Structural, thermal and electrical characterization of NdLiMo2O8 electroceramics, using impedance spectroscopy

Journal of Physics and Chemistry of Solids 73 (2012) 357–362

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Structural, thermal and electrical characterization of NdLiMo2O8 electroceramics, using impedance spectroscopy Sanjaya Brahma a,n, R.N.P. Choudhary b, S.A. Shivashankar a a b

Materials Research centre, Indian Institute of Science, Bangalore 560012, India Department of Physics, Institute of Technical Education and Research, S. O. A. University, Bhubaneswar 751030, India

a r t i c l e i n f o

abstract

Article history: Received 23 January 2011 Received in revised form 18 July 2011 Accepted 30 September 2011 Available online 12 October 2011

We report electrical property of a polycrystalline NdLiMo2O8 ceramics using complex impedance analysis. The material shows temperature dependent electrical relaxation phenomena. The d.c. conductivity shows typical Arrhenius behavior, when observed as a function of temperature. The a.c. conductivity is found to obey Jonscher’s universal power law. The material was prepared in powder form by a standard solid-state reaction technique. Material formation and crystallinity have been confirmed by X-ray diffraction studies. Impedance measurements have been performed over a range of temperatures and frequencies. The results have been analyzed in the complex plane formalism and suitable equivalent circuits have been proposed in different regions. The role of bulk and grain boundary effect in the overall electrical conduction process is discussed with proper justification. & 2011 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics C. Thermogravimetric analysis C. X-ray diffraction D. Electrical properties

1. Introduction Rare earth based molybdates (R¼ La, Ce, Pr, Nd etc.) having different structural forms, have attracted lot of attention among researchers. These materials exhibit a variety of physical properties such as application in laser host [1], ferroelectric/ferroelastic behavior [2], phosphors [3], ferromagnetism [4], anomalous semiconductor to metal transition [5], spin glass behavior [6], ionic conductors [7,8] and semiconducting behavior [9]. Rare earth molybdates often show polymorphism due to the flexibility of coordination, and geometry of both rare earth cation (R þ ) and molybdate ion (Mo þ 6). This property permits possibility of 6–12 and 4–7 coordinates with various coordination polyhedra for both trivalent rare earth cation and hexavalent molybdate cation, respectively [10,11]. The detailed literature survey indicates the existence of some scheelite like (CaWO4) structure related molybdates having compositions AMoO4 (A¼alkaline earth ion) or A0 A00 Mo2O8 (A0 ¼alkali metal ion and A00 ¼lanthanide ion). One such compound is AgLnMo2O8 (Ln¼La, Nd, Sm etc), whose crystal structure and physical properties have been reported [12,13]. However, molybdates based on combination of rare earth and alkali metals have not received much attention. Here, we report electrical properties of such a compound i.e., NdLiMo2O8, using complex impedance

n

Corresponding author. Tel.: þ91 80 22932782; fax: þ91 80 2360 7316. E-mail addresses: [email protected], [email protected] (S. Brahma). 0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.09.028

spectroscopy technique. The synthesis conditions have been optimized by thermal analysis (TGA) and the details of the optimization condition for the solid-state synthesis of NdLiMo2O8 is reported here.

2. Experimental A standard solid-state reaction method was used for the synthesis of NdLiMo2O8 powder materials. The synthesis procedure involves high purity (AR grade) precursors (Li2CO3, MoO3 and Nd2O3) taken in appropriate stoichiometric ratio and mixed mechanically in an agate mortar for 2–3 h. This is followed by further mechanical grinding in methanol to achieve homogenous mixing of the constituents. The mixture in powder form was calcined at a temperature of 570 1C (optimized from thermo gravimetry analysis) in air atmosphere. The calcination step was repeated twice under similar conditions in order to allow the completion of the solid-state reaction. The calcined powder was pressed into cylindrical pellets (10 mm diameter and 1.3 mm thickness) with polyvinyl alcohol (PVA) as the binder. An isostatic pressure of 3  106 N/m2 was applied for pelletization. The sample pellets were sintered at a temperature of 550 1C for 6 h to obtain dense sample. Both sides of the pellet were polished by fine emery paper to make their faces smooth and parallel. It was finally coated with conductive silver paint followed by slow drying (150 1C for 2 hs), before carrying out the impedance measurement.

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Material formation was confirmed by X-ray diffraction (XRD) studies. XRD pattern of the powder sample was recorded at room temperature by using an X-ray diffractometer (PHILIPS, model: ˚ as the source. Thermal analysis PW-1710) with CuKa (1.5418 A) was carried out by using a simultaneous TGA/DTA/DTG thermal analysis system (PERKIN ELMER, air flow at 100 ml/min). Impedance measurements were carried out by using a computer controlled frequency response analyzer (HIOKI LCR HI Tester, Model: 3532). An input a.c. signal of voltage  10 mV was applied across the sample pellet for electrical measurements.

3. Results and discussion

Therefore, it may be concluded that 570 1C is the suitable temperature for calcinations of as prepared NdLiMo2O8. The electrical properties have been studied by complex impedance spectroscopy over a range of frequencies and temperatures. The method ensures separation of the bulk, grain boundary and electrode properties, giving an insight about the electrical processes taking place within the system and their correlation with sample microstructure, when modeled in terms of their equivalent electrical circuit. Fig. 3 shows complex impedance spectrum (Nyquist Plot) of NdLiMo2O8 measured at different temperatures. The pattern shows the variation of real part of impedance (Z0 ¼ Z cos y) with the imaginary part of impedance (Z00 ¼Z cos y). At low temperature, only arcs are observed and with the increase in the temperature, arcs were transformed into semicircles. The complete pattern can be analyzed by considering three different temperature regions, starting from 100 1C to 250 1C, intermediate temperature range (300 1C–400 1C) and finally at 430 1C. There is a clear indication of a single semicircular arc at 150 1C (Fig. 3a(i)).

561°C

158°C

100000

539°C

(112)

323°C 154°C 439°C

(303) (224)

(116)

20000

570°C

(204)

523°C

40000

(220)

347°C

60000

(004)

Intensity (a.u.)

80000

166°C

Δm/m(μgmin )

5703 C 2NdLiMo2 O8 þ CO2 m D, air

(211) (114)

Endothermic

356°C

Nd2 O3 þLi2 CO3 þ MoO3

(200)

Exothermic

265°C

Δm/ΔT(μgmin )

Differential Heat Flow

Thermogravimetric analysis determines the weight gain or loss of the material due to gas release or absorption as a function of temperature. We systematically optimized the preparation conditions, such as calcination temperature by thermal analysis. Fig. 1(a) shows the TGA pattern of as prepared precursor mixture, which shows a steady loss of mass of the powder sample when heated from 50 1C onwards and reaches saturation at 570 1C. It also shows small depressions at temperatures  166 1C and 347 1C. These may be attributed to the release of absorbed moisture present in the material and intermediate reaction stages [14,15]. The total mass loss up to 570 1C was estimated to be 9.9%. The maximum mass loss occurring between 523 1C and 570 1C in the TGA pattern may be attributed to the release of CO2 gas [16]. The observations recorded in the TGA curve is found to be in close agreement with DTG pattern (Fig. 1(b)) and DTA pattern (Fig. 1(c)). DTG pattern shows four maxima, which may be assumed to be the mass loss due to release of water content, intermediate reaction steps and evolution of CO2 at the end of the reaction The removal of the water content present in the material and CO2 at the completion of the reaction, appear to be

endothermic processes and these are indicated by two endotherms at 158 1C and 561 1C in the DTA pattern. The presence of small exothermic peaks at about 265 1C and 356 1C may be attributed to the beginning of reactions corresponding to the intermediate reaction steps. Fig. 2 shows the powder XRD pattern of NdLiMo2O8. The pattern shows diffraction peaks of varying magnitude in intensity at different Bragg angles. The absence of any other peak either from the starting precursor material or from any impurity (if there in) confirms the phase purity of the material, and completion of the solid-state reaction, leading to the formation of NdLiMo2O8. The powder material shows better crystallinity as revealed by the high intensity peaks in the X-ray powder diffraction pattern. The peaks have been indexed and the values are shown in the pattern. A preliminary structural analysis indicates that the material is crystallized to tetragonal ˚ c ¼11.46 A). ˚ unit cell structure (lattice parameters: a ¼5.25 A, These values appear to be in good agreement with JCPDS data base [17]. Based on XRD and thermal analysis results, we believe that the following solid-state reaction might be a possibility, when the precursors are calcined in the air atmosphere

0 100

200

300

400 500 600 Temperature (°C)

Fig. 1. TGA/DTG/DTA of NdLiMo2O8.

700

800

20

30

40

50 2θ

Fig. 2. X-ray diffraction pattern of NdLiMo2O8.

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S. Brahma et al. / Journal of Physics and Chemistry of Solids 73 (2012) 357–362

359

20

150 200

-Z" (MΩ)

15

10

5

0 0

5

10

15

20

Z'(MΩ )

1000

Log (R )KΩ

100

10

1 1.4

1.5

1.6

1.7

1.8

1.9

2.0

(1000/T)K Fig. 3. (a) Complex impedance spectrum as a function of temperature with electrical equivalent circuit (inset). (b) Variation of grain boundary resistance as a function of temperature.

The capacitance was found to be (  10  12F), which indicates the beginning of intergranular activities with definite contribution from the bulk material [18,19]. The trend continues up to 230 1C.

Each semicircular arc in impedance pattern can be attributed to a parallel combination of bulk resistance (Rb) and bulk capacitance (Cb). This semicircular behavior can be explained by considering

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the following mathematical equation:

30000

  R 2 R2 Z 02  þ Z 002 ¼ 2 4

25000

Z' (M Ω)

20000

15000 0

10000

5000

100 150 200

0 0.1

25000

ð1Þ

1

10 Frequency (kHz)

100

1000

Fig. 4. Variation of real part of impedance (Z0 )as a function of frequency.

20000 -Z"(M Ω)

where Z0 and Z00 are the real and imaginary part of impedance, respectively. Low frequency intercepts of these semicircles with the real axis gives the bulk resistance. With increase in temperature, the intercept moves towards origin, which indicates the decrease in bulk resistance. In the temperature range (300 1C– 400 1C), two semicircular arcs are observed in high and low frequency regions (Fig. 3a(ii)). This additional semicircular arc starts to appear from 240 1C. This phenomena may be attributed to the inter and intra grain (bulk and grain boundary) phenomena. Such an electrical process can be modeled in terms of equivalent electrical circuit according to the ‘‘Brick layer model’’ [20], comprising of series combination of two parallel R–C circuits of both grain and grain boundary effects. As the temperature increased, third semicircular arc was observed at  430 1C as shown in (Fig. 3a(iii)). The appearance of the third semicircular arc at higher temperatures ( 4400 1C), may be attributed to the beginning of the polarization effects (polarization at the material–electrode interface) [21]. The grain boundary resistance (Rgb) was estimated from the complex impedance spectrum. Fig. 3(b) shows the logarithm of grain boundary resistance (log10 Rgb) with the inverse of temperature ((103/T)K  1), which shows a monotonic decrease in grain boundary resistance with rise in temperature,.i.e, arhenious type behavior. A decrease in the grain boundary resistance with rise in temperature suggests the enhanced hopping electrical conduction due to the lowering of barrier height/potential. Fig. 4 shows the variation of real part of impedance (Z0 ) with frequency at different temperatures. It shows a monotonous decrease of Z0 with a frequency followed by a plateau type behavior in the high frequency region. The curve shows a sigmoidal type behavior with increase in temperature. The frequency at which the merging of Z0 takes place, appears to shift towards high frequency side of the spectrum. The impedance is higher at low temperatures in the low frequency domain but gradually decreases with increasing temperature. The decrease in Z0 with rise in temperature and frequency indicates the possibility of an increase in the a.c. conductivity with increase in temperature and frequency [19]. The impedance Z0 merges at higher frequencies for all the temperatures, which may be due to release of space charge at higher temperatures [22].

100 150 200

15000 10000 5000 0 0.1

1

10

100

1000

Frequency (kHz) Fig. 5. Variation of imaginary part of impedance (Z00 ) as a function of frequency.

Fig. 5 shows variation of imaginary part of impedance Z00 as a function of frequency (typically called loss spectrum) at different sets of temperatures. It highlights the elements with the largest resistance in accordance with the relations " Z0 ¼ R

#

"

oRC ðoRCÞ2 , Z 00 ¼ R 2 1 þðoRCÞ2 1 þðoRCÞ

# ð2Þ

The loss spectrum has the following important features: (1) monotonous decrease of Z00 with increase in frequency in the low frequency region and merge at high frequency region showing a plateau type behavior up to a temperature of 150 1C, (2) above 150 1C, the pattern shows peaks at unique frequency and the peak shifts to high frequency region with increase in temperature, (3) appearance of assymetric peak broadening, (4) decrease of Z00 value with increase in temperature and (5) decrease of height of the peaks with increase in temperature. Absence of peaks up to 150 1C in the loss spectrum indicates no dissipation in the material. Peaks in the pattern show the type and strength of electrical relaxation in the material [23]. The relaxation species in the material may possibly be immobile species/electrons at low temperatures and defects/vacancies at higher temperatures. The asymmetric broadening of peaks in the pattern suggests that there is a spread of relaxation time (indicated by the changes of peak width) with two equilibrium positions. The magnitude of Z00 decreases gradually, with shift of peak frequency and finally merges at the high frequency region. This may be due to the accumulation of space charge in the material. The impedance data was used to evaluate the relaxation time (t) of the material at different temperatures and is plotted as a function of temperature (Fig. 6). The relaxation time was calculated using the relation omaxt ¼ omaxRbCb ¼1, and is found to be independent of the sample geometrical factors and depends basically on the intrinsic properties of the material sample. The graph shows a steady increase in the relaxation time with temperature. This result suggests the presence of temperature dependent electrical relaxation phenomena in the material, which may be due to the migration of immobile species/defects. The typical variation appears to be of Arrhenius nature governed by the relation t ¼ t0exp[ Ea/kT]. An estimation of activation energy using—vs. 103/T plot gives a value of Et is (  0.8 eV). The electrical conductivity of the material was investigated at different temperatures over a range of frequencies. The bulk conductivity of the material was evaluated from the complex

S. Brahma et al. / Journal of Physics and Chemistry of Solids 73 (2012) 357–362

Relaxation Time (τ)

1E-3

1E-4

1E-5

1E-6

1E-7 1.25

1.50

1.75

2.00 2.25 2.50 (1000/T) K-1

2.75

3.00

3.25

Fig. 6. Variation of relaxation time (t) as a function of temperature.

conductivity ( 10  9 S cm  1) indicating a jump of nearly five orders of magnitude. This large variation of conductivity may be due to the creation of defects/vacancies at the higher temperature. This may be related to the lowering of grain boundary resistance (Rgb) resulting in lowering of the barrier to the mobility of charge carriers assisted by grain boundary conduction with rise in temperature. Fig. 7(inset) shows the variation of grain boundary conductivity as a function of temperature. It also shows Arrhenius type behavior indicating the thermally activated nature of grain boundary conduction, in close agreement with the results of (Fig. 3b). The activation energy was estimated to be  1.1 eV. This value of activation energy suggests a possibility of the mobility of oxide ion (O2  ) or oxide ion vacancies V00 o at higher temperatures (above 280 1C). Fig. 8(a) shows the variation of a.c. conductivity as a function of frequency at different temperatures. In the temperature range (250 1C–400 1C), the conductivity spectrum shows continuous dispersion in conductivity up to a certain frequency followed by a plateau at the high frequency region. The plateau region corresponds to the frequency independent d.c. conductivity and dispersion region corresponds to the frequency dependent part. The particular frequency at which change in slope of the conductivity

1E-4

150 200 250 300 350 400

1E-8

C C C C C C

-1

-1

Ac Conductivity σac(Ω cm )

D.C conductivity

1E-5

361

1E-6

1E-7

1E-8

1E-9

1E-10

1E-9 1.4

1.6

1.8

2.0

2.2 2.4 -1 (1000/T)K

2.6

2.8

3.0

3.2

1E-11 0.1

10

100

1000

1E-4 1kHz 10kHz 100kHz

1E-5

-1

-1

AC Condudctivity σac(Ω cm )

impedance spectrum by using the formula: sd.c. ¼1/Rnbl/A where Rb ¼bulk resistance, l¼sample thickness and A¼sample area [24]. Fig. 7 shows the variation of d.c.conductivity of NdLiMo2O8 as a function of temperature. A careful observation reveals two types of electrical conduction processes in the material over two different regions of temperatures. At higher temperature (130 1C–450 1C), the conductivity variation indicates an increase of conductivity with rise in temperature with a typical Arrhenius type behavior having linear dependence of logarithm of conductivity (log10 sdc) with the inverse of temperature ((103/T)K  1). This type of temperature dependence of d.c. conductivity indicates that the electrical conduction in the material is a thermally activated process. It can be explained in accordance with the relation:sd.c. ¼ s0exp(Ea/kT), where s0 ¼pre-exponential factor, Ea ¼activation energy and k¼Boltzmann constant. At the low temperature region (50 1C–130 1C), the conductivity variation shows small departure from linear behavior with change of slope. The activation energy value calculated from d.c. conductivity pattern was found to be 0.84 eV. A maximum conductivity (3.5942  10  4 S cm  1) was observed at 450 1C. This is a very high conductivity in comparison with room temperature

1

Frequency (kHz)

Fig. 7. Variation of d.c. conductivity as a function of temperature.

1E-6

1E-7

1E-8

1.5

2.0

2.5

(1000/T) K

3.0

3.5

-1

Fig. 8. a.c conductivity (a) Variation of a.c. conductivity as a function of frequency. (b) Variation of a.c. conductivity as a function of temperature.

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pattern takes place is known as the ‘‘hopping frequency’’ op, which shifts to higher frequency with rise in temperature. The physical phenomena gives an intuitive idea about the mechanism of the electrical conduction in the material. The electrical conduction takes place by the hopping mechanism described by Jonscher’s universal power law [25] governed by the relation: s(o) ¼ sd.c. þ A(o)n with 0ono1, where A is a pre-exponential factor dependent on temperature. The conductivity behavior is governed by the relation sa(o)n up to a temperature 200 1C. Above this temperature, Jonscher’s universal power law governs the conductivity relation. Further rise in the conductivity value with temperature indicates that the electrical conduction in the material is a thermally activated process. These results agree well with the observations from impedance spectrum analysis. Fig. 8(b) represents the variation of a.c. conductivity of the materials observed as a function of temperature at different frequencies. As temperature rises, the electrical conductivity at different frequencies approaches each other in the higher temperature region. These features in the conductivity pattern indicate that the electrical conduction in the material sample is a thermally activated process governed by the release of space charge [23].

4. Conclusions The article reports the systematic procedure of synthesis of NdLiMo2O8 polycrystalline ceramics. Electrical properties of NdLiMo2O8 were characterized using complex impedance spectroscopy. Impedance spectra lead to conclude that the conduction contribution is mainly due to bulk material (up to T¼270 1C) and grain boundary effects (for TZ280 1C). Frequency dependence of impedance pattern shows temperature dependent relaxation phenomena in the material. Evidences of electrode polarization effects of such ionically conducting polycrystalline material have also been noticed at T 4400 1C. The impedance spectrum results have been used to estimate the electrical conductivity properties. Variation of d.c. conductivity as a function of temperature shows non-linear variation for Tr130 1C and almost a linear variation for all TZ130 1C. The linear variation in the conductivity pattern is attributed to Arrhenius type thermally activated electrical transport phenomena. Sample activation energies estimated from the conductivity pattern and relaxation time pattern are almost the same. This indicates that the same type of charge carrier is responsible for both the electrical conduction and electrical

relaxation phenomena in the sample. The activation energy (  1.1 eV) estimated from grain boundary conduction plot suggests, the possibility of electrical conduction due to the mobility of oxide ion (O2  ) or oxide ion vacancies V00 o at higher temperatures. The frequency dependence of a.c. conductivity is found to follow Jonscher’s Universal power law.

Acknowledgment Sanjaya Brahma thanks Council of Scientific and Industrial Research (CSIR) for the award of research associateship and IIT Kharagpur for some initial experimental work. The author is also grateful to Dr Debakanta Samal for the valuable suggestions. References [1] H.J. Borchardt, P.E. Bierstedt, Appl. Phys. Lett. 8 (1966) 50. [2] (a) K. Aizu, A. Kumada, H. Yumoto, S. Ashida, J. Phys. Soc. Jpn. 27 (1969) 511; (b) W. Jeitschko, Acta Crystallogr., Sect. B: Struct. Sci. 28 (1972) 60. [3] G. Blasse, A. Brill, J. Chem. Phys. 45 (1966) 2350. [4] N. Ali, M.P. Hill, S. Labroo, J.E. Greedan, J. Solid State Chem. 83 (1989) 178. [5] P. Gall, P. Gougeon, M. Greeenblat, E.B. Jones, W.H. McCarroll, K.V. Ramanujachary, Croat. Chem. Acta 68 (1995) 849. [6] J.S. Gardner, B.D. Gaulin, S.H. Lee, C. Broholm, N.P. Raju, J.E. Greedan, Phys. Rev. Lett. 83 (1999) 211. [7] P. Lacorre, F. Goutenoire, O. Bohnke, R. Retoux, Y. Laligant, Nature 404 (2000) (2000) 856. [8] F. Goutenoire, O. Isnard, R. Retoux, P. Lacorre, Chem. Mater. 12 (2000) 2575. [9] R. Gautier, O.K. Anderson, P. Gougeon, J.F. Halet, E. Canadell, J.D. Martin, Inorg. Chem. 41 (2002) 4689. [10] R.D. Shannon, Acta Crystallogr., Sect. A: Found. Crystallogr. 32 (1976) 751. [11] J. Huang, J. Lories, P. Porcher, J. Solid State Chem. 43 (1982) 87. [12] S.H.I. Fanian, J. Meng, Y. Ren, Mater. Res. Bull. 30 (1995) 1401. [13] N. Taira, Y. Hinatsu, J. Mater. Chem. 12 (2002) 148. [14] M. de, F.V. de Moura, J. do, R. Matos, R.F. de Farias, Thermochim. Acta 414 (2004) 159. [15] S.S. Kandil, G.B. El-Hefnawy, E.A. Baker, Thermochim. Acta 414 (2004) 113. [16] J.M. Luiz, J.R. Matos, I. Giolito, M. Ionashiro, Thermochim. Acta 254 (1995) 209. [17] JCPDS NO-23-1195. [18] J.T.C. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 138. [19] S. Selvasekarapandian, M. Vijaykumar, Mater. Chem. Phys. 80 (2003) 29. [20] J.R. Macdonald, Impedance Spectroscopy Emphasizing Solid Materials and Systems, John Wiley & Sons, 1987 Chapter 4. [21] D.P. Almond, C.C. Hunter, A.R. West, J. Mater. Sci. 19 (1984) 3236. [22] A.R. James, K. Srinivas, Mater. Res. Bull. 34 (1999) 1301. [23] S. Chatterjee, P.K. Mahapatra, R.N.P. Choudhary, A.K. Thakur, Phys. Status Solidi A 201 (2004) 588. [24] A.J. Campbell, D.D.C. Bradley, J. Laubender, M. Sokolowski, J. Appl. Phys. 86 (1999) 5004. [25] A.K. Jonscher, Nature 267 (1977) 673.