Luminescence of germanate garnets with terbium on dodecahedral and octahedral lattice sites

Abstract

{Ca(Tb)} 3Ga 2Ge 3O 12 and Sr 3[Y(Tb)] 2Ge 3O 12 garnets provide a cubic lattice with trivalent terbium, a luminescent rare earth, in dodecahedral (8 oxygen neighbors) and in octahedral (6 oxygen neighbors) sites, respectively. In the materials examined, Tb 3+ in octahedral sites has a narrower fluorescence emission spectrum, higher luminescence efficiency and a longer lifetime than in dodecahedral sites. The luminescence properties and the unit cell size of the Ca 3Y 2Ge 3O 12 garnet, where Tb 3+ is expected to be located on octahedral sites, are between those of the CaGa and SrY garnets. All these garnets are phosphorescent. On the basis of the variation with temperature of the phosphorescence decay law, it is proposed that the radiative electron-hole recombination process proceeds via a tunneling mechanism at temperatures below the thermal glow peaks, and via a free carrier diffusion mechanism at temperatures above the thermal glow peaks. All three Tb 3+-doped germanate garnets have sufficiently high efficiency and persistence under electron beam excitation to be suitable for practical phosphor applications. The strontium yttrium germanate-Tb material has a cathodoluminescence decay time nearly three times that of the commercial P53 garnet phosphor.