Phosphors are useful in numerous applications including imaging, detection, and lighting. They come in forms of thin films, monoliths, or powders for miscellaneous devices, or dispersible nano particles for applications such as biomedical imaging. Oxide phosphors, as apposed to chalcogenides or pnictides, have inherent chemical and thermal stability and minimal toxicity to the biosphere. Tantalates are very promising materials for these applications because they are especially robust and resistant to chemical degradation. Studies have shown that rare-earth tantalates are excellent host lattices for europium (Eu)-doped red-emitting phosphors, excited by blue light.

The lithium lanthanum tantalate Li5La3Ta2O12 has a cubic garnet structure, denoted LiLaTa-garnet. Blue is octahedral Ta, pink is 8-coordinate La, and green is tetrahedral and octahedral Li.
The specific application of using phosphors for improved color rendering of blue light-emitting diodes (LEDs) for solid-state lighting was explored. Discovery of an ideally suitable red phosphor for this application could potentially revolutionize this highly-competitive, advanced technology industry.

The lithium lanthanum tantalate LiTaLa2O6 has a perovskite structure, denoted LiLaTa-perovskite. The octahedral Li site is a green sphere. Pink spheres are La and blue spheres are Ta.
The use of lithium lanthanum tantalate materials for blue-excitation, red-emitting phosphors for solid-state lighting was explored. These oxide host lattices, Li5La3Ta2O12 and LiTaLa2O6, are well-known solid-state electrolytes for lithium conductivity, yet they have not been investigated for phosphor applications. They potentially offer great flexibility in that the lattice can accommodate substitutions and vacancies. These variations provide opportunity to optimize the luminescence characteristics such as by shifting and broadening adsorption and emission peaks, and increasing the quantum yield, or brightness of emission. For red-emission, Eu-doping, manganese (Mn)-doping, and self-activation were explored.

While Eu-doped LiLaTa phases proved to not be better than previously reported phases, the Mn-doped LiLaTa phases warrant further investigation. In particular, the significant adsorption of blue light via the Mn-doped LiLaTa-perovskite provides a promising material for blue LED down-converters for solid-state lighting. Additionally, this LiLaTa-perovskite phosphor is formed optimally by a unique ion-exchange process not previously reported in the scientific literature. Further work to optimize this material and related materials is planned.

This work was done by May D. Nyman and Lauren E. S. Rohwer of Sandia National Laboratories. For more information, download a technical support package from Green Design & Manufacturing.

Lighting Technology Magazine

This article first appeared in the August, 2011 issue of Lighting Technology Magazine.

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