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Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation
Applied Surface Science 253 (2006) 1856–1860 www.elsevier.com/locate/apsusc
Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation Huiling Du a,*, Xi Yao a, Xianfeng Zhang b, Huimin Weng b b
a Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an 710049, China Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
Received 4 January 2006; received in revised form 12 March 2006; accepted 13 March 2006 Available online 2 May 2006
Abstract Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi1.5Zn0.5)(Zn0.5x/3TixNb1.52x/3)O7 (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I2 in the Tirich sample. At 1 MHz their dielectric constant (e0 ) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan d) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant. # 2006 Elsevier B.V. All rights reserved. PACS: 78.70.Bj; 74.62.Dh; 77.22.Ch Keywords: Positron annihilation; Pyrochlore; Complex defects; Dielectric constant
1. Introduction Dielectric ceramics are widely used with advances in microelectronic technologies microwave communication, and high dielectric constant ceramics have received intensive attention in reducing the size of microelectronic circuits, where high permittivity and low dielectric loss are required for many applications [1,2]. Pyrochlore-type oxides with the general formula Bi1.5ZnNb1.5O7 (BZN) have been attracting a lot of attention because of their excellent dielectric constant and low firing temperature. Much work has been performed on improving the dielectric properties, especially in enhancing the permittivity and reducing dielectric loss [3–7]. Most of this attention has focused on pyrochlore-based ceramics that contain highly polarizable cations such as Ti4+ and Ta5+ on the B-sites. Among them, (Bi1.5Zn0.5)(Ti1.5Nb0.5)O7 (BZNT) is one of the most promising materials due to its stability and high dielectric constant (210). We have shown the beneficial
* Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (H.L. Du). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.026
aspects of the tetravalent Ti, Sn, Zr and Ce incorporation into Bsite of BZN pyrochlore [8,9]. An understanding of the relationship between dielectric properties and structure, especially the structural origin of high permittivity and dielectric relaxation, is thus of great relevance to the development of materials appropriate of such systems. Lattices of crystalline materials are not perfect on the atomic scale since point defects always exist. Defects can dramatically change physical properties of materials, even if their concentration is very low. Therefore, knowing the defects in dielectrics and understanding the structural origin of dielectric properties are of great importance. Although preliminary results on the crystal imperfections are traditionally obtained by the conventional analytical tools, such as XRD, SEM and TEM [10–12], the analysis of the point defects and lattice disorder at the ppm level in the high-Q dielectric ceramics obviously requires a novel approach. In this situation positron annihilation spectroscopy is a powerful technique to detect a small amount of related defects, especially a very small vacancy-type of defects and others, which are beyond the resolution limit of the electron microscopy [13]. Thus, this technique has been widely used for detecting point defects in various materials, such as
H.L. Du et al. / Applied Surface Science 253 (2006) 1856–1860
metals and semiconductors [14–16]. With the Doppler broadening parameters and positron lifetime, one can get various information about defect properties. Point defects in perovskite-type oxide such as BT and PLZT have been investigated by using PAT recently and the results show that positrons can be a useful probe for studying vacancytype defects in such materials [17–20]. Unlike the simple perovskites, the point defect chemistry of the pyrochlores is largely unknown and never been reported. Hence, by applying positron annihilation technology, it would be possible to detect and characterize trace amounts of these lattice defects. The present study is mainly aimed at evaluating the nature of microdefects introduced by Ti ions occupied at B-site of BZN dielectrics. In this contribution, we further outline the influence of structural defects in the pyrochlores on dielectric properties.
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Fig. 1. Dielectric constant and lattice constant of BZTN as a function of amounts of Ti.
2. Materials and experimental technique
3. Results and discussion
constant generally diminishes linearly with increasing temperature. The tangent loss increases with increase of temperatures particularly, beyond 375 8C. The increase in tangent loss at higher temperature might be due to the formation of higher concentration of charge carriers. The base ceramic BZN (x = 0) has a dense crystal microstructure with bigger grains from 4 to 7 mm at 1000 8C. With the substitution of Ti4+, the extent of grain abnormally growing up was effectively controlled. For example, the grain size of BZNT2 (x = 0.5) and BZNT4 (x = 1.5) is about 2–3 and 1 mm at 1000 8C, respectively. That indicates Ti4+ substitutions are effective for depressing the grain growing. In general, the concentration of grain boundaries is mainly determined by grain size, the sample with larger grain has lower grain boundary concentration. The difference of grain size results mainly from the influences of Ti incorporation on the grain growth because the series samples were prepared at same conditions. There may have two causes needing consideration, the one is difference in the sintering mechanism induced due to Ti concentration, the other is the affect of defects induced due to Ti concentration. It is known that the diffusivity of vacancy-related defects is slowest during sintering. Thus an increase of vacancy concentration in the Ti-rich samples would
Considering the ionic radius of Zn2+, Nb5+ and Ti4+ with eightcoordination number, we assumed that Ti4+ can co-substitute Zn2+ and Nb5+ at B-site. The mechanism of substitution can be presented: 3Ti4+ ! Zn2+ + 2Nb5+. Therefore, the criterion of the charge neutrality can also be satisfied. Our recent research [9] revealed that titanium ions can co-substitute for Zn and Nb ions at B-site as solid solutions for 0 x 1.5 according to the X-ray diffraction results, inducing the decrease of lattice constant linear while remaining cubic pyrochlore phase. The composition dependence of the lattice constant and dielectric constant is shown in Fig. 1. At 1 MHz their dielectric constant (e0 ) varies from 150 for Ti-free system to 210 for Ti-rich system and loss tangent (tan d) remains rather low level. The temperature dependency of the dielectric constant and the dissipation factor at 1 MHz is shown in Fig. 2. There is no significant peak observed in these curves, and the dielectric
Fig. 2. The temperature dependence of dielectric behaviors of BZNT pyrochlores.
All samples of the base composition (Bi1.5Zn0.5)(Zn0.5x/ 3TixNb1.52x/3)O7 (x = 0, 0.25, 0.5,1.0, 1.5) were prepared by conventional solid-state reaction methods from mixtures of Bi2O3, ZnO, Nb2O5 and TiO2. All of the reagents were of 99.9% purity. The mixtures were calcined at 800 8C in air for 2 h. The obtained powders were milled for 4 h, pressed into pellets and sintered at 1000–1080 8C for 2 h in air and then furnace-cooled. The obtained ceramic density accounted for above 95% of the theoretical value. The positron lifetimes were measured with a fast–fast coincidence spectrometer with a time resolution of 210 ps. We collected 1.5 106 annihilation events in each spectrum. As a positron source 22Na was sealed in a Ni-foil and the experiments were sandwiched between two identical samples. All measurements were carried out at 300 K and the positron annihilation lifetime spectra were analyzed using the program Positronfit-Extended. Observations of Doppler broadening distributions with an energy dispersive Ge detector were also performed. The Doppler broadening of the annihilation peak is characterized by the lineshape parameters S and W.
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H.L. Du et al. / Applied Surface Science 253 (2006) 1856–1860
suppress the growth of grains, which results in small-grain microstructure. In order to investigate the defect properties, PAS measurement on the BZNT ceramics was performed at room temperature. According to the two-state trapping model, the average positron lifetime tave and the bulk lifetime tb are calculated from the fitted lifetimes and their intensities by the following equations t ave ¼ I1 t1 þ I2 t2 tb ¼
1 1 ¼ lb ðI1 =t1 Þ þ ðI2 =t2 Þ
and the positron trapping rate k k¼
Fig. 3. Relationship of positron lifetime t2 and I2 parameters to Ti content.
tm tb 1 t2 tm tb
In general, for the ceramic materials, the t2 should be regarded as the weighted average value of positron lifetimes at grain boundaries and in vacancies, because the positron lifetime at grain boundaries is longer than that in vacancies. Thus, when we discuss the defect variation by means of the positron lifetime parameters, the grain boundaries, vacancies-related defect must be considered. Table 1 summarizes the results of the samples studied, the last two column in the table giving the values of the S and W parameters. It is well known that the t2 is assigned to the positron annihilated in the defect state and I2 is its intensity. So the following discussion is mainly concerned with this lifetime component. The value of I2 is usually associated with the defect concentration. The higher defect concentration corresponds to the larger I2. So the increase in the I2 with an increase of Ti concentration shows the increase of the defects concentration (shown in Fig. 3). For the Ti-rich samples, the concentration of the grain boundary increased because the grain size become smaller, thus an increase in I2 with increasing Ti indicates both rise of vacancy-related defect concentration and grain boundaries in the samples. The defect variation can be also explained from the variation in the t2, which is the weighted value of the annihilation lifetimes at grain boundaries and in vacancies. There was a systematic and substantial decrease of annihilation lifetime with Ti content, saturating at a decrease of 50% for BZNT3, confirming the presence of vacancy-related defects in these series pyrochlores. Considering the annihilation lifetime at grain boundaries is longer than one in vacancies, a decrease in t2 should results certainly from in an increase in the vacancyrelated defect concentration with Ti incorporation. By Table 1 Positron lifetime data of the BZTN compounds Samples
t1 (ps)
t2 (ps)
I2 (%)
tave (ps)
S
W
BZN BZNT1 BZNT2 BZNT3 BZNT4
297.3 264.2 218.7 147.3 173.0
529.1 594.7 341.1 298.2 295.2
7.4 14.8 59.6 77.0 64.6
308.8 310.1 288.1 263.3 249.1
0.4787 0.4754 0.4741 0.4723 0.4708
0.2303 0.2281 0.2285 0.2290 0.2298
comparing this result with known previous data for other kinds of electroceramics, such as BaTiO3 and SrTiO3 [17–20], we can put forward the following supposition considering the pyrochlore structure characteristics: (1) To explain the variation of t2 and I2, complex defects models were proposed. (2) For low content of Ti ions introduced, small amount of defects such as TiNb1, VZn2, VNb5 formed. Here, larger t2 and smaller I2 value for Ti-poor samples should attribute to the annihilation of grain boundaries (which is a long lifetime component). (3) When Ti content increases further, defects concentration is constantly increasing, making it the predominant positron trapping center. The shorter lifetime of those defects causes a decrease in t2 and increase in I2. (4) For Ti-rich compounds, tb reaches a saturation value, which indicates that the electron concentration increases to a constant value and the predominant defect is complex defects. According to the complex defect theory [14,15], the vacancies may pair with each other to form double vacancies in the form of TiZn2+VZn2, 2TiNb1Zni2+, which will lead to a saturation in the positron lifetime component t2 and a decrease in the positron annihilation density I2. As the concentration of the complex defect is enhanced, the concentration of free vacancies is decreased quickly because of defect accumulation. Consequently, the total defect concentration is decreased, and correspondingly the intensity I2 is decreased. (5) Although it is quite difficult to decompose the positron lifetime spectra for a multi-defect systems, our PAS data show the presence of significant amount of vacancy-related defects in all studied compounds. According to the PAS data, the Ti-rich compounds show the larger concentration of the vacancy-related defects than that of Ti-poor compounds. The BZNT3, which the vacancy compensation mechanism is dominant, also exhibits the highest trapping rate (k = 2.54 ns1). As expected, the bulk positron lifetime of the series compounds increases linearly with the lattice constant from 240 to 310 ps (Fig. 4). The rapid decrease in tb means an increase in the electronic density of the
H.L. Du et al. / Applied Surface Science 253 (2006) 1856–1860
Fig. 4. Relationship of bulk lifetime parameters tb and positron trapping rate k to Ti content.
materials from BZN to BZNT4. This is probably because the concentration of vacancy is increased with Ti content. tb decreases toward a saturation value, which indicates that the electron concentration is increased to a constant value due to the vacancy compensation mechanism for Ti-rich samples. In Fig. 5, the value of S and W for the series samples can be represented by a straight line except the BZN sample, which suggests that the main origin of a decrease in S (or an increase in W) is a decrease in the concentration of vacancy-related defects. It is located apart from the line for the BZN sample, means that the species of the defects in the sample containing Ti were different from the Ti-free samples. Because the S parameter includes the composite information on the size of the defects and their concentration, more detailed discussion cannot be made only from these data. The defect behavior, the grain size and the dielectric property of the BZNT ceramics are significantly influenced by the Ti content. It is interesting to understand the physical origin of the novel high permittivity of BZNT pyrochlores. We know that dielectrics with high e are based on structures with BO6octahedra joined one to another by their tops. The key ion B
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located in the center of octahedra plays an important role in determining dielectric properties. The high dielectric constant in ceramics containing Ti4+ is generally attributed to rattling of Ti4+ within its TiO6 octahedron [21]. Vugmeister and Glinchuk [22] suggested that the substituting ions with smaller ionic radius or greater polarization forces than the lattice ions will lead to loosening state of cations located in the center of octahedral, so favor off-center displacement in dipole glass and ferroelectrics. Although we can rationalize the high dielectric constant of BZNT based on its atomic structure, there is good reason to suspect that the dielectric constant of this phase is enhanced by its microstructure. In the cubic pyrochlore structure encountered in BZNT series samples, besides the above polarization mechanism, the influence of defect structure on the dielectric properties should be considered. The occurrence of the complex defect causes distortion of the oxygen octahedron due to its strong inner coupling force. The distortion of the octahedron makes the internal field stronger and produces more than one off-center site for the weakly binding Ti ions. The loosening state of Ti4+ favors off-center displacement and provides a high permittivity and low dissipation factor. The high permittivity and low loss for BZNT samples could be mainly attributed to the loosening state of Ti4+, which derive from the high polarizability of Ti itself and the occurrence of complex defects. Thus, the annihilation characteristics are able to provide information about the structural defects of pyrochlores. More experimental and theoretical work is, however, needed to confirm the above viewpoints. 4. Conclusions Positron annihilation lifetime technique has been applied to study the microstructural changes in the sintered BZTN oxides with different amount of Ti. The results indicate that positron lifetime varies for different samples, which is ascribed to the various types and amounts of defects. The positron annihilation lifetime spectra provide evidence of the pairing of the complex defect and confirm that its concentration increases and the concentration of free defects decreases for Ti-rich compounds. In BZNT series samples, the dielectric properties differ from one another which may be related to different defect structures. The occurrence of complex defects help to enhance the dielectric constant. The high permittivity and low loss for BZNT samples could be mainly attributed to the loosening state of Ti4+, which derive from the high polarizability of Ti itself and the occurrence of complex defects. Acknowledgments This work was both supported by the Ministry of Science and Technology of China through 973-project under grant 2002CB613302 and China Postdoctoral Science Foundation. References
Fig. 5. S–W plots for BZNT series samples. It is located apart from the line for the BZN sample.
[1] D. Xue, K. Betzler, H. Hesse, J. Phys.: Condens. Matter 12 (2000) 3113. [2] A. Chen, Y. Zhi, R. Guo, A.S. Bhalla, J. Appl. Phys. 93 (2003) 3475.
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[3] D.P. Cann, C.A. Randall, T.R. Shrout, Solid State Commun. 100 (1996) 529. [4] V.P. Sirotinkin, A.A. Bush, Inorg. Mater. 39 (2003) 974. [5] A. Chen, Y. Zhi, H.J. Youn, C.A. Randall, A.S. Bhalla, L.E. Cross, M. Lanagan, Appl. Phys. Lett. 82 (2003) 3734. [6] M. Valant, P.K. Davies, J. Am. Ceram. Soc. 83 (2000) 147. [7] P. Prabhakar Rao, S.J. Liji, K. Ravindaran Nair, P. Koshy, Mater. Lett. 58 (2004) 1924. [8] H. Du, X. Yao, Phys. B: Condens. Matter 324 (2002) 121. [9] H. Du, X. Yao, Mater. Res. Bull. 40 (2005) 1527. [10] I. Levin, T.G. Amos, J.C. Nino, T.A. Vanderah, C.A. Randall, M.T. Lanagan, J. Solid State Chem. 168 (2002) 69. [11] R.L. Withers, T.R. Welberry, A.K. Larsson, Y. Liu, L. Nore’n, H. Rundlof, F.J. Brinka, J. Solid State Chem. 177 (2004) 231. [12] J.C. Nino, I.M. Reaney, M.T. Lanagan, C.A. Randall, Mater. Lett. 57 (2002) 414.
[13] M.J. Puska, R.M. Nieminen, Rev. Mod. Phys. 66 (1994) 841. [14] C. Corbel, P. Bernede, H. Pascard, F. Rullier-Albenque, R. Korman, J.F. Marucco, Appl. Phys. A: Solids Surf. 48 (1989) 335. [15] F. Hori, K. Koike, R. Oshima, Appl. Surf. Sci. 242 (2005) 304. [16] V.S. Mikhalenkov, E.A. Tsapko, S.S. Polishchuk, A.I. Ustinov, J. Alloys Compd. 386 (2005) 192. [17] A. Uedono, K. Shimayama, M. Kiyohara, J. Appl. Phys. 92 (2002) 2697. [18] A. Chen, Y. Zhi, J. Appl. Phys. 71 (1992) 4451. [19] C. Macchi, A. Somoza, A. Dupasquier, A.L. Garcia, M. Castro, J. Phys.: Condens. Matter 13 (2001) 5717. [20] T. Friessnegg, S. Aggarwal, R. Ramesh, B. Nielsen, E.H. Poindexter, D.J. Keeble, Appl. Phys. Lett. 77 (2000) 127. [21] M.A. Subramanian, Dong Li, N. Duan, B.A. Reisner, A.W. Sleight, J. Solid State Chem. 151 (2000) 323. [22] B.E. Vugmeister, M.D. Glinchuk, Solid State Commun. 48 (1983) 503.
Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation Huiling Du a,*, Xi Yao a, Xianfeng Zhang b, Huimin Weng b b
a Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an 710049, China Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
Received 4 January 2006; received in revised form 12 March 2006; accepted 13 March 2006 Available online 2 May 2006
Abstract Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi1.5Zn0.5)(Zn0.5x/3TixNb1.52x/3)O7 (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I2 in the Tirich sample. At 1 MHz their dielectric constant (e0 ) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan d) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant. # 2006 Elsevier B.V. All rights reserved. PACS: 78.70.Bj; 74.62.Dh; 77.22.Ch Keywords: Positron annihilation; Pyrochlore; Complex defects; Dielectric constant
1. Introduction Dielectric ceramics are widely used with advances in microelectronic technologies microwave communication, and high dielectric constant ceramics have received intensive attention in reducing the size of microelectronic circuits, where high permittivity and low dielectric loss are required for many applications [1,2]. Pyrochlore-type oxides with the general formula Bi1.5ZnNb1.5O7 (BZN) have been attracting a lot of attention because of their excellent dielectric constant and low firing temperature. Much work has been performed on improving the dielectric properties, especially in enhancing the permittivity and reducing dielectric loss [3–7]. Most of this attention has focused on pyrochlore-based ceramics that contain highly polarizable cations such as Ti4+ and Ta5+ on the B-sites. Among them, (Bi1.5Zn0.5)(Ti1.5Nb0.5)O7 (BZNT) is one of the most promising materials due to its stability and high dielectric constant (210). We have shown the beneficial
* Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (H.L. Du). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.026
aspects of the tetravalent Ti, Sn, Zr and Ce incorporation into Bsite of BZN pyrochlore [8,9]. An understanding of the relationship between dielectric properties and structure, especially the structural origin of high permittivity and dielectric relaxation, is thus of great relevance to the development of materials appropriate of such systems. Lattices of crystalline materials are not perfect on the atomic scale since point defects always exist. Defects can dramatically change physical properties of materials, even if their concentration is very low. Therefore, knowing the defects in dielectrics and understanding the structural origin of dielectric properties are of great importance. Although preliminary results on the crystal imperfections are traditionally obtained by the conventional analytical tools, such as XRD, SEM and TEM [10–12], the analysis of the point defects and lattice disorder at the ppm level in the high-Q dielectric ceramics obviously requires a novel approach. In this situation positron annihilation spectroscopy is a powerful technique to detect a small amount of related defects, especially a very small vacancy-type of defects and others, which are beyond the resolution limit of the electron microscopy [13]. Thus, this technique has been widely used for detecting point defects in various materials, such as
H.L. Du et al. / Applied Surface Science 253 (2006) 1856–1860
metals and semiconductors [14–16]. With the Doppler broadening parameters and positron lifetime, one can get various information about defect properties. Point defects in perovskite-type oxide such as BT and PLZT have been investigated by using PAT recently and the results show that positrons can be a useful probe for studying vacancytype defects in such materials [17–20]. Unlike the simple perovskites, the point defect chemistry of the pyrochlores is largely unknown and never been reported. Hence, by applying positron annihilation technology, it would be possible to detect and characterize trace amounts of these lattice defects. The present study is mainly aimed at evaluating the nature of microdefects introduced by Ti ions occupied at B-site of BZN dielectrics. In this contribution, we further outline the influence of structural defects in the pyrochlores on dielectric properties.
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Fig. 1. Dielectric constant and lattice constant of BZTN as a function of amounts of Ti.
2. Materials and experimental technique
3. Results and discussion
constant generally diminishes linearly with increasing temperature. The tangent loss increases with increase of temperatures particularly, beyond 375 8C. The increase in tangent loss at higher temperature might be due to the formation of higher concentration of charge carriers. The base ceramic BZN (x = 0) has a dense crystal microstructure with bigger grains from 4 to 7 mm at 1000 8C. With the substitution of Ti4+, the extent of grain abnormally growing up was effectively controlled. For example, the grain size of BZNT2 (x = 0.5) and BZNT4 (x = 1.5) is about 2–3 and 1 mm at 1000 8C, respectively. That indicates Ti4+ substitutions are effective for depressing the grain growing. In general, the concentration of grain boundaries is mainly determined by grain size, the sample with larger grain has lower grain boundary concentration. The difference of grain size results mainly from the influences of Ti incorporation on the grain growth because the series samples were prepared at same conditions. There may have two causes needing consideration, the one is difference in the sintering mechanism induced due to Ti concentration, the other is the affect of defects induced due to Ti concentration. It is known that the diffusivity of vacancy-related defects is slowest during sintering. Thus an increase of vacancy concentration in the Ti-rich samples would
Considering the ionic radius of Zn2+, Nb5+ and Ti4+ with eightcoordination number, we assumed that Ti4+ can co-substitute Zn2+ and Nb5+ at B-site. The mechanism of substitution can be presented: 3Ti4+ ! Zn2+ + 2Nb5+. Therefore, the criterion of the charge neutrality can also be satisfied. Our recent research [9] revealed that titanium ions can co-substitute for Zn and Nb ions at B-site as solid solutions for 0 x 1.5 according to the X-ray diffraction results, inducing the decrease of lattice constant linear while remaining cubic pyrochlore phase. The composition dependence of the lattice constant and dielectric constant is shown in Fig. 1. At 1 MHz their dielectric constant (e0 ) varies from 150 for Ti-free system to 210 for Ti-rich system and loss tangent (tan d) remains rather low level. The temperature dependency of the dielectric constant and the dissipation factor at 1 MHz is shown in Fig. 2. There is no significant peak observed in these curves, and the dielectric
Fig. 2. The temperature dependence of dielectric behaviors of BZNT pyrochlores.
All samples of the base composition (Bi1.5Zn0.5)(Zn0.5x/ 3TixNb1.52x/3)O7 (x = 0, 0.25, 0.5,1.0, 1.5) were prepared by conventional solid-state reaction methods from mixtures of Bi2O3, ZnO, Nb2O5 and TiO2. All of the reagents were of 99.9% purity. The mixtures were calcined at 800 8C in air for 2 h. The obtained powders were milled for 4 h, pressed into pellets and sintered at 1000–1080 8C for 2 h in air and then furnace-cooled. The obtained ceramic density accounted for above 95% of the theoretical value. The positron lifetimes were measured with a fast–fast coincidence spectrometer with a time resolution of 210 ps. We collected 1.5 106 annihilation events in each spectrum. As a positron source 22Na was sealed in a Ni-foil and the experiments were sandwiched between two identical samples. All measurements were carried out at 300 K and the positron annihilation lifetime spectra were analyzed using the program Positronfit-Extended. Observations of Doppler broadening distributions with an energy dispersive Ge detector were also performed. The Doppler broadening of the annihilation peak is characterized by the lineshape parameters S and W.
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suppress the growth of grains, which results in small-grain microstructure. In order to investigate the defect properties, PAS measurement on the BZNT ceramics was performed at room temperature. According to the two-state trapping model, the average positron lifetime tave and the bulk lifetime tb are calculated from the fitted lifetimes and their intensities by the following equations t ave ¼ I1 t1 þ I2 t2 tb ¼
1 1 ¼ lb ðI1 =t1 Þ þ ðI2 =t2 Þ
and the positron trapping rate k k¼
Fig. 3. Relationship of positron lifetime t2 and I2 parameters to Ti content.
tm tb 1 t2 tm tb
In general, for the ceramic materials, the t2 should be regarded as the weighted average value of positron lifetimes at grain boundaries and in vacancies, because the positron lifetime at grain boundaries is longer than that in vacancies. Thus, when we discuss the defect variation by means of the positron lifetime parameters, the grain boundaries, vacancies-related defect must be considered. Table 1 summarizes the results of the samples studied, the last two column in the table giving the values of the S and W parameters. It is well known that the t2 is assigned to the positron annihilated in the defect state and I2 is its intensity. So the following discussion is mainly concerned with this lifetime component. The value of I2 is usually associated with the defect concentration. The higher defect concentration corresponds to the larger I2. So the increase in the I2 with an increase of Ti concentration shows the increase of the defects concentration (shown in Fig. 3). For the Ti-rich samples, the concentration of the grain boundary increased because the grain size become smaller, thus an increase in I2 with increasing Ti indicates both rise of vacancy-related defect concentration and grain boundaries in the samples. The defect variation can be also explained from the variation in the t2, which is the weighted value of the annihilation lifetimes at grain boundaries and in vacancies. There was a systematic and substantial decrease of annihilation lifetime with Ti content, saturating at a decrease of 50% for BZNT3, confirming the presence of vacancy-related defects in these series pyrochlores. Considering the annihilation lifetime at grain boundaries is longer than one in vacancies, a decrease in t2 should results certainly from in an increase in the vacancyrelated defect concentration with Ti incorporation. By Table 1 Positron lifetime data of the BZTN compounds Samples
t1 (ps)
t2 (ps)
I2 (%)
tave (ps)
S
W
BZN BZNT1 BZNT2 BZNT3 BZNT4
297.3 264.2 218.7 147.3 173.0
529.1 594.7 341.1 298.2 295.2
7.4 14.8 59.6 77.0 64.6
308.8 310.1 288.1 263.3 249.1
0.4787 0.4754 0.4741 0.4723 0.4708
0.2303 0.2281 0.2285 0.2290 0.2298
comparing this result with known previous data for other kinds of electroceramics, such as BaTiO3 and SrTiO3 [17–20], we can put forward the following supposition considering the pyrochlore structure characteristics: (1) To explain the variation of t2 and I2, complex defects models were proposed. (2) For low content of Ti ions introduced, small amount of defects such as TiNb1, VZn2, VNb5 formed. Here, larger t2 and smaller I2 value for Ti-poor samples should attribute to the annihilation of grain boundaries (which is a long lifetime component). (3) When Ti content increases further, defects concentration is constantly increasing, making it the predominant positron trapping center. The shorter lifetime of those defects causes a decrease in t2 and increase in I2. (4) For Ti-rich compounds, tb reaches a saturation value, which indicates that the electron concentration increases to a constant value and the predominant defect is complex defects. According to the complex defect theory [14,15], the vacancies may pair with each other to form double vacancies in the form of TiZn2+VZn2, 2TiNb1Zni2+, which will lead to a saturation in the positron lifetime component t2 and a decrease in the positron annihilation density I2. As the concentration of the complex defect is enhanced, the concentration of free vacancies is decreased quickly because of defect accumulation. Consequently, the total defect concentration is decreased, and correspondingly the intensity I2 is decreased. (5) Although it is quite difficult to decompose the positron lifetime spectra for a multi-defect systems, our PAS data show the presence of significant amount of vacancy-related defects in all studied compounds. According to the PAS data, the Ti-rich compounds show the larger concentration of the vacancy-related defects than that of Ti-poor compounds. The BZNT3, which the vacancy compensation mechanism is dominant, also exhibits the highest trapping rate (k = 2.54 ns1). As expected, the bulk positron lifetime of the series compounds increases linearly with the lattice constant from 240 to 310 ps (Fig. 4). The rapid decrease in tb means an increase in the electronic density of the
H.L. Du et al. / Applied Surface Science 253 (2006) 1856–1860
Fig. 4. Relationship of bulk lifetime parameters tb and positron trapping rate k to Ti content.
materials from BZN to BZNT4. This is probably because the concentration of vacancy is increased with Ti content. tb decreases toward a saturation value, which indicates that the electron concentration is increased to a constant value due to the vacancy compensation mechanism for Ti-rich samples. In Fig. 5, the value of S and W for the series samples can be represented by a straight line except the BZN sample, which suggests that the main origin of a decrease in S (or an increase in W) is a decrease in the concentration of vacancy-related defects. It is located apart from the line for the BZN sample, means that the species of the defects in the sample containing Ti were different from the Ti-free samples. Because the S parameter includes the composite information on the size of the defects and their concentration, more detailed discussion cannot be made only from these data. The defect behavior, the grain size and the dielectric property of the BZNT ceramics are significantly influenced by the Ti content. It is interesting to understand the physical origin of the novel high permittivity of BZNT pyrochlores. We know that dielectrics with high e are based on structures with BO6octahedra joined one to another by their tops. The key ion B
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located in the center of octahedra plays an important role in determining dielectric properties. The high dielectric constant in ceramics containing Ti4+ is generally attributed to rattling of Ti4+ within its TiO6 octahedron [21]. Vugmeister and Glinchuk [22] suggested that the substituting ions with smaller ionic radius or greater polarization forces than the lattice ions will lead to loosening state of cations located in the center of octahedral, so favor off-center displacement in dipole glass and ferroelectrics. Although we can rationalize the high dielectric constant of BZNT based on its atomic structure, there is good reason to suspect that the dielectric constant of this phase is enhanced by its microstructure. In the cubic pyrochlore structure encountered in BZNT series samples, besides the above polarization mechanism, the influence of defect structure on the dielectric properties should be considered. The occurrence of the complex defect causes distortion of the oxygen octahedron due to its strong inner coupling force. The distortion of the octahedron makes the internal field stronger and produces more than one off-center site for the weakly binding Ti ions. The loosening state of Ti4+ favors off-center displacement and provides a high permittivity and low dissipation factor. The high permittivity and low loss for BZNT samples could be mainly attributed to the loosening state of Ti4+, which derive from the high polarizability of Ti itself and the occurrence of complex defects. Thus, the annihilation characteristics are able to provide information about the structural defects of pyrochlores. More experimental and theoretical work is, however, needed to confirm the above viewpoints. 4. Conclusions Positron annihilation lifetime technique has been applied to study the microstructural changes in the sintered BZTN oxides with different amount of Ti. The results indicate that positron lifetime varies for different samples, which is ascribed to the various types and amounts of defects. The positron annihilation lifetime spectra provide evidence of the pairing of the complex defect and confirm that its concentration increases and the concentration of free defects decreases for Ti-rich compounds. In BZNT series samples, the dielectric properties differ from one another which may be related to different defect structures. The occurrence of complex defects help to enhance the dielectric constant. The high permittivity and low loss for BZNT samples could be mainly attributed to the loosening state of Ti4+, which derive from the high polarizability of Ti itself and the occurrence of complex defects. Acknowledgments This work was both supported by the Ministry of Science and Technology of China through 973-project under grant 2002CB613302 and China Postdoctoral Science Foundation. References
Fig. 5. S–W plots for BZNT series samples. It is located apart from the line for the BZN sample.
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