Analysis of the electrical transport, conductivity and dielectric relaxation behavior of La0.75 Ba0.10 Sr0.15 Fe O2.875-δ (δ = 0.375 and 0.50) brownmillerite oxides

The topotactic reduction of La 0.75 Ba 0.10 Sr 0.15 FeO 2.875 with titanium metal leads to a new isostructural material of the composition La 0.75 Ba 0.10 Sr 0.15 FeO 2.875−δ (δ = 0.375 and 0.50). XRD analysis of phases conrms that the obtained compound adopts a brownmillerite-type structure. A slight distorted monoclinic P2/m was found to describe the crystal structure. The transport properties have been investigated by current–voltage (I–V). The electric and dielectric measurements were carried out covering a wide range of temperature (300–600 K). Our materials display semiconducting properties as well as mixed ionic and electronic conductivity. At high temperatures, the activation energy values proved to be around 907 − 630 meV, which refers basically to oxygen vacancies conduction. The analysis of dielectric properties and dielectric losses (ε’, tanδ) of both compounds vs temperature at different frequencies demonstrates two relaxer attitudes. A low dielectric loss and low electrical conductance were displayed. Relying upon these values, these materials stand for perfect candidates for micro-electronics devices.


Introduction
Much ink has been spilled upon oxygen de cient perovskite oxides owing to their multiple merits [1].
Indeed, they exhibit a mixed ionic and electronic conductivity [2]. Besides, these ceramics are promising materials for a wide range of technological applications [3][4][5][6][7]. Moreover, ferroelectrics oxides are gaining much popularity in recent years in view of their high colossal dielectric constant (ε' 10 3 ) [8]. This behavior has potential enhancements in microelectronics devices, such as random access memories and capacitors [9][10][11]. They are basically used in reducing of the size of electronic components such as capacitive components [12].
In ABO 3−δ perovskites, B-site cations conventionally create BO 6 , BO 5 , or BO 4 polyhedral depending on the structure. Their crystal structure depends largely on the arrangement of defects generated from oxygen vacancy creation. These defects can follow a disordered or ordered pattern [13,14] leading to a new material. One of the common structures resulting from oxygen ordering is brownmillerite type structure with A 2 BB ' O 5 formula. In a typical brownmillerite, the octahedral B and tetrahedral B' sites are exhibited in layers parallel to the long axis. The brownmillerite structure can be derived from the cubic phase by removing 1/6 of all oxygen atoms ordered in the pseudocubic direction [110] [5] [15][16][17]. Previous studies reported [18,19] that the brownmillerite structure with composition A 2 B 2 O 5 , remains ambiguous between Pnma, Imma, C2/c and I2mb space group owing to different orientations of tetrahedral chains with respect to each other.
This structural change leads to the enhancement of electric and dielectric properties of these materials presenting ionic and electronic conductivity [2].
Previously, oxygen de cient perovskites could be formed through quenching the perovskite from high temperature to room temperature [20] or using a forming gas of H 2 in Ar [21]. In this work, our central focus is upon a new method to create nontoxic, low costed and simply processed multiferroic samples.
In our previous papers [22], we con ned our study to the oxygen de cient samples La 0.75 Ba 0.10 Sr 0.15 FeO 2.875−δ for de ciency rates less than 0.25. In order to extend this work and build upon it, we attempted to increase the de ciency oxygen concentration so as to investigate their effect on the purity of phases (to examine the degree of non-stoichiometry to which the perovskite structure is presented). In our laboratories and for the rst time, we have successfully elaborated perovskite de cient oxygen samples with a rate higher than 0.25. The basic objective of this research paper lies in investigating the structure as well as electric and dielectric properties of samples.
In order to maintain overall electrical neutrality and owing to the creation of oxygen vacancies, the amounts (2δ) of Fe 3+ are reduced by (2δ) Fe 2+ cations. The developed electronic formula of these materials is expressed in terms of

Experimental Details
Our samples were obtained from an already made perovskite La 0.75 Ba 0.10 Sr 0.15 FeO 2.875 , which was synthesized via sol gel method followed by thermal active process using oxygen acceptor metal (titanium Ti). This procedure was well described in our previous work [22]. Following the same procedure, we prepared the samples with δ = 0.375 and δ = 0.50.
Phase identi cation was undertaken through powder X-ray diffraction (XRD), where data were collected by Burker 8D Advance X-ray powder diffractometer with CuKα radiation (λ = 0.15406 nm) in steps of 0.02° for 10 °≤2θ ≤ 100 °. A structure re nement was carried out using the Rietveld method. The scanning electron microscopy (SEM) observations and compositional analysis were recorded using TESCAN VEGA3 SBH equipped with an energy dispersive X-ray system (EDX). To determine the different vibrational bands, the Fourier transform infrared (FTIR) spectra were speci ed in the wavenumber range 550-1000 cm − 1 , using a Perkin Elmer spectrometer. The spectra were elaborated at room temperature.
The current-voltage (I-V) measurements were set forward using a voltmeter (Agilent 34,401) and a current source (Keithley 220). The impedance measurements were estimated on pellets with a diameter of 5 mm and 1.5 mm of thickness on using an Agilent 4294A Precision Impedance Analyzer and cold plate of a liquid nitrogen cooled cryostat to measure the capacitance and conductance over a wide frequency and temperature range.

X-ray diffraction and morphological characterization
Room-temperature XRD data collected from the as-prepared samples with δ = 0.375 and δ = 0.50 are depicted in Fig. 1(a). The crystallographic study demonstrated that those samples adopt the brownmillerite-type structure with no traces of secondary phase. A structural transition from orthorhombic structure (Pnma) with the splitting of peaks can be observed on X-ray diffraction spectra.
The structural transition appears for δ = 0.375 and δ = 0.5 corresponding to an Fe 2+ concentration between 75 % and 100 %. As the amount of oxygen de ciency increases, the intensities of peaks decrease, as revealed with the most intense peak in Fig. 1(a).
The pro le re nement was carried out using the Fullproof software [23] and the pseudo-Voigt functions were invested for peak pro le tting. The re ned lattice parameters and the discrepancy factors are exhibited in Table. 1. The Rp and R Wp factors displayed much higher values than expected, which is accounted for in terms of the peaks with small intensities [21]. The difference between the tted and experimental intensities of the re ection peaks may be assigned to preferred orientation effects [23,24]. The re ection peaks were crystallized in the monoclinic system with P2/m space group. The average crystallite size was determined by well-known Scherer equation [25] and the results are outlined in Table. 1.
The elemental composition of the compound was measured by Energy dispersive X-ray analysis. EDX spectrum (

FTIR characterization
The crystal structure of the perovskite was identi ed through Fourier transform infrared (FTIR) spectra highlighting its characteristic absorption bands [26]. The vibration recorded at 900 cm − 1 can be attributed to C-H bending vibrations [28].
For both samples, the spectra exhibit two bands characteristics of the metal cations' stretching vibrations inside the tetrahedral and octahedral sites. The FTIR spectra of the above mentioned samples display two main absorption bands corresponding to the vibrations of tetrahedral and octahedral site complexes [29]. The stretching vibration of the tetrahedral site lies in the wave-number range 550-800 cm − 1 and the low frequency band below 550 cm − 1 is assigned to the stretching vibrations of octahedral complexes.
Referring to pervious state of art works, the origin of the splitting of the main absorption band is not well processed. However, this can be inferred through the complex vibrations of the tetrahedral complexes which are highly correlated to the octahedral site complexes.

Current-voltage analysis
Before discussing dielectric properties, the current-voltage characteristic (I-V) was undertaken in order to test the contact quality. Figure 4 portrays (I-V) and (J-V) (where J is the current density) variations for both structures recorded under different temperatures. The I-V curves exhibit two symmetric fragments of lines. This behavior con rms the presence of no depletion layer [34]. In other words, the electrons can drift between the two electrodes without being reduced by depletion layer. As a matter of fact, we infer that the contact is of ohmic type. This attitude was also con rmed by the linear variation of double log-log I-V variation. The log-log plots of I vs V is tted with the relation I α V n , where n is found to be equal to 1 (ohmic relation between current-voltage).

Electrical conductance analysis
Most of the brownmillerite compounds [2], where the total conductivity includes the contribution of electronic and ionic conductivity, stand for mixed conductors. Our compounds possess the contribution of both electron and oxide-ion conductivity in view of the presence of oxygen vacancies and mixed valence states of iron ions (Fe 2+ /Fe 3+ ) [35].
The electrical conductance investigation over a wide temperature (300-620 K) and frequency (40-10 6 Hz) ranging for δ = 0.375 and 0.50 samples are illustrated in Fig. 5(a). This entity is indicated by the relation [36] where σ is the conductivity, A is cross sectional area of material and l is its length.
The conduction is ensured by the hopping of electron from Fe 3+ to Fe 2+ ions and the presence of oxygen vacancy [35]. The increase in temperature enhances the hopping phenomenon resulting in an increase in the conduction process and therefore leading to the conductance increase [37].
The frequency dependence of conductance is examined by Jonscher's law [38, 39] G = G dc + G ac = G dc + Aω S Eq. 3 Referring to the expression stated above, G dc and G ac represent the conductance in continuous and alternative current, respectively. A is a pre-exponential factor dependent on temperature, ω is the angular frequency and s is the frequency exponent (0 ≤ s ≤ 1).
We can infer from the conductance measurements that for each temperature, the spectrum can be divided into two parts. A Plateau appears at low frequencies (frequency independent conductance (40 Hz to 10 3 Hz)) attributed to dc-conductance bulk. The low-frequency plateau gets larger proportionally to the increase of temperature. The second part corresponds to the higher frequency dispersion region. This part corresponds to ac-conductance [40].
The experimental conductance data were tted by Eq. 3 for the investigated samples. A typical example of this tting at T = 500 K is illustrated in Fig. 5(b) (red solid line). A close agreement is traced between the experimental data and the tted one. As noticed, the conductance decreases with the increase in δ rate. Departing from the electronic neutrality of δ = 0.375 (25% Fe 3+ , 75% Fe 3+ ) and δ = 0.50 (100% Fe 2+ ) compounds, we detect that the oxygen de ciency reduces the Fe 2+ /Fe 3+ ratio. Thus, the number of hopping sites decreases. The presence of oxygen vacancy is responsible for the excess of Fe 2+ ions over Fe 3+ ions.
DC conductance ndings resulting from conductance measurements at low frequencies are portrayed in Fig. 6(a) for both studied compounds. The activation energy values E a corresponding to G dc were estimated from the Arrhenius law as described by the following expression [41] where A 0 is the pre-exponential factor, E a is the activation energy and K B is the Boltzmann constant.
In view of the creation of oxygen defects (vacancies), a wide variation of conductivity is noticed. In addition to the three straight lines observed in the whole explored temperature range 300-620 K, we detect an interior region representing the plateau (as portrayed in Fig. 6(b)). Each one corresponds to a particular E a which rises up proportionally to the corresponding temperature. The estimated values of E a were highlighted in Table. 2. The determined values are very close to those previously obtained in oxygen vacancy ferroelectrics compounds [42].
The activation energy proves to be around 908 meV − 630 meV at high temperature (region VI). This nding can be assigned to the bigger charge than that of an electron. In other words, oxygen vacancies motion is the main responsible factor for conduction at high temperature [43,44]. Relying upon other reports [45], the E a of oxygen vacancy is around 1 eV. For region (II), the E a attitude refers primarily to octahedral distortion obtained after oxygen vacancy creation [42]. The activation energy of lowtemperature region (I) is attributed to the hopping process.
In region (III), we notice a metallic behavior con rming the existence of two charge carriers. Indeed, as temperature increases, the free electrons density increases as well. This enhances collision frequency between charge carriers and induces metallic behavior. At higher temperature, ion conduction appears and becomes predominant.
The creation of oxygen vacancies gives rise to additional properties of the compounds.

Permittivity analysis
The variation of dielectric (Fig. 7) constant with frequency demonstrates a normal behavior of polar dielectric materials [46] with giant constant (in the range of 10 4 at low frequency for δ = 0.375 sample) and shows that it is temperature dependent. This colossal dielectric constant is mainly assigned to interfacial polarization ameliorated after oxygen vacancy creation [11]. The value of ε' decreases when frequency increases until it slowly reaches a constant value at higher frequencies. The observed behavior can be explained by Maxwell-Wagner polarization [47,48]. It is noticeable that the magnitude of ε' Moreover this decrease can be explained by an increase in FeO 6 titling due to the insertion of oxygen vacancy [11].
The thermal variation of dielectric properties are examined at different frequencies (10 3 , 10 4 , 10 5 and 10 6 Hz) in Fig. 8. Two dielectric relaxations are observed. These dielectric peaks are seen around 500 K and 620 K for all frequencies. This critical temperature is known as transition temperature. The charge carriers hopping between Fe 3+ and Fe 2+ ions play the main role behind the dielectric relaxation at low temperature. Furthermore, the low temperature peak may refer to the transition from ferroelectric to antiferroelectric state.
However, at high temperature, the dielectric relaxation is associated with oxygen vacancy and the peak corresponds to the transition from anti-ferroelectric state to paraelectric state.
The sample displays a peaking behavior at a speci c temperature, owing to the jumping frequency of an electron between Fe 3+ and Fe 2+ , which becomes equal to the applied eld frequency at this speci c temperature. In fact, when both frequencies are the same, the polarization increases gradually until reaching its maximum. Then, the applied frequency keeps increasing while the other one can't maintain this rise. This leads eventually to polarization decreasing. Thus, the dielectric constant decreases. Yet, this evolution can be assigned to intrinsic effects [11]. The oxygen vacancy creation increases the coulombic repulsive force between iron ions. When samples are exposed to an ac electric eld, the iron ions can be displaced around without distorting the octahedral FeO 6 . From this perspective, this effect proves to be the cause of the increase in the dielectric constant proportional to the temperature.
The loss factor variation (Fig. 9) indicates a very similar trend as the dielectric constant exhibits two relaxer behaviors at low and high temperature. As reported in Fig. 9, the oxygen vacancy decreases the dielectric loss leading to a better use in future applications [8]. The decrease of dielectric loss with oxygen vacancy concentration is associated with the decrease of electrical conductance.

Conclusion
This research paper presents an overview of structural, electrical and dielectric investigation of oxygen de cient perovskites. Samples with chemical compositions La 0.75 Ba 0.10 Sr 0.15 FeO 2.875−δ (δ = 0.375 and 0.50) con rm the brownmillerite-type structure. The compounds crystallize in the monoclinic system with P2/m space group. Conductance plots reveal the electrical properties as a function of temperature (300-600 K) and frequency 40-10 6 Hz. A semiconducting behavior is recorded with the important contribution of ionic conductance to the total conductance (electron and holes electron). The activation energy at high temperature is basically assigned to the oxygen vacancy. The study of dielectric properties (ε', tanδ) of both compounds vs temperatures at different frequencies characterizes two relaxer attitudes. At this stage of analysis, we would assert that our materials prove to be a promising candidate for different micro electric devices thanks to their low dielectric loss as well as low conductance.     (a,b) Temperature dependence of loss factor of La0.75Ba0.10Sr0.15FeO2.875-δ at different frequencies respectively for δ=0.375 and 0.50