Discovery of a new phosphor via aliovalent cation substitution: DFT predictions, phase transition and luminescence properties for lighting and anti-counterfeiting applications

Abstract

Over the last two decades, suitable compositions have been investigated to make new phosphors because they are key materials for use in solid-state lighting and security applications. Aliovalent cations have different radii, electronegativity and valence states, and can thus be used to remarkably modify crystal structures. Here, we report a new Sr₃₃Lu₆(PO₄)₂₈ phase produced from Sr₃(PO₄)₂via the method of aliovalent cation substitution. Density functional theory calculations were used to predict the possibility of the formation of a new phase when substituting three Sr²⁺ for two Lu³⁺ ions in the material. The new phase, Sr₃₃Lu₆(PO₄)₂₈, was confirmed by X-ray diffraction, electron diffraction and photoluminescence spectroscopy. Different from the purple phosphor Sr₃(PO₄)₂:Eu²⁺, Sr₃₃Lu₆(PO₄)₂₈:Eu²⁺ exhibits broad green emission under near-UV excitation due to the multi-site occupancy of Eu²⁺. Combining near-UV chip, blue, red and Sr₃₃Lu₆(PO₄)₂₈:Eu²⁺ green phosphors, a white light-emitting diode with a high colour rendering index (∼92) was made, with potential lighting applications. Additionally, a transparent security ink fabricated using the Sr₃₃Lu₆(PO₄)₂₈:Eu²⁺ phosphor shows high concealment and recognition. This work is not confined to the report of a new Sr₃₃Lu₆(PO₄)₂₈:Eu²⁺ phosphor and its corresponding applications; it is expected that aliovalent cation substitution will be useful as a general method for the discovery of new materials.