Christine Reinhard, Rafael Valiente and Hans U. Güdel
Exchange-Induced Upconversion in Rb₂MnCl₄:Yb³⁺
J. Phys. Chem. B 106, 10051-10057 (2002)                 

Abstract: Crystals of Yb³⁺ doped Rb₂MnCl₄ exhibit very intense yellow-orange Mn^{2+ 4}T_1g ⁶A_1g upconversion luminescence under near-infrared ²F_7/2 ²F_5/2 Yb³⁺ excitation around 1 mm 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⁺.