First-Principles Study of Point Defect Formation in AgNbO3

AgNbO 3 exhibits an antiferroelectric phase transition to the orthorhombic P b c m phase at ∼626 K. At room temperature it exhibits ferroelectric behavior with a large spontaneous polarization. However, the ferroelectric structure is still not well understood, with different groups reporting it to have P b c m symmetry, with its ferroelectricity attributed to local symmetry breaking and/or a defective structure, while others propose space group P m c 2 1 on the basis of converged beam electron diffraction and neutron and X-ray diffraction results. Here, we report first-principles calculations of point defect formation in AgNbO 3 using the projector augmented wave method within the density functional theory. Our calculations reveal that the material should contain abundant [2V Ag ' + V O •• ] × complexes (i.e., two Ag vacancies charge compensated by formation of an O vacancy) in AgNbO 3 crystals. We show that the presence of this defect complex in an antiferroelectric P b c m matrix could act as the origin of the weak ferroelectricity in AgNbO 3 .