An X-ray diffraction study of AgNbO3 and comparison with NaNbO3

The powder diffraction pattern of the perovskite AgNbO3 has been measured using CuK alpha(1) radiation with an incident beam focusing monochromator to eliminate the K alpha(2) component. Indexing the pattern shows that the multipartite cell is 2X2X4 times that of the pseudocubic subcell. Comparison is made with the diffraction pattern of NaNbO3, which has a similar multipartite unit cell. There are strong similarities, but close inspection shows that the structures are not isomorphous. The payer concludes with a discussion of the figure of merit F-N for pseudosymmetric structures. It is suggested that two figures of merit be reported. The first should be the standard one using either all measured reflections or just the first 30. The proposed second figure of merit does not include any superlattice reflections. These superlattice reflections tend to be very weak, resulting in a low completeness factor and relatively large error in the measurement of their position. This effect produces an unrealistically low value of the standard figure of merit. By including only "main" reflections, i.e., those reflections that are common to both the low-symmetry and high-symmetry parent phase (if it exists), a much better estimate of the quality of the fitting of the measured diffraction pattern is obtained. (C) 1999 International Centre for Diffraction Data. [S0885-7156(99)00301-2].