Chem. Mater. 2007, 19, 4077-4082
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Structure and Microwave Dielectric Properties of Sr2+nCe2Ti5+nO15+3n (n e 10) Homologous Series Ganesanpotti Subodh,† Jose James,† Mailadil T. Sebastian,† Roberto Paniago,‡ Anderson Dias,§ and Roberto L. Moreira*,‡ Materials and Minerals DiVision, Institute of Interdisciplinary Sciences and Technology, TriVandrum 695 019, India, Departamento de Fı´sica, Instituto de Cieˆ ncias Exatas, UniVersidade Federal de Minas Gerais, CP 702, Belo Horizonte MG 30123-970, Brazil, and Departamento de Quı´mica, Instituto de Cieˆ ncias Exatase e Biolo´ gicas, UniVersidade Federal de Ouro Preto, Ouro Preto MG 35400-000, Brazil ReceiVed March 31, 2007. ReVised Manuscript ReceiVed May 23, 2007
Nominally Sr2+nCe2Ti5+nO16+3n dielectric ceramics were prepared by conventional solid-state ceramic route. The structure and microstructural features of the ceramics were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. At optimized sintering conditions, it is shown that Ce4+ ions reduce to Ce3+ ions, which leads to the correct formula Sr2+nCe2Ti5+nO15+3n for the materials. The ceramics present a cubic SrTiO3-like structure, where Ce3+ ions and their associated vacancies randomly share the A-sites with the Sr2+ cations. Therefore, this solid solution could be alternatively described by Sr1-3x/2CexTiO3, where x e 0.40 is not necessarily an integer. For 0 e n e 10 (0.40 > x > 0.133), these materials have dielectric constant and Quxf in the range of 113-185 and 6000-11000 GHz (at 2 GHz), respectively. However, the relatively high positive temperature coefficients of resonance frequencies decrease substantially to values five times lower than those commonly observed for SrTiO3.
Introduction The dramatic progress in wireless communication and microelectronic systems has stimulated the demand for new microwave dielectric resonator (DR) materials with high dielectric constant (r), high-unloaded quality factor (Qu), and low-temperature coefficient of resonant frequency (τf).1-6 One of the most suitable ways for miniaturization of microwave devices is to use high-dielectric-constant and lowloss DRs. Communication systems operating in the microwave frequency range require low-loss and high-dielectricconstant (r > 20) materials as basic components in oscillators, filters, and antennas.4,6,7 The application of such DR materials ensures better performance along with reduction of weight and overall dimensions of the microwave devices.5,6,8 In general, as the dielectric constant increases, Qu decreases and τf increases.5,6 The development of microwave ceramics with high dielectric constant and good thermal stability together with small dielectric loss is a challenging problem in the area of DR materials research. * Corresponding author. Tel: 55-31-3499-5624. E-mail: fisica.ufmg.br. † Institute of Interdisciplinary Sciences and Technology. ‡ Universidade Federal de Minas Gerais. § Universidade Federal de Ouro Preto.
(1) (2) (3) (4) (5) (6) (7) (8)
Among the polytitanate dielectric materials, Srn+1TinO3n+1 and Can+1TinO3n+1 and their solid solutions have been extensively studied9-12 because of their high r and low loss with the potential to miniaturize the devices. The microwave dielectric properties of SrTiO3 (r ) 290, Quxf ) 3000 GHz, and τf ) 1650 ppm/°C) and CaTiO3 (r ) 162, Quxf ) 13 000 GHz, and τf ) 859 ppm/°C) are reported.9-12 However, these materials have relatively high positive τf values. Okawa et al.13 studied the microwave dielectric properties of the BanLa4Ti3+nO12+3n homologous series. For n ) 1, these materials possess r ) 46, Quxf ) 46 000 GHz, and low τf (-11 ppm/°C).13 Recently, Sreemoolanadhan et al.14 reported the microwave dielectric properties of the ceramics in a BaO-2CeO2-nTiO2 system with reasonably good microwave dielectric properties. However, the number of low-loss ceramic materials with high dielectric constant (r > 100) remains very small. Bamberger et al.15 reported the existence of a new compound Sr2Ce2Ti5O16 and a series of solid solutions of the type Sr2+nCe2Ti5+nO16+3n (n e 7). However, the crystal structure and phase purity of these
bmoreira@
Higuchi, Y.; Tamura, H. J. Eur. Ceram. Soc. 2003, 23, 2683. Wersing, W. Curr. Opin. Solid State Mater. Sci. 1996, 1, 715. Cava, R. J. J. Mater. Chem. 2001, 11, 54. Sebastian, M. T.; Santha, N.; Bijumon, P. V.; Axelsson, A. K.; Alford, N. M. J. Eur. Ceram. Soc. 2004, 24, 2583. Nenasheva, E. A.; Kartenko, N. F. J. Eur. Ceram. Soc. 2001, 21, 2697. Reaney, I. M.; Iddles, D. J. Am. Ceram. Soc. 2006, 89, 2063. Sebastian, M. T.; Ohsato, H. Mater. Integr. 2005, 18, 6. Fiedziuszko, S. J.; Hunter, I. C.; Itoh, T.; Kobayashi, Y.; Nishikawa, T.; Snitzer, S. N.; Wakino, K. IEEE Trans. MicrowaVe Theory Tech. 2002, 50, 706.
(9) Wise, P. L.; Reaney, I. M.; Lee, W. E.; Price, T. J.; Iddles, D. M.; Cannell, D. S. J. Eur. Ceram. Soc. 2001, 21, 2629. (10) Wise, P. L.; Reaney, I. M.; Lee, W. E.; Price, T. J.; Iddles, D. M.; Cannell, D. S. J. Eur. Ceram. Soc. 2001, 21, 1723. (11) Wu, L.; Chen, Y. C.; Chen, L. J.; Chou, Y. P.; Tsai, Y. T. Jpn. J. Appl. Phys. 1999, 38, 5612. (12) Chen, X. M.; Li, L.; Liu, X. Q. Mater Sci. Eng., B 2002, 99, 255. (13) Okawa, T.; Kiuchi, K.; Okabe, H.; Ohsato, H. Jpn. J. Appl. Phys. 2001, 40, 5779. (14) Sreemoolanadhan, H.; Sebastian, M. T.; Ratheesh, R.; Blachnik, R.; Woehlecke, M.; Schneider, B.; Neumann, M.; Mohanan, P. J. Solid State Chem. 2004, 177, 3995. (15) Bamberger, C. E.; Haverlock, T. J.; Kopp, O. C. J. Am. Ceram. Soc. 1994, 77, 1659.
10.1021/cm070894n CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007
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Table 1. Chemical and Structural Parameters of Sr2+nCe2Ti5+nO15+3n Ceramics n
fully oxidized material
sintering T (˚C)
Ce4+/Cetotal
fully reduced material
(Å)
ttheor
a (Å)
tobs
0 1 2 3 4 5 6 7 8 9 10
Sr2Ce2Ti5O16 Sr3Ce2Ti6O19 Sr4Ce2Ti7O22 Sr5Ce2Ti8O25 Sr6Ce2Ti9O28 Sr7Ce2Ti10O31 Sr8Ce2Ti11O34 Sr9Ce2Ti12O37 Sr10Ce2Ti13O40 Sr11Ce2Ti14O43 Sr12Ce2Ti15O46
1300 1375 1375 1375 1375 1375 1375 1375 1400 1400 1400
0.10 0.07 0.07 0.06 0.06 0.08 0.05 0.05 0.04 0.04 0.03
Sr2/5Ce2/501/5TiO3 Sr3/6Ce2/601/6TiO3 Sr4/7Ce2/701/7TiO3 Sr5/8Ce2/801/8TiO3 Sr6/9Ce2/901/9TiO3 Sr7/10Ce2/1001/10TiO3 Sr8/11Ce2/1101/11TiO3 Sr9/12Ce2/1201/12TiO3 Sr10/13Ce2/1301/13TiO3 Sr11/14Ce2/1401/14TiO3 Sr12/15Ce2/1501/15TiO3
1.380 1.394 1.402 1.408 1.412 1.414 1.418 1.420 1.422 1.423 1.425
0.9874 0.9926 0.9954 0.9976 0.9999 0.9998 1.0012 1.0019 1.0026 1.0031 1.0036
3.878(0) 3.879(8) 3.884(6) 3.887(0) 3.890(0) 3.891(0) 3.892(0) 3.892(5) 3.892(8) 3.894(1) 3.894(4)
0.9854 0.9863 0.9907 0.9933 0.9960 0.9968 0.9977 0.9977 0.9984 0.9996 0.9998
compounds were not well-established. Because of the similarity between these materials with the BaO-2CeO2nTiO2 compounds, we decided to investigate the Sr2+nCe2Ti5+nO16+3n solid solutions, prepared under the same conditions employed by Bamberger et al.15 The material with n ) 0, assumed to be Sr2Ce2Ti5O16, presented high r ) 113, Quxf ) 8000 GHz, and τf ) 306 ppm/°C.16 In the present paper, we report the preparation, crystal structure, and microwave dielectric properties of the Sr2+nCe2Ti5+nO16+3n ceramics for the first time. At optimized sintering conditions, it was shown that Ce4+ ions reduce to Ce3+ ions, which lead to the correct formula Sr2+nCe2Ti5+nO15+3n for these solid solutions. This result explains its structural similarity with SrTiO3, already presented in the work of Bamberger et al.15 and, consequently, the dielectric response of the system.
Low-frequency dielectric properties were measured by using a LCR meter (Hioki 3532-50). Silver paste was used as electrodes for samples having a diameter of about 11 mm and a thickness of 1 mm, for low-frequency measurement. The microwave dielectric properties were measured by a vector network analyzer (8753 ET, Agilent Technologies). The unloaded quality factor and the dielectric constant of the samples were measured by the resonance method using the TE01δ mode.17 The specimens were placed on a low-loss quartz spacer inside a copper cavity, whose inner side was silver-plated. The use of low-loss single-crystal quartz spacer reduces the effect of losses due to the surface resistivity of the cavity.18 The diameter of the cavity was about 4 times larger than that of the sample for better isolation of the excited TE01δ mode.18 The τf was measured by noting the variations of TE01δ mode frequency with a temperature in the range of 25-75 °C.
Results and Discussion Experimental Section Nominally pure Sr2+nCe2Ti5+nO16+3n (n e 10) ceramics were prepared by the solid-state ceramic route. High-purity SrCO3 and TiO2 (99.9+%, Aldrich Chemical Co., Milwaukee, WI) and CeO2 (99.99%, Indian Rare Earth Ltd., Udyogamandal, India) were used as starting materials. Stoichiometric amounts of powder mixtures were ball-milled in distilled water medium using yttria-stabilized zirconia balls in a plastic container for 24 h. The slurry was dried, ground, and calcined at 1100 °C for 5 h. The calcined material was ground into a fine powder and divided into different batches for optimizing the sintering temperature. Around 4 wt % polyvinyl alcohol (PVA) (molecular weight 22 000, BDH Lab Suppliers, England) was added to the dried powders and again ground into fine powder. Cylindrical pucks of about 6-7 mm height and about 14 mm diameter were made by applying a pressure of 100 MPa. These compacts were then fired at 600 °C for 30 min to burn the binder before sintering at temperatures ranging from 1300 to 1400 °C, for 2 h. Crystal structure and phase purity of the powdered samples were studied by X-ray diffraction (XRD), using Ni filtered Cu-KR radiation (Rigaku X-ray diffractometer, D-max, Japan). X-ray photoelectron spectroscopic (XPS) measurements were performed in a VG ESCALAB 220i-XL system, using Al-KR radiation (1486.6 eV) and base pressure of 1 × 10-10 mbar. Survey XPS spectra were collected with pass energy of 50 eV, whereas Ce-3d spectra were collected with pass energy of 20 eV. Measurements were done before and after (in situ) soft sputtering procedure (2 keV, Ar+), which removes approximately 20 Å of surface contaminants. Sintered samples were thermally etched for 20 min at a temperature of about 25 °C below the sintering temperature, and the surface morphology was studied using a scanning electron microscope (SEM, JEOL-JSM 5600 LV, Tokyo, Japan). (16) Subodh, G.; Sebastian, M. T. Mater. Sci. Eng., B 2007, 136, 50.
Figure 1 shows the XRD patterns for several Sr2+nCe2Ti5+nO16+3n ceramics sintered at 1300 °C/2 h (n = 0), 1375 °C/2 h (n ) 1-7), and 1400 °C/2 h (n ) 8-10). The results showed that the materials present a perovskite-type structure, quite similar to cubic SrTiO3, as already noticed by Bamberger et al.15 Neither secondary phase nor the presence of superstructure peaks were detected within the experimental accuracy. Also, the Bragg peaks are relatively sharp, indicating a high degree of crystallinity. If some samples belong to a non-cubic system, the ferroelastic distortion would be faint enough to not being discerned by conventional XRD measurements. The measured cubic cell parameters, presented in Table 1, appeared to increase with n, from 3.878(0) Å (n ) 0) to 3.894(4) Å (n ) 10). SrTiO3 was prepared under similar conditions and using the same chemicals. The lattice parameter of SrTiO3 was found to be 3.901 Å. Hence, it can be concluded that as the value of n increases, the lattice parameter (a) and the cell volume of Sr2+nCe2Ti5+nO16+3n materials approach those of SrTiO3. The fact that these complex materials could belong to a perovskite structure raises an intriguing question. For instance, the n ) 0 material would be represented by Sr0.4Ce0.4TiO3+δ in the perovskite form (δ ) 0.2). How could one account for the oxygen excess? The situation for the other materials (with larger n values) would be less dramatic (smaller δ). Some amount of Ce+4 could be reduced to the Ce+3, as observed in some Ba-polytitanates sintered in air (17) Krupka, J.; Derzakowski, K, D.; Riddle, B.; Jarvis, J. B. Meas. Sci Technol. 1998, 9, 1751. (18) Surendran, K. P.; Varma, M. R.; Mohanan, P.; Sebastian, M. T. J. Am. Ceram. Soc. 2003, 86, 1695.
Structure and Properties of Sr2+nCe2Ti5+nO15+3n (n e 10)
Figure 1. XRD patterns of nominally prepared Sr2+nCe2Ti5+nO16+3n ceramics sintered at 1300 °C for n ) 0, 1375 °C, for 1 e n e 7, and 1400 °C for n ) 8-10.
Figure 2. Survey XPS spectra for the sample n ) 0, before and after sputtering for removing surface contaminants.
at high temperatures.14,19 To investigate the oxidation states of cerium ions in our materials, we undertook XPS measurements. The XPS experiments allowed the quantification of Ce3+ and Ce4+, as well as the determination of the relative atomic concentrations of Sr, Ce, Ti, and O in the samples. Survey spectra (see Figure 2 for the sample with n ) 0) have shown that all samples contain, as expected, Sr, Ce, Ti, and O and a small amount of carbon contaminant, even after soft argon ion sputtering. The obtained atomic concentrations of Sr, Ti, and O relative to Ce, as calculated from the main photoemission peaks of those elements in XPS spectra, agree well with the stoichiometry for all materials, within the experimental errors. For these calculations, a background subtraction by the Shirley’s method was employed, and we have used the sensitivity factors from Scofield.20 The spectra performed in the Ce-3d binding (19) (20) (21) (22)
Kolar, D. Mater. Res. Soc. Symp. Proc. 1997, 453, 425. Scofield, J. H.; Hartree-Slater J. Electron. Spectrosc. 1976, 8, 129. Tsunekawa, S.; Fukuda, T.; Kasuya, A. Surf. Sci. 2000, 457, L437. Vercaemst, R.; Poelman, D.; VanMeirhaaeghe, R. L.; Fiermans, L.; Lafle`re, W. H.; Cardon, F. J. Luminesc. 1995, 63, 19.
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Figure 3. XPS spectra in the Ce-3d binding energy region for selected samples, from n ) 0-10. Peaks ascribed to Ce3+ and Ce4+ cations are indicated.
energy region showed that all samples contain more than 90% of Ce3+ on the surface. The presence of Ti+3 ions was not detected in any sample. Figure 3 shows the results for samples with n ) 0, 1, 5, 7, 9, and 10. The quantification of the amounts of Ce3+ and Ce4+ was done according to refs 14, 21, and 22, and the results are presented in Table 1. The results from XPS demonstrate that the compounds are Sr2+nCe2Ti5+nO15+3n instead of Sr2+nCe2Ti5+nO16+3n, with almost fully reduced Ce ions. This explains why they could form stable perovskite structures without oxide segregation to secondary phases. Cerium reduction under annealing has been observed by different authors.19,23 For the present case, well-sintered samples with good high-density dielectric properties were obtained only for sintering temperatures in the range 1300-400 °C. As can be observed in Table 1 for samples with optimized dielectric response (and also in ref 19 for Ba polytitanate), the residual amount of Ce4+ decreases with increasing sintering temperature. In the worst case, with 10% of Ce4+ for the n ) 0 sample sintered at 1300 °C, this perovskite has the formula Sr0.4Ce0.4TiO3.02. For the other samples, the amount of excess oxygen (δ) is still lower. Thus, we will alternatively describe the fully reduced system by Sr1-3x/2CexTiO3. We notice that the reduction of the lattice constants with decreasing n (or increasing x) presented in Table 1 supports this formula and the attribution of Ce ions in the perovskite A-sites, because Ce ions are smaller than Sr, but larger than Ti ions.24 Once we have determined the chemical features that support the crystalline perovskite structure of the wellsintered materials, let us present the investigations of their microstructure and dielectric properties, to show how the materials properties evolve with sintering temperature. SEM images of thermally etched Sr2+nCe2Ti5+nO15+3n ceramics with n ) 1, 3, 5, and 7 are shown in Figure 4. All the materials have homogeneous grains (regular shapes) with no secondary phases (similar tonalities), in agreement with our (23) Beyreuther, E.; Grafstro¨m, S.; Eng, L. M.; Thiele, C.; Dorr, K. Phys. ReV. B 2006, 73, 155425. (24) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
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Figure 4. SEM micrographs for the Sr2+nCe2Ti5+nO15+n samples sintered at 1375 °C/2 h, for (a) n ) 1, (b) n ) 3, (c) n ) 5, and (d) n ) 7.
XRD results. Also, the micrographs reveal well-packed grains with sizes in the range 3-10 µm and very low porosities. However, a relatively larger grain size is observed for n ) 1 ceramics compared to other materials. The sintering temperatures of these Sr2+nCe2Ti5+nO15+3n ceramics were optimized on the basis of the high-density dielectric constant (r) and unloaded quality factor (Quxf). Figure 5 shows the variation of density, microwave r, and Quxf for the solid solutions as functions of the sintering temperature. For 1 e n e 7, as the sintering temperature increases, the density, r and Quxf increase and reach their maxima at 1375 °C. However, for n ) 5, the maximum density and r are observed at 1400 °C, decreasing subsequently for higher sintering temperatures. For n ) 0, the best dielectric response was obtained by sintering at 1300 °C,16 whereas for the samples with n ) 8-10 (not shown in Figure 5), the optimized temperature was 1400 °C. The Sr2+nCe2Ti5+nO15+3n ceramics showed 94-96% densification at the optimized sintering temperature. However, the densities of the specimens decreased for temperatures above 1375 °C. The increasing of the dielectric constant and Quxf with sintering temperature is due to the improved density achieved. On the other hand, the dielectric properties were degraded because of the exaggerated grain growth observed at elevated temperatures. The dielectric constants and electrical conductivities (σ) for the optimized materials at 1 MHz and the microwave dielectric properties at 2 GHz are given in Table 2. The radio frequency dielectric constant increases with n, as also observed in the microwave region. We noticed that the values at 1 MHz and 2 GHz are very close for each sample, indicating a nonpolar character for their structures. However, the conductivity of the specimens decreases with n, which can be explained by the reduction of A-site vacancies and Ce ions with n. The conduction mechanism in this system could be quite complex because of the presence of several
Table 2. Radio Frequency (1 MHz) and Microwave (2 GHz) Dielectric Properties of Sr2+nCe2Ti5+nO15+3n (or Sr1-3x/2CexTiO3) Ceramics 1 MHz n
x
r
σ (µS/m)
0 1 2 3 4 5 6 7 8 9 10 SrTiO3 (single crystal)
0.4 0.333 0.286 0.25 0.222 0.2 0.182 0.167 0.154 0.143 0.133 0
110.0 119.8 134.8 134.8 148.0 153.2 166.3 170 201 203 231 280
10.30 19.76 12.06 8.67 7.454 5.776 5.495 4.612 2.080 0.512 0.184 228.3
2 GHz r
Quxf
τf (ppm/°C)
113 8000 123 10000 136 10800 143 11000 150 9600 157 9300 167 8000 173 7800 179 8000 185 6000 no resonance 290 3000
306 392 428 478 497 544 601 637 724 789 1650
charged species besides the possibility of charge transfer between Ce3+ and Ce4+ ions. This point deserves further investigation, being beyond the scope of the present work. The variations of the microwave dielectric constant and Quxf for the Sr2+nCe2Ti5+nO15+3n (or Sr1-3x/2CexTiO3) ceramics at the optimized sintering temperature are presented in Figure 6 as functions of the cerium content (x). r decreases monotonically with x (from 290 to 113) for values varying from 0 (pure SrTiO3) to 0.40. Initially, the Quxf for the materials increases with x, reaches a maximum value of 11 000 GHz for x ) 0.25, and then decreases for larger x. As is well-known, SrTiO3 is an incipient ferroelectric,25 which is responsible for its relatively high dielectric constant. For that reason, it presents a large deviation from the Shannon’s additive polarizability rule for determining dielectric constants in oxides.26 By using that rule and the Clausius-Mosotti equation, we only obtain a dielectric (25) Mu¨ller, K. A.; Burkard, H. Phys. ReV. B 1979, 19, 3593. (26) Shanon, R. D. J. Appl. Phys. 1993, 73, 348.
Structure and Properties of Sr2+nCe2Ti5+nO15+3n (n e 10)
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for the A-site ions and their molecular volumes. The variations in both variables with x are very small, so that the dielectric constant predicted by this method varies only from 40.0 (x ) 0) to 36.7 (x ) 0.40). Because the measured dielectric constants of all materials investigated here are far above the predicted values, we concluded that the Sr1-3x/2CexTiO3 materials should also be incipient ferroelectrics. The observed variations in their dielectric constants would likely be due to the hardening of the soft phonon mode by the influence of the extrinsic Ce ions and their associated vacancies. The behavior of the phonon modes for these materials is being investigated in detail by infrared and Raman techniques and will be published elsewhere. Concerning the quality factor behavior, we believe that its first increase with x is a direct consequence of the soft mode hardening described above (r decreasing). However, above x ) 0.25, the Quxf decreasing should be due to the disorder introduced in the system, which would reduce the phonon lifetimes of the different polar phonon modes. Let us now discuss the behavior of the temperature coefficient of the resonance frequency. This parameter is often discussed in association with the tolerance factor (t), which for our system can be written as t)
Figure 5. Density, microwave dielectric constant and quality factor as functions of sintering temperature for Sr2+nCe2Ti5+nO15+n ceramics with 1 e n e 7.
〈RA〉 + RX
x2(RB + RX)
(1)
where is the average ionic radius in the A-site cation, RB and RX are the ionic radius of the B (Ti) cation and X (O) anion, respectively. In Table 1, we present two sets of t values. The first one (ttheor) was obtained by averaging the amount of Ce3+, Ce4+, and Sr2+ in the A-site. Here, conversely to the calculations of dielectric polarizabilities, the vacancies do not enter in the average, because they do not introduce attractive nor repulsive forces. The second one (tobs) was obtained on the basis of a treatment proposed by Jiang et al.27 and improved by some of us.28 In this model, the predicted lattice parameter (apred) for cubic perovskites is given by apred ) 2β(rB + rX) + γt - δ
(2)
where the parameters β, γ, and δ were determined for a large set of materials within 1% accuracy as 0.9109, 1.1359, and 0.7785, respectively. Therefore, by knowing the lattice parameters and the radii of B and X, eq 2 allows us to obtain the tolerance factors without using information on the A-site. For SrTiO3, using the parameters above and the ionic radii for Ti and O, we obtained apred ) a + 0.024.28 Therefore, to obtain a higher-precision tolerance factor, we used for our SrTiO3-based materials t ) [a + 0.024 - 2β (rB + rX) + δ]/γ Ce3+
(3)
Figure 6. Dependence of microwave r and Quxf with content (x) for the Sr2+nCe2Ti5+nO15+n (renamed Sr1-3x/2CexTiO3) solid solutions, at optimized sintering temperatures. SrTiO3 values are included for x ) 0.
The values of ttheor and tobs presented in Table 1 for the solid solutions are very close, increasing our confidence in
constant of 40.0 for SrTiO3. For all the materials studied here, we have also calculated the predicted dielectric constants from this method using the average polarizability
(27) Jiang, L. Q.; Guo. J. K.; Liu, H. B.; Zhu, M.; Zhou, X.; Wu, P.; Li, C. H. J. Phys. Chem. Sol. 2006, 67, 1531. (28) Moreira, R. L.; Dias, A. J. Phys. Chem. Solids 2007, 68, DOI: 10.1016/ j. jpc.2007.03.050.
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with decreasing tolerance factors, i.e., positive τf, which decreases down to 306 ppm/°C when the tolerance factor approaches 0.985. In this picture, the decreasing of this parameter with increasing Ce content in the solid solutions has a quite simple structural origin. Anyway, even though the τfs of the materials are still relatively high, they represent a good advance aiming to technological applications in the microwave region. Conclusions
Figure 7. Variation of temperature coefficient of resonant frequency with the observed tolerance factor for Sr2+nCe2Ti5+nO15+3n materials. Pure SrTiO3 is also showed for comparison.
the methods. We noticed that the experimental values (tobs) were obtained without any hypothesis about the ions that are occupying the A-site. Therefore, we prefer to use these values in the subsequent analysis. We can see from Table 1 that for x increasing from 0.133 (n ) 10) to 0.40 (n ) 0), t decreases monotonically from about 1.00 to 0.9854. This latest value is slightly above the inferior limit for cubic perovskites (0.985).6 Therefore, according to this result, all materials appear to belong to a cubic perovskite structure, as assumed previously. Finally, Figure 7 presents τf as a function of the “observed” tolerance factors for all materials (including pure SrTiO3). This figure shows the classical behavior for cubic systems
In Sr2+nCe2Ti5+nO16+3n dielectric ceramics sintered at temperatures above 1300 °C, Ce4+ ions reduce to Ce3+, which explains the perovskite-like structure (prototype SrTiO3) shown by the materials. These compounds can be written as Sr1-3x/2CexTiO3, for x e 0.40. For all materials, the unit-cell parameters remain close to that of SrTiO3, so that the Ce3+ ions and their associated vacancies could not present longrange ordering, but rather, must randomly share the A-site with the Sr2+ cations, which would give an average cubic structure, compatible with the observed behavior of the structural (t) and dielectric (r, τf) parameters investigated. The absence of ordering and of indications of any structural change lead us to believe that x must be a real number and that the average structure could be the aristotype Pm3hm. The measured dielectric properties are competitive to that of the existing high-dielectric-constant microwave ceramics. Acknowledgment. The authors are grateful to the Department of Science and Technology, New Delhi, India, and to Brazilian agencies MCT/CNPq, FINEP, and FAPEMIG for the financial assistance. G.S. is thankful to Council of Scientific and Industrial Research, India, for a Junior Research Fellowship. CM070894N
