Position of the optical absorption edge of alkaline earth borates

Based on the results of luminescent measurements on MB2O4 (M = Ca, Sr), Ca2B5O9Cl, SrB6O10, SrAl2B2O7 and the literature data, the influence of the crystal structure of alkaline earth borates on the position of their optical absorption edge is discussed. It is found that the optical absorption edge tends to shift to higher energy from β-BaB2O4 (6.4 eV) to M3(BO3)2, MB2O4 (6.9–7.2 eV) to SrB6O10 (7.7 eV), SrB4O7 (>7.8 eV). It is shown that this tendency can be interpreted as a result of (a) the removal of M2+ states from the top of the valence band; (b) a more efficient stabilization of the O 2p states by B–O covalent bonding with increasing condensation degree of the borate anions.