Crystal structure of ferroelectric silver niobate AgNbO3 was determined (Pmc21) by convergent beam electron, electron, neutron, and synchrotron diffraction techniques and first-principles calculations. The atomic displacements along the c axis in Pmc21 AgNbO3 are responsible for the spontaneous polarization, ferroelectricity, and the paraelectric−ferroelectric phase transition.

Silver niobate AgNbO 3 is the important mother compound of lead-free ferroelectric AgNbO 3 −based materials and photocatalyts. The AgNbO 3 has the ferrielectric structure with orthorhombic Pmc2 1 space group, but the origin of the ferrielectricity of AgNbO 3 is unclear. Here, we report the results of electronic-structure calculations of the ferrielectric orthorhombic Pmc2 1 AgNbO 3 in order to discuss the origin of the ferroelectricity and visible-light photocatalytic activity. We found the covalent Nb−O and Ag−O bonds in the ferrielectric AgNbO 3 . The covalent character is ascribed to the Nb 4d-O 2p and Ag 4d-O 2p hybridizations in AgNbO 3 . The Nb−O and Ag−O hybridization would be a key factor of the off-center displacements of Nb and Ag cations along the c axis, which induces the ferrielectricity in AgNbO 3 . The covalent bonds and the existence of Ag atom are responsible for the visible-light response of AgNbO 3 photocatalyst.

Structure of Ferroelectric Silver Niobate AgNbO 3 . -The structure of AgNbO 3 is determined by synchrotron and powder neutron diffraction, convergent-beam electron diffraction, SAED, and DFT calculations. The compound crystallizes in the non-centrosymmetric orthorhombic space group Pmc21 with Z = 8, and has a perovskite-type structure consisting of corner-linked NbO6 octahedra and Ag atoms. The net spontaneous polarization in AgNbO 3 is essentially due to atomic displacements along the c-axis in Pmc21. The ferroelectric-paraelectric transition around 67°C is suggested to be a Pmc21-Pbcm transition. These results are important for the understanding of the physical and chemical properties of the title compound and for the design of AgNbO3-based piezoelectric materials and photocatalysts. -(YASHIMA*, M.; MATSUYAMA, S.; SANO, R.; ITOH, M.; TSUDA, K.; FU, D.; Chem. Mater. 23 (2011) 7, 1643-1645, http://dx.doi.org/10.1021/cm103389q ; Dep. Mater. Sci. Eng., Grad. Sch. Sci. Eng., Tokyo Inst. Technol., Midori, Yokohama 226, Japan; Eng.) -W. Pewestorf 25-006