G. M. Salley, R. Valiente and H. U. Güdel
Phonon-assisted cooperative sensitization of Tb³⁺ in SrCl₂:Yb, Tb
J. Phys.: Condens. Matter 14, 5461-5475 (2002)                 

Abstract: Excitation into the Yb^{3+ 2}F_5/2 excited states leads to visible-by-eye green luminescence spanning the spectral region from 490 to 790 nm, with a quadratic power dependence. Optical absorption, luminescence, and excitation spectroscopy as well as pulsed measurements on single-crystal SrCl₂:Yb(1%), Tb(1%) are used to determine the upconversion (UC) properties of this system. The upconverted luminescence is easily detectable by eye from RT to 100 K, at which point the intensity drops significantly and a change in colour from green to blue is observed. Pulsed measurements coupled with excitation spectroscopy lead to the unambiguous assignment of a phonon-assisted cooperative sensitization mechanism as the dominant UC process for T>50 K with VIS/NIR photon ratios on the order of 10^–2% for a laser power of 56 W mm^–2. Below 50 K, the dominant UC emission becomes the well-known Yb-Yb cooperative luminescence around 500 nm, with a consequential reduction of Tb³⁺ emission by more than three orders of magnitude from RT to 10 K.