Besides, the PL intensity has been found to be quenched at
Eu3+ ions concentration more than 0.5 at.%. Concentration
quenching is a well known phenomenon where, for example,
the quenching concentrations of Eu3+ ions luminescence
in GaN:Eu31 was 2–3 at.% and 4 at.% in Y2O3:Eu.32
This concentration quenching might be associated with an
appearance of Eu3+–Eu3+ pairs in the system because of
the decrease in the average distance between Eu3+ ions.
Transitions of electrons within the 4f -shell are restricted
by the selection rules applicable to electric and magnetic
dipole processes. According to the theory of Judd33 and
Ofelt,34 5D0 →7F2 transition is forbidden electric dipole