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Reabsorption lines of molecular oxygen and water in natural apatite
July 1997
ELSEVIER
Optical Materials 8 (1997) 143-148
Reabsorption lines of molecular oxygen and water in natural apatite Michael Gaft a,*, Renata Reisfeld b,1, Gerard Panczer c, Shlomo Shoval a, Bernard Champagnon c a Physics and Geology Groups, The Open University oflsrael, 16 Klausner St., Ramat-Aviv, 61392 Tel-Aviv, Israel b Department of Inorganic andAnalytical Chemistry, The Hebrew University of Jerusalem, Givat-Ram, 91904 Jerusalem, Israel c Physico-Chimie des Mat£riaux Luminescents, Universit£ Claude Bernard Lyon L UMR 5620, CNRS, 43 Bd. du 11 November 1918, F-69622 Villeurbanne, France Received 25 November 1996; revised 31 January 1997; accepted 1 May 1997
Abstract An atmospheric absorption system of molecular oxygen and water has been detected and identified in apatites. Such a phenomenon is firstly described in solids. Our results confirm that molecular oxygen and water may be accommodated by structural incorporation in the channels of the apatite structure. A reabsorption spectrum represents a new spectroscopic tool which gives direct evidence of such existence. © 1997 Elsevier Science B.V.
1. Introduction Luminescent and optical properties of natural and artificial apatite have been the subject of numerous investigations. The synthetic haloapatite was one of the first phosphors applied for fluorescent lamps. Synthetic apatites activated by REE and Mn 5+ are used as potential laser materials. Application of the laser action of rare-earth doped oxyapatite single crystals to microchip lasers have been investigated. Apatite is considered as possible migration barrier in nuclei waste storage [l-4].
The laser-induced luminescence spectra of natural apatite contain lines and bands of REE 3÷, Mn 2+, Mn 5+, (UO2) 2÷. Its luminescent properties have found many applications such as prospecting by lidars or ore enrichment by radiometric sorting [5,6]. In this paper, we report on the reabsorption lines in laser-induced luminescent spectra of natural fluor-apatite Cas(POa)3(F, C1, OH) and carbonatefluor apatite (francolite) Cas(PO 4, CO3)3(F, C1, OH) which are here detected and identified for the first time.
2. Experimental * Corresponding author. Fax: +972 3 6460582; e-mail: michael @ shaked.openu.ac.il. I Enrique Berman Professor of Solar Energy; member of the Farcas Center for Light Induced Processes.
The luminescence spectra were investigated under excimer UV laser (308 nm) excitation which deliver
00925-3467/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 5 - 3 4 6 7 ( 9 7 ) 0 0 0 4 0 - 2
144
M. Gaff et al. / Optical Materials 8 (1997) 143-148
pulses of 10 ns duration and 0.1 cm - l spectral width. The pulse energy has been maintained to about 80 mJ. The luminescence observed at the geometry of 90 ° was analyzed with a Hilger and Watts monochromator with a grating of 1200 g r o o v e s / m m blazed at 500 nm. The luminescence in the range of 400-900 nm was detected by a fast response GaAs photomultiplier (RCA 31034) and the signal was fed into a Canberra multichannel analyzer for the lifetime data. For the short (less then 1 /zs) fluorescence spectroscopy the signal was fed into a SR250 Boxcar and digitized, while the decay time was measured by a Lecroy 9410 digital oscilloscope. The experimental set-up was controlled by a PC computer. All samples were investigated at room (300 K) and liquid nitrogen (77 K) temperatures. The emission spectra are corrected according to the spectral function of the equipment.
The following results allow us to conclude that these lines are not connected with noise or artifacts: - the spectral features are 'negative', thus they may not be connected with second order lines or incidental source of light; - the spectrum is presented without any smoothing or other mathematical treatment. It is clearly seen that negative lines are much stronger then the noise. Besides that, the negative lines are always situated at the same places and the invariability of the spectral positions provides the evidence that they are not connected with fluctuations of the laser pulses and detection system.
3. Results
This phenomenon can be attributed to the absorption of the broad luminescence band with long decay time of 5 - 6 ms arising probably from Fe 3+. It will be shown later that the characteristic absorption belongs to molecular oxygen and water. Our conclusions are based on the following facts. The optical absorption spectra of natural apatites in the range 600-900 nm contain several lines and bands connected with Nd 3+, Pr 3+, Mn 5+, SO 3 [7], but they do not coincide with negative lines detected in our study. The closely situated Nd 3+ lines are near 740 and not 760 nm and the other characteristic lines are also absent. The reabsorption lines are very narrow and a part of them may be connected with f - f transitions of actinides. It is known that they are also present in the apatite lattice, namely, U 4+ which have been detected by EPR [7]. The reabsorption of this center is known in zircon, but with the strongest line at 656 nm [8]. The strongest lines at 600 and 660 nm are detected in the absorption spectrum of LiYF4 activated by U 4+ [9]. It is interesting to note that these lines are also present in the reabsorption spectrum (Fig. l) and may be connected with U 4+. The optical spectroscopy data connected with other minerals and solids have also been checked, but all attempts were unsuccessful. Nevertheless, these absorption lines, though not previously men-
The laser-induced time-delayed (more then 1 /zs) luminescence spectra of magmatic and sedimentary apatites contain several 'negative' lines at the red part of the visible spectrum (Fig. 1). The correlation analyses reveals that this group is subdivided into two: - the strongest line at 760 nm accompanied by the weaker line at 687 nm; - a doublet at 720 nm accompanied by a triplet at 823 nm. 10
z~ o~ ~)
760
,
400
i 500
,
i 600
,
i
700
i
763
i
800
i
900
nm
Fig. 1. Reabsorption lines in the laser-induced luminescence of apatite with long decay time of more then 1 /xs (Ae, c = 308 nm,
r = 3 0 0 K).
4. Discussion
4.1. Spectroscopic interpretation of reabsorption bands
M. Gaff et al. / Optical Materials 8 (1997) 143-148
tioned in solids, are well known as the visible light absorption in the atmosphere [10]. The strongest absorption lines of molecular oxygen are named as A-band, or 760 nm, and B-band, or 687 nm. The 760 nm band is very famous in astronomy, because its presence in the atmosphere is used as a test of photosynthetic activity on a distant planet. The strongest absorption lines of water in the visible range consist of a doublet at 718 and 729 nm and a triplet at 818, 823 and 829 nm which exactly coincides with reabsorption lines in apatite. Their is a striking identity between the reabsorption spectrum of the apatite and the absorption spectrum of the atmosphere determined with high spectral resolution and signal to noise ratio [ 11,12]. Not only the strong lines, but also the weak ones are the same. Even the splitting of the A-band is similar in both cases. It consists of the narrower and more intensive line situated at 760 nm and the broader and less intensive one situated at 763 nm (Fig. 2). The absorption of molecular oxygen and water in the air is very weak. It is explained by the energy levels schemes of molecular oxygen and water [13,14]. The corresponding electron transitions to excited states which give rise to absorption bands of molecular oxygen and water are forbidden on the bases of spin and symmetry. The low transition probability is reflected in the long radiative lifetime of 7 s f r o m t h e l Eg+ state of molecular oxygen. Why is in such case the absorption in apatite relatively strong? The possible explanation is that this forbiddeness, strictly observed in the spectra of free molecules, is less stringent inside crystals where forbidden transitions often occur owing to interaction with heavy metal impurities, such as U, Fe, Mn, which are responsible for spin-orbit coupling relaxing and to some extent the forbiddeness of the spinforbidden transitions. It is worthwhile to note that the long lifetimes are in the limit of the zero pressure and may be considerably modified even by perturbations from surrounding gases [13]. The next interesting problem is why reabsorption lines disappear at liquid nitrogen temperature? In emission-reabsorption processes both centers behave as independent systems and do not interact directly. Thus, energy migration is not temperature dependent. The possible explanation is that at liquid nitrogen temperature (77 K) the oxygen and water exist
145
not as gas and liquid, but as liquid and solid, correspondingly. It is known that in the spectra of condensed or compressed oxygen there are bands that are not due to transitions of isolated 02 molecules. They are attributed to transitions in molecular complexes, perhaps short-lived collision complexes. The strongest band near 633 nm is attributed to the (0-0) transition associated with simultaneous excitation of two 3E~- oxygen molecules to the lag state [15]. We do not see such lines at 77 K. Another possibility is that the interaction between molecular oxygen and water may be decreased at lower temperature and electron transitions remain strictly forbidden. Thus it is possible to conclude that the optical active centers in apatite which are responsible for the reabsorption lines are molecular oxygen and water. 4.2. Crystallochemical considerations
Apatite structure is characterized by the existence of channels, running along the c-axis of the hexagonal structure. It has been proposed that the space existing in the channels of non-stoichiometric apatites may be available for molecular trapping. The most evident example is given by oxygen containing apatites. It has been shown, in the case of phosphor-calcium apatites, that they occupy all the available space of the channels and their maximum amount is approximately 0.5 tool%. Water has been suggested to exist in vacancies of the channels. However, quantitative data are scarce because of the difficulty to distinguish molecules inside the crystal and molecules adsorbed on its surface [16]. Besides that, investigation of E u 3+ luminescence in different kinds of artificial apatites leads to the analogous conclusion. It is apparent that the charge compensating species for the trivalent rare-earth elements occupying the divalent calcium may appear in the form of halogen being replaced by oxygen. However, the profound dependence of the Eu 3+ spectra on the type of halogen ion involved in apatites makes to conclude that the halogen in the second type of calcium position remains intact. Hence, it is suggested that the charge compensating species, namely free oxygen, should occur as an interstitial in the vicinity of the E u 3+ center [17]. The strong connection of molecular oxygen and water with apatite is confirmed by thermal behavior
I11
o
o
s
o
o
~o
m C)
I
°l
o
~o
I~d~ce
I W / l l r m ! .urn) 0
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O)
~ ÷
v
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Q-
rff,>
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O
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,
'
v
-"s
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O
O) (J1 O)
CA)
O)
I
r~ r~
M. Gaft et al. / Optical Materials 8 (1997) 143-148
W• '
46 <
8
, x=l~5ns
'
,
,
"-~
13
<
N
r-
4
2 ,
40(
I
500
,
I
600
,
I
--v-v?~r,
700
8
'/
6
~
4
500
600
700
800
500
600
700
800
= 20 ns
O9 C 2
2 0 400
4(
6
, Ce3+
e<
2 8
10
c~ c
t---
e8
tt-
147
500
600
700
nm Fig. 3. Laser-induced luminescence spectra with short decay (less then 1 /xs) of Ce 3+ in magmatic apatite (lower) and supposedly H 2 0 in sedimentary apatite (upper) (hex c = 308 nm).
of reabsorption lines mentioned above. The high thermostability up to 800°C may testify to structural incorporation by apatite lattice. The lower thermostability up to 400°C is well corresponding with thermodesorption spectra of many minerals, where chemically adsorbed molecular oxygen leaves the mineral's surface at approximately this temperature [18]. Thus the spectroscopic conclusion is in accordance with the crystallochemistry of apatite, namely with possible accommodation of molecular oxygen and water in different ways: by structural incorporation and by adsorption. 4.3. Luminescence of molecular oxygen and water
Besides reabsorption lines, several narrow luminescence lines appear in oxygen containing apatites
400
nm Fig. 4. Reabsorption lines of molecular oxygen and water in laser-induced luminescence spectra with long decay times of more then 1 p~s in natural diamonds (Aexc = 308 nm).
with the strongest one at 703 nm. The interpretation attempts using traditional approaches (rare earth elements, actinides) have been unsuccessful. Nevertheless, within the framework of oxygen model they may be ascribed. The spectrum of the red chemiluminescence of molecular oxygen has been studied by several researches. A prominent line at 703 nm has been observed, as well as weaker lines at 633, 578, 762 and 786 nm [19]. The strongest line at 703 nm has been connected with double molecule transitions, two molecules with one photon, from the ground and first vibrational state of the lag level. The luminescence of molecular oxygen in a solid matrix, namely solid krypton, is also known [20]. It is characterized by narrow lines at 620, 676 and 741 nm and a strong green band with a maximum at 560 nm. It is interesting to note that the narrow line at
Fig. 2. Reabsorption lines in the laser-induced spectrum of apatite (upper) and absorption spectrum of the atmosphere (lower) (from Ref. [11,12]).
148
M. Gaff et al. / Optical Materials 8 (1997) 143-148
620 nm is also encountered in the apatite laser-induced luminescence spectra. The green luminescence band may be hidden in our case by uranyl luminescence. Apatites and francolites are often characterized by violet-blue laser-induced luminescence with short decay times. It is known that in apatite it is connected with Ce 3÷ [5]. Nevertheless, in the case of francolite the spectrum is different (Fig. 3). By analogy with violet-blue luminescence of zeolites and ice [21] it is possible to suppose that this luminescence is connected with adsorbed water molecules.
4.4. Other minerals Several other minerals have also been checked on the presence of molecular oxygen and water reabsorption. At present time, besides apatite, they are detected only in one, but rather important, mineral: diamond. The intensity of lines varies strongly from sample to sample, but sometimes they are relatively strong (Fig. 4). The problem needs farther clarification but the determination of the form of oxygen incorporation may be interesting for the discussion of diamond genesis.
5. Conclusions
5.1. Spectroscopic An atmospheric absorption system of molecular oxygen and water is detected and interpreted in sedimentary and magmatic apatites. Such a phenomenon is firstly described in minerals and, according to our knowledge, in solids in general.
5.2. Crystallochemical The results confirm the existing ideas that molecular oxygen and water may be accommodated by
structural incorporation in the channels of the apatite structure. The reabsorption spectrum represents a new spectroscopic tool which gives direct evidence of such existence.
References [1] R. Moncorge, H. Manaa, G. Boulon, Opt. Mater. 4 (1994) 139. [2] S. Payne, W. Krupke, The International Conference on Luminescence, Connecticut M3C- 1, 1993. [3] J. Aubert, C. Wyon, C. Borel, The French-Israeli Workshop on Apatites and Lasers, Jerusalem, December, 1996. [4] P. Martin, A. Chevarier, N. Chevaruier, G. Panczer, The French-Israeli Workshop on Apatites and Lasers, Jerusalem, 1996. [5] R. Reisfeld, M. Gaft, G. Boulon, G. Panczer, C. Jorgensen, J. Lumin. 69 (1996) 343. [6] M. Gaft, B. Pregerson, J. Rabinovitz, Rev. Chem. Eng. 9 (1993) 267. [7] L. Gilinskaya, R. Mashkovtsev, 16 General Meeting Int. Miner. Assoc., vol. 145, 1994. [8] M. Gaft, J. Therm. Anal. 40 (1993) 67. [9] S. Hubert, E. Simoni, M. Louis, W. Zhang, J. Gesland, J. Lumin. 60/61 (1994) 245. [10] J. Peixoto, A. Oort, Physics of Climate, American Institute of Physics, 1993, p. 93. [11] A. Kuze, K. Chance, J. Geophys. Res. 99 (1994) 14. [12] A. Kuze, K. Chance, J. Geophys. Res. 99 (1994) 481. [13] L. Wallace, D. Hnnten, J. Geophys. Res. 73 (1968) 4813. [14] G. Castellan, Physical Chemistry, Addison-Wesley, 1993, p. 646. [15] E. Ogryzlo, J. Chem. Educ. 42 (1965) 647. [16] C. Rey, Phosphorus Res. Bull. 1 (1991) 1. [17] R. Jagannathan, M. Kottaisamy, Phys. Condens. Matter 7 (1995) 8453. [18] O. Kotova, Surface Processes in Physical Separation Methods, Russian Acad. of Science, 1993 (in Russian). [19] A. Khan, M. Kasha, J. Am. Chem. Soc. 92 (1970) 92. [20] L. Berg, P. Lindblom, T. Olsson, Phys. B: Mol. Opt. Phys. 27 (1994) 5241. [21] A. Taraschan, Luminescence of Minerals, Naukova Dumka, 1978 (in Russian).
ELSEVIER
Optical Materials 8 (1997) 143-148
Reabsorption lines of molecular oxygen and water in natural apatite Michael Gaft a,*, Renata Reisfeld b,1, Gerard Panczer c, Shlomo Shoval a, Bernard Champagnon c a Physics and Geology Groups, The Open University oflsrael, 16 Klausner St., Ramat-Aviv, 61392 Tel-Aviv, Israel b Department of Inorganic andAnalytical Chemistry, The Hebrew University of Jerusalem, Givat-Ram, 91904 Jerusalem, Israel c Physico-Chimie des Mat£riaux Luminescents, Universit£ Claude Bernard Lyon L UMR 5620, CNRS, 43 Bd. du 11 November 1918, F-69622 Villeurbanne, France Received 25 November 1996; revised 31 January 1997; accepted 1 May 1997
Abstract An atmospheric absorption system of molecular oxygen and water has been detected and identified in apatites. Such a phenomenon is firstly described in solids. Our results confirm that molecular oxygen and water may be accommodated by structural incorporation in the channels of the apatite structure. A reabsorption spectrum represents a new spectroscopic tool which gives direct evidence of such existence. © 1997 Elsevier Science B.V.
1. Introduction Luminescent and optical properties of natural and artificial apatite have been the subject of numerous investigations. The synthetic haloapatite was one of the first phosphors applied for fluorescent lamps. Synthetic apatites activated by REE and Mn 5+ are used as potential laser materials. Application of the laser action of rare-earth doped oxyapatite single crystals to microchip lasers have been investigated. Apatite is considered as possible migration barrier in nuclei waste storage [l-4].
The laser-induced luminescence spectra of natural apatite contain lines and bands of REE 3÷, Mn 2+, Mn 5+, (UO2) 2÷. Its luminescent properties have found many applications such as prospecting by lidars or ore enrichment by radiometric sorting [5,6]. In this paper, we report on the reabsorption lines in laser-induced luminescent spectra of natural fluor-apatite Cas(POa)3(F, C1, OH) and carbonatefluor apatite (francolite) Cas(PO 4, CO3)3(F, C1, OH) which are here detected and identified for the first time.
2. Experimental * Corresponding author. Fax: +972 3 6460582; e-mail: michael @ shaked.openu.ac.il. I Enrique Berman Professor of Solar Energy; member of the Farcas Center for Light Induced Processes.
The luminescence spectra were investigated under excimer UV laser (308 nm) excitation which deliver
00925-3467/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 5 - 3 4 6 7 ( 9 7 ) 0 0 0 4 0 - 2
144
M. Gaff et al. / Optical Materials 8 (1997) 143-148
pulses of 10 ns duration and 0.1 cm - l spectral width. The pulse energy has been maintained to about 80 mJ. The luminescence observed at the geometry of 90 ° was analyzed with a Hilger and Watts monochromator with a grating of 1200 g r o o v e s / m m blazed at 500 nm. The luminescence in the range of 400-900 nm was detected by a fast response GaAs photomultiplier (RCA 31034) and the signal was fed into a Canberra multichannel analyzer for the lifetime data. For the short (less then 1 /zs) fluorescence spectroscopy the signal was fed into a SR250 Boxcar and digitized, while the decay time was measured by a Lecroy 9410 digital oscilloscope. The experimental set-up was controlled by a PC computer. All samples were investigated at room (300 K) and liquid nitrogen (77 K) temperatures. The emission spectra are corrected according to the spectral function of the equipment.
The following results allow us to conclude that these lines are not connected with noise or artifacts: - the spectral features are 'negative', thus they may not be connected with second order lines or incidental source of light; - the spectrum is presented without any smoothing or other mathematical treatment. It is clearly seen that negative lines are much stronger then the noise. Besides that, the negative lines are always situated at the same places and the invariability of the spectral positions provides the evidence that they are not connected with fluctuations of the laser pulses and detection system.
3. Results
This phenomenon can be attributed to the absorption of the broad luminescence band with long decay time of 5 - 6 ms arising probably from Fe 3+. It will be shown later that the characteristic absorption belongs to molecular oxygen and water. Our conclusions are based on the following facts. The optical absorption spectra of natural apatites in the range 600-900 nm contain several lines and bands connected with Nd 3+, Pr 3+, Mn 5+, SO 3 [7], but they do not coincide with negative lines detected in our study. The closely situated Nd 3+ lines are near 740 and not 760 nm and the other characteristic lines are also absent. The reabsorption lines are very narrow and a part of them may be connected with f - f transitions of actinides. It is known that they are also present in the apatite lattice, namely, U 4+ which have been detected by EPR [7]. The reabsorption of this center is known in zircon, but with the strongest line at 656 nm [8]. The strongest lines at 600 and 660 nm are detected in the absorption spectrum of LiYF4 activated by U 4+ [9]. It is interesting to note that these lines are also present in the reabsorption spectrum (Fig. l) and may be connected with U 4+. The optical spectroscopy data connected with other minerals and solids have also been checked, but all attempts were unsuccessful. Nevertheless, these absorption lines, though not previously men-
The laser-induced time-delayed (more then 1 /zs) luminescence spectra of magmatic and sedimentary apatites contain several 'negative' lines at the red part of the visible spectrum (Fig. 1). The correlation analyses reveals that this group is subdivided into two: - the strongest line at 760 nm accompanied by the weaker line at 687 nm; - a doublet at 720 nm accompanied by a triplet at 823 nm. 10
z~ o~ ~)
760
,
400
i 500
,
i 600
,
i
700
i
763
i
800
i
900
nm
Fig. 1. Reabsorption lines in the laser-induced luminescence of apatite with long decay time of more then 1 /xs (Ae, c = 308 nm,
r = 3 0 0 K).
4. Discussion
4.1. Spectroscopic interpretation of reabsorption bands
M. Gaff et al. / Optical Materials 8 (1997) 143-148
tioned in solids, are well known as the visible light absorption in the atmosphere [10]. The strongest absorption lines of molecular oxygen are named as A-band, or 760 nm, and B-band, or 687 nm. The 760 nm band is very famous in astronomy, because its presence in the atmosphere is used as a test of photosynthetic activity on a distant planet. The strongest absorption lines of water in the visible range consist of a doublet at 718 and 729 nm and a triplet at 818, 823 and 829 nm which exactly coincides with reabsorption lines in apatite. Their is a striking identity between the reabsorption spectrum of the apatite and the absorption spectrum of the atmosphere determined with high spectral resolution and signal to noise ratio [ 11,12]. Not only the strong lines, but also the weak ones are the same. Even the splitting of the A-band is similar in both cases. It consists of the narrower and more intensive line situated at 760 nm and the broader and less intensive one situated at 763 nm (Fig. 2). The absorption of molecular oxygen and water in the air is very weak. It is explained by the energy levels schemes of molecular oxygen and water [13,14]. The corresponding electron transitions to excited states which give rise to absorption bands of molecular oxygen and water are forbidden on the bases of spin and symmetry. The low transition probability is reflected in the long radiative lifetime of 7 s f r o m t h e l Eg+ state of molecular oxygen. Why is in such case the absorption in apatite relatively strong? The possible explanation is that this forbiddeness, strictly observed in the spectra of free molecules, is less stringent inside crystals where forbidden transitions often occur owing to interaction with heavy metal impurities, such as U, Fe, Mn, which are responsible for spin-orbit coupling relaxing and to some extent the forbiddeness of the spinforbidden transitions. It is worthwhile to note that the long lifetimes are in the limit of the zero pressure and may be considerably modified even by perturbations from surrounding gases [13]. The next interesting problem is why reabsorption lines disappear at liquid nitrogen temperature? In emission-reabsorption processes both centers behave as independent systems and do not interact directly. Thus, energy migration is not temperature dependent. The possible explanation is that at liquid nitrogen temperature (77 K) the oxygen and water exist
145
not as gas and liquid, but as liquid and solid, correspondingly. It is known that in the spectra of condensed or compressed oxygen there are bands that are not due to transitions of isolated 02 molecules. They are attributed to transitions in molecular complexes, perhaps short-lived collision complexes. The strongest band near 633 nm is attributed to the (0-0) transition associated with simultaneous excitation of two 3E~- oxygen molecules to the lag state [15]. We do not see such lines at 77 K. Another possibility is that the interaction between molecular oxygen and water may be decreased at lower temperature and electron transitions remain strictly forbidden. Thus it is possible to conclude that the optical active centers in apatite which are responsible for the reabsorption lines are molecular oxygen and water. 4.2. Crystallochemical considerations
Apatite structure is characterized by the existence of channels, running along the c-axis of the hexagonal structure. It has been proposed that the space existing in the channels of non-stoichiometric apatites may be available for molecular trapping. The most evident example is given by oxygen containing apatites. It has been shown, in the case of phosphor-calcium apatites, that they occupy all the available space of the channels and their maximum amount is approximately 0.5 tool%. Water has been suggested to exist in vacancies of the channels. However, quantitative data are scarce because of the difficulty to distinguish molecules inside the crystal and molecules adsorbed on its surface [16]. Besides that, investigation of E u 3+ luminescence in different kinds of artificial apatites leads to the analogous conclusion. It is apparent that the charge compensating species for the trivalent rare-earth elements occupying the divalent calcium may appear in the form of halogen being replaced by oxygen. However, the profound dependence of the Eu 3+ spectra on the type of halogen ion involved in apatites makes to conclude that the halogen in the second type of calcium position remains intact. Hence, it is suggested that the charge compensating species, namely free oxygen, should occur as an interstitial in the vicinity of the E u 3+ center [17]. The strong connection of molecular oxygen and water with apatite is confirmed by thermal behavior
I11
o
o
s
o
o
~o
m C)
I
°l
o
~o
I~d~ce
I W / l l r m ! .urn) 0
Q O
0
0 O
O
O)
~ ÷
v
3
"s
"M
Q-
rff,>
I
O
(C)
,
'
v
-"s
0"
O
O) (J1 O)
CA)
O)
I
r~ r~
M. Gaft et al. / Optical Materials 8 (1997) 143-148
W• '
46 <
8
, x=l~5ns
'
,
,
"-~
13
<
N
r-
4
2 ,
40(
I
500
,
I
600
,
I
--v-v?~r,
700
8
'/
6
~
4
500
600
700
800
500
600
700
800
= 20 ns
O9 C 2
2 0 400
4(
6
, Ce3+
e<
2 8
10
c~ c
t---
e8
tt-
147
500
600
700
nm Fig. 3. Laser-induced luminescence spectra with short decay (less then 1 /xs) of Ce 3+ in magmatic apatite (lower) and supposedly H 2 0 in sedimentary apatite (upper) (hex c = 308 nm).
of reabsorption lines mentioned above. The high thermostability up to 800°C may testify to structural incorporation by apatite lattice. The lower thermostability up to 400°C is well corresponding with thermodesorption spectra of many minerals, where chemically adsorbed molecular oxygen leaves the mineral's surface at approximately this temperature [18]. Thus the spectroscopic conclusion is in accordance with the crystallochemistry of apatite, namely with possible accommodation of molecular oxygen and water in different ways: by structural incorporation and by adsorption. 4.3. Luminescence of molecular oxygen and water
Besides reabsorption lines, several narrow luminescence lines appear in oxygen containing apatites
400
nm Fig. 4. Reabsorption lines of molecular oxygen and water in laser-induced luminescence spectra with long decay times of more then 1 p~s in natural diamonds (Aexc = 308 nm).
with the strongest one at 703 nm. The interpretation attempts using traditional approaches (rare earth elements, actinides) have been unsuccessful. Nevertheless, within the framework of oxygen model they may be ascribed. The spectrum of the red chemiluminescence of molecular oxygen has been studied by several researches. A prominent line at 703 nm has been observed, as well as weaker lines at 633, 578, 762 and 786 nm [19]. The strongest line at 703 nm has been connected with double molecule transitions, two molecules with one photon, from the ground and first vibrational state of the lag level. The luminescence of molecular oxygen in a solid matrix, namely solid krypton, is also known [20]. It is characterized by narrow lines at 620, 676 and 741 nm and a strong green band with a maximum at 560 nm. It is interesting to note that the narrow line at
Fig. 2. Reabsorption lines in the laser-induced spectrum of apatite (upper) and absorption spectrum of the atmosphere (lower) (from Ref. [11,12]).
148
M. Gaff et al. / Optical Materials 8 (1997) 143-148
620 nm is also encountered in the apatite laser-induced luminescence spectra. The green luminescence band may be hidden in our case by uranyl luminescence. Apatites and francolites are often characterized by violet-blue laser-induced luminescence with short decay times. It is known that in apatite it is connected with Ce 3÷ [5]. Nevertheless, in the case of francolite the spectrum is different (Fig. 3). By analogy with violet-blue luminescence of zeolites and ice [21] it is possible to suppose that this luminescence is connected with adsorbed water molecules.
4.4. Other minerals Several other minerals have also been checked on the presence of molecular oxygen and water reabsorption. At present time, besides apatite, they are detected only in one, but rather important, mineral: diamond. The intensity of lines varies strongly from sample to sample, but sometimes they are relatively strong (Fig. 4). The problem needs farther clarification but the determination of the form of oxygen incorporation may be interesting for the discussion of diamond genesis.
5. Conclusions
5.1. Spectroscopic An atmospheric absorption system of molecular oxygen and water is detected and interpreted in sedimentary and magmatic apatites. Such a phenomenon is firstly described in minerals and, according to our knowledge, in solids in general.
5.2. Crystallochemical The results confirm the existing ideas that molecular oxygen and water may be accommodated by
structural incorporation in the channels of the apatite structure. The reabsorption spectrum represents a new spectroscopic tool which gives direct evidence of such existence.
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