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

Scheelite nanostructures Ca1−2x(Eu,Na)2xWO4 (0 < x ≤ 0.135) were prepared from 5 nm Ca0.968(Eu,Na)0.032WO4 by hydrothermal treatment. The preparation of 5 nm Ca0.968(Eu,Na)0.032WO4 at room temperature and subsequent hydrothermal treatment allow control over chemical compositions and particle size of CaWO4-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 Eu3+ and Na+ at Ca2+ sites were possible using this methodology, which allows one to vary the local symmetry surrounding Eu3+ and moreover the energy transfer from O2− to Eu3+ and tungstate groups to Eu3+ for optimum luminescence. As a consequence, the obtained CaWO4-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.