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European Journal of Mineralogy DOI: 10.1127/0935-1221/2013/0025-2260 Title: Laser-induced time-resolved luminescence of natural margarosanite Pb(Ca,Mn)2Si3O9, swedenborgite NaBe4SbO7 and walstromite BaCa2Si3O9 You receive only the enclosed proofs for correction. Please do not change the text against the manuscript. Only corrections of spelling mistakes will be accepted. For other changes or corrections we have to charge you. If the proofs/correction notes do not reach us in time the paper will be printed without further corrections. Kindly return the corrected proofs with your ready for print within 3 days.
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Eur. J. Mineral. Fast Track DOI: 10.1127/0935-1221/2013/0025-2260
Fast Track Article
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Laser-induced time-resolved luminescence of natural margarosanite Pb(Ca,Mn)2Si3O9, swedenborgite NaBe4SbO7 and walstromite BaCa2Si3O9 MICHAEL GAFT1,*, H. YEATES2 and LEV NAGLI1 1
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Laser Distance Spectrometry (LDS), 11 Granit, Petach Tikva, Israel *Corresponding author, e-mail:
[email protected] 2 1707 Vestal Drive, Coral Springs, FL, 33071, USA
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Abstract: Visual luminescence of natural margarosanite, swedenborgite and walstromite is well known for mineralogists, but their spectral characteristics and interpretation were still missing. We studied those minerals by laser-induced time-resolved luminescence technique, described the corresponding spectral and kinetic parameters and interpreted the luminescence centers as follows: margarosanite – Mn2þ, Mn2þ clusters, Pb2þ and Ce3þ; swedenborgite – Sb3þ; walstromite – Mn2þ, Eu2þ and trivalent rare-earth elements. Key-words: luminescence, time-resolved technique, Raman spectroscopy, margarosanite, swedenborgite, walstromite, rare-earth elements.
1. Introduction
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The three title minerals have in common that visually their luminescence is well known (e.g., 6), while the spectral composition data and luminescence centers identification are still lacking. The aim of this paper is therefore to study the luminescence of swedenborgite, walstromite and margarosanite by laser-induced time-resolved spectroscopy in order to ascribe the emissions to luminescence centers.
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2. Experimental setup
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spectra above. One hundred points have been measured in each kinetic series with different time intervals, namely 10 ms for long-lived components (with 100 ms steps), 1 ms (with 10 ms steps) and 100 ms (with 1 ms steps) for shortlived components. By this method we have been able to measure specific decay times without interferences from closely spaced spectral features (cf. Gaft & Nagli, 2009). The following samples have been studied: swedenborgite in calcite matrix from La˚ngban, Sweden, margarosanite from Franklin, New Jersey, and walstromite from Big Creek deposit, Fresno County, California. For luminescence and Raman spectroscopy measurements the corresponding mineral location was found using UV mercury lamp excitation (Fig. 1) and the narrow excitation laser spot was directed exactly on the studied mineral. By this way the potential influence of accompanying minerals luminescence was removed.
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3. Experimental results
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In order to confirm the minerals identity, their Raman spectra have been measured. Fig. 2 presents the corresponding data. Raman spectra of all those minerals agree very well to known database (http:\\RRUFF.info) making their mineralogical identification definite. Fig. 3 presents time-resolved luminescence spectra of margarosanite obtained under 355 nm excitation at room (300 K) and low (100 K) temperatures. At 300 K it is characterized by broad band peaking at 635 nm (Fig. 3a) with long decay time of 1.2 ms (Fig. 3d). At 100 K the band
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The luminescence excitation sources were pulsed lasers, utilizing two UV harmonics of Nd-YAG (266, 355 nm), which delivers pulses of 5–10 ns duration. The spectra, observed at a 90 geometry, are detected by an intensified ANDOR iStar CCD camera synchronized to the laser pulses. It enables spectral measurements in adjustable time windows which are determined by a delay time, namely the time between the end of the laser pulse and the beginning of the measurement, and by gate width, namely the time between the beginning and the end of the measurement. Delay time and gate width may be changed from 4 ns to 19 ms. Spectral resolution is determined by the monochromator gratings (from 300 to 2400 lines/ mm) and the minimal value was 0.1 nm. The spectra have not been corrected for the differential sensitivity as a function of wavelength. Decay times have been measured by a kinetic series of different delay and gates, as described for the excitation
0935-1221/13/0025-2260 $ 3.15 DOI: 10.1127/0935-1221/2013/0025-2260
# 2012 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
M. Gaft, H. Yeates, L. Nagli
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maximum shifts to 655 nm (Fig. 3b) while its decay of 2.1 ms becomes even longer (Fig. 3d). Besides, at low temperature, additional band may be detected using a very narrow gate of 750 ns, enabling to detect the weak emission with short decay time. It has a maximum at 460 nm and a short decay time of approximately 1.5 ms. Figure 4 presents timeresolved luminescence spectra of margarosanite under excitation by 266 nm wavelength at 300 and lower (200 and 100 K) temperatures. At room temperature, the three luminescence bands are detected: the orange peaking at 630 nm with decay time of 540 ms (Fig. 4a); the blue peaking at 450 nm with short decay time of 10.6 ms and the UV one peaking at 325 nm with very short decay time of 1.3 ms (Fig. 4b). At lower temperature of 200 K, the orange band becomes more intensive (Fig. 4c), the blue one becomes weaker and the UV one remains approximately the same (Fig. 4d). At temperature of approximately 150 K, the last two bands become very weak. At even lower temperature of 100 K, the orange band increases its intensity and its decay time of 410 ms is shorter compared to the room temperature (Fig. 4e), while a new UV emission appears peaking at 375 nm with very short decay time of 150 ns (Fig. 4f). Swedenborgite in our collection was not luminescent under excitation by 355 nm wavelength. Figure 5 presents its time-resolved luminescence spectra under excitation by 266 nm wavelength at 300 and 100 K temperatures. At 300 K, emission consists of violet band peaking at 410 nm with short decay time of 4.5 ms (Fig. 5 a,b). At 100 K, the spectral form remains actually the same, but the decay time of 135 ms becomes substantially longer (Fig. 5 c,d). Figure 6 presents time-resolved luminescence spectra of walstromite under excitation by 355 nm wavelength at 300 K temperature. Three types of luminescence are clearly seen: relatively narrow orange band peaking at 590 nm with long decay time of 1 ms, relatively narrow blue band peaking at 480 nm with very short decay time of 1 ms and several narrow lines peaking at 363, 384, 419, 425, 434, 450 and 459 nm with intermediate decay time of 75–100 ms. Luminescence was also detected during Raman measurements under excitation by 437 nm wavelength. The spectrum is characterized by intensive narrow emission lines peaking at 482, 491, 564, 567, 599, 603 and 645 nm. Besides, weak narrow lines are found in the IR spectral region between 800 and 900 nm (Fig. 7)
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4. Interpretation of experimental results
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4.1. Margarosanite Pb(Ca,Mn)2Si3O9
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Two different types of orange luminescence may be detected. The first one is under excitation by 266 nm, where the emission band at 100 K becomes a little narrower compared to the 300 K, but its intensity, spectral position and decay time remain actually without change. Such behavior is typical for Mn2þ luminescence with its
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Fig. 1. Visual luminescence of margarosanite (upper), walstromite (middle) and swedenborgite (down) under UV lamp excitation.
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forbidden d-d luminescent transition and correspondingly relatively long decay time. At first glance, orange luminescence under excitation by 355 nm looks the same, but several details are different. The emission band at 100 K is nearly twice more intensive and moves approximately 20 nm in long-wavelength direction compared to the luminescence at 300 K. Decay time becomes nearly twice longer (Fig. 3d). The possible explanation is that both emission bands are connected to Mn2þ luminescence centers, while Mn2þ substitutes for Ca2þ and calcium has two different positions in margarosanite crystal structure (Freed & Peacor, 1969). The substantial decay-time change of luminescence under excitation by 355 nm at lower temperature may be ascribed to Mn2þ clusters known in many Mn-bearing minerals (Tarashchan, 1978; Gorobets & Rogojine, 2002; Gaft et al. 2005). It is quite natural for margarosanite from Franklin, New Jersey, with elevated Mn concentrations of 0.2–0.6% (Dunn, 1985) and even 1.14–2.17% (Armstrong, 1963), which are far higher than trace level. In such a case, the excitation does not stay on the same ion of Mn2þ, but can travel readily through the sub-lattice of in-resonance Mn2þ ions. The emission originates from Mn2þ ions associated with impurities and other
defects. They occupy regular cation sites in the lattice and perturb the surrounding Mn2þ ions, lowering their energy levels relative to those of the unperturbed (intrinsic) Mn2þ ions. At low temperatures the excitation cannot return to the exciton state; the excited, perturbed Mn2þ ions decay radiatively with a spectrum characteristic of the particular trap. It explains the substantial red shift of luminescence at low temperature. Deeper traps are also present and are effective as quenching states, i.e. traps from which no emission occurs, but where the excitation is lost non-radiatively. At low temperatures the traps are effective, but at higher ones they begin to lose their trapped excitation energy by thermally activated back-transfer to the exciton level. From here deeper traps may trap the energy. Finally all the emitting traps are emptied and only the deep, nonemitting traps are operative. As a consequence the luminescence has been quenched. These quenching traps may be Ni or Fe ions (Gaft et al. 2005). Such radiative and nonradiative decay may explain the shorter decay time at 300 K compared to the low temperature one. Blue very broad luminescence band is found peaking at 460 nm with relatively short decay time of 10.6 ms. It becomes less intensive with temperature lowering and
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Fig. 4. Laser-induced time-resolved luminescence of margarosanite under excitation by 266 nm wavelength: at 300 K with intermediate delay and broad gate (a) and zero delay and narrow gate (b), at 200 K with intermediate delay and broad gate (c) and zero delay and narrow gate (d), at 100 K with intermediate delay and broad gate (e) and zero delay and narrow gate (f). 1 2 3
nearly disappears at approximately 150 K. Simultaneously a new UV band appears peaking at 375 nm with very short decay time of 150 ns. It may be supposed that such
luminescence is connected to another potential luminescent ion present in margarosanite, namely to Pb2þ. In some cases two emission bands are typical for s2 ions, where Pb2þ
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Fig. 5. Laser-induced time-resolved luminescence of swedenborgite under excitation by 266 nm wavelength: emission spectrum (a) and decay time (b) at 300 K and emission spectrum (c) and decay time (d) at 100 K.
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Fig. 6. Laser-induced time-resolved luminescence of walstromite under excitation by 266 nm wavelength at 100 K: with zero delay and broad gate (a), zero delay and intermediate gate (b and c) and zero delay and very narrow gate (d).
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belongs, while their relative intensities change with temperature. The two emissions arise from different minima on the potential energy surface of the relaxed excited state. At low temperatures the high energy level which is populated by optical excitation emits with resulting UV luminescence.
At elevated temperatures the barrier between the minima may be overcome and emission from minimum with lower energy occurs with resulting visible blue luminescence (Blasse & Grabmaier, 1994; Gaft et al. 2005). Very similar UV luminescence was connected with Pb2þ in calcite,
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problem is that from classification of luminescent minerals position (Gorobets & Rogojine, 2002) it is difficult to 603 suppose the potential luminescent impurity. There are no known luminescence centers that may substitute for the Sm ˚ or for the Sb5þ Be2þ with its very small radius of 0.3–0.4 A with its very high valence state. The main luminescent 2 cation substituting for Naþ is Mn2þ, but its luminescence properties are not consistent with those found in swedenborgite. 645 Another possibility is intrinsic and not impurity-related 1 luminescence. Trivalent Sb is a well known luminescence center in artificial phosphors, for example in calcium halophosphate, activated by Sb3þ and Mn2þ, which has been used for many years in fluorescent lamps (Blasse, 1984). 0 For minerals Sb participation in luminescence was not 500 600 700 800 900 Wavelength, nm considered before. In swedenborgite Sb presents in þ5 state, which is not luminescent. If the minute presence of Fig. 7. Continuous wave luminescence of walstromite under excitaSb3þ may be assumed, it is the most probable luminestion by 437 nm wavelength at 300 K. cence center. In order to prove it, the best way is to study artificially synthesized nominally pure NaBe4SbO7 and to confirm that such luminescence is really taking place. hardystonite and pyromorphite (Gaft et al. 2002, 2005). In margarosanite, where Pb is a major element, concentration quenching and corresponding energy migration adds to the 4.3. Walstromite BaCa Si O 2 3 9 complexity of this phenomenon. The deep UV band peaking at 330 nm may be ascribed to The situation as to the potential presence of luminescent Ce3þ luminescence because of its spectral position and ions in walstromite is much easier than in swedenborgite. very short decay time. Electric-dipole transitions between Substitutions for Ba2þ (Bi3þ, Agþ), Ca2þ (Mn2þ, REE3þ, the 4f ground state and the 5d excited state of Ce3þ are REE2þ) and Si4þ (Fe3þ) are well known (Tarashchan, parity- and spin-allowed and have a large oscillator 1978; Gorobets & Rogojine, 2002; Gaft et al. 2005). strength and very short decay time. Optical excitation in Based on this evaluation, orange band peaking at 590 nm the Ce-containing materials is normally performed in the with long decay time of 1.5 ms may be ascribed to Mn2þ range between 300 and 350 nm. The excitation arises from luminescence center. The presence of Mn impurities in optical inter-shell Ridberg transitions between 4s and 5d 0.18–0.33% levels is known in walstromite from levels. Substitution by Ce3þ in Ca2þ structural position is California (Alfors et al. 1965). well known in many calcium-bearing minerals, such as Blue band peaking at 480 nm with very short decay time apatite, fluorite, calcite and others (Tarashchan, 1978; of 650 ns is very typical to Eu2þ luminescence center. The Gorobets & Rogojine, 2002; Gaft et al. 2005). absorption and emission spectra of divalent europium are due to electronic transitions between the 4f7þ and 4f65d1 electronic configurations. An approximate energy level 4.2. Swedenborgite NaBe4SbO7 scheme was proposed for the electronic transitions in Eu2þ Its blue luminescence at 300 K consists of broad band by using strong-field formalism to describe the 5d levels and to describe the 4f orbitals. The peaking at 410 nm with short decay time of 4.5 ms. At the weak-field formalism 2þ 8 4f7 electronic 100 K the spectrum is actually the same, but the decay time ground state of Eu is S7/2 because of6 the 1 of 135 ms is much longer. Such luminescence behavior is configuration. In the configuration 4f 5d , one electron very typical for the heavy 6s2 ions where the transitions occupies a 5d orbital which is split into two orbital sets, t2g eg, by a cubic crystal field. Thus the energy terms are between the ground state and the 3P1 state become addi- and 2 2 tionally allowed by spin-orbit mixing of the 3P1 and 1P1 Eg and T2g in full cubic symmetry (Blasse & Grabmaier, states. After excitation at low temperature, the system 1994; Gaft et al. 2005). In the group of divalent RE lumi2þ relaxes to the lowest excited state. Consequently, the emis- nescence centers, Eu is the best known in minerals. It sion at low temperatures can be ascribed to the forbidden shows a 5d-4f emission, which varies usually from UV to transition 3P0-1S0 and has a long decay time. Nevertheless, blue. The host lattice dependence of the emission color of 2þ both 3P1 and 3P0 are emitting levels and they are very close the Eu is mainly connected with covalence, which will 3 so that at higher temperatures the luminescence from P1 reduce the energy difference between the 4f and 5d configlevel may appear with similar spectrum, but shorter decay. urations, crystal field splitting of the 5d configuration and The best known example in minerals is Pb2þ (Tarashchan, the Stokes shift (Tarashchan, 1978; Gorobets & Rogojine, 1978; Gaft et al. 2002, 2005; Gorobets & Rogojine, 2002). 2002; Gaft et al. 2005). The narrow lines are typical for trivalent REE luminesNevertheless, it is difficult to find the suitable place for such so called mercury-type centers in swedenborgite. The cence centers. The characteristic absorption and emission 599
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Laser-induced time-resolved luminescence
Acknowledgements: Raman data were provided by G. Panczer (Lyon 1 University) and for margarosanite confirmed by G.R. Rossman (Caltech).
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Alfors, J., Stinson, M., Matthews, R. (1965): Seven new barium minerals from eastern Fresno County, California. Am. Mineral., 50, 314–340. Armstrong, R. (1963): New data on margarosanite. Am. Mineral., 48, 698–703. Blasse, G. (1984): Luminescence of calcium halophosphaste-Sb3þ, Mn2þ at low temperatures. Chem. Phys. Lett., 104, 160–162. Blasse, G. & Grabmaier, B. (1994): Luminescent materials. Springer, Berlin, 232 p. Dunn, P. (1985): The lead silicates from Franklin, New Jersey: occurrence and composition. Mineral. Mag., 49, 721–727. Freed, R. & Peacor, D. (1969): Determination and refinement of the crystal structure of margarosanite, PbCa2Si3O9. Zeitschrift Fur Kristallographie, 128, 213–228. Gaft, M., Nagli, L. (2009): Gated Raman spectroscopy: potential for fundamental and applied mineralogy. Eur. J. Mineral., 21, 33–42. Gaft, M., Seigel, H., Panczer, G., Reisfeld, R. (2002): Laser-induced time-resolved luminescence spectroscopy of Pb2þ in minerals. Eur. J. Mineral., 14, 1041–1048. Gaft, M., Reisfeld, R., Panczer, G. (2005): Luminescence spectroscopy of minerals and materials. Springer, Berlin, 350 p. Gorobets, B. & Rogojine, A. (2002): Luminescent spectra of minerals. All-Russia Institute of Mineral Resources (VIMS), Moscow, 300 p. http://www.fluomin.org and http:\\RRUFF.info Tarashchan, A. (1978): Luminescence of minerals. Naukova Dumka, Kiev, 296 p. (in Russian).
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Received 30 April 2012 Modified version received 10 September 2012 Accepted 5 October 2012
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References
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spectra of lanthanide compounds in the visible, near ultraviolet and infra-red is attributed to transitions between 4f levels due to the fact that they present sharp line with oscillators strengths typically of the order of 106. These transitions are electric dipole forbidden but become partially allowed as forced electric dipole transitions (Blasse & Grabmaier, 1994; Gaft et al. 2005). The narrow lines under UV excitation take place at 364, 384, 419, 425, 434, 450 and 459 nm. The strongest lines at 450 and 459 nm have the similar intermediate decay time of 75 ms. The remaining lines are weak and their decay time was very difficult to measure, but comparing their temporal behavior in comparison with 450 and 459 nm lines, namely the relative intensities in different time windows, it may be concluded that their decay time is longer. The strong narrow line peaking at 450–453 nm with relatively short decay time connected with electronic transition from 1D4 level in Tm3þ is known in apatite, scheelite and other minerals (Gaft et al. 2005). Other lines are weak and difficult to interpret besides their belonging to the trivalent REE group. Narrow lines under visible blue excitation are much easier to interpret. The lines at 564, 567, 599, 603 and 645 nm belong to 4G5/2-6H5/2, 4G5/2-6H7/2 and 4G5/2-6H9/2 electronic transitions in Sm3þ luminescence center while the excitation by 437 nm well corresponds to its excitation spectrum (Gaft et al. 2005). The weak line in NIR spectral range may be ascribed to Nd3þ luminescence.
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