Scheelite luminescence

Figure 4.8a represents scheelite luminescence in the near IR ranges of the spectrum. The usual characteristic fines of Nd " are detected in the spectral... 

Figure 4.10a presents scheelite luminescence in the near IR ranges of the spectrum. The usual characteristic tines of Nd are detected in the spectral range up to 1.6 pm. Strong reabsorptirai tines of Nd are characteristic for luminescence of many minerals, but were firstly detected in scheelite (Gorobets 1975). 

At the third level, the most detailed partition of luminescence minerals is carried out on the basis of metals in the mineral formulae, hi rare cases we have minerals with host luminescence, such as uranyl minerals, Mn minerals, scheelite, powellite, cassiterite and chlorargyrite. Much more often luminescent elements are present as impurities substituting intrinsic cations if their radii and charges are close enough. Thus, for example, Mn + substitutes for Ca and Mg in many calcium and magnesium minerals, REE + and REE substitutes for Ca, Cr substitutes for AP+ in oxygen octahedra, Ee substitutes for Si in tetrahedra and so on. Luminescence centers presently known in solid-state spectroscopy are summarized in Table 4.2 and their potential substitutions in positions of intrinsic cations in minerals in Table 4.3. 

The natural scheelite in our study consisted of 90 samples from a variety of geologic environments. Concentrations of potential luminescence impurities in several samples are presented in Table 4.5. 

Fig. 4.8, a-d Laser-induced time-resolved luminescence spectra of scheelite demonstrating Nd ", Yb, Tm ", Er " and centers... 

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

Luminescence spectrum of scheelite with a broad gate width of 9 ms is shown in Fig. 4.9d. The narrow lines at 490 and 572 nm are usually ascribed to Dy and the lines at 607 and 647 nm to Sm " ". Nevertheless, the relative intensity of the line at 607 nm compared to the line at 647 nm is lower at longer delay times (Fig. 4.9e,f). Besides that with a shorter gate width of 1 ps, when the relative contribution of the short lived centers is bigger, the characteristic lines of Sm " " at 647 nm and Dy " at 575 nm disappear while the lines at 488 and 607 nm remain. Such luminescence is characteristic of Pr ", which was confirmed by a time-resolved luminescence study of scheelite artificially activated by Pr " and Sm " (Fig. 5.5). Unlike in apatite, the phenomenon exists not only under 308 nm, but also under 337 nm excitation due to the higher energy... 

Eigures 5.14a,b represent luminescence spectra of scheelite enriched by Eu. Luminescence of Eu " is well known in steady-state spectra of scheelite (Tarash-chan 1978 Gorobets and Kudrina 1980). In time-resolved spectroscopy its relative intensity is stronger after a long delay time, which is explained by the longest decay time of Eu " in scheelite compared to other REE. 

The possible luminescence of Eu " in scheelite is a very interesting problem. It was not detected by steady-state luminescence spectroscopy. The possible reason is that the very strong intrinsic luminescence of scheehte is situated in the same spectral range, which covers the weaker emission of Eu ". We tried to solve this problem by the time-resolved method using different decay times for intrinsic and Eu bands. Time-resolved spectroscopy... 

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... 

The fines of Er " " are difficult to point out because they have short decay times, which are comparable with the decay of broad band luminescence of scheelite at the same spectral region. Using a dye laser with 357 nm emission solved this problem. It is effective for Er " " but not suitable for the excitation of the WO4-center (Fig. 4.8c). These fines of Er " " are most clearly seen with a narrower gate width when the luminescence of short-lived centers dominates. Investigation of the time-resolved luminescence of synthetic CaW04 Er confirmed our interpretation (Fig. 5.19). 

A possible candidate may be Tm ". For example, the doublets at 803 and 817 nm and at 796 and 813 nm are the strongest ones in cathodoluminescence spectra of fluorite and scheelite activated by Tm " (Blank et al. 2000). It is possible to suppose that the strong fines at 805 and 820 nm with a relatively short decay time of 60 ps in the titanite luminescence spectrum belong to Tm " ". They appear under 532 nm excitation and are evidently connected with the electron transition. Similar emission of Tm " was also detected in... 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is... 

Identification of minerals is not a trivial question when dealing with natural objects. Luminescent minerals received from different mineralogists, museums and collectors are often not correctly identified. It is a potential source of serious errors, because the presence of a certain luminescence center in one mineral maybe trivial, while its luminescence in another mineral may represent a certain interest. For example, emission of Mn " is common in calcite, but its absence in scheelite is an interesting problem. Thus, when you find the band... 

Tungsten is usually identified by atomic spectroscopy. Using optical emission spectroscopy, tungsten in ores can be detected at concentrations of 0.05—0.1%, whereas x-ray spectroscopy detects 0.5—1.0%. Scheelite in rock formations can be identified by its luminescence under ultraviolet excitation. In a wet-chemical identification method, the ore is fired with sodium carbonate and then treated with hydrochloric acid addition of zinc, aluminum, or tin produces a beautiful blue color if tungsten is present. 

A variety of related structures can be identified with 6,8, and 12-fold coordination of the A cation and four or sixfold coordination of the anion. In fact, the chemistry of ABO4 temarys is extremely complicated with solid solutions and phase transitions being common. Lattice defects may be introduced easily by appropriate dopings. Scheelites and its relatives have been studied intensively for their properties as heterogeneous catalysts, as host materials for impurity activated luminescent materials, and for specialized optical uses see Oxide Catalysts in Solid-state Chemistry and Section 4.4). 

For the production of phosphores for lasers, fluorescence lamps, oscilloscopes, luminescence colors, X-ray observation screens, luminous paints, and scintillation counters. The property of fluorescence is very important in prospecting and mining of scheelite ore. 

In oxides the Mo ion is usually four-coordinated, as, for example, in the most well-known luminescent molybdate CaMo04 with scheelite structure. This, by the way, is the only luminescent molybdate whose luminescence has been investigated in some detail. 

As far as we know the tetrahedral niobate group occurs only in the fergusonite structure of YNbO4 which is a distorted version of the scheelite structure of CaMo04 This luminescence is bright blue and has a relatively high Tq, viz. 500 jfg decay time is 15 MS at 11 which is short in comparison with the plateau value for the molybdate tetrahedron. It is very unfortunate that no more data exist on this complex with its efficient luminescence. 

In the AVO4 (A = Yb, Y, Lu and Nd), the band gaps increase in the low-pressure zircon phase (by 1-2 meV/kbar) and decrease with pressure in the high-pressure scheelite phase (by 0.7-2.2 meV/kbar) [71]. In wurtzite and rocksalt InN [72], the absorption threshold energy was found to increase by 3.0-3.2 meV/kbar, which corresponds to the increase of the band gap with increasing pressure. A similar dependence of 2.1-2.7 meV/kbar was obtained from the luminescence measurements [73]. 

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