Luminescence and thermoluminescence of alkaline earth metaborates

The luminescent properties of undoped MB2O4 (M=Ca, Sr) have been studied upon X-ray and synchrotron excitation. The X-ray induced emission spectra of CaB2O4 consist mainly of two overlapping emission bands with maxima at about 3.19 and 3.65 eV. In addition to these features, there are several weak broad bands at lower energies. The excitation spectra of the dominant bands show an asymmetrical band with a sharp edge on the lower energy side and a maximum at 7.8 eV. This excitation band is ascribed to O 2p→B 2s, 2p transitions within BO3 groups. The emission bands at 3.19 and 3.65 eV are tentatively ascribed to self-trapped excitons. Other emission bands with complicated excitation spectra are attributed to defects or impurities. TSL glow curve of CaB2O4 after X-ray irradiation at 80 K consists of a stronger peak at about 132 K and weaker ones at 227, 272, 354 and 409 K. A comparison with the results on SrB2O4 and the literature data on α (β)-modifications of BaB2O4 is also made.