The persistent luminescence spectra of the CaO and SrOglasses, recorded at room temperature, are shown in Fig. 2a andb, respectively. They exhibit a strong band centered at 490 nmand a weak one at 410 nm. These bands are observed due to the4f65d1→ 4f7transition of Eu2+ions located in two differentcation sites in the Sr4Al14O25structure. As expected, the inten-sity of the emission depends on the concentration of the MPsin the glass: the higher the concentration, the stronger the emis-sion. While it seems that the CaO glasses exhibit more intenseemission than the SrO glasses, such change could not be unam-biguously discussed as the MPs are not well distributed in theglass matrix making the comparison of the emission intensitydifficult.