(Digital Presentation) Growth of Mn4+-Activated Red-Emitting Photoluminescent Crystals

Abstract

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 Mn4+ 2E→4A2 transition energy in oxides. Figure 1