Study On Electronic Structure And Physical Properties Of Perovskite Ferroelectrics

The perovskite ferroelectrics are important functional materials. Theoretical researches are not only helpful to understand the phase transition mechanism of ferroelectrics, but also advantaged to the application of ferroelectric materials. However, the origin of ferroelectric behavior is not very clear nowadays.The ferroelectric distortions involve small displacements of the cations relative to the anions, leading to a net dipole moment per unit volume, i.e. a polarization. Lower lattice symmetry makes the description more difficult for conventional methods such as density functional theory and molecular dynamics simulations. But it is convenient for the valence bond theory to deal with low lattice symmetry problems.In this dissertation, the valence electronic structures of five kinds of typical perovskite ferroelectrics are investigated according to the valence bond theory, the empirical electron theory (EET) of solids and molecules.In simple perovskite ferroelectrics, the bond of atom B and atom O is the strongest bond, which is responsible for the tendency to the ferroelectricity. It is found that the valence electronic structure is different between cubic phase and tetragonal phase and the B-O bond changes into two kinds of different bonds along the tetragonal axis direction as the transition occurs.In lead-free ferroelectrics, the difference of the valence electronic structures between the lead titanant and lead-free materials is found. In tetragonal ferroelectric phase, the B-O bond is less weaker than that in PbTiO3. B-O bond is essential to the ferroelctricity of these materials. The spontaneous polarizations of KNbO3 and BaTiO3 are smaller than PbTiO3.In solid solution ferroelectrics, the B-site atoms make different contributions to the ferroelectricity. In PZT, atom Ti is more important than atom Zr. With the increase of Ti content, the B-O bond becomes stronger. In KTN, atom Ni is more important than atom Ta.In new single crystal relaxor ferroelectrics, valence electronic structures of the Morphotropic phase boundary (MPB) will change with the temperature and the composition of these materials.In single phase multiferroelectric, the possibility of the co-existence of the magnetism and ferroelectricity is found after the calculation of valence electronic structure. The Fe-O bond is not the shortest bond in the crystal but is the strongest co-valence bond, which is the origin of the ferreoelectricity and atom Fe has intrinsic magnetic moment from d electron.It is showed by EET that the bond B-O is very important to the ferroelctricity in these perovskite ferroelectrics.