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).

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Luminescence offers a sensitive probe of the quality of Nd:YAG laser material both in the bulk and, via cathodoluminescence, in the near surface layers. The spectral signals are primarily from the Nd dopants, but the thermoluminescence spectra reveal traces of impurities such as Tb, Cr and Mn. Control of the electron energy of the cathodoluminescence demonstrates that the outer few micron layers differ significantly in luminescence response from the bulk crystal. The cathodoluminescence signals are influenced by near surface dislocations and solvents from cleaning treatments. The effects are often apparent as discontinuities in the Nd signal intensities at temperatures which match the solid-liquid or liquid-gas phase transitions of contaminants. Additionally there is strong evidence for the inclusion of carbon dioxide, in the form of nanoparticles. At 202 K, the CO2 sublimation temperature, the Nd line intensities change discretely and there are wavelength shifts for some of the emission lines consistent with pressure driven changes in the lattice parameter. Data for x-ray lattice parameters identify a complex and sudden change and expansion of the lattice in this temperature region. The luminescence methods used suggest identification of impurities and gaseous inclusions can be exploited in other insulator materials and examples are cited.