X-ray structural determination of the quasi-commensurate phase of barium sodium niobate

Abstract

In order to provide an accurate basis for the interpretation of the physical properties of this complex incommensurate system, the structure of the room temperature quasi-commensurate phase of barium sodium niobate is determined by taking into account, for the first time, the satellite reflections present in the diffraction spectrum of this material. The quasi-commensurate phase considered is assimilated to a strictly commensurate phase, the satellite reflections being considered as superlattice reflections and referred to a crystalline supercell with a=35.187 AA, b=17.620 AA and c=7.987 AA. This approximation gives an excellent agreement index R=0.031 for the fit to the intensities of the 2432 basic reflections and the 1691 satellite reflections collected at 294 K ( lambda Mo K alpha =0.71069 AA, mu 1=111.47 cm-1, space group Bbm2). The structural results show that the commensurate modulation consists of a tilting and a shearing of all the NbO6 octahedra in the structure in contrast to previous inferences which had only suggested the occurrence of a shear. The framework of the structure may be described on the basis of the existence of two different Nb5O24 units of five corner-sharing octahedra each. The structure obtained by spatially averaging the modulation is consistent with previous structural studies of the basic reflections of this material. Physical characteristics pertaining to the polar character, or the domain structure, are also deduced.