Prediction on Mn⁴⁺-Doped Germanate Red Phosphor by Crystal Field Calculation on Basis of Exchange Charge Model: A Case Study on K₂Ge₄O₉:Mn⁴⁺

Blue excitable red phosphor is the key component to improve the quality of lighting and display which is based on InGaN blue chips. Because of the potential in the area, Mn⁴⁺ red phosphors have recently got rising interests. However, most of them were found by trial and error. It remains very challenging to predict which kind of compound can stabilize Mn⁴⁺ and which wavelengths Mn⁴⁺ ions if they could survive in it will exhibit the excitation and emission at. Here, we first propose to use crystal field calculation on basis of exchange charge model to predict the energy levels of Mn⁴⁺ ion in germanate K₂Ge₄O₉ since Mn⁴⁺ and Ge⁴⁺ are almost identical in size and charge, and the local field around Mn⁴⁺ will experience less distortion after substitution for Ge⁴⁺. The calculation shows the red emission peaking at 663 nm and the blue absorption of ⁴A_2g (⁴F) → ⁴T_2g (⁴F) in 450~470 nm, which matches better to blue chips than commercial phosphor 3.5MgO·0.5MgF₂·GeO₂:Mn⁴⁺. This inspires the synthesis of Mn-doped K₂Ge₄O₉, the optical properties of which confirm the existence of Mn⁴⁺ and consist with the prediction. Comparison between theoretical and experimental results implies that no obvious preference of Mn⁴⁺ substitution over two different types of octahedral germanium sites, Ge1 and Ge2. Consequent systematic explorations have been made on the effects of flux content, preparation temperature, Mn content, and holding time to find the ways to enhance the Mn⁴⁺ luminescence for promotion of practical application. The results reveal the optimal sample can be made under much mild optimal condition (850°C for 4 h in air) with a quantum yield of >30% upon blue excitation of 460~470 nm. As temperature rises from 8 to 573 K, zero photon line (ZPL) emission redshifts along with gradual appearance of anti-Stokes side phonon bands due to the enhanced interaction of Mn⁴⁺ with host. This work demonstrates it possible to find Mn⁴⁺ red phosphors by guide of crystal field calculation.