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Effect of oxygen activity on Nb solubility in BaTiO3
Journal of Physics and Chemistry of Solids 62 (2001) 537±541
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Effect of oxygen activity on Nb solubility in BaTiO3 K. Kowalski a,1, M. Ijjaali a, T. Bak b, B. Dupre a, J. Nowotny b,*, M. Rekas b, C.C. Sorrell b b
a Universite Nancy I, Faculte des Sciences, Laboratoire de Chimie du Solide Mineral, 54506 Vandoeuovre-les-Nancy, France Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
Received 29 July 1999; accepted 21 March 2000
Abstract This paper describes the effect of oxygen partial pressure on Nb solubility in polycrystalline BaTiO3 at 1673 K. It was found that an increase of p(O2) in the gas phase results in an increased Nb solubility in BaTiO3. Annealing of polycrystalline BaTiO3, covered with a thin layer of NbCl5, in air at 1573 K results in Nb incorporation to the level of 0.1 at.% while annealing at the same temperature in nitrogen (involving 2% H2) does not lead to Nb incorporation into BaTiO3. The observed effect of p(O2) on Nb solubility in BaTiO3 is consistent with the defect disorder model of BaTiO3. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Defects; D. Dielectric properties; D. Diffusion transport properties
1. Introduction Processing technologies of electronic ceramic materials based on BaTiO3 require introduction of small amounts of donors and/or acceptors either into the BaTiO3 lattice or into intergranular spaces. Donor-type additions are important components of dielectric materials based on BaTiO3 [1± 5]. However, despite a number of papers reporting properties of donor-doped BaTiO3 [2±14], so far little is known on processing of donor-doped BaTiO3 and, speci®cally, the impact of the gas phase on the processing. Therefore, there is a need to understand the impact of gas phase composition on the formation of BaTiO3 ±Nb2O5 solid solution. Based on defect chemistry of BaTiO3 [1,15] one should expect that the concentration of the predominant defects in undoped BaTiO3 of stoichiometric composition (Ba/Ti 1) is determined by oxygen partial pressure. Speci®cally, the concentration of oxygen vacancies and cation vacancies are the predominant defects at low and high values of p(O2), * Corresponding author. Tel.: 161-2-9385-6465; fax: 161-29385-6467. E-mail address: [email protected] (J. Nowotny). 1 Present address: Surface Spectroscopy Laboratory, University of Mining and Metallurgy and Joint Center for Chemical Analysis and Structural Research of Jaqiellonian University, ul. Reymonta 23, 30-059 Krakow, Poland.
respectively. Moreover, based on defect equilibrium constants derived from defect equilibria one may evaluate the dependence of defects concentrations as a function of p(O2) [15,16]. However, these dependences are not valid for non-stoichiometric BaTiO3 involving either excess of Ba (Ba/Ti . 1) or excess of Ti (Ba/Ti , 1). Speci®cally, excess of Ba results in the formation of Ti vacancies those are formed according to the reaction: zz BaO O BaBa 1 V 0000 Ti 1 OO 1 2VO :
1
In analogy, excess of Ti leads to the formation of Ba vacancies: TiO2 O TiTi 1 V 00Ba 1 2OO 1 VOzz :
2
Then the concentration of cation vacancies is determined by the cation non-stoichiometry and is practically independent of p(O2). In the preceding papers we have reported diffusion coef®cient of Nb in BaTiO3 in air [17,18]. The purpose of this paper is to evaluate the effect of gas phase composition, and speci®cally of oxygen partial pressure, on Nb solubility in BaTiO3. This information is essential for establishment of optimal processing conditions, such as temperature, time of annealing and gas phase composition, of donor-doped BaTiO3.
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00212-2
538
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
2. Experimental Polycrystalline BaTiO3 of stoichiometric composition was provided by Cookson Technology Centre, UK. The powder was cold pressed into disc in an isostatic press (diameter 10 mm) and then hot pressed for 1 h at 1723 K under 21 MPa (hot isostatic press). Then the specimen was annealed at 1673 K for 70 h. The density of thus formed disc was 99.8% of the theoretical density. The Nb-doped BaTiO3 was prepared by impregnation of the BaTiO3 disc with NbCl5. The procedure of doping involved deposition of a thin layer of NbCl5 on the discs of undoped BaTiO3 and then annealing at 1573 K for 6 h in (a) an oxidizing atmosphere (air) and (b) reducing atmosphere (nitrogen involving 2% of hydrogen). In order to minimize the effect of Nb evaporation during annealing the specimen was closed in a quartz ampoule. Fig. 1a±c show SEM image of the BaTiO3 surface after 6 h annealing at 1573 K in air after deposition of NbCl5. This image indicates a lack of pores and con®rms that the specimen has a high density. As seen the grains are well shaped and grain boundaries are well visible. The average grain size is about 10 mm. Analysis of the Nb penetration pro®le was made using X-Ray Microprobe (Camebax) made by Cameca. Before the determination the crystal was polished with diamond paste involving 0.5 mm diamond powder. The analysis was made along a linescan that was perpendicular to the surface on which Nb was deposited. The measurements were taken at 2 mm distances within a strong concentration gradient (close to the surface) and 5 mm at low concentration gradient (far from the surface). The diameter of the analysed beam was 2 mm.
3. Results and discussion Figs. 2 and 3 show the Nb concentration as a function of the distance from the surface along the scanning distance 250±350 mm after annealing in air and in reducing atmosphere, respectively. As seen on both ®gures Nb concentration exhibits peaks. In both cases the average distance between the peaks is the same as average grain size (10 mm). Concordantly, one may assume that maxima and minima of the Nb peaks correspond to the grain boundary area and the bulk of grains, respectively. As seen from Figs. 2 and 3 the ratio between the maxima and the minima of the peaks depends essentially on the gas phase environment under the specimens were annealed. The average Nb ratio for the specimens annealed in oxidizing atmosphere (air) is about two. In the case of the specimens annealed in reducing atmosphere (nitrogen 1 2% hydrogen) the peaks assume the values that are substantially larger than those for the specimen annealed in air. The maxima of the Nb peaks for specimens annealed at oxidising and reducing atmospheres remain in the ranges between 0.1±0.25 and
1±3, respectively. As seen in Figs. 2 and 3 the gas phase composition has a substantial impact on the bulk Nb content. Speci®cally, annealing in air results in about 0.05±0.1 at.% while annealing in reducing atmosphere does not allow for Nb-incorporation into the BaTiO3 grains. Results of the X-ray microprobe analysis are summarised in Table 1. As seen, the bulk Nb concentration in BaTiO3 annealed in air is visibly elevated with the surface layer being slightly enriched in Nb. As seen the bulk Nb solubility under reducing atmosphere is zero or negligibly low. The effect of gas phase composition on the bulk and grain boundary solubility of Nb may be considered in terms of defect chemistry of BaTiO3 [1] and, speci®cally, the impact of p(O2) on concentrations of defects and related charge neutrality condition. Interactions between oxygen and BaTiO3 may be expressed according to the equilibria [1]: OO O 3 2
1 2
O2 1 2e 0 1 VOzz
00 O2 1 6e 0 O 3OO 1 V 0000 Ti 1 V Ba
3 4
Concordantly, the charge neutrality for undoped BaTiO3 assumes the following form: 0 0 2VOzz 1 hz 2V 00Ba 1 4V 0000 Ti 1 A 1 e
5
0
where [A ] denotes the concentration of acceptor-type elements including both intentionally introduced additions and impurities. It has been generally agreed that the predominant defects in very reduced BaTiO3 (undoped) are oxygen vacancies that are compensated by acceptor-type impurities and electrons [1,18]. This is the case when BaTiO3 is annealed at very low p(O2). Then BaTiO3 exhibits n-type properties and the condition (5) assumes the following form: 2VOzz A 0 1 e 0
6
In this case the concentration of cation vacancies in undoped BaTiO3 is reduced to a negligibly low level. Doping with Nb results in n-type properties. Nb incorporation into BaTiO3 can be expressed by following equilibrium: 5BaO 1 2Nb2 O5 O 5BaBa 1 4NbTi z 1 15OO 1 V 0000 Ti
7
Then the lattice charge neutrality requires that: 0 NbTi z 1 2VOzz 4V 0000 Ti 1 e
8
Concordantly, assuming that Nb may enter the BaTiO3 lattice only via Ti vacancies, there are not available diffusion tracks for Nb incorporation in reduced BaTiO3. This is in agreement with the experiment performed in this study. Speci®cally, it was observed that there is no detectable Nbincorporation into BaTiO3 that is annealed under reduced conditions (nitrogen and 2% of hydrogen). This observation is in con¯ict with the report of Buessem and Kahn [5] who claim that homogeneous Nb distribution may be achieved by reducing atmospheres.
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
539
Fig. 1. SEM micrograph of BaTiO3 hot pressed at 1723 K after polishing impregnation with NbCl5 and annealing at 1573 K for 2 h; enlargement: 300 (a), 1000 (b), 3000 (c).
540
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541 Table 1 Average concentration of Nb (in at.%) in polycrystalline BaTiO3 in the bulk phase of individual grains and within intergranular regions after annealing at 1573 K during 6 h in oxidised and reducing atmospheres Bulk Oxidation 0.05±0.1 Reduction 0.0
Fig. 2. The Nb concentration pro®les across the specimen annealed at 1673 K for 70 h in air.
At high p(O2) the predominant defects in BaTiO3 are both Ba and Ti vacancies [1,18]. Then the charge neutrality condition (5) assumes the form: z
h
2V 00Ba
1
4V 0000 Ti
9
In this case BaTiO3 assumes p-type properties. Assuming that Nb can only be incorporated into the Ti sublattice one may expect that increase of p(O2) leads to increase of Nb incorporation and, consequently, the Nb content. This is in agreement with the experiment which shows that Nb can be incorporated into the bulk of BaTiO3 only at high p(O2) while Nb incorporation is blocked at low p(O2). The results obtained in this work allow to select the most optimised processing conditions aiming at the preparation of Nb-doped BaTiO3. Speci®cally, increase of p(O2) results in increase of the concentration of Ti vacancies and, consequently, creates more favourable conditions for Nb incorporation. However, annealing of BaTiO3 under reducing conditions results in reduction of Ti vacancies to negligibly low level and, consequently, leads to blocking of Nb incorporation. Then all of Nb introduced to the specimen by
Fig. 3. The Nb concentration pro®les across the specimen annealed at 1673 K for 70 h in nitrogen±hydrogen (2%) mixture.
Grain boundary 0.1±0.18 3.0
impregnation remains within grain boundary area while the bulk of BaTiO3 remain undoped and Nb-free.
4. Conclusions There is a substantial effect of oxygen partial pressure on Nb concentration in BaTiO3 at 1573 K. It appears that Nb may be incorporated into the BaTiO3 lattice according to the Ti vacancy mechanism. These Ti vacancies can be generated at high oxygen partial pressure. The presence of Ti vacancies is also required to enable ionic compensation of donors formed as a result of Nb incorporation. In contrast Nb does not enter the lattice of reduced BaTiO3 when the concentration of Ti vacancies assumes negligibly low level and, consequently, the incorporation process is blocked. Then rapid transport of Nb along grain boundaries results in ®lling intergranular spaces with Nb2O5 addition while the bulk of grains remains undoped. The results obtained in the present work may serve in selecting appropriate preparation conditions of Nb-doped BaTiO3. High values of equilibrium p(O2) allows the formation of Nb2O5 ±BaTiO3 solid solution. The impregnation procedure described in this work results in Nb bulk concentration at the level of 0.05±0.1 at.% although the Nb solubility limit is substantially larger. In contrast annealing at very reduced atmosphere results in reduction of the concentration of Ti vacancies to negligibly low level and, consequently, the bulk transport of Nb is blocked. One should expect that solubility of Nb in BaTiO3 may be enhanced not only by temperature, which is already close to the stability limit of the cubic phase [1], but also by application of high oxygen pressures. It has been postulated that the interface layer of undoped BaTiO3 exhibits different structure than that of the bulk phase [1,19,20]. It was also reported that Nb has a tendency to segregate towards surfaces and grain boundaries [20]. A strong Nb concentration gradient within the surface layer was also observed by Buessem and Kahn [5]. The observed effect of gas phase composition on Nb concentration in BaTiO3 is in agreement with defect disorder models of undoped and Nb-doped BaTiO3. The observed effect is in disagreement with the report of
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
Buessem and Kahn [5] who have claimed that homogeneous Nb distribution in BaTiO3 may be enhanced under reducing atmospheres. References [1] J. Nowotny, M. Rekas, in: J. Nowotny (Ed.), Electronic Ceramic Materials, Trans Tech Public, Zurich, 1991, p. 1. [2] M. Rekas, in: P. Barret, L.C. Dufour (Eds.), Materials Science Monographs, vol.28, Elsevier, Amsterdam, 1985 (367pp.). [3] M. Kahn, J. Am. Ceram. Soc. 54 (1971) 452. [4] M. Kahn, J. Am. Ceram. Soc. 54 (1971) 455. [5] R.W. Buessem, M. Kahn, J. Am. Ceram. Soc. 54 (1971) 458. [6] A.M.J.H. Seuter, Philips Res.Rept. Suppl. (1974) 1. [7] R. Wernicke, Phys. Stat. Sol. A 47 (1978) 139. [8] G.H. Jonker, E.E. Havinga, Mater. Res. Bull. 17 (1982) 345. [9] N.H. Chan, D.M. Smyth, J. Am. Ceram. Soc. 67 (1984) 285. [10] N.H. Chan, M.P. Harmer, D.M. Smyth, J. Am. Ceram. Soc. 69 (1986) 507.
541
[11] J.M. Millet, R.S. Roth, L.D. Ettlinger, H.S. Parker, J. Solid State Chem. 67 (1987) 259. [12] R.S. Roth, L.D. Ettlinger, H.S. Parker, J. Solid State Chem. 68 (1987) 330. [13] D.M. Smyth, M.P. Harmer, P. Peng, J. Am. Ceram. Soc. 72 (1989) 2276. [14] C. Gillot, J.P. Michenaut, M. Maglione, B. Jannot, Solid State Commun. 84 (1992) 1033. [15] J. Nowotny, M. Rekas, Solid State Ionics 49 (1991) 135. [16] J. Nowotny, M. Rekas, Intern. Ceram. Monogr. 2 (1999) 269. [17] B. Dupre, M. Ijjaali, K. Kowalski, J. Nowotny, Intern. Ceram. Monogr. 2 (1999) 104±109. [18] K. Kowalski, M. Ijjaali, T. Bak, B. Dupre, J. Nowotny, M. Rekas, C.C. Sorrell, Y. Zhao, J. Phys. Chem. Solids 62 (3) (2000) (this issue). [19] Z. Zhang, P.J. Pigram, J. Nowotny, R.N. Lamb, J. Aust. Ceram. Soc. 34 (1998) 254. [20] A. Bernasik, W. Hirschwald, J. Nowotny, F. Stolze, Ceram. Int. 21 (1999) 263.
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Effect of oxygen activity on Nb solubility in BaTiO3 K. Kowalski a,1, M. Ijjaali a, T. Bak b, B. Dupre a, J. Nowotny b,*, M. Rekas b, C.C. Sorrell b b
a Universite Nancy I, Faculte des Sciences, Laboratoire de Chimie du Solide Mineral, 54506 Vandoeuovre-les-Nancy, France Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
Received 29 July 1999; accepted 21 March 2000
Abstract This paper describes the effect of oxygen partial pressure on Nb solubility in polycrystalline BaTiO3 at 1673 K. It was found that an increase of p(O2) in the gas phase results in an increased Nb solubility in BaTiO3. Annealing of polycrystalline BaTiO3, covered with a thin layer of NbCl5, in air at 1573 K results in Nb incorporation to the level of 0.1 at.% while annealing at the same temperature in nitrogen (involving 2% H2) does not lead to Nb incorporation into BaTiO3. The observed effect of p(O2) on Nb solubility in BaTiO3 is consistent with the defect disorder model of BaTiO3. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Defects; D. Dielectric properties; D. Diffusion transport properties
1. Introduction Processing technologies of electronic ceramic materials based on BaTiO3 require introduction of small amounts of donors and/or acceptors either into the BaTiO3 lattice or into intergranular spaces. Donor-type additions are important components of dielectric materials based on BaTiO3 [1± 5]. However, despite a number of papers reporting properties of donor-doped BaTiO3 [2±14], so far little is known on processing of donor-doped BaTiO3 and, speci®cally, the impact of the gas phase on the processing. Therefore, there is a need to understand the impact of gas phase composition on the formation of BaTiO3 ±Nb2O5 solid solution. Based on defect chemistry of BaTiO3 [1,15] one should expect that the concentration of the predominant defects in undoped BaTiO3 of stoichiometric composition (Ba/Ti 1) is determined by oxygen partial pressure. Speci®cally, the concentration of oxygen vacancies and cation vacancies are the predominant defects at low and high values of p(O2), * Corresponding author. Tel.: 161-2-9385-6465; fax: 161-29385-6467. E-mail address: [email protected] (J. Nowotny). 1 Present address: Surface Spectroscopy Laboratory, University of Mining and Metallurgy and Joint Center for Chemical Analysis and Structural Research of Jaqiellonian University, ul. Reymonta 23, 30-059 Krakow, Poland.
respectively. Moreover, based on defect equilibrium constants derived from defect equilibria one may evaluate the dependence of defects concentrations as a function of p(O2) [15,16]. However, these dependences are not valid for non-stoichiometric BaTiO3 involving either excess of Ba (Ba/Ti . 1) or excess of Ti (Ba/Ti , 1). Speci®cally, excess of Ba results in the formation of Ti vacancies those are formed according to the reaction: zz BaO O BaBa 1 V 0000 Ti 1 OO 1 2VO :
1
In analogy, excess of Ti leads to the formation of Ba vacancies: TiO2 O TiTi 1 V 00Ba 1 2OO 1 VOzz :
2
Then the concentration of cation vacancies is determined by the cation non-stoichiometry and is practically independent of p(O2). In the preceding papers we have reported diffusion coef®cient of Nb in BaTiO3 in air [17,18]. The purpose of this paper is to evaluate the effect of gas phase composition, and speci®cally of oxygen partial pressure, on Nb solubility in BaTiO3. This information is essential for establishment of optimal processing conditions, such as temperature, time of annealing and gas phase composition, of donor-doped BaTiO3.
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00212-2
538
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
2. Experimental Polycrystalline BaTiO3 of stoichiometric composition was provided by Cookson Technology Centre, UK. The powder was cold pressed into disc in an isostatic press (diameter 10 mm) and then hot pressed for 1 h at 1723 K under 21 MPa (hot isostatic press). Then the specimen was annealed at 1673 K for 70 h. The density of thus formed disc was 99.8% of the theoretical density. The Nb-doped BaTiO3 was prepared by impregnation of the BaTiO3 disc with NbCl5. The procedure of doping involved deposition of a thin layer of NbCl5 on the discs of undoped BaTiO3 and then annealing at 1573 K for 6 h in (a) an oxidizing atmosphere (air) and (b) reducing atmosphere (nitrogen involving 2% of hydrogen). In order to minimize the effect of Nb evaporation during annealing the specimen was closed in a quartz ampoule. Fig. 1a±c show SEM image of the BaTiO3 surface after 6 h annealing at 1573 K in air after deposition of NbCl5. This image indicates a lack of pores and con®rms that the specimen has a high density. As seen the grains are well shaped and grain boundaries are well visible. The average grain size is about 10 mm. Analysis of the Nb penetration pro®le was made using X-Ray Microprobe (Camebax) made by Cameca. Before the determination the crystal was polished with diamond paste involving 0.5 mm diamond powder. The analysis was made along a linescan that was perpendicular to the surface on which Nb was deposited. The measurements were taken at 2 mm distances within a strong concentration gradient (close to the surface) and 5 mm at low concentration gradient (far from the surface). The diameter of the analysed beam was 2 mm.
3. Results and discussion Figs. 2 and 3 show the Nb concentration as a function of the distance from the surface along the scanning distance 250±350 mm after annealing in air and in reducing atmosphere, respectively. As seen on both ®gures Nb concentration exhibits peaks. In both cases the average distance between the peaks is the same as average grain size (10 mm). Concordantly, one may assume that maxima and minima of the Nb peaks correspond to the grain boundary area and the bulk of grains, respectively. As seen from Figs. 2 and 3 the ratio between the maxima and the minima of the peaks depends essentially on the gas phase environment under the specimens were annealed. The average Nb ratio for the specimens annealed in oxidizing atmosphere (air) is about two. In the case of the specimens annealed in reducing atmosphere (nitrogen 1 2% hydrogen) the peaks assume the values that are substantially larger than those for the specimen annealed in air. The maxima of the Nb peaks for specimens annealed at oxidising and reducing atmospheres remain in the ranges between 0.1±0.25 and
1±3, respectively. As seen in Figs. 2 and 3 the gas phase composition has a substantial impact on the bulk Nb content. Speci®cally, annealing in air results in about 0.05±0.1 at.% while annealing in reducing atmosphere does not allow for Nb-incorporation into the BaTiO3 grains. Results of the X-ray microprobe analysis are summarised in Table 1. As seen, the bulk Nb concentration in BaTiO3 annealed in air is visibly elevated with the surface layer being slightly enriched in Nb. As seen the bulk Nb solubility under reducing atmosphere is zero or negligibly low. The effect of gas phase composition on the bulk and grain boundary solubility of Nb may be considered in terms of defect chemistry of BaTiO3 [1] and, speci®cally, the impact of p(O2) on concentrations of defects and related charge neutrality condition. Interactions between oxygen and BaTiO3 may be expressed according to the equilibria [1]: OO O 3 2
1 2
O2 1 2e 0 1 VOzz
00 O2 1 6e 0 O 3OO 1 V 0000 Ti 1 V Ba
3 4
Concordantly, the charge neutrality for undoped BaTiO3 assumes the following form: 0 0 2VOzz 1 hz 2V 00Ba 1 4V 0000 Ti 1 A 1 e
5
0
where [A ] denotes the concentration of acceptor-type elements including both intentionally introduced additions and impurities. It has been generally agreed that the predominant defects in very reduced BaTiO3 (undoped) are oxygen vacancies that are compensated by acceptor-type impurities and electrons [1,18]. This is the case when BaTiO3 is annealed at very low p(O2). Then BaTiO3 exhibits n-type properties and the condition (5) assumes the following form: 2VOzz A 0 1 e 0
6
In this case the concentration of cation vacancies in undoped BaTiO3 is reduced to a negligibly low level. Doping with Nb results in n-type properties. Nb incorporation into BaTiO3 can be expressed by following equilibrium: 5BaO 1 2Nb2 O5 O 5BaBa 1 4NbTi z 1 15OO 1 V 0000 Ti
7
Then the lattice charge neutrality requires that: 0 NbTi z 1 2VOzz 4V 0000 Ti 1 e
8
Concordantly, assuming that Nb may enter the BaTiO3 lattice only via Ti vacancies, there are not available diffusion tracks for Nb incorporation in reduced BaTiO3. This is in agreement with the experiment performed in this study. Speci®cally, it was observed that there is no detectable Nbincorporation into BaTiO3 that is annealed under reduced conditions (nitrogen and 2% of hydrogen). This observation is in con¯ict with the report of Buessem and Kahn [5] who claim that homogeneous Nb distribution may be achieved by reducing atmospheres.
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
539
Fig. 1. SEM micrograph of BaTiO3 hot pressed at 1723 K after polishing impregnation with NbCl5 and annealing at 1573 K for 2 h; enlargement: 300 (a), 1000 (b), 3000 (c).
540
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541 Table 1 Average concentration of Nb (in at.%) in polycrystalline BaTiO3 in the bulk phase of individual grains and within intergranular regions after annealing at 1573 K during 6 h in oxidised and reducing atmospheres Bulk Oxidation 0.05±0.1 Reduction 0.0
Fig. 2. The Nb concentration pro®les across the specimen annealed at 1673 K for 70 h in air.
At high p(O2) the predominant defects in BaTiO3 are both Ba and Ti vacancies [1,18]. Then the charge neutrality condition (5) assumes the form: z
h
2V 00Ba
1
4V 0000 Ti
9
In this case BaTiO3 assumes p-type properties. Assuming that Nb can only be incorporated into the Ti sublattice one may expect that increase of p(O2) leads to increase of Nb incorporation and, consequently, the Nb content. This is in agreement with the experiment which shows that Nb can be incorporated into the bulk of BaTiO3 only at high p(O2) while Nb incorporation is blocked at low p(O2). The results obtained in this work allow to select the most optimised processing conditions aiming at the preparation of Nb-doped BaTiO3. Speci®cally, increase of p(O2) results in increase of the concentration of Ti vacancies and, consequently, creates more favourable conditions for Nb incorporation. However, annealing of BaTiO3 under reducing conditions results in reduction of Ti vacancies to negligibly low level and, consequently, leads to blocking of Nb incorporation. Then all of Nb introduced to the specimen by
Fig. 3. The Nb concentration pro®les across the specimen annealed at 1673 K for 70 h in nitrogen±hydrogen (2%) mixture.
Grain boundary 0.1±0.18 3.0
impregnation remains within grain boundary area while the bulk of BaTiO3 remain undoped and Nb-free.
4. Conclusions There is a substantial effect of oxygen partial pressure on Nb concentration in BaTiO3 at 1573 K. It appears that Nb may be incorporated into the BaTiO3 lattice according to the Ti vacancy mechanism. These Ti vacancies can be generated at high oxygen partial pressure. The presence of Ti vacancies is also required to enable ionic compensation of donors formed as a result of Nb incorporation. In contrast Nb does not enter the lattice of reduced BaTiO3 when the concentration of Ti vacancies assumes negligibly low level and, consequently, the incorporation process is blocked. Then rapid transport of Nb along grain boundaries results in ®lling intergranular spaces with Nb2O5 addition while the bulk of grains remains undoped. The results obtained in the present work may serve in selecting appropriate preparation conditions of Nb-doped BaTiO3. High values of equilibrium p(O2) allows the formation of Nb2O5 ±BaTiO3 solid solution. The impregnation procedure described in this work results in Nb bulk concentration at the level of 0.05±0.1 at.% although the Nb solubility limit is substantially larger. In contrast annealing at very reduced atmosphere results in reduction of the concentration of Ti vacancies to negligibly low level and, consequently, the bulk transport of Nb is blocked. One should expect that solubility of Nb in BaTiO3 may be enhanced not only by temperature, which is already close to the stability limit of the cubic phase [1], but also by application of high oxygen pressures. It has been postulated that the interface layer of undoped BaTiO3 exhibits different structure than that of the bulk phase [1,19,20]. It was also reported that Nb has a tendency to segregate towards surfaces and grain boundaries [20]. A strong Nb concentration gradient within the surface layer was also observed by Buessem and Kahn [5]. The observed effect of gas phase composition on Nb concentration in BaTiO3 is in agreement with defect disorder models of undoped and Nb-doped BaTiO3. The observed effect is in disagreement with the report of
K. Kowalski et al. / Journal of Physics and Chemistry of Solids 62 (2001) 537±541
Buessem and Kahn [5] who have claimed that homogeneous Nb distribution in BaTiO3 may be enhanced under reducing atmospheres. References [1] J. Nowotny, M. Rekas, in: J. Nowotny (Ed.), Electronic Ceramic Materials, Trans Tech Public, Zurich, 1991, p. 1. [2] M. Rekas, in: P. Barret, L.C. Dufour (Eds.), Materials Science Monographs, vol.28, Elsevier, Amsterdam, 1985 (367pp.). [3] M. Kahn, J. Am. Ceram. Soc. 54 (1971) 452. [4] M. Kahn, J. Am. Ceram. Soc. 54 (1971) 455. [5] R.W. Buessem, M. Kahn, J. Am. Ceram. Soc. 54 (1971) 458. [6] A.M.J.H. Seuter, Philips Res.Rept. Suppl. (1974) 1. [7] R. Wernicke, Phys. Stat. Sol. A 47 (1978) 139. [8] G.H. Jonker, E.E. Havinga, Mater. Res. Bull. 17 (1982) 345. [9] N.H. Chan, D.M. Smyth, J. Am. Ceram. Soc. 67 (1984) 285. [10] N.H. Chan, M.P. Harmer, D.M. Smyth, J. Am. Ceram. Soc. 69 (1986) 507.
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