The influence of rare-earth ions on the low-temperature thermoluminescenceof Bi4Ge3O12

Low-temperature (20-290 K) thermoluminescence spectra of B4Ge3O12 reveal a range of trapping levels, some of which are common to both undoped and dope d material. The emission spectra for undoped and transition-metal-doped Bi4 Ge3O12 indicate that at low temperatures, intrinsic luminescence centres re sult in broad-band emission typical of signals from relaxed excitons or pos sibly excited bismuth ions. For material containing rare-earth ions, the si gnals are characteristic of the rare-earth dopants, even when the rare-eart h ions are present in concentrations as low as 3 ppm, The temperatures of t he glow peaks seen at similar to 54, 105 and 141 K for undoped material are strongly modified by the rare-earth ions. The trapping and recombination s ites, monitored by rare-earth emission, are intimately linked, probably wit hin large complex structures. For these three glow peaks the peak temperatu re varies smoothly with the ionic radii of the rare-earth impurities. These movements are substantial, with changes of up to 50 K, as a function of th e rare-earth radii. Of all the rare-earth ions, europium farms the most sta ble recombination centres. This is probably because the trivalent europium ion is similar in size to the host (bismuth) ion for which it substitutes. Tentative models for trapping sites and thermoluminescence mechanisms are p roposed.