Optical properties of red-emitting long afterglow phosphor Mg2Si1-xGexO4: Mn2+/Mn4+

For decades, materials with persistent luminescence (PersL) have triggered interest among scientists and industry. A considerable amount of research has been done on multiple commercially available materials emitting PersL in the blue and green spectral regions. Despite the wide variety of applications, the number of publications on the afterglow in the red spectral range is considerably lower. In pursuit of a competitive red PersL emitter, manganese doped Mg2SiO4, Mg2GeO4, and their solid solutions of Mg2Si1-xGexO4 (x = 0.1–0.9) were prepared by the solid-state synthesis in ambient atmosphere. The materials were comprehensively analysed by X-ray diffraction (XRD), optical spectroscopy, thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR). Bright red PL and subsequent PersL of manganese ions in the red spectral region have been detected. In the case of the best persistent phosphor - Mg2Si0.1Ge0.9O4: 0.1 mol% Mn afterglow was detected for more than 16 h. Although both Mn2+ and Mn4+ are present in samples, only Mn2+ related PersL with broadband emission between 600 and 800 nm and a maximum around 640 nm is observed. An in-depth analysis of TSL measurements showed that three distinct trap centres with trap depth values of 0.87 eV, 0.99 eV, and 1.84 eV are present in the material. Additionally, a quasi-continuous distribution of closely overlapping trap levels appears with trap depth values around 1.1–1.8 eV. EPR measurements have confirmed the presence of two distinct charge traps associated with trapped hole O− and trapped electron F+ centres. The thermal stability of these centres was analysed and correlated to PersL processes.