Upconversion luminescence of cubic and hexagonal structured SrTa4O11:Er3+/Yb3+ phosphors

Abstract

Cubic and hexagonal structured SrTa₄O₁₁(STO):Er³⁺/Yb³⁺ phosphors were synthesized by a solid state reaction (SSR) and molten salt synthesis (MSS). The upconversion luminescence (UCL) intensity of these samples was investigated. Hexagonal STO:Er³⁺/Yb³⁺ with much or a little β-Ta₂O₅ can be synthesized by SSR in air or vacuum, respectively, and the UCL intensity of the sample synthesized by SSR in a vacuum is higher. Cubic STO:Er³⁺/Yb³⁺ can be synthesized by MSS with KCl flux, and hexagonal STO:Er³⁺/Yb³⁺ can be synthesized by MSS with B₂O₃ flux, which has the strongest UCL intensity among all the samples compared with samples prepared by SSR. The sample by MSS with B₂O₃ flux was acid pickled (AP) with HCl solution, and the green UCL intensity increased by 2.18 times, which reached 32.95% for β-NaYF₄:Er³⁺/Yb³⁺. The UCL intensity of the hexagonal STO:Er³⁺/Yb³⁺ is much higher than that of the cubic structure, which is due to the layered structure and the non-central symmetry of the Er³⁺/Yb³⁺ doped sites in hexagonal STO:Er³⁺/Yb³⁺. The temperature sensitivity of samples is evaluated by luminescence intensity ratio (LIR) technology. The maximum relative sensitivity is 0.0099 K⁻¹ at 303 K. All the results show that hexagonal STO:Er³⁺/Yb³⁺ has excellent pure green UCL intensity and high temperature sensitivity, which can be used in UCL display and temperature sensing.