Prediction and explanation of the self-reduction of Eu3+ → Eu2+ and Mn4+ → Mn2+ using Site Occupancy Theory (SOT)

Abstract

This study introduces a novel application of Site Occupancy Theory (SOT) to predict and elucidate the self-reduction processes of doped ions. The stable valence state of doped ions at the cationic site is determined by analyzing the deviations in bond energy when these ions occupy specific sites within the matrix. Self-reduction occurs when the bond energy of the ion in a lower oxidation state is closer to that of the matrix cation than in a higher oxidation state. To provide experimental validation for the application of SOT in predicting and explaining self-reduction, a series of compounds were synthesized using a high-temperature solid-state method. These include Lithium Calcium Borate (Li₄Ca₂B₈O₁₆:Eu) and Sodium Yttrium Tungstate (Na₅Y(WO₄)₄:Mn). Synthesis was carried out under different atmospheric conditions to investigate their impact on the self-reduction behavior of doped ions. Photoluminescence (PL) spectra were analyzed from samples prepared in varying atmospheres, confirming that self-reduction occurs from Eu³⁺ to Eu²⁺ in Li₄Ca₂B₈O₁₆:Eu and from Mn⁴⁺ to Mn²⁺ in Na₅Y(WO₄)₄:Mn under air. Additionally, Li₄Ca₂B₈O₁₆:Eu synthesized under a reducing atmosphere demonstrates significant potential for applications in White Light Emitting Diodes (W-LEDs). Likewise, Na₅Y(WO₄)₄:Mn synthesized under air conditions exhibits promising applications in photodynamic therapy (PDT) and photorejuvenation (DPL). In conclusion, the experimental results support the effectiveness of SOT in predicting the self-reduction behavior of doped ions within a matrix, providing insights into the conditions under which these processes occur. The ability to predict self-reduction could accelerate the development of new materials and reduce reliance on extensive trial-and-error experimentation.