Oliver S. Wenger and Hans U. Güdel
Photon Upconversion Properties of Ni²⁺ in Magnetic and Nonmagnetic Chloride Host Lattices
Inorg. Chem. 40, 157-164 (2001)                 

Abstract: Near-infrared to visible upconversion luminescence in Ni²⁺:CsCdCl₃, Ni²⁺:CsMnCl₃, and Ni²⁺:RbMnCl₃ is presented and analyzed. In all three materials upconversion occurs via a sequence of ground-state absorption/excited-state absorption processes, which are both formally spin-forbidden transitions. Consequently, in the diamagnetic Ni²⁺:CsCdCl₃ they are weak, and the efficiency of the upconversion process is relatively low. This is in clear contrast to the isostructural Ni²⁺:RbMnCl₃ where the spin selection rule relaxes because of Ni²⁺ - Mn²⁺ exchange interactions, leading to an intensity enhancement of the spin-flip transitions involved in the Ni²⁺ upconversion mechanism. This results in an exchange-induced enhancement of the upconversion rate in Ni²⁺:RbMnCl₃ relative to Ni²⁺:CsCdCl₃ by 2 orders of magnitude after two-color excitation into the maxima of the ground-state and excited-state absorption bands. In Ni²⁺:CsMnCl₃ the Ni²⁺ - Mn²⁺ exchange interaction does not play a significant role. This is due to the different Ni²⁺ - Cl^– - Mn²⁺ bridging geometry relative to Ni²⁺:RbMnCl₃. In contrast to Ni²⁺:CsCdCl₃ and Ni²⁺:RbMnCl₃ where the upconversion luminescence occurs from Ni²⁺, in Ni²⁺:CsMnCl₃ the upconverted energy is emitted from Mn²⁺ in the visible spectral region. This leads to an enhanced visible upconversion luminescence in Ni²⁺:CsMnCl₃, relative to the other two samples where Ni²⁺ near-infrared inter-excited-state emissions compete with the visible upconversion luminescence.