Relationship between Luminescence and Crystal Structure of Eu³⁺doped Fluorite

According to the relationship between luminescent properties and local symmetry of Eu³⁺, together with the site symmetry of symmetrically equivalent points in space group, it can be inferred that there are mainly two kinds of emission centers, which have the C_4v or C_3v site symmetry respectively, in fluorite:Eu³⁺ system. The Eu³⁺ ion requires charge compensation, because it enters the cubic site in a heterovalent substitution for the Ca²⁺ ion. The charge-compensating defects, such as the predominant interstitial F_i^-, influence the local environment of the Eu³⁺ ions and, in particular, modify the site symmetry. The existence of F_i^- round the Eu³⁺ ion reduces the O_h symmetry to C_4v or C_3v, which breaks the parity selection rules and promotes the luminescence of fluorite:Eu³⁺ samples.