Controlling the europium oxidation state in diopside through flux concentration

This paper explores the connection between the H₃BO₃ flux concentration and the co-existence of Eu²⁺ and Eu³⁺ dopants within CaMgSi₂O₆ crystals (diopside). The samples were synthesised using a solid-state synthesis method under varying atmospheric conditions, including oxidative (air), neutral (N₂), and reductive (H₂/N₂ mixture) environments. Additionally, some materials underwent chemical modification by partially substituting Si⁴⁺ with Al³⁺ ions acting as charge compensation defects stabilizing Eu³⁺ luminescence. Depending on the specific synthesis conditions, the materials predominantly displayed either the orange-red luminescence of Eu³⁺ (under oxidising conditions) or the blue luminescence of Eu²⁺; however, the comprehensive results confirmed the co-existence of Eu³⁺/Eu²⁺ luminescence in both cases. This work shows that varying flux concentrations added during synthesis significantly affect the relative strength of Eu²⁺ and Eu³⁺ emissions in a manner dependent on the synthesis atmosphere. The emission of Eu²⁺ 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 Eu³⁺ emission intensity depended on the presence or absence of Al³⁺ ions namely the increase of flux increased the Eu³⁺ 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 Eu³⁺ emission.