In order to clarify the nature of the crystals present inthe MP-containing glasses, XRD patterns of the investigatedglasses were measured. However, no reflections from theSr4Al14O25:Eu2+,Dy3+were observed in the XRD patterns ofthe MPs-containing glasses as expected due to the low MP con-centration. Except for the reflections which can be attributed tothe sample holder, no additional peaks were observed in the XRDpatterns (not shown here). Our on-going study of the crystalliza-tion mechanism of these base glasses suggests that two phasescrystallize in the base glass upon heat treatment: Ca(PO3)2andNaCa(PO3)3in the CaO glass, and Sr(PO3)2and NaSr(PO3)3inthe SrO glass. A complete study of the nucleation mechanismof these glasses is ongoing and will be published separately. Wesuspect that the same crystals precipitate in the MP-containingglasses. As discussed above, the diffusion of Al and Eu fromthe MPs causes the SrO glasses to exhibit a shorter persistentluminescence duration than the CaO glasses.