Evaluation of the mechanism of luminescence and structure-efficiency relations of vanadate phosphors

Over the past decades, research on novel vanadate phosphors has gained increasing attention. The widely accepted mechanism that explains their broad absorption in the ultraviolet and their broad luminescence in the visible spectrum is based on energy levels derived from the molecular orbitals of isolated

tetrahedra, in which the excitation is described as a charge transfer excitation. In this work, we critically examine both this mechanism of luminescence in vanadates and two mechanisms that are often used to explain their luminescent efficiency. By correlating published optical properties (e.g. excitation energies, Stokes shifts, and emission bandwidths) with structural properties (e.g. bond lengths and bond angles) on 77 different vanadate phosphors, we find that there is no strong evidence in favour of the proposed mechanisms used to describe luminescence as well as quenching thereof. Instead, we suggest a mechanism in which the luminescent charge transfer state is not directly formed upon photoexcitation but rather formed after initial electron trapping following bandgap excitation. The resulting luminescent state is, therefore, likely to be more appropriately termed a self-trapped exciton.