The light induced valence change of europium in Sr2SiO4 ? Eu involving transient crystal structureElectronic supplementary information (ESI) available: details of the experiment under different irradiation conditions, characterization by XRD and PL spectra of Ba2SiO4 ? Eu3+ and Ca2SiO4 ? Eu3+. See http://www.rsc.org/suppdata/jm/b4/b400515e/

Sr2SiO4 ∶ Eu3+ shows orange-red emission of Eu3+ substitutively present in two different Sr sites. The light-induced spectral changes from orange-red sharp line emission to yellow-white broad band are observed in Sr2SiO4 ∶ Eu at room temperature under irradiation with short UV or X-rays. The spectral changes are attributed to the optically assisted reduction of Eu3+ → Eu2+. The photoreduced Sr2SiO4 ∶ Eu shows emission containing contributions from both Eu2+ and Eu3+ in comparison to chemically reduced samples. This is explained on the basis of preferential reduction of Eu3+ present in Sr(1) sites under irradiation due to unsatisfied EuSr–O–Si bonds. The absence of photoactivity for Ba2SiO4 ∶ Eu3+ (space group = Pnam) as well as Ca2SiO4 ∶ Eu3+ (space group = P21/n) indicates that crystal structure plays an important role in the photoreduction of Sr2SiO4 ∶ Eu3+ 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 Sr2SiO4, resulting in the positionally disordered Sr atoms and orientationally disordered SiO4 tetrahedra. Electron paramagnetic resonance studies confirm the electron trapping by dynamically disordered (SiO4)4− under high energy photon illumination resulting in the formation of radical anion (SiO4)5−. The substitutional studies indicate that the [Eu3+ ← O2−] charge-transfer (CT) state is directly involved in the photoreduction process. The excitation of Sr2SiO4 ∶ Eu3+ produces the [Eu3+ ← O2−] CT state which relaxes and transfers electrons to SiO4 groups due to optically assisted rearrangement of local environment and mediates the electron transfer process to cause photoreduction of Eu3+ to Eu2+. 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 Eu2+ to Eu3+. The thermally activated conversion of Eu2+ → Eu3+ in Sr2SiO4 is optically reversible, thereby resulting in a highly efficient material for application as an optical storage medium.