Inorganic Chemistry Communications 40 (2014) 5–7

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Inorganic Chemistry Communications

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Influence of Sm3+ doping on the dielectric properties of CaCu3Ti4O12

ceramics synthesized via autocombustion

Alex Lopera a, Miguel Angel Ramirez b, Claudia García c,⁎, Carlos Paucar d, Jairo Marín c

a Universidad Nacional de Colombia, Faculty of Mines, Carrera 80, No. 65-223 Núcleo Robledo, Medellín, Colombiab Universidade Estadual Paulista, Câmpus Guaratinguetá, Av. Dr. Ariberto Pereira da Cunha, 333, Pedregulho, Guaratinguetá, 12516-410, Brazilc Universidad Nacional de Colombia, Physics School, Calle 59A, # 63-20, Medellín, Colombiad Universidad Nacional de Colombia, Chemistry School, Calle 59A, # 63-20, Medellín, Colombia

⁎ Corresponding author. Tel./fax: +57 4 4309327.E-mail addresses: aalopera@unal.edu.co (A. Lopera), m

(M.A. Ramirez), cpgarcia@unal.edu.co (C. García), cgpaucajhmarin@unal.edu.co (J. Marín).

1387-7003/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.inoche.2013.11.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 July 2013Accepted 20 November 2013Available online 26 November 2013

Keywords:CCTOSamariumCitrate–nitrate routeDielectric properties

Ca1 − xSmxCu3Ti4O12 (x = 0.0, 0.2, 0.3) electronic ceramics were fabricated via the chemical route using metalnitrate solutions in order to improve the dielectric properties of this ceramic. X-ray diffraction (XRD) analysisindicated the formation of a single CaCu3Ti4O12 (CCTO). Grain size of the samples doped with Sm3+ was in therange of 1–2 μm, as opposed to 50–100 μm in the pure samples of CCTO. The cutoff frequency with the dopingwas remarkably shifted, from 1 MHz (pure CCTO) to 10 MHz (doped CCTO). Meanwhile, the real dielectric (εr)and imaginary dielectric (ε″) constants showed a decrease as the doping was increased.

© 2013 Elsevier B.V. All rights reserved.

CaCu3Ti4O12 (CCTO) is an attractivematerial due to its high dielectricpermittivity (approximately 104–105), which is practically independentof temperature and frequency throughout the ranges from 100 K to400 K and from DC to 1 MHz, respectively [1,2].

This material does not exhibit a phase transition in the temperaturerange from 100 to 600 K [3]. The origin of the high dielectric constant inCCTO has not been properly explained. Some authors have attributedthe origin of the high permittivity of CCTO to its intrinsic crystal struc-ture [3,4]. One of the most accepted models to explain the electrical be-havior of CCTO is the internal barrier capacitance layer (IBCL) [5–7],where the special dielectric behavior of CCTO seems to be linked tothe presence of electrical in-homogeneities, such as semiconductinggrains and highly insulating grain boundaries [8].

High dielectric constant (εr)materials are very important in themin-iaturization of modern electronic devices such as capacitors, resonators,and filters [9,10]. In order to apply CCTO to microelectronic devices, it isvery important to improve its dielectric loss and dielectric constantwithin operating frequencies (above 1 MHz). In this regard, preliminarystudies have shown that the dielectric properties of CCTO are stronglydependent on the processing conditions aswell as the route of synthesisand dopant concentration [11,12]. CCTO doped with rare-earth ele-ments has been implemented in order to improve its dielectric proper-ties at high frequencies [13]. The decrease of the dielectric loss and the

argbrasil@yahoo.esr@unal.edu.co (C. Paucar),

ghts reserved.

shift of the cutoff frequency by introducing dopants such as La3+, Y3+,and Gd3+ has been previously reported [13–15].

Copper nitrate (Cu(NO3)2), Calcium nitrate (Ca(NO3)2) Ti-isopropoxide(Ti[OCH(CH3)2]4,) Samarium nitrate (Sm(NO3)2), and acid citric(C6H8O7H2O) were used as the starting materials. Standard solutionsof copper and samarium nitrates and Ti-isopropoxide were preparedand mixed using distilled water. The solution was heated on a hotplate magnetic stirrer at 70 °C until auto combustion occurred.

Ceramics of Ca1 − xSmxCu3Ti4O12 with x = 0.0 (CCTO), x = 0.2(CSCTO1), and x = 0.3 (CSCTO2) were sintered at 1050 °C for 10 h. Inthe diffractograms (Fig. 1), the formation of cubic perovskite CCTO as themain phase was confirmed. The lattice parameters were calculated byusing the least-squares refinement method. The lattice parameters forCCTO, CSCTO1, and CSTO2 are 7.392 Å, 7.394 Å, and 7.395 Å, respectively.These values are in good agreement with those previously reported [15].

Fig. 2 shows the SEM images of the surface of the CCTO doped andundoped with Sm+3. Fig. 2a illustrates an excessive grain growth withgrains of 50 μm, in comparison with grains of ∼10 μm in undopedCCTO. This phenomenon is associated with a process of liquid phasesintering assisted by CuO [16,17]. The presence of the Sm+3 produceda decrease in grain size (∼2 μm) and a narrower distribution of the par-ticle size (Fig. 2b).

Fig. 3a and b shows the real (ε′) and imaginary (ε″) parts of the per-mittivity complex for the CCTO dopedwith Sm3+ at room temperature.

The dielectric constant diminished with the presence of the dopant(i.e. at 100 kHz). ε′ for CCTO was 15,474, and for CCTO doped with0.2% and 0.3% of Sm+3, ε′ values were 5158 and 4126, respectively.The imaginary part of the dielectric constant (ε″) showed a decreaseat high frequency (i.e. at 2.5 MHz). ε″ of CCTO was 12,705, and ε′ for

Fig. 1. X-ray diffraction patterns for the system Ca1 − xSmxCu3Ti4O12 CSCTO2 (x = 0.2), CSCTO1 (x = 0.3), and CCTO (x = 0.0).

6 A. Lopera et al. / Inorganic Chemistry Communications 40 (2014) 5–7

CCTO doped with 0.2% and 0.3% of Sm+3 was 1378 and 1511, respec-tively. On the other hand, the constant dielectric of doped samplesshowed good stability between∼104–106 Hz, and the value of the cutofffrequency (f0) was shifted to 10 MHz, in comparison with the cutofffrequency for pure CCTO, which is near to 1 MHz.

Fig. 3c shows the dependence of M″ on the frequency at roomtemperature. As the dopant (Sm3+) content increases, M″ increasesat low frequencies. This indicates that the grain boundary capacitance

Fig. 2. SEM images of surface morphologies for (a)

decreases sharply with the presence of the dopant. Meanwhile, thegrain boundary relaxation frequency decreases with the content ofSm3 + .

In conclusion, pure phase Ca1 − xSmxCu3Ti4O12 ceramicswere prepared via the chemical route. XRD studies indicated an in-crease in the lattice parameter of the cubic structure of CSCTOdoped with Sm3+, due to the relative size difference between Ca2+

and Sm3+.

undoped CCTO, (b) CSCTO1, and (c) CSCTO2.

Fig. 3. Plots of (a) real dielectric constant (ε′), (b) imaginary dielectric constant vs.frequency and (c) electric modulus imaginary part (M″) for ceramics of doped CCTO CCTO(undoped) CSCTO1 (doped with samarium 0.2%) CSCTO2 (doped with samarium 0.3%).

7A. Lopera et al. / Inorganic Chemistry Communications 40 (2014) 5–7

The grain size of CCTO doped with Sm3+tended to decrease. Thepresence of the dopant in the structure of the CCTO inhibited abnormalgrain growth.

The dielectric constant of CCTO doped with Sm3+decreased. Thisphenomenon could be attributed to Maxwell_Wagner relaxation. Thiseffect arises from the charge accumulation at the interface of materialswith different conductivities.

Doping with Sm3+ shifted the CCTO cutoff frequency, f0, toward ε′values near 10 MHz, in comparison with pure CCTO, which exhibiteda cutoff frequency value near 1Mhz. Doping with Sm3+ can optimizethe dielectric properties of CCTO.

Acknowledgment

This paperwas supported by project 3028 of the Fundación Banco dela Republica, Colombia.

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