Glasses are prepared by quenching of the melts as well as via the modern sol-gel process.
The rare-earth ions doped into the host materials in moderate amounts are used as structural probes. The changes induced in the optical spectra of the dopants is evidence of the changes of the glass structure as function of composition. Most useful in this regard are induced intensity and shape changes of the so-called "hypersensitive transitions". On the other hand rare-earth doped glasses constitute materials with very interesting optical properties which can be used, e.g., as solid state lasers.
In connection with the glasses prepared via the sol-gel process the emphasis is on maintaining the local structure of molecular
complexes which show magnetic exchange interaction between paramagnetic centres.