Origin of the Ferrielectricty and Visible-Light Photocatalytic Activity of Silver Niobate AgNbO3

Silver niobate AgNbO3 is the important mother compound of lead-free ferroelectric AgNbO3-based materials and photocatalyts. The AgNbO3 has the ferrielectric structure with orthorhombic Pmc2(1) space group, but the origin of the ferrielectricity of AgNbO3 is unclear. Here, we report the results of electronic-structure calculations of the ferrielectric orthorhombic Pmc2(1). AgNbO3 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 ferrielectic AgNbO3. The covalent character is ascribed to the Nb 4d-O 2p and Ag 4d-O 2p hybridizations in AgNbO3. The Nb-O and Ag-O hybridization would be a key factor of the off-center displacement's of Nb and Ag cations along the c axis, which induces the ferrielectricity in AgNbO3. The covalent bonds and the existence of Ag atom are responsible for the visible-light response of AgNbO3 photocatalyst.