Compared with Mn⁴⁺-activated polycrystalline phosphors in micro powder form, the Mn⁴⁺-activated red-emitting crystals with considerable size exhibit improved resistance to deterioration under water attack. Besides, large sized fluorescent crystal can be packaged on LED in the remote configuration. We recently developed a series of methods for growing Mn⁴⁺-doped red-emitting crystals which have different solubilities.

(1) For the fluoride hosts that have large solubility in acid solution such as (NH₄)₂SiF₆, (NH₄)₂TiF₆, K₂TiF₆, a series of centimeter-sized crystalline ingots were prepared by a cooling induced crystallization method (cooling from 20 °C to minus 40 °C). During cooling, the solubility decreases and the centimeter-scale Na₂SiF₆:Mn⁴⁺ ingots as well as the fluoride phosphors with similar physicochemical property including (NH₄)₂SiF₆:Mn⁴⁺, (NH₄)₂TiF₆:Mn⁴⁺ and K₂TiF₆:Mn⁴⁺ were successfully grown through the supersaturated crystallization process by cooling down.

(2) For the fluoride hosts that are slightly soluble such as BaTiF₆, we firstly prepared BaTiOF₄ as precursor, which is then mixed with K₂MnF₆ and gradually fluorinated in HF acid solution at room temperature. After a continuous feeding process, epitaxial growth of the BaTiF₆:Mn⁴⁺ ended up with ingots up to a record-breaking length of 200−300 μm, which exhibit better water resistance than commercial K₂SiF₆:Mn⁴⁺ micro phosphor.

(3) For the oxide hosts that are insoluble such as MgAl₂O₄, we use the molten salt method, and by optimizing the type of molten salt to adjust the dissolution and migration of Mg²⁺/Al³⁺/Mn⁴⁺ ions in it, the MgAl₂O₄:Mn⁴⁺ crystals in tetragonal bipyramid shape with uniform size of 2−4 μm were prepared under relatively mild conditions at 950 °C. The phosphor exhibits a broadband emission peaking at 651 nm under the 450 nm excitation, which is the highest energy of the Mn^{4+ 2}E→⁴A₂ transition energy in oxides.