Molten salt synthesized LaTa7O19:Er3+/Yb3+ with superior upconversion luminescence using KCl flux

Abstract

A LaTa₇O₁₉(LTO):Er³⁺/Yb³⁺ upconversion luminescence (UCL) phosphor with a hexagonal structure was successfully prepared using a molten salt synthesis method using KCl as solvent. Increasing the holding temperature from 825 to 1150 °C, the LTO structure is transformed towards K₂LaTa₅O₁₅(KLTO). At 1150 °C, the sample is pure phase KLTO:Er³⁺/Yb³⁺. Increasing the holding time at 825 °C from 2 to 48 h, the LTO phase first increases and then decreases. Increasing the KCl flux mass at 825 °C for 24 h, the KLTO structure is first transformed towards LTO, then the KLTO structure increases, and LTO:Er³⁺/Yb³⁺ grain size decreases. The highest green UCL integral intensity of LTO:Er³⁺/Yb³⁺ reached 89% of β-NaYF₄:Er³⁺/Yb³⁺. The maximum relative temperature sensitivity of 0.866% K⁻¹ is reached by using the luminescence intensity ratio (LIR) technique. LTO:Er³⁺/Yb³⁺ phosphors with excellent pure green UCL can be prepared by molten salt with KCl flux, which can be applied in display and temperature sensing.