Etude par spectroscopie de vibration des hexahydroxodialuminate et digallate de potassium

Infrared and Raman spectra of K AlO₂, 1,5 H₂O and K GaO₂, 1,5 H₂O are interpreted on the basis of [Al₂O(OH)₆]²⁻ and [Ga₂O(OH)₆]²⁻ ions. They support a previous X-ray study on the potassium hexahydroxodialuminate. These ions are constituted by two MO₄ tetrahedra sharing one oxygen atom. Spectra show very little difference between the three external MO bonds and the internal one, around each metal atom. On the other hand, due to the presence of a cristallographic symmetry axis on the oxygen atom of the MOM bridge, the spectra of isotopic mixed crystals allow us to characterize only three OH (or OD) bonds of different lengths. The hydrogen bonds lengths obtained from the v(OH) frequencies agree very well with the observed external oxygen-hydrogen distances. The MO₄ tetrahedra of a M₂O₇ unit are slightly coupled and the study of MO₃ group frequencies shows that the MO bond strength increases when aluminium is replaced by gallium. This fact explains increase and decrease of bending and stretching OH vibrations.