Hydrothermal synthesis and upconversion luminescent properties of Sr₂LaF₇ doped with Yb³⁺ and Er³⁺ nanophosphors

Upconversion phosphors Sr₂LaF₇ co-doped with Yb³⁺ and Er³⁺ are synthesized via hydrothermal method. The phase compositions of Sr₂LaF₇ phosphors with different amounts of Yb³⁺ and Er³⁺ dopants, pH values, and complexing agents are characterized by X-ray diffraction (XRD). The crystal structure and morphology of Sr₂LaF₇ phosphor doped with 15% Yb³⁺/ 1% Er³⁺ is refined by the Rietveld method and transmission electron microscopy (TEM). Under 980 nm laser excitation, Yb³⁺ and Er³⁺ co-doped Sr₂LaF₇ phosphors exhibit red emission around 670 nm generated by the ⁴F_9/2 → ⁴I_15/2 transition and green emissions centering around 550 nm and 523 nm generated by the ⁴S_3/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The optimum synthetic conditions for upconversion luminescence for Sr₂LaF₇ co-doped with Yb³⁺ and Er³⁺ are discussed and determined to be 15% Yb³⁺/ 1% Er³⁺ with EDTA-2Na as the complexing agent under pH value = 8, and the corresponding Commission International de L′Eclairage (CIE) coordinates are calculated to be x = 0.4293 and y = 0.5589 with reddish yellow color visible to naked eyes under the 980 nm laser excitation. The related upconversion mechanisms of Yb³⁺ and Er³⁺ co-doped Sr₂LaF₇ depending on the pump power are also studied in detail.