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Vol. 8. Issue 6.
Pages 5396-5404 (November - December 2019)
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Vol. 8. Issue 6.
Pages 5396-5404 (November - December 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.09.006
Open Access
Enhancement in dielectric and optical properties of La1-xCexFeO3 nanoparticles
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M.M. Armana,
Corresponding author
mmarmsci@gmail.com

Corresponding author.
, S.I. El-Dekb
a Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt
b Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, (PSAS), Beni-Suef University, Beni-Suef, Egypt
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Tables (3)
Table 1. The values of the lattice parameters a, b, c, the unit cell volume, the theoretical density (Dx), the crystallite size and the tolerance factor (t) for the investigated samples.
Table 2. The activation energies EI and EII of the investigated samples.
Table 3. The values of optical energy band gap for the investigated samples.
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Abstract

La1-xCexFeO3; 0×0.15 nanoparticles were prepared in single phase using flash combustion technique. The crystalline structure and morphology of the investigated samples were researched through XRD and HRTEM. As a result, the dielectric constant of LaFeO3 was enhanced by Ce3+ ion substitution due to some the Ce3+ ions changes its valence to Ce4+ ions. The conduction mechanism in La1-xCexFeO3 samples is the small polaron (SP) tunneling. The type of optical transition is direct allowed transition and the band gap values are in the range of (2.8–1.89eV). ε/ values of La0.90Ce0.10FeO3 decreases by increasing the external applied pressure at different frequencies.

Keywords:
Orthoferrite
LaFeO3
Dielectric
Optical properties
Perovskite
Nanoparticles
Full Text
1Introduction

The general formula of perovskite oxides is ABO3, where A is rare earth, alkaline, or alkaline-earth cations and B transition metal cations. ABO3 have been attracting increasing attention in the last years due to a huge importance and applications [1,2]. The most important feature of ABO3 is the extraordinary structural stability where a wide range of cations can be replaced on the A and/or B sites without destroying the matrix structure. This benefit allows by a great flexibility to tailoring their physicochemical properties to better target their applications [3].

There are different preparation methods for perovskite-type oxides, such as the polymerization, thermoelectric conversion, citrate method, sol-gel, flash, heteronuclear complex, precipitation method [4–6]. However, the predominant method for synthesis of perovskites is the solid state reaction between precursor metal oxides and/or carbonates. Additionally, electrospinning plays a role in obtaining nano-materials in fiber form as simple, convenient and low cost technique [7]. A great number of nanomaterials could be prepared using flash combustion technique which give fast results and small size nanoparticles [8].

LaFeO3 crystallized in distorted orthorhombic perovskite structure with centro-symmetric space group and exhibit colossal dielectric constant. It has anti-ferromagnetic character with Neel temperature 467°C. As LaFeO3 belongs to centrosymmetric group, ferroelectricity may be outlawed in this material. But, improper ferroelectricity was reported with the Curie temperature greater than 202°C [9]. Some published articles also reported the multiferroicity in LaFeO3[10,11]. Lanthanum orthoferrite (LaFeO3) has a huge importance due to its various technological applications including gas sensors, catalysts and electrode materials for solid oxide fuel cell [12–14]. The electronic and magnetic properties of LaFeO3 are enhanced either by changing oxygen partial pressure or by substitution with foreign elements at the A or B lattice sites.

The band gap, (Eg) is the separation between the energy of the lowest conduction band and that of the highest valence band [15]. The band gap value is always calculated from UV–vis absorption spectroscopy. According to the UV spectra analysis, the type of transition band considered, directly or indirectly, can be determined also. UV–vis diffuse reflectance spectroscopy is used also to determine the type of electronic transitions between the different orbitals in the solid.

LaFeO3 and their substituted nanoparticles have attracted great attention in studying their characteristics and physical properties. The role of vacancy on A cation was previously studied [16]. We reported an enhancement in the molar magnetic susceptibility, saturation magnetization, exchange bias and ferroelectric properties [16]. On the other hand, the effect of vacancies on Fe cation was investigated resulting in that χM of the sample LaFe0.990.01O was increased by 2.5 times than that of LaFeO3[17]. The ac conductivity for the sample La0.95Sb0.05FeO3 increases 12.5 time than that of the parent LaFeO3 at T=553K and frequency 1MHz [18]. For the first time, La0.95Sb0.05FeO3 is concluded to be a novel single phase multiferroic material. In this work we focus on tuning in the electrical and optical properties of LaFeO3 by substitution Ce3+ ions at different doping levels.

2Experimental techniques

The samples La1-xCexFeO3; 0x0.15 were prepared in single phase using flash combustion technique. The precursor metal nitrates with purity 99.9% (Sigma–Aldrich) were used without any further purification and then good mixed in stoichiometric ratios using glycine as a fuel. The (glycine/nitrate) ratio is kept constant for all samples and is equal to 1. The mixture was heated on the magnetic stirrer until a highly viscous liquid was formed. Upon further heating, the viscous liquid transferred to a fluffy powder of nanoparticles. The obtained samples were heated at 500°C for 2h using heating/cooling rate of 4°C/min. Fig. 1 shows a flowchart of the preparation of the investigated samples.

Fig. 1.

Flowchart of the preparation of the La1-xCexFeO3.

(0.12MB).

The crystalline phases were studied by the X-ray powder diffraction (XRD) using a Proker D8 advance X-ray diffractometer with CuKα radiation (λ=1.5418Å). The XRD of the investigated samples were indexed according to the International Centre for Diffraction Data (ICDD) card number 74-2203. The high-resolution transmission electron microscope (HRTEM) model (JEOl-2100) was used to analyze the particle shape and distribution.

The powder of the samples was pressed to form pellets and were heated at 500°C for 2h by rate 4°C/min. After that, the two surfaces were coated by silver paste and checked for good conduction. The electrical properties measurements were studied at different frequencies using LCR meter (Hioki model 3532 Japan). Dc conductivity and I–V characteristic data were measured in the temperature range 300–750K by using Picoammeter Keithley 485. The optical properties were analyzed by transmittance spectra (UV–Visible-NIR spectrometer).

Ac conductivity and dielectric constant were measured for the investigated samples under the effect of external pressure. A homemade holder was connected to the LCR meter (Hioki model 3532 Japan) to study the dependence of ε/ and σ on the applied pressure at constant frequency 10kHz by the two probe method.

3Results and discussion3.1Structure and microstructure

Fig. 2 shows the XRD pattern of the samples La1-xCexFeO3 where 0.0×0.15. The observed peaks are compared and indexed with ICDD card number 74-2203. The XRD pattern confirmed that the samples were obtained in single phase without any impurity peaks. The XRD pattern demonstrates that the investigated sample possesses the orthorhombic structure with Pnma space group. The lattice parameter values were calculated using Debye-Scherrerformula reported in Eq. (1)[19]

L = (0.9 λ)/(β cosθ)
Where L is the average crystallite size, λ is the wave length of the x-ray radiation, θ is the Bragg angle and β is the corrected full width in radians subtended by the corrected half maximum intensity of the powder pattern peak. Table 1 represented the values of lattice parameters, unit cell volume and crystallite size. The values in Table 1 indicate that the samples were prepared in nano scale. The crystallite size decreases by increasing Ce3+ ions as a result of replacing La3+ large ionic radius (1.216 Å) ion by the smaller Ce3+one (1.196 Å)

Fig. 2.

XRD patterns of La1-xCexFeO3 where x=0.0, 0.05, 0.10, 0.15.

(0.12MB).
Table 1.

The values of the lattice parameters a, b, c, the unit cell volume, the theoretical density (Dx), the crystallite size and the tolerance factor (t) for the investigated samples.

Samples  a(Å)  b(Å)  c(Å)  V(Å)3  Dx(g/cm3Crystallite size (nm)  Particle size (nm)  Tolerance factor (t) 
LaFeO3  5.5736  5.4896  7.9863  244.355  6.5977  35  27  0.9549 
La0.95Ce0.05FeO3  5.5552  5.5331  7.7992  239.728  6.7267  25  25  0.9546 
La0.90Ce0.10FeO3  5.6117  5.4025  8.1735  247.798  6.5093  21  33  0.9542 
La0.85Ce0.15FeO3  5.5530  5.5501  7.8225  241.087  6.6921  21  24  0.9538 

The tolerance factor (t) was calculated from the following Goldsmith formula [20]

where rA, rFe and rO are the ionic radii of the A, iron cations and the oxygen anion respectively. The ionic radius of A site cation (rA) was calculated according to the relation rA=(1-x) rLa + (x) rCe where x=0.00, 0.05, 0.10 and 0.15. The values of (t) reported in the Table 1 are in the range 0.9549<t<0.9538. They assure that the investigated samples have orthorhombic structure and confirm the results obtained from XRD. By increasing the Ce content, the tolerance factor decreases which leads to increasing the distortion in the crystal structure thereby, decreasing the angle between FeOFe and increasing the tilting of<FeO6> octahedron.

The HRTEM was used to photograph the investigated samples La1-xCexFeO3. The particle sizes (L) of the samples were measured from HRTEM images and reported in Table 1. The values of (L) illustrate that the samples were prepared in nano size scale and agree with the results obtained from XRD. The particles seemed to have geometric platelet shape with slight agglomeration due to the absence of capping agent. The size is found to depend on the Ce content. The inset in the Fig. 3 is selected area electron diffraction (SAED) pattern for the samples La1-xCexFeO3. The SAED is seen as clear diffraction concentric rings indicating that all samples are formed in the polycrystalline form with excellent crystallinity despite the reduced size of the crystallites

Fig. 3.

HRTEM of samples La1-xCexFeO3, x=0.00, 0.05, 0.10, 0.15.

(0.61MB).
3.2Dielectric constant and loss factor

Fig. 4 shows the relation between dielectric constant with the temperature at different frequencies for the sample La0.90Ce0.10FeO3. The values of ε/ in the first temperature region (300–650K) are nearly frequency independent. While the former increases with rising temperature where the thermal energy causes a transition at about 750K. In the second temperature range (750–870K), by increasing the temperature the electric dipoles are aligned in the direction of applied field leads to enhancement of the polarization and increasing the values of ε/. The investigated samples are characterized by large values of dielectric constant and can be used in many applications such that multilayer capacitor, memory devices, resonators, low magnetic field sensors [21].

Fig. 4.

Dependence of the real part of dielectric constant (ε/) on absolute temperature T (K) of the sample La0.90Ce0.10FeO3 as a function of frequency.

(0.08MB).

The dependence of the dielectric loss factor (ε//) on the temperature as a function of frequencies for the sample La0.90Ce0.10FeO3 is shown in Fig. 5. The trend of ε// is the same as of ε/ as the general trend of dielectrics.

Fig. 5.

Dependence of dielectric loss factor (ε//) on absolute temperature T(K) of the sample La0.90Ce0.10FeO3 as a function of frequency.

(0.08MB).

The large values of dielectric constant (ε/) in the low frequency region are instigated from grain boundaries, interfacial polarization and oxygen vacancies. While at higher frequencies, the values of (ε/) nearly remain constant, this was in fact due to that the dipoles do not have enough time to follow up the applied ac electric field variations. Agreeing with the well-known Maxwell-Wagner model [22], the dielectric material structure is regarded as well conducting grains separated by non-conducting thin grain boundaries. As a consequence, the space charge polarization is built at the grain boundaries. By applying an external voltage on the sample, it drops on grain boundaries. Conferring Koop’s [22] model, at low frequencies the influence of grain boundaries is leading where higher dielectric constant is owing to ultra-thin layer of grain boundaries. Hence at low frequencies, the space charges trail the frequency of the applied electric field while at high frequencies there is no sufficient time to build up.

The dielectric constant is using the complex function as: ε* (ω) = ε/ (ω) – iε// (ω). The real part of dielectric constant ε/ (ω) represents the elastic reaction of the material to an external applied ac electric field. While the imaginary part ε// (ω) is related to the conductivity according to the relation σ(ω) = εoωε// = εoωε/tanδ [23].

The imaginary part of dielectric constant ε// (ω) increases slowly with increasing the temperature and then rapidly from 700K due to the improvement in the conductivity [23].

Fig. 6 represents the temperature dependence of ε/ at 100kHz for the investigated samples. By doping the LaFeO3, ε/ is growing to be greater than that of the parent sample. The enhancement in the values of dielectric constant ε/ is a result of the valence exchange of Ce3+ ions into Ce4+. This in turns increases the polarization in the perovskite orthorhombic lattice. We could not also neglect the change of Fe3+ ions to Fe2+ ions to allow charge neutrality in the investigated samples. Further increasing Ce3+, ε/ decreases.

Fig. 6.

Dependence of ε/ on the absolute temperature at 100kHz for the samples La1-xCexFeO3, (0.0x0.15 in step of 0.05).

(0.18MB).
3.3AC and dc conductivity measurements

Fig. 7 correlates lnσ (σ: conductivity) with the reciprocal of absolute temperature (1000/T) at frequency 10kHz for the sample LaFeO3. The conductivity increased with increasing the temperature exhibiting a semiconducting like behavior. The data were found to follow the known Arrhenius relation [24]:

σ=σoexp (-E/kT)
where E is the activation energy, T is the absolute temperature and k is the Boltzmann’s constant. The activation energies values were calculated and reported in Table 2. The activation energy EΠ at high temperature region is greater than that at low temperature (EІ). The values of the former assure the semiconducting trend of the multiferroics. By increasing the concentration of Ce3+ ions, the values of the activation energies decrease as a result of the valence exchange of both Fe3+ and Ce3+ ions which increases the conductivity of the multiferroic samples, thereby decreasing the activation energies.

Fig. 7.

The dependence of Lnσ on the reciprocal of the absolute temperature at frequency 10kHz of the sample LaFeO3.

(0.07MB).
Table 2.

The activation energies EI and EII of the investigated samples.

Samples  EI (eV)  EII (eV) 
LaFeO3  0.697  2.329 
La0.95Ce0.05FeO3  0.604  1.473 
La0.80Ce0.10FeO3  0.558  1.451 
La0.85Ce0.15FeO3  0.546  0.971 

The ac conductivity obeyed the power law: σac(ω)=A ωS[25] where σac is the ac conductivity, A is a temperature dependent constant, ω=2πf is the angular frequency and S is the frequency dependent exponent with values 0S1. Fig. 8 shows the dependence of Lnσ on y-axis and Lnω on x-axis at different selected temperatures. The slope of the straight lines represents the exponent S. Fig. 9 shows the dependence of the later on absolute temperature. This plot indicated the conduction mechanisms in the sample. The values of S increased with absolute temperature pointing to the small polaron (SP) tunneling conduction mechanism [25].

Fig. 8.

The dependence of Lnσ on Lnɷ for the sample La0.90Ce0.10FeO3 as a function of the absolute temperature.

(0.09MB).
Fig. 9.

Dependence of the exponent S on absolute temperature (T).

(0.05MB).

Fig. 10 shows the I–V characteristic plot for the investigated samples. The relation between I–V plot is an ohmic relation where the electric current intensity increases as the potential difference increases. From this plot, we considered 100V as constant value during dc conductivity-temperature measurement.

Fig. 10.

I–V characteristic curve of dc conductivity of the samples La1-xCexFeO3.

(0.1MB).

Fig. 11 shows the relation between dc conductivity and absolute temperature for the investigated samples. The values of σdc are nearly constant in the first temperature region up to 500K and then increases rapidly. The inset of Fig. 11 elucidates the values of σdc up to T =500K. There is a peak for the sample La0.95Ce0.05FeO3. At high temperature region, the thermal energy increases the distance between the molecules and makes the molecules and ions more flexible to be aligned in the direction of applied field.

Fig. 11.

The relation between dc conductivity and absolute temperature.

(0.15MB).
3.4Optical band gap studies

The determination of energy gap (Eg) can be carried out using different methods but in our work, we focused on Kubelka–Munk (K–M or F (R)) method [26]. The equation of K–M method is:

Where F (R) is proportional to the extinction coefficient (α) and R is the reflectance. This method is applied to a materials characterized by high light scattering and absorbing particles in a matrix.

Equation (5) represent a modified Kubelka–Munk function obtained by multiplying F (R) function by the energy of photon (hυ), using the coefficient (n) which is related to an electronic transition.

(F (R)* hυ)n

For the Eg calculation and determination the type of transition in the investigated samples, the following well-known Tauc’s equation [27] was used:

α (hυ) ≈ B (hυ - Eg)n
where (α) is the extinction coefficient, which is proportional to F(R), h is the Planck’s constant (J.s), υ is the light frequency (s−1), B is the absorption constant, Eg is the band gap (eV). The value of n for the specific transition can be experimentally determined from the best linear fit as the following: n=12 for direct allowed transition, n=32 for a direct forbidden transition, n=2 for an indirect allowed transition and n=3 for an indirect forbidden transition.

Fig. 12 shows the Tauc plot for the investigated samples. The optical band gap of the samples La1-xCexFeO3, was estimated from the extrapolation of F(R) hν=0. The type of transition is direct allowed transition where n=½. The values of band gap were reported in Table 3.

Fig. 12.

Tauc plot for the investigated samples.

(0.08MB).
Table 3.

The values of optical energy band gap for the investigated samples.

The samples  Eg (eV) 
LaFeO3  2.54 
La0.95Ce0.05FeO3  2.80 
La0.90Ce0.10FeO3  2.54 
La0.85Ce0.15FeO3  1.89 

It is clear that the values of Eg decreases from 2.54eV for the parent sample LaFeO3 to 1.89eV for the sample La0.85Ce0.15FeO3. The reasons for the enhancement of optical properties are:

  • 1

    The release of excess charge from transferring some Ce3+ to Ce4+cations which counter by replacing Fe3+ ions to Fe2+ ions to keep a balanced columbic interaction.

  • 2

    Ce3+ ions dopant form a donor level at a lower potential than the top of the valence band composed of O 2p orbitals. Thus, the band-gap energy became narrowed by replacing Ce3+ ions on the Fe3+ site [28].

The decrease in the optical band gap of the investigated samples by Ce3+ doping increases the ability of exploiting these samples in gas sensing and photo catalytic activity as it is compared to the improved TiO2[29].

3.5Effect of pressure on electrical properties

Fig. 13 shows the relation between dielectric constant (ε/) and the external pressure as a function of frequencies at room temperature for the sample La0.90Ce0.10FeO3 as an example. It is clear that the values of ε/ at low frequencies are greater than that at high frequencies due to the electric dipoles need more time to follow the applied electric field. ε/ values decreases by increasing the external applied pressure at different frequencies.

Fig. 13.

Dependence of ε/ on applied pressure as a function of frequency at room temperature for La0.90Ce0.10FeO3.

(0.08MB).

Fig. 14 illustrates the relation between ε/ and external pressure at frequency 10kHz for the Ce3+ doped samples. ε/ increases in the samples containing Ce3+ ions as a result of releasing more ions and charge carriers due to converting Ce3+ ions to Ce4+ ions and some Fe3+ ions converting to Fe2+ ions. The change in valences of the Ce3+ and Fe3+ ions lead to increasing the polarization, dielectric constant as well as the ac conductivity. The enhancement in the ac conductivity with Ce3+ ion doped is illustrated in Fig. 15

Fig. 14.

The relation between ε/ and pressure at frequency 10kHz for the samples.

(0.17MB).
Fig. 15.

The enhancement in the ac conductivity as a function of external pressure at 10kHz of the investigated samples.

(0.09MB).
4Conclusion

  • 1

    XRD confirmed that the samples were prepared in single phase orthorhombic structure.

  • 2

    HRTEM shows that the particle size of the samples are in the range of (24–33) nm.

  • 3

    The dielectric constant and conductivity of LaFeO3 were increased by Ce3+ ions doping

  • 4

    The conduction mechanism in La1-xCexFeO3 samples is the small polaron (SP) tunneling.

  • 5

    La1-xCexFeO3 samples obey optical direct allowed transition and Eg decreases by increasing Ce3+ ions.

  • 6

    ε/ values of La0.90Ce0.10FeO3 decreases by increasing the external applied force at different frequencies.

  • 7

    We recommend the use of the samples of low optical band gap in gas sensing and improved magnetic photocatalysis.

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Journal of Materials Research and Technology

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