Reversible conversion of the valence states of Eu ions in ABaPO₄ (A = Li, Na, and K) in terms of change in luminescence property

We investigated the emission properties of Eu ion-doped ABaPO₄ (ABaPO₄:Eu, A = Li, Na, and K) in which the valence state of the Eu ions was controlled by annealing in various ambients, gamma-ray (γ-ray) irradiation, and hydrogen ion (H⁺) implantation. We synthesized the ABaPO₄:Eu samples by solid-state reaction in air. The as-synthesized samples showed sharp red emission near 600 nm, which originated from Eu³⁺ ions. Interestingly, annealing under H₂ ambient changed the valence state of the Eu ions from 3+ to 2+, which was indicated by the disappearance of the red emission and concurrent emergence of strong blue emission. After subsequent annealing in O₂ atmosphere, the red emission of Eu³⁺ was recovered. The reversible switching between the 2+ and 3+ states was found to be repeatable by alternating H₂ and O₂ annealing. The valence state was also changed from 3+ to 2+ by H⁺ implantation and γ-ray irradiation of the as-synthesized samples, possibly owing to the formation of oxygen vacancies. Our results suggest that the luminescence color of the ABaPO₄:Eu phosphors can be controlled within a range from red to blue by simple annealing in various ambients.