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 AgNbO
3 and 0.4 PbTiO
3-0.6 BiFeO
3. The space group of ferroelectric AgNbO
3 was found to be non-centrosymmetric
Pmc2
1. 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 AgNbO
3. The net spontaneous polarization in AgNbO
3 is essentially ascribed to the atomic displacements along the c axis in
Pmc2
1. The ferroelectric-antiferroelectric transition of AgNbO
3 is suggested to be a
Pmc2
1-
Pbcm transformation. In ferroelectric tetragonal 0.4 PbTiO
3-0.6 BiFeO
3, 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 PbTiO
3-0.6 BiFeO
3.
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