Influence of Al³⁺ doping on the energy levels and thermal property of the 3.5MgO·0.5MgF₂·GeO₂:Mn⁴⁺ red-emitting phosphor

A series of Al³⁺-doped 3.5MgO·0.5MgF₂·GeO₂:Mn⁴⁺ red-emitting phosphors is synthesized by high temperature solid-state reaction. The broad excitation band at 300 nm-380 nm, resulting from the ⁴A₂ → ⁴T₁ transition of Mn⁴⁺, exhibits a blue shift with the increase of Al₂O₃ content. The observation of the decreased Mn⁴⁺-O^2- distance is explained by the crystal field theory. The temperature-dependent photoluminescence spectra with various amounts of Al₂O₃ content are comparatively measured and the calculation shows that the activation energy increases up to 0.41 eV at the Al₂O₃ content of 0.1 mol. The maximum phonon densities of state for these samples are calculated from Raman spectra and they are correlated with the thermal properties.