Infrared studies of spinels-II. The experimental bases for solving the assignment problem

The problem of interpreting the i.r. spectrum of spinels is discussed. Since the four i.r. active fundamentals belong to the same representation T1u vibrational interactions must be expected and any assignment of the observed frequencies to localized vibrations is necessarily an approximation. The validity of this approximation is discussed for different types (weak or strong) of vibrational interactions, and emphasis is put on the identification of the cations which are involved in either "localized" or "complex" vibrations. These assignment problems may be solved by appropriate experimental methods, namely the systematic investigation of the vibrational behaviour of: (i) homogeneous families of pure compounds; (ii) isotopic species, including isotopes of medium-weight cations; (iii) solid solutions. © 1971.