Electrical and dielectric properties of the Ca2MnO4−δ system

Solid State Communications 151 (2011) 1331–1335

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Electrical and dielectric properties of the Ca2 MnO4−δ system N. Chihaoui a , R. Dhahri a , M. Bejar a , E. Dharhi a,∗ , L.C. Costa b , M.P.F. Graça b a

Laboratoire de Physique Appliquée, Faculté des Sciences, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisie

b

I3N and Physics Department, University of Aveiro, 3810-193 Aveiro, Portugal

article

info

Article history: Received 28 February 2011 Accepted 18 June 2011 by T. Kimura Available online 29 June 2011 Keywords: A. Small polarons C. Oxygen vacancies D. Activation energy E. Impedance spectroscopy

abstract The materials Ca2 MnO4−δ (δ = 0.0–0.5) were prepared in two steps: the preparation of the parent compound Ca2 MnO4 by the sol–gel method followed by the creation of oxygen vacancies δ . The morphology has been studied by scanning electron microscopy (SEM) technique and has revealed that the free surface morphology becomes more densified when increasing the oxygen vacancies δ . The electrical properties have been studied using dc conductivity and ac impedance spectroscopy techniques. From the dc conductivity measurements it was found that, at high temperatures, the conduction mechanism is controlled by small polarons. The activation energy increases with δ , indicating an increase in the height of the potential barriers. The dielectric properties were studied using the modulus formalism, showing a non-Debye relaxation process, which is thermally activated presenting activation energy independent of δ . © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Recently there has been a growing interest in the study of layered perovskites, which acquire a wide variety of interesting physical properties such as colossal magnetoresistance [1–4], superconductivity [5], ferroelectricity [6], and for their possible use in spintronic devices [7–9]. Currently, magnetocaloric effect (MCE) offers an alternate technology for refrigeration, with enhanced efficiency and without environmental hazards [10–14]. Many studies have revealed that the properties of polycrystalline perovskite materials are greatly dependent on microstructures. In fact, it was found that the electrical response of a material is not intrinsic but dominated by many extrinsic factors such as grain boundaries and electrode–material interfaces [15–17]. On the other hand, with the recent progress of technological integrated circuits, high dielectric constant materials are increasingly required for commercial devices, as they permit smaller capacitive components. Perovskite materials, having high dielectric constants, are used for technological applications such as wireless communication systems, cellular phones and global positioning systems in the form of capacitors, resonators and filters. Recently a great number of studies have been made on various issues in Ruddlesden–Popper manganites (R, A)n+1 Mnn O3n+1

∗

Corresponding author. Tel.: +216 98373734; fax: +216 74676609. E-mail addresses: [email protected] (N. Chihaoui), [email protected] (R. Dhahri), [email protected] (M. Bejar), [email protected] (E. Dharhi), [email protected] (L.C. Costa), [email protected] (M.P.F. Graça). 0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.06.023

(R = trivalent lanthanides, A = divalent alkaline-earth ions). The electric transport and magnetic properties of the Ruddlesden–Popper phases are governed by the nature and size of the cations at the sites A and B, the width of n perovskite slabs, the B–O–B angle and the oxygen content. Many studies have highlighted the important role played by the size of the A-site substitution in determining the dielectric properties of these materials [18]. The aim of this work is to study the effect of the creation of vacancies in oxygen sites on the morphologic and electrical/dielectric properties of Ca2 MnO4−δ Ruddlesden–Popper (RP) compounds using dc conductivity and ac impedance spectroscopy measurements. 2. Experimental details Polycrystalline sample Ca2 MnO4 is synthesized by the sol–gel method. Stoichiometric amounts of Ca(NO3 ) (98%) and MnO2 (99%) powders were dissolved in a dilute HNO3 solution with citric acid and ethylene glycol. The obtained mixture was then heated until a dark gel was formed. This gel was subsequently fired at 500 °C and then at 700 °C to decompose the organic residual. Then the resultant powder was subjected to several cycles of sintering with intermediate grinding. Finally, the powder was sintered at 900 °C for 10 days and slow cooled to the room temperature. Before every cycle, the sample was carefully grinded and pelletized by applying a uniaxial pressure of approximately 2 tones/cm2 . In order to create vacancies in oxygen sites, the oxide was placed in a quartz tube containing titanium in stoichiometric

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Ca2 MnO4 +

δ 2

Ti → Ca2 MnO4−δ +

δ 2

TiO2 .

(1)

To control the reaction, the obtained products are weighted, which allows to determine the real rate of the created oxygen vacancies. The X-ray diffraction (XRD) patterns data were obtained on an Siemens D5000 diffractometer (Cu Kα radiation, λ = 1.54056 Å) at 40 kV, and 30 mA, with a curved graphite monochromator, an automatic divergence slit (irradiated length 20.00 mm), a progressive receiving slit (height 0.05 mm) and a flat plane sample holder in a Bragg–Brentano parafocusing optics configuration. Intensity data were collected by the step counting method (step 0.02° in 1 s) in the 2θ angle range of 10°–70°. The morphologies of the obtained ceramics were observed by scanning electron microscopy (SEM) on a Hitachi S4100-1, on the free and fracture surfaces. The samples were covered with carbon before microscopic observation. For the electrical measurements the opposite sides of the samples were painted with silver conducting paste. During the electrical measurements, the samples, with a thickness about 1 mm, were maintained in a helium atmosphere in order to improve the heat transfer and eliminate the moisture. The dc conductivity (σdc ) was measured with a Keithley electrometer, model 617, as a function of the temperature (100–370 K) and the impedance spectroscopy measurements were performed in the frequency range of 40 Hz–110 MHz using an Agilent 4294A, in the same temperature range. 3. Results and discussion Fig. 1 presents the XRD patterns of the polycrystalline Ca2 MnO4−δ (δ = 0.0–0.5) samples. It was found that all the samples present a single phase and also that sample with δ = 0.0 crystallizes in the tetragonal structure with the I41 /acd space group (a = b = 5.183 Å; c = 24.117 Å). This structure becomes orthorhombic with Bbcm space group for δ > 0.0 (a = 5.30 Å; b = 10.05 Å; c = 12.23 Å). The morphology of the samples is shown in Fig. 2. With the increase of the oxygen vacancies, δ it can be observed that the free surface morphology evidences a more densified structure, i.e., the quantity of superficial pores decrease. It should be noticed that the morphology of the sample with δ = 0.0 is different from the morphology of the other samples, where the grain coalescence is higher. This behavior can be related with the structural change from the quadratic phase, in sample with δ = 0.0, to orthorhombic, for samples with δ > 0.0. In the fracture surface, with the exception of the δ = 0.0 one, it is visible the presence of a grain size distribution. With the increase of δ value the quantity of smalls grains increase. The dependence of the dc conductivity with the temperature, for the range between 290 and 360 K, is shown in Fig. 3, where the relation between ln(σdc ) and 1000/T is linear and thus the Arrhenius equation (Eq. (2)) can be applied in order to obtain the activation energy of this conduction mechanism. The sample with δ = 0.5 is not shown because it presents a very low conductivity (<10−12 S/m). The Arrhenius expression [11–16] used to adjust the temperature dependence of the dc conductivity (σdc ) is given by:

  Ea(dc ) σdc = σ0 exp − kB T

(2)

where σ0 is a pre-exponential factor, Ea(dc ) the activation energy, kB the Boltzmann constant (kB = 8.617 eV K−1 ) and T the temperature. The good linear behavior suggests that the

Intensity (arb. units)

proportion according to the following reaction (Eq. (1)):

f"= δ =0.5 0.5 f"= δ =0.4 0.4 f"= 0.3

δ = 0.3

f"= δ =0.2 0.2 f"= δ =0.1 0.1

δ =0.0 0.0 f"=

10

20

30

40

50

60

70

2 Theta

Fig. 1. XRD patterns of the system Ca2 MnO4−δ (δ = 0.0–0.5) samples.

conduction mechanism is controlled by small polarons [19]. The achieved activation energy (Ea(dc ) ), shown in Fig. 3, revealed an increases with δ until 0.2, indicating an increase in the height of the potential barriers, suggesting higher difficulty of the charge carriers mobility. It was observed that with the increase of the oxygen vacancies content, from 0.15 to 0.4, the σdc decreases. According to the electrical neutrality, the increase of the oxygen 3+ 2− vacancies content δ in Ca22+ Mn41+ −2δ Mn2δ O4−δ compounds leads 3+ to the increase of the Mn ions number. This augment must be accompanied by an enhancement of the double-exchange (DE) interaction mechanism, more delocalized electrons and, as result, a reduction of the activation energy. In our case, the increase of that energy proves that the DE mechanism cannot alone explain this behavior and there are surely other factors to be considered. According to the magnetic study, we have found that for δ ≥ 0.1 samples, there is an apparition of a geometric frustration leading to a robust antiferromagnetic interaction. This component enhances with δ and, as result, the electrons become more localized, which can explain the increase of the activation energy. Figs. 4 and 5 show the real (M ′ ) and imaginary (M ′′ ) parts of the complex dielectric modulus. The complex modulus formalism (M ∗ = M ′ + jM ′′ = 1/ε∗ ), was adopted to analyze dielectric relaxation mechanisms, because it minimizes the electrode interface capacitance contribution and others interfacial effects [20]. The representation of M ′′ vs. frequency (in logarithmic scale) often show peak(s) associated with the contribution(s) of small capacitance(s). The relaxation time is defined by τ = (2π .Fr )−1 , where Fr represents the frequency of the M ′′ peak(s) [21–24]. In those figures we observe a maximum in the imaginary part, which correspond to an inflexion in the curve of the real part. This behavior is indicative of the presence of a relaxation phenomenon. The Nyquist plot of Fig. 6 shows a decentred semi-circle, that is the curve as its center below the abscissa axe. This profile indicates that the simple exponential decay, corresponding to a Debye relaxation, is inappropriate to describe the relaxation and should be replaced by a frequency-dependent electric modulus (Eq. (3)):

[

M (ω) = M∞ 1 −

∞

∫



exp (−iωt ) − 0

dΦ ( t ) dt



] dt

(3)

where φ(t ) denotes the dielectric relaxation function. In order to analyze the data, we have used the Cole–Cole formalism, as presented. The components of the dielectric modulus can be expressed as (Eqs. (4) and (5)): M′ = M ′′ =

M∞ Ms A [AMs + (1M ) cos ϕ ] A2 Ms2 + 2A 1M 1 Ms cos ϕ + 1M 2 M∞ Ms A [(1M ) sin ϕ ] A2 Ms2 + 2A 1MMs cos ϕ + 1M 2

(4)

(5)

N. Chihaoui et al. / Solid State Communications 151 (2011) 1331–1335

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Fig. 2. SEM micrographs of the free surface (s) and of fracture (f ) of the system Ca2 MnO4−δ (δ = 0.0–0.5).

M´

0.06

T = 290 K

0.00

Fig. 3. ln(σdc ) vs. 1000/T of the system Ca2 MnO4−δ with δ = 0.0 (), δ = 0.15 (N), δ = 0.2 (1), δ = 0.3 (•) and δ = 0.4 (◦).

T = 360 K

0.03

2

4 log (frequency) (Hz)

6

Fig. 4. M ′ vs. frequency at different temperatures, for δ = 0.0 sample.

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16

M"(× 10-3)

12 360 K

8

ΔT = 10 K 290 K

4

0 2

3

4

5

6

7

log (frequency) (Hz)

Fig. 7. Relaxation parameters vs. T , for δ = 0.0.

Fig. 5. M ′′ vs. frequency at different temperatures, for δ = 0.0 sample. 0.08

106

M´ 105 δ = 0.10

0.04

Fr (Hz)

M´ an d M´´

0.06

0.02

δ = 0.20 104

δ = 0.30

M´´

δ = 0.00

0.00 10

1

10

2

3

10

10

4

5

10

6

10

103 0.0027

Frequency (Hz)

0.0030

0.0033

0.0036

1/T (K–1) 0.05

Fig. 8. Relaxation frequency vs. 1/T , in logarithmic scale.

M´´

0.04

the chi-square error between the experiment and simulation. When α is close to 0, the material is more homogeneous, and the semi-circle is practically centered in the abscissa axe. An angle of depression can be defined as:

0.03

Θ=α

0.02

0.01

0.00

0.02

0.04

0.06

0.08

M´

Fig. 6. M ′ and M ′′ vs. frequency and the Nyquist diagram for δ = 0.0 sample, at the temperature of 350 K.

π 2

.

(8)

Fig. 7 shows the relaxation parameters as a function of the temperature, for a pure sample δ = 0.0. Fig. 8 presents the relaxation frequency in function of 1/T and as expected, it increases with temperature. The relaxation processes are thermally activated, as well as, Arrhenius law can be used to calculate the ac activation energies associated to it. The variation of relaxation frequency, Fr , with temperature can be expressed by Eq. (9):

 Fr = B exp −

where M∞ and Ms are the values of the real part of the complex modulus in the high and low frequency regime respectively, 1M = M∞ − Ms and A and ϕ parameters defined by Eqs. (6) and (7) [25]:



A = 1 + 2 (ωτ )1−α sin

 ϕ = tan−1 

πα

+ (ωτ )2(1−α) 2 

(ωτ )1−α cos πα 2

 1 + (ωτ )1−α sin πα 2

.

1/2

(6)

(7)

The fit to the measured data was performed using the above equations and it was extracted the parameter α , which gauges the broadening of the loss spectrum, the relaxation frequency and the modulus strength. The best parameters were those that minimized

E(ac ) kB T

 (9)

where B is the relaxation frequency for high temperature, Ea(ac ) the activation energy for the relaxation process and kB the Boltzmann constant. In a logarithmic representation of Fr as a function of the inverse temperature, linear fits are obtained, as we can observe in Fig. 9. The activation energy is independent of δ , with a value of 0.24 eV and lower than the observed for the dc conduction process (Fig. 3). Assuming the potential barrier model, these results indicate that the long range conduction process is more difficult that the short range. In Fig. 9 we can see the depression angle as a function of temperature for different values of δ . In general, higher values of δ present lower angles.

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Acknowledgments This work, within the collaboration Tuniso–Portuguese framework, is supported by the Tunisian National Ministry of Higher Education, Scientific Research and Technology and the Portuguese Ministry of Science, Technology and Higher Education. References

Fig. 9. Angle vs. T , for different samples.

4. Conclusions In this work Ca2 MnO4−δ , with δ = 0.0–0.5, polycrystalline samples were synthesized in two steps: the preparation of the parent compound Ca2 MnO4 by the sol–gel method followed by the creation of oxygen. The sample with δ = 0.0 crystallizes with tetragonal structure and for δ > 0.0 it changes to orthorhombic. The increase of the oxygen vacancies promotes a decrease of the quantity of superficial pores and an increase of the quantity of small grains. The electrical measurements showed that the dc activation energy increases from the sample with δ = 0.0 (0.33 eV) to the sample 0.20 (0.38 eV) and then decreases to 0.33E with the increase of δ . This behavior shows that the double-exchange interaction mechanism alone cannot explain the observed behavior. The dc conductivity decreases with the increase of the δ value. Dielectric relaxation analysis, performed using the modulus formalism showed decentred semi-circles, indicating the existence of a relaxation time distribution. This relaxation processes are thermally activated. The ac activation energy is independent of δ , with a value of 0.24 eV.

[1] G. Huo, S. Liu, Y. Wang, Y. Wang, J. Alloys Compd. 428 (2007) L1. [2] J. Androulakis, J. Giapintzakis, Physica B 405 (2010) 107. [3] R.N. Mahato, K. Sethupathi, V. Sankaranarayanan, R. Nirmala, J. Magn. Magn. Mater. 322 (2010) 2537. [4] L. Han, C. Chen, J. Mater. Sci. Technol. 26 (3) (2010) 234. [5] P.A. Salvador, K.B. Greenwod, J.R. Mawdsley, K.R. Poeppelmeier, T.O. Mason, Chem. Mater. 11 (1999) 1760. [6] P.W. Rehrig, S.E. Park, S. Trolier-McKinstry, G.L. Messing, B. Jones, T.R. Shrout, J. Appl. Phys. 86 (1999) 1657. [7] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [8] Y. Tokura, Y. Tomioka, J. Magn. Magn. Mater. 200 (1999) 23. [9] X.X. Zhang, J. Tejada, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596. [10] M. Bejar, R. Dhahri, F. El Halouani, E. Dhahri, J. Alloys Compd. 414 (1–2) (2006) 31. [11] N.A. de Oliveira, P.J. von Ranke, Phys. Rep. 489 (4–5) (2010) 89. [12] M. Bejar, R. Dhahri, E. Dhahri, M. Balli, E.K. Hlil, J. Alloys Compd. 442 (1–2) (2007) 136. [13] V.S. Kumar, R. Mahendiran, Solid State Commun. 150 (31–32) (2010) 1445. [14] S. Othmani, R. Blel, M. Bejar, M. Sajieddine, E. Dhahri, E.K. Hlil, Solid State Commun. 149 (25–26) (2009) 969. [15] N. Kumar, A. Dutta, S. Prasad, T.P. Sinha, Physica B 405 (2010) 4413. [16] N. Sdiri, R. Jemai, M. Bejar, M. Hussein, K. Khirouni, E. Dhahri, S. Mazen, Solid State Commun. 148 (2008) 577. [17] G. Garcia-Belmonte, J. Bisquert, F. Fabregat, V. Kozhukharov, J.B. Carda, Solid State Ion. 107 (1998) 203. [18] A. Castro-Couceiro, M. Sánchez-Andújar, B. Rivas-Murias, J. Mira, J. Rivas, M.A. Señarís-Rodríguez, Solid State Sci. 7 (2005) 905. [19] N.F. Mott, E.A. Davis, Electronic Process in Non-Crystalline Materials, Claredon, Oxford, 1979. [20] P.B. Macedo, C.T. Moynihan, R. Bose, Phys. Chem. Glasses 13 (6) (1972) 171–179. [21] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. [22] J.R. Macdonald, Impedance Spectroscopy, John Wiley & Sons, New York, 1987. [23] C.C. Silva, M.P.F. Graça, M.A. Valente, A.S.B. Sombra, J. Non-Cryst. Solids 352 (2006) 1490–1494. [24] C.C. Silva, D.X. Gouveia, M.P.F. Graça, L.C. Costa, A.S.B. Sombra, M.A. Valente, J. Non-Cryst. Solids 356 (2010) 602–606. [25] J. Belattar, M.P.F. Graça, L.C. Costa, M.E. Achour, C. Brosseau, J. Appl. Phys. 107 (2010) 124111.