Magnesium halides as a lead-free family with unique optoelectronic properties†

Abstract

Metal halides have superior optoelectronic properties because of their special electronic structures; these include ns² metal halides (Pb²⁺, Sb³⁺ and Sn²⁺) and d^x metal halides (Zn²⁺, Cd²⁺, and Mn²⁺). Mg halides, which have unfavorable electronic structures with high excited state energy levels, conflict with the basic requirements of luminescent metal halides. In this study, we found that Mg halides exhibit efficient luminescent properties. A series of Mg halides were synthesized and studied. Under ultraviolet (UV) excitation, Li₂MgCl₄ and RbMgCl₃ exhibit violet light emissions at 350 nm and 430 nm, and CsMgCl₃ and Cs₂MgCl₄ exhibit blue light emissions at 465 nm and 480 nm. CsMgCl₃, RbMgCl₃, and Li₂MgCl₄ show high thermal stability due to the strong rigidity of the crystal structures. By doping Mn(II) ions into the Mg sites, their emission colors could be further tuned. Based on the unique photoluminescence of the Mg halides, we successfully applied the as-synthesized materials into green lighting and anti-counterfeiting devices. Our study shows that Mg halides are also a family worth studying, shedding light on the exploration of metal halides as efficient optoelectronic semiconductors.