Orderly-Layered Tetravalent Manganese-Doped Strontium Alummate Sr_4Al_(14)O_(25):Mn~(4+): An Efficient Red Phosphor for Warm White Light Emitting Diodes

摘要

Searching for an efficient non rare earth-based oxide red phosphor, particularly excitable by light in the wavelength from 380 to 480 nm and unexcitable by green light, is essential for the development of warm white light emitting diodes (WLEDs). Here, we report a promising and orderly-layered candidate: Sr_4Al_(14)O_(25):Mn~(4+) with CIE color coordinates (0.722, 0.278). It has higher luminescence efficiency particularly upon blue excitation and is much cheaper than the commercial red phosphor 3.5MgO·0.5MgF_2·GeO_2:Mn~(4+) (MMG:Mn~(4+)). In sharp contrast to Eu~(2+)-doped (oxy)nitrides, the phosphor can be synthesized by a standard solid-state reaction at 1200℃ in air. The effects of flux boron content, environment, and preparation temperature, sintering dwelling time as well as Mn concentration have been systematically investigated for establishing the optimal synthesis conditions. The low temperature emission spectra reveal that there are at least three types of Mn~(4+) ions in Sr_4Al_(14)O_(25):Mn~(4+) due to the substitution for the distorted octahedral Al~(3+) sites. The AlO_6 layers where Mn~(4+) prefers to reside are well separated from one another by AlO_4 tetrahedra in one dimension parallel to axis a. This scenario can efficiently isolate Mn~(4+) ions from local perturbations, thereby enabling the high efficiency of luminescence. The energy transfer rates and mechanism are discussed.