Optimization of the mechanochemical conditions for the synthesis of the xBiFeO3–(1  x)PbTiO3 multiferroic system

https://doi.org/10.1016/j.jallcom.2011.02.097Get rights and content

Abstract

The xBiFeO3–(1  x)PbTiO3 solid solution is nowadays one of the most promising multiferroic systems. However, the obtaining of single phases with a high crystallinity is difficult by the conventional methods. In this work, the mechanochemical method has been optimized, testing different parameters in the reaction to obtain the suitable conditions of synthesis. The investigated parameters were: the atmosphere conditions and the material of the reaction medium. Single phases with compositions belonging to the whole system (0  x  1) have been obtained, being the mechanochemical reaction with tungsten carbide vessel the best synthesis medium. The structural characterization of the obtained powders has been carried out by XRD and microstructures of powdered samples have been studied by SEM and TEM.

Highlights

► High-energy ball milling with tungsten carbide vessel as an optimum approach to mechanosynthesize the xBiFeO3–(1  x)PbTiO3 solid solution. ► Oxygen atmosphere avoids reduction of starting materials and formation of secondary phases during milling process in steel medium. ► Obtained xBiFeO3–(1  x)PbTiO3 solid solution with a clear coexistence of rhombohedral and tetragonal perovskite phases in the MPB region.

Introduction

Nowadays, one of the most promising multiferroic materials is the BiFeO3, an oxide with high Curie (825 °C) and Néel (370 °C) temperatures [1]. This compound is ferroelectric and antiferromagnetic at room temperature, and it is a good candidate to study the magnetoelectric effect, interesting property from the point of view of its numerous applications [2]. The most common problems of this rhombohedral perovskite (space group R3c, No. 161) are related to its high conductivity and antiferromagnetic character, which does not allow a net magnetization [1], [3].

In order to improve the ferroelectric and the magnetic properties, many cationic substitutions have been carried out in the perovskite structure [4], being one of the most promising possibilities the solid solution xBiFeO3–(1  x)PbTiO3 (BF–PT) [5]. The PbTiO3 is a tetragonal perovskite (S.G. P4mm, No. 99) with a TC = 490 °C and very good ferroelectric properties [6]. The substitution of Pb2+ for Bi3+ improves the ferroelectric properties [7], [8], while the substitution of Ti4+ for Fe3+ increases the stability of the perovskite structure and the magnetic properties, since the modulated antiferromagnetic spiral structure is destroyed and a weakly ferromagnetic state appears [3]. This solid solution exhibits a morphotropic phase boundary (MPB) between the rhombohedral symmetry (BiFeO3-rich region) and the tetragonal one (PbTiO3-rich region) both with perovskite structure, although there is still some controversy about the localization and the width of this range of coexistence of both polymorphs [9].

In the bibliography, the synthesis of the xBiFeO3–(1  x)PbTiO3 system has been reported in many works, but in most of them the apparition of secondary phases (Bi2Fe4O9 and Bi24FeO39) in the synthesis process of the BF-rich compositions is reported [10]. Several synthesis routes have been employed to obtain different compositions of this system, some examples are: solid-state route [11], co-precipitation [12], sol–gel combustion [13] or microwave-hydrothermal synthesis [14]. In most of them low temperature is used in the synthesis, because of the BiFeO3 decomposition at 830 °C or higher temperatures [15].

In this sense, mechanochemical methods help the formation of perovskite-type oxides at low temperature, due to the increase of the reactivity, during the milling process, both by mechanoactivation of reactants and further annealing at moderate temperatures [16], [17] or by direct mechanosynthesis at room temperature [18], [19], [20], [21], even allowing high-pressure metastable phases to be isolated without the application of external pressure [22]. Other authors have previously used the high-energy milling as synthesis method only for one composition in the MPB region (0.7BiFeO3–0.3PbTiO3) of the present solid solution. Different results were obtained depending on the mechanochemical conditions, Khan et al. [23] obtained mechanically activated powders, which transform to the single phase of 0.7BiFeO3–0.3PbTiO3 after low-temperature calcinations, while Zhang et al. [24] obtained this mechanosynthesized powder, employing a different milling system.

In this work the optimization of the mechanochemical conditions for the synthesis of the whole xBiFeO3–(1  x)PbTiO3 multiferroic system has been carried out. Three different milling approaches have been compared with the ceramic method. The optimized parameters in the mechanochemical process are the atmosphere and the material of the milling medium and they are discussed and reported in increasing effectiveness order. Also, the obtained phases have been characterized from the structural point of view.

Section snippets

Material and methods

Different compositions of the system xBiFeO3–(1  x)PbTiO3 were prepared from a stoichiometric mixture of analytical grade Bi2O3, Fe2O3, PbO and TiO2, for x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.625, 0.65, 0.675, 0.7, 0.75, 0.8, 0.9 and 1. Initial mixtures were homogenized by hand in an agate mortar. The samples were synthesized by two different routes: solid state reaction and mechanochemical method. In the case of the mechanical treatment, different conditions were investigated until the synthesis

Traditional ceramic route

Table 1 summarizes the synthesis conditions for different members of the xBiFeO3–(1  x)PbTiO3 system and Fig. 1 shows the XRD patterns of the final products obtained for each composition. The final temperature of the synthesis depends on the composition. It is possible to observe that only the compositions far from the MPB were obtained as well-crystallized phases by this method, while the compositions belonging or close to the MPB are constituted by a mixture of low crystalline perovskites.

Conclusions

The high-energy ball milling, in air, with tungsten carbide vessel and balls is the optimum approach to mechanosynthesize the whole xBiFeO3–(1  x)PbTiO3 solid solution. By using this method it is possible to obtain nanocrystalline single phases of all compositions of the system, and their crystallinity can easily increase with a subsequent thermal treatment. Moreover, by this method it is possible to locate the MPB region between the compositions with x = 0.625 and 0.75.

Finally, although the

Acknowledgements

This work has been founded by the MICINN (Spain) through the MAT2007-61884 and MAT2010-18543 projects. Ms. C.C. and Dr. T.H. thank the financial support by the Spanish MICINN (BES-2008-005409) and CSIC (JAEDoc082), respectively.

References (26)

  • I.O. Troyanchuk et al.

    Phys. B: Condens. Matter

    (2009)
  • R. Mazumder et al.

    J. Alloys Compd.

    (2009)
  • T.P. Comyn et al.

    J. Eur. Ceram. Soc.

    (2008)
  • T.T. Carvalho et al.

    Mater. Lett.

    (2008)
  • J. Prado-Gonjal et al.

    Mater. Res. Bull.

    (2009)
  • L.B. Kong et al.

    Prog. Mater. Sci.

    (2008)
  • T. Hungria et al.

    J. Alloys Compd.

    (2007)
  • L. Zhang et al.

    Mater. Lett.

    (2007)
  • A. Hasanpour et al.

    Phys. B: Condens. Matter

    (2007)
  • G. Catalan et al.

    Adv. Mater.

    (2009)
  • S. Picozzi et al.

    J. Phys.: Condens. Matter

    (2009)
  • J. Chaigneau et al.

    Phys. Rev. B

    (2009)
  • D.I. Woodward et al.

    J. Appl. Phys.

    (2003)
  • Cited by (4)

    View full text