Dependence of the optical properties of Mn4+ activated A2Ge4O9 (A=K,Rb) on temperature and chemical environment

The optical properties of the novel red-emitting K2Ge4O9:Mn4+ and Rb2Ge4O9:Mn4+ phosphors exhibit an unusually strong dependence on temperature. The temperature dependence of Racah parameters B and C as well as crystal field factor Dq was determined. It was found that C exhibits an inverse trend in regard to B. This observation was linked to a potential change in bond angles of the Mn4+ dopant site. Mn4+ occupies two sites in the material, causing a double Fermi-Dirac quenching behavior. Mn4+ on the site with larger Mn-O distances shows a lower thermal quenching temperature. Decay constants were calculated from temperature dependent decay measurements. The constants decrease steadily with temperature, most likely not due to thermal quenching but an increase of the 2E→4A2 transition probability.