Valence state switching and reversible emission tunability of A(2)SiO(4) (A = Ba, Sr, and Ca) with two-site substitution of Eu ions through simple thermal treatment

We investigated the changes in the structural and luminescent properties of Eu-ion-doped A(2)SiO(4) (A(2)SiO(4):Eu, A = Ba, Sr, and Ca) by annealing in oxidizing and reducing atmospheres. The initially synthesized samples displayed distinct, intense red emissions at approximately 600 and 700 nm, which can be attributed to the presence of Eu(3+) ions. The emission intensity of Eu(3+) was the strongest in Ca(2)SiO(4):Eu, which exhibited the lowest lattice symmetry among the three samples. Remarkably, following annealing in a reducing atmosphere (H(2)), the previously observed red emission vanished, and instead, a strong green emission at around 500 nm, which is characteristic of Eu(2+) ions. Because of the two occupation sites of the Eu ions in A(2)SiO(4), the emission of Eu(2+) strongly depends on the excitation wavelength, which is the most evident in Ca(2)SiO(4):Eu. Conversely, after annealing in an oxidizing atmosphere (O(2)), the emission in the green region was suppressed and the emission in the red region returned. The reversible transition between two oxidation states occurred repeatedly by alternating H(2) and O(2) annealing, resulting in good color tunability in wide visible region with a simple ambient annealing process in a single compound.