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

The powder diffraction pattern of the perovskite AgNbO3 has been measured u sing CuK alpha(1) radiation with an incident beam focusing monochromator to eliminate the K alpha(2) component. Indexing the pattern shows that the mu ltipartite cell is 2X2X4 times that of the pseudocubic subcell. Comparison is made with the diffraction pattern of NaNbO3, which has a similar multipa rtite unit cell. There are strong similarities, but close inspection shows that the structures are not isomorphous. The payer concludes with a discuss ion of the figure of merit F-N for pseudosymmetric structures. It is sugges ted 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 propos ed second figure of merit does not include any superlattice reflections. Th ese superlattice reflections tend to be very weak, resulting in a low compl eteness factor and relatively large error in the measurement of their posit ion. This effect produces an unrealistically low value of the standard figu re 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 me asured diffraction pattern is obtained. (C) 1999 International Centre for D iffraction Data. [S0885-7156(99)00301-2].