強誘電セラミックスの結晶構造解析

抄録

Ferroelectric ceramic materials with large spontaneous polarization and high dielectric and piezoelectric responses are attractive due to a wide range of applications. Here I describe the crystal structure analysis of ferroelectric silver niobate AgNbO₃ and 0.4 PbTiO₃-0.6 BiFeO₃. The space group of ferroelectric AgNbO₃ was found to be non-centrosymmetric Pmc2₁. Through the neutron and synchrotron powder diffraction analyses, and first-principles Density Functional Theory (DFT) calculations we have determined the atomic positions in the unit cell of AgNbO₃. The net spontaneous polarization in AgNbO₃ is essentially ascribed to the atomic displacements along the c axis in Pmc2₁. The ferroelectric-antiferroelectric transition of AgNbO₃ is suggested to be a Pmc2₁-Pbcm transformation. In ferroelectric tetragonal 0.4 PbTiO₃-0.6 BiFeO₃, we have demonstrated the experimental and theoretical evidences for the hybridization between (Bi,Pb)(6s,6p) and O(2p) orbitals, which is the key for the extremely large tetragonality (axial ratio c/a), high Curie temperature TC and ferroelectricity of 0.4 PbTiO₃-0.6 BiFeO₃.