Abstract
It has been shown that the optical and electrical properties of 
depend on composition. A composition with excess alumina was found to have the highest CR efficiency, photoconductivity, and a long‐lasting persistence. A mechanism is proposed which assigns the delayed luminescence to a process involving the conduction of charged particles. Evidence is presented which indicates that holes are trapped at lattice defects and the Eu2+ emission is associated with thermal deactivation of these traps.