No abstract

The substitution of rare-earth ions into insulating host crystals introduces lattice strains and, for non-trivalent sites, a need for charge compensation. Such effects alter the site symmetry and this is reflected in properties such as the wavelength, linewidth, lifetime and relative intensity of the rare-earth transitions. Equally clear, but less well documented, is the influence on second-harmonic generation (even from cubic crystal lattices). For example, in bismuth germanate, secondharmonic generation efficiency varies by factors of more than 100 as a result of different rare-earth dopant ions. The ions are variously incorporated as substitutional ions, pairs, clusters, or even as precipitates of new phases, but the detailed modelling is often speculative. This article summarizes some recent studies which explore the role of rare-earth ions in thermoluminescence and second-harmonic generation. There are numerous differences in glow peak temperature, for nominally the same defect sites, which are thought to indicate charge trapping and recombination within coupled defect sites, or within a large complex. Size and cluster effects can be modified by heat treatments. This review considers the similarity and trends seen between numerous host lattices which are doped with rare-earth ions. For thermoluminescence there are trends in the variation in glow peak temperature with ion size, with movements of 20 to 50 K. Examples are seen in many hosts with extreme effects being suggested for zircon, with peak shifts of 200 K (probably from precipitate phases).

Low-temperature (20-290 K) thermoluminescence spectra of Bi4 Ge3 O12 reveal a range of trapping levels, some of which are common to both undoped and doped material. The emission spectra for undoped and transition-metal-doped Bi4 Ge3 O12 indicate that at low temperatures, intrinsic luminescence centres result in broad-band emission typical of signals from relaxed excitons or possibly excited bismuth ions. For material containing rare-earth ions, the signals are characteristic of the rare-earth dopants, even when the rare-earth ions are present in concentrations as low as 3 ppm. The temperatures of the glow peaks seen at ~54, 105 and 141 K for undoped material are strongly modified by the rare-earth ions. The trapping and recombination sites, monitored by rare-earth emission, are intimately linked, probably within large complex structures. For these three glow peaks the peak temperature 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 the rare-earth radii. Of all the rare-earth ions, europium forms the most stable 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 proposed.

The effect of the photoinitiator branching on polymerisation thresholds in a series of triphenylamine ethyl ester derivatives is studied.