(502b) Design Rules for Finding New Oxide Phosphors for Solid-State Lighting

Design rules for
finding new oxide phosphors for solid-state lighting

Nathan C. George,¹ Bradley F. Chmelka¹, Ram
Seshadri²

¹Department of Chemical Engineering, ²Materials
Department & Materials Research Laboratory, University of California, Santa
Barbara, California, U.S.A. 93106

Solid-state lighting systems consisting of a
yellow oxide phosphor, in conjunction with a blue InGaN
source, are a promising way of creating efficient white light with devices that
are long-lived and durable. In general, the oxide phosphor consists of a host
crystal lattice, which is doped with small amounts of rare-earth or transition-metal
ions.  These ions in the host
crystal "activate" the lattice, enabling down-conversion of blue or UV light to
yellow-orange light.  Many efforts
have been directed towards finding new phosphor materials, but little has been
done to understand the relationships between optical phenomena and the
structure of phosphor materials. Structural and compositional reasons for the
high efficiencies of the canonical yellow phosphor, Y_3-xCe_xAl₅O₁₂ (YAG:Ce,
x<0.12) are established by using a
synergistic combination of characterization techniques, including NMR, ESR, synchrotron
X-ray and neutron scattering, and XANES/EXAFS.  In particular, an important feature of
the YAG:Ce lattice is determined
to be its high rigidity, which reduces the number of accessible phonon modes at
typical operating temperatures, resulting in its high quantum efficiency. EXAFS
and Reverse Monte Carlo (RMC) simulations of total neutron scattering show that
the Ce³⁺–O polyhedra are furthermore
highly compressed in YAG:Ce.
The lattice rigidity and relatively small Y site into which Ce³⁺ ions
substitute contribute to the yellow emission that is unusual for Ce-doped oxides. Both solid-state NMR and ESR enable
quantification of the small amount of Ce³⁺ ions that substitute into
nearest-neighbor Y³⁺ sites. 
These Ce³⁺ sites begin to significantly decrease quantum
efficiency above 2 mol% Ce in YAG, which suggests Ce
substitution levels should be below this threshold. Though specific for YAG:Ce, these analyses provide general
design criteria for the compositions and structures of  phosphor materials, including non-oxide
systems, that are expected to lead to new materials with improved properties.