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

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/N2 mixture. 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.