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

Highlights: • Both Eu²⁺ and Eu³⁺ luminescences were investigated in ABaPO₄ (A = Li, Na, and K). • The valence states of Eu ions changed reversibly through the H₂- and O₂-ambient post-annealing. • The valence states of Eu ions could be changed by gamma-ray irradiation and H⁺-implantation. • The color of Eu ion doped ABaPO₄ phosphors can be tuned easily in a range from red to blue by post-treatments. -- Abstract: 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.