Inorder to provide anaccurate basis forthe interpretationofthe physical properties of this complex incommensurate system, the structure of the room temperature quasicommensurate 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 A. b = 17.620 A and c = 7.987 A. This approximation gives an excellent agreement index R = 0.031 for the fit to the intensities of the 2432 basic reflections andthe 1691satellitereflectionscollectedat294 K(AMo Ka = 0.71069A,p, = 111.47cm.'. space group : Bbm2). The structural resultsshow that the commensurate modulation consists of a tilting and a shearing of all the NbO, 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 Nb50?4 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.

The x-ray-diffraction spectra of Ba2NaNb50&5 have been investigated by means of i precession camera between room temperature and 500'C, across the orthorhombic-tetragon il transition near Tt =300'C. Long exposures have revealed reflections, onsetting with the orthorhombic phase, which are incommensurate with respect to the periodicity of the basic crystal lattice.Referred to the pseudotetragonal lattice vectors, the modulation vector is k = (1+5)(a + b )/ 4+e /2 where 5 increases, on heating, from a value of 0.01 at room temperature, up to 0.115 at T~. Its variations exhibit an upward jump it Tlat=2'75'C from 0.035 to 0.085. These results show for the first time, the existence of two incommensurate phases in birium sodium niob ite.These incommensurate phases have the same modulation direction. They both h &ve &n iver &ge orthorhombic symmetry and are therefore ferroelastic on symmetry grounds. Above T~, precursor effects are disclosed in the form of strong diffuse sc ittering rods which condense it Tĩ nto the satellite reflections. Birefringence and DTA (differential thermal inalysis) me isurements are consistent with the x-ray results.

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