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Nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance
Journal of Physics and Chemistry of Solids 61 (2000) 1623–1632 www.elsevier.nl/locate/jpcs
Nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance Y. Bensimon a,*, B. Deroide a, F. Dijoux a, M. Martineau b a
Laboratoire de Physico-chimie de la Matie`re Condense´e, UMR CNRS 5617, Universite´ de Montpellier II, Place Euge`ne Bataillon, 34095 Montpellier Cedex 5, France b Institut Universitaire de Technologie, 99, avenue d’Occitanie, 34075 Montpellier Cedex, France Received 2 December 1999; accepted 3 February 2000
Abstract A natural clay of South of France is studied by electron spin resonance (ESR). Two radiation-induced defects are revealed, certainly localized in the quartz contained in clay. The first is the peroxy-center, associated with oxygen, formed by the action of an O22 ion on a silicon atom to lead to Si–O–O z. The second is the E 0 -center, where an electron was trapped in oxygen vacancy to give xSi z. The spectroscopic parameters were determined. The thermal stability of these defects is studied by isothermal annealing experiments. Second order kinetics best explain the results, although more complex mechanisms must occur. The two defects have very different behaviors: the peroxy-center seems more stable at high temperature than the E 0 -center, although its activation energy is much weaker. They could be both used for the ESR dating of old clay. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electron spin resonance; Clay; D. Defects; Thermal stability
1. Introduction Electron spin resonance (ESR) is a technique very much used in physicochemistry of solid and biology. Its use extends now in the field of geological and archaeological sciences. Archaeology uses this technique more and more particularly for the ESR dating of lithic tools and the knowledge of their prehistoric heating conditions. These archaeological materials indeed contain species, which give an ESR signal. They can be transition metal ions like Fe 31 or Mn 21 which have unpaired electrons and which give ESR spectra to fine or hyperfine structures, the spectrum being strongly influenced by the paramagnetic centers’ environment [1]. An external phenomenon, like a thermal process, can involve chemical reactions such as an oxydoreduction or dehydration and can shift the signals and modify its form. It allows to specify the influence of the
* Corresponding author. Present address: E.N.S.C.M., 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France. Tel.: 1 334-67-147-257; fax: 1 33-4-67-144-353. E-mail address: [email protected] (Y. Bensimon).
heating effects on old materials [2–5]. Elsewhere the main part of the old objects contains traces of organic compounds. The latter break up with temperature and can produce another type of paramagnetic defect resulting from pyrolysis, such as for example, the homolytic break of C–C-bond to lead to the radical carbon. The evolution of this radical depends on the temperature and the pressure of oxygen. Thus, the observation of the ESR spectra evolution of these materials give information on their thermal history [2,6,7–10]. Finally a third type of paramagnetic center can be observed in old materials: radiation-induced defects generated by natural radioactivity and cosmic radiation effects on these non-conductive solids. The latter, submitted to high energies, have some electrons ejected of their energy level that leads to the formation of trapped electrons and holes. The majority recombine quickly but few remain trapped in well-defined sites within the lattice and form paramagnetic centers. These centers are very stable at room temperature but disappear quickly on heating [11,12]. They are used for the ESR dating of various old objects which were heated at one time of their history [11,13–16] according to the principle that the older the object is, the more numerous these centers will be. This
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(00)00048-2
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technique allows us to obtain the age of the geological material that was never heated [14]. This paper relates to this last type of center (radiationinduced centers) which are present in common clay of the South of France. As the majority of natural clays, it contains quartz [17]. Quartz is a good reserve of paramagnetic defects [18] that appear under the effect of radioactive radiation. These defects are allotted to the E 0 -centers associated with silicon (xSi z; “ z” indicate an unpaired electron) [19– 21], to the “oxygen hole center” (Si–Ox z) and the peroxycenters (xSi–O–O z) [22–24], to the centers associated with aluminum [16,25,26]. These centers are very well observed with the ESR technique when they are contained in pure quartz or silica. However, they pose a problem in clays because of the interference of a very broad and very intense signal allotted to iron [27]. Indeed, clays generally contain iron traces [28–30] in sufficient quantity to generate an intense ESR signal. Paramagnetism is due to the Fe 31 species, which has an electronic spin S 5=2; sometimes responsible for complex absorption bands. Thus, in the X-band (9,8 GHz), an ESR spectrum of a clay reveals two broad signals at g 4 and g 2 [31] and a signal much thinner about g 2 [32]. The first two are due to the Fe III species in different environments. Resonance at g 4 comes from Fe 31 located in the orthorhombic sites [2]. The very broad signal located around g 2 is allotted to the hydrated Fe 31 species that give rise to the FexOy or FeOOH oxides [33]. The thin signal at g 2; which relates to natural radiation-induced defects, is often not well defined and its interpretation is not easy in raw clay. In the studied sample, the two centers responsible for the signal due to natural radiation-induced defects were identified. The study of their thermal stability at various temperatures allows us to obtain the energy parameters that characterize them. Let us specify that these defects exist in a natural state in this clay and that the latter was thus submitted to no irradiation.
2. Experimental 2.1. Samples The clay used was taken out in a layer at Argelliers, near Montpellier, South of France. It is a common silicoaluminous clay belonging to the smectites family, which was used for a long time for potteries. It contains, expressed as a percentage mass, silicon 32%, aluminum 9% and iron 4.5% [34]. First of all, the samples were washed with a 15% H2O2 solution (clay 1 g for 50 cm 3 of solution) for 24 h and at 508C in order to eliminate the organic impurities. Several washings with water then eliminate H2O2. The clay is then dried at 608C after sedimentation and filtration. The ESR analyses on raw clay are carried out after this first treatment.
In order to eliminate most of non-structural iron, clay is then treated by a 6 mol l 21 solution of HCl (clay 1 g for 50 cm 3 of solution) for 2 h and at 508C. After this treatment, clay contains nothing but 0.9% of iron [34]. Several washings with water are then necessary to eliminate the acid and the formed iron complexes. The sample is then dried at 608C after sedimentation and filtration. The ESR analyses are carried out on the treated clay after this second treatment. 2.2. ESR experiments The establishment of the ESR spectra in the X-band (9,8 GHz) was made using a spectrometer Bruker ER100 by adopting a modulation frequency of 100 kHz whose amplitude was fixed at 0.4 mT for the broad lines and was lowered up to 0.05 mT for the narrow lines. The ESR spectrum intensity of the DPPH standard, recorded under the same conditions of modulation, was measured to calibrate the intensity of each signal. The ESR signals were located in experiments by their g-value defined by geff
hn ; bB
where h is the Planck’s constant, n the resonance frequency, b the Bohr magneton, B the magnetic field at which resonance occurs. These values are calibrated by comparison with the DPPH standard g 2:0036: The spectra are calculated by means of a program elaborated at the laboratory [35] and allows us to obtain the parameters values gx, gy and gz and the linewidths s . 2.3. Isothermal annealing experiments To carry out the decay kinetic study of the signals at defined temperature, for each experiment, 100 mg of the treated clay are introduced into a quartz tube. This tube was put in a furnace connected to a power supply, controlled in order to have a well-defined temperature. At the end of each heating time, the tube is immersed in a cold water bath in order to stop the reaction. The recording of the ESR spectrum is carried out at room temperature and the signal intensity is then measured. These experiments were carried out in air at 350, 450 and 5008C. Measurements were stopped when the studied signal disappeared completely or did not move any more. 3. Results and discussion 3.1. Analysis of the ESR spectra 3.1.1. The total spectrum The ESR spectrum of Argelliers’ clay (Fig. 1a), presents three distinct signals. As already indicated, signal I at g 4 comes from the Fe 31 species in an orthorhombic environment. Let us note that signal II due to iron oxides present in
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Fig. 1. The X-band ESR spectra of (a) raw clay and (b) acid treated clay recorded at 298 K.
the clay clusters, and not in the material structure, is extremely broad. It interferes very much with signal III, which is much thinner. The lines, due to the paramagnetic centers likely used for dating, appear in this spectrum region, around g 2: These last signals are very thin and in general of low amplitude. Thus they should be partially masked by a signal as intense as signal II. In order to decrease the important interference of signal II with signal III, it was necessary to eliminate most of the iron oxides. For which, raw clay has undergone treatment with acid so that signal III, which includes the radiation defects, is not modified so that the dating possibilities of the sample are maintained. The nature of clay, on the other hand, can be some disturbed, but that has no consequences on the study of these defects. This treatment, carried out using a 6 mol l 21 HCl solution, reveals a major reduction in signal II amplitude and an increased signal III resolution (Fig. 1b). Thus by analyzing the ESR spectrum of clay, in a restricted domain of the magnetic field around g 2; it will be much easier to study the paramagnetic centers of this region.
3.1.2. The peroxy-center To improve the characterization of signal III, the ESR spectrum of raw clay, treated by HCl, was recorded in an interval of reduced magnetic field (Fig. 2, exp). This signal is then well solved and it seems interesting to simulate it. Although it seems to have an axial symmetry, its spectral characteristics were determined according to the orthorhombic symmetry which gives the best results. Calculation was carried out by using a Lorentzian function. The results (Fig. 2, calc) indicate a good agreement between the experimental and calculated spectra. The spectroscopic parameters, the g-tensor and the linewidth s , are given in Table 1. The identification of the species responsible for this signal is carried out from the values gx, gy and gz according to the nature of material. The studied clay, formed of aluminosilicates, also contains quartz [17]. The work undertaken on silicas, showed that, after irradiation, the ESR spectra of these materials revealed the existence of defects associated with oxygen [22]. More precisely two types of
Fig. 2. The X-band experimental and calculated ESR spectra of peroxy-center, recorded at 298 K.
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Table 1 ESR spectroscopic parameters of peroxy and E 0 centers: g-tensor and linewidth s
Peroxy E0
gx
gy
gz
s x (mT)
s y (mT)
s z(mT)
2:0018 ^ 1 × 10 24 2:0003 ^ 1 × 1024
2:0033 ^ 1 × 1024 2:0004 ^ 1 × 1024
2:051 ^ 1 × 1023 2:0017 ^ 11024
1:3 ^ 0:1 0:35 ^ 0:02
1:3 ^ 0:1 0:38 ^ 0:02
1:0 ^ 0:1 0:20 ^ 0:02
paramagnetic centers have been identified [23]. The first is the “nonbridging-oxygen hole center” (NBOHC), which comes from the breaking of the Si–O–Si-bond or the breaking of the OH-bond in Si–OH to lead in both cases to the Si–O z entity. The second is the peroxy radical already identified in silica glasses after irradiation [23,24]. Its comes from the formation of the asymmetrical bond between the O22 ion and a silicon atom to lead to the Si–O–O z entity. According to the values of the g-tensor g x 2:0018; gy 2:0033 and gz 2:051 and the linewidth s , it appears that signal III of Argelliers’ clay is due to a peroxy radical. Indeed, for this last, the actual values in silica glasses vary slightly according to the nature of the irradiation: 2:0018 , g x , 2:0020; 2:0070 , gy , 2:0085; 2:027 , gz , 2:067 [23,24]. One notes, concerning signal III, a value slightly lower for the component gy. This is explained by the very different nature, as well from the studied clay matrix as the rough quartz present. 3.1.3. The E 0 -center Signal III, however, seems to be of composite nature because it is difficult to explain correctly its form at geff 2:0021 (Fig. 2, exp). It could then be that two paramagnetic defects are responsible, the peroxy-center and a center much narrower being in the same magnetic field region. For a better characterization of this very narrow signal, the latest spectrum was recorded between 349.7 and 350.5 mT. An
axial line (Fig. 3, exp) is then underlined. Simulation (Fig. 3, calc) was carried out by using a Lorentzian function and allowed to determine the spectroscopic parameter values (Table 1). Among the defects observed in silica glasses or quartz, the E 0 -centers show spectroscopic characteristics very close to those given herein 2:0003 , gx , 2:0004; 2:0004 , gy , 2:0006; gz 2:0018 [36]. These centers are intrinsic in SiO2. They can be formed by the breaking of the Si–Sibonds to lead to a silicon atom with an unpaired spin according to xSi–Six ! xSi " 1 xSi 1 e– 37 or xSi–Six 1 H ! xSi" 1 xSiH 23: By considering the g values obtained for this line gx 2:0003; gy 2:0004; gz 2:0017 and the very low linewidth, the observed signal can be allotted unambiguously to an E 0 -center. It seems that these two centers, peroxy and E 0 , are described in a natural clay for the first time. Some were observed in potteries [32] and in unburnt soil [2]. They were not underlined during a test of ESR dating of old pottery [38]. For better characterization, their thermal stability was studied to deduce some thermodynamic parameters.
Fig. 3. The X-band experimental and calculated ESR spectra of E 0 -center, recorded at 298 K.
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3.2. Annealing experiments
Fig. 4. Various ESR spectra of peroxy-center observed in 4508C annealing experiment.
For each studied temperature, the study of the decay of the number of peroxy and E 0 centers was carried out by isothermal measuring of each ESR signal intensity. This intensity is measured in arbitrary units. Instead of taking these arbitrary units, the samples were calibrated at the initial time of the decay kinetics by using a standard containing a known number of spins. The standard used is a synthetic sample of lazurite, used like pigment, obtained by heating in a reducing medium of a mixture of sulfates and sulfides of sodium, kaolin and carbonates. This standard contains 8:8 × 1016 spins. By comparing the integral of its absorption with those of the studied centers, the number of peroxy and E 0 centers present at the initial moment in 100 mg of sample have been determined. Measurements on the sample and the standard must be performed under the same modulation conditions to be able to compare absorptions. Thus, 100 mg of sample contain, at the initial time of the decay kinetics, 8:7 × 1015 peroxy-centers and 1:1 × 1013 E 0 -centers. Three temperatures were used to study the isothermal decrease of the number of centers: 350, 450 and 5008C. As an example, we show the evolution of the ESR signal of the peroxy (Fig. 4) and E 0 (Fig. 5) centers, for a few moments, at 4508C temperature. The decrease process of these centers is certainly not simple. It is usually described for the same type of centers by equations of the first or second order [39]. The first order can be described by the equation dN 2lt dt
1
where N is the instantaneous defect number and l the decay constant. By integrating Eq. (1) one obtains N N0 exp 2lt
2
where N0 is the initial number of centers. In this case the N Neperian logarithm decreases linearly with the heating time according to the law ln N ln N0 2 lt
3
The following relation gives the half-life time t :
t
0:69 l
4
For a second order mechanism, in which a center recombines with a complementary center, the law can have the following form: dN 2lN 2 dt
5
By integrating Eq. (5) one obtains the following expression: Fig. 5. Various ESR spectra of the E 0 -center observed in 4508C annealing experiment.
1 1 lt 1 N N0
6
It appears that for a second order reaction, the reciprocal
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Fig. 6. The peroxy-center number as a function of heating time: isothermal annealing experiments were performed at 350, 450 and 5008C.
number of sites is an increasing linear function versus the heating time. The following relation gives the half-life time
t
1 lN0
7
According to the undertaken experiments, it seem that the peroxy and E 0 centers decaying, follows a second order mechanism. Expression (5) applies correctly to their isothermal treatment, in particular during the first part of the process. Then the phenomena are certainly more complicated and the same law cannot be applied any more for the end of the reaction. However, one can completely consider that the beginning of the process follows a second order law for the two studied defects, which is in agreement with the previous observations of the same type of defects [12,40,41]. In any case, the decay constant follows the Arrhenius’s law E 8 l l0 exp 2 a kT
where l 0 is the pre-exponential factor, T the temperature (K), Ea the activation energy, k the Boltzman’s constant 8:6 × 10-5 eV K21 : This last expression can be put under the form ln l ln l0 2
Ea kT
9
The l values are obtained from Eq. (6) and the parameters l 0 and Ea by fitting the linear expression of T 21 in Eq. (9). 3.2.1. Peroxy-center Fig. 6 presents the evolution of the number of peroxycenters as a function of the heating time, for the three studied temperatures. It is noted that the reaction progress increases versus temperature. At 3508C the number of centers do not move any more after 120 min and is maintained at 4:2 × 1015 spins. At 450 and 5008C, the progress of the reaction is much more important. There remains only 15% of sites at 4508C after 267 min and 12% at 5008C after 188 min.
Fig. 7. The ESR peroxy-center reciprocal number versus heating time: the data points lie on straight lines indicating that they are defined by second order decay kinetics.
Y. Bensimon et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1623–1632 Table 2 Kinetic parameters for peroxy and E 0 centers in clay from Argelliers: decay constants at 350, 450, 5008C and at room temperature, activation energies, pre-exponential factor and half-life
l 350 (spins 21 min 21) l 450 (spins 21 min 21) l 500 (spins 21 min 21) Ea (eV) l 0 (spins 21 min 21) l 298 (spins 21 min 21) t (years)
Peroxy
E0
3:18 × 10218 4:56 × 10218 5:38 × 10218 0:14 ^ 0:01 4:68 × 10217 ind. ind.
8:63 × 10217 2:23 × 10214 1:23 × 10213 2:01 ^ 0:01 1:46 × 10234 1:30 × 10221 1:18 × 1015
Fig. 7 shows, for the three studied temperatures, the linear evolution of the reciprocal number of peroxy-centers as a function of the heating time. This law remains valid in an interval of well-defined time. Thus it is no more valid beyond 11 min at 3508C, 43 min at 4508C and 84 min at 5008C. Thus it seems that the more the heating temperatures are raised, the more the second order law applies. By considering the intervals of time when the second order law is valid, expression (6) gives access to the decay constants l by fitting each straight line (Table 2). The relating parameters to the Arrhenius’s equation are obtained by fitting relation (9) (Fig. 8). The obtained Ea value (0.14 eV) seems low according to those observed for other paramagnetic defects, which are between 1.6 and 2.0 eV [41–43]. Nevertheless, the defects and material were different. With such activation energy, one could think that the peroxy-center is easily destroyed even at room temperature. However, it is extremely stable at 298 K. It could be that the kinetic aspect is dominating in its case and that the g irradiation, responsible for its creation, produces a defect in the deep layers of the structure. At this time a chemical reaction, which in general takes place only on the valence layers, would not occur easily. It is difficult, for all these reasons to define a half-life time at 298 K for the peroxy-center. Indeed, by
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considering Fig. 9, which represents time when the second order decay kinetics is valid, versus temperature, one notes that this kinetics is not valid any more at a temperature around 2008C. Under these conditions, it is useless to extrapolate the decay constant value at room temperature and to deduce a half-life time from expression (7). 3.2.2. E 0 -center It is described usually that the E 0 -centers formed by the g irradiation at room temperature, see their number increasing if the material is heated from 1708C to reach a maximum at 3008C [11]. This is explained by the formation of oxygen vacancies during the irradiation which trap an electron and are transformed into E 0 -centers when the material is heated. These latter disappear completely if the material is heated at 4408C [44]. These descriptions are in agreement with the phenomena that are observed herein. Fig. 10 indicates the evolution of the number of E 0 -centers (for 100 mg of clay) as a function of the heating time. One notes, for the three temperatures, an increase in the number of sites during first minutes. Then this number decreases, much less for the temperature of 3508C. The decay kinetics of the E 0 -center were studied by considering that the initial number of sites corresponded to the maximum noted on the curves (Fig. 10). This initial number of sites was calibrated in a way already indicated. It is noticed that at 3508C the number of E 0 centers always remains very important. On the other hand, these centers disappear after 70 min at 4508C and 39 min at 5008C. Fig. 11 shows the reciprocal number of E 0 -centers as a function of the heating time, when a second order decay kinetics occurs. This law is valid for 40 min at 3508C, 23 min at 4508C and 19 min at 5008C. Thus it appears that the lower the temperature is, the more the second order kinetics applies a long time. This result is contrary to the one of the peroxy-center and point out the different nature of the two centers.
Fig. 8. The Arrhenius plots of the decay factors of the peroxy-center: the activation energy and the pre-exponential factor are shown in Table 2 and were obtained from the linear regression fits to the data.
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Fig. 9. Peroxy-center: second order kinetic validity time as an increasing function of the heating temperature.
Fig. 10. The E 0 -center number as a function of heating time: isothermal annealing experiments were performed at 350, 450 and 5008C.
Fig. 11. The ESR E 0 -center reciprocal number versus heating time: the data points lie on straight lines indicating that they are defined by second order decay kinetics.
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Fig. 12. The Arrhenius plots of the decay factors of the E 0 -center: the activation energy and pre-exponential factor are shown in Table 2 and were obtained from the linear regression fits to the data.
Fig. 13. E 0 -center: second order kinetic validity time as an decreasing function of the heating temperature.
These results gave access to the decay constants l for the three temperatures. By fitting the curve (Fig. 12), resulting from expression (9), the pre-exponential factor l 0 and the activation energy Ea are determined (Table 2). The found activation energy (2.01 eV) is in agreement with those already observed for the same types of centers in other materials [12]. Fig. 13 indicates that the time during which the second order decay kinetics remains valid is a function of the temperature. It is well seen that this time is large since the temperature is low. It is then possible to determine the halflife time at room temperature in the case of this experiment from expression (7). The found value (Table 2) indicates that the E 0 -center is extremely stable at room temperature.
4. Conclusion The observed X-band spectra of natural clay, clearly show that two paramagnetic defects are present: the
peroxy-center and the E 0 -center which come from the environmental and cosmic irradiation. They were studied such as they were naturally in clay. They disappear under the temperature effect and their thermal stability was interpreted. The parameters deduced from this study are given. It appears that these two centers have a completely different behavior. Initially the peroxy-center is much more numerous than the E 0 -center, nearly thousand times more. Then, the very weak activation energy, found for the peroxy-center decay, seems to indicate that this last is formed from a modification of deep energy levels. The formation of paramagnetic defects requires much energy. Thus, for example, the formation of E 0 -centers requires 3 × 107 eV=defect in quartz and 10 5 eV/defect in fused silica [21,45]. Therefore this energy is different according to the nature of material and it is also different according to the nature of the center. It appears that, in the studied clay, formation of the peroxy-center and its thermal degradation are closely related to the nature of the material which
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contains it. Moreover, the kinetic aspect, during its heating, seems to predominate the thermodynamic aspect. In any event, its decaying, at low temperatures, certainly concerns complex mechanisms. The E 0 -center has, in this clay, a thermal behavior which corresponds to the already described results. The E 0 -center was formed in the crystalline environment of quartz contained in the clay [17]. Then one finds the behavior of the defects induced by the artificial g irradiation in quartz. In particular, the half-time is higher than 10 15 years. Such a half-life shows that this type of center can be used for the ESR radiation dosimetry on natural clay, at room temperature. References [1] J.R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon, Oxford, 1990. [2] J. Bartoll, A. Tani, M. Ikeya, T. Inada, Appl. Magn. Reson. 11 (1996) 577. [3] A. Tani, J. Barstool, M. Ikeya, K. Komura, H. Kajiwara, Ionics 22 (1996) 3. [4] G.V. Robins, N.J. Seeley, M.C.R. Symons, D.A.C. McNeil, Archaeometry 23 (1981) 103. [5] Y. Matsuoka, M. Ikeya, Jpn. J. Appl. Phys. 34 (1995) 6068. [6] D. Robins, New Scientist 117 (1988) 49. [7] G.C. Hillman, G.V. Robins, A.D. Oduwole, K.D. Sales, D.A.C. McNeil, J. Archaeol. Sci. 12 (1985) 49. [8] K.D. Sales, A.D. Oduwole, J. Convert, G.V. Robins, Archaeometry 29 (1987) 103. [9] G.V. Robins, C. Del Re, N.J. Seeley, A.G. Davies, J.A.-A. Hawari, J. Archeol. Sci. 10 (1983) 385. [10] K.D. Sales, J. Oduwole, G.V. Robins, S. Olsen, Nucl. Tracks 10 (1985) 845. [11] M. Ikeya, New Applications of Electron Spin ResonanceDating, Dosimetry and Microscopy, World Scientific, Singapore, 1993. [12] S. Toyoda, M. Ikeya, Geochem. J. 25 (1991) 437. [13] M. Jonas, Radiat. Meas. 27 (1997) 943. [14] W.J. Rink, Radiat. Meas. 27 (1997) 975. [15] M. Laurent, C. Falgue`res, J.J. Bahain, Y. Yokoyama, C.R. Acad. Sci. Paris 318 (1994) 521.
[16] Y. Yokoyama, C. Falgue`res, J.P. Quaegebeur, Nucl. Tracks Radiat. Meas. 10 (1985) 921. [17] Y. Bensimon, B. Deroide, S. Clavel, J.V. Zanchetta, Jpn. J. Appl. Phys. 37 (1998) 4367. [18] E.V. Garrison, Phys. Chem. Miner. 16 (1989) 767. [19] R.A. Weeks, C.M. Nelson, J. Am. Ceram. Soc. 43 (1960) 399. [20] D.W. McMorris, Nature 226 (1970) 146. [21] D.L. Griscom, Rev. Solid State Sci. 4 (1990) 565. [22] M. Stapelbroek, D.L. Griscom, E.J. Friebele, G.H. Sigel, J. Non-Cryst. Solids 32 (1979) 313. [23] E.J. Friebele, D.L. Griscom, M. Stapelbroek, R. Weeks, Phys. Rev. Lett. 42 (1979) 1346. [24] D.L. Griscom, E.J. Friebele, Phys. Rev. B 24 (1981) 4896. [25] D.W. McMorris, J. Geophys. Res. 76 (1971) 7875. [26] J.A. Weil, Radiat. Eff. 26 (1975) 261. [27] G.V. Robins, J. Anal. Appl. Pyrolysis 6 (1984) 31. [28] L. Gatineau, C. Tchoubar, Bull. Gr. Fr. Argiles 25 (1975) 1. [29] M. Che, J. Fraissard, J.C. Vedrine, Bull. Gr. Fr. Argiles 26 (1975) 1. [30] D. Olivier, J.C. Vedrine, H. Pezerat, J. Solid State Chem. 20 (1977) 267. [31] B.R. Angel, W.E.J. Vincent, Clays Clay Miner. 26 (1978) 263. [32] T. Warashina, T. Higashimura, Y. Madea, Occ. Pap.-Br. Museum 19 (1981) 117. [33] D. Bonnin, S. Muller, G. Calas, Bull. Mine´r. 105 (1982) 467. [34] Y. Bensimon, B. Deroide, M. Martineau, J.V. Zanchetta, C.R. Acad. Sci. Paris 2 (1999) 119. [35] B. Deroide, Y. Bensimon, P. Belougne, J.V. Zanchetta, J. Phys. Chem. Solids 52 (1991) 853. [36] D.L. Griscom, Nucl. Instrum. Methods B1 (1984) 481. [37] F.J. Feigl, W.B. Fowler, K.L. Yip, Solid State Commun. 14 (1974) 225. [38] J. Bartoll, M. Ikeya, Appl. Radiat. Isot. 48 (1997) 981. [39] C. Furetta, Nucl. Tracks Radiat. Meas. 14 (1988) 413. [40] P.W. Levy, Nucl. Instrum. Method Phys. B1 (1984) 436. [41] B. Clozel, T. Allard, J.P. Muller, Clays Clay Miner. 42 (1994) 657. [42] K. Shimokawa, N. Imaı¨, Geochim. Cosmochim. Acta 51 (1987) 115. [43] A. Wieser, H.Y. Go¨ksu, D.F. Regulla, Nucl. Tracks Radiat. Meas. 10 (1985) 831. [44] M.G. Jani, R.B. Bossoli, L.E. Halliburton, Phys. Rev. B. 27 (1983) 2285. [45] D.L. Griscom, Phys. Rev. B. 20 (1979) 1823.
Nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance Y. Bensimon a,*, B. Deroide a, F. Dijoux a, M. Martineau b a
Laboratoire de Physico-chimie de la Matie`re Condense´e, UMR CNRS 5617, Universite´ de Montpellier II, Place Euge`ne Bataillon, 34095 Montpellier Cedex 5, France b Institut Universitaire de Technologie, 99, avenue d’Occitanie, 34075 Montpellier Cedex, France Received 2 December 1999; accepted 3 February 2000
Abstract A natural clay of South of France is studied by electron spin resonance (ESR). Two radiation-induced defects are revealed, certainly localized in the quartz contained in clay. The first is the peroxy-center, associated with oxygen, formed by the action of an O22 ion on a silicon atom to lead to Si–O–O z. The second is the E 0 -center, where an electron was trapped in oxygen vacancy to give xSi z. The spectroscopic parameters were determined. The thermal stability of these defects is studied by isothermal annealing experiments. Second order kinetics best explain the results, although more complex mechanisms must occur. The two defects have very different behaviors: the peroxy-center seems more stable at high temperature than the E 0 -center, although its activation energy is much weaker. They could be both used for the ESR dating of old clay. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electron spin resonance; Clay; D. Defects; Thermal stability
1. Introduction Electron spin resonance (ESR) is a technique very much used in physicochemistry of solid and biology. Its use extends now in the field of geological and archaeological sciences. Archaeology uses this technique more and more particularly for the ESR dating of lithic tools and the knowledge of their prehistoric heating conditions. These archaeological materials indeed contain species, which give an ESR signal. They can be transition metal ions like Fe 31 or Mn 21 which have unpaired electrons and which give ESR spectra to fine or hyperfine structures, the spectrum being strongly influenced by the paramagnetic centers’ environment [1]. An external phenomenon, like a thermal process, can involve chemical reactions such as an oxydoreduction or dehydration and can shift the signals and modify its form. It allows to specify the influence of the
* Corresponding author. Present address: E.N.S.C.M., 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France. Tel.: 1 334-67-147-257; fax: 1 33-4-67-144-353. E-mail address: [email protected] (Y. Bensimon).
heating effects on old materials [2–5]. Elsewhere the main part of the old objects contains traces of organic compounds. The latter break up with temperature and can produce another type of paramagnetic defect resulting from pyrolysis, such as for example, the homolytic break of C–C-bond to lead to the radical carbon. The evolution of this radical depends on the temperature and the pressure of oxygen. Thus, the observation of the ESR spectra evolution of these materials give information on their thermal history [2,6,7–10]. Finally a third type of paramagnetic center can be observed in old materials: radiation-induced defects generated by natural radioactivity and cosmic radiation effects on these non-conductive solids. The latter, submitted to high energies, have some electrons ejected of their energy level that leads to the formation of trapped electrons and holes. The majority recombine quickly but few remain trapped in well-defined sites within the lattice and form paramagnetic centers. These centers are very stable at room temperature but disappear quickly on heating [11,12]. They are used for the ESR dating of various old objects which were heated at one time of their history [11,13–16] according to the principle that the older the object is, the more numerous these centers will be. This
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(00)00048-2
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technique allows us to obtain the age of the geological material that was never heated [14]. This paper relates to this last type of center (radiationinduced centers) which are present in common clay of the South of France. As the majority of natural clays, it contains quartz [17]. Quartz is a good reserve of paramagnetic defects [18] that appear under the effect of radioactive radiation. These defects are allotted to the E 0 -centers associated with silicon (xSi z; “ z” indicate an unpaired electron) [19– 21], to the “oxygen hole center” (Si–Ox z) and the peroxycenters (xSi–O–O z) [22–24], to the centers associated with aluminum [16,25,26]. These centers are very well observed with the ESR technique when they are contained in pure quartz or silica. However, they pose a problem in clays because of the interference of a very broad and very intense signal allotted to iron [27]. Indeed, clays generally contain iron traces [28–30] in sufficient quantity to generate an intense ESR signal. Paramagnetism is due to the Fe 31 species, which has an electronic spin S 5=2; sometimes responsible for complex absorption bands. Thus, in the X-band (9,8 GHz), an ESR spectrum of a clay reveals two broad signals at g 4 and g 2 [31] and a signal much thinner about g 2 [32]. The first two are due to the Fe III species in different environments. Resonance at g 4 comes from Fe 31 located in the orthorhombic sites [2]. The very broad signal located around g 2 is allotted to the hydrated Fe 31 species that give rise to the FexOy or FeOOH oxides [33]. The thin signal at g 2; which relates to natural radiation-induced defects, is often not well defined and its interpretation is not easy in raw clay. In the studied sample, the two centers responsible for the signal due to natural radiation-induced defects were identified. The study of their thermal stability at various temperatures allows us to obtain the energy parameters that characterize them. Let us specify that these defects exist in a natural state in this clay and that the latter was thus submitted to no irradiation.
2. Experimental 2.1. Samples The clay used was taken out in a layer at Argelliers, near Montpellier, South of France. It is a common silicoaluminous clay belonging to the smectites family, which was used for a long time for potteries. It contains, expressed as a percentage mass, silicon 32%, aluminum 9% and iron 4.5% [34]. First of all, the samples were washed with a 15% H2O2 solution (clay 1 g for 50 cm 3 of solution) for 24 h and at 508C in order to eliminate the organic impurities. Several washings with water then eliminate H2O2. The clay is then dried at 608C after sedimentation and filtration. The ESR analyses on raw clay are carried out after this first treatment.
In order to eliminate most of non-structural iron, clay is then treated by a 6 mol l 21 solution of HCl (clay 1 g for 50 cm 3 of solution) for 2 h and at 508C. After this treatment, clay contains nothing but 0.9% of iron [34]. Several washings with water are then necessary to eliminate the acid and the formed iron complexes. The sample is then dried at 608C after sedimentation and filtration. The ESR analyses are carried out on the treated clay after this second treatment. 2.2. ESR experiments The establishment of the ESR spectra in the X-band (9,8 GHz) was made using a spectrometer Bruker ER100 by adopting a modulation frequency of 100 kHz whose amplitude was fixed at 0.4 mT for the broad lines and was lowered up to 0.05 mT for the narrow lines. The ESR spectrum intensity of the DPPH standard, recorded under the same conditions of modulation, was measured to calibrate the intensity of each signal. The ESR signals were located in experiments by their g-value defined by geff
hn ; bB
where h is the Planck’s constant, n the resonance frequency, b the Bohr magneton, B the magnetic field at which resonance occurs. These values are calibrated by comparison with the DPPH standard g 2:0036: The spectra are calculated by means of a program elaborated at the laboratory [35] and allows us to obtain the parameters values gx, gy and gz and the linewidths s . 2.3. Isothermal annealing experiments To carry out the decay kinetic study of the signals at defined temperature, for each experiment, 100 mg of the treated clay are introduced into a quartz tube. This tube was put in a furnace connected to a power supply, controlled in order to have a well-defined temperature. At the end of each heating time, the tube is immersed in a cold water bath in order to stop the reaction. The recording of the ESR spectrum is carried out at room temperature and the signal intensity is then measured. These experiments were carried out in air at 350, 450 and 5008C. Measurements were stopped when the studied signal disappeared completely or did not move any more. 3. Results and discussion 3.1. Analysis of the ESR spectra 3.1.1. The total spectrum The ESR spectrum of Argelliers’ clay (Fig. 1a), presents three distinct signals. As already indicated, signal I at g 4 comes from the Fe 31 species in an orthorhombic environment. Let us note that signal II due to iron oxides present in
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Fig. 1. The X-band ESR spectra of (a) raw clay and (b) acid treated clay recorded at 298 K.
the clay clusters, and not in the material structure, is extremely broad. It interferes very much with signal III, which is much thinner. The lines, due to the paramagnetic centers likely used for dating, appear in this spectrum region, around g 2: These last signals are very thin and in general of low amplitude. Thus they should be partially masked by a signal as intense as signal II. In order to decrease the important interference of signal II with signal III, it was necessary to eliminate most of the iron oxides. For which, raw clay has undergone treatment with acid so that signal III, which includes the radiation defects, is not modified so that the dating possibilities of the sample are maintained. The nature of clay, on the other hand, can be some disturbed, but that has no consequences on the study of these defects. This treatment, carried out using a 6 mol l 21 HCl solution, reveals a major reduction in signal II amplitude and an increased signal III resolution (Fig. 1b). Thus by analyzing the ESR spectrum of clay, in a restricted domain of the magnetic field around g 2; it will be much easier to study the paramagnetic centers of this region.
3.1.2. The peroxy-center To improve the characterization of signal III, the ESR spectrum of raw clay, treated by HCl, was recorded in an interval of reduced magnetic field (Fig. 2, exp). This signal is then well solved and it seems interesting to simulate it. Although it seems to have an axial symmetry, its spectral characteristics were determined according to the orthorhombic symmetry which gives the best results. Calculation was carried out by using a Lorentzian function. The results (Fig. 2, calc) indicate a good agreement between the experimental and calculated spectra. The spectroscopic parameters, the g-tensor and the linewidth s , are given in Table 1. The identification of the species responsible for this signal is carried out from the values gx, gy and gz according to the nature of material. The studied clay, formed of aluminosilicates, also contains quartz [17]. The work undertaken on silicas, showed that, after irradiation, the ESR spectra of these materials revealed the existence of defects associated with oxygen [22]. More precisely two types of
Fig. 2. The X-band experimental and calculated ESR spectra of peroxy-center, recorded at 298 K.
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Table 1 ESR spectroscopic parameters of peroxy and E 0 centers: g-tensor and linewidth s
Peroxy E0
gx
gy
gz
s x (mT)
s y (mT)
s z(mT)
2:0018 ^ 1 × 10 24 2:0003 ^ 1 × 1024
2:0033 ^ 1 × 1024 2:0004 ^ 1 × 1024
2:051 ^ 1 × 1023 2:0017 ^ 11024
1:3 ^ 0:1 0:35 ^ 0:02
1:3 ^ 0:1 0:38 ^ 0:02
1:0 ^ 0:1 0:20 ^ 0:02
paramagnetic centers have been identified [23]. The first is the “nonbridging-oxygen hole center” (NBOHC), which comes from the breaking of the Si–O–Si-bond or the breaking of the OH-bond in Si–OH to lead in both cases to the Si–O z entity. The second is the peroxy radical already identified in silica glasses after irradiation [23,24]. Its comes from the formation of the asymmetrical bond between the O22 ion and a silicon atom to lead to the Si–O–O z entity. According to the values of the g-tensor g x 2:0018; gy 2:0033 and gz 2:051 and the linewidth s , it appears that signal III of Argelliers’ clay is due to a peroxy radical. Indeed, for this last, the actual values in silica glasses vary slightly according to the nature of the irradiation: 2:0018 , g x , 2:0020; 2:0070 , gy , 2:0085; 2:027 , gz , 2:067 [23,24]. One notes, concerning signal III, a value slightly lower for the component gy. This is explained by the very different nature, as well from the studied clay matrix as the rough quartz present. 3.1.3. The E 0 -center Signal III, however, seems to be of composite nature because it is difficult to explain correctly its form at geff 2:0021 (Fig. 2, exp). It could then be that two paramagnetic defects are responsible, the peroxy-center and a center much narrower being in the same magnetic field region. For a better characterization of this very narrow signal, the latest spectrum was recorded between 349.7 and 350.5 mT. An
axial line (Fig. 3, exp) is then underlined. Simulation (Fig. 3, calc) was carried out by using a Lorentzian function and allowed to determine the spectroscopic parameter values (Table 1). Among the defects observed in silica glasses or quartz, the E 0 -centers show spectroscopic characteristics very close to those given herein 2:0003 , gx , 2:0004; 2:0004 , gy , 2:0006; gz 2:0018 [36]. These centers are intrinsic in SiO2. They can be formed by the breaking of the Si–Sibonds to lead to a silicon atom with an unpaired spin according to xSi–Six ! xSi " 1 xSi 1 e– 37 or xSi–Six 1 H ! xSi" 1 xSiH 23: By considering the g values obtained for this line gx 2:0003; gy 2:0004; gz 2:0017 and the very low linewidth, the observed signal can be allotted unambiguously to an E 0 -center. It seems that these two centers, peroxy and E 0 , are described in a natural clay for the first time. Some were observed in potteries [32] and in unburnt soil [2]. They were not underlined during a test of ESR dating of old pottery [38]. For better characterization, their thermal stability was studied to deduce some thermodynamic parameters.
Fig. 3. The X-band experimental and calculated ESR spectra of E 0 -center, recorded at 298 K.
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3.2. Annealing experiments
Fig. 4. Various ESR spectra of peroxy-center observed in 4508C annealing experiment.
For each studied temperature, the study of the decay of the number of peroxy and E 0 centers was carried out by isothermal measuring of each ESR signal intensity. This intensity is measured in arbitrary units. Instead of taking these arbitrary units, the samples were calibrated at the initial time of the decay kinetics by using a standard containing a known number of spins. The standard used is a synthetic sample of lazurite, used like pigment, obtained by heating in a reducing medium of a mixture of sulfates and sulfides of sodium, kaolin and carbonates. This standard contains 8:8 × 1016 spins. By comparing the integral of its absorption with those of the studied centers, the number of peroxy and E 0 centers present at the initial moment in 100 mg of sample have been determined. Measurements on the sample and the standard must be performed under the same modulation conditions to be able to compare absorptions. Thus, 100 mg of sample contain, at the initial time of the decay kinetics, 8:7 × 1015 peroxy-centers and 1:1 × 1013 E 0 -centers. Three temperatures were used to study the isothermal decrease of the number of centers: 350, 450 and 5008C. As an example, we show the evolution of the ESR signal of the peroxy (Fig. 4) and E 0 (Fig. 5) centers, for a few moments, at 4508C temperature. The decrease process of these centers is certainly not simple. It is usually described for the same type of centers by equations of the first or second order [39]. The first order can be described by the equation dN 2lt dt
1
where N is the instantaneous defect number and l the decay constant. By integrating Eq. (1) one obtains N N0 exp 2lt
2
where N0 is the initial number of centers. In this case the N Neperian logarithm decreases linearly with the heating time according to the law ln N ln N0 2 lt
3
The following relation gives the half-life time t :
t
0:69 l
4
For a second order mechanism, in which a center recombines with a complementary center, the law can have the following form: dN 2lN 2 dt
5
By integrating Eq. (5) one obtains the following expression: Fig. 5. Various ESR spectra of the E 0 -center observed in 4508C annealing experiment.
1 1 lt 1 N N0
6
It appears that for a second order reaction, the reciprocal
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Fig. 6. The peroxy-center number as a function of heating time: isothermal annealing experiments were performed at 350, 450 and 5008C.
number of sites is an increasing linear function versus the heating time. The following relation gives the half-life time
t
1 lN0
7
According to the undertaken experiments, it seem that the peroxy and E 0 centers decaying, follows a second order mechanism. Expression (5) applies correctly to their isothermal treatment, in particular during the first part of the process. Then the phenomena are certainly more complicated and the same law cannot be applied any more for the end of the reaction. However, one can completely consider that the beginning of the process follows a second order law for the two studied defects, which is in agreement with the previous observations of the same type of defects [12,40,41]. In any case, the decay constant follows the Arrhenius’s law E 8 l l0 exp 2 a kT
where l 0 is the pre-exponential factor, T the temperature (K), Ea the activation energy, k the Boltzman’s constant 8:6 × 10-5 eV K21 : This last expression can be put under the form ln l ln l0 2
Ea kT
9
The l values are obtained from Eq. (6) and the parameters l 0 and Ea by fitting the linear expression of T 21 in Eq. (9). 3.2.1. Peroxy-center Fig. 6 presents the evolution of the number of peroxycenters as a function of the heating time, for the three studied temperatures. It is noted that the reaction progress increases versus temperature. At 3508C the number of centers do not move any more after 120 min and is maintained at 4:2 × 1015 spins. At 450 and 5008C, the progress of the reaction is much more important. There remains only 15% of sites at 4508C after 267 min and 12% at 5008C after 188 min.
Fig. 7. The ESR peroxy-center reciprocal number versus heating time: the data points lie on straight lines indicating that they are defined by second order decay kinetics.
Y. Bensimon et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1623–1632 Table 2 Kinetic parameters for peroxy and E 0 centers in clay from Argelliers: decay constants at 350, 450, 5008C and at room temperature, activation energies, pre-exponential factor and half-life
l 350 (spins 21 min 21) l 450 (spins 21 min 21) l 500 (spins 21 min 21) Ea (eV) l 0 (spins 21 min 21) l 298 (spins 21 min 21) t (years)
Peroxy
E0
3:18 × 10218 4:56 × 10218 5:38 × 10218 0:14 ^ 0:01 4:68 × 10217 ind. ind.
8:63 × 10217 2:23 × 10214 1:23 × 10213 2:01 ^ 0:01 1:46 × 10234 1:30 × 10221 1:18 × 1015
Fig. 7 shows, for the three studied temperatures, the linear evolution of the reciprocal number of peroxy-centers as a function of the heating time. This law remains valid in an interval of well-defined time. Thus it is no more valid beyond 11 min at 3508C, 43 min at 4508C and 84 min at 5008C. Thus it seems that the more the heating temperatures are raised, the more the second order law applies. By considering the intervals of time when the second order law is valid, expression (6) gives access to the decay constants l by fitting each straight line (Table 2). The relating parameters to the Arrhenius’s equation are obtained by fitting relation (9) (Fig. 8). The obtained Ea value (0.14 eV) seems low according to those observed for other paramagnetic defects, which are between 1.6 and 2.0 eV [41–43]. Nevertheless, the defects and material were different. With such activation energy, one could think that the peroxy-center is easily destroyed even at room temperature. However, it is extremely stable at 298 K. It could be that the kinetic aspect is dominating in its case and that the g irradiation, responsible for its creation, produces a defect in the deep layers of the structure. At this time a chemical reaction, which in general takes place only on the valence layers, would not occur easily. It is difficult, for all these reasons to define a half-life time at 298 K for the peroxy-center. Indeed, by
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considering Fig. 9, which represents time when the second order decay kinetics is valid, versus temperature, one notes that this kinetics is not valid any more at a temperature around 2008C. Under these conditions, it is useless to extrapolate the decay constant value at room temperature and to deduce a half-life time from expression (7). 3.2.2. E 0 -center It is described usually that the E 0 -centers formed by the g irradiation at room temperature, see their number increasing if the material is heated from 1708C to reach a maximum at 3008C [11]. This is explained by the formation of oxygen vacancies during the irradiation which trap an electron and are transformed into E 0 -centers when the material is heated. These latter disappear completely if the material is heated at 4408C [44]. These descriptions are in agreement with the phenomena that are observed herein. Fig. 10 indicates the evolution of the number of E 0 -centers (for 100 mg of clay) as a function of the heating time. One notes, for the three temperatures, an increase in the number of sites during first minutes. Then this number decreases, much less for the temperature of 3508C. The decay kinetics of the E 0 -center were studied by considering that the initial number of sites corresponded to the maximum noted on the curves (Fig. 10). This initial number of sites was calibrated in a way already indicated. It is noticed that at 3508C the number of E 0 centers always remains very important. On the other hand, these centers disappear after 70 min at 4508C and 39 min at 5008C. Fig. 11 shows the reciprocal number of E 0 -centers as a function of the heating time, when a second order decay kinetics occurs. This law is valid for 40 min at 3508C, 23 min at 4508C and 19 min at 5008C. Thus it appears that the lower the temperature is, the more the second order kinetics applies a long time. This result is contrary to the one of the peroxy-center and point out the different nature of the two centers.
Fig. 8. The Arrhenius plots of the decay factors of the peroxy-center: the activation energy and the pre-exponential factor are shown in Table 2 and were obtained from the linear regression fits to the data.
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Fig. 9. Peroxy-center: second order kinetic validity time as an increasing function of the heating temperature.
Fig. 10. The E 0 -center number as a function of heating time: isothermal annealing experiments were performed at 350, 450 and 5008C.
Fig. 11. The ESR E 0 -center reciprocal number versus heating time: the data points lie on straight lines indicating that they are defined by second order decay kinetics.
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Fig. 12. The Arrhenius plots of the decay factors of the E 0 -center: the activation energy and pre-exponential factor are shown in Table 2 and were obtained from the linear regression fits to the data.
Fig. 13. E 0 -center: second order kinetic validity time as an decreasing function of the heating temperature.
These results gave access to the decay constants l for the three temperatures. By fitting the curve (Fig. 12), resulting from expression (9), the pre-exponential factor l 0 and the activation energy Ea are determined (Table 2). The found activation energy (2.01 eV) is in agreement with those already observed for the same types of centers in other materials [12]. Fig. 13 indicates that the time during which the second order decay kinetics remains valid is a function of the temperature. It is well seen that this time is large since the temperature is low. It is then possible to determine the halflife time at room temperature in the case of this experiment from expression (7). The found value (Table 2) indicates that the E 0 -center is extremely stable at room temperature.
4. Conclusion The observed X-band spectra of natural clay, clearly show that two paramagnetic defects are present: the
peroxy-center and the E 0 -center which come from the environmental and cosmic irradiation. They were studied such as they were naturally in clay. They disappear under the temperature effect and their thermal stability was interpreted. The parameters deduced from this study are given. It appears that these two centers have a completely different behavior. Initially the peroxy-center is much more numerous than the E 0 -center, nearly thousand times more. Then, the very weak activation energy, found for the peroxy-center decay, seems to indicate that this last is formed from a modification of deep energy levels. The formation of paramagnetic defects requires much energy. Thus, for example, the formation of E 0 -centers requires 3 × 107 eV=defect in quartz and 10 5 eV/defect in fused silica [21,45]. Therefore this energy is different according to the nature of material and it is also different according to the nature of the center. It appears that, in the studied clay, formation of the peroxy-center and its thermal degradation are closely related to the nature of the material which
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contains it. Moreover, the kinetic aspect, during its heating, seems to predominate the thermodynamic aspect. In any event, its decaying, at low temperatures, certainly concerns complex mechanisms. The E 0 -center has, in this clay, a thermal behavior which corresponds to the already described results. The E 0 -center was formed in the crystalline environment of quartz contained in the clay [17]. Then one finds the behavior of the defects induced by the artificial g irradiation in quartz. In particular, the half-time is higher than 10 15 years. Such a half-life shows that this type of center can be used for the ESR radiation dosimetry on natural clay, at room temperature. References [1] J.R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon, Oxford, 1990. [2] J. Bartoll, A. Tani, M. Ikeya, T. Inada, Appl. Magn. Reson. 11 (1996) 577. [3] A. Tani, J. Barstool, M. Ikeya, K. Komura, H. Kajiwara, Ionics 22 (1996) 3. [4] G.V. Robins, N.J. Seeley, M.C.R. Symons, D.A.C. McNeil, Archaeometry 23 (1981) 103. [5] Y. Matsuoka, M. Ikeya, Jpn. J. Appl. Phys. 34 (1995) 6068. [6] D. Robins, New Scientist 117 (1988) 49. [7] G.C. Hillman, G.V. Robins, A.D. Oduwole, K.D. Sales, D.A.C. McNeil, J. Archaeol. Sci. 12 (1985) 49. [8] K.D. Sales, A.D. Oduwole, J. Convert, G.V. Robins, Archaeometry 29 (1987) 103. [9] G.V. Robins, C. Del Re, N.J. Seeley, A.G. Davies, J.A.-A. Hawari, J. Archeol. Sci. 10 (1983) 385. [10] K.D. Sales, J. Oduwole, G.V. Robins, S. Olsen, Nucl. Tracks 10 (1985) 845. [11] M. Ikeya, New Applications of Electron Spin ResonanceDating, Dosimetry and Microscopy, World Scientific, Singapore, 1993. [12] S. Toyoda, M. Ikeya, Geochem. J. 25 (1991) 437. [13] M. Jonas, Radiat. Meas. 27 (1997) 943. [14] W.J. Rink, Radiat. Meas. 27 (1997) 975. [15] M. Laurent, C. Falgue`res, J.J. Bahain, Y. Yokoyama, C.R. Acad. Sci. Paris 318 (1994) 521.
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