Size driven phase transition of barium titanate nanoparticles prepared by plasma chemical vapor deposition

In the displacive-type ferroelectrics such as barium titanate (BaTiO3), it is well known that the crystal structure at room temperature transforms from tetragonal (ferroelectric phase) to cubic (paraelectric phase) with decreasing particle size. This phenomenon is called the “size driven phase transition” in ferroelectrics, and is one of the important phenomena in terms of industry as well as science. Since BaTiO3 nanoparticles have conventionally been prepared by liquid phase methods, numerous researchers have investigated the phase transition behavior using liquid phase synthesized BaTiO3 nanopowder. Various critical sizes of phase transition in the range from 17 to 190 nm have been reported [1–10]. The inconsistency of the critical size is ascribed to the difficulty in measurement, because the difference in lattice constants is less than 0.1 . Furthermore, factors such as defects, impurities, agglomeration and crystallinity also affect the critical size and total free energies of BaTiO3 nanoparticles. For example, Wada et al. reported that the correlational size of dipoles introduced by the hydroxyl groups in the lattice can determine the crystal structure [8, 16, 20]. Buscaglia et al. indicated that transition elements doped in BaTiO3 affect the lattice constants [11]. Li et al. suggested that agglomeration of BaTiO3 nanoparticles promotes the stability of the tetragonal phase [6]. These are extrinsic factors originating in an imperfection of BaTiO3. Surface energy [10], stress [9] and softening of lattice vibration mode (soft mode) [12] are considered as intrinsic origins of the phase transition. Recently, we have succeeded in the preparation of BaTiO3 nanoparticles using radio-frequency plasma chemical vapor deposition (RF-plasma CVD) [13]. Well-crystallized BaTiO3 nanoparticles with particle sizes of about 15 nm can be synthesized by this method. The powder has impurities such as strontium of 0.04 ± 0.01at%, calcium of 0.08 ± 0.02at% iron of 0.05 ± 0.03at% and nickel of 0.08 ± 0.02at%. Hydroxyl groups were detected on the surface of the particles. The objective of the present work is to clarify the size driven phase transition behavior of the BaTiO3 nanoparticles obtained by the plasma CVD method. BaTiO3 nanoparticles prepared by the plasma CVD method (as-prepared BaTiO3 nanoparticles) were used as starting materials. The particle sizes were controlled by heating in air at various temperatures of 550, 600, 700, 800, 900 and 1000 ◦C for 1 hr. The treated powders were identified by X-ray diffraction (XRD) (Model RINT2000 and PSA200, RIGAKU) using CuKα radiation (1.54051 , 40 kV, 50 mA). The powder morphology was observed using a scanning electron microscope (SEM) (Model S-4000, Hitachi). The degree of grain growth was investigated by specific surface area measurement (SSA) (Model Microtrac surface area analyzer 4200, Nikkisou) with nitrogen absorption. The Ba/Ti atomic ratio was estimated by X-ray fluorescence (XRF) (Model ZSX100E, RIGAKU) using RhKα radiation (0.61324 , 50 kV, 70 mA). The lattice constants of the treated powders were refined using a Rietveld method. The Rietveld analysis program RIETAN2000 was employed and the fitting of the XRD profile was performed using a pseudo-Voigt function. The crystal structure was also investigated by Raman spectroscopy (Model JRS-system 2000, JEOL) using an Ar laser (514.5 nm, 25 mW). Fig. 1 shows XRD patterns of the samples treated at various temperatures. An almost single phase of BaTiO3 with a slight amount of barium carbonate (BaCO3) was identified in the samples as-prepared and treated at the temperatures of 550, 600 and 700 ◦C. On the other hand, single phases of BaTiO3 were obtained by heat treatment at 800, 900 and 1000 ◦C. Fig. 2 shows SEM images of the samples treated at various temperatures. A remarkable grain growth occurred above 700 ◦C and the shape of particles in all products was observed to be spherical. The particle sizes were estimated from SEM images (dSEM) and SSA measurements (dSSA). Diameters of 100 particles from each of the SEM images were measured and the particle size distribution was fitted using log-normal function. The D50 value of particle size distribution was defined as dSEM. The results of measured dSEM and dSSA are summarized in Fig. 3. Ba/Ti atomic ratios estimated by XRF measurement are also described in Fig. 3. Fig. 3 shows that the dSEM and dSSA increased with increasing heat treatment temperature. The dSEM was almost comparable with the dSSA below 800 ◦C, whereas, the dSSA became much larger than dSEM above 900 ◦C. This result indicates that particles agglomerate with increasing heat treatment temperature above 900 ◦C. The phase transition from cubic to tetragonal causes the peak splitting of the {2 0 0}c into the {2 0 0}t and {0 0 2}t in the XRD profile. Therefore, the phase transition can be confirmed by {2 0 0} peak splitting. To investigate the peak splitting, XRD measurement at a scanning rate of 0.1 ◦/min in the 2θ range from 44 to 46 ◦ was carried out. Fig. 4 shows dependence of the