Giant Enhancement in Upconversion Luminescence of β-Ba₂ScAlO₅:Yb³⁺/Er³⁺ Phosphor by the Intermediate Band through Ca²⁺ Doping

Luminescence intensity is a critical issue for upconversion oxides with high phonon energy. Herein, an effective enhancement in red upconversion luminescence (UCL) is implemented via an intermediate band (IB) in the Ba2ScAlO5:Yb3+/Er3+ phosphor after Ca2+ doping. The downshifting luminescence spectra under the irradiation of a 320 nm laser verify the existence of the IB. The physical mechanism is that the valence band (VB) → IB transition increases the absorption of the excitation energy and provides plentiful electrons from the host to Er3+. Therefore, the red and green emission intensities are increased by 45.2 and 7.5 times, respectively. The dependence of luminescent intensity on power and an effective fluorescence lifetime of Ca2+-doped Ba2ScAlO5:Yb3+/Er3 also confirm the proposed energy transfer mechanism. Based on the excellent red emission, the temperature-dependent luminescent properties of the title material are explored in the temperature range from 298 to 358 K. The linear relationship between the luminescence and the temperature and the high sensitivity suggest the promising potential in biomedical temperature-sensing applications. This work paves a new way for enhancing UCL and will arouse extensive interest in noncontact temperature-sensing applications with a single red upconversion emission.