Wang Yuan-Xu

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN), we calculated the electronic structure of CaTiO3, BaTiO3 and KTN by first principles calculation. From total energy analysis, it is shown that, with increasing cell volume, the crystals (CaTiO3, SrTiO3) will have a ferroelectric instability. For BaTiO3, the ferroelectricity will disappear as the cell volume is decreased. From the density of states analysis, it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics. This is consistent with the analysis of band structure.

An Electronic Structure of Ferroelectric PbZr0.5Ti0.5O3

An electronic structure of ferroelectric PbZr1/2Ti1/2O3 is presented. The full potential linearized augmented plane wave method is used and the exchange correlation effects are treated by the generalized gradient approximation. In order to understand the ferroelectricity of PbZr1/2Ti1/2O3, the density of states, charge density distribution and band structure are calculated. The results show that the atoms Pb and Ti act similarly as in pure PbTiO3. Like other perovskite ABO3 ferroelectrics, there is a hybridization between the Ti d and O p states, which is responsible for the tendency to ferroelectricity.

Size Driven Ferroelectric-Paraelectric Phase Transition from the Surface Energy Viewpoint

The size driven ferroelectric-paraelectric phase transition in a ferroelectric of small size is studied within the framework of Landau phase transition theory. The transition is a consequence of the competition between decreasing the volume free energy by polarization and increasing the surface energy of the ferroelectric phase, which has a surface energy density higher than that in the paraelectric phase. A simple expression for the ferroelectric critical size as a function of the Landau free energy coefficients and the surface energy density is derived.