Zircon luminescence

It was proposed that yellow zircon luminescence is connected not with one, but with many centers, which have similar luminescence and excitation spectra, but different decay times and thermal stability (Gaft et al. 1986 Shinno 1987 ... 

It is interesting that yellow zircon luminescence is very specific and different from other Zr-bearing minerals such as catapleite, keldyshite, vlasovite, khibinskite and others, which are usually characterized by blue luminescence evidently connected with titanium impurity, namely TiOe complexes (Gaft et al. 1981). 

Figure 4.39a represents the liuninescence band detected in all investigated natural zircons. It has an excitation band peaking at 310 nm. The broad yellow band at 575 nm with a decay time of 25-35 ps represents classical zircon luminescence. The picture is not principally changed with different delay times and gate width. After heating the sample up to 700 °C the luminescence intensity is nearly the same, but after that it is strongly reduced and at 800 °C the yellow luminescence disappears. Luminescence spectra at 77 and 4.2 K are not substantially different. 

Part of solid state chemistry is presently involved with what is called soft chemistry or soft materials. As a matter of fact these are not expected to luminescence, at least not when the luminescent centers are broadband emitters. This has been shown to be the case, for example, for the isomorphous Al2(W04)3, Sc2(W04)3, and Zr2(P04)2S04. The Stokes shift of the tungstate and zirconate luminescence in these materials is enormous, viz., some 2 eV. The quantum efficiencies, even at 4.2 K are low (101). The exact structural explanation has been discussed in the literature (101). 

The most convincing case of zirconate luminescence is the emission observed for BaZrSi3O9 5). This compound has the benitoite structure with Zr in octahedral coordination. This emission is situated in the ultraviolet the emission band has its maximum at 285 nm. Excitation is only possible with X < 220 nm. The compound ZrP2O7 shows a similar behaviour ). No further data are available for these compounds. From... 

Besides REE, broad spectral bands characterize the luminescence of zircon. They are structureless down to 4.6 K, which makes difficult the correct interpretation of the nature of the luminescent centers. Different suppositions are made in previous studies and even the question about a yellow luminescence connection with intrinsic or impurity defect remains open. For example, the yellow band ( C-band ) was ascribed to SiO -defects (Votyakov et al. 1993 Krasnobayev et al. 1988) while the same emission ( band VII ) was explained by impurity luminescence, namely by Yb " " created by radioactive reduction of Yb " " (Kempe et al. 2000). 

Fig. 4.39. a-d Laser-induced time-resolved luminescence spectra of zircon demonstrating intrinsic radiation induced, luanyl and Fe center... 

Table 4.15. Concentrations of potential luminescent impurities (ppm) in several zircon samples ...

Thorite and orangite (orange thorite) have a tetragonal structure and are isostructural with zircon. Steady-state spectra under X-ray and laser (337 nm) excitations are connected with REE " ", namely Sm " ", Tb ", Dy " " and Eu ". Reabsorption lines of Nd " have been also detected (Gorobets and Rogojine 2001). Laser-induced time-resolved luminescence enables us to detect Eu " and uranyl emission centers (Eig. 4.70). 

Fig. 5.5. a -f Laser-induced time-resolved luminescence spectra of synthesized apatite, zircon and scheelite artificially activated by Pr... 

Luminescence of Pr + in zircon is very difficult to detect under UV excitation even by time-resolved spectroscopy. The reason is that it has a relatively short decay time similar to those of radiation-induced centers. Visible excitation, which is not effective for broadband luminescence, allows the revealing of Pr + luminescence lines, using high-resolution steady-state spectroscopy. Under such experimental conditions each element has individual lines, enabling confident identification of the spectrum to be possible (Gaft et al. 2000a). Only if radiation-induced luminescence in zircon is relatively weak, the lines of Pr may be detected by UV excitation (Fig. 4.38c). 

Time-resolved luminescence spectroscopy of zircon revealed luminescence lines, which maybe confidentially ascribed to a Eu center (Fig. 4.38d). Usually they are hidden by a broad band yellow emission of zircon and may be detected only with a long delay time using its much longer decay time compared to yellow luminescence. 

Fig. 5.11. Laser-induced time-resolved luminescence spectra of synthetic zircon artificially activated by Eu (a, b) and excitation spectra of different Eu " centers (c, d)...

Zircon belongs to the tetragonal system and is a positive uniaxial. The typical form shows the ill and the 110 planes. The two orientations selected for luminescence polarization study were the (110) plane, parallel to the basal section and the [100] row. In such cases the axis perpendicular to the (110) plane will be called X. The orientation notation is made according to the so-called Porto notation (Porto et al. 1956). The Xi(ZX2)Xi orientation means that the laser light entered parallel to the Xi axis of the crystal and is polarized in the Z direction, while the emission is collected along the Xi axis with X2 polarization. By polarization spectroscopy with a high spectral resolution (less then 0.1 nm) six lines are observed for the Dq- Fi transition of the Eu-II center instead of the maximum three allowed for an unique site (Fig. 5.12). In Z(XX)Z geometry which corresponds to observation of a-polarized luminescence we... 

Table 5.1. Different Eu luminescence centers in artificially activated zircon...

In all minerals the gadolinium luminescence spectra are completely located in the UV part of the spectrum and consist of several lines at 310-315 nm, corresponding to transition P7/2- S7/2- The main line is characterized by a long decay time and is especially prominent in the spectra with a long delay. Gd " is known as a good sensitizer of the other rare-earth ions luminescence. It is detected in spectra of fluorite (Fig. 4.12a), zircon (Fig. 4.38h), anhydrite (Fig. 4.17a), and hardystonite (Fig. 4.20b). 

Fig. 5.19. a-d Laser-induced time-resolved (a, d) and steady-state (b, c) luminescence spectra of synthetic zircon and scheelite artificially activated by Er... 

Luminescence spectra at low temperature, together with very short decay times are rather characteristic for Cr " luminescence, which was not known until recently, and which is especially similar to the luminescence of CrO " doped YPO4 with zircon structure (Brunold et al. 1997). At 15 K it also consists... 

In the time-resolved luminescence, Fe dominates spectra with long delay times. The examples may be seen in feldspars and obsidian (Fig. 4.43), woUastonite (Fig. 4.42b), zircon (Fig. 4.39d), and beryl (Fig. 4.52b). 

Another example is the luminescence of in YPO4 with tetragonal zircon structure (Oomen et al. 1988). The emission consists of two bands, one in the UV region and one in the visible part of the spectrum. The intensity ratio of these bands is strongly temperature dependent (Fig. 5.57). 

The reabsorption lines of have been found in zircon (Gaft et al. 1986) luminescence spectra. The strongest reabsorption lines at 592 and 656 nm are (Fig. 4.39b) clearly seen, which are totally identical to absorption lines of in zircon (Platonov et al. 1989). 

Natural zircons heated at 800 °C during one hour when natural yellow broadband luminescence nearly totally disappears and irradiated by different doses of alpha particles. 

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