LUMINESCENCE MECHANISMS OF AG+ CUBIC CENTERS IN STRONTIUM FLUORIDE-CRYSTALS

The spectroscopic properties of Ag+-doped strontium fluoride crystals were investigated at various temperatures, using absorption and fluorescence spectroscopies. The system exhibits a strong ultraviolet emission upon excitation into the two principal absorption bands. The azimuthal dependence of the degree of polarization of this luminescence is analysed, as well as its dynamics. The monovalent silver ions are shown to substitute for a host cation, with cubic symmetry. This is the first reported example of a cubic coordination for the Ag+ ions in an insulator. This cubic field, together with the strong ionic character of the framework, confers rather original spectroscopic properties to this system. The luminescence mechanisms are interpreted on the basis of the measured decay times and with the aid of energy diagram calculations. Two closed thermalized spin-orbit levels, with symmetry A2g and T2g respectively, are involved in the luminescence processes. The pure spin triplet A2g only emits at low temperatures (T < 15 K), whereas the T2g level (approximately 2% spin singlet character) emits in turn upon warming the crystal. One-dimensional configuration coordinate diagrams are proposed to interpret the peculiar temperature dependence of the emission band maximum.