Ba（Zr, Ti）O3is a lead-free relaxor ferroelectric. Using the first-principles method, the ferroelectric dipole moments for pure BaTiO3 and Ba（Zr, Ti）O3supercells are studied. All possible ion configurations of Ba Zr0.5Ti0.5O3 and Ba Zr0.25Ti0.75O3 are constructed in a 2 × 2 × 2 supercell. For the half-substituted case, divergence of ferroelectric properties is found from these structures, which greatly depends on the arrangements of Ti and Zr ions. Thus our results provide a reasonable explanation to the relaxor behavior of Ba（Zr, Ti）O3. In addition, a model based on the thermal statistics gives the averaged polarization for Ba（Zr, Ti）O3, which depends on the temperature of synthesis. Our result is helpful to understand and tune the relaxor ferroelectricity of lead-free Ba（Zr, Ti）O3.
Bi2Te3single crystals were prepared by the solid-state reaction method. The effect of the vacuum on the growth of Bi2Te3 single crystals was studied with varying the oxygen content by controlling the air pressure in the silica tube. High quality Bi2Te3 single crystals have been obtained and there is no influence on the growth by an extremely small amount of oxygen in a high vacuum at 1.0 × 10^-3Pa. As the air pressure is increased at 1.0 × 10^-2Pa, oxygen only mainly impacts on the growth of the surface for the prepared samples. Micron-sized rod-like structure and flower-like clusters are observed on the surface. For the samples prepared at 1.0 × 10^-1Pa, x-ray diffraction data show that the yellow part on the surface is Bi2TeO5, while the Bi2Te3 single crystal is still the major phase as the inside part. More interestingly, various crystal morphologies are observed by scanning electron microscope for Bi2Te3 near the boundary between Bi2Te3 and Bi2TeO5.Possible growth mechanisms for Bi2Te3 with different morphologies are discussed in detail.
PbTiO3 is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [001]-orientated films. There are few studies on PbTiO3 films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO3 films strained by SrTiO3 and LaAlO3 substrates. Our results show that the polarization of PbTiO3 films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO3 mainly depends on the tetragonality of the lattice, instead of the simple strain.
