Cooperative Near‐IR to Visible Photon Upconversion in Yb3+‐Doped MnCl2 and MnBr2: Comparison with a Series of Yb3+‐Doped Mn2+ Halides

Yb 3 + -doped MnCl 2 and MnBr 2 crystals exhibit strong red upconversion luminescence under near-infrared excitation around 10000 cm - 1 at temperatures below 100 K. The broad red luminescence band is centred around 15200 cm - 1 for both compounds and identified as the Mn 2 + 4 T 1 g → 6 A 1 g transition. Excitation with 10 ns pulses indicates that the upconversion process consists of a sequence of ground-state and excited-state absorption steps. The experimental VIS/ NIR photon ratio at 12 K for an excitation power of 191 mW focused on the sample with a 53 mm lens is 4.1 % for MnCl 2 :Yb 3 + and 1.2% for MnBr 2 :Yb 3 + . An upconversion mechanism based on exchange coupled Yb 3 + -Mn 2 + ions is proposed. Similar upconversion properties have been reported for RbMnCl 3 : Yb 3 + , CsMnCl 3 :Yb 3 + , CsMnBr 3 :Yb 3 + , RbMnBr 3 :Yb 3 + , Rb 2 MnCl 4 :Yb 3 + . The efficiency of the upconversion process in these compounds is strongly dependent on the connectivity between the Yb 3 + and Mn 2 + ions. The VIS/NIR photon ratio decreases by three orders of magnitude along the series of corner-sharing Yb 3 + -Cl - -Mn 2 + , edge-sharing Yb 3 + -(Cl - ) 2 -Mn 2 + to face-sharing Yb 3 + -(Br - ) 3 -Mn 2 + bridging geometry. This trend is discussed in terms of the dependence of the relevant super-exchange pathways on the Yb 3 + -Mn 2 + bridging geometry.