Structural and dielectric properties of Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3 manganite system are investigated. The XRD analysis confirms that all compounds crystallized in a rhombohedral structure ( space group). Using complex impedance spectroscopy, we confirm that our sample proves a semiconductor behavior which is governed by the increase of the conductance as function of temperature. From the complex impedance spectra, we conclude that the electrical mechanism for each temperature indicate a non-Debye type relaxation behavior. The achieved activation energy is found to be Ea = 0.117 ev. Indeed, we proved that the abrupt decrease of dielectric lost a δ with frequency is related to the decrease of the grain resistance at high frequencies.
Impedance spectroscopy, Electrical conductivity, Complex impedance spectra, Dielectric properties
Manganite perovskite have an important impact on technological advances due to their wide range of magnetic and transport properties [1]. Based on the comprehension of their fundamental physical properties, such as electrical and dielectric characteristics, may contribute to the development of devices. In particular, manganite achieved vast domain of requests such as magnetic refrigeration [2,3] and spintronic technologies [4]. For that the main challenge is to develop new promoting in the microelectronic industry by improving their dielectric constants.
For instance, most the physical properties of the manganite are interpreted by the double-exchange mechanism which underlies the happening of electron from Mn3+ to Mn4+ ions via the oxygen atom. Importantly, the physical properties of these materials can be controlled by changing the sintering preparation conditions method and either by the type of the metal ion substitutions on La and/or Mn sites [5]. Among the wide literature on manganite, the substitution of metal ion has an important effect which affects the magnetic properties with respect to the electrical conduction properties. These behaviors were generally interpreted from the grains (bulk) and grain boundaries effect [6].
Moreover, studies of the dielectric and electrical properties of materials have been developed in recent years due to their wide field of application. Infact, there are many researches about these properties in different materials type such as ZnO [7], ZrO2 [8], and the tris (8-hydroxyquinoline) aluminum (Alq3) material [9]. In electronics, the applications of dielectric and ferroelectric materials are in the field of memories. The best known are non-FRAM memories (Ferroelectric Random Access Memory) integrating a ferroelectric material for the storage of information. Perovskite offer interesting advantages for the intended application as high values of electrical permittivity, for example and seem favorable to a “super polarizability" One of the techniques most used for the exploration and the interpretation of the dielectric properties of materials is impedance spectroscopy. It allows in particular detecting the contribution of each microstructure constituent, grain, grain boundaries and effect of the electrodes to the different types of electrical polarization. This method of measuring complex impedance consists in studying the response of a material to the submission to the action of an alternating electric field of variable frequency. In this context; we studied the dielectric properties as a function of the frequency at different temperatures for the sample Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3 in order to see the effect of temperature on its dielectric properties.
The Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3 sample synthesized by the Sol-gel method. Stoichiometric amounts of Pr(NO3)3.6H2O, Y(NO3)3.6H2O, Ca(NO3)2, Sr(NO3)2, Mn2O3 and Fe (NO3)3.9H2O were dissolved in a distilled water with citric acid. This solution was mixed under regular stirring at 90 ℃ in the presence of ethylene glycol to obtain a homogeneous gel. Then, the obtained gel was calcined at 500 K for 8 h to eliminate the organic material. After that, the obtained powders were pelletized into pellets with diameter 13 mm and thickness of 1 mm under a pressure of 4 tons/cm2. Finally, the pellets were sintered at 1000 ℃ for 24 h. The structural characterization carried out by X-ray diffraction using an automatic two-circle diffractometer. Analysis of the X-ray diffractograms obtained at room temperature showed that the lines are intense and fine, proof of good crystallization of the compounds. The X-ray diffraction spectra of sample Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3 are refined by the Rietveld method using Full prof software. It was found that the compound crystallizes in the rhombohedra structure with the symmetry of R3¯c (see Figure 1 and Table 1). In addition, the electrical measurements were carried out on PSM1735 model impedance spectroscopy in the temperature varying between 295 and 420 K.
The study of electrical conductance is one of the most significant characteristics of manganite which describes the phenomena of charge carrier's hopping. Figure 2 shows the variation of conductance versus frequency recorded in the temperature range [295-420] K for our sample. The increase of conductance is clear with increasing frequency. Importantly, the conductance curve present two regions part. The first one is located at a low frequency region described by the presence of a Plateau region independent of frequency; Named σdc. Additionally, the second region; named σac is located at a high frequency. It is noteworthy that the total conductance is the sum of σdc and σac explained by the "universal" Jonscher power law [10] as follow:
G(ω) = GdC + GaC = GdC + Aωn (1)
Where A is a constant describing the strength of polarizability and n is the power exponent with 0 ≤ n ≤ 1.
Using Equation 1, the experimental conductivity data were well fitted as shown in Figure 3. It is clear that the fit curve coincidences well with the experimental values. It can be seen from the fitted result presented in Table 2 that the achieved n values are all superior than 1 for the whole temperature range. This behavior indicates that there are hops between neighboring sites [11].
Figure 4 displays the temperature dependence GDC conductance measurement in temperature range 295-420 K for Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3 sample. Throughout the temperature range, we observe the increase of the conductance as function of temperature which proves only a semiconductor behavior in our compounds.
In our case the experimental data of the conductance were well fitted by the Mott and Davis law given by:
GDC = G0exp(−EaKBT)
Where G0 is a pre-exponential factor, Ea is the activation energy and kB is the Boltzmann constant.
From the linear fit of the experimental data as shown in the inset of Figure 4, the determined activation energy is found to be Ea = 0.117 ev.
For our composition the impedance analysis is carried out in order to investigate the microstructure contributions like grains and grain boundaries with the relaxation process. For that we plot the Nyquist diagram which is defined as the plot between real part of the impedance (Z') and imaginary part of the impedance (Z"), in the temperature range 295-420 K as shown in Figure 5. It is clearly from this diagram the appearance of semicircular arcs which is characterized by the decreasing of their diameter with increasing temperature. From the achieved single semicircle, we can conclude that the electrical mechanism for each temperature indicate a non-Debye type relaxation behavior [12]. This, decrease is related to the increase of dc conduction. Further, to determine the corresponding resistance (R) and capacitance (C) values we have employed an equivalent circuit model by fitting the Nyquist diagram for all temperature. The fitted results are shown by solid lines in which the best fit is composed with a three parallel RC circuits combined in series as shown in Figure 6. The fitting results are grouped illustrated in Table 3 for each temperature.
From the results fit we can deduce that the Nyquist plot is due to the superposition of three contributions. In other word three distinct semicircular arcs are not observed; one corresponds to the grain resistance contribution located at high frequency RC element; the medium corresponds to grain boundary resistance and the low is associated to electrode contributions [13].
The grain-boundary resistance Rgb appears to be larger than the grain resistance Rgb. Those assigned to the disordered atomic arrangement located near the grain boundary region which assure a significant increase in electron scattering.
Interestingly, we remark the decrease of the Rgb values with the increase of temperature. Thus, this decrease suggests semiconductor behavior for this sample [14]. Additionally this behavior can be interpreted by the lowering of barrier which lead to a significant increase in the concentration of mobile charges and then enhance the electrical transport as rising temperature [15].
Figure 7 presents the variation of real part of impedance (Z') as a function with the frequency at various temperatures range. It is determined that Z' value decreases as increasing frequency whose merges for all the temperatures in the high frequency range. This behavior suggests the presence of the space charge polarization [16].
We also note in lower frequency region, that the Z' value decreases with increasing temperature, indicating reduction of barrier potential which is responsible for the enhancement of conductivity [17,18].
In Figure 8 we present the frequency dependence of the imaginary part Z" of the impedance in the temperature region 295-420 K.
The higher peak of Z'' decreases gradually, with increasing temperature and its broadening increases. This variation indicates the existence of immobile species in our compounds. Further, at the high-frequency range, the merging in all the Z'' plots can be explained by the accumulation of space charge. It should also be mentioned the appearance of a maximum peak Z"max at particular relaxation frequency as increasing temperature. This peak shift towards higher frequencies as temperature increases. This behavior is related to the polarization phenomena due to the contribution the mobility of the charge carriers on the relaxation frequency which correspond to the thermally activated relaxation [19]. Additionally, the coincidence of the Z" curves at high frequencies region approves the liberation of charge at the grain.
The complex permittivity ε*(ω) is known as ε*(ω) = ε′(ω) − jε"(ω) [20], in which the real part ε′ describes the stored energy and the imaginary one ε"gives a detail of the dissipation energy, respectively. These values are achieved from the following expression: [21]
ε∗ = 1jwC0Z∗
Where the geometric capacitance C0 = ε0S/e. ε0 is the free space permittivity and S and e are the electrodes area and the thickness of the sample, respectively.
In Figure 9 we display the frequency dependence of the permittivity ε' in the temperatures ranges 295-420 K for sample. It is clear at low frequencies thatε' reach a large value which decreases abruptly until the coincidence of ε' values with all temperatures at high frequencies.
This behavior suggests a change in the dielectric evolution in our compounds [22]. In fact, the dipoles cannot respect the orientation of the electric field as the frequency increases which assure the strongly decrease of the permittivity. In our case the increase in temperature contribute to disorder the dipoles which induces the decrease of ε' values [23].
In addition, we studied the dielectric loss factor tanδ which is given as '' tanδ = ε"/ε'. Figure 10 displays the tanδ variation versus frequency in the temperature range 295-420 K. An abrupt decrease of tanδ is clearly observed from the curves with the increase of frequency assuming the usual dielectric dispersion. It is well known that when the resistivity value is high, at low frequencies, the grain boundary contribution is dominant due to the contribution from the interfacial polarization [24]. For that we need more electrical energy to assure the mobility of charges carriers. On the other hand, the abrupt decrease of tanδ with frequency is related to the decrease of the grain resistance at high frequencies. Nevertheless, tanδ increase with the temperature increase. This behavior confirms the thermal activation of the dielectric relaxation [24].
Based on the complex modulus measurements we can separate the grain from the grain boundary contribution without any intervention of the electrode response.
The complex electrical modulus is defined as: M* =M' + jM'', which can be expressed as:
M = εε2 + ε′′2 and M = ε′′ε2 + ε′′2
In Figure 11 we present the frequency dependence evolution of the real part M' at different temperatures. At low frequency it is evident that M' values reach zero. This behavior can be interpreted in terms of long-range mobility of charge which consequently indicates the negligible contribution of electrode influence [25]. However, at higher frequency range we detect a continuous dispersion evolution which indicates the occurrence of short-range mobility charge conduction [26].
In Figure 11 we present the frequency dependence evolution of the real part M' at different temperatures. At low frequency it is evident that M' values reach zero. This behavior can be interpreted in terms of long-range mobility of charge which consequently indicates the negligible contribution of electrode influence [25]. However, at higher frequency range we detect a continuous dispersion evolution which indicates the occurrence of short-range mobility charge conduction [26].
The structural and dielectric properties were investigated using complex impedance spectroscopy versus frequency and temperature for the system Pr0.525Y0.075Ca0.1Sr0.3Mn0.975Fe0.025O3. Powder XRD patterns revealed the crystallization of our sample in a rhombohedral structure ( space group). The increase of the conductance as function of temperature confirms the semiconductor behavior in our system. The complex impedance spectra revealed the appearance of semicircle arcs, well modeled in terms of electrical equivalent circuit. Additionally, the decrease of the diameter of semicircular arcs with increasing temperature confirms the non-Debye type relaxation behavior in our compound.