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The present paper deals with the applicability of IR spectroscopy to problems related to ceramics. High refractory oxides, such as alumina and many alumina silicates, often consist of a tightly bound three-dimensional network, where coupling between the atoms is strong. If, in addition, the unit cell is small and its symmetry high, as in the case of MgO, most of the lattice vibrations merge into a broad band, with no distinct band structure.

Changes introduced, for instance into alumina ceramics by neutron irradiation, show up in the IR spectra as small shoulders or mere band broadening (Kostyukov et al., 1967). The defects in alumina, introduced for instance, by isomorphous substitution of Al3+ by V4+ and Co2+ (Wong et al., 1967) are best studied in the very far IR region, where these impurities give rise to sharp bands at 28 and 52·6 cm−1 and to a band at 110 cm−1 respectively. Fluorine-containing alumina, doped with transition metal cations in the trivalent state, reacts with molecular hydrogen to form OH-groups and transition metal cations in the divalent state. These OH-groups can easily be seen in the IR spectrum. Above 900°C, fluorine escapes as AlF. In order to restore stoichiometry based on trivalent Al, oxygen has to be given off leaving oxygen vacancies (Novak and Confova, 1968), but these defects are hardly detected in the IR spectra.

As soon as definite atomic groups are contained in the lattice, for instance Si04 tetrahedra in orthosiIicates and arnets (McDevitt, 1969),

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