Benitoite luminescence

Eigure 5.41 summarizes the temperature behavior of decay time and quantum efficiency of red benitoite luminescence at 660 nm in the forms ln(r) and ln(q) as a functions of 1/T. In such case the luminescence may be explained using simple scheme of two levels, namely excited and ground ones. The relative quantum yield (q) and decay time (r) of the red emission may be described by simple Arhenius equations ... 

Fig. 5.62.a. Calculated energy levels cheme for blue benitoite luminescence... 

Benitoite is characterized by very intensive blue luminescence (White 1990). Laser-induced time-resolved technique enables us to detect three broad bands and one narrow line, connectet with TiOe, Ti " " and Cr or Mn, luminescence centers (Fig. 4.35). 

Combination of broad emission band and narrow line is typical for elements with d electronic configuration, such as Cr +, Mn and Manganese participation is supported by chemical analyses of benitoite, where chromium was never mentioned as micro-impurity, while Mn is known with concentrations changing from 0.03 to 0.11% (Laurs et al. 1997). Such concentrations are quite enough for luminescence generation. Substitution in Mn +form substituting... 

Comparison of this luminescence intensity in different samples reveals that any correlation is absent any impurity concentration. Thus it was supposed that the mostly probable luminescence center is Ti, which presence is quite natural in Ti bearing benitoite. The wide occurrence of Ti " minor impurities in minerals was detected by EPR. Like the other d ions (V, Mo ), Ti ions occur often in minerals as electron center (Marfunin 1979). It may be realized in benitoite, which does have some natural exposure to gamma rays in its natural setting. There could be radiation centers, such as, for example, Ti + gamma ray + electron donor Ti + electron hole. Benitoite color does not change with gamma irradiation to quite high doses (Rossman 1997) but luminescence is much more sensitive compared to optical absorption and can occur from centers at such low concentration that they do not impact the color of a benitoite. 

Figure 1. Photoluminescence emission and excitation spectrum for benitoite showing Ti + luminescence.

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... 

Fig. 4.82 (a-c) Laser-induced time-resolved luminescence spectra of benitoite demonstrating TiOg, Ti and Cr or Mn centers... 

Gaft M, Nagli L, Waychunas G (2004) The nature of blue luminescence of natural benitoite BaTiSisOg. Phys Chem Miner 31 365-373... 

Under 355 and 532 nm excitations at 300 K the certain benitoite samples exhibit red band peaking at 720 nm with half-width of 125 nm accompanied by narrow line at 680 nm (Fig. 4.82c). Time resolved spectroscopy with different decays and gates revealed that both band and Une behave in a similar way, namely they have similar decay time of 170 ps. It may be supposed that band and line belong to the same luminescence center. The broad band disappears at low temperatures. The line is much stronger at lower temperatures, where two additional weak lines appear at 695 and 702 nm. Decay time at low temperature is very long reaching x = 1.1 ms (Gaft et al. 2004). 

Luminescence of Ti was not confidently detected in steady-state luminescence spectra of minerals. In Ti minerals studied by laser-induced time resolved spectroscopy broad read band have been found with decay time of several ps at 660 nm in benitoite (Fig. 4.82) and 750 nm in titanite (Fig. 4.801). At room temperature the benitoite band with a maximum at 660 nm has half-width of 135 nm and may be approximated by one Gaussian (Gaft et al. 2004). One exponent with decay time of 1.1 ps approximates well its decay curve at room temperature in all spectral range of luminescence band. At lower temperatures up to 30 K this red luminescence intensity becomes approximately ten times higher and the spectrum undergoes certain changes, namely its maximum shifts in long wave direction to 668 mn and the band becomes a little narrower with half-width of 105 mn. Such red emission is not excited by laser sources in the visible part of the spectrum, such as 488, 514 and 532 mn. Excitation spectrum at lower temperatures, when... 

The luminescence of many titanium minerals was studied by steady-state luminescence spectroscopy and it was proposed that blue luminescence bands mutual for these minerals is connected with TiOe complex luminescence (Gaft et al. 1981a White 1990). Figure 4.82a presents spectral properties of the blue emission from benitoite at 300 K. Under short and middle-wave UV laser excitation, such as at 266 and 308 nm, respectively, an intensive broad blue emission band peaking at approximately 420 nm with half-width of 80 nm is detected. Spectra with different excitations, delays and gates revealed that this band consists of only one... 

According to our experience all benitoite samples have intensive blue emissimis under short wave and middle wave UV, and we have never seen or heard of any that did not. On the other hand, efficient luminescence has been observed from the Ti04 , TiOe and even TiOs complexes. In addition, bazirite BaZrSisOg with the benitoite stmcture demonstrates luminescence of zirconate complexes ZrOe (Blasse 1980). The blue luminescence of impurity Ti centers has been found in synthetic BaSnSisOg-Ti" and BaZTSisOg-Ti with the benitoite structure (Konijenendijk 1981). 

We thus have strong evidences in favor of an intrinsic model for the blue luminescence in benitoite, connected with the TiOg octahedra. Hence we have tried to determine if the spectroscopic experimental data may be explained based on such a model (Gaft et al. 2004). 

Figure 5.99 summarizes the temperature behavior of decay time x and quantum efficiency q of the blue luminescence from benitoite in the forms ln(x) and ln(q) as a function of reciprocal temperature 1/T. Figure 5.100 demonstrates a suitable energy levels scheme. After excitation the metastable level 1 is populated due to nomadiative fast transition from excited level. Between levels 1 and 2 the... 

Fig. 5.101 Energy levels scheme for TlOg luminescence center in benitoite...

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