Photoluminescence properties of MgO powders with coordinatively unsaturated surface ions

The photoluminescence spectrum of MgO degassed at high temperature has been reinvestigated using a standard JRC-MgO sample because the contribution of low-coordination surface sites (Mg2+LC—O2–LC) to the observed photoluminescence of the degassed MgO samples has been recently questioned. The JRC-MgO-I sample exhibits two different types of photoluminescence, i.e. one short-lived with a lifetime of ca. 10–4 s, the other long-lived with a lifetime of 1–104 s. The effect of the degassing temperature of the sample and of added quencher molecules indicates that the short-lived photoluminescence observed under u.v. excitation is a radiative decay process from the charge-transfer-excited complex (Mg+LC—O–LC)* with a lower coordination number of four. However, the luminescence observed after u.v. excitation, i.e. a long-lived emission, is a radiative recombination process of photo-produced electrons and holes via defects such as F+centres. Thus, both charge transfer and defect mechanisms account for the photoluminescence of the MgO degassed at high temperatures, although the long-lived emission is not directly measured in the present work owing to its much smaller contribution.