  The synthesis, structural, and luminescence properties have been carried out
for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for
application in pc-NIR LED. Upon blue excitation, these compounds are capable of
exhibiting broadband NIR emission stems primarily from 4T2-->4A2 transition in
the range of 670-1200 nm (maxima ~808 nm, FWHM ~140 nm) for AWO:Cr and of
700-1300 nm (maxima ~870 nm, FWHM ~164 nm) for SWO:Cr. The significant shift of
NIR emission is attributed to the substitution of AlO6 with larger ScO6
octahedrons. To gain insight into the luminescence the crystal field strength,
Racah parameters, nephelauxetic effect, and electron-phonon coupling have been
analyzed based on spectroscopic results. The electron-phonon coupling parameter
S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr,
which is in accordance with its strong thermal quenching. The abrupt changes
occurring at 275 K in temperature-dependent luminescence spectra and decay
lifetime of AWO:Cr is associated with temperature-driven phase transformation
from low-temperature monoclinic to high-temperature orthorhombic phase.
Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was
confirmed by employing high pressure evolution of Raman spectra. A high-power
NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip,
shows good performance with an output power of 17.1 mW driven by a current of
320 mA, revealing potential application of studied materials for NIR light
source.
