Boron oxide influencing phase transformations in Eu and Dy co‐doped Sr₄Al₁₄O₂₅ exhibiting persistent luminescence

The addition of boron oxide is well‐known to dramatically extend the afterglow persistence (PersiL) from minutes to hours in Sr 4 Al 14 O 25 (S 4 A 7 ) co‐doped with 1 at% Eu and 1 at% Dy (S 4 A 7 ED x B), while the longest duration occurs when 30–40 mol% B 2 O 3 is used. To elucidate the influence of B 2 O 3 on the S 4 A 7 ED x B structure supporting PersiL, the thermodynamics and kinetics of phase transformation from amorphous preceramics of S 4 A 7 ED x B are investigated in the present work. Powders of S 4 A 7 ED x B containing x = 0, 10, 20, 30, and 40 mol% B 2 O 3 were prepared by Pechini processing, and their phase evolution was analyzed at characteristic exothermic reaction temperatures. Differential thermal analysis and thermogravimetry at different heating rates, X‐ray diffraction, micro‐Raman, and scanning electron microscopy methods are used to reveal that S 4 A 7 forms along different reaction pathways involving SrAl 2 O 4 , γ‐Al 2 O 3 , and SrAl 4 O 7 , with the pathway determined primarily by the amount of boron oxide present. Evidence is presented correlating the energy barrier of different phase transformations to the relative stability of Al 3+ coordination polyhedral networks that form the scaffolding in each crystal phase. To lift the kinetic limitations for direct crystallization of S 4 A 7 , at least 30 mol% B 2 O 3 is required for sufficient liquid borate to enable both the Sr 2+ diffusion and the reorganization in topological connectivity in the Al 3+ polyhedral scaffolding.