Unveiling the role of the hexagonal polymorph on SrAl₂O₄-based phosphors

In persistent luminescence materials, the SrO-Al₂O₃ system has been mainly studied due to its chemical stability, higher photoluminescence response and longest green-afterglow times. Specifically, the research has focused on SrAl₂O₄ doped with europium and dysprosium. SrAl₂O₄ has two polymorphs: monoclinic polymorph (space group P2₁) and hexagonal polymorph (space group P6₃22). Besides, the coexistence of these two phases, monoclinic and hexagonal, appears in almost all the results. However, it is not clear how this coexistence influences optical response. Some authors have reported that only the monoclinic structure exhibits luminescence properties, while another suggests that the hexagonal SrAl₂O₄ polymorph has a higher emission efficiency than the monoclinic polymorph. Here we report a systematic evaluation of the effects of the stabilization of the hexagonal SrAl₂O₄ polymorph. We show that an interrelationship between the hexagonal polymorph and phosphorescent properties is the linchpin for the development of good luminescence properties. Remarkably, the stabilization of the hexagonal SrAl₂O₄ polymorph on the monoclinic-hexagonal polymorphic coexistence appears to be related to the preservation of the nanometric nature of the SrAl₂O₄-based system. Our results will help to understand the role of the hexagonal polymorph in the polymorphic coexistence on SrAl₂O₄-based systems and may facilitate the development of luminescent nanometric particles for the design and preparation of new light emitting materials.