The light induced valence change of europium in $Sr_2SiO_4$ : Eu involving transient crystal structure

$Sr_2SiO_4 : Eu^{3+}$ shows orange-red emission of $Eu^{3+}$ substitutively present in two different Sr sites. The lightinduced spectral changes from orange-red sharp line emission to yellow-white broad band are observed in $Sr_2SiO_4$ : Eu at room temperature under irradiation with short UV or X-rays. The spectral changes are attributed to the optically assisted reduction of $Eu^{3+} \rightarrow Eu^{2+}.$ The photoreduced $Sr_2SiO_4$ : Eu shows emission containing contributions from both $Eu^{2+}$ and $Eu^{3+}$ in comparison to chemically reduced samples. This is explained on the basis of preferential reduction of $Eu^{3+}$ present in Sr(1) sites under irradiation due to unsatisfied $Eu_{Sr}–O–Si$ bonds. The absence of photoactivity for $Ba_2SiO_4$ : $Eu^{3+}$ (space group = Pnam) as well as $Ca_2SiO_4 : Eu^{3+}$ (space group = $P2_{1/n}$) indicates that crystal structure plays an important role in the photoreduction of $Sr_2SiO_4 : Eu^{3+}$ because of the prevailing orientational as well as the positional disorder in the latter. Further, the orientationally disordered monoclinic random domains persist within the orthorhombic lattice of $Sr_2SiO_4,$ resulting in the positionally disordered Sr atoms and orientationally disordered $SiO_4$ tetrahedra. Electron paramagnetic resonance studies confirm the electron trapping by dynamically disordered $(SiO_4)^{4}$ under high energy photon illumination resulting in the formation of radical anion $(SiO_4)^{5}.$ The substitutional studies indicate that the $[Eu^{3+} \leftarrow O^{2}]$ charge-transfer (CT) state is directly involved in the photoreduction process. The excitation of $Sr_2SiO_4 : Eu^{3+}$ produces the $[Eu^{3+} \leftarrow O^{2}]$ CT state which relaxes and transfers electrons to $SiO_4$ groups due to optically assisted rearrangement of local environment and mediates the electron transfer process to cause photoreduction of $Eu^{3+}$ to $Eu^{2+}.$ The yellow emission is stable at room temperature and reverts to red on annealing at elevated temperature in Ar atmosphere due to thermally activated detrapping of charge carriers present at the defect centers which, in turn, convert $Eu^{2+}$ to $Eu^{3+}$. The thermally activated conversion of $Eu^{2+} \rightarrow Eu^{3+}$ in $Sr_2SiO_4$ is optically reversible, thereby resulting in a highly efficient material for application as an optical storage medium