Synthesis and Optimum Luminescence of CaWO₄-Based Red Phosphors with Codoping of Eu³⁺and Na⁺

Scheelite nanostructures Ca 1−2 x (Eu,Na) 2 x WO 4 (0 < x ≤ 0.135) were prepared from 5 nm Ca 0.968 (Eu,Na) 0.032 WO 4 by hydrothermal treatment. The preparation of 5 nm Ca 0.968 (Eu,Na) 0.032 WO 4 at room temperature and subsequent hydrothermal treatment allow control over chemical compositions and particle size of CaWO 4 -based red phosphors that has not yet possible when using traditional preparation methods. By careful structural and electronic characterization, it is shown that simultaneous substitutions of Eu 3+ and Na + at Ca 2+ sites were possible using this methodology, which allows one to vary the local symmetry surrounding Eu 3+ and moreover the energy transfer from O 2− to Eu 3+ and tungstate groups to Eu 3+ for optimum luminescence. As a consequence, the obtained CaWO 4 -based nanocrystals displayed excellent luminescence properties as demonstrated by luminescence lifetimes of milliseconds, abnormally narrowed emissions, and maximum quantum efficiencies of 92%. The results reported in this work show that it is possible to control chemical composition of oxide nanostructures for structural decoration and luminescence property tailoring via codoping aliovalent ions.