Infrared studies of spinels-V. Lithium spinels of the type LiXY4O8

The i.r. spectrum of ordered spinels LiXIIIYIII4O8(XIII = Ga, Fe, In; YIII = Cr, Rh) are discussed and assigned with the help of 6Li-7Li isotopio shifts. Two bands only may be assigned to definite motions of specific cations or coordinated groups: one band in the 450-350 cm-1 region exhibits an important 6Li-7Li isotopic shift and is assigned to a displacement of the lithium cation. The lowest-lying frequency strongly depends on the mass of the XIII tetrahedral cation and thus is assigned to a translation of this cation. All the remaining bands, in the 700-500 cm-1 region, are related to mixed vibrations involving the simultaneous participation of XIII and YIII cations. These assignments completely disagree with those of DeAngelis et al., which are shown to rely upon erroneous considerations. The disordered phase of LiGaCr4O8 (statistical distribution of Li and Ga cations on tetrahedral sites) has also been investigated and the main features of its i.r. spectrum are briefly discussed. © 1973.