Photon upconversion properties of Ni2+ in magnetic and nonmagnetic chloride host lattices

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 Ni(2+)-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 Ni(2+)-Mn2+ exchange interaction does not play a significant role. This is due to the different Ni(2+)-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.