To this aim, the present article concerns the influence of the
optical basicity variation, due to addition of different halides to
the glass backbone, on the local state (coordination and valence)
of transition metal ions in the glass matrix and finally the glass color
itself. The theoretical optical basicity of a glass may be derived
from its chemical composition, using the optical basicity values
of the individual component oxides [17,18]. The theory of hard
and soft acid and bases is used in contexts where only a qualitative
description would help in understanding the predominant factors
that drive chemical properties and reactions. Hence optical basicity
is considered as a measure of the acid–base properties of oxides
and glasses [5,9,19,20]. That is actually the case in transition metal
chemistry, where numerous experiments have been done