Ab initio study of silver niobate

Using DFT calculations, we investigate the local structure and the distortion patterns of the perovskite AgNbO3 to examine the feasibility of using silver on the perovskite A-site in lead-free piezoelectrics. Our calculations show that in a 5-atom AgNbO 3 unit cell, silver atoms can off-center by about 0.5 A, forming short covalent bonds similar to the short Pb-O bonds in Pb-based ferrroelectrics. In the more realistic 40-atom supercell, Ag behaves similar to Pb in PbZrO 3, forming short covalent Ag-O bonds through a combination of large octahedral rotations and small Ag offcenter distortions. Unlike PbZrO3, AgNbO3 is not antiferroelectric due to the presence of large Nb off-center distortions. In the AgNbO 3-PbTiO3 solid solution, both A-cations off-center significantly and display a preference for distortion away from Nb atoms and toward Ti atoms. The interplay between this preference and maximization of local dipole alignment should lead to an MPB in the AgNbO3-PbTiO3 phase diagram.