielectric constant ε′ ((a), (c), (e), and (g)) and the dielectric loss tangent tan δ ((b), (d), (f) and (h)) of the La_0.95Fe_0.05TiO_3+δ, La_0.9Fe_0.1TiO_3+δ, La_0.7Fe_0.3TiO_3+δ, and La_0.6Fe_0.4TiO_3+δ samples, the temperature range is 77–360 K.

As shown in Fig. 4(a) and (c), the low doped content samples become frequency and temperature independent at temperature lower than ∼270 K. But which increases monotonously with temperature and shows obviously temperature dependence when the temperature is above 270 K. The phenomenon may be caused by the thermal exciting more electron–hole pairs when the temperature over 270 K. It can be seen that the dielectric constant ε′ of La_0.7Fe_0.3TiO_3+δ sample obviously increases with temperature when the temperature lower than 270 K. From Fig. 4(g), we can easily find that the dielectric constant ε′ of La_0.6Fe_0.4TiO_3+δ sample rapidly increases in all measured frequency range, which shows distinguished temperature and frequency dependence. At lower temperature (around 150 K), there is a polaronic relaxation originated from Fe ions,³⁸ which becomes more obviously and stronger with Fe concentration increasing. The above results reveal that the Fe-doped concentration can significantly affect the dielectric properties.

Fig. 4(b), (d), (f) and (h) show the temperature dependence of the dielectric loss tangent tan δ (tan δ = ε′′/ε′). For the low doped samples, obvious plateau exists in the curves at temperature lower than 270 K, which correspond to the sharp changes in ε′ mentioned above. In order to further reveal the low temperature dielectric varieties of the samples, we enlarge the dielectric loss measured data at the temperature lower than 300 K, which shown as insets in Fig. 4(b), (d) and (f). From the insets, we can find some dielectric loss peaks. As the measured frequency increasing, the loss peak positions move to high temperatures, which show a thermally excited relaxation process.^39–41 Because of Fe doping, the conductivity and polarization of the samples may change to some degree, which jointly determine the dielectric loss.^21,33 As a result, the dielectric losses for all samples are quite different.

Fig. 5 shows the log–log plots of ε′ and ε′′ versus frequency of the four Fe-doped samples sequentially at fixed temperature. As shown in Fig. 5(a), (c), (e) and (g), we can divide the curves into two groups according to the measured temperature. Most of the curves drop nonlinearly with the increasing frequency when the temperature above 270 K. The dielectric constants are strongly dependent on the temperature and the frequency, which is a clear signature of dielectric properties being dictated by the regions of different conductivities. But the curves become frequency and temperature independent at temperature lower than ∼270 K for low doped concentrations bulks. For all samples, the ε′ decrease more quickly at lower frequencies, especially in the higher temperature. While for the heavily Fe-doped samples, it is clearly observed that extraordinarily high low-frequency dielectric constants appear at room temperature in La_0.6Fe_0.4TiO_3+δ (ε′ > 10⁵), which is ∼1000 times larger than that of La_0.95Fe_0.05TiO_3+δ (ε′ ≈ 100). The heavy doped bulk (La_0.6Fe_0.4TiO_3+δ) shows different properties with the other bulks. In the high temperature (300–360 K), ε′ shows two stepwise decreases occurred at lower- and high-frequency range, between which there are plateaus. In the temperature range of 300–360 K, ε′ seems to behave as a dielectric plateau independent of frequency and temperature.^22–27

Fig. 5 The frequency dependence of the dielectric constant ε′ ((a), (c), (e) and (g)) and the imaginary dielectric constant ε′′ ((b), (d), (f) and (h)) of the La_0.95Fe_0.05TiO_3+δ, La_0.9Fe_0.1TiO_3+δ, La_0.7Fe_0.3TiO_3+δ, and La_0.6Fe_0.4TiO_3+δsamples, the frequency range is 10² Hz to 10⁶ Hz.

Fig. 5(b), (d), (f) and (h) show the ε′′, which is the imaginary part of the ε*, of the as-prepared samples. Obviously, the higher temperature curves demonstrate that most of the experimental data fall on a straight line in the frequency range covered for the low doped concentrations samples, indicating that the conductivity has overwhelming contribution to energy dissipation. However, the several data points of ε′ deviate from the straight line at lower temperature and higher frequency, which shows that localized polarization charge carriers have contribution to ε′′ except for the conductivity.^4,5,50 It is well known that the condensation of the polarized clusters are more easily emerges at low temperature relatively and their domain walls cause an obvious delay of the response to the external alternating field at higher frequency.^51,52 Shown as Fig. 4(f), it can be seen that the high temperature curves decreased rapidly with the increasing frequencies when the temperature above 300 K, but the low temperature curves show the opposite characteristics, which decrease slowly in the low frequency region, but change rapidly in the high frequency region when the temperature lower than 180 K. Between 210 K and 270 K, the curves remains unchanged, showing a strong independent frequency. The above facts indicate that there is one thermally activated dielectric relaxation in the samples.

These ceramics in all measured frequencies show a wide dielectric dissipation peak. As shown in Fig. 4(b), (d), (f) and (h), they express Debye-like relaxation peaks that shift to higher temperature with the increase of measured frequency, which indicates that the dielectric properties mainly originate from the thermally activated relaxation process.^17,50–52 The dielectric properties of the relaxation follows the Arrhenius law:

f = f₀ exp(−E_a/K_BT) (1)

where f is the measuring frequency, f₀ is the pre-exponential factor, K_B is the Boltzmann constant, and T is the temperature where the maximum loss tangent occurs, E_a is the activation energy. Fig. 6 displays the Arrhenius plots of the measured frequency (log f is the logarithm of the measurement frequency) versus 1/T (The T is defined as the temperature where d(tan δ)/dT = 0), as indicated by the solid lines. The values of activation energy are shown in Table 1. These experimental results imply that the nature of charge carries is responsible for dielectric relaxation peaks and dc conduction which belongs to same category.

Fig. 6 The Arrhenius plots (a)–(d) of the relaxation time as a function of inverse temperature from tan δ maximum for the polycrystalline La_0.95Fe_0.05TiO_3+δ, La_0.9Fe_0.1TiO_3+δ, La_0.7Fe_0.3TiO_3+δ, and La_0.6Fe_0.4TiO_3+δ samples.

Table 1 The values of activation energy of La_1−xFe_xTiO_3+δ

	x = 0.05	x = 0.1	x = 0.3	x = 0.4
E (eV)	0.62	0.79	0.94	0.89

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Visible light modulate dielectric properties

The band gap energy of LaTiO_3+δ is ∼3.8 eV, which makes it impossible to absorb any visible light (λ > 400 nm). Therefore, the band gap energy of the Fe-doped LaTiO_3+δ is ∼2.6 eV, which indicates that the Fe-doped LaTiO_3+δ compound has the advantage of absorbing visible light, since the band gap energy is smaller than 3.0 eV.²¹ Despite the structure and role of dopants have not been well understood, the dielectric properties of the Fe-doped LaTiO_3+δ compound can be improved under visible light irradiation owing to light excited electron–hole pairs.^31–33

Fig. 7 shows room temperature I–V characteristics of the Fe-doped samples under dark and visible light condition. It is obvious that the current absolute value in visible light is bigger than those in dark for all samples under same voltage. As shown in Fig. 7(a), for La_0.6Fe_0.4TiO_3+δ, the high doped concentration sample, the maximum current absolute value is ∼0.6 mA under visible light, which increases by 50% than that in dark. From Fig. 7(b), we can easily find that the maximum current absolute value is about 16 nA, which increases by 50% than that in dark also. The above results indicate that the visible light can significantly increase the current value under the condition of constant voltage. Compare Fig. 7(a) with Fig. 7(b), the Fe-doped concentration shows a significantly impact on the I–V properties, the current value of the La_0.6Fe_0.4TiO_3+δ is more than 10⁴ times than that of the La_0.7Fe_0.3TiO_3+δ.

Fig. 7 The I–V characteristics of the Fe-doped samples under visible light conditions, (a) La_0.6Fe_0.4TiO_3+δ and (b) La_0.7Fe_0.3TiO_3+δ. The inset in (a) is a schematic of visible light on samples.

In order to have a further study on these problems, the visible light dependent on the dielectric constants ε′ (La_0.7Fe_0.3TiO_3+δ and La_0.6Fe_0.4TiO_3+δ) in room temperature is shown in Fig. 8(a) and (b). And the visible light dependent on the dielectric loss tangent tan δ is shown in Fig. 8(c) and (d). From Fig. 8(a) and (b), it is very easily for us to conclude that the dielectric constant ε′ is bigger under the conditions of the visible light than that in dark at all measured frequency. As shown in Fig. 8(c) and (d), it is obvious that the visible light can affect the dielectric loss tangent tan δ too. For the two samples, the dielectric loss tangent tan δ increases significantly in the low frequency region, which indicates that the visible light activates the carriers. As shown in Fig. 8(d), the dielectric loss tangent tan δ moves to high frequency under visible light, which indicates that the visible light has the same effect as the increased temperature. The results should be attributed to photo-induced carriers under visible light irradiation.^21,23

Fig. 8 The dielectric constant ε′ ((a) and (b)) and the dielectric loss tangent tan δ ((c) and (d)) as a function of visible light for the La_0.7Fe_0.3TiO_3+δ and La_0.6Fe_0.4TiO_3+δ samples, the frequency range is 10² to 10⁶ Hz.

The impedance properties have been used to investigate the origin of conducting mechanism and to find out the electronic, ionic or even mixed electronic–ionic conductivity of the doped ceramics. As shown in Fig. 7, the visible light shows the obvious regulation on the I–V characteristics, which is bound to affect the impedance of the samples. This will inevitably lead to impedance variety of the bulks. The above discussion suggests that the dielectric behavior of Fe-doped LaTiO_3+δ ceramics is associated with the carrier polarization. We now turn our attention to the impedance dependent of the Fe-doped sample under visible light.

The complex impedance Z* associated with single R–C cell is given by Z* = Z′ + jZ′′, where Z′ is the real part, and Z′′ is the imaginary part of Z*. The insets of Fig. 9 are the complex impedance plots (Z′′ vs. Z′) for the La_0.6Fe_0.4TiO_3+δ ceramic at room temperature. The cole–cole curve usually appears in the form of succession of semi-circles represents the various conductions. Two semicircular arcs are observed in all the insets. It is well known that when the carriers hop to the vicinity of blocking grain boundaries or electrodes to form space charges, the relaxation of the space charges will result in apparent giant dielectric constant due to the internal barrier-layer capacitor mechanism. The presence of two semicircular arcs in impedance patterns in Fig. 9 is located in the low- and high-frequency ranges, which theoretically considered to represent the dielectric contributions from the grain boundaries (interfaces) and bulk (grain interior), respectively.^50–52 What displayed in the main panel of Fig. 9 is an alternative presentation of Z′ vs. Z′′/f at room temperature, where two well defined regions is divided by f = 22 kHz in the measured frequency range in dark. But the frequency dividing point is f = 37 kHz in light, which is significantly higher than the value in dark. From the insets of Fig. 9, we can easily find that both Z′ and Z′′ value of the ceramic in dark bigger than that in visible light. The peak height of the complex impedance spectra in visible light is lower than that in dark. These results indicate that Fe-doped ceramics have light dependent at room temperature. Hwang²¹ reported that the most significant feature in the electronic band structure of the Fe-doped LaTiO_3+δ was the formation of a partially filled 3d band in the band gap of LaTiO_3+δ, while the contribution of these dopants on the valence band was negligible. Excitation of electrons from this localized interband to the conduction band of LaTiO_3+δ was responsible for visible light absorption. The above results suggest that the impedance characteristics of the Fe-doped LaTiO_3+δ ceramics are light dependent.

Fig. 9 Representation of Z′ vs. Z′′/f at room temperature for La_0.6Fe_0.4TiO_3+δ as a function of visible light. The insets display the corresponding impedance spectrum. (a) In dark, and (b) in visible light.

Conclusions

The low frequency dielectric properties of Fe-doped LaTiO_3+δ ceramics have been investigated in detail. In the as-prepared samples, the dielectric constants of samples can be improved with increasing Fe content. It is reliable that visible light can improve the dielectric properties of the ceramics. Complex impedance analysis reveals that electrical processes of ceramic can be attributed to the combined actions of bulk properties and grain boundary effects. The visible light response properties make La_1−xFe_xTiO_3+δ ceramics a versatile candidate for constructing multifunctional devices.

Acknowledgements

We acknowledge the financial support from National Natural Science Foundation of China (no. 51331002 & 51571006).

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