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 Sr4Al14O25 (S4A7) co-doped with 1 at% Eu and 1 at% Dy (S4A7EDxB), while the longest duration occurs when 30–40 mol% B2O3 is used. To elucidate the influence of B2O3 on the S4A7EDxB structure supporting PersiL, the thermodynamics and kinetics of phase transformation from amorphous preceramics of S4A7EDxB are investigated in the present work. Powders of S4A7EDxB containing x = 0, 10, 20, 30, and 40 mol% B2O3 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 S4A7 forms along different reaction pathways involving SrAl2O4, γ-Al2O3, and SrAl4O7, 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 Al3+ coordination polyhedral networks that form the scaffolding in each crystal phase. To lift the kinetic limitations for direct crystallization of S4A7, at least 30 mol% B2O3 is required for sufficient liquid borate to enable both the Sr2+ diffusion and the reorganization in topological connectivity in the Al3+ polyhedral scaffolding.