Application de la spectrometrie vibrationnelle à un probleme de non-stoechiométrie par insertion dans les tunnels pentagonaux des réseaux (M6X4O26) (M = Nb, Ta; X = Si, Ge)

The ir spectra of A3M6Si4O26 (A = Ba, Sr; M = Nb, Ta) and K6M6Si4O26 oxides, whose structure contains linear Si2O7 groups, are discussed with particular emphasis on the peculiar behavior of the antisymmetric stretching frequency of the linear SiOSi bridge. In accord with previous data, this frequency is the highest of the spectrum (near 1200 cm-1), but it is significantly lowered (by about 75 cm-1) when passing from the A3M6Si24O26 to the K6M6Si4O26 compounds. This is readily explained by the peculiar structure of the K6 compounds, in which three (out of the six) K+ cations are located near the bridge oxygen (A2 sites), these sites remaining empty in the A3M6Si4O26 compounds. The resulting KO bonding weakens the SiO bond, thus leading to a lowering of the corresponding bridge frequency. The same type of explanation holds for the presence of a new band at an intermediate frequency (about 1150 cm-1) in phases of intermediate composition K6-2xBaxM6Si4O26, this new band being correlated with a partial occupancy of the A2 sites. This has been applied to, and is a sensitive means of, detecting nonstoichiometry in the A2 sites of other compounds with (M6X4O26) layers (X = Si, Ge) such as Ba6+xNb14Si4O47, K8M14Si4O47, and K10M22X4O68 (M = Nb, Ta). © 1978.