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.