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Properties of the solid solution (1−x)Na0.5Bi0.5TiO3–(x)BiFeO3
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1762–1766
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Properties of the solid solution (1x)Na0.5Bi0.5TiO3–(x)BiFeO3 ˜ a c, G. Trolliard b V. Dorcet a,b,, P. Marchet b, O. Pen a b c
´chal Juin, F-14050-Caen Cedex, France Laboratoire de Cristallographie et Sciences des Mate´riaux, CRISMAT-CNRS UMR 6508, 6 Bd. du Mare ´de´s Ce´ramiques et des Traitements de Surface, SPCTS-CNRS UMR 6638, 123 Av. Albert Thomas, F-87060-Limoges Cedex, France Laboratoire Sciences des Proce Laboratoire des Sciences Chimiques de Rennes, UMR 6226 CNRS, Campus de Beaulieu, F-35042-Rennes Cedex, France
a r t i c l e in f o
a b s t r a c t
Available online 8 February 2009
This study shows that remarkable electric and magnetic properties are encountered within the (1x)Na0.5Bi0.5TiO3 (NBT)–(x)BiFeO3 (BF) solid solution. Dual ferroelectric and magnetic properties are observed in the BF-rich part of the solid solution implying intrinsic multiferroic character of the compounds. In addition, a relaxation phenomenon is evidenced within the overall compositional domain of the solid solution. This study emphasizes that in the NBT-rich part, the relaxor behaviour is very similar to that of NBT, while beyond x ¼ 0.5, it turns to a different mechanism of relaxation probably induced by the presence of oxygen vacancies resulting from the mixed valence of the iron cations. & 2009 Elsevier B.V. All rights reserved.
Keywords: NBT BF Multiferroic Dielectric property Magnetic property
1. Introduction Na0.5Bi0.5TiO3 (NBT) and BiFeO3 (BF) are two perovskite compounds showing R3c space group at ambient temperature. A previous study [1] has shown that they form a continuous R3c solid solution at room temperature. These two compounds exhibit a ferroelectric behaviour which is maintained within all such solid solution [1]. However, important discrepancies in their physical properties arise too, that made this solid solution an interesting case study. On one hand, BF is also antiferromagnetic until its Ne´el temperature at about TN ¼ 370 1C [2] while NBT does not present any magnetic properties. So the solid solution may show interesting intrinsic multiferroic behaviour. On the other hand, NBT presents a ferroelectric–antiferroelectric transition at about 200 1C [3] associated to a small frequency-dependent dielectric anomaly appearing as a hump in the permittivity curves [3] testifying the relaxor behaviour of NBT [3,4] while BF neither presents antiferroelectric nor relaxor properties [5,6]. In addition, a recent in-situ TEM study [7] of the phase transition sequence demonstrated that the first dielectric anomaly of NBT (200 1C) is associated to the beginning of a first order reconstructive rhombohedral to orthorhombic phase transition, which proceeds between 200 and 290 1C, via the formation of a modulated phase [7]. The authors suggested that the modulated phase as well as
Corresponding author at: Laboratoire de Cristallographie et Sciences des Mate´riaux, CRISMAT-CNRS UMR 6508, 6, Bd. du Mare´chal Juin, F-14050-Caen Cedex, France. Tel.: +33 0 2 31 45 26 33; fax: +33 0 2 31 95 16 00. E-mail address: [email protected] (V. Dorcet).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.02.014
the orthorhombic one is in fact antiferroelectric [7]. The same authors have also concluded that the second dielectric anomaly of NBT (maximum of the permittivity 320 1C), corresponding to the antiferroelectric–paraelectric transition, is in fact associated to a second order phase transition arising from the orthorhombic to the tetragonal phase [8]. At higher temperature (520 1C), the last phase transition through the prototype cubic form is not associated to any dielectric anomaly, the tetragonal phase being already paraelectric. Concerning BF, the ferroelectric–paraelectric transition occurs at 830 1C [5] straight from the rhombohedral phase [6] to a high temperature phase whose symmetry is still controversial. As a consequence, contrasted properties are expected all along the solid solution and the aim of this paper is then to present the results of both electrical and magnetic properties all over the solid solution.
2. Experimental Pure samples of the solid solution (1x)Na0.5Bi0.5TiO3– (x)BiFeO3, named hereafter as NBT(1x)–BFx (x ¼ 0, 0.1,y0.9 and x ¼ 1) were prepared by classical solid state route. Stoichiometric amounts of highly pure reagents (Na2CO3, Bi2O3, TiO2 and Fe2O3) were mixed and fired for 4 h between 800 and 900 1C depending on the composition. Then, after grinding, the obtained powders were pressed into pellets and then sintered in the 900–1120 1C temperature range for 1 h. Ceramics were then electroded with a platinum paint. Dielectric and piezoelectric resonance measurements were performed with an HP4194A Impedance Analyser.
ARTICLE IN PRESS V. Dorcet et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1762–1766
Pyroelectric measurements were carried out using the thermal depolarisation method. Magnetic hysteresis loops were measured from pieces of ceramic samples with a Quantum Design MPMS 5XL SQUID magnetometer.
3. Results and discussion Fig. 1 shows the typical evolution of the dielectric permittivity versus temperature for different frequencies and for different NBT(1x)–BFx compositions. The corresponding loss curves (tan d) are presented in Fig. 2. These curves exhibit three main characteristics (which are denoted as ‘‘1’’, ‘‘2’’ and ‘‘3’’, respectively, in Fig. 1.a): (1) an abrupt increase of the permittivity value at high temperature (2) a frequency-dependent hump in the 200–230 1C temperature range and (3) a large maximum around Tm ¼ 320 1C for NBT. These three characteristics will now be described separately.
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temperature is presented in Fig. 3 for x ¼ 0.7. Two different behaviours are observed (i) at low temperature the conductivity remains quite constant and (ii) at high temperature the conductivity increases, following an Arrhenius law. The calculated activation energy of 1.2 eV (Fig. 3) is very close to those observed in other titanate ceramics (0.9–1.0 eV) [9] in which the conductivity (mobility of oxygen) was attributed to a mechanism of oxygen vacancy jumps [9]. In the present case, we suggest that the increase of the conductivity in the solid solution is correlated to an increase of the oxygen vacancies concentration induced by the reduction of Fe3+ into Fe2+, the iron concentration being more important as x increases. The assumption of the presence of oxygen vacancies in the studied samples is corroborated by some annealing treatments conducted under oxygen atmosphere. Indeed, the amount of vacancies is then supposed to be reduced during such treatments and as expected, a characteristic decrease of the conductivity was evidenced in these samples. 3.2. The frequency-dependent hump
3.1. Abrupt increase of the permittivity Such abrupt increase of the permittivity at high temperature (above 600–700 1C) is generally related to an increase of the electrical conductivity. However, Fig. 1 shows that this phenomenon appears at lower temperature as the BF content increases. A plot of the logarithm of the AC conductivity versus the inverse
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In NBT, this anomaly of the permittivity is associated to a relaxor ferroelectric behaviour [1,3,7]. Such behaviour must however disappear with increasing BF content. Nevertheless, this frequency-dependent anomaly, better observed in the dielectric loss curves (tan d) (Fig. 2), is observed whatever the composition. In Fig. 4a (x ¼ 0), this ferroelectric–antiferroelectric transition is
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Fig. 1. Evolution of dielectric permittivity measured from 100 Hz to 631 kHz as a function of temperature for (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7. Ellipses denoted as 1, 2 and 3 in (a) define the three characteristic features discussed in the text.
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Fig. 2. Evolution of dielectric losses measured from 100 Hz to 631 kHz as a function of temperature for (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7.
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Fig. 3. Evolution of the logarithm of the conductivity versus inverse temperature, for x ¼ 0.7.
attested by the location of a maximum of the pyroelectric coefficient ‘‘p’’ at 200 1C while the polar character falls abruptly at the phase transition temperature, the antiferroelectric phase being unpolar. The same experiments were done on compounds of the solid solution (Fig. 4b, c). For xp0.5, the hump temperature still corresponds to the depolarisation temperature while above x ¼ 0.5, the frequency-dependent anomaly is no longer associated to the depolarisation of the sample. Indeed, in Fig. 4c (x ¼ 0.6), ‘‘p’’ does not evolve up to 150 1C and then strangely evolves, passing though negative values. This artefact let suppose that, above
150 1C, the high conductivity of the x ¼ 0.6 sample does not allow to perform any correct measurements of ‘‘p’’. The depolarisation temperature of the BF-rich samples was then estimated by the disappearance of the piezoelectric resonance signal, the latter being the testimony of the ferroelectric behaviour of the compounds. Such a resonance signal was observed for the BFrich samples well above the temperature of the frequencydependent anomaly and up to Tm. The relaxation phenomenon observed in the BF-rich part of the solid solution has then a different origin than the ferroelectric–antiferroelectric transition observed in NBT [3–7]. For the BF-rich compounds, we suggest that the oxygen vacancies presented above to explain the temperature conductivity, may play a key role on the relaxation phenomenon as previously reported in the literature [10,11]. In fact, the relaxation mechanism linked to the oxygen vacancies is obviously present within all the solid solution, but its contribution increases with the iron amount. The analysis and quantification of the dielectric relaxation phenomenon in the solid solution were already developed in Ref. [11] (in French) and will be published elsewhere in a forthcoming publication.
3.3. The maximum of permittivity In NBT, this maximum is attributed to the antiferroelectric– paraelectric phase transition [3], i.e. to the orthorhombic–tetragonal phase transition [8]. However, BF does not exhibit any antiferroelectric properties so that, for BF-rich compounds, this
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V. Dorcet et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1762–1766
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Fig. 4. Evolution of the pyroelectric coefficient p and spontaneous polarisation PS versus temperature for (a) NBT, (b) x ¼ 0.4 and (c) x ¼ 0.6.
ferroelectric–antiferroelectric–paraelectric, for xp0.5, ferroelectric–paraelectric, for x40.5, resulting in an important difference in the Tm evolution below and above x ¼ 0.5. Finally, Fig. 6 shows the magnetic hysteresis loops obtained on BF-rich compounds (0.7pxp1). A very narrow loop (low HC coercive field) is obtained for pure BF (Fig. 6d), in spite of its antiferromagnetic character. As the same loop has still been observed above TN ¼ 370 1C (Fig. 7), it is attributed to the occurrence of a small amount of ferromagnetic Fe3O4 impurity which has a Curie temperature of about 580 1C [12]. This impurity is obviously present in restricted amount because it has not been evidenced by X-ray diffraction experiments (see Ref. [1]). The hysteresis loops are wider for the other compounds (HCE4 kOe
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maximum may better correspond to a ferroelectric–paraelectric phase transition as for pure BF [6]. Then, whatever the composition, the maximum of the permittivity is always associated to the Curie temperature (i.e. the achievement of the paraelectric phase). As shown in Fig. 5, the value of Tm increases with the BF content. This evolution can easily be explained considering the role of the Bi3+ cation in the R3c solid solution. Indeed, Bi3+ possesses an electronic lone pair which induces a strong displacement out of the centre of the cuboctahedral cavity, leading to a strong ferroelectric (when x40.5) or antiferroelectric (when xp0.5) character. In summary, the higher is the concentration of Bi3+, the higher the polar character and the higher the Curie temperature. However, the high conductivity of the BF-rich compounds does not allow to determine Tm for x40.7 and we remark that the Curie temperature of BF (830 1C) is well above the Tm values measured in the compounds of the solid solution (520 1C for x ¼ 0.7). We can then expect a rapid increase of Tm between x ¼ 0.7 and 1. For the two composition domains (xp0.5 and x40.5) the phase transition sequences are different:
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Fig. 5. Evolution of the temperature of the permittivity maximum (Tm) versus composition (x).
and MRE0.11 emu/g for x ¼ 0.8) indicating that the magnetic behaviour has changed. Since the hysteresis loops can be described as a superposition of a ferromagnetic component (coercive field and remanent magnetization) and an antiferromagnetic component (linear increase of the magnetization with the applied field, at high fields), we may conclude that these compounds rather present a ferri or ferromagnetic behaviour. Note that for x ¼ 0.9, the hysteresis loop presents deformations near zero field that are again attributable to Fe3O4 impurity. This latter have apparently disappeared for x ¼ 0.7 and 0.8. As these compounds also exhibit ferroelectric properties, they are thus potential intrinsic multiferroic materials at ambient temperature.
4. Conclusion This study shows the evolution of the electric and magnetic properties of the R3c solid solution NBT1x–BFx. Although every
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Fig. 6. Magnetic hysteresis loops of BF rich compounds at 77 1C. (a) x ¼ 0.7, (b) x ¼ 0.8, (c) x ¼ 0.9 and (d) x ¼ 1.
Finally, starting from BF which is antiferromagnetic, the appropriate substitution of iron by titanium leads to ferri or ferromagnetic behaviour. Compounds with composition 0.7pxp0.9 are thus multiferroics and may present magnetoelectric properties.
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Acknowledgements The authors gratefully acknowledge the financial assistance of the Re´gion Limousin (France) and Fame-NoE and Dr. Mario Maglione (ICMCB CNRS, Bordeaux) for his help in pyroelectric measurements. References
Fig. 7. Magnetic hysteresis loops of BF at 377 1C.
compound of the solid solution possesses the same structure, two distinct parts ought to be distinguished:
for xp0.5, the compounds exhibit a relaxor ferroelectric
behaviour associated with a ferroelectric–antiferroelectric transition as for pure NBT, for x40.5, the relaxation is not associated to a ferroelectric–antiferroelectric phase transition and is related to the presence of oxygen vacancies.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
V. Dorcet, P. Marchet, G. Trolliard, J. Eur. Ceram. Soc. 27 (2007) 4371. S.V. Kiselev, R.P. Ozerov, G.S. Zhdanov, Sov. Phys.–Doklady 7 (1963) 742. K. Sakata, Y. Masuda, Ferroelectrics 7 (1974) 347. V. Dorcet, G. Trolliard, Acta Mater. 56 (8) (2008) 1753. Y.P. Wang, G.L. Yuan, X.Y. Chen, J.-M. Liu, Z.G. Liu, J. Phys. D: Appl. Phys. 39 (2006) 2019. W. Prellier, M.P. Singh, P. Murugavel, J. Phys.: Condens. Matter 17 (2005) R803. V. Dorcet, G. Trolliard, P. Boullay, Chem. Mater. 20 (2008) 5061. G. Trolliard, V. Dorcet, Chem. Mater. 20 (2008) 5074. W.L. Warren, K. Vanheusden, D. Dimos, G.E. Pike, B.E. Tuttle, J. Am. Ceram. Soc. 79 (1996) 536. W. Li, K. Chen, Y. Yao, J. Zhu, Y. Wang, Appl. Phys. Lett. 85 (20) (2004) 4717. V. Dorcet, Ph.D. Thesis, 2008 – Universite´ de Limoges-France – In French, 240p. R.M. Cornell, U. Schwertmann, The Iron Oxides, Wiley-VCH, Weinheim, 2003.
Contents lists available at ScienceDirect
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Properties of the solid solution (1x)Na0.5Bi0.5TiO3–(x)BiFeO3 ˜ a c, G. Trolliard b V. Dorcet a,b,, P. Marchet b, O. Pen a b c
´chal Juin, F-14050-Caen Cedex, France Laboratoire de Cristallographie et Sciences des Mate´riaux, CRISMAT-CNRS UMR 6508, 6 Bd. du Mare ´de´s Ce´ramiques et des Traitements de Surface, SPCTS-CNRS UMR 6638, 123 Av. Albert Thomas, F-87060-Limoges Cedex, France Laboratoire Sciences des Proce Laboratoire des Sciences Chimiques de Rennes, UMR 6226 CNRS, Campus de Beaulieu, F-35042-Rennes Cedex, France
a r t i c l e in f o
a b s t r a c t
Available online 8 February 2009
This study shows that remarkable electric and magnetic properties are encountered within the (1x)Na0.5Bi0.5TiO3 (NBT)–(x)BiFeO3 (BF) solid solution. Dual ferroelectric and magnetic properties are observed in the BF-rich part of the solid solution implying intrinsic multiferroic character of the compounds. In addition, a relaxation phenomenon is evidenced within the overall compositional domain of the solid solution. This study emphasizes that in the NBT-rich part, the relaxor behaviour is very similar to that of NBT, while beyond x ¼ 0.5, it turns to a different mechanism of relaxation probably induced by the presence of oxygen vacancies resulting from the mixed valence of the iron cations. & 2009 Elsevier B.V. All rights reserved.
Keywords: NBT BF Multiferroic Dielectric property Magnetic property
1. Introduction Na0.5Bi0.5TiO3 (NBT) and BiFeO3 (BF) are two perovskite compounds showing R3c space group at ambient temperature. A previous study [1] has shown that they form a continuous R3c solid solution at room temperature. These two compounds exhibit a ferroelectric behaviour which is maintained within all such solid solution [1]. However, important discrepancies in their physical properties arise too, that made this solid solution an interesting case study. On one hand, BF is also antiferromagnetic until its Ne´el temperature at about TN ¼ 370 1C [2] while NBT does not present any magnetic properties. So the solid solution may show interesting intrinsic multiferroic behaviour. On the other hand, NBT presents a ferroelectric–antiferroelectric transition at about 200 1C [3] associated to a small frequency-dependent dielectric anomaly appearing as a hump in the permittivity curves [3] testifying the relaxor behaviour of NBT [3,4] while BF neither presents antiferroelectric nor relaxor properties [5,6]. In addition, a recent in-situ TEM study [7] of the phase transition sequence demonstrated that the first dielectric anomaly of NBT (200 1C) is associated to the beginning of a first order reconstructive rhombohedral to orthorhombic phase transition, which proceeds between 200 and 290 1C, via the formation of a modulated phase [7]. The authors suggested that the modulated phase as well as
Corresponding author at: Laboratoire de Cristallographie et Sciences des Mate´riaux, CRISMAT-CNRS UMR 6508, 6, Bd. du Mare´chal Juin, F-14050-Caen Cedex, France. Tel.: +33 0 2 31 45 26 33; fax: +33 0 2 31 95 16 00. E-mail address: [email protected] (V. Dorcet).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.02.014
the orthorhombic one is in fact antiferroelectric [7]. The same authors have also concluded that the second dielectric anomaly of NBT (maximum of the permittivity 320 1C), corresponding to the antiferroelectric–paraelectric transition, is in fact associated to a second order phase transition arising from the orthorhombic to the tetragonal phase [8]. At higher temperature (520 1C), the last phase transition through the prototype cubic form is not associated to any dielectric anomaly, the tetragonal phase being already paraelectric. Concerning BF, the ferroelectric–paraelectric transition occurs at 830 1C [5] straight from the rhombohedral phase [6] to a high temperature phase whose symmetry is still controversial. As a consequence, contrasted properties are expected all along the solid solution and the aim of this paper is then to present the results of both electrical and magnetic properties all over the solid solution.
2. Experimental Pure samples of the solid solution (1x)Na0.5Bi0.5TiO3– (x)BiFeO3, named hereafter as NBT(1x)–BFx (x ¼ 0, 0.1,y0.9 and x ¼ 1) were prepared by classical solid state route. Stoichiometric amounts of highly pure reagents (Na2CO3, Bi2O3, TiO2 and Fe2O3) were mixed and fired for 4 h between 800 and 900 1C depending on the composition. Then, after grinding, the obtained powders were pressed into pellets and then sintered in the 900–1120 1C temperature range for 1 h. Ceramics were then electroded with a platinum paint. Dielectric and piezoelectric resonance measurements were performed with an HP4194A Impedance Analyser.
ARTICLE IN PRESS V. Dorcet et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1762–1766
Pyroelectric measurements were carried out using the thermal depolarisation method. Magnetic hysteresis loops were measured from pieces of ceramic samples with a Quantum Design MPMS 5XL SQUID magnetometer.
3. Results and discussion Fig. 1 shows the typical evolution of the dielectric permittivity versus temperature for different frequencies and for different NBT(1x)–BFx compositions. The corresponding loss curves (tan d) are presented in Fig. 2. These curves exhibit three main characteristics (which are denoted as ‘‘1’’, ‘‘2’’ and ‘‘3’’, respectively, in Fig. 1.a): (1) an abrupt increase of the permittivity value at high temperature (2) a frequency-dependent hump in the 200–230 1C temperature range and (3) a large maximum around Tm ¼ 320 1C for NBT. These three characteristics will now be described separately.
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temperature is presented in Fig. 3 for x ¼ 0.7. Two different behaviours are observed (i) at low temperature the conductivity remains quite constant and (ii) at high temperature the conductivity increases, following an Arrhenius law. The calculated activation energy of 1.2 eV (Fig. 3) is very close to those observed in other titanate ceramics (0.9–1.0 eV) [9] in which the conductivity (mobility of oxygen) was attributed to a mechanism of oxygen vacancy jumps [9]. In the present case, we suggest that the increase of the conductivity in the solid solution is correlated to an increase of the oxygen vacancies concentration induced by the reduction of Fe3+ into Fe2+, the iron concentration being more important as x increases. The assumption of the presence of oxygen vacancies in the studied samples is corroborated by some annealing treatments conducted under oxygen atmosphere. Indeed, the amount of vacancies is then supposed to be reduced during such treatments and as expected, a characteristic decrease of the conductivity was evidenced in these samples. 3.2. The frequency-dependent hump
3.1. Abrupt increase of the permittivity Such abrupt increase of the permittivity at high temperature (above 600–700 1C) is generally related to an increase of the electrical conductivity. However, Fig. 1 shows that this phenomenon appears at lower temperature as the BF content increases. A plot of the logarithm of the AC conductivity versus the inverse
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In NBT, this anomaly of the permittivity is associated to a relaxor ferroelectric behaviour [1,3,7]. Such behaviour must however disappear with increasing BF content. Nevertheless, this frequency-dependent anomaly, better observed in the dielectric loss curves (tan d) (Fig. 2), is observed whatever the composition. In Fig. 4a (x ¼ 0), this ferroelectric–antiferroelectric transition is
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Fig. 1. Evolution of dielectric permittivity measured from 100 Hz to 631 kHz as a function of temperature for (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7. Ellipses denoted as 1, 2 and 3 in (a) define the three characteristic features discussed in the text.
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Fig. 2. Evolution of dielectric losses measured from 100 Hz to 631 kHz as a function of temperature for (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7.
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Fig. 3. Evolution of the logarithm of the conductivity versus inverse temperature, for x ¼ 0.7.
attested by the location of a maximum of the pyroelectric coefficient ‘‘p’’ at 200 1C while the polar character falls abruptly at the phase transition temperature, the antiferroelectric phase being unpolar. The same experiments were done on compounds of the solid solution (Fig. 4b, c). For xp0.5, the hump temperature still corresponds to the depolarisation temperature while above x ¼ 0.5, the frequency-dependent anomaly is no longer associated to the depolarisation of the sample. Indeed, in Fig. 4c (x ¼ 0.6), ‘‘p’’ does not evolve up to 150 1C and then strangely evolves, passing though negative values. This artefact let suppose that, above
150 1C, the high conductivity of the x ¼ 0.6 sample does not allow to perform any correct measurements of ‘‘p’’. The depolarisation temperature of the BF-rich samples was then estimated by the disappearance of the piezoelectric resonance signal, the latter being the testimony of the ferroelectric behaviour of the compounds. Such a resonance signal was observed for the BFrich samples well above the temperature of the frequencydependent anomaly and up to Tm. The relaxation phenomenon observed in the BF-rich part of the solid solution has then a different origin than the ferroelectric–antiferroelectric transition observed in NBT [3–7]. For the BF-rich compounds, we suggest that the oxygen vacancies presented above to explain the temperature conductivity, may play a key role on the relaxation phenomenon as previously reported in the literature [10,11]. In fact, the relaxation mechanism linked to the oxygen vacancies is obviously present within all the solid solution, but its contribution increases with the iron amount. The analysis and quantification of the dielectric relaxation phenomenon in the solid solution were already developed in Ref. [11] (in French) and will be published elsewhere in a forthcoming publication.
3.3. The maximum of permittivity In NBT, this maximum is attributed to the antiferroelectric– paraelectric phase transition [3], i.e. to the orthorhombic–tetragonal phase transition [8]. However, BF does not exhibit any antiferroelectric properties so that, for BF-rich compounds, this
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ferroelectric–antiferroelectric–paraelectric, for xp0.5, ferroelectric–paraelectric, for x40.5, resulting in an important difference in the Tm evolution below and above x ¼ 0.5. Finally, Fig. 6 shows the magnetic hysteresis loops obtained on BF-rich compounds (0.7pxp1). A very narrow loop (low HC coercive field) is obtained for pure BF (Fig. 6d), in spite of its antiferromagnetic character. As the same loop has still been observed above TN ¼ 370 1C (Fig. 7), it is attributed to the occurrence of a small amount of ferromagnetic Fe3O4 impurity which has a Curie temperature of about 580 1C [12]. This impurity is obviously present in restricted amount because it has not been evidenced by X-ray diffraction experiments (see Ref. [1]). The hysteresis loops are wider for the other compounds (HCE4 kOe
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maximum may better correspond to a ferroelectric–paraelectric phase transition as for pure BF [6]. Then, whatever the composition, the maximum of the permittivity is always associated to the Curie temperature (i.e. the achievement of the paraelectric phase). As shown in Fig. 5, the value of Tm increases with the BF content. This evolution can easily be explained considering the role of the Bi3+ cation in the R3c solid solution. Indeed, Bi3+ possesses an electronic lone pair which induces a strong displacement out of the centre of the cuboctahedral cavity, leading to a strong ferroelectric (when x40.5) or antiferroelectric (when xp0.5) character. In summary, the higher is the concentration of Bi3+, the higher the polar character and the higher the Curie temperature. However, the high conductivity of the BF-rich compounds does not allow to determine Tm for x40.7 and we remark that the Curie temperature of BF (830 1C) is well above the Tm values measured in the compounds of the solid solution (520 1C for x ¼ 0.7). We can then expect a rapid increase of Tm between x ¼ 0.7 and 1. For the two composition domains (xp0.5 and x40.5) the phase transition sequences are different:
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and MRE0.11 emu/g for x ¼ 0.8) indicating that the magnetic behaviour has changed. Since the hysteresis loops can be described as a superposition of a ferromagnetic component (coercive field and remanent magnetization) and an antiferromagnetic component (linear increase of the magnetization with the applied field, at high fields), we may conclude that these compounds rather present a ferri or ferromagnetic behaviour. Note that for x ¼ 0.9, the hysteresis loop presents deformations near zero field that are again attributable to Fe3O4 impurity. This latter have apparently disappeared for x ¼ 0.7 and 0.8. As these compounds also exhibit ferroelectric properties, they are thus potential intrinsic multiferroic materials at ambient temperature.
4. Conclusion This study shows the evolution of the electric and magnetic properties of the R3c solid solution NBT1x–BFx. Although every
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Finally, starting from BF which is antiferromagnetic, the appropriate substitution of iron by titanium leads to ferri or ferromagnetic behaviour. Compounds with composition 0.7pxp0.9 are thus multiferroics and may present magnetoelectric properties.
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Acknowledgements The authors gratefully acknowledge the financial assistance of the Re´gion Limousin (France) and Fame-NoE and Dr. Mario Maglione (ICMCB CNRS, Bordeaux) for his help in pyroelectric measurements. References
Fig. 7. Magnetic hysteresis loops of BF at 377 1C.
compound of the solid solution possesses the same structure, two distinct parts ought to be distinguished:
for xp0.5, the compounds exhibit a relaxor ferroelectric
behaviour associated with a ferroelectric–antiferroelectric transition as for pure NBT, for x40.5, the relaxation is not associated to a ferroelectric–antiferroelectric phase transition and is related to the presence of oxygen vacancies.
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