Phase transition and ferroelectric properties of xBiFeO3–(1−x)BaTiO3 ceramics
Introduction
Multiferroic BiFeO3 (BFO) has a high Curie temperature (TC) of 830–850 °C and Néel temperature (TN) of ∼370 °C [1], [2]. The crystallographic structure of BiFeO3 is a rhombohedrally distorted perovskite structure with lattice constants of a = 3.96 Å and αR = 89.4° [3]. The BiFeO3 has a spontaneous polarization of 3.5 μC/cm2 along the (100) direction and 6.1 μC/cm2 along the (111) direction in single crystal at 77 K and varies from 2.5 to 16.6 μC/cm2 in ceramics [4], [5]. In addition, BiFeO3 possess a coupling of ferroelectric and antiferromagnetic (or weak ferromagnetic) parameters at room temperature due to a residual moment from spin structure as previously reported [6], [7].
One of the major problems of BiFeO3 materials is high current leakage, which affects the measurement of dielectric/ferroelectric properties. The high electric leakage of BiFeO3 materials is attributed to the oxidation reduction of Fe ions (Fe2+ → Fe3+ + e−), creating oxygen vacancies for charge compensation [8], [9]. To overcome the intrinsic problems of BiFeO3 materials, the processing of solid solutions with substitutions of ABO3 perovskite materials at the Bi and Fe sites has been investigated [10], [11]. BaTiO3 (BT) is a well known ferroelectric material with a Curie temperature Tc ∼120 °C. The mixed solid solutions (x)BiFeO3–(1−x)BaTiO3 show a rhombohedral and cubic structures in the range of 0.67 ≤ x ≤ 1.0 and 0.075 ≤ x ≤ 0.67 respectively in the low-temperature region [12], [13]. xBiFeO3–(1−x)BaTiO3 exhibits a tetragonal symmetry for x < 0.075 [12], [13].
Though BFO has been studied extensively in recent years, its phase transition and dielectric properties still lack consistency and are not fully understood. The main focus of this work is to study phase transitions of xBiFeO3–(1−x)BaTiO3 ceramics (x = 1.0, 0.9, 0.8, and 0.7) by in-situ high-resolution synchrotron x-ray diffraction (XRD). Another task is to examine the effect of adding BaTiO3 on dielectric properties as functions of temperature and frequency.
Section snippets
Experimental procedure
The BFO ceramic was prepared by the solid-state-reaction method. The dried starting powders of Bi2O3 and Fe2O3 (purity ≥ 99.0%) were weighed in 1.1:1 ratio to compensate the low melting point of Bi, and then mixed in an agate mortar for more than 24 h using alcohol as a medium. The mixture was dried and mixed with polyvinyl acetate as a binder for granulation. The ground mixture was pressed into a 1.0 cm-diameter disk. The pressed BFO disk was sintered in the region of 850–870 °C for 1–3 h. The
Results and discussion
Fig. 1 shows room-temperature XRD spectra of BFO and BFO-10–30%BT. The XRD spectrum of BFO exhibits an obvious splitting in the higher 2θ peaks and indicates a rhombohedral structure. The splitting behavior of XRD peak gradually disappears with increasing BT and becomes a nearly single peak in BFO-30%BT, indicating the appearance of cubic phase. Minor second phases of possible Bi25FeO39 or Bi2Fe4O9 occur in BFO and BFO-10–30%BT as indicated by “∗”.
Fig. 2 shows temperature-dependent synchrotron
Conclusions
A ferroelectric rhombohedral (R)-orthorhombic (O)-paraelectric cubic (C) phase transition takes place at 825 and 850 °C in BFO upon heating. The Curie temperature shifts toward lower temperature as BT content increases. The Curie temperatures of BFO-10%BT, and BFO-20%BT appear near 760 and 740 °C, respectively. A C(R)–C transition takes place near 680 °C in BFO-30%BT. The (110) R-phase peaks of BFO-10%BT and BFO-20%BT are much broader than in BFO and BFO-30%BT, possibly due to the random
Acknowledgments
This work was supported by National Science Council of Taiwan Grant No. 97-2112-M-030-003-MY3.
References (20)
- et al.
Solid State Commun.
(1970) - et al.
Scr. Mater.
(2009) - et al.
Cer Int.
(2008) - et al.
J. Magn. Magn. Mater.
(1998) - et al.
Sov. Phys. Solid State
(1961) - et al.
Sov. Phys. JETP
(1963) - et al.
Acta Crystallogr. B
(1990) - et al.
Appl. Phys. Express
(2008) - et al.
J. Phys. C
(1982) - et al.
Sov. Phys. Usp
(1982)