Upconversion luminescence properties of Li⁺-doped ZnWO₄:Yb,Er

Upconversion luminescence (UPL) characteristics and effects of Li⁺ion on the UPL of ZnWO₄:Yb,Er polycrystalline phosphors were investigated. It was shown that introduction of Li⁺ions could reduce the calcination temperature by about 200 °C and increase the crystallinity of ZnWO₄:Yb,Er by a liquid-phase sintering process via formation of Li₂WO₄and other intermediates. UPL efficiency is remarkably promoted by Li⁺ions. Moreover, the UPL spectrum of Li⁺-doped ZnWO₄:Yb,Er presents a red shift, and the strongest peak position shifts from 553 to 559 nm. These can be attributed to a shift in the 4f level barycenter to lower energy, which results from lowering of the symmetry of the crystal field around Er³⁺.