Correlating Local Compositions and Structures with the Macroscopic Optical Properties of Ce 3+ -Doped CaSc 2 O 4 , an Efficient Green-Emitting Phosphor
Calcium scandate (CaSc 2 O 4 ) substituted with small amounts (<1%) of Ce 3+ is a recently discovered bright-green-emitting phosphor with favorable light absorption and emission properties and robust temperature stability that make it well-suited for solid-state white-lighting applications. Combined analyses of scattering, solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and photoluminescence measurements establish the compositional and structural origins of the macroscopic optical properties of this phosphor material. Simultaneous refinements of synchrotron X-ray and neutron diffraction data of Ce 3+ -doped CaSc 2 O 4 enable the average crystal structure to be determined, which is shown to correspond to an exceedingly rigid host structure, as corroborated by density functional theory (DFT) calculations. Such structural rigidity leads to high quantum efficiency, which is optimized by the substitution of as little as 0.5 mol % of Ce 3+ for Ca 2+ ions, with higher extents of Ce 3+ substitution leading to decreased photoluminescent quantum yields. Solid-state 43 Ca and 45 Sc magic-angle spinning (MAS) NMR spectra are sensitive to the effects of the paramagnetic Ce 3+ dopant ions on nearby atoms in the host structure and yield evidence for local structural distortions. EPR measurements provide direct insights on structures of the Ce 3+ ions, as a function of Ce 3+ substitution. The combined scattering and spectroscopic analyses yield detailed new understanding of the local and long-range structures of Ce 3+ -doped CaSc 2 O 4 , which account for the sensitive composition-dependent optical properties of this important phosphor material.