Rare-earth-size effects on thermoluminescence and second-harmonic generation

The substitution of rare-earth ions into insulating host crystals introduce s lattice strains and, for non-trivalent sites, a need for charge compensat ion. Such effects alter the site symmetry and this is reflected in properti es such as the wavelength, linewidth, lifetime and relative intensity of th e rare-earth transitions. Equally clear, but less well documented, is the i nfluence on second-harmonic generation (even from cubic crystal lattices). For example, in bismuth germanate, second-harmonic generation efficiency va ries by factors of more than 100 as a result of different rare-earth dopant ions. The ions are variously incorporated as substitutional ions, pairs, c lusters, or even as precipitates of new phases, but the detailed modelling is often speculative. This article summarizes some recent studies which exp lore the role of rare-earth ions in thermoluminescence and second-harmonic generation. There are numerous differences in glow peak temperature, for no minally the same defect sites, which are thought to indicate charge trappin g and recombination within coupled defect sites, or within a large complex. Size and cluster effects can be modified by heat treatments. This review c onsiders the similarity and trends seen between numerous host lattices whic h are doped with rare-earth ions. For thermoluminescence there are trends i n the variation in glow peak temperature with ion size, with movements of 2 0 to 50 It. Examples are seen in many hosts with extreme effects being sugg ested for zircon, with peak shifts of 200 K (probably from precipitate phas es).