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Effect of anion interstitials on the thermoluminescent properties of CaSO4:Dy
Journal of Luminescence 142 (2013) 212–219
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Effect of anion interstitials on the thermoluminescent properties of CaSO4:Dy A.R. Lakshmanan n, V. Sivakumar, R. Sangeetha Rani, S. Kalpana Saveetha Engineering College, Thandalam, Chennai 602105, India
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2013 Received in revised form 20 March 2013 Accepted 29 March 2013 Available online 22 April 2013
Efforts were made to synthesise CaSO4 based thermoluminescent (TL) phosphors by solid state reaction route. ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice as witnessed from the high TL intensity peak at about 100 1C following sintering at 750 1C in air. Its TL intensity is even higher than that of the 260 1C peak appearing in recrystallised CaSO4:Dy. Similar low temperature TL peak was found in recrystallised CaSO4:Dy samples sintered at 1000 1C in air. The results were explained on the basis of incorporation of interstitial oxygen anions which act as hole traps. Firing in sulphur or ammonium sulphate atmosphere did not shift the low temperature TL glow peak. But firing in carbon atmosphere at 750 1C with ZnSO4 flux enhanced the intensity of TL glow peaks at 250 1C and at 400 1C due to the partial re-conversion of oxygen ions to sulphate ions. Firing at 850 1C in reduced atmosphere, however, quenched the intensity of all TL peaks due to the removal of oxygen i.e. partial reduction of CaSO4 to CaS. The slow decline in the intensity of 2501 TL peak on prolonged annealing at 400 1C in recrystallised CaSO4:Dy indicates the thermal migration of defects causing the TL peak. Redox mechanism involving such interstitial ions and anion vacancies in the presence or absence of cation vacancies could lead to the emission of high ( 250 1C) or low ( 100 1C) temperature TL peaks, respectively. Certain other co-dopants tried such as Al3+ and SiO44− simply quenched the TL efficiencies of CaSO4:Dy and CaSO4:Mn, respectively. Among the monovalent sulphates tried, Na2SO4:Dy gave a TL peak around 100 1C which is 20% of the 250 1C TL intensity of recrystallised CaSO4:Dy. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence CaSO4:Dy Solid state synthesis ZnSO4 flux Oxygen interstitials
1. Introduction CaSO4:Dy is a well known thermoluminescent (TL) phosphor used widely for radiation dosimetry [1]. It exhibits an intense and stable TL peak near 250 1C and a feeble one near 100 1C. The effect of cation co-dopants (e.g., Na+,K+,Li+,Ag+,Zn2+, etc.) and anion co-dopants (P5−) at substitutional (Ca++and S6− respectively) sites on the thermoluminescent (TL) characteristics of CaSO4 is well studied. However, the effect of anion co-dopants such as O2−, Cl− etc. at interstitial sites has not been well studied so far. By introducing such defects one can play with charge compensation mechanism and tune the intensity of different TL peaks. Such studies would also throw more insight into the TL production mechanism and in the identification of electron and hole traps in CaSO4:RE which remain so far mystical. Yet another way is thermal dissociation of host anions since very few metal sulphates are sufficiently stable at high temperatures to have a measurable melting point [2]. All decompose eventually with the evolution
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of SO3. The latter in turn dissociates to SO2 and O2. Irradiation (radiolysis) also dissociates SO4 radicals leaving behind anion vacancies (F+ centres) but subsequent heating restores them unlike the case with thermally induced dissociation. All these defects create a variety of hole traps at interstitial sites giving rise to a number of TL peaks in CaSO4:Dy. SO42− radicals are unstable against heat as well as irradiation. Radiolysis products in CaSO4 based phosphors reportedly form a variety of hole centres which are related to oxygen and oxy-sulphur radicals (SO4−, SO3−, SO2−, O3− and O−) as observed in electron spin resonance (ESR) and optical absorption (OA) studies. Alternately, the thermal dissociation of some SO42− radicals at high temperatures ( 1000 1C) is also probable, yielding the electrons for the reduction of oxygen atoms at interstitial sites. These two decomposition processes apparently give rise to two types of defects. While the thermally induced defects are labelled host related, the radiation induced defects are attributed to impurities and radiolysis. The role of sintering temperature (650–750 1C) and sintering duration (1–2 h) in the case of CaSO4:Dy, has never been understood fully. Irrespective of the method of preparation, the above sintering treatment has been found to be essential to obtain optimal TL glow curve structure and sensitivity. The TL sensitivity
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systematically increased with sintering temperature in the range 300-750 1C, a result which has not been explained so far [3]. It was assumed that the high temperature treatment resulted in better crystallinity of the host lattice, despite the fact no experimental result has been published so far to support this hypothesis. By intentional doping of anions, one can tune the intensity of the low temperature peak. Incorporation of anions (Oi2−, Cl− etc.) along with Dy3+ dopant in CaSO4 during phosphor synthesis should provide the charge compensation and should result in intense low temperature TL peak at the expense of the 250 1C peak. In this work ZnSO4 was used as the flux as well as oxygen source since it is reported to dissociate at 600 1C. ZnCl2 was used as the chlorine source since it is reported to boil at 732 1C. Preparation of highly sensitive CaSO4 based TL phosphors with a desired glow curve structure using a solid state diffusion technique has not been successful so far. The reasons for this enigma will also be analysed in this paper.
Fig. 1. Thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O.
Table 1 Weight loss on heating, ZnSO4 7H2O. Sample
2. Experimental details Thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O was carried out by weight measurement following successive heat treatments in the temperature range in a furnace for 5 min duration. For phosphor synthesis, CaSO4 2H2O (E. Merck, precipitated and Qualigens grade), ZnSO4/ZnCl2 (all E. Merck grade) and Dy2O3 (Indian Rare Earth Chemicals) were wet mixed and dried at 150 1C and then sintered at high temperatures in a muffle furnace for 1 h duration. Other co-dopants studied include Na, Mn and Al, all in sulphate form. The effects of Si (added in silicic acid form) as well as (NH4)2SO4 and sulphur co-dopants were also studied. Firing was carried out in air as well as in reduced (carbon) atmosphere. Dy doped monovalent sulphates were made by solid state reactions in air near their melting points using Li2SO4 H2O, Na2SO4, K2SO4 and Dy2O3 (all Merck grade) as raw materials. The synthesised samples were characterised by TL and photoluminescence (PL). The TL glow curves were recorded using Riso system reader (model TL/OSL-DA-20). The system allows 48 samples to be individually heated immediately after 90Sr beta irradiation (radiation dose ¼1 Gy in all cases studied). The heater is made of low-mass Kanthal strip. 10 mg of phosphor powder was uniformly spread on the heater strip. The heating system could heat the samples up to 700 1C at constant heating rates from 0.1 to 10 1C/s. To minimise thermal lag between samples and heater strip, a heating rate of 5 1C/s was employed. The heating strip can be cooled by a nitrogen flow. The glow curve has been recorded by running the programme installed in the computer. TL emission spectra were recorded after 60Co gamma irradiation (dose¼ 10 Gy) with a Fluorolog Jobin Yvon-Spex spectrofluorimeter by keeping the irradiated samples at a constant temperature of 180 1C.
3. Results 3.1. Thermogravimetric analysis Results of thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O is shown in Fig. 1. A reduction in the weights observed in ZnSO4 7H2O and CaSO4 2H2O in the temperature region 200–400 1C (by 44% and 21%, respectively) correspond to their dehydration (loss of water of crystallisation). No such weight loss occurs in BaSO4 as no water molecules are attached to it. The minor weight loss by about 6% is seen in ZnSO4 when the annealing temperature was increased from 400 1C to 750 1C (Table 1) which shows that little dissociation of ZnSO4 takes place
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Hydrated, ZnSO4 7H2O Anhydrate, ZnSO4 ZnO After 750 1C 1 h, heat treatment in air
Relative weight Theory
Experiment
1.00 0.56 0.28 –
1.00 0.56 ( 400 1C) 0.53
Table 2 Weight loss on heating, CaSO4 2H2O. Sample
Hydrated, CaSO4 2H2O Anhydrate, CaSO4 CaO After 750 1C, 1 h , heat treatment in air
Relative weight Theory
Experiment
1.000 0.790 0.325 –
1.000 0.796 ( 400 1C) – 0.794
at 750 1C, but this may be well sufficient to influence TL properties! However, in the case of ZnCl2 vigorous fuming and the formation of a ZnO residue as witnessed by its thermochromic behaviour (yellow colour at high temperatures) on heating to 750 1C. In the case of ZnCl2 boiling can introduce oxygen as well as chlorine ion interstitials. Comparatively, the weight loss seen in CaSO4 when the annealing temperature was increased from 400 to 700 1C was negligible ( 0.25%; Table 2). BaSO4 is thermally more stable than the other two sulphates studied (Table 3). 3.2. Effect of high temperature heat treatment on TL of CaSO4:Dy Fig. 2 shows the effect TL glow curves of recrystallised CaSO4: Dy on 1000 1C heat treatment. Air firing shifted the TL glow curve from high temperature to low temperature region. While recrystallised CaSO4:Dy exhibits a major TL glow peak around 260 1C, 1000 1C air fired sample showed a major peak around 110 1C with a high temperature shoulder around 160 1C in agreement with the results reported earlier [4]. However, firing in reduced (carbon) atmosphere at 1000 1C quenched the TL intensities of nearly all the TL peaks; a feeble TL peak around 80 1C is all what that remains in the latter case. 3.3. Solid state synthesis of CaSO4 based TL phosphors 3.3.1. CaSO4:Mn Fig. 3 compares the TL glow curves of CaSO4:Mn,Na and CaSO4: Mn,Si prepared by the solid state diffusion at 650 1C in air with that of the standard recrystallised CaSO4:Dy. MnSO4 H2O, silicic
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Table 3 Weight loss on heating, BaSO4. Sample
BaSO4 BaO After 750 1C, 1 h , heat treatment in air After 820 1C 1 h, heat treatment in air
Relative weight Theory
Experiment
1.000 0.656 – –
1.000 – 0.994 0.992
Fig. 2. TL glow curves of: a—CaSO4:Dy (std—recrystallised), b—(a) +1000 1C, 1 h in air and c—(a)+1000 1C, 1 h in carbon atmosphere. 90Sr beta dose ¼1 Gy.
form of silicic acid) proved to be a very good flux, yielding a bright white powder but the incorporation of Si co-dopant was found to quench the TL intensity of CaSO4:Mn. Addition of sulphur helps in preventing the oxidation of Mn in air. When inserted inside the furnace at 450 1C, the ingredients got fire due to combustion of sulphur in air. The samples were then ground and refired at 650 1C in air for 1 h duration. 3.3.2. CaSO4:Dy,Zn Incorporation of Dy dopant into CaSO4 crystal lattice using solid state diffusion reaction lattice also requires a suitable flux. For this purpose both ZnSO4 and ZnCl2 were tried. ZnSO4 reportedly decompose at 600 1C eventually forming ZnO, but other reports put its decomposition temperature well beyond 800 1C. ZnCl2 reportedly melts at 283 1C and boils at 732 1C and vaporised at 756 1C. Only ZnO residue remains in this case as well. Starting materials include CaSO4 2H2O (Merck as well Qualigens make), ZnSO4 7H2O and ZnCl2, all in analar grade form. ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice (Merck make) as witnessed from the high TL intensity peak observed in Fig. 4. Its TL intensity is even higher than that of the 260 1C peak of recrystallised CaSO4:Dy but the glow peak occurred at about 100 1C. The influence of firing atmosphere (air or carbon) at 750 1C on the TL was very little unlike that observed at 1000 1C (Fig. 2). Zn added as ZnCl2 produced similar results. Results obtained with CaSO4 (Qualigens make) were similar (Fig. 5) but the high temperature tails were much more pronounced in the TL glow curve structure than those seen with Merck samples (Fig. 4). Firing in carbon atmosphere at 750 1C with ZnSO4 flux enhanced the intensity of TL glow peaks at 230 1C and at 400 1C while firing at 850 1C quenched the intensity of all TL glow peaks (Fig. 6). TL emission spectra (Fig. 7) of all CaSO4:Dy samples were found to be characteristic of Dy3+ with emission peaks at about 480 and 570 nm. Differences in emission spectra observed with co-dopants were minor. 3.3.3. Effect of sulphur and NH4SO4 Effects of sulphur and NH4SO4 co-dopants on the TL glow curve structure of CaSO4:Dy was studied in the presence or absence of
Fig. 3. TL glow curves of: a—CaSO4:Dy (Std—recrystallised), b—CaSO4:Mn,Na (solid state synthesis) and c—CaSO4:Mn,SiO2 (solid state synthesis). 90Sr beta dose¼ 1 Gy.
acid and Na2SO4 which were used as raw materials for this purpose. CaSO4:Mn,Na exhibits a simple glow peak around 160 1C whose TL intensity is about 80% of that of CaSO4:Dy but the peak is much narrower in the former sample. Incorporation of MnSO4 into CaSO4 lattice poses the problem of oxidation of Mn in air at high temperatures. Without a flux, incorporation of any impurity into CaSO4 was not found to be feasible. Na2SO4 serves as a good flux in solid state synthesis of CaSO4 based phosphors. But the solubility of Na in CaSO4 host lattice is quite high and so one always ends up with the TL glow peak characteristic of CaSO4:Mn, Na whose TL peak appears slightly on the high temperature when compared to that (Tmax ¼ 100–130 1C), of CaSO4:Mn prepared by the recrystallisation technique as reported in literature. Si (in the
Fig. 4. left: TL glow curves of: a—CaSO4:Dy, Zn(Zn added as ZnSO4) , 750 1CC, 1 h in air, b—CaSO4:Dy,Zn (Zn added as ZnSO4, 750 1C, 1 h carbon atmosphere). All made from CaSO4 2H2O (E. Merck). c—CaSO4:Dy (0.2 mol%) (recrystallised). 90Sr beta dose¼ 1 Gy.
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Fig. 5. right: TL glow curves of: a—CaSO4:Dy,Zn (Zn added as ZnCl2+ZnSO4), 750 1C, 1 h in air, (b) CaSO4:Dy,Zn (Zn added as ZnCl2), 750 1C, 1 h in air. All made from CaSO4 2H2O (Qualigens). 90Sr beta dose ¼ 1 Gy.
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Fig. 7. Thermoluminescent emission spectrum of CaSO4:Dy,Zn (Zn added as ZnCl2+ZnSO4) (a), CaSO4:Dy,Zn (Zn added as ZnSO4) (b) and CaSO4:Dy (recrystallised) (c) recorded at 180 1C. 60Co-gamma dose ¼ 10 Gy. The PL intensities are not to be inter-compared.
750 1C for 2 h in air. The sample exhibited a weak TL peak at 100 1C (Fig. 8) while no high temperature TL peaks are produced. 3.3.5. Monovalent sulphates Dy doped monovalent sulphates were made by firing the intimate mixture of samples in air at 900–1070 1C for 1 h duration since the melting points of Li2SO4, Na2SO4 and K2SO4 were respectively, 845 1C, 884 1C and 1069 1C. Among them Na2SO4:Dy gave the maximum TL sensitivity, roughly 20% of that of recrystallised CaSO4:Dy but with the glow peak at 100 1C (Fig. 9).
4. Discussion 4.1. Electron and hole traps
Fig. 6. a—TL glow curves of CaSO4:Dy,ZnSO4 (750 1C, 1 h in carbon atmosphere), b—CaSO4:Dy,ZnSO4 (850 1C, 1 h in carbon atmosphere). The TL intensities of sample b are magnified by a factor of 10. All made from CaSO4 2H2O (Qualigens). The enhancement of high temperature peaks with qualigens makes gypsum chemical more pronounced than that obtained with Merck gypsum precipitated chemical. 90Sr beta dose¼1 Gy.
ZnSO4 flux by firing in air an intimate mixture of the constituent powder mixture at 700 1C for 1 h in air. However, results showed that none of these efforts succeeded in shifting the major TL peak to high temperatures (200–250 1C) desired in radiation dosimetry.
3.3.4. Trivalent co-dopants Due to its trivalency, incorporation of Al3+ at Ca2+ should produce cation vacancies similar to the case of Dy3+. This should in principle enhance the high temperature TL peak near 250 1C. However, Al2(SO4)3 is reported to dissociate at 770 1C which should introduce oxygen interstitials. To confirm this view, solid state synthesis of CaSO4:Dy3+,Al3+ with dysprosium in the form of dysprosium oxide and aluminium in the form of Al2(SO4)3 18H2O was carried out at
While the hole trap corresponding to 250 1C peak in CaSO4:Dy phosphor sintered at 700 1C has been assigned tentatively to sulphate radicals stabilized by Ca2+ vacancies created by the incorporation of Dy3+ cations at Ca2+ lattice sites, the electron traps in this phosphor have not yet been identified. This proposal is supported by the fact that the incorporation of monovalent cations such as Na+, Li+ or K+ along with Dy3+ removes the cation vacancies while the 250 1C TL peak vanishes. Instead, the intensity of host related low temperature TL peak near 100 1C is enhanced. Pre-irradiation annealing at temperatures ≥800 1C in air drastically reduces the 250 1C TL peak in CaSO4:Dy while the low temperature peaks near 100 1C are strongly enhanced similar to that of the Na co-doped sample. In CaSO4:Dy samples sintered at 1000 1C, the 250 1C peak totally vanished while the TL peaks near 100 1C showed nearly the same intensity as that of the 250 1C peak found in samples sintered at 700 1C. This was explained earlier on the basis of thermal dissociation of CaSO4 to CaO or diffusion of atmospheric oxygen (O2−) ions from air into the CaSO4 host lattice which get stabilized at interstitial sites in CaSO4:Dy samples near Dy ions when sintered at 1000 1C in air. CaSO4 starts decomposing above 800 1C, which should also introduce oxygen interstitials. This has resulted in the elimination of cation vacancies since oxygen ions provide the charge compensation for Dy3+ incorporation. Redox mechanism (O2i2−/Oi2−)-(O2i−/Oi−) – oxidation during irradiation and (O2i−/Oi−-O2i2−/Oi2−) – reduction during TL
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dissociated species seem to act like electron traps. Unlike the case with 100 1C TL peak, the TL process of 250 1C peak does not merely involve a redox process but also involve the recombination of the radiolysis products, e.g. SO3−+O−-SO42−. We shall see later that, prior to irradiation, there is also a thermal component involved in stabilizing the defects giving rise to 250 1C TL peak. 4.2. Decomposition of metal sulphates
Fig. 8. TL glow curves of: a—CaSO4:Dy (0.2 mol%) (Std—recrystallised) and b—CaSO4:Dy,Al (solid state synthesis, 750 1C, 1 h in air). 90Sr beta dose ¼ 1 Gy.
Fig. 9. TL glow curves of Na2SO4:Dy (a), K2SO4:Dy (b) and undoped CaSO4 (c) and Li2SO4:Dy (d). 90Sr beta dose ¼1 Gy. X marks magnification.
readout has been proposed to explain the TL process of the 100 1C peak. This means O2i2−/Oi2− acts as a hole trap. The anion vacancies (F+ centres) created by the dissociation and subsequent migration of sulphate radicals should act as electron traps. Similar proposal has been made recently to explain the TL process in undoped BaSO4 [5]. The fact that 612 nm optical absorption (OA) band attributed to O− ions has been observed in CaSO4:Dy single crystals along with other host lattice related radicals only after Xirradiation supports the involvement oxygen interstitials in the TL process. The intensity of the 612 nm band increased proportionately with radiation dose and the post-irradiation thermal decay of this band did really show two major annealing steps, one near 100 1C and the other near 250 1C supporting the involvement of redox mechanism in oxygen ions in both these TL peaks. It is likely that the hole traps near cation vacancies are more stable due to a lack of positive charge and the corresponding TL peak occurs at a relatively higher temperature (i.e. at 250 1C) while the hole traps without neighbouring cation vacancies are relatively less stable and give rise to the low temperature TL peak near 100 1C. The hole traps related to radiolysis products at interstitial sites seem to promote 250 1C TL peak while oxygen interstitials promote low temperature TL peak near 100 1C. In both cases, anion vacancies left behind by the radiolysis products or thermally
If the TL glow peak temperature of undoped (though quite weak in intensity) and impurity doped material (with strong TL intensity) remain the same, the defect giving rise to the peak is labelled as host related. This is the case with BaSO4. Both undoped and Eu doped BaSO4 give TL peak near 200 1C [5]. Same case is true with undoped CaSO4 and Mn doped CaSO4. Both give TL peak near 100 1C. When host related excitons (self-trapped) decay under impurity emission, the TL intensities are strong. This happens to be the case when the STE emission spectra overlaps well with the PL excitation spectra of the impurity. The major TL glow peaks in CaSO4:Dy3+ and CaSO4:Ag+,Dy3+ occur near 250 1C and 400 1C, respectively. The hole traps in the latter two phosphors are related to impurities which do not have same valency as that of the host. Recently, an electron spin resonance (ESR) study by Sanyal et al. [6] was carried out to understand the change in TL glow curve structure of CaSO4:Dy on high temperature firing (900 1C, 2 h in air). ESR spectra recorded at 77 K exhibited three radiation induced radicals which were attributed to SO3−, SO4− and O3−. On the basis of enhancement in ESR signal/TL intensities and thermal decay characteristics, SO4− radical was attributed to the low temperature TL peak near 130 1C. Surprisingly all the above three radicals disappeared on thermal annealing below 130 1C which means that no ESR signal corresponding to the high temperature TL peak near 250 1C has been observed! Another study by Gundu Rao et al. [7] also indicated that the decay of radiation induced SO3− radical to occur at the same temperature as that for the SO4− centre which influences the 1001 TL peak in CaSO4:Dy. Therefore they commented that further investigations are needed to verify whether SO3− centres play any role in the TL emission. In contrast, Huzimura et al. [8] correlated the TL peaks at 220 1C and 285 1C in CaSO4:Tm to the stimulated relaxation of SO3− ions and relaxed SO3− centres respectively. The contradictions in these results show that the defect structure in RE doped phosphors is far from exact identification. Superoxide anion is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. With one unpaired electron, the superoxide ion is a free radical, and, like dioxygen, it is paramagnetic. Superoxides are compounds in which the oxidation number of oxygen is −½ and the valence ½. The O–O bond distance in O2− is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O22−. A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion ([O–O]2−). The peroxide is not a radical and is also not paramagnetic which explains the difficulty in detecting the O22− species through ESR. The O–O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1. The simplest stable peroxide is hydrogen peroxide. A reason for the quenching of TL efficiency of CaSO4:Dy on Al co-doping (Fig. 8) is due to the decomposition of Al2(SO4)3 which directly proceeds to oxide without any intermediate oxysulphate as follows: Al2(SO4)3 ¼ Al2O3+3SO2+3/2O2. Weight loss has been measured during decomposition between 650 1C and 950 1C. Cation vacancies apparently disappear when additional oxygen ions released by the decomposition of Al2(SO4)3 are incorporated in CaSO4 crystal lattice which leads to the quenching of 250 1C TL
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peak. In principle for every Al3+ ion incorporated, 6Oi−, or 3O2i2− defects can be created. In addition, ionic radius of Al3+ ion (53.5 pm) is too small compared to that of Ca2+ ion (100 pm) which makes it as a highly incompatible structure leading to quenching of luminescence efficiency. In contrast, Zn2+ has a relatively higher ionic radius (72 pm) and more importantly it has the same valency as that of the host cation. A reason for the relatively high TL sensitivity of Na2SO4:Dy among the monovalent metal sulphates tried is a better match of ionic radius of Dy3+ (91.2 pm) with that of Na+ (102 pm) as compared to those of Li+ (76 pm) or K+ (138 pm). Incorporation of trivalent Dy3+ at monovalent Na+ cation site would, however, introduce a large number of cation vacancies while the incorporation of oxygen interstitial anions when Dy is incorporated in the form of Dy2O3 would remove them, thereby quenching the high temperature TL peak which explains the relatively poor TL efficiency of Na2SO4:Dy when compared to that of CaSO4:Dy. 4.3. Thermal migration of defects The fact that sintering treatment near 700–750 1C is perhaps essential to create the defects responsible for causing TL indicates the creation/stabilisation of electron and hole traps through thermal the decomposition process. It is relevant to consider that on heating all of the anhydrous sulphates of type RE2(SO4)3, decompose, without melting to basic salts (oxysulphates) of the type RE2O3.SO3, then to an oxide. The oxide final product is RE2O3. A major anomaly arose from the experimental results reported in literature that the sensitivity of 2501 TL peak CaSO4:Dy sintered at 700 1C decreased slowly on thermal annealing at 400 1C (roughly 1% per 1 h annealing) only to recover fully following sintering at 700 1C. No such reduction in TL was seen on thermal annealing at 300 1C [9]. This result clearly indicated thermally induced migration of defects/interstitials causing the 2501 TL peak and that high temperature sintering stabilizes the defect structure giving rise to the dosimetric TL peak near 250 1C in CaSO4:Dy. While 4001 annealing disturbs the defect, possibly by way of thermal migration, the 700 1C annealing restores it. Thermal annealing much below 400 1C does not affect TL since the activation energy required to migrate the defect was not achieved at lower temperatures. Our earlier attempt to explain this anomolous behaviour on the basis of damage to the trap structure caused by hydration of Dy2(SO4)3 at room temperature and its dehydration at 400 1C when doped within CaSO4 is not supported by the experimental result that dehydration of Dy2(SO4)3 in free state occurs in two stages, one at 105 1C and the other at 175 1C but both these steps occur much below 400 1C. A change in the trap structure, however, explains this result well, though the exact mechanism is still speculative. One such premise includes thermal migration of O2i2− into a sulphate ion vacancy which would eliminate electron and hole traps thereby reducing the TL intensity as indicated in Fig. 8 c; Thermal migration below 400 1C is less probable while high temperature i.e. 700 1C annealing could remove the O2i2− ions from the sulphate ion vacancy and restore the two defects to its original state thereby restoring the original TL intensity. CaSO4 decomposes directly to CaO. The existence of an intermediate oxysulfate and discrepant values for the melting point of CaSO4 are explained by a eutectic between CaO and CaSO4 near 1265 1C, with less than 20% CaO. Such high temperatures are, however, not encountered in this study and therefore one can rule out decomposition of CaSO4 as the cause for the above changes. But the situation could be quite different with the sulphates associate with Dy which are relatively less stable thermally. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) have shown that on heating, all of the anhydrate sulphates of the trivalent rare earth elements (R) and yttrium,
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type formula R2(SO4)3, decompose, first without melting, to basic salts (oxysulphates) of the type R2O3.SO3, then to an oxide. The oxide final product is R2O3 for all these elements. Hence thermal decomposition of R2(SO4)3 could constitute major interstitial defect(s). The melting point of MnSO4, 700 1C, quoted in handbooks, is reported to be erroneous. Decomposition is reported to begin before melting. The product usually observed on pyrolysis in air is Mn3O4, as in TGA, though Mn2O3 may appear mixed with it in the lower temperature range, complicating the kinetic picture [2]. 4.4. Thermal stability of monovalent and divalent suphates In the short time of TGA runs, K2SO4 appeared stable to 900 1C. A slight loss in weight at 1000 1C was attributed to sublimation. Na2SO4 appeared stable beyond 900 1C. No weight loss occurred up to 900 1C, 0.04% at 1000 1C and 1.5% at 1200 1C. This result agrees with the TGA studies in literature. In the short time of TGA runs, Na2SO4 appears stable to beyond 900 1C. No weight loss was observed in maintaining Na2SO4 for 2 h up to 900 1C, 0.04% at 1000 1C and 1.05% at 1200 1C. The composition of the residue showed that both decomposition and volatilisation has occurred [2]. A study on the high temperature behaviour of sulphates suggests that the thermal decomposition products in CaSO4, BaSO4 and SrSO4 are always SO2 and O2, independent of temperature via the route MSO4 ¼ MO+SO3 ¼ MO+SO2+1/2O2. Initial vaporisation step involves the evolution of an SO3 molecule, rather than the equilibrium mixture of SO2 and 1/2O2 on the surface. There are several factors which influence the kinetics of final conversion of SO3 to SO2 and 1/2O2 which includes the thickness of the residual MO layer on the sample, possible catalytic effects of the sample surface and the temperature. For this reason the second reaction is much slower than the first reaction which explains the need for prolonged sintering duration (about 60 min) at high temperatures to achieve optimal TL sensitivity. Catalysis by the MgO layer is presumed to be much less effective than that by CaO, SrO or BaO; therefore the apparent products of the reaction are SO3 for MgSO4 and SO2 and O2 for the other sulphates. In addition, temperature plays a major role since the decomposition of SO3 is a thermally activated reaction. 4.5. Phenomenological models Fig. 10a shows a tentative structure of CaSO4:Dy after 700 1C, 1 h annealing. Incorporation of Dy3+ creates cation (Ca2+) vacancy. Thermal dissociation of sulphates (perhaps associated with Dy) presumably could create oxygen interstitial ions (O2i2−) and F+centres, i.e., anion vacancies as shown. The assignment of O2i2− as a hole trap is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation viz., SO4−, SO3−, SO2−, O3− and O− etc. In any case, only hole traps stabilized near cation vacancies lead to their enhanced thermal stability and their thermal activation result in 250 1C TL peak. Other hole traps give rise to low temperature TL peaks. Fig. 10b shows a tentative structure of CaSO4:Dy after 1000 1C, 1 h annealing. Cation vacancies disappear when oxygen from air enters into the host lattice or due to the formation of CaO due to thermal decomposition of CaSO4 which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. Fig. 10c shows a tentative structure of CaSO4:Dy after 400 1C (several hours) annealing. Thermal migration of interstitial oxygen ions into F+ centres at 400 1C would destroy TL. A 700 1C, 1 h annealing could, however, restore the crystal to its original status, i.e. transform Fig. 10c to a while
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Fig. 10. (a) CaSO4:Dy after 700 1C, 1 h annealing. Incorporation of Dy3+ creates cation (Ca2+) vacancy. Thermal dissociation of sulphates associated with Dy presumably or even the presence of oxygen ions attached to Dy (i.e. Dy2O3) could create oxygen interstitial ions (O2i2−) and F+-centres, i.e., anion vacancies as shown. The assignment of O2i2 − as a hole trap is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation viz., SO4−, SO3−, SO2−, O3− and O− etc. (b) CaSO4:Dy after 1000 1C, 1 h annealing. Cation vacancies disappear when oxygen from air enters into the host lattice or due to thermal dissociation of CaSO4 to CaO which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. (c) Recrystallised CaSO4:Dy after 400 1C (several hours) annealing. Thermal migration of O2i2− into F+ centres at 400 1C would destroy TL. A 700 1C, 1 h annealing could, however, restore the crystal to its original status, i.e. transforms c to a while thermal migration of O2i2− ions at 300 1C is negligible. (d) CaSO4:Dy,Na after 700 1C, 1 h annealing. Na+ incorporation removes cation vacancies. (e) CaSO4:Mn after 700 1C, 1 h annealing. Thermal dissociation of sulphates associated with Mn is presumed. (f) Introduction of SiO44− ion at SO42− site would introduce anion vacancies which would act as radiationless recombination sites thereby quenching TL efficiency of CaSO4: Mn,Si as seen in Fig. 3 c. (g) CaSO4:Dy,Zn (added as ZnSO4) after 700 1C, 1 h annealing. Thermal dissociation of sulphates associated with Zn is presumed. Incorporation of ZnO–Zn at substitutional site and oxygen at interstitial site, removes cation vacancies. (h) CaSO4:Dy,Zn (added as ZnCl2) after 700 1C, 1 h annealing. Chlorine gets incorporated during the boiling of ZnCl2 which results in the elimination of cation vacancies.
thermal migration of O2i2− ions at 300 1C is presumed to be negligible. Fig. 10d and e gives a phenomenological picture of the defects in CaSO4:Dy,Na and CaSO4:Mn, respectively. Incorporation of Na+ at Ca2+ sites removes the cation vacancies created by Dy3+ incorporation at Ca2+ sites. Fig. 10f gives the picture of CaSO4: Mn,Si. Introduction of SiO44− ion at SO42− site would introduce anion vacancies which would act as radiationless recombination sites thereby quenching the TL efficiency of CaSO4:Mn,Si as seen in Fig. 3c. Fig. 10g and h gives a picture of the defects in CaSO4:Dy,Zn with ZnSO4 and ZnCl2 as the source of Zn, respectively. While ZnSO4 serves as a good flux, its dissociation at high temperatures changes the defect structure of CaSO4:Dy resulting in the shifting of its TL glow peak to low temperatures. The effect of Cl− ion interstitials also yields the same result. Firing in carbon atmosphere near 700 1C should in principle convert all the ZnO incorporated into ZnSO4 and restore the structure shown in Fig. 10a thereby restoring the 230 1C TL peak to its original intensity. In practice, however, the above reaction seems to be taking place only partially which results in the presence of both 100 1C and 230 1C TL peaks as shown in Fig. 6 a. Firing in carbon atmosphere at 1000 1C or 850 1C removes all the oxygen interstitials including those responsible for the creation of low temperature TL peaks which results in the partial reduction of CaSO4 to CaS, thereby quenching the TL intensities of all TL peaks as seen in Figs. 2 c and 6 b. Firing in sulphur atmosphere or addition of (NH4)2SO4 failed to convert the interstitial oxygen ions into SO42− and as a result the
TL peaks continue to occur in the low temperature region. The evaporation of these compounds at low temperatures may be the reason for their ineffectiveness. However, in the recrystallisation technique which involves the evaporation of totally dissolved CaSO4 and Dy in hot concentrated H2SO4 to dryness, presence of excess of sulphate ions (SO42−) in acidic (pH o7) conditions throughout the crystallisation process prevents thermal dissociation and incorporation of oxygen interstitial ions. This is the prima facie reason for the presence of intense 250 1C TL peak in recrystallised CaSO4:Dy.
5. Conclusion ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice as witnessed from the high TL intensity of CaSO4:Dy,Zn prepared by solid state reaction at 750 1C but its glow peak occurred at about 100 1C. Cation vacancies disappear when oxygen ions enters into the host lattice which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. The source of these oxygen ions could be incorporation of atmospheric oxygen or thermal decomposition of CaSO4 into CaO at 1000 1C or thermal decomposition of ZnSO4 at about 700 1C or even the presence of oxygen ions attached to Dy (i.e. Dy2O3) at interstitial sites. Firing in carbon atmosphere above 850 1C, however, removes all the oxygen interstitials including those responsible for the creation of low
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temperature TL peaks which result in the partial reduction of CaSO4 to CaS, thereby quenching the TL intensities of all TL peaks as seen in Fig. 2 c. Firing in sulphur atmosphere or addition of (NH4)2SO4 failed to convert the interstitial oxygen ions into SO42− and as a result the TL peaks continue to occur in the low temperature region. The evaporation of these compounds at low temperatures may be the reason for their ineffectiveness. Firing in carbon atmosphere at 750 1C with ZnSO4 flux, however, enhanced the intensity of CaSO4:Dy TL glow peaks at 230 1C and at 400 1C. It should in principle convert all the ZnO incorporated into ZnSO4 and restore the 230 1C TL peak to its original intensity. In practice, however, the above reaction seems to be taking place only partially which results in the presence of both 100 1C and 230 1C TL peaks as shown in Fig. 6 a. O2i2−/Oi2− interstitials have been tentatively assigned as hole traps while F+-centres, i.e., anion vacancies have been assigned as electron traps. This means redox process following irradiation should simultaneously convert F+-F (reduction) and O2i2−/Oi2− -O2i−/Oi− (oxidation). A reversal of this process should occur during heating (TL readout). The assignment of O2i2−/Oi2− as a hole trap and F+-centres, i.e., anion vacancies as electron traps is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation by the radiolysis process viz., SO4−, SO3−, SO2−, O3− and O− etc. which is in consonant with the multipeak structure of CaSO4:Dy. In any case, only hole traps stabilized near cation vacancies lead to their enhanced thermal stability and their thermal activation results in 250 1C TL peak. Other hole traps give rise to low temperature TL peaks near 100 1C. CaSO4:Dy,Al exhibited a weak TL peak at 100 1C while no high temperature TL peaks are produced. Among the monovalent sulphates tried, Na2SO4:Dy gave the maximum TL sensitivity, roughly 20% of that of recrystallised CaSO4:Dy but with the glow peak at 100 1C. Sintering treatment near 700–750 1C for 1 h duration is perhaps essential to create the electron and hole traps responsible for 250 1C through the thermal decomposition process. The anomalous reduction in the TL intensity of 250 1C peak in recrystallised CaSO4:Dy on prolonged annealing at 400 1C clearly indicates thermally induced migration of defects/interstitials causing this peak and that high temperature sintering stabilizes the defect
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structure. While 400 1C annealing disturbs the defect, by way of thermal migration, the 700 1C annealing restores it. Thermal annealing much below 400 1C does not affect TL since the activation energy required to migrate the defect was not achieved at lower temperatures. In short, the art of CaSO4:Dy phosphor synthesis with a high temperature (250 1C) TL peak by solid state reaction route lies in controlling the number of oxygen interstitials. There is also a thermal component in stabilizing the defects giving rise to this peak. Similarly, the purpose of ZnSO4 as flux would have been well served provided thermal dissociation of ZnSO4 to ZnO had not taken place. To conclude, hole traps related to radiolysis products at interstitial sites seem to promote 250 1C TL peak while oxygen interstitials due to thermal dissociation promote low temperature TL peak near 100 1C. In both cases, anion vacancies seem to act as electron traps. The TL process of 250 1C peak does not merely involves a redox process but also involve the recombination of the radiolysis products, e.g. SO3−+O−-SO42−.
Acknowledgements We wish to express our profound gratitude to Dr. M.T. Jose and his group members from RSD, IGCAR, Kalpakkam for arranging to take all TL measurements. References [1] A.R. Lakshmanan, Prog. Mater. Sci. 44 (1999) 1. [2] K.H. Stern, E.L. Weise, NSRDS-NBS 7, High Temperature Properties and Decomposition of Inorganic Sulfates, Part 1. Sulfates, USA, Issued October 1, 1966. [3] A.R. Lakshmanan, M.T. Jose, V. Ponnusamy, P.R. Vivek Kumar., J.Phys.D:Appl. Phys. 35 (2002) 386. [4] A.R. Lakshmanan, D. Lapraz, H. Prvost, N. Benabdesselam, Phys. Status Solidi A 201 (2005) 131. [5] V. Ramaswamy, R.M. Vimalathithan, V. Ponnusamy, M.T. Jose, J. Lumin., doi: 10.1016/j.jlumin.2012.06.047, S0022-2313(12)00395-X. [6] B. Sanyal, S.P. Chawla, A. Sharma, Int. J. Lumin. Appl. 3 (10) (2013) 32. [7] T.K. Gundu Rao, B.C. Bhatt, J.K. Srivastava, K.S.V. Nambi, J. Phys. C 5 (1993) 1791. [8] R. Huzimura, K. Asahi, M. Takenga., Nucl. Instrum. Methods 175 (1980) 8. [9] A.K. Bakshi, A.S. Pradhan, K. Srivastava, D.H. Kolambe, Radiat. Prot. Dosim. 100 (1–4) (2002) 293.
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Effect of anion interstitials on the thermoluminescent properties of CaSO4:Dy A.R. Lakshmanan n, V. Sivakumar, R. Sangeetha Rani, S. Kalpana Saveetha Engineering College, Thandalam, Chennai 602105, India
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2013 Received in revised form 20 March 2013 Accepted 29 March 2013 Available online 22 April 2013
Efforts were made to synthesise CaSO4 based thermoluminescent (TL) phosphors by solid state reaction route. ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice as witnessed from the high TL intensity peak at about 100 1C following sintering at 750 1C in air. Its TL intensity is even higher than that of the 260 1C peak appearing in recrystallised CaSO4:Dy. Similar low temperature TL peak was found in recrystallised CaSO4:Dy samples sintered at 1000 1C in air. The results were explained on the basis of incorporation of interstitial oxygen anions which act as hole traps. Firing in sulphur or ammonium sulphate atmosphere did not shift the low temperature TL glow peak. But firing in carbon atmosphere at 750 1C with ZnSO4 flux enhanced the intensity of TL glow peaks at 250 1C and at 400 1C due to the partial re-conversion of oxygen ions to sulphate ions. Firing at 850 1C in reduced atmosphere, however, quenched the intensity of all TL peaks due to the removal of oxygen i.e. partial reduction of CaSO4 to CaS. The slow decline in the intensity of 2501 TL peak on prolonged annealing at 400 1C in recrystallised CaSO4:Dy indicates the thermal migration of defects causing the TL peak. Redox mechanism involving such interstitial ions and anion vacancies in the presence or absence of cation vacancies could lead to the emission of high ( 250 1C) or low ( 100 1C) temperature TL peaks, respectively. Certain other co-dopants tried such as Al3+ and SiO44− simply quenched the TL efficiencies of CaSO4:Dy and CaSO4:Mn, respectively. Among the monovalent sulphates tried, Na2SO4:Dy gave a TL peak around 100 1C which is 20% of the 250 1C TL intensity of recrystallised CaSO4:Dy. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence CaSO4:Dy Solid state synthesis ZnSO4 flux Oxygen interstitials
1. Introduction CaSO4:Dy is a well known thermoluminescent (TL) phosphor used widely for radiation dosimetry [1]. It exhibits an intense and stable TL peak near 250 1C and a feeble one near 100 1C. The effect of cation co-dopants (e.g., Na+,K+,Li+,Ag+,Zn2+, etc.) and anion co-dopants (P5−) at substitutional (Ca++and S6− respectively) sites on the thermoluminescent (TL) characteristics of CaSO4 is well studied. However, the effect of anion co-dopants such as O2−, Cl− etc. at interstitial sites has not been well studied so far. By introducing such defects one can play with charge compensation mechanism and tune the intensity of different TL peaks. Such studies would also throw more insight into the TL production mechanism and in the identification of electron and hole traps in CaSO4:RE which remain so far mystical. Yet another way is thermal dissociation of host anions since very few metal sulphates are sufficiently stable at high temperatures to have a measurable melting point [2]. All decompose eventually with the evolution
n
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of SO3. The latter in turn dissociates to SO2 and O2. Irradiation (radiolysis) also dissociates SO4 radicals leaving behind anion vacancies (F+ centres) but subsequent heating restores them unlike the case with thermally induced dissociation. All these defects create a variety of hole traps at interstitial sites giving rise to a number of TL peaks in CaSO4:Dy. SO42− radicals are unstable against heat as well as irradiation. Radiolysis products in CaSO4 based phosphors reportedly form a variety of hole centres which are related to oxygen and oxy-sulphur radicals (SO4−, SO3−, SO2−, O3− and O−) as observed in electron spin resonance (ESR) and optical absorption (OA) studies. Alternately, the thermal dissociation of some SO42− radicals at high temperatures ( 1000 1C) is also probable, yielding the electrons for the reduction of oxygen atoms at interstitial sites. These two decomposition processes apparently give rise to two types of defects. While the thermally induced defects are labelled host related, the radiation induced defects are attributed to impurities and radiolysis. The role of sintering temperature (650–750 1C) and sintering duration (1–2 h) in the case of CaSO4:Dy, has never been understood fully. Irrespective of the method of preparation, the above sintering treatment has been found to be essential to obtain optimal TL glow curve structure and sensitivity. The TL sensitivity
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systematically increased with sintering temperature in the range 300-750 1C, a result which has not been explained so far [3]. It was assumed that the high temperature treatment resulted in better crystallinity of the host lattice, despite the fact no experimental result has been published so far to support this hypothesis. By intentional doping of anions, one can tune the intensity of the low temperature peak. Incorporation of anions (Oi2−, Cl− etc.) along with Dy3+ dopant in CaSO4 during phosphor synthesis should provide the charge compensation and should result in intense low temperature TL peak at the expense of the 250 1C peak. In this work ZnSO4 was used as the flux as well as oxygen source since it is reported to dissociate at 600 1C. ZnCl2 was used as the chlorine source since it is reported to boil at 732 1C. Preparation of highly sensitive CaSO4 based TL phosphors with a desired glow curve structure using a solid state diffusion technique has not been successful so far. The reasons for this enigma will also be analysed in this paper.
Fig. 1. Thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O.
Table 1 Weight loss on heating, ZnSO4 7H2O. Sample
2. Experimental details Thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O was carried out by weight measurement following successive heat treatments in the temperature range in a furnace for 5 min duration. For phosphor synthesis, CaSO4 2H2O (E. Merck, precipitated and Qualigens grade), ZnSO4/ZnCl2 (all E. Merck grade) and Dy2O3 (Indian Rare Earth Chemicals) were wet mixed and dried at 150 1C and then sintered at high temperatures in a muffle furnace for 1 h duration. Other co-dopants studied include Na, Mn and Al, all in sulphate form. The effects of Si (added in silicic acid form) as well as (NH4)2SO4 and sulphur co-dopants were also studied. Firing was carried out in air as well as in reduced (carbon) atmosphere. Dy doped monovalent sulphates were made by solid state reactions in air near their melting points using Li2SO4 H2O, Na2SO4, K2SO4 and Dy2O3 (all Merck grade) as raw materials. The synthesised samples were characterised by TL and photoluminescence (PL). The TL glow curves were recorded using Riso system reader (model TL/OSL-DA-20). The system allows 48 samples to be individually heated immediately after 90Sr beta irradiation (radiation dose ¼1 Gy in all cases studied). The heater is made of low-mass Kanthal strip. 10 mg of phosphor powder was uniformly spread on the heater strip. The heating system could heat the samples up to 700 1C at constant heating rates from 0.1 to 10 1C/s. To minimise thermal lag between samples and heater strip, a heating rate of 5 1C/s was employed. The heating strip can be cooled by a nitrogen flow. The glow curve has been recorded by running the programme installed in the computer. TL emission spectra were recorded after 60Co gamma irradiation (dose¼ 10 Gy) with a Fluorolog Jobin Yvon-Spex spectrofluorimeter by keeping the irradiated samples at a constant temperature of 180 1C.
3. Results 3.1. Thermogravimetric analysis Results of thermogravimetric analysis of BaSO4, CaSO4 2H2O and ZnSO4 7H2O is shown in Fig. 1. A reduction in the weights observed in ZnSO4 7H2O and CaSO4 2H2O in the temperature region 200–400 1C (by 44% and 21%, respectively) correspond to their dehydration (loss of water of crystallisation). No such weight loss occurs in BaSO4 as no water molecules are attached to it. The minor weight loss by about 6% is seen in ZnSO4 when the annealing temperature was increased from 400 1C to 750 1C (Table 1) which shows that little dissociation of ZnSO4 takes place
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Hydrated, ZnSO4 7H2O Anhydrate, ZnSO4 ZnO After 750 1C 1 h, heat treatment in air
Relative weight Theory
Experiment
1.00 0.56 0.28 –
1.00 0.56 ( 400 1C) 0.53
Table 2 Weight loss on heating, CaSO4 2H2O. Sample
Hydrated, CaSO4 2H2O Anhydrate, CaSO4 CaO After 750 1C, 1 h , heat treatment in air
Relative weight Theory
Experiment
1.000 0.790 0.325 –
1.000 0.796 ( 400 1C) – 0.794
at 750 1C, but this may be well sufficient to influence TL properties! However, in the case of ZnCl2 vigorous fuming and the formation of a ZnO residue as witnessed by its thermochromic behaviour (yellow colour at high temperatures) on heating to 750 1C. In the case of ZnCl2 boiling can introduce oxygen as well as chlorine ion interstitials. Comparatively, the weight loss seen in CaSO4 when the annealing temperature was increased from 400 to 700 1C was negligible ( 0.25%; Table 2). BaSO4 is thermally more stable than the other two sulphates studied (Table 3). 3.2. Effect of high temperature heat treatment on TL of CaSO4:Dy Fig. 2 shows the effect TL glow curves of recrystallised CaSO4: Dy on 1000 1C heat treatment. Air firing shifted the TL glow curve from high temperature to low temperature region. While recrystallised CaSO4:Dy exhibits a major TL glow peak around 260 1C, 1000 1C air fired sample showed a major peak around 110 1C with a high temperature shoulder around 160 1C in agreement with the results reported earlier [4]. However, firing in reduced (carbon) atmosphere at 1000 1C quenched the TL intensities of nearly all the TL peaks; a feeble TL peak around 80 1C is all what that remains in the latter case. 3.3. Solid state synthesis of CaSO4 based TL phosphors 3.3.1. CaSO4:Mn Fig. 3 compares the TL glow curves of CaSO4:Mn,Na and CaSO4: Mn,Si prepared by the solid state diffusion at 650 1C in air with that of the standard recrystallised CaSO4:Dy. MnSO4 H2O, silicic
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Table 3 Weight loss on heating, BaSO4. Sample
BaSO4 BaO After 750 1C, 1 h , heat treatment in air After 820 1C 1 h, heat treatment in air
Relative weight Theory
Experiment
1.000 0.656 – –
1.000 – 0.994 0.992
Fig. 2. TL glow curves of: a—CaSO4:Dy (std—recrystallised), b—(a) +1000 1C, 1 h in air and c—(a)+1000 1C, 1 h in carbon atmosphere. 90Sr beta dose ¼1 Gy.
form of silicic acid) proved to be a very good flux, yielding a bright white powder but the incorporation of Si co-dopant was found to quench the TL intensity of CaSO4:Mn. Addition of sulphur helps in preventing the oxidation of Mn in air. When inserted inside the furnace at 450 1C, the ingredients got fire due to combustion of sulphur in air. The samples were then ground and refired at 650 1C in air for 1 h duration. 3.3.2. CaSO4:Dy,Zn Incorporation of Dy dopant into CaSO4 crystal lattice using solid state diffusion reaction lattice also requires a suitable flux. For this purpose both ZnSO4 and ZnCl2 were tried. ZnSO4 reportedly decompose at 600 1C eventually forming ZnO, but other reports put its decomposition temperature well beyond 800 1C. ZnCl2 reportedly melts at 283 1C and boils at 732 1C and vaporised at 756 1C. Only ZnO residue remains in this case as well. Starting materials include CaSO4 2H2O (Merck as well Qualigens make), ZnSO4 7H2O and ZnCl2, all in analar grade form. ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice (Merck make) as witnessed from the high TL intensity peak observed in Fig. 4. Its TL intensity is even higher than that of the 260 1C peak of recrystallised CaSO4:Dy but the glow peak occurred at about 100 1C. The influence of firing atmosphere (air or carbon) at 750 1C on the TL was very little unlike that observed at 1000 1C (Fig. 2). Zn added as ZnCl2 produced similar results. Results obtained with CaSO4 (Qualigens make) were similar (Fig. 5) but the high temperature tails were much more pronounced in the TL glow curve structure than those seen with Merck samples (Fig. 4). Firing in carbon atmosphere at 750 1C with ZnSO4 flux enhanced the intensity of TL glow peaks at 230 1C and at 400 1C while firing at 850 1C quenched the intensity of all TL glow peaks (Fig. 6). TL emission spectra (Fig. 7) of all CaSO4:Dy samples were found to be characteristic of Dy3+ with emission peaks at about 480 and 570 nm. Differences in emission spectra observed with co-dopants were minor. 3.3.3. Effect of sulphur and NH4SO4 Effects of sulphur and NH4SO4 co-dopants on the TL glow curve structure of CaSO4:Dy was studied in the presence or absence of
Fig. 3. TL glow curves of: a—CaSO4:Dy (Std—recrystallised), b—CaSO4:Mn,Na (solid state synthesis) and c—CaSO4:Mn,SiO2 (solid state synthesis). 90Sr beta dose¼ 1 Gy.
acid and Na2SO4 which were used as raw materials for this purpose. CaSO4:Mn,Na exhibits a simple glow peak around 160 1C whose TL intensity is about 80% of that of CaSO4:Dy but the peak is much narrower in the former sample. Incorporation of MnSO4 into CaSO4 lattice poses the problem of oxidation of Mn in air at high temperatures. Without a flux, incorporation of any impurity into CaSO4 was not found to be feasible. Na2SO4 serves as a good flux in solid state synthesis of CaSO4 based phosphors. But the solubility of Na in CaSO4 host lattice is quite high and so one always ends up with the TL glow peak characteristic of CaSO4:Mn, Na whose TL peak appears slightly on the high temperature when compared to that (Tmax ¼ 100–130 1C), of CaSO4:Mn prepared by the recrystallisation technique as reported in literature. Si (in the
Fig. 4. left: TL glow curves of: a—CaSO4:Dy, Zn(Zn added as ZnSO4) , 750 1CC, 1 h in air, b—CaSO4:Dy,Zn (Zn added as ZnSO4, 750 1C, 1 h carbon atmosphere). All made from CaSO4 2H2O (E. Merck). c—CaSO4:Dy (0.2 mol%) (recrystallised). 90Sr beta dose¼ 1 Gy.
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Fig. 5. right: TL glow curves of: a—CaSO4:Dy,Zn (Zn added as ZnCl2+ZnSO4), 750 1C, 1 h in air, (b) CaSO4:Dy,Zn (Zn added as ZnCl2), 750 1C, 1 h in air. All made from CaSO4 2H2O (Qualigens). 90Sr beta dose ¼ 1 Gy.
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Fig. 7. Thermoluminescent emission spectrum of CaSO4:Dy,Zn (Zn added as ZnCl2+ZnSO4) (a), CaSO4:Dy,Zn (Zn added as ZnSO4) (b) and CaSO4:Dy (recrystallised) (c) recorded at 180 1C. 60Co-gamma dose ¼ 10 Gy. The PL intensities are not to be inter-compared.
750 1C for 2 h in air. The sample exhibited a weak TL peak at 100 1C (Fig. 8) while no high temperature TL peaks are produced. 3.3.5. Monovalent sulphates Dy doped monovalent sulphates were made by firing the intimate mixture of samples in air at 900–1070 1C for 1 h duration since the melting points of Li2SO4, Na2SO4 and K2SO4 were respectively, 845 1C, 884 1C and 1069 1C. Among them Na2SO4:Dy gave the maximum TL sensitivity, roughly 20% of that of recrystallised CaSO4:Dy but with the glow peak at 100 1C (Fig. 9).
4. Discussion 4.1. Electron and hole traps
Fig. 6. a—TL glow curves of CaSO4:Dy,ZnSO4 (750 1C, 1 h in carbon atmosphere), b—CaSO4:Dy,ZnSO4 (850 1C, 1 h in carbon atmosphere). The TL intensities of sample b are magnified by a factor of 10. All made from CaSO4 2H2O (Qualigens). The enhancement of high temperature peaks with qualigens makes gypsum chemical more pronounced than that obtained with Merck gypsum precipitated chemical. 90Sr beta dose¼1 Gy.
ZnSO4 flux by firing in air an intimate mixture of the constituent powder mixture at 700 1C for 1 h in air. However, results showed that none of these efforts succeeded in shifting the major TL peak to high temperatures (200–250 1C) desired in radiation dosimetry.
3.3.4. Trivalent co-dopants Due to its trivalency, incorporation of Al3+ at Ca2+ should produce cation vacancies similar to the case of Dy3+. This should in principle enhance the high temperature TL peak near 250 1C. However, Al2(SO4)3 is reported to dissociate at 770 1C which should introduce oxygen interstitials. To confirm this view, solid state synthesis of CaSO4:Dy3+,Al3+ with dysprosium in the form of dysprosium oxide and aluminium in the form of Al2(SO4)3 18H2O was carried out at
While the hole trap corresponding to 250 1C peak in CaSO4:Dy phosphor sintered at 700 1C has been assigned tentatively to sulphate radicals stabilized by Ca2+ vacancies created by the incorporation of Dy3+ cations at Ca2+ lattice sites, the electron traps in this phosphor have not yet been identified. This proposal is supported by the fact that the incorporation of monovalent cations such as Na+, Li+ or K+ along with Dy3+ removes the cation vacancies while the 250 1C TL peak vanishes. Instead, the intensity of host related low temperature TL peak near 100 1C is enhanced. Pre-irradiation annealing at temperatures ≥800 1C in air drastically reduces the 250 1C TL peak in CaSO4:Dy while the low temperature peaks near 100 1C are strongly enhanced similar to that of the Na co-doped sample. In CaSO4:Dy samples sintered at 1000 1C, the 250 1C peak totally vanished while the TL peaks near 100 1C showed nearly the same intensity as that of the 250 1C peak found in samples sintered at 700 1C. This was explained earlier on the basis of thermal dissociation of CaSO4 to CaO or diffusion of atmospheric oxygen (O2−) ions from air into the CaSO4 host lattice which get stabilized at interstitial sites in CaSO4:Dy samples near Dy ions when sintered at 1000 1C in air. CaSO4 starts decomposing above 800 1C, which should also introduce oxygen interstitials. This has resulted in the elimination of cation vacancies since oxygen ions provide the charge compensation for Dy3+ incorporation. Redox mechanism (O2i2−/Oi2−)-(O2i−/Oi−) – oxidation during irradiation and (O2i−/Oi−-O2i2−/Oi2−) – reduction during TL
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dissociated species seem to act like electron traps. Unlike the case with 100 1C TL peak, the TL process of 250 1C peak does not merely involve a redox process but also involve the recombination of the radiolysis products, e.g. SO3−+O−-SO42−. We shall see later that, prior to irradiation, there is also a thermal component involved in stabilizing the defects giving rise to 250 1C TL peak. 4.2. Decomposition of metal sulphates
Fig. 8. TL glow curves of: a—CaSO4:Dy (0.2 mol%) (Std—recrystallised) and b—CaSO4:Dy,Al (solid state synthesis, 750 1C, 1 h in air). 90Sr beta dose ¼ 1 Gy.
Fig. 9. TL glow curves of Na2SO4:Dy (a), K2SO4:Dy (b) and undoped CaSO4 (c) and Li2SO4:Dy (d). 90Sr beta dose ¼1 Gy. X marks magnification.
readout has been proposed to explain the TL process of the 100 1C peak. This means O2i2−/Oi2− acts as a hole trap. The anion vacancies (F+ centres) created by the dissociation and subsequent migration of sulphate radicals should act as electron traps. Similar proposal has been made recently to explain the TL process in undoped BaSO4 [5]. The fact that 612 nm optical absorption (OA) band attributed to O− ions has been observed in CaSO4:Dy single crystals along with other host lattice related radicals only after Xirradiation supports the involvement oxygen interstitials in the TL process. The intensity of the 612 nm band increased proportionately with radiation dose and the post-irradiation thermal decay of this band did really show two major annealing steps, one near 100 1C and the other near 250 1C supporting the involvement of redox mechanism in oxygen ions in both these TL peaks. It is likely that the hole traps near cation vacancies are more stable due to a lack of positive charge and the corresponding TL peak occurs at a relatively higher temperature (i.e. at 250 1C) while the hole traps without neighbouring cation vacancies are relatively less stable and give rise to the low temperature TL peak near 100 1C. The hole traps related to radiolysis products at interstitial sites seem to promote 250 1C TL peak while oxygen interstitials promote low temperature TL peak near 100 1C. In both cases, anion vacancies left behind by the radiolysis products or thermally
If the TL glow peak temperature of undoped (though quite weak in intensity) and impurity doped material (with strong TL intensity) remain the same, the defect giving rise to the peak is labelled as host related. This is the case with BaSO4. Both undoped and Eu doped BaSO4 give TL peak near 200 1C [5]. Same case is true with undoped CaSO4 and Mn doped CaSO4. Both give TL peak near 100 1C. When host related excitons (self-trapped) decay under impurity emission, the TL intensities are strong. This happens to be the case when the STE emission spectra overlaps well with the PL excitation spectra of the impurity. The major TL glow peaks in CaSO4:Dy3+ and CaSO4:Ag+,Dy3+ occur near 250 1C and 400 1C, respectively. The hole traps in the latter two phosphors are related to impurities which do not have same valency as that of the host. Recently, an electron spin resonance (ESR) study by Sanyal et al. [6] was carried out to understand the change in TL glow curve structure of CaSO4:Dy on high temperature firing (900 1C, 2 h in air). ESR spectra recorded at 77 K exhibited three radiation induced radicals which were attributed to SO3−, SO4− and O3−. On the basis of enhancement in ESR signal/TL intensities and thermal decay characteristics, SO4− radical was attributed to the low temperature TL peak near 130 1C. Surprisingly all the above three radicals disappeared on thermal annealing below 130 1C which means that no ESR signal corresponding to the high temperature TL peak near 250 1C has been observed! Another study by Gundu Rao et al. [7] also indicated that the decay of radiation induced SO3− radical to occur at the same temperature as that for the SO4− centre which influences the 1001 TL peak in CaSO4:Dy. Therefore they commented that further investigations are needed to verify whether SO3− centres play any role in the TL emission. In contrast, Huzimura et al. [8] correlated the TL peaks at 220 1C and 285 1C in CaSO4:Tm to the stimulated relaxation of SO3− ions and relaxed SO3− centres respectively. The contradictions in these results show that the defect structure in RE doped phosphors is far from exact identification. Superoxide anion is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. With one unpaired electron, the superoxide ion is a free radical, and, like dioxygen, it is paramagnetic. Superoxides are compounds in which the oxidation number of oxygen is −½ and the valence ½. The O–O bond distance in O2− is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O22−. A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion ([O–O]2−). The peroxide is not a radical and is also not paramagnetic which explains the difficulty in detecting the O22− species through ESR. The O–O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1. The simplest stable peroxide is hydrogen peroxide. A reason for the quenching of TL efficiency of CaSO4:Dy on Al co-doping (Fig. 8) is due to the decomposition of Al2(SO4)3 which directly proceeds to oxide without any intermediate oxysulphate as follows: Al2(SO4)3 ¼ Al2O3+3SO2+3/2O2. Weight loss has been measured during decomposition between 650 1C and 950 1C. Cation vacancies apparently disappear when additional oxygen ions released by the decomposition of Al2(SO4)3 are incorporated in CaSO4 crystal lattice which leads to the quenching of 250 1C TL
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peak. In principle for every Al3+ ion incorporated, 6Oi−, or 3O2i2− defects can be created. In addition, ionic radius of Al3+ ion (53.5 pm) is too small compared to that of Ca2+ ion (100 pm) which makes it as a highly incompatible structure leading to quenching of luminescence efficiency. In contrast, Zn2+ has a relatively higher ionic radius (72 pm) and more importantly it has the same valency as that of the host cation. A reason for the relatively high TL sensitivity of Na2SO4:Dy among the monovalent metal sulphates tried is a better match of ionic radius of Dy3+ (91.2 pm) with that of Na+ (102 pm) as compared to those of Li+ (76 pm) or K+ (138 pm). Incorporation of trivalent Dy3+ at monovalent Na+ cation site would, however, introduce a large number of cation vacancies while the incorporation of oxygen interstitial anions when Dy is incorporated in the form of Dy2O3 would remove them, thereby quenching the high temperature TL peak which explains the relatively poor TL efficiency of Na2SO4:Dy when compared to that of CaSO4:Dy. 4.3. Thermal migration of defects The fact that sintering treatment near 700–750 1C is perhaps essential to create the defects responsible for causing TL indicates the creation/stabilisation of electron and hole traps through thermal the decomposition process. It is relevant to consider that on heating all of the anhydrous sulphates of type RE2(SO4)3, decompose, without melting to basic salts (oxysulphates) of the type RE2O3.SO3, then to an oxide. The oxide final product is RE2O3. A major anomaly arose from the experimental results reported in literature that the sensitivity of 2501 TL peak CaSO4:Dy sintered at 700 1C decreased slowly on thermal annealing at 400 1C (roughly 1% per 1 h annealing) only to recover fully following sintering at 700 1C. No such reduction in TL was seen on thermal annealing at 300 1C [9]. This result clearly indicated thermally induced migration of defects/interstitials causing the 2501 TL peak and that high temperature sintering stabilizes the defect structure giving rise to the dosimetric TL peak near 250 1C in CaSO4:Dy. While 4001 annealing disturbs the defect, possibly by way of thermal migration, the 700 1C annealing restores it. Thermal annealing much below 400 1C does not affect TL since the activation energy required to migrate the defect was not achieved at lower temperatures. Our earlier attempt to explain this anomolous behaviour on the basis of damage to the trap structure caused by hydration of Dy2(SO4)3 at room temperature and its dehydration at 400 1C when doped within CaSO4 is not supported by the experimental result that dehydration of Dy2(SO4)3 in free state occurs in two stages, one at 105 1C and the other at 175 1C but both these steps occur much below 400 1C. A change in the trap structure, however, explains this result well, though the exact mechanism is still speculative. One such premise includes thermal migration of O2i2− into a sulphate ion vacancy which would eliminate electron and hole traps thereby reducing the TL intensity as indicated in Fig. 8 c; Thermal migration below 400 1C is less probable while high temperature i.e. 700 1C annealing could remove the O2i2− ions from the sulphate ion vacancy and restore the two defects to its original state thereby restoring the original TL intensity. CaSO4 decomposes directly to CaO. The existence of an intermediate oxysulfate and discrepant values for the melting point of CaSO4 are explained by a eutectic between CaO and CaSO4 near 1265 1C, with less than 20% CaO. Such high temperatures are, however, not encountered in this study and therefore one can rule out decomposition of CaSO4 as the cause for the above changes. But the situation could be quite different with the sulphates associate with Dy which are relatively less stable thermally. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) have shown that on heating, all of the anhydrate sulphates of the trivalent rare earth elements (R) and yttrium,
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type formula R2(SO4)3, decompose, first without melting, to basic salts (oxysulphates) of the type R2O3.SO3, then to an oxide. The oxide final product is R2O3 for all these elements. Hence thermal decomposition of R2(SO4)3 could constitute major interstitial defect(s). The melting point of MnSO4, 700 1C, quoted in handbooks, is reported to be erroneous. Decomposition is reported to begin before melting. The product usually observed on pyrolysis in air is Mn3O4, as in TGA, though Mn2O3 may appear mixed with it in the lower temperature range, complicating the kinetic picture [2]. 4.4. Thermal stability of monovalent and divalent suphates In the short time of TGA runs, K2SO4 appeared stable to 900 1C. A slight loss in weight at 1000 1C was attributed to sublimation. Na2SO4 appeared stable beyond 900 1C. No weight loss occurred up to 900 1C, 0.04% at 1000 1C and 1.5% at 1200 1C. This result agrees with the TGA studies in literature. In the short time of TGA runs, Na2SO4 appears stable to beyond 900 1C. No weight loss was observed in maintaining Na2SO4 for 2 h up to 900 1C, 0.04% at 1000 1C and 1.05% at 1200 1C. The composition of the residue showed that both decomposition and volatilisation has occurred [2]. A study on the high temperature behaviour of sulphates suggests that the thermal decomposition products in CaSO4, BaSO4 and SrSO4 are always SO2 and O2, independent of temperature via the route MSO4 ¼ MO+SO3 ¼ MO+SO2+1/2O2. Initial vaporisation step involves the evolution of an SO3 molecule, rather than the equilibrium mixture of SO2 and 1/2O2 on the surface. There are several factors which influence the kinetics of final conversion of SO3 to SO2 and 1/2O2 which includes the thickness of the residual MO layer on the sample, possible catalytic effects of the sample surface and the temperature. For this reason the second reaction is much slower than the first reaction which explains the need for prolonged sintering duration (about 60 min) at high temperatures to achieve optimal TL sensitivity. Catalysis by the MgO layer is presumed to be much less effective than that by CaO, SrO or BaO; therefore the apparent products of the reaction are SO3 for MgSO4 and SO2 and O2 for the other sulphates. In addition, temperature plays a major role since the decomposition of SO3 is a thermally activated reaction. 4.5. Phenomenological models Fig. 10a shows a tentative structure of CaSO4:Dy after 700 1C, 1 h annealing. Incorporation of Dy3+ creates cation (Ca2+) vacancy. Thermal dissociation of sulphates (perhaps associated with Dy) presumably could create oxygen interstitial ions (O2i2−) and F+centres, i.e., anion vacancies as shown. The assignment of O2i2− as a hole trap is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation viz., SO4−, SO3−, SO2−, O3− and O− etc. In any case, only hole traps stabilized near cation vacancies lead to their enhanced thermal stability and their thermal activation result in 250 1C TL peak. Other hole traps give rise to low temperature TL peaks. Fig. 10b shows a tentative structure of CaSO4:Dy after 1000 1C, 1 h annealing. Cation vacancies disappear when oxygen from air enters into the host lattice or due to the formation of CaO due to thermal decomposition of CaSO4 which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. Fig. 10c shows a tentative structure of CaSO4:Dy after 400 1C (several hours) annealing. Thermal migration of interstitial oxygen ions into F+ centres at 400 1C would destroy TL. A 700 1C, 1 h annealing could, however, restore the crystal to its original status, i.e. transform Fig. 10c to a while
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Fig. 10. (a) CaSO4:Dy after 700 1C, 1 h annealing. Incorporation of Dy3+ creates cation (Ca2+) vacancy. Thermal dissociation of sulphates associated with Dy presumably or even the presence of oxygen ions attached to Dy (i.e. Dy2O3) could create oxygen interstitial ions (O2i2−) and F+-centres, i.e., anion vacancies as shown. The assignment of O2i2 − as a hole trap is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation viz., SO4−, SO3−, SO2−, O3− and O− etc. (b) CaSO4:Dy after 1000 1C, 1 h annealing. Cation vacancies disappear when oxygen from air enters into the host lattice or due to thermal dissociation of CaSO4 to CaO which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. (c) Recrystallised CaSO4:Dy after 400 1C (several hours) annealing. Thermal migration of O2i2− into F+ centres at 400 1C would destroy TL. A 700 1C, 1 h annealing could, however, restore the crystal to its original status, i.e. transforms c to a while thermal migration of O2i2− ions at 300 1C is negligible. (d) CaSO4:Dy,Na after 700 1C, 1 h annealing. Na+ incorporation removes cation vacancies. (e) CaSO4:Mn after 700 1C, 1 h annealing. Thermal dissociation of sulphates associated with Mn is presumed. (f) Introduction of SiO44− ion at SO42− site would introduce anion vacancies which would act as radiationless recombination sites thereby quenching TL efficiency of CaSO4: Mn,Si as seen in Fig. 3 c. (g) CaSO4:Dy,Zn (added as ZnSO4) after 700 1C, 1 h annealing. Thermal dissociation of sulphates associated with Zn is presumed. Incorporation of ZnO–Zn at substitutional site and oxygen at interstitial site, removes cation vacancies. (h) CaSO4:Dy,Zn (added as ZnCl2) after 700 1C, 1 h annealing. Chlorine gets incorporated during the boiling of ZnCl2 which results in the elimination of cation vacancies.
thermal migration of O2i2− ions at 300 1C is presumed to be negligible. Fig. 10d and e gives a phenomenological picture of the defects in CaSO4:Dy,Na and CaSO4:Mn, respectively. Incorporation of Na+ at Ca2+ sites removes the cation vacancies created by Dy3+ incorporation at Ca2+ sites. Fig. 10f gives the picture of CaSO4: Mn,Si. Introduction of SiO44− ion at SO42− site would introduce anion vacancies which would act as radiationless recombination sites thereby quenching the TL efficiency of CaSO4:Mn,Si as seen in Fig. 3c. Fig. 10g and h gives a picture of the defects in CaSO4:Dy,Zn with ZnSO4 and ZnCl2 as the source of Zn, respectively. While ZnSO4 serves as a good flux, its dissociation at high temperatures changes the defect structure of CaSO4:Dy resulting in the shifting of its TL glow peak to low temperatures. The effect of Cl− ion interstitials also yields the same result. Firing in carbon atmosphere near 700 1C should in principle convert all the ZnO incorporated into ZnSO4 and restore the structure shown in Fig. 10a thereby restoring the 230 1C TL peak to its original intensity. In practice, however, the above reaction seems to be taking place only partially which results in the presence of both 100 1C and 230 1C TL peaks as shown in Fig. 6 a. Firing in carbon atmosphere at 1000 1C or 850 1C removes all the oxygen interstitials including those responsible for the creation of low temperature TL peaks which results in the partial reduction of CaSO4 to CaS, thereby quenching the TL intensities of all TL peaks as seen in Figs. 2 c and 6 b. Firing in sulphur atmosphere or addition of (NH4)2SO4 failed to convert the interstitial oxygen ions into SO42− and as a result the
TL peaks continue to occur in the low temperature region. The evaporation of these compounds at low temperatures may be the reason for their ineffectiveness. However, in the recrystallisation technique which involves the evaporation of totally dissolved CaSO4 and Dy in hot concentrated H2SO4 to dryness, presence of excess of sulphate ions (SO42−) in acidic (pH o7) conditions throughout the crystallisation process prevents thermal dissociation and incorporation of oxygen interstitial ions. This is the prima facie reason for the presence of intense 250 1C TL peak in recrystallised CaSO4:Dy.
5. Conclusion ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice as witnessed from the high TL intensity of CaSO4:Dy,Zn prepared by solid state reaction at 750 1C but its glow peak occurred at about 100 1C. Cation vacancies disappear when oxygen ions enters into the host lattice which explains the disappearance of 250 1C TL peak and enhancement of low temperature peaks associated with charge transfer of electrons from interstitial oxygen ions to F+ centres. The source of these oxygen ions could be incorporation of atmospheric oxygen or thermal decomposition of CaSO4 into CaO at 1000 1C or thermal decomposition of ZnSO4 at about 700 1C or even the presence of oxygen ions attached to Dy (i.e. Dy2O3) at interstitial sites. Firing in carbon atmosphere above 850 1C, however, removes all the oxygen interstitials including those responsible for the creation of low
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temperature TL peaks which result in the partial reduction of CaSO4 to CaS, thereby quenching the TL intensities of all TL peaks as seen in Fig. 2 c. Firing in sulphur atmosphere or addition of (NH4)2SO4 failed to convert the interstitial oxygen ions into SO42− and as a result the TL peaks continue to occur in the low temperature region. The evaporation of these compounds at low temperatures may be the reason for their ineffectiveness. Firing in carbon atmosphere at 750 1C with ZnSO4 flux, however, enhanced the intensity of CaSO4:Dy TL glow peaks at 230 1C and at 400 1C. It should in principle convert all the ZnO incorporated into ZnSO4 and restore the 230 1C TL peak to its original intensity. In practice, however, the above reaction seems to be taking place only partially which results in the presence of both 100 1C and 230 1C TL peaks as shown in Fig. 6 a. O2i2−/Oi2− interstitials have been tentatively assigned as hole traps while F+-centres, i.e., anion vacancies have been assigned as electron traps. This means redox process following irradiation should simultaneously convert F+-F (reduction) and O2i2−/Oi2− -O2i−/Oi− (oxidation). A reversal of this process should occur during heating (TL readout). The assignment of O2i2−/Oi2− as a hole trap and F+-centres, i.e., anion vacancies as electron traps is tentative. It could very well be supplemented with a variety of other hole centres observed in ESR spectra after irradiation by the radiolysis process viz., SO4−, SO3−, SO2−, O3− and O− etc. which is in consonant with the multipeak structure of CaSO4:Dy. In any case, only hole traps stabilized near cation vacancies lead to their enhanced thermal stability and their thermal activation results in 250 1C TL peak. Other hole traps give rise to low temperature TL peaks near 100 1C. CaSO4:Dy,Al exhibited a weak TL peak at 100 1C while no high temperature TL peaks are produced. Among the monovalent sulphates tried, Na2SO4:Dy gave the maximum TL sensitivity, roughly 20% of that of recrystallised CaSO4:Dy but with the glow peak at 100 1C. Sintering treatment near 700–750 1C for 1 h duration is perhaps essential to create the electron and hole traps responsible for 250 1C through the thermal decomposition process. The anomalous reduction in the TL intensity of 250 1C peak in recrystallised CaSO4:Dy on prolonged annealing at 400 1C clearly indicates thermally induced migration of defects/interstitials causing this peak and that high temperature sintering stabilizes the defect
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structure. While 400 1C annealing disturbs the defect, by way of thermal migration, the 700 1C annealing restores it. Thermal annealing much below 400 1C does not affect TL since the activation energy required to migrate the defect was not achieved at lower temperatures. In short, the art of CaSO4:Dy phosphor synthesis with a high temperature (250 1C) TL peak by solid state reaction route lies in controlling the number of oxygen interstitials. There is also a thermal component in stabilizing the defects giving rise to this peak. Similarly, the purpose of ZnSO4 as flux would have been well served provided thermal dissociation of ZnSO4 to ZnO had not taken place. To conclude, hole traps related to radiolysis products at interstitial sites seem to promote 250 1C TL peak while oxygen interstitials due to thermal dissociation promote low temperature TL peak near 100 1C. In both cases, anion vacancies seem to act as electron traps. The TL process of 250 1C peak does not merely involves a redox process but also involve the recombination of the radiolysis products, e.g. SO3−+O−-SO42−.
Acknowledgements We wish to express our profound gratitude to Dr. M.T. Jose and his group members from RSD, IGCAR, Kalpakkam for arranging to take all TL measurements. References [1] A.R. Lakshmanan, Prog. Mater. Sci. 44 (1999) 1. [2] K.H. Stern, E.L. Weise, NSRDS-NBS 7, High Temperature Properties and Decomposition of Inorganic Sulfates, Part 1. Sulfates, USA, Issued October 1, 1966. [3] A.R. Lakshmanan, M.T. Jose, V. Ponnusamy, P.R. Vivek Kumar., J.Phys.D:Appl. Phys. 35 (2002) 386. [4] A.R. Lakshmanan, D. Lapraz, H. Prvost, N. Benabdesselam, Phys. Status Solidi A 201 (2005) 131. [5] V. Ramaswamy, R.M. Vimalathithan, V. Ponnusamy, M.T. Jose, J. Lumin., doi: 10.1016/j.jlumin.2012.06.047, S0022-2313(12)00395-X. [6] B. Sanyal, S.P. Chawla, A. Sharma, Int. J. Lumin. Appl. 3 (10) (2013) 32. [7] T.K. Gundu Rao, B.C. Bhatt, J.K. Srivastava, K.S.V. Nambi, J. Phys. C 5 (1993) 1791. [8] R. Huzimura, K. Asahi, M. Takenga., Nucl. Instrum. Methods 175 (1980) 8. [9] A.K. Bakshi, A.S. Pradhan, K. Srivastava, D.H. Kolambe, Radiat. Prot. Dosim. 100 (1–4) (2002) 293.