Unveiling the role of the hexagonal polymorph on SrAl 2 O 4 -based phosphors.

In persistent luminescence materials, the SrO-Al 2 O 3 system has been mainly studied due to its chemical stability, higher photoluminescence response and longest green-afterglow times. Specifically, the research has focused on SrAl 2 O 4 doped with europium and dysprosium. SrAl 2 O 4 has two polymorphs: monoclinic polymorph (space group P 2 1 ) and hexagonal polymorph (space group P 6 3 22). 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 SrAl 2 O 4 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 SrAl 2 O 4 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 SrAl 2 O 4 polymorph on the monoclinic-hexagonal polymorphic coexistence appears to be related to the preservation of the nanometric nature of the SrAl 2 O 4 -based system. Our results will help to understand the role of the hexagonal polymorph in the polymorphic coexistence on SrAl 2 O 4 -based systems and may facilitate the development of luminescent nanometric particles for the design and preparation of new light emitting materials.