Vibrational study of A3In2B"O9 (A = Ba, Sr; B" = U, W) double perovskites

Raman scattering and infrared transmittance techniques were used to investigate the room-temperature phonon spectra of the polycrystalline Ba3In2UO9, Sr3In2UO9, Ba3In2WO9 and Sr3In2WO9 double perovskites. Although X-ray diffraction has shown that both Ba3In2UO9 and Ba3In2WO9 crystallize in a disordered cubic (Pm (3) over barm-O-h(1)) structure with one formula unit per unit cell (Z = 1), Raman measurements revealed that both Ba3In2UO9 and Ba3In2WO9 compounds should exhibit a 1:1 ordered domains with Fm (3) over barm symmetry, embedded in the Pm (3) over barm matrix. Observation of a nu(1)-like vibration of the InO6 octahedron reflects the occurrence of a two-phonon-like behavior due to the intrinsic occupational disorder at octahedral symmetry sites. Assignment of most of the Raman and infrared phonons is also given. Copyright (C) 2009 John Wiley & Sons, Ltd.