Dielectric property change of ferroelectrics and electronic structures

Electronic structures were investigated in relation to the relative permittivity of ferroelectrics such as the ABO(3)-type and A- and/or B-substituted materials, using a molecular orbital method. The A-site ions were Ba, Ph, Sr and Ca, and the B-site ion was Zr. Calculation was performed using the DV-X alpha (discrete variational Xa) method and the overlap population, which is related to the covalent bonding nature, was discussed together with the effective charge. As a result, the change from ferroelectric to paraelectric was found to be associated with the covalency between the A-site ion and Ti or Zr: Furthermore, the energy for pi-bonding between O2p and Ti3d (or Zr4d in AZrO(3)) shifted toward the lower energy level by substitution of the A site with Ca to give a lower relative permittivity. Then, we studied the effect of quantitative changes of the amount (x = 0.25, 0.5, 0.75) of A-site ions in the (Ba1-x, Sr-x)TiO3 system on the electronic structures, and suggested that the bonding nature between O and Ti is related to the relative permittivity of the system.