The low efficiency of the host luminescence can be ascribed to
the strong phonon coupling of the transition with the host lattices
[13], resulting in the occurrence of nonradiative process. Its broad
emission band extended from 380 nm to 600 nm also indicates
such a strong electron–phonon coupling in the radiative
transition. In general the host luminescence is not as efficient as
that of the properly doped activators. Nevertheless it provides an
optical approach to examine the intrinsic properties of the host
defects for comparison with the doped activators.