Thermoluminescence and laser Raman studies on RbI:Gd2+

216

Nuclear

THERMOLUMINESCENCE S.B.S. SASTRY Department

Instruments

and Methods

AND LASER RAMAN STUDIES

in Physics Research B32 (1988) 216-221 North-Holland, Amsterdam

ON RbI : Gd*+

and G. MURALIDHARAN

of Physics, Indian Institute of Technology, Madras 600 036, India

TL glow of RbI : Gd2+ shows a single p eak. During a TL process aggregation of V-centres is observed. Laser Raman spectra show two lines at 111 cm-’ and around 150 cm-‘. Doping of the Gd*+ Ions enhances the aggregation. TL emission contains two bands at 2.6 and 2.9 eV. The 2.9 eV band in TL emission is attributed to the impurity. From photostimulated emission studies the TL process is identified as due to the thermal release of F-electrons.

1. Introduction

400 o C to achieve a better ions in the host crystals.

Among the alkali halides, the iodides differ from others in some aspects. Iodides are more difficult to be coloured than bromides and chlorides. From studies on RbI, it was found that the thermoluminescence process in alkali halides leads to aggregation of V-centres or in other words the aggregation of halogen molecular ions [l]. Rzepka and his co-workers [2-41 recently used the laser Raman scattering (LRS) technique to identify the V-centres and their aggregates in potassium halides, and they suggested that divalent cationic impurities stabilize the X; ions (X - halogen ions). In order to study the effect of Gd2+ on the aggregation process and also on the thermoluminescence (TL) of RbI, TL emission, optical absorption, fluorescence and LRS of RbI : Gd2+ are studied.

2. Experimental The crystals (0.05 and 0.1 wt.% Gd) were grown in our laboratory by the Bridgemann technique. The samples used for the studies had a size of 5 X 5 X 1 mm3. They were irradiated at RT with a 6oCo gamma source of 0.15 MR/h. The TL setup used is described in an earlier work [5]. The glow curves were recorded at a heating rate of 110 K/min. This heating rate, although it appears to be a little fast, is chosen so as to enable a comparison of TL intensity of pure RbI (which is known to be a poor TL emitter) with that of samples doped with different kinds of impurities, some being TL enhancers (e.g., Tl+, Eu2+) and some TL killers (e.g., OH-, Ni’+). TL emission spectra were recorded using a Jarrel-Ash monochromator with omnidrive in conjunction with an R955 photomultiplier and a Pacific model 129 laboratory photometer. LRS were recorded using a Cary-82 Raman spectrometer. All experiments were carried out after quenching the samples from 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

homogeneity

of the impurity

3. Results and discussions TL glow of a crystal gamma-irradiated with different doses is given in fig. 1. It consists of a single peak which shifts to higher temperatures on increasing the dose. This is the usual glow which appears as an F-centre recombines with V-centres. When the dose is increased further (for 95 min irradiation), a shoulder appears at the low temperature side on the main glow. The TL intensity is less in quenched crystals than in “as grown” crystals, other factors remaining the same. The reduction in glow intensity after quenching the crystals suggests that the uniform distribution of impurity ions leads to a reduction in luminescent recombination. This can be explained on the basis of V-centre aggregation processes. In an “as grown” crystal there are more aggregated impurity-vacancy complexes. When one quenches the crystals after annealing them at temperatures high enough to break the aggregates, the impurities are more uniformly distributed. It is known that the I; ion is a linear molecular ion with C,, symmetry and it occupies two anion sites and a cation vacancy along the [lOO] direction in alkali iodides [6]. When the impurities (and hence the cation vacancies) are homogeneously distributed, the V-aggregate formation is also enhanced and hence a reduction in luminescent recombination. The shoulder appearing in the glow curve on prolonged irradiation is likely to be due to Z,-centres. From previous studies on rubidium halides doped with alkaline earth and rare earth ions it is found that Z,-centres are formed even during the irradiation process itself [7,8] (though they are normally produced by shining F-light on crystals containing F-centres). The glow peak attributable to Z,-centres is normally ob-

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/ Thermoluminescence

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Fig. 1. TL glow of RbI : Gd*+: (1)as grown irradiated for 35 ruin; Quenched crystals irradiated for (2) 35 min, (3) 10 min, (4) 95 min, (5) 17 h. served at lower temperatures compared to the main glow peaks because of their lower thermal stability. Comparing the present results with them, the new glow peak can be attributed to Z,-centres. Because of the poor colourability of iodides the same could not be confirmed through optical absorption studies as is done in other cases [l]. The total glow does not increase with dose though there is a proportional increase in F-centre concentration (in fact on very heavy irradiation the glow is completely suppressed). Even after a full thermal run (upto 525 K) the crystals appear still coloured which means all the thermally released centres are not recombining with complementary centres; instead some complex centres are formed. The crystals lose colour completely when heated to 675 K. Optical absorption of the

crystals after a full thermal run did not show any absorption in the F-band region whereas it does in the UV region. In (KI + I,) aqueous solution [9] and electrolytically coloured KI crystals [lo] absorption bands due to I; ions were reported at 3.5 and 4.29 eV. Laser Raman studies on these systems showed lines due to I; ions confirming their presence and its consequent 3.5 and 4.29 eV absorption. In RbI an absorption at 3.6 eV was attributed to Vs-centres [ll]. In ultrapure RbI, Nagarajan observed these Vs-centres [l]. He found that there is a residual absorption after a complete TL run even in the ultrapure samples. The absence of F-centres and the presence of an absorption in the UV region indicates that the F-electrons are thermally released and a portion of them lead IV. HALIDES

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/ Thermoluminescence

and laser Raman studies

to aggregation of V-centres. The V-centres (other than V, and H) are diamagnetic and hence not much information could be obtained about their structure, and optical absorption was the only tool available to study them earlier. These V-centres (V,, V,, V,) were suggested to be X; ions (X denotes halogen) [12]. Because of their molecular nature the study of Raman scattering throws light on them. Rzepka and his co-workers [2-41 studied the V-centre by the laser Raman scattering technique (LRS). Our results on LRS of RbI : Gd*+ irradiated for two different doses and also after a partial thermal cleaning

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Fig. 3. Laser Raman spectrum of crystals irradiated for 50 days and stored in the dark for 20 days. (1) 0.1 wt.%, (2) 0.05 wt.% of Gd*+, and (3) 0.05 wt.% of Gd*+ crystals after a thermal run.

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Fig. 2. Laser Raman spectrum of crystals irradiated for 9 days (1) 0.1 wt.% and (2) 0.05 wt.% of Gd*+.

are shown in figs. 2 and 3. It is observed that in ail cases there is a strong line at 111 cm-’ and a shoulder around 150 cm-‘. (The line around 220 cm-’ is the first overtone of 111 cm-l.) The intensity of the 111 cm-’ Iine is higher in the impurity doped crystals and also it increases with concentration of the impurity. In X-irradiated KI at 145 K Rzepka et aI. [13-151 observed an intense line at 111 cm-’ and a weak line at 173 cm-‘. On prolonged irradiation new lines are found to appear at 180 and 189 cm-’ while the 111 cm-’ line has almost disappeared and the 173 cm-’ line is reduced in intensity. They attributed the 173 cm-’ line to

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vicinity of the divalent cation. The increase of 1; in crystals doped with Gd2+ supports this idea of stabilisation. Now we have to see why the intensity is less in 0.1% Gd2+ crystals than in 0.05% Gd2+ crystals after storage for 20 days. There seem to be two ways of explaining this. (1) The aggregation of impurity ions, which will be naturally more for a higher concentration of impurity ions, can make the available cation vacancies fewer thus destabilising the I; ions. (2) When one stores the irradiated crystals, F-electrons may become mobile, and by recombining with the 1; ions they may form higher V-aggregates reducing the I; ion concentration through the probable reaction:

I-I stretching vibration of the I; complex formed during irradiation and the 111 cm-’ line to the I; ions. In RbI, Nagarajan observed lines at 111 and 150 cm-’ and attributed them to I; ions [l]. In the present study (RbI : Gd2+) we did not observe any lines at 180 and 189 cm-’ even after very large doses (one sample 9 days and another 50 days y-irradiated) of continuous irradiation. On storing the crystals irradiated for 50 days in dark for 20 days, it was observed that the 111 cm-’ Raman line is less intense in crystals having a larger concentration of impurity. Comparing the intensity of 111 and 150 cm-’ in the 9 day irradiated and the 50 day irradiated crystals we find that the 150 cm-’ line becomes prominent in the latter crystals. The same was reported in pure RbI [I] as well. Even after a thermal cleaning process the line around 150 cm-’ remains unchanged in intensity which means that the centres responsible for this line are more stable than the ones which lead to the 111 cm-’ line. It is known that the larger aggregates are thermally more stable than the I; and hence the 150 cm-’ line is likely to be due to 1, type aggregates. The increase in the concentration of X; ions is attributable to the compensating vacancies created by the doping of the divalent ion. Taurel et al. [3] studied the effect of divalent cations on the interstitial stabilisation in KBr : Sr2+ and KI : Ca2+. They found that the interstitials are stabilised as Br; and I; ions in the

X;

+e--+X-+X;.

Xi (V=centre) is known to be unstable at RT. They may further break up to give Xi and an electron: Xi

+ X! + electron.

These X: molecules when they are near I; ions facilitate the formation of higher aggregates like I;, I;, etc.: x,

+ x; + x,,,

.

Such higher aggregates are easily formed in a higher concentration of impurity doped crystals since there is a large concentration of I; ions initially (before storage).

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Fig. 4. TL emission of 5; h irradiated RbI : Gd2+.

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07

06

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Fig. 5. Curve 1: photostimulated

36

40

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emission on F-stimulation. Curves 2: fluorescence (a) emission on excitation at 3.65 eV, (b) excitation spectrum corresponding to the 2.95 eV emission.

To understand the mechanism of recombination, the spectral distributions of TL and F-light stimulated emission were studied. The TL emission of a crystal irradiated for 18 h shows a band at 2.9 eV with a shoulder around 2.6 eV (fig. 4). Nagarajan reported three emission bands at 2.2, 2.4 and 2.6 eV in pure RbI [l]. The 2.6 eV band in the present case is attributed to the thermally released F-electrons recombining with hole centres. The 2.9 eV emission which was not observed in the pure sample may be due to inpurity ions. The emission on F-stimulation is seen as a single band at 2.9 eV (curve 1, fig. 5). Fluorescence of unirradiated crystals exhibits an emission band at 2.9 eV when excited at 3.65 eV (curve 2(a), fig. 5). The excitation spectrum for this emission shows two bands, at 4.43 and 3.65 eV. The 4.43 eV band is weak compared to the 3.65 eV band. The emission is attributed to Gd’+ ions. Hills and Scott made a study on the incorporation of trivalent cations (La3+, Y 3+, Gd3+, Dy3+, Sm3+ and Nd3+) in KC1 [16]. From their study they concluded that these ions can be incorporated into the KC1 lattice only in the divalent state. In fact Lidiard showed that if the ionisation potential associated with the removal of an electron from an atom or ion to yield a given oxidation state is greater than about 22 V, the element would not exist in the given oxidation state, but converts into a lower one [17]. As the third ionisation potential of Gd is likely to be

more than 22 V, we believe that the impurity has entered the lattice only in the divalent state and the fluorescence is attributable to Gd’+ ions. From the results on TLE, PSE and fluorescence the 2.9 eV band observed in TLE is attributed to the recombination of thermally released F-electrons with Gd3+ ions, and the TL process is due to the thermal release of F-electrons.

4. Conclusions The doping of Gd’+ aids the formation of V-aggregate centres. TL emission contains the characteristic emission of the impurity. The TL process is identified as due to the thermal release of F-electrons and their recombination with complementary V-centres and Gd3+ ions. One of the authors (G.M.) thanks the Council of Scientific and Industrial Research, India, for awarding a Senior Research Fellowship.

References [l] S. Nagarajan, Ph. D Thesis, Indian Institute of Technology, Madras, India (1985). [2] E. Rzepka, J.L. Doualan, S. Lefrant and L. Taurel, J. Phys. Cl5 (1982) L119.

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/ Thermoluminescence

[3] L. Taurel, E. Rzepka and S. Lefrant, Radiat. Eff. 11 (1983) 111. [4] S. Lefrant and E. Rzepka, J. Phys. Cl2 (1979) L5373. [5] S.B.S. Sastry and S. Sapru, Phys. Status Solidi B94 (1979) K149. [6] S. Radhakrishna and B.V.R. Chowdari, Forts&r. Phys. 25 (1977) 511. [7] S.B.S. Sastry and S. Nagarajan, Phys. Status Solidi B117 (1983) 171. [8] R.K. Gartia and B.S. Acharya, Phys. Status Solidi A47 (1978) K165. [9] W. Kiefer and H.J. Bernstein, Chem. Phys. Lett. 16 (1972) 5.

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[lo] T. Okada, J. Phys. Sot. Jpn. 50 (1981) 582. [ll] E.M. Winter, D.R. Wolfe and R.W. Christy, Phys. Rev. 186 (1969) 949. [12] H.N. Hersh, Phys. Rev. 105 (1957) 1410. [13] E. Rzepka, J.L. Doualan and L. Taurel, J. Phys. Cl6 (1983) 4769. [14] E. Rzepka, S. Lefrant, L. Tame1 and A.E. Hughes, J. Phys. Cl4 (1981) L767. [15] B.V. Shanabrook and J.S. Lannin, Solid State Commun. 38 (1981) 49. [16] N.E. Hills and B.A. Scott, Nature 192 (1961) 1086. [17] A.B. Lid&d, Handbuch der Physik, vol. 20 (1957) p. 246.

IV. HALIDES