Light is one of the central factors in the life of mankind, which is why the various natural light phenomena and the materials that display them have been of interest to mankind since the beginning of history. Numerous attempts have been and are still being made to understand and reproduce these phenomena and materials. Today, it is possible to imitate, study and model almost any natural phenomenon or substance, and thus to increase our knowledge of light phenomena and substances exhibiting light phenomena. Nevertheless, the functioning of some phenomena is still only partially understood and their understanding is still ongoing. New properties of new materials, which are constantly being developed, are gradually shedding light on analogies, thus also helping to deepen our understanding of the phenomena we experience. Among the different types of luminescence, phosphorescence is one of the most complex phenomena, in which the individual sub-processes and their roles are still only partially understood.
With our current knowledge, we can therefore design in advance the luminescent materials that best meet our needs. These needs can vary widely depending on the application, so it may be necessary to manipulate the wavelengths emitted and absorbed for materials with extremely long or even short afterglow times. With the advance of the green approach, new demands have been made on luminescent materials such as long lifetime, reusability, extreme chemical and physical stability, safer use or simply cheap and large-scale production. To meet these needs, new materials can be developed or old materials can be improved, and new applications can be associated with known materials.
Today, a large number of activated photoluminescent inorganic oxides are known, just to mention a few examples: MAl2O4, M4Al14O25 M2ZnSiO7, MAlSiN3, MBPO5, M3PO4 (M = Mg, Ca, Sr, Ba). The presence of impurity (activating/co-activating) ions is usually required; these can be one or more rare earth metals, depending on the host crystal structure, but also transition metals. One of the main advantages of these materials, besides their durability, is that they do not contain any radioactive component. However, of the hundreds of known phosphorescent materials, only a few can be charged in sunlight and then emit light for hours, so materials that can do this together are of particular interest.