Structure twinning and photoluminescence properties of sodium dysprosium phosphate Na₃Dy(PO₄)₂

Rare-earth phosphate with the general formula Na₃Ln(PO₄)₂ (Ln = lanthanoids) have been known as good luminescent materials for a long time, but their crystal structures have not been recognized completely. In this work, compound Na₃Dy(PO₄)₂ was prepared using molten salt (flux) method and was structurally determined on X-ray single-crystal diffraction for the first time. Interestingly, its structure solution was complicated by non-merohedral twinning, whose twin law can be characterized by a 180° rotation about the (1 0 0) direction of reciprocal lattice. Furthermore, the excitation and emission spectra, decay time and CIE chromaticity index of Na₃Dy(PO₄)₂ were studied. Under near-UV light excitation (350 nm), the powdered Na₃Dy(PO₄)₂ sintered at 1000 °C shows the strongest emission at around 484 and 574 nm, which originates from the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transformations of Dy³⁺ ion, respectively. The decay curve at 350 nm was measured and the decay curve can be well fitted by bi-exponential function, which can be explained by two crystallographic sites of Dy atom in the structure.