Sustainable rare-earth-free phosphors for the visible range

Energy conservation, recycling and sustainable raw materials management are key issues of the 21st century. For energy-saving white light illumination and display technology, inorganic ceramic luminescent materials, which are excited with a blue-emitting semiconductor-based LED and glow either yellow or green and red due to the lanthanide ions Ce³⁺ or Eu²⁺ contained in the luminescent material, have now become established. Together with the blue light of the semiconductor LED, this results in white light. The luminescence of the lanthanide ions Ce³⁺ and Eu²⁺ can be adjusted across the entire visible spectral range on the basis of an interconfigurational 4f^n-15d¹ → 4fⁿ transition via the surrounding ligand field. The efficiency and width of the emission band can also be appropriately tuned by choosing the structure of the host compound. 

As the export of rare earths becomes increasingly dependent on individual countries, the question arises as to whether more sustainable alternatives exist. The much more readily available 3d⁵ ion Mn²⁺ offers a promising alternative here. Depending on the choice of coordination geometry and also the ionicity or covalency of the chemical Mn-ligand bond, the color of the luminescence of this transition metal ion can be tuned between green and red. Due to the smaller size of the 3d orbitals compared to the 5d orbitals relevant for lanthanides, the corresponding emission bands should be even narrower than for Eu²⁺ if the surrounding host compound is chosen correctly, which promises high color fastness. At the same time, they should be characterized by high internal quantum yields.