Exchange-induced upconversion in Rb2MnCl4:Yb3+

Crystals of Yb3+ doped Rb2MnCl4 exhibit very intense yellow-orange Mn2+ 4T1g → 6A1g upconversion luminescence under near-infrared 2F7/2 → 2F5/2 Yb3+ excitation around 1 μm at 15 K. Excitation with 10 ns pulses indicates that the upconversion process consists of a sequence of ground-state and excited-state absorption steps. The upconversion mechanism is cooperative, involving both Yb3+ and Mn2+ ions in the crystal, and we postulate an exchange mechanism between adjacent Yb3+ and Mn2+ ions to account for the unusually high efficiency of the process. The efficiency of the upconversion process is dependent on the nature of the Yb3+−Mn2+ connectivity. The upconversion in Rb2MnCl4:Yb3+, with a linear Yb3+−Cl-−Mn2+ arrangement, is 3 orders of magnitude more efficient than in the Yb3+−(Br-)3−Mn2+ arrangement of face-sharing octahedra in CsMnBr3:Yb3+.