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Dielectric properties of A- and B-site doped BaTiO3: Effect of La and Ga
ARTICLE IN PRESS Physica B 404 (2009) 1799–1805
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Physica B journal homepage: www.elsevier.com/locate/physb
Dielectric properties of A- and B-site doped BaTiO3: Effect of La and Ga Devidas Gulwade, Prakash Gopalan Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, India
a r t i c l e in fo
abstract
Article history: Received 17 January 2009 Accepted 22 February 2009
Extremely small amounts of La and Ga doping on the A- and B-site of BaTiO3, respectively, resulting in a solid solution of the type Ba13xLa2xTi13yGa4yO3 have been investigated. The present work dwells on the influence of the individual dopants, namely La and Ga, on the dielectric properties of BaTiO3. The compositions have been prepared by solid-state reaction. X-ray diffraction (XRD) reveals the presence of tetragonal (P4/mmm) phase. The XRD data has been analyzed using FULLPROF, a Rietveld refinement package. The microstructure have been studied by orientation imaging microscopy (OIM). The compositions have been characterized by dielectric spectroscopy between room temperature and 250 1C. Further, the nature of phase transition has been studied using high temperature XRD. The resulting compounds exhibit high dielectric constant, enhanced diffuseness and low temperature coefficient of capacitance. & 2009 Elsevier B.V. All rights reserved.
PACS: 77.80.e 61.05.C 64.60.i Keywords: Ferroelectric properties Diffuse phase transition Doped BaTiO3 High temperature XRD
1. Introduction Doped barium titanate are continually being explored as a possible lead-free relaxor materials. Various dopants have been explored to modify properties of pure BaTiO3. Some of the dopants shift one or more of the transition temperatures of BaTiO3. The shift in the transitions or shape of transitions cannot be explained by simple ionic size effects. Many dopants give rise to a diffuse transition accompanied by a deviation from Curie–Weiss (C–W) law, a spontaneous polarization over an extended temperature range, and frequency dispersion; termed as a relaxor behavior [1]. The relaxor behavior is usually explained as resulting from the formation of a short-range ordered phase within the matrix of a high temperature long range ordered phase [2,3]. Also, relaxors were believed to exhibit compositional inhomogeneity [4,5], inter- and intra-grain stresses leading to the formation of polar nano domains and their reduced interaction. In addition to these various conventional explanations, models have been proposed [1,6]. The widely investigated dopants are Ta, Nb, Sn [7,8], Sr [9], La [10–12], Zr [13,14], Ce [15–19], Ca, Pb and Y [20,21]. In general it is believed that shape (i.e. diffuseness) of the transition is defined by the dopant on the Ti site. In literature, tetravalent and pentavalent dopants on the Ti site are widely explored. The Ti site dopants widely explored are the ferroelectrically active Nb5+ and Ta5+ and the ferroelctrically inactive Zr4+, Sn4+, Ce4+. The dopants causes
Corresponding author. Tel.: +91 22 25767635; fax: +91 22 25723480.
E-mail address: [email protected] (P. Gopalan). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.02.026
loss of long range ordering by diluting the ferroelectric characteristics [22] or by impurity induced clusters triggered by soft mode correlation length [23]. The effect of Ti site dopants are commonly evaluated with or without dopant at Ba site co-doped BaTiO3. It is generally commented that B-site dopants play a predominant role in the extent of diffuseness. However, the effects cannot be categorized as A- or B-site whereas the defect reaction involved predominantly plays a role. Morrsion et al. [11] has explained the effect of La on the basis of Ti vacancy model and further they compared observed broadening of the transition with Zr doping. The doping of La and Zr brings about the same effects by disturbing the long range ordering due to the presence of Zr, a ferroelectric inactive ion or Ti vacancies. In the present study we embarked on evaluating effects of trivalent dopant (Ga3+ and La3+) at Ti and Ba sites. The main objective was to study effect of Ga3+ in order to suppress the Ti vacancies produced by La3+ at Ba site and thereby to study the effect on extent of diffuseness of the transition. Recently, we reported the effect of La and Ga (and Al) on the properties of the BaTiO3. Solid solutions of the type Ba13xLa2x Ti13xGa4xO3 [24] and Ba13xLa2xTi13xAl4xO3 [25] were investigated. The resulting compositions exhibited a diffuse transition as well as a deviation from C–W law, a high dielectric constant and low loss. In the present work, the focus is on ascertaining the individual roles of each dopant, La and Ga, on the properties of BaTiO3 that shows very interesting behavior. Following our earlier work [25], we have chosen Ba13xLa2xTi13yGa4yO3 (x ¼ y ¼ 0.006) as the starting composition that exhibits high dielectric constant and
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enhanced diffuseness. We report here on the synthesis, as well as the structure and dielectric property correlations. An increase in La concentration results in a decrease in the transition temperature and increase in diffuseness. However, a change in Ga concentration alters the transition temperature to a much smaller extent. Further, we have observed that an increase in the Ga concentration results in a classical ferroelectric transition exhibiting least diffuseness and a higher dielectric constant, due to suppressing Ti vacancies defects. Further, the nature of transition is studied with high temperature X-ray diffraction (HTXRD).
2. Experimental work Two series of nominal compositions corresponding to Ba13xLa2xTi13yGa4yO3 were prepared. In the first series, with x ¼ 0.006 fixed, six compositions with values of y varying between 0.002 and 0.014, and in the second series, for y ¼ 0.006 fixed, three compositions with values of x varying between 0 and 0.01 were synthesized by conventional solid-state reaction. The starting materials, BaCO3 (99.95%), TiO2 (99.9+%), La2O3 (99.99%) and Ga2O3 (99.99%), were procured from Aldrich Chemicals (USA). Stoichiometric amounts of the starting materials were mixed in a ball-mill with zirconia as a grinding media in ethanol. The powder was calcined at 1100 1C for 12 h, followed by repetitive stages of ball-milling and calcinations. The powder was characterized by X-ray diffraction (XRD) at room temperature and a few compositions were investigated using HTXRD between room temperature and 250 1C. The lattice parameters were extracted by indexing the data in the tetragonal space group P4/mmm, using the FULLPROF least square refinement software [26]. The powder was ball-milled for 24 h, dried, pelletized using a 10 mm diameter tungsten carbide die and sintered at 1350 1C for 4 h. The dielectric measurements were recorded in the temperature range between room temperature and 300 1C at different frequencies using a HP impedance analyzer (4192A). The microstructural aspects were characterized by orientation imaging microscopy (OIM) using a commercial TSL system on a FEI Quanta 200 HV SEM. The data are indexed in cubic space group as the tetragonality was less than 1% in all compositions.
Fig. 1. XRD pattern for Ba13xLa2xTi13yGa4yO3 for x ¼ 0.006 (fixed) and varying y ¼ 0.004 (a), y ¼ 0.006 (b), y ¼ 0.008 (c), y ¼ 0.012 (d) and y ¼ 0.016 (e).
3. Results and discussion The XRD patterns for a few representative compositions are exhibited in Fig. 1(a)–(e); all the compositions exhibit a single phase. The XRD data could be indexed in the tetragonal space group P4/mmm. The lattice parameters have been extracted from the refined XRD pattern. The tetragonality (c/a) is plotted in Fig. 2. It can be observed that the extent of tetragonality decreases with increasing La dopant concentration. However, the tetragonality varies to lesser extent with increase in Ga concentration. The change in tetragonality is explained qualitatively by the corresponding change in tolerance factor, which decreases with higher rate in case of La dopant. Tolerance factor decreases with increase in Ga concentration at a lower rate; the decrease in tetragonality with initial increase in Ga dopant concentration between y ¼ 0 and 0.004 corroborates with corresponding decrease in tolerance factor. With further increase in Ga concentration tetragonality increases, this increase in tetragonality is related with increase in grain size, which is addressed later in reference with OIM data. The dielectric behaviors as a function of temperature for different frequencies are plotted in Figs. 3–8. The composition x ¼ 0.006, y ¼ 0 exhibits anomalous dispersion of dielectric constant as a function of frequency. This is a characteristic of the space charge agglomera-
Fig. 2. Variation in tetragonality as a function of dopant concentration (x and y).
tion at grain boundaries (see Fig. 8). None of the other compositions exhibit such dispersion with frequency. The dielectric constant at 10 KHz as a function of temperature for all the compositions is provided in Figs. 9 and 10. For an increase in La concentration, the diffuseness increases and the transition temperature decreases. The transition temperature shows a weak dependence on Ga concentration; the transition temperature decreases with increase in Ga concentration between y ¼ 0 and 0.004 (at constant La, x ¼ 0.006) and increases with further increase in Ga concentration. The change in Tc is in good agreement with a corresponding change in tetragonality. The tetragonality as a function of the transition temperature is plotted in Fig. 11, which depicts a linear relationship. To compare the extent of diffuseness, normalized dielectric constant as a function of the normalized temperature is exhibited in Fig. 12(a) and (b). The diffuseness increases with an increase in La concentration. The composition (x ¼ 0.010, y ¼ 0.006) exhibits the highest diffuseness. For a fixed La (x ¼ 0.006), the diffuseness increases
ARTICLE IN PRESS D. Gulwade, P. Gopalan / Physica B 404 (2009) 1799–1805
Fig. 3. Dielectric constant as a function of temperature for x ¼ y ¼ 0.006.
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Fig. 6. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.014.
Fig. 4. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.008.
Fig. 7. Dielectric constant as a function of temperature for x ¼ 0.010 and y ¼ 0.006.
Fig. 5. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.012.
Fig. 8. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.
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Fig. 9. Dielectric constant at 10 KHz as a function of temperature for all the compositions for fixed La (x ¼ 0.006).
Fig. 11. Transition temperature as a function of tetragonality ’x ¼ 0, y ¼ 0.006; x ¼ 0.002, y ¼ 0.006; mx ¼ 0.006, y ¼ 0.006; .x ¼ 0.010, y ¼ 0.006; ~x ¼ 0.006, y ¼ 0.004; bx ¼ 0.006, y ¼ 0.008; cx ¼ 0.006, y ¼ 0.014; *x ¼ 0.006, y ¼ 0.016.
Fig. 10. Dielectric constant at 10 KHz as a function of temperature for all the compositions with fixed Ga (y ¼ 0.006).
initially for Ga increasing from y ¼ 0 to 0.004, thereafter up to y ¼ 0.008 the diffuseness deceases, and the transition appears as for a classical ferroelectric transition. The compositions with a higher diffuseness exhibit a deviation from C–W law. To describe the behavior, the modified C–W law [27], provided below (Eq. (1)), has been used for further analysis. 1
1
max
¼
ðT T max Þg C
(1)
In Eq. (1), C and g are constants, and emax is the maximum dielectric constant at the transition temperature Tmax. The value of the constant g varies between 1 (classical ferroelectric) and 2 (relaxor). The constant g represents the slope of the plot between log (1/e1/emax) and log (TTmax), and is a measure of the diffuseness of the transition. A representative plot for two of the compositions is provided in Fig. 13. The extracted values of g are exhibited in Fig. 14. For a fixed Ga concentration (y ¼ 0.006), with an increase in the La concentration, g varies between 1.01 (x ¼ 0) and 1.72 (x ¼ 0.010). Thus, an increase in the La doping is responsible for destroying the long range ordering. However, as
Fig. 12. Plot of normalized dielectric constant at 10 KHz as a function of normalized temperature.
discussed above, for a constant value of La (x ¼ 0.006), an increase in the concentration of Ga leads to an initial increase in diffuseness and then diffuseness decreases with further increase in Ga; approaching the classical ferroelectric transition. The value
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Table 1 Transition temperature and dielectric constant for all the Ba13xLa2xTi13yGa4yO3 compositions. Composition x
y
0.006
0.00 0.002 0.004 0.006 0.008 0.012 0.016 0.006 0.006 0.006
0 0.002 0.010
Fig. 13. Plot of log (TTmax) vs. log (1/e1/emax) for representative compositions.
Fig. 14. Variation in g as a function of dopant concentration (x and y).
of g decreases from 1.8 for the (x ¼ 0.006, y ¼ 0.004) composition to 1.01 for the (x ¼ 0.006, y ¼ 0.008) composition. Thus, a classical ferroelectric state is attained by a restoration of the long range ordering with increase in Ga dopant concentration. Thereafter, with further increase in Ga, g increases marginally. This conclusion agrees with the inferences drawn from the normalized dielectric constant vs. normalized temperature (Fig. 12). The compositions x ¼ 0.006, y ¼ 0.004 and x ¼ 0.0010, y ¼ 0.006 exhibit the highest diffuseness; g1.8. The initial increase in diffuseness for lower Ga concentration is also observed in Ga doped BaTiO3 compositions, the details of which are under investigation at present and will be reported later. The increase in diffuseness at lower Ga concentration is due to the presence of two kinds of ions present at the ferroelectric active site and it reverses the trend with further increase in Ga concentration. The possible defect models governing the observed behavior are addressed later. The transition temperatures and the dielectric constant data for the various compositions are summarized in
Tc (1C)
Dielectric constant at Tc
95 85 65 80 90 90 90 120 110 45
27 500 5032 3695 5422 10 320 9301 8839 6368 8338 3561
Table 1. For a fixed Ga concentration (y ¼ 0.006), the dielectric constant increases initially in going from x ¼ 0 to 0.002, and thereafter the dielectric constant decreases. The initial increase in the dielectric constant followed by enhanced diffuseness is well established in La doped BaTiO3 [10]. The decrease in dielectric constant and an enhanced diffuseness with an increase in La concentration is a result of breakage of long range ordering as a result of Ti vacancies, which is an accepted defect model in La modified BaTiO3 (see Eq. (2)). The highest La concentration composition, x ¼ 0.010 for y ¼ 0.006, appears to exhibit the onset of relaxor type behavior, with a frequency dispersion near transition temperature, and a slight change in Tc with frequency (Fig. 9). However, we are unable to substantiate the observations due to the frequency span limitations, and a change in Tc is not observed in other compositions. For a constant La concentration (x ¼ 0.006), the dielectric constant decreases with increase in Ga concentration up to y ¼ 0.004 and thereafter increases with increase in the Ga concentration from y ¼ 0.004 to 0.008, and with further increase in y, the dielectric constant decreases at a slower rate. The effect of Ga doping may be divided into two zones: one as a La rich region, the other as a Ga rich region. The Ga rich region y40.006 (at constant La x ¼ 0.006) results in high dielectric constant accompanied by less diffuseness. For yo0.006 i.e. in La rich region, compositions exhibit higher diffuseness and low dielectric constant. The La doping in BaTiO3 acts as a donor doping giving rise to Ti vacancies [11]. Also, La acts as grain growth inhibitor. However, on other hand Ga doping (i.e. acceptor doping) is supposed to suppress Ti vacancies and thereby may give rise to grain growth. The increased diffuseness is also related to the grain size effect that is addressed latter. However, donor effect is predominant in the La rich region; grain growth is inhibited and compositions exhibit high diffuseness. However, in the Ga rich region, enhanced grain size is observed predominantly after crossover from La rich region to Ga rich region, at x ¼ y ¼ 0.006. The possible defect models are provided in Eqs. (2)–(4). Eq. (2), as discussed earlier, governs the defect model due to La doping. In Ga rich compositions, oxygen vacancies (Eq. (3)) and selfcompensation (Eq. (4)) are the possible mechanisms. The higher valent ion at Ba site (La) and the lower valent ion at Ti site (Ga) may lead to self-compensation, restoring the long range ordering and as a consequence the diffuseness decreases, followed by an enhancement in the dielectric constant. Further, the formation of oxygen vacancies cannot be ruled out and in fact both the mechanisms may be operative. La2 O3 ! 2LaBad þ 12V kTi
(2)
Ga2 O3 ! 2GaTid þ V Od d
(3)
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La2 O3 þ Ga2 O3 ! 2LaBad þ 2GajTi
(4)
In the representative OIM plots exhibited in Fig. 15, none of the compositions exhibit preferred orientation. The average grain size
obtained from OIM data are provided in Table 2. The grain size decreases with increase in La concentration (see x ¼ 0, y ¼ 0.006; x ¼ 0.002, y ¼ 0.006 and x ¼ y ¼ 0.006). However, the grain size increases with increase in Ga concentration from y ¼ 0.006 to 0.008 at constant La (x ¼ 0.006). Thereafter grain size decreases slowly with further increase in Ga concentration, this explains the marginal increase in diffuseness at higher Ga compositions. This corroborates with decrease in dielectric constant at higher Ga concentration. The smaller grain sizes along with coarser grains are observed in Fig. 15. Satisfactory data cannot be recorded for other samples, as smaller grain size is possibly comparable to electron spot diameter, resulting in higher overlap of diffraction patterns and weak patterns. In order to interpret crystallographic changes, HTXRD experiments were carried out between room temperature and 250 1C. The compositions exhibited a tetragonal splitting for higher angle peaks well above the temperature where the dielectric constant exhibited a maximum (Tmax). However, the splitting disappeared at Tmax for pure BaTiO3. The changes in volume and lattice parameter as a function of temperature are plotted in Figs. 16 and 17 respectively. Pure BaTiO3 and the composition x ¼ 0.002, y ¼ 0.006 exhibit a sharp change in
Fig. 16. Change in volume as a function of temperature.
Fig. 15. Inverse pole figure of Ba13xLa2xTi13xGa4xO3 compositions: (a) x ¼ 0, y ¼ 0.006; (b) x ¼ 0.002, y ¼ 0.006; (c) x ¼ 0.006, x ¼ 0.012; (d) x ¼ 0.006, y ¼ 0.014. Table 2 Average grain size for the Ba13xLa2xTi13yGa4yO3 compositions obtained from OIM. Composition
Average grain size (mm)
x
y
0 0.002 0.006 0.006 0.006 0.006
0.006 0.006 0.006 0.008 0.012 0.014
52 4.7 1.5 7 4.5 3.7 Fig. 17. Change in lattice parameters as a function of temperature.
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volume. The compositions exhibiting diffuse transition, namely x ¼ 0.006 ¼ 0.006 and x ¼ 0.010, y ¼ 0.006, do not exhibit a sudden change in volume; the nature of the transition is rather continuous. In Fig. 17, BaTiO3 and x ¼ 0.002, y ¼ 0.006 exhibit a sudden change in lattice parameters near to their respective Tmax. However, the change in lattice parameters for x ¼ 0.01, y ¼ 0.006 is continuous and the structure is tetragonal well above Tmax, unlike pure BaTiO3; this corroborates with the observed diffuse phase transition and higher g (see Fig. 9). The higher value of g represents the deviation from C–W law and is indicative of formation of polar domains; our experimental results are in agreement with theory. The similar effects are observed in an investigation of effect of hetero- and iso-valent doped BaTiO3 on phase transition [28]. Also, this is in agreement with the related work on La and Zr modified PbTiO3; the existence of lower temperature polar phase is observed till the tetragonal to cubic phase transition temperature (490 1C) of pure PbTiO3 [29]. The decrease in Tmax results in a wide difference between the Tmax and crystallographic transition temperature, resulting in a diffuse phase transition. This is an experimental evidence that the transition observed in dielectric constant as a function of temperature does not correspond to crystallographic phase change. The experimental findings corroborate with the higher diffuseness observed in the compositions.
Acknowledgment
4. Conclusions
[17]
In the present investigations, the changes in dielectric properties as a result of co-doping of La and Ga in barium titanate are addressed. Also the individual role of each of the dopant, La and Ga, is established and the defect mechanism is addressed. The La dopant is effective in lowering the Tc and enhancing the diffuseness. However, on the other hand, Ga increases the dielectric constant making the transition more prominent and restores long range ordering. The composition x ¼ y ¼ 0.006 exhibits the highest dielectric constant at room temperature as a compromise between diffuseness and dielectric constant. Our work establishes a methodology to develop a diffuse transition and higher dielectric constant, and also on the nature of the crystallographic changes in doped BaTiO3 materials exhibiting diffuse phase transition. Further work may lead to the development of a novel material as a result of simultaneous doping of La and Ga.
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The authors thank the ER and IPR Division, DRDO, Government of India for the generous research support that helped in the execution of this work.
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Dielectric properties of A- and B-site doped BaTiO3: Effect of La and Ga Devidas Gulwade, Prakash Gopalan Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, India
a r t i c l e in fo
abstract
Article history: Received 17 January 2009 Accepted 22 February 2009
Extremely small amounts of La and Ga doping on the A- and B-site of BaTiO3, respectively, resulting in a solid solution of the type Ba13xLa2xTi13yGa4yO3 have been investigated. The present work dwells on the influence of the individual dopants, namely La and Ga, on the dielectric properties of BaTiO3. The compositions have been prepared by solid-state reaction. X-ray diffraction (XRD) reveals the presence of tetragonal (P4/mmm) phase. The XRD data has been analyzed using FULLPROF, a Rietveld refinement package. The microstructure have been studied by orientation imaging microscopy (OIM). The compositions have been characterized by dielectric spectroscopy between room temperature and 250 1C. Further, the nature of phase transition has been studied using high temperature XRD. The resulting compounds exhibit high dielectric constant, enhanced diffuseness and low temperature coefficient of capacitance. & 2009 Elsevier B.V. All rights reserved.
PACS: 77.80.e 61.05.C 64.60.i Keywords: Ferroelectric properties Diffuse phase transition Doped BaTiO3 High temperature XRD
1. Introduction Doped barium titanate are continually being explored as a possible lead-free relaxor materials. Various dopants have been explored to modify properties of pure BaTiO3. Some of the dopants shift one or more of the transition temperatures of BaTiO3. The shift in the transitions or shape of transitions cannot be explained by simple ionic size effects. Many dopants give rise to a diffuse transition accompanied by a deviation from Curie–Weiss (C–W) law, a spontaneous polarization over an extended temperature range, and frequency dispersion; termed as a relaxor behavior [1]. The relaxor behavior is usually explained as resulting from the formation of a short-range ordered phase within the matrix of a high temperature long range ordered phase [2,3]. Also, relaxors were believed to exhibit compositional inhomogeneity [4,5], inter- and intra-grain stresses leading to the formation of polar nano domains and their reduced interaction. In addition to these various conventional explanations, models have been proposed [1,6]. The widely investigated dopants are Ta, Nb, Sn [7,8], Sr [9], La [10–12], Zr [13,14], Ce [15–19], Ca, Pb and Y [20,21]. In general it is believed that shape (i.e. diffuseness) of the transition is defined by the dopant on the Ti site. In literature, tetravalent and pentavalent dopants on the Ti site are widely explored. The Ti site dopants widely explored are the ferroelectrically active Nb5+ and Ta5+ and the ferroelctrically inactive Zr4+, Sn4+, Ce4+. The dopants causes
Corresponding author. Tel.: +91 22 25767635; fax: +91 22 25723480.
E-mail address: [email protected] (P. Gopalan). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.02.026
loss of long range ordering by diluting the ferroelectric characteristics [22] or by impurity induced clusters triggered by soft mode correlation length [23]. The effect of Ti site dopants are commonly evaluated with or without dopant at Ba site co-doped BaTiO3. It is generally commented that B-site dopants play a predominant role in the extent of diffuseness. However, the effects cannot be categorized as A- or B-site whereas the defect reaction involved predominantly plays a role. Morrsion et al. [11] has explained the effect of La on the basis of Ti vacancy model and further they compared observed broadening of the transition with Zr doping. The doping of La and Zr brings about the same effects by disturbing the long range ordering due to the presence of Zr, a ferroelectric inactive ion or Ti vacancies. In the present study we embarked on evaluating effects of trivalent dopant (Ga3+ and La3+) at Ti and Ba sites. The main objective was to study effect of Ga3+ in order to suppress the Ti vacancies produced by La3+ at Ba site and thereby to study the effect on extent of diffuseness of the transition. Recently, we reported the effect of La and Ga (and Al) on the properties of the BaTiO3. Solid solutions of the type Ba13xLa2x Ti13xGa4xO3 [24] and Ba13xLa2xTi13xAl4xO3 [25] were investigated. The resulting compositions exhibited a diffuse transition as well as a deviation from C–W law, a high dielectric constant and low loss. In the present work, the focus is on ascertaining the individual roles of each dopant, La and Ga, on the properties of BaTiO3 that shows very interesting behavior. Following our earlier work [25], we have chosen Ba13xLa2xTi13yGa4yO3 (x ¼ y ¼ 0.006) as the starting composition that exhibits high dielectric constant and
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enhanced diffuseness. We report here on the synthesis, as well as the structure and dielectric property correlations. An increase in La concentration results in a decrease in the transition temperature and increase in diffuseness. However, a change in Ga concentration alters the transition temperature to a much smaller extent. Further, we have observed that an increase in the Ga concentration results in a classical ferroelectric transition exhibiting least diffuseness and a higher dielectric constant, due to suppressing Ti vacancies defects. Further, the nature of transition is studied with high temperature X-ray diffraction (HTXRD).
2. Experimental work Two series of nominal compositions corresponding to Ba13xLa2xTi13yGa4yO3 were prepared. In the first series, with x ¼ 0.006 fixed, six compositions with values of y varying between 0.002 and 0.014, and in the second series, for y ¼ 0.006 fixed, three compositions with values of x varying between 0 and 0.01 were synthesized by conventional solid-state reaction. The starting materials, BaCO3 (99.95%), TiO2 (99.9+%), La2O3 (99.99%) and Ga2O3 (99.99%), were procured from Aldrich Chemicals (USA). Stoichiometric amounts of the starting materials were mixed in a ball-mill with zirconia as a grinding media in ethanol. The powder was calcined at 1100 1C for 12 h, followed by repetitive stages of ball-milling and calcinations. The powder was characterized by X-ray diffraction (XRD) at room temperature and a few compositions were investigated using HTXRD between room temperature and 250 1C. The lattice parameters were extracted by indexing the data in the tetragonal space group P4/mmm, using the FULLPROF least square refinement software [26]. The powder was ball-milled for 24 h, dried, pelletized using a 10 mm diameter tungsten carbide die and sintered at 1350 1C for 4 h. The dielectric measurements were recorded in the temperature range between room temperature and 300 1C at different frequencies using a HP impedance analyzer (4192A). The microstructural aspects were characterized by orientation imaging microscopy (OIM) using a commercial TSL system on a FEI Quanta 200 HV SEM. The data are indexed in cubic space group as the tetragonality was less than 1% in all compositions.
Fig. 1. XRD pattern for Ba13xLa2xTi13yGa4yO3 for x ¼ 0.006 (fixed) and varying y ¼ 0.004 (a), y ¼ 0.006 (b), y ¼ 0.008 (c), y ¼ 0.012 (d) and y ¼ 0.016 (e).
3. Results and discussion The XRD patterns for a few representative compositions are exhibited in Fig. 1(a)–(e); all the compositions exhibit a single phase. The XRD data could be indexed in the tetragonal space group P4/mmm. The lattice parameters have been extracted from the refined XRD pattern. The tetragonality (c/a) is plotted in Fig. 2. It can be observed that the extent of tetragonality decreases with increasing La dopant concentration. However, the tetragonality varies to lesser extent with increase in Ga concentration. The change in tetragonality is explained qualitatively by the corresponding change in tolerance factor, which decreases with higher rate in case of La dopant. Tolerance factor decreases with increase in Ga concentration at a lower rate; the decrease in tetragonality with initial increase in Ga dopant concentration between y ¼ 0 and 0.004 corroborates with corresponding decrease in tolerance factor. With further increase in Ga concentration tetragonality increases, this increase in tetragonality is related with increase in grain size, which is addressed later in reference with OIM data. The dielectric behaviors as a function of temperature for different frequencies are plotted in Figs. 3–8. The composition x ¼ 0.006, y ¼ 0 exhibits anomalous dispersion of dielectric constant as a function of frequency. This is a characteristic of the space charge agglomera-
Fig. 2. Variation in tetragonality as a function of dopant concentration (x and y).
tion at grain boundaries (see Fig. 8). None of the other compositions exhibit such dispersion with frequency. The dielectric constant at 10 KHz as a function of temperature for all the compositions is provided in Figs. 9 and 10. For an increase in La concentration, the diffuseness increases and the transition temperature decreases. The transition temperature shows a weak dependence on Ga concentration; the transition temperature decreases with increase in Ga concentration between y ¼ 0 and 0.004 (at constant La, x ¼ 0.006) and increases with further increase in Ga concentration. The change in Tc is in good agreement with a corresponding change in tetragonality. The tetragonality as a function of the transition temperature is plotted in Fig. 11, which depicts a linear relationship. To compare the extent of diffuseness, normalized dielectric constant as a function of the normalized temperature is exhibited in Fig. 12(a) and (b). The diffuseness increases with an increase in La concentration. The composition (x ¼ 0.010, y ¼ 0.006) exhibits the highest diffuseness. For a fixed La (x ¼ 0.006), the diffuseness increases
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Fig. 3. Dielectric constant as a function of temperature for x ¼ y ¼ 0.006.
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Fig. 6. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.014.
Fig. 4. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.008.
Fig. 7. Dielectric constant as a function of temperature for x ¼ 0.010 and y ¼ 0.006.
Fig. 5. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.012.
Fig. 8. Dielectric constant as a function of temperature for x ¼ 0.006 and y ¼ 0.
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Fig. 9. Dielectric constant at 10 KHz as a function of temperature for all the compositions for fixed La (x ¼ 0.006).
Fig. 11. Transition temperature as a function of tetragonality ’x ¼ 0, y ¼ 0.006; x ¼ 0.002, y ¼ 0.006; mx ¼ 0.006, y ¼ 0.006; .x ¼ 0.010, y ¼ 0.006; ~x ¼ 0.006, y ¼ 0.004; bx ¼ 0.006, y ¼ 0.008; cx ¼ 0.006, y ¼ 0.014; *x ¼ 0.006, y ¼ 0.016.
Fig. 10. Dielectric constant at 10 KHz as a function of temperature for all the compositions with fixed Ga (y ¼ 0.006).
initially for Ga increasing from y ¼ 0 to 0.004, thereafter up to y ¼ 0.008 the diffuseness deceases, and the transition appears as for a classical ferroelectric transition. The compositions with a higher diffuseness exhibit a deviation from C–W law. To describe the behavior, the modified C–W law [27], provided below (Eq. (1)), has been used for further analysis. 1
1
max
¼
ðT T max Þg C
(1)
In Eq. (1), C and g are constants, and emax is the maximum dielectric constant at the transition temperature Tmax. The value of the constant g varies between 1 (classical ferroelectric) and 2 (relaxor). The constant g represents the slope of the plot between log (1/e1/emax) and log (TTmax), and is a measure of the diffuseness of the transition. A representative plot for two of the compositions is provided in Fig. 13. The extracted values of g are exhibited in Fig. 14. For a fixed Ga concentration (y ¼ 0.006), with an increase in the La concentration, g varies between 1.01 (x ¼ 0) and 1.72 (x ¼ 0.010). Thus, an increase in the La doping is responsible for destroying the long range ordering. However, as
Fig. 12. Plot of normalized dielectric constant at 10 KHz as a function of normalized temperature.
discussed above, for a constant value of La (x ¼ 0.006), an increase in the concentration of Ga leads to an initial increase in diffuseness and then diffuseness decreases with further increase in Ga; approaching the classical ferroelectric transition. The value
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Table 1 Transition temperature and dielectric constant for all the Ba13xLa2xTi13yGa4yO3 compositions. Composition x
y
0.006
0.00 0.002 0.004 0.006 0.008 0.012 0.016 0.006 0.006 0.006
0 0.002 0.010
Fig. 13. Plot of log (TTmax) vs. log (1/e1/emax) for representative compositions.
Fig. 14. Variation in g as a function of dopant concentration (x and y).
of g decreases from 1.8 for the (x ¼ 0.006, y ¼ 0.004) composition to 1.01 for the (x ¼ 0.006, y ¼ 0.008) composition. Thus, a classical ferroelectric state is attained by a restoration of the long range ordering with increase in Ga dopant concentration. Thereafter, with further increase in Ga, g increases marginally. This conclusion agrees with the inferences drawn from the normalized dielectric constant vs. normalized temperature (Fig. 12). The compositions x ¼ 0.006, y ¼ 0.004 and x ¼ 0.0010, y ¼ 0.006 exhibit the highest diffuseness; g1.8. The initial increase in diffuseness for lower Ga concentration is also observed in Ga doped BaTiO3 compositions, the details of which are under investigation at present and will be reported later. The increase in diffuseness at lower Ga concentration is due to the presence of two kinds of ions present at the ferroelectric active site and it reverses the trend with further increase in Ga concentration. The possible defect models governing the observed behavior are addressed later. The transition temperatures and the dielectric constant data for the various compositions are summarized in
Tc (1C)
Dielectric constant at Tc
95 85 65 80 90 90 90 120 110 45
27 500 5032 3695 5422 10 320 9301 8839 6368 8338 3561
Table 1. For a fixed Ga concentration (y ¼ 0.006), the dielectric constant increases initially in going from x ¼ 0 to 0.002, and thereafter the dielectric constant decreases. The initial increase in the dielectric constant followed by enhanced diffuseness is well established in La doped BaTiO3 [10]. The decrease in dielectric constant and an enhanced diffuseness with an increase in La concentration is a result of breakage of long range ordering as a result of Ti vacancies, which is an accepted defect model in La modified BaTiO3 (see Eq. (2)). The highest La concentration composition, x ¼ 0.010 for y ¼ 0.006, appears to exhibit the onset of relaxor type behavior, with a frequency dispersion near transition temperature, and a slight change in Tc with frequency (Fig. 9). However, we are unable to substantiate the observations due to the frequency span limitations, and a change in Tc is not observed in other compositions. For a constant La concentration (x ¼ 0.006), the dielectric constant decreases with increase in Ga concentration up to y ¼ 0.004 and thereafter increases with increase in the Ga concentration from y ¼ 0.004 to 0.008, and with further increase in y, the dielectric constant decreases at a slower rate. The effect of Ga doping may be divided into two zones: one as a La rich region, the other as a Ga rich region. The Ga rich region y40.006 (at constant La x ¼ 0.006) results in high dielectric constant accompanied by less diffuseness. For yo0.006 i.e. in La rich region, compositions exhibit higher diffuseness and low dielectric constant. The La doping in BaTiO3 acts as a donor doping giving rise to Ti vacancies [11]. Also, La acts as grain growth inhibitor. However, on other hand Ga doping (i.e. acceptor doping) is supposed to suppress Ti vacancies and thereby may give rise to grain growth. The increased diffuseness is also related to the grain size effect that is addressed latter. However, donor effect is predominant in the La rich region; grain growth is inhibited and compositions exhibit high diffuseness. However, in the Ga rich region, enhanced grain size is observed predominantly after crossover from La rich region to Ga rich region, at x ¼ y ¼ 0.006. The possible defect models are provided in Eqs. (2)–(4). Eq. (2), as discussed earlier, governs the defect model due to La doping. In Ga rich compositions, oxygen vacancies (Eq. (3)) and selfcompensation (Eq. (4)) are the possible mechanisms. The higher valent ion at Ba site (La) and the lower valent ion at Ti site (Ga) may lead to self-compensation, restoring the long range ordering and as a consequence the diffuseness decreases, followed by an enhancement in the dielectric constant. Further, the formation of oxygen vacancies cannot be ruled out and in fact both the mechanisms may be operative. La2 O3 ! 2LaBad þ 12V kTi
(2)
Ga2 O3 ! 2GaTid þ V Od d
(3)
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La2 O3 þ Ga2 O3 ! 2LaBad þ 2GajTi
(4)
In the representative OIM plots exhibited in Fig. 15, none of the compositions exhibit preferred orientation. The average grain size
obtained from OIM data are provided in Table 2. The grain size decreases with increase in La concentration (see x ¼ 0, y ¼ 0.006; x ¼ 0.002, y ¼ 0.006 and x ¼ y ¼ 0.006). However, the grain size increases with increase in Ga concentration from y ¼ 0.006 to 0.008 at constant La (x ¼ 0.006). Thereafter grain size decreases slowly with further increase in Ga concentration, this explains the marginal increase in diffuseness at higher Ga compositions. This corroborates with decrease in dielectric constant at higher Ga concentration. The smaller grain sizes along with coarser grains are observed in Fig. 15. Satisfactory data cannot be recorded for other samples, as smaller grain size is possibly comparable to electron spot diameter, resulting in higher overlap of diffraction patterns and weak patterns. In order to interpret crystallographic changes, HTXRD experiments were carried out between room temperature and 250 1C. The compositions exhibited a tetragonal splitting for higher angle peaks well above the temperature where the dielectric constant exhibited a maximum (Tmax). However, the splitting disappeared at Tmax for pure BaTiO3. The changes in volume and lattice parameter as a function of temperature are plotted in Figs. 16 and 17 respectively. Pure BaTiO3 and the composition x ¼ 0.002, y ¼ 0.006 exhibit a sharp change in
Fig. 16. Change in volume as a function of temperature.
Fig. 15. Inverse pole figure of Ba13xLa2xTi13xGa4xO3 compositions: (a) x ¼ 0, y ¼ 0.006; (b) x ¼ 0.002, y ¼ 0.006; (c) x ¼ 0.006, x ¼ 0.012; (d) x ¼ 0.006, y ¼ 0.014. Table 2 Average grain size for the Ba13xLa2xTi13yGa4yO3 compositions obtained from OIM. Composition
Average grain size (mm)
x
y
0 0.002 0.006 0.006 0.006 0.006
0.006 0.006 0.006 0.008 0.012 0.014
52 4.7 1.5 7 4.5 3.7 Fig. 17. Change in lattice parameters as a function of temperature.
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volume. The compositions exhibiting diffuse transition, namely x ¼ 0.006 ¼ 0.006 and x ¼ 0.010, y ¼ 0.006, do not exhibit a sudden change in volume; the nature of the transition is rather continuous. In Fig. 17, BaTiO3 and x ¼ 0.002, y ¼ 0.006 exhibit a sudden change in lattice parameters near to their respective Tmax. However, the change in lattice parameters for x ¼ 0.01, y ¼ 0.006 is continuous and the structure is tetragonal well above Tmax, unlike pure BaTiO3; this corroborates with the observed diffuse phase transition and higher g (see Fig. 9). The higher value of g represents the deviation from C–W law and is indicative of formation of polar domains; our experimental results are in agreement with theory. The similar effects are observed in an investigation of effect of hetero- and iso-valent doped BaTiO3 on phase transition [28]. Also, this is in agreement with the related work on La and Zr modified PbTiO3; the existence of lower temperature polar phase is observed till the tetragonal to cubic phase transition temperature (490 1C) of pure PbTiO3 [29]. The decrease in Tmax results in a wide difference between the Tmax and crystallographic transition temperature, resulting in a diffuse phase transition. This is an experimental evidence that the transition observed in dielectric constant as a function of temperature does not correspond to crystallographic phase change. The experimental findings corroborate with the higher diffuseness observed in the compositions.
Acknowledgment
4. Conclusions
[17]
In the present investigations, the changes in dielectric properties as a result of co-doping of La and Ga in barium titanate are addressed. Also the individual role of each of the dopant, La and Ga, is established and the defect mechanism is addressed. The La dopant is effective in lowering the Tc and enhancing the diffuseness. However, on the other hand, Ga increases the dielectric constant making the transition more prominent and restores long range ordering. The composition x ¼ y ¼ 0.006 exhibits the highest dielectric constant at room temperature as a compromise between diffuseness and dielectric constant. Our work establishes a methodology to develop a diffuse transition and higher dielectric constant, and also on the nature of the crystallographic changes in doped BaTiO3 materials exhibiting diffuse phase transition. Further work may lead to the development of a novel material as a result of simultaneous doping of La and Ga.
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The authors thank the ER and IPR Division, DRDO, Government of India for the generous research support that helped in the execution of this work.
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