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

Near-infrared to visible upconversion luminescence in Ni2+:CsCdCl3, Ni2+:Cs MnCl3, and Ni2+:RbMnCl3 is presented and analyzed. In all three materials u pconversion occurs via a sequence of ground-state absorption/excited-state absorption processes, which are both formally spin-forbidden transitions. C onsequently, in the diamagnetic Ni2+:CsCdCl3 they are weak, and the efficie ncy of the upconversion process is relatively low. This is in clear contras t to the isostructural Ni2+:RbMnCl3 where the spin selection rule relaxes b ecause of Ni2+-Mn2+ exchange interactions, leading to an intensity enhancem ent of the spin-flip transitions involved in the Ni2+ upconversion mechanis m. This results in an exchange-induced enhancement of the upconversion rate in Ni2+:RbMnCl3 relative to Ni2+:CsCdCl3 by 2 orders of magnitude after tw o-color excitation into the maxima of the ground-state and excited-state ab sorption bands. In Ni2+:CsMnC3 the Ni2+-Mn2+ exchange interaction does not play a significant role. This is due to the different Ni2+-Cl--Mn2+ bridgin g geometry relative to Ni2+:RbMnCl3. In contrast to Ni2+:CsCdCl3 and Ni2+:R bMnCl3 where the upconversion luminescence occurs from Ni2+, in Ni2+:CsMnCl 3 the upconverted energy is emitted from Mn2+ in the visible spectral regio n. This leads to an enhanced visible upconversion luminescence in Ni2+:CsMn Cl3, relative to the other two samples where Ni2+ near-infrared inter-excit ed-state emissions compete with the visible upconversion luminescence.