DISORDER PHENOMENA IN THE INCOMMENSURATE COMPOUND BI2-XFE2O9+DELTA(XSR3)

The refinement of the modulated structure of Bi2+xSr3-xFe2O9+delta, us ing the our-dimensional formalism and single-crystal x-ray-diffraction data, has been performed. Different models, already used for the copp er compound, have been tested to describe the [BiO] layers. A discussi on pointing out the qualities and the failings of each one led us to c onsider a model assuming a static disorder both on the bismuth and oxy gen atom positions. Then, an extra oxygen atom has been located within the [BiO] layers and its occupancy probability has been refined. Beca use of this model, two disordered regions, governing the behavior of a ll the atoms in the crystal, have been evidenced. A comparison with th e supercell description previously reported shows the interest of the four-dimensional formalism in this case. Finally, a Mossbauer study of Bi2+xSr3-xFe2O9+delta specifies the trivalent state of iron and confi rms the various surroundings in oxygen atoms of the iron atoms. The co rrelation between the incommensurate character of the structure and th e Mossbauer spectroscopy data is discussed.