As seen in Fig. 9b, the micro-Raman spectra of the two dif-ferent regions near the microparticles also depict sharp peakswhich are shifted to longer wavenumbers compared to thoseseen in Fig. 9a. The spectra are similar to that of the Sr2P2O7crystal28indicating that the crystals around the MPs are prob-ably strontium phosphate-based crystals. One can notice thatthe spectra measured in the crystal with the needle-like shape and in the MP–glass interface are similar except for the addi-tional peaks at 415 and 640 cm−1in the spectrum measured atthe interface. Those peaks can be related to the Raman band ofthe Al O vibration as shown in.29These results are in agree-ment with the data presented in Fig. 8 confirming that an Al-richshell has been formed on the Sr4Al14O25:Eu2+,Dy3+core due toelemental diffusion from the MP to the glass.