No abstract

tant was decanted. This peptization step was repeated 3±4 times. Finally, the combined supernatants were centrifuged at 12 500g for 5 min and the weakly opalescent colorless supernatant, containing the smallest nanoparticles, was carefully decanted.Thin fibers of LaPO 4 :Eu were synthesized in acidic solution as follows: A solution of La(NO 3 ) 3 ×6H 2 O (12.34 g; 28.5 mmol) and Eu(NO 3 ) 3 ×5H 2 O (0.642 g; 1.5 mmol) in water (100 mL) was adjusted to pH 4.5 and combined with stirring with (NH 4 ) 2 HPO 4 (3.96 g; 30 mmol) dissolved in water (100 mL). The resulting suspension having pH 1.7 was poured into a Teflonlined autoclave (Berghof, HR-500) and treated as described above.Similarly, fibers of LaPO 4 :Ce(5 %) and La 0.4 Ce 0.45 Tb 0.15 PO 4 were prepared by employing a total amount of 30 mmol of the respective nitrates. Prior to heating, the suspension in the autoclave was purged with forming gas (N 2 :H 2 = 9:1) for 60 min, in order to prevent oxidation of Ce 3+ to Ce 4+ .Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C).Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples.UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin±Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm.[**] We thank the Royal Society (YQZ, WKH, MT), the DERA (NG), Conacyt-MØxico and DGAPA-UNAM IN 107-296 (HT), and the EPSRC for financial support. We are grateful to J. Thorpe and D. Randall (Sussex) for assistance with TEM and SEM facilities.

Spectrally resolved thermoluminescence of co-doped Dy:Tm: shows that the maximum peak temperature for the nominal C dosimetry peak differs by C for Dy and Tm emission. This is interpreted as evidence that the rare earth ions form part of a complex defect which variously provides both the charge trapping and, during heating, the radiative decay. The peaks have the same activation energies but different pre-exponential factors. Modifications of the material by thermal treatments using furnace or laser pulse heating convert the state of dispersion of the rare earth ions between isolated, pair or defect clusters, which alter the dosimetry efficiency. In some cases the modified geometries are detectable via movement of the emission lines. For rapidly quenched materials, discontinuities in the thermoluminescence responses are suggested to be indicative of new microcrystalline phases. Slow cooling degrades the efficiency but also indicates the presence of further thermoluminescence glow peaks within the region of the main dosimetry signal. Pulsed laser heating with a UV laser altered the glow curve and resulted in strong signals. Mechanisms for this process are considered.

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).

The radioluminescence and thermoluminescence spectra of synthetic zircon crystals doped with individual trivalent rare earth element (REE) ions (Pr, Sm, Eu, Gd, Dy, Ho Er, and Yb) and P are reported in the temperature range 25 to 673 K. Although there is some intrinsic UV/blue emission from the host lattice, the dominant signals are from the rare-earth sites, with signals characteristic of the REE 3+ states. The shapes of the glow curves are different for each dopant, and there are distinct differences between glow peak temperatures for different rare-earth lines of the same element. Within the overall set of signals there are indications of linear trends in which some glow peak temperatures vary as a function of the ionic size of the rare earth ions. The temperature shifts of the peaks are considerable, up to 200°, and much larger than those cited in other rare-earth-doped crystals of LaF 3 and Bi 4 Ge 3 O 12 . The data clearly suggest that the rare-earth ions are active both in the trapping and luminescence steps, and hence the TL occurs within localized defect complexes that include REE 3+ ions.

No abstract

No abstract

Thermoluminescence (TL) and radio-thermoluminescence spectral analysis techniques have been applied to doped calcium sulphate samples designed for radiation measurements at elevated temperatures. CaSO 4 :Dy, when co-doped with Ag, provides a TL dosimetric peak near 350 • C which is useful for radiation measurements at high temperatures. Dopants of Ce, Mn and Dy variously move the peak temperature from 400 • C to 200 • C. Each dopant ion gives a characteristic emission spectra, which for CaSO 4 :Ce, Mn samples indicate that there is a systematic temperature difference of ∼7 • C between the glow peaks from the Ce and Mn sites. The CaSO 4 :Dy samples show a discontinuity in the emission wavelength from the Dy ions near T = 200 • C and a decrease in the radioluminescence fluorescence in the same temperature region. In each case it is proposed that the dopants form part of large, complex defects, instead of isolated trapping and recombination centres. The data offer further evidence for a localized phase transition of the defect complex at 200 • C. Low-temperature data, from 20 K, show similar differences in the peak temperature from the various dopants and additionally indicate reproducible discontinuities in the wavelength positions and intensities, for all samples, at T = 230 K. This again suggests structural phase adjustments of the defect sites.