Hydrothermal synthesis and upconversion luminescence of NaGd(WO₄)₂ co-doped with Ho³⁺ and Yb³⁺

The Ho³⁺/Yb³⁺ co-doped NaGd(WO₄)₂ phosphors with different concentrations of Yb³⁺ were prepared by a facile hydrothermal route. X-ray diffraction and scanning electron microscopy were used to study the structure of the samples. The upconversion (UC) emission spectra of the samples were measured under 980 nm excitation with different pumping power. Intense red luminescence at 625–675 nm, weak blue luminescence at 458–492 nm, green luminescence at 523–565 nm and near infrared luminescence at 736–770 nm emitted by Ho³⁺ were observed, and their emission intensities changed with the Yb³⁺ concentration. The transition mechanisms of the UC luminescence were also discussed. - Highlights: • NGW: Ho³⁺–Yb³⁺ phosphors were successfully synthesized by a hydrothermal method. • Highly intense red upconversion emission around 659 nm was observed under 980 nm excitation. • Upconversion emission efficiency was enhanced greatly with the presence of the Yb³⁺ ions. • The transition mechanisms of the upconversion emission can be ascribed to a two-photon process