Infrared studies of spinels-IV. Normal spinels with a high-valency tetrahedral cation

The investigation of normal II-IV germanates, I-II-V vanadates and normal I-VI molybdate and tungstate spinels show that the high-frequency band of their i.r. spectrum must be assigned to a vibration of the tetrahedral group. A comparison is made with the assignment previously proposed for the normal II-III spinels, and it is suggested that the high-frequency absorption band of a spinel must be assigned to a vibration between the oxygen and the highest-valency cation, irrespective of the co-ordination of this cation. The medium- and low-frequency bands are often related to complex vibrations, as evidenced from the study of isotopic species. The Raman spectrum of the molybdates has also been investigated and the fundamentals have been assigned to the different symmetry species with the help of 92Mo-100Mo isotopic shifts. The i.r. spectrum of the I-II-V vanadates exhibits several abnormal features: the stretching vibration of the V04 tetrahedron is represented by a broad, complex absorption, and the low-frequency band is missing. © 1972.