Vibrational studies of molybdates, tungstates and related c compounds-III. Ordered cubic perovskites A2BIIBVIO6

Most of the bands predicted by a group theoretical analysis have been observed in the i.r. and Raman spectra of a number of ordered, cubic perovskites A2BIIBVIO6. With the help of cartesian symmetry coordinates, and from the wealth of experimental data including 24Mg26Mg, 40Ca44Ca and 122Te130Te isotopic frequencies, it appears that the separation of the observed vibrations in internal and external modes is a good approximation for the high-frequency bands, which are assigned to the stretching vibrations of the BVIO6 octahedron. This approximation is not so satisfactory for the corresponding bending vibrations, since these vibrations may be more or less mixed up with translational, external modes which belong to the same symmetry species (T1u in i.r.; T2g in Raman). The behaviour of the totally symmetric A1g mode has been investigated in some detail: its frequency is predominantly determined by the nature of the hexavalent BVI cation; but it is also influenced by the nature of the bivalent BII octahedral cation, acting through its bonding force and ionic radius, and by the size of the large A cation, because of its influence on the BIIO distance. Abnormal values of the A1g frequency are observed when the BII cation is rather large: this is qualitatively explained by a displacement of the oxygen towards the BVI cation, with or without distortion of the structure. © 1974.