Infra-red spectra of inorganic aluminates and characteristic vibrational frequencies of AlO4 tetrahedra and AlO6 octahedra

Some aspects of the interpretation of IR spectra of inorganic solids are discussed, with emphasis on the factors influencing the vibrational frequencies of cation-oxygen co-ordinated groups, namely the value of the co-ordination number, “isolated” or “condensed” state of the co-ordinated groups and vibrational interactions with neighbouring groups. These considerations are applied to the study of AlO stretching frequencies in a series of aluminates. Characteristic frequency ranges are as follows: “Condensed” AlO4 tetrahedra 900-700 cm−1, “Isolated” AlO4 tetrahedra 800-650 cm−1, “Condensed” AlO6 octahedra 680-500 cm−1, “Isolated” AlO6 octahedra 530-400 cm−1. Several cases of mixed vibrations (AlO + LiO) are found in the particular case of lithium aluminates from abnormal or erratic 6Li7Li isotopic shifts.