The chemical bond properties and ferroelectricity studies of SrBi4Ti4O15

Spontaneous polarization as the most immediate parameter in ferroelectricity is always an emphasis in ferroelectric research. Some ferroelectric microscopic theory such as Berry-phase method and first principles calculation are used to study the spontaneous polarization of perovskite type ferroelectrics. SrBi4Ti4O15 is a typical bismuth layered structure ferroelectric, the complexity of its crystal structure makes the ferroelectric research more difficult. This study, from the perspective of chemical bond, analyzes the relationship between the chemical bond properties and the spontaneous polarization, and further explores the atomic bonding state in ferroelectric crystal and its impact on ferroelectricity.By starting from the crystal structure data of SrBi4Ti4O15, the atomic local cluster structure including bond length, atomic coordination situation and the number of atoms in a crystal structure unit are obtained by the atomic environment calculation (AEC). Calculation results show that there are 13 atomic local cluster structures in SrBi4Ti4O15. Then combined with the crystal decomposition method, the SrBi4Ti4O15crystal is decomposed into 38 pseudo-binary crystals, and each pseudo-binary crystal corresponds to a chemical bond. Accordding to the dielectric theory of chemical bond that used in binary crystal, chemical bond properties such as the number of effective valence electron, the effective valence electron density and the bond ionicity are calculated. Through improvement of the bond dipole moment model, the relationship among bond dipole moment, bond properties, and bond parameter is established, and the bond dipole moment of each bond in SrBi4Ti4O15 is calculated.The spontaneous polarization of an ferroelectric can be approximated as the superposition of the spontaneous polarization of various chemical bonds in the crystal, where the spontaneous polarization of chemical bond is due to the bond dipole moment. Based on the traditional polarization theory, the spontaneous polarization can be expressed as the average bond dipole moment per unit volume, and considering the number of molecules in unit cell (Z) and the atomic site occupation factor (SOF), the correlation between bond dipole moment and spontaneous polarization of the bismuth layered ferroelectrics is established. On the basis of this, the calculated spontaneous polarization along a axis in the ferroelectric SrBi4Ti4O15 is 28.03 C/cm2.This study simplifies the complex crystal research by AEC and crystal decomposition method, and studies the ferroelectricity of the bismuth layered ferroelectrics from the viewpoint of chemical bond. The bond dipole moment as the bridge in this study for connecting spontaneous polarization with chemical bond properties, which is a new micro study method in macro-properties of bismuth layered ferroelectrics. Based on this method, the spontaneous polarization of other relevant ferroelectrics such as orthogonal SrBi2Ta2O9, orthogonal Bi4Ti3O12, and tetragonal SrBi4Ti4O15 are calculated, all the results are in good agreement with the experimental values and other theoretically calculated values.