Controlling the europium oxidation state in diopside through flux concentration.

This paper explores the connection between the H ₃ BO ₃ flux concentration and the co-existence of Eu ²⁺ and Eu ³⁺ dopants within CaMgSi ₂ O ₆ crystals (diopside). The samples were synthesised using a solid-state synthesis method under varying atmospheric conditions, including oxidative (air), neutral (N ₂ ), and reductive (H ₂ /N ₂ mixture) environments. Additionally, some materials underwent chemical modification by partially substituting Si ⁴⁺ with Al ³⁺ ions acting as charge compensation defects stabilizing Eu ³⁺ luminescence. Depending on the specific synthesis conditions, the materials predominantly displayed either the orange-red luminescence of Eu ³⁺ (under oxidising conditions) or the blue luminescence of Eu ²⁺ ; however, the comprehensive results confirmed the co-existence of Eu ³⁺ /Eu ²⁺ luminescence in both cases. This work shows that varying flux concentrations added during synthesis significantly affect the relative strength of Eu ²⁺ and Eu ³⁺ emissions in a manner dependent on the synthesis atmosphere. The emission of Eu ²⁺ increases with a higher flux concentration in materials synthesised under oxidative and neutral atmospheres independent of the chemical modification. In contrast, for materials obtained under a reductive atmosphere, the changes in the Eu ³⁺ emission intensity depended on the presence or absence of Al ³⁺ ions namely the increase of flux increased the Eu ³⁺ intensity in the case of unmodified materials and decreased in the Al-modified ones. All observed effects were qualitatively explained considering the double role of the flux in the studied system, which besides facilitating the diffusion of chemical species during synthesis acts as a charge compensating agent by creating B' _Si centres stabilizing Eu ³⁺ emission.