Structure-property relations of co-doped bismuth layer-structured Bi3.25La0.75(Ti1-xMo x )3O12 ceramics
© Siriprapa et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
In this work, the fabrication and investigation of substituting higher-valence Mo6+ for Ti4+ ion on the B-site of La3+-doped Bi4Ti3O12 [BLT] structure to form Bi3.25La0.75(Ti1-xMo x )3O12 [BLTM] (when x = 0, 0.01, 0.03, 0.05 0.07, 0.09, and 0.10) ceramics were carried out. X-ray diffraction patterns of BLTM ceramics indicated an orthorhombic structure with lattice distortion, especially with a higher concentration of a MoO3 dopant. Microstructural investigation showed that all ceramics composed mainly of plate-like grains. An increase in MoO3 doping content increased the length and thickness of the grain but reduced the density of the ceramics. Electrical conductivity was found to decrease, while the dielectric constant increased with Mo6+ doping concentration. Ferroelectric properties were found to be improved with increasing MoO3 content and were optimized at x = 0.1.
In recent years, the family of bismuth layer-structured ferroelectrics has received much attention as the candidate for ferroelectric random access memories. An extensively studied bismuth titanate (Bi4Ti3O12 [BIT]) is a member of the Aurivillius family that can be represented by a general formula (Bi2O3)[Am-1(B) m O3m+1] which consists of (Bi2O2)2+ sheets alternating with (Bi2Ti3O10)2- perovskite-like layers . BIT has large spontaneous polarization along the a axis ( approximately 50 μC/cm2), low processing temperature, high Curie temperature, and is a Pb-free material . However, it still has high leakage current and domain pinning due to defects, such as Bi vacancies accompanied by oxygen vacancies [3, 4]. To overcome these problems, A-site substitution by a replacement of volatile Bi with rare earth or other metal oxide additives is often necessary for ferroelectric property improvement. For example, ions of La , V , Nd , and Pr  have been used to substitute the Bi ion in a BIT bulk material or thin film without destroying its layered structure. Bu et al.  prepared La-doped BIT thin films by pulsed laser deposition and reported that these films were appropriate for non-volatile random access memory devices because of their high remanent polarization and low leakage current. In recent years, SimÕes et al.  reported that a doping content of x = 0.75 in Bi4-xLa x Ti3O12 [BLT] showed an improvement of the fatigue endurance upon a repeated cyclic electric field which emphasized its possible use in FRAM applications. However, these BLT ceramics still showed a rather high leakage current. Attention has thus been paid to investigate this material in order to overcome this disadvantage. Wang et al.  reported that substitution of high-valence ions, such as Mo6+, in BLT thin films or known as Mo6+ co-doped BLT thin films led to polarization fatigue improvement, high remanent polarization, and lower leakage current density.
In terms of ceramics, there has been no detailed study on a new co-doped bismuth-layered structure based on Mo6+-doped Bi3.25La0.75Ti3O12. In this study, therefore, effects of MoO3 doping concentration on the phase, microstructure, and electrical properties (i.e., conductivity, dielectric, and ferroelectric properties) of BLT ceramics produced by a conventional solid-state mixed-oxide method are reported and discussed.
A perovskite bismuth-layered structure based on Bi3.25La0.75(Ti1-xMo x )3O12 [BLTM] (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, and 0.10) powders was prepared using a solid-state mixed-oxide method. Starting binary oxide powders, i.e., Bi2O3 ( > 98%, Fluka; Sigma-Aldrich Corporation, St. Louis, MO, USA), La2O3 (99.98%, Fluka), TiO2 ( > 99%, Riedel-de Haën; Sigma-Aldrich Corporation, St. Louis, MO, USA), and MoO3 (99.9%, Fluka), were ball-milled and calcined at 750°C for 4 h. The slurry was transferred to a spherical flask and placed in a shell freezer. The flask was rotated in an ethanol bath for at least 1 h. The flask of frozen slurry was then immediately transferred to a vacuum dryer and dried for 24 h. After the ice was sublimated, fine dried powder was produced. The BLTM powders were then pressed under a uniaxial hydraulic pressure of 5.5 MPa with a few drops of 3 wt.% polyvinyl alcohol used as a binder. The pressed samples were sintered at temperatures in a range of 1,000 to 1,150°C for 4 h. Optimum sintering temperature for producing highest-density ceramics was determined, and the samples were selected for further characterization. Phases of selected ceramics were characterized using an X-ray diffractometer [XRD] (X-pert, PANalytical B.V., Almelo, The Netherlands) with CuKα radiation. Density was measured by Archimedes' method. The ceramics were polished and thermally etched at a temperature of 150°C below the optimum sintering temperature for 15 min dwell time prior to microstructural investigation using a scanning electron microscope [SEM] (JEOL JSM-6335F, JEOL Ltd., Akishima, Tokyo, Japan). Average grain size was determined using a mean linear intercept method from SEM micrographs. Electrical conductivity measurement was done at 1 kHz using an LCZ meter. Dielectric constant [εr] and loss tangent [tanδ] were measured at room temperature with a frequency between 1 to 100 kHz using LCR Hitester 3532-50 (Hioki, Ueda, Nagano, Japan). Ferroelectric hysteresis polarization-electric field [P-E] loops were determined using a computer-controlled modified Sawyer-Tower circuit. Remanent polarization [Pr], maximum polarization [Pmax], coercive field [Ec], maximum field [Emax], and loop squareness [Rsq] values were evaluated from the loops.
Results and discussion
Physical, dielectric, and ferroelectric properties of Bi3.25La0.75(Ti1- x Mo x )3O12 ceramics
P r /P max
E c /E max
7.46 ± 0.03
2.59 ± 0.61
7.21 ± 0.01
2.66 ± 0.79
7.16 ± 0.03
4.61 ± 2.13
6.74 ± 0.02
5.94 ± 2.11
6.94 ± 0.02
7.73 ± 3.13
6.79 ± 0.01
8.04 ± 3.31
6.83 ± 0.02
8.13 ± 2.97
A new system of co-doped bismuth titanate-layered structure ceramics, i.e., Bi3.25La0.75(Ti1-xMo x )3O12 or BLTM (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, and 0.10), was successfully prepared by a solid-state mixed-oxide method. X-ray diffraction analysis indicated that the MoO3 dopant induced a preferred orientation of the BLT ceramics with changes in lattice constant of its orthorhombic structure. Grain size was found to increase with increasing MoO3 doping content. Electrical conductivity of BLTM was slightly decreased by Mo6+ donor doping due to the charge compensation between the provided excess electrons and inherent holes. This led, consequently, to an increased dielectric constant from 127 for BLT to a maximum of 414 for 0.03 mol MoO3 doping content. Addition of MoO3 content also increased the remanent polarization of over 10 μC/cm2, and this value was shown to be much larger than that of the BLT ceramic, due to a reduction of domain pinning oxygen vacancies and leakage current loss with optimum ferroelectric properties obtained for the composition of x = 0.1 mol. It is suggested that molybdenum-doped BLT could be an alternative material for potential applications in electronic industries which require a lead-free material having large remanent polarization and lower processing temperatures.
This work is financially supported by the Thailand Research Fund (TRF) and the Nation Research University Project under Thailand's Office of the Higher Education Commission (OHEC). PS would like to thank the Commission on Higher Education for their support through a grant fund under the program Strategic Scholarships for Frontier Research Network for the Ph.D. Program Thai Doctoral degree, Faculty of Science and Graduate School, Chiang Mai University for this research.
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