Phase Transitions A Multinational Journal
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Above room temperature complex impedance analysis of properties of La0.33Sr0.67Mn0.33Ti0.67O3±δ perovskite Dhahbi Tlili, Nejeh Hamdaoui, Sobhi Hcini, Mohamed Lamjed Bouazizi & Sadok Zemni To cite this article: Dhahbi Tlili, Nejeh Hamdaoui, Sobhi Hcini, Mohamed Lamjed Bouazizi & Sadok Zemni (2016): Above room temperature complex impedance analysis of properties of La0.33Sr0.67Mn0.33Ti0.67O3±δ perovskite, Phase Transitions, DOI: 10.1080/01411594.2016.1252984 To link to this article: http://dx.doi.org/10.1080/01411594.2016.1252984
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Date: 10 November 2016, At: 07:46
PHASE TRANSITIONS, 2016 http://dx.doi.org/10.1080/01411594.2016.1252984
Above room temperature complex impedance analysis of properties of La0.33Sr0.67Mn0.33Ti0.67O3§d perovskite Dhahbi Tlilia, Nejeh Hamdaouib,c, Sobhi Hcinid,e, Mohamed Lamjed Bouazizif and Sadok Zemnia a
Laboratory of Physical Chemistry of Materials, Department of Physics, Faculty of Science of Monastir, University of Monastir, Monastir, Tunisia; bLaboratory of Energy and Materials, Department of Physics, High School of Sciences and Technologies of Hammam Sousse, University of Sousse, Hammam Sousse, Tunisia; cLaboratory of Energy and Materials, Department of Physics, Research Group of Nano-Materials for Telecommunications, Higher Institute of Computer and Communication Technologies, University of Sousse, Hammam Sousse, Tunisia; dResearch Unit of Valorization and Optimization of Exploitation of Resources, Faculty of Science and Technology of Sidi Bouzid, University Campus Agricultural City, University of Kairouan, Sidi Bouzid, Tunisia; eDepartment of Physics, Faculty of Sciences of Gafsa, University Campus – Sidi Ahmed Zarroug, University of Gafsa, Gafsa, Tunisia; fMechanical Department, College of Engineering, Prince Sattam Bin Abdulaziz University, AlKharj, KSA
ABSTRACT
ARTICLE HISTORY
La0.33Sr0.67Mn0.33Ti0.67O3§d perovskite was prepared by the standard solid state reaction at 1773 K. The sample crystallizes in the cubic Pm3m structure. The electrical properties of the sample have been investigated above room temperature in 293–373 K range with varying frequency between 102 and 106 Hz using complex impedance analysis. The sample exhibit a metal–semiconductor transition temperature at TMS D 303 K. The conductance curves were well fitted by Jonscher power law G(v) D Gdc C Avn, which shows that with increasing temperature, the exponent (n) decreases below TMS, and increases above TMS. The activation energy deduced from the analysis of the conductance curves matches very well with the value estimated from the relaxation time indicating that relaxation process and electrical conductivity are attributed to the same defect. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.
Received 10 September 2016 Accepted 19 October 2016 KEYWORDS
LSMTO; complex impedance spectroscopy; conductance; metal–semiconductor transition; relaxation time
1. Introduction Oxide perovskite systems Ln1¡xAxMnO3 (Ln D La, Nd, Pr, Y, etc… and A D Ca, Sr, Ba, etc…) have attracted lot of attention of the scientific community due to their wide range of the technological applications particularly in Solid Oxide Fuel Cells due to their high operation temperature (above room temperature) [1]. Perovskite compounds have also found their application in magnetic refrigeration technology [2,3] and in electronic devices such as magnetic field sensors [4,5], hard disk read heads [6], infrared devices [7], spintronics and microwave active components. General consensus is that the physical properties of perovskite materials, such as electrical, magnetic and dielectric properties can be improved or tuned for technological purposes by proper choice of method of preparation, doping element and doping level [8]. A way to study these electrical properties of perovskite materials can be achieved by the complex impedance spectroscopy (CIS) analysis. In order to investigate the nature of ionic motion, the
CONTACT Dhahbi Tlili
[email protected]
© 2016 Informa UK Limited, trading as Taylor & Francis Group
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relaxation phenomenon and the metal–semiconductor temperature transition (TMS), we have studied in this work the electrical properties of La0.33Sr0.67Mn0.33Ti0.67O3§d (LSMTO) perovskite sample using a CIS technique over a wide range of frequency above room temperature in 293–373 K range.
2. Experimental LSMTO1 sample was prepared using the conventional solid-state reaction method at 1773 K. Microstructure and composition were analyzed by scanning electron microscopy (SEM) using a Philips XL30 microscope with an energy dispersive X-ray spectrometer working at 20 kV. Powder X-ray diffraction (XRD) were collected using Cu-Ka1 radiation in the 2u range 10 –80 with a step size of 0.017 and a counting time of 18 s per step at room temperature (RT). Rietveld structure refinement was carried out using the FullProf software [9]. For electrical measurements, pure silver is deposited on the surface of pellet to ensure ohmic contact. The frequency and temperature dependent electrical measurements were carried out using a N4L-NumetriQ (model PSM1735) connected to a computer.
3. Results and discussions 3.1. Microstructure and structural analysis Figure 1 presents the XRD Rietveld refinement for LSMTO perovskite sample. This figure indicates that our sample exhibits a single phase without any detectable of secondary phase. The SEM micrograph (given in the inset of Figure 1) shows uniform grain size distribution with average particle size of about 3 mm. Indexing of the XRD pattern and Rietveld structure refinement was performed using the cubic structure with Pm3m space group (see Figure 1). The obtained lattice parameters are a D 3.9194 (4) (A) and V D 60.207 (1) A3 where the number in parenthesis is estimated standard deviation with the last significant digit. 3.2. Electrical conductance study Figure 2 shows the log–log plot of the total conductance G vs. frequency at different temperatures for LSMTO perovskite sample. The study of the frequency dependence of the conductance is a
Figure 1. XRD pattern of LSMTO perovskite sample. All peaks are indexed in the cubic structure with Pm3m space group. The inset shows the SEM image.
PHASE TRANSITIONS
3
-5
10
-3
10
Gdc (S)
f= 100 Hz TMS= 303 K -6
10
-4
G (S)
10
280 -5
10
300
320 340 T (K)
360
380 293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K 373 K
-6
10
2
10
3
10
4
10
5
10
6
10
f (Hz) Figure 2. Variation of the total conductance as a function of frequency at different temperatures for LSMTO perovskite sample. Inset: variation of dc conductance Gdc as a function of temperature.
well-established method for characterizing the hopping dynamics of the charge carrier for our sample. The total conductance G of the sample is the superposition of the dc conductance (Gdc) and the ac conductance (Gac) according to the following Jonscher power law [10]: GðvÞ D Gdc C Gac
(1)
In the low frequency region, an independent frequency behavior of the conductance is observed for each temperature (as shown by a plateau at low frequencies corresponding to the dc conductance Gdc). In this plateau region, the curves prove the presence of metal–semiconductor transition at TMS D 303 K for our sample (see the inset of Figure 2). This is in good agreement with the reported results in Refs. [11–13]. The experimental data of dc conductance were well fitted by the Mott and Davis law which describes small polaron hopping [14]: Ea Gdc T D Bexp kB T
(2)
where B is a pre-exponential factor, Ea is the activation energy, T is the absolute temperature and kB is the Boltzmann constant. The plot of log(GdcT) vs. (1000/T) shown in Figure 3 for LSMTO perovskite sample is linear in all the temperature range above TMS, confirming that conduction process is thermally activated. The Ea value estimated from the slope of the linear fit plot is equal to 0.45 eV. For high frequency region, a second behavior is characterized by a change in slope of the conductance. In this region, the curves become monotonous almost linear growing. This part of conductance corresponds to the ac conductance (Gac) which can be described by the following power law [10]: Gac D Avn
(3)
where A is coefficient dependent on temperature and n is the frequency exponent which depends on both frequency and temperature. According to Funke [15], n 1 means that the electron hopping involves a translational motion with a sudden hopping, whereas n > 1 means that the motion involves localized hopping between neighboring sites.
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D. TLILI ET AL.
-5,5 experimentel data fitting curve
ln(GdcT) (S.K)
-6,0
Ea= 0.45 eV
-6,5 -7,0 -7,5 -8,0 -8,5 -9,0 2,6
2,7
2,8
2,9
3,0
3,1
3,2
3,3
3,4
3,5
-1
1000/T (K ) Figure 3. Variation of the log(GdcT) as a function of (1000/T) for LSMTO perovskite sample.
The experimental data of total conductance as a function of frequency at different temperatures for LSMTO were well fitted using Equation (3) (red solid line in Figure 4). In the fitting procedure, the (A) and (n) factors have been varied simultaneously to get the best fit. It can be seen that the fit matches well with the experimental values. The goodness of the fit is usually evaluated by comparing the squared coefficient of linear correlation coefficient (R2) obtained for each temperature (see Table 1). We can notice from this table (see also the inset of Figure 4) that n decreases with increasing temperature below TMS. However, above TMS, it increases with the increasing temperature. This result is in good agreement with the Gdc(T) curves. On the other hand, we found that the obtained values of n are all lower than 1 in the whole temperature range which, according to Funke [15], corresponds to a hopping process through long distance.
-3
10
0,84 0,80
G (S)
n
0,76 -4
10
0,72
TMS= 303 K
0,68 0,64 280 -5
10
300
320 340 T(K)
360
380
293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K 373 K
-6
10
2
10
3
10
4
10
5
10
6
10
f (Hz) Figure 4. Variation of the total conductance as a function of frequency at different temperatures for LSMTO perovskite sample. Red solid lines represent the fitting to the experimental data using the universal Jonscher power law. Inset: variation of the exponent n as a function of temperature.
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Table 1. The fitting parameters obtained from experimental data of the total conductance as a function of frequency using Jonscher power law. N A £ 10¡9 R2 T (K) Gdc £ 10¡7(s) 293 6.94 0.666 5.574 0.999 303 4.96 0.657 4.902 0.999 313 5.79 0.674 4.636 0.999 323 8.36 0.685 4.421 0.999 333 12.40 0.690 5.305 0.998 343 18.75 0.722 3.742 0.998 353 29.21 0.773 1.669 0.999 363 49.01 0.795 1.748 0.995 373 68.38 0.833 1.233 0.999
3.3. Complex impedance analysis Figure 5 shows the variation of the real part of impedance (Z0 ) with frequency at various temperatures for LSMTO perovskite sample. We can notice from this figure that Z0 increases with increasing temperature below TMS; however, above TMS, it decreases with the increasing temperature. Figure 5 also shows that the magnitude of Z0 is typically higher in the low frequency region, and then it decreases gradually with increasing frequency. The value of Z0 appears to merge in the high frequency region irrespective of temperature. This result may possibly be related to the release of space charge as a result of reduction in the barrier properties of material with rise in temperature [11,16], and may be a responsible factor for the enhancement of conductance of the material with temperature at high frequencies. The merge of the value of Z0 for all temperatures at higher frequencies can be interpreted by the presence of space charge polarization. This interpretation was confirmed by the higher impedance values at lower frequencies. The behavior of Z0 observed for our sample at lower and higher frequencies is in good agreement with the reported results in Refs. [11,16]. The variation of Z00 with frequency at different temperatures is depicted in Figure 6. These curves exhibit peaks with characteristic frequency maxima known as relaxation frequency (fr). It is noted that the peaks are shifted towards lower frequencies below TMS (respectively shifted towards higher frequencies above TMS) when increasing the temperature. This behavior leads to the increase of the relaxation time (t D 1=2pfr ) below TMS (respectively the decrease of t above TMS). The temperature
6
1.8x10
293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K 373 K
6
1.6x10
6
1.4x10
6
Z' (Ω )
1.2x10
6
1.0x10
5
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
0.0 5
-2.0x10
2
10
3
4
10
10
5
10
6
10
f (Hz) Figure 5. Variation of real part of the impedance (Z0 ) for LSMTO perovskite sample as a function of frequency for different temperatures.
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D. TLILI ET AL.
5
9x10
293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K 373 K
5
8x10
5
7x10
5
Z′′ (Ω )
6x10
5
5x10
5
4x10
5
3x10
5
2x10
5
1x10
0 2
3
10
4
10
5
10
6
10
10
f (Hz) Figure 6. Variation of imaginary part of the impedance (Z00 ) for LSMTO perovskite sample as a function of frequency for different temperatures.
dependent characteristics of t follow the Arrhenius relation as mentioned below: t D to exp
Ea kB T
(4)
where Ea is the activation energy. The Ea value estimated from the slope of the linear fit plot (see Figure 7) is equal to 0.34 eV. This value is in good agreement with that previously derived from the analysis of the dc conductance. This indicates that the same type of charge carriers is responsible for both electrical conduction and relaxation phenomena. The complex impedance spectrum gives the direct correlation between the response of a real system and an idealized model circuit composed of discrete electrical components. The variation of Z00
-7,5
experimentel data fitting curve
-8,0
ln(τ )
Ea= 0.34 eV -8,5
-9,0
-9,5
-10,0 2,6
2,7
2,8
2,9
3,0
3,1
3,2
3,3
3,4
3,5
-1
1000/T (K ) Figure 7. Variation of the ln(t) as a function of (1000/T) for LSMTO perovskite sample. Red solid line is the linear fit for our data.
PHASE TRANSITIONS
6
293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K 373 K
1.4x10
6
1.2x10
6
Z'' (Ω )
7
1.0x10
5
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
0.0 0.0
5
5.0x10
6
1.0x10
6
1.5x10
6
2.0x10
6
2.5x10
6
3.0x10
Z' (Ω ) Figure 8. Complex impedance spectrum for LSMTO perovskite sample at different temperatures.
vs. Z0 for our samples is represented as Nyquist plots in Figure 8. It can be seen from this figure that the complex impedance plot for our compound is characterized by the appearance of semicircles with a diameter that varies with temperature. The impedance data are fitted using Zview software [17] and the best fit (red solid line in Figure 8) is obtained when employing an equivalent circuit, as shown in the inset of Figure 8, formed by a grain resistance (Rg) in series with a parallel combination of grain boundary resistance (Rgb) and constant phase element impedance (ZCPE). The CPE impedance (ZCPE) is given by the following relation [18–20]: ZCPE D
1 T ðjvÞp
(5)
where T indicates the value of capacitance of the CPE element (expressed in Farad units), and p is the factor exponent (0 p 1). The p factor represents the capacitive nature of the element: if p D 1, the element is an ideal capacitor and if p D 0, it behaves as a frequency independent ohmic resistor [21]. T and p can be temperature dependent. The expressions of real (Z0 ) and imaginary (Z00 ) components of impedance related to the equivalent circuit are [22] Rgb C 1 C Rgb Tvp cosðpp 2 Þ 2 Z D Rg C pp 2 p 1 C Rgb Tvp cosðpp 2 Þ C ðRgb Tv sinð 2 ÞÞ
(6)
R2gb Tvp sinðpp 2 ÞÞ Z D pp 2 2 p 1 C Rgb Tv cosð 2 Þ C ðRp Tvp sinðpp 2 ÞÞ
(7)
0
00
The Rg, Rgb, T and p parameters of each fitting are summarized in Table 2. We can notice that the variation of the resistance of grains Rg and grain boundaries Rgb increases with increasing temperatures below TMS (metallic behavior). Whereas, Rg and Rgb decrease with temperature, above TMS, indicating the presence of thermally activated conduction mechanism in this system which is similar to a semiconducting behavior. Such behavior has been reported in other perovskite systems [11,23].
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D. TLILI ET AL.
Table 2. Electrical parameters of equivalent circuit deduced from complex impedance spectrum for LSMTO perovskite sample. Rgb £ 106 (V) T (nF) p T (K) Rg (V) 293 209 1.46 0.523 0.780 303 317 2.25 0.601 0.81 313 261 1.81 0.619 0.815 323 237 1.26 0.678 0.818 333 193 0.86 0.729 0.873 343 171 0.56 0.815 0.85 353 140 0.36 0.895 0.845 363 103 0.20 0.910 0.875 373 96 0.13 1.122 0.856
4. Conclusion Electrical conductance and complex impedance analysis of LSMTO perovskite sample have been studied above room temperature as a function of both frequency and temperature. Electrical conductance curves show a metal–semiconductor transition temperature at TMS D 303 K. The activation energy deduced from analysis of the conductance curves is Ea D 0.45 eV. This value matches very well with that estimated from the relaxation time determination (Ea D 0.34 eV). The exponent (n) deduced from the fitting of conductance curves using the Jonscher power law (G(v) D Gdc C Avn) decreases below TMS, then it increases above TMS. The n values are all lower than 1 in the whole temperature range corresponding to a hopping process through long distance. The impedance study using Nyquist representation revealed the appearance of semicircle arcs, well modeled in terms of electrical equivalent circuit formed by a grain resistance (Rg) in series with a parallel combination of grain boundary resistance (Rgb) and constant phase element impedance (ZCPE).
Notes 1. It should be noted that the oxygen composition in the sample is O3§d since no control of oxygen content is available in our synthesis experiment. So, the developed chemical formula of our sample should be written as C C C C 2 Sr20:67 Mn30:33 Ti40:67 O3 § d . La30:33 2. The number in parenthesis is estimated standard deviation with the last significant digit.
Disclosure statement No potential conflict of interest was reported by the authors.
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