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 (Pm3 m − O_h¹) 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 Fm3 m symmetry, embedded in the Pm3 m matrix. Observation of a ν₁‐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 © 2009 John Wiley & Sons, Ltd.