Method to Measure the Degree of Reduction of Eu3+to Eu2+: How Anion and Cation Vacancies Influence the Degree of Reduction

Abstract

Phosphors for white light-emitting diode (LED) light sources are very often alkaline earth silicate matrices doped with Eu2+. As europium occurs in nature only in the +3 oxidation state, the synthesis of such phosphors requires annealing in a reducing atmosphere. Unfortunately, sometimes the reduction is not efficient. Therefore, it is necessary to precisely determine the reduction degree of Eu3+to Eu2+and to recognize all of the factors affecting the reduction process. This paper shows new insight into how defects such as calcium and oxygen vacancies affect the Eu3+→ Eu2+reduction degree and the quantum efficiency of the Eu2+emission. This paper explains the mechanisms of reduction of Eu3+both in samples synthesized in sol-gel and solid state and the formation of defects and presents a method for identifying such defects using IR measurements. A useful method based on magnetic measurements to determine the degree of reduction of Eu3+is also presented. For this purpose, the åkermanite-gehlenite matrix was synthesized using two methods, solid-state and sol-gel. Annealing was carried out in two different atmospheres: vacuum and the H2/N2mixture. The samples produced by the solid-state method are free from calcium and oxygen vacancies, while those produced by the sol-gel method always have an abundance of them. The number of defects depends on the reducing atmosphere, stoichiometry, and the introduced codopants, either Li+or Zr4+ions. The results are universal and can be applied to other phosphors. The presented approach allows to develop a phosphor with high quantum efficiency.