Thermoluminescene and optical absorption of BaFCl crystals

zyxwvutsrq zyxwvutsr zyxwv zyxwvutsrqpo K. SOXAIAH e t al. : Thermoluminescence and Optical Absorption of BaFCl 737 phys. stat. sol. (a) 56, 737 (1979) Subject classification: 20.3; 10.2; 20.1; 22.5.4 Solid State and Materials Science Laboratories, Department of Physics, College of Science, Osmania University, Hyderabadl) Thermoluminescence and Optical Absorption of BaFCl Crystals BY K. SO&AIAH,P. VEERESHAM, K. L. N. PRASAD~), and V. HARI BABU Thermoluminescence and optical absorption spectra of X- and y-irradiated BaFCl crystals are studied as function of irradiation time and after annealing at room temperature for different periods. Four glow peaks in the temperature ranges 330 t o 350, 365 t o 370, 380 t o 385, and 400 to 415 K are obtained. The glow curves obtained are resolved by thermal cleaning technique and later analyzed assuming first order kinetics. The trap depth and frequency factor of 415 K peak are calculated employing various methods. From the thermoluminescence and optical absorption studies of irradiated and annealed crystals the first two low-temperature peaks are attributed to the detrapping of electrons from impurities and subsequent recombination with holes whereas t h e other two peaks at 385 and 415 K are associated with F(E1)and F(F) centers respectively. Es werden Thermolumineszenz- und optische Absorptionsspektren von Rontgen- und y-bestrahlten BsFCl-I<ristallen als Funktion der Bestrahlungsdauer und nach Temperung bei Zimmertemperatur mit verschiedener Dauer untersucht. Vier Glowmaxima in den Temperaturbereichen 330 biu 350, 365 bis 370, 380 bis 385 und 400 bis 415 K werden gefunden. Die Glowkurven werden durch thermisches Entleeren aufgelost und darauf unter der Annahme einer Kinetik erster Ordnung analysiert. Die Haftstellentiefe und der Frequenzfaktor des 415 K-Maximums werden mit verschiedenen Nethoden berechnet. Aus den Untersuchungen der Thermolumineszenz und optischen Absorption von bestrahlten und getemperten Kristallen werden die ersten beiden Tieftemperaturmaxima dcr Befrciung der Elektronen aus Storstellen und der nachfolgenden Rekombination mit Lochern zugeschrieben, wahrend die anderen beiden Maxima bei 385 nnd 415 K mit F(E)bzw. F(@-Zentren verknupft sind. zyxw zyx zyxwvutsr 1. Introduction I n recent years single crystals of mixed dihalides such as BaFCl have been grown by various techniques [l]. Somaiah and Hari Babu [Z] have grown BaFC1, BaFBr, SrFCl, and SrFBr by direct and indirect flux techniques. Characterization of these crystals has shown that the crystals are of fairly good quality. F-centres in BaFCl have been studied by Fong and Yucorn [3] and Nicklaus and Fischer [4]by optical absorption. Yuste et al. [ 5 ] have employed optipal absorption and ESR t o study F-centers in both BaFCl and SrFC1. The thermoluminescence of irradiated alkali halides has been studied for many years. An important aspect of these studies has been to establish a correlation between thermoluniinescence and thermal stability of the radiation induced color centers, mainly F-centers. Thernioluininescenee of BaFBr crystals have been reported earlier [6] and in the present investigation, therrnoluminescence and optical absorption measurements of X-ray and y-ray irradiated BaFCl crystals have been undertaken t o obtain l) 2, Hyderabad 500007, India. Department of Physics, Indian Institute of Technology, Madras. zyxwvu 738 K. SOIMAIAEI, P. VEERESHAM, K. L. N. PRASAD, and V. HARIBAEV more information about the F-centers. Glow peaks have been analyzed by various methods and the activation energy and frequency factor have been calculated. The various trapping centers responsible for the occurrence of glow peaks have been identified. 2. Experimental The BaFCl crystals were grown by flux method and the details have been reported earlier [2]. X-rays from a n iron target operated a t 30 kV and 15 mA and y-rays from a 6oCo source, dose rate being 36MR h-l are used for irradiation purpose. The light emitted during the heating of irradiated crystals was measured by means of RCB 931 d and EM1 6256 S photomultiplier tubes. The glow curves were recorded at a heating rate of 40 K m-l. A 65 W heating element powered from a s a t a b l y adjusted variac was used for this purpose. The output of the photomultiplier tube was fed t o a dc amplifier and finally recorded by Sarvogar two-channel or DIGLOG-2000 X - Y recorders. The optical absorption spectra were taken on a Cary 14(R) recording spect rop hot omet er. zyxwvut zyxwvut zy zyxwvu zyxwvutsrq zyxwv 3. Results 3.1 Glow curves The glow curves obtained by exposing the BaFCl crystal to X-irradiation for different tiines are shown in Fig. 1. I n general it is observed that the intensity of the glow peaks is enhanced with irradiation time up t o a certain extent and then decreases. I n the as-grown crystals irradiated for 30 s two broad and overlapping peaks positioned a t 345 and 365 K are observed. The intensity of the 345 K peak is larger than that of the 365 K peak. Irradiating for 2 min results in a n increase in intensity of both peaks. The intensity of 345 peak however is smaller than that of the 3 6 5 K peak. Increasing the irradiation time to 5 inin results in a overall increase in intensity of both the peaks. The 345 K peak now appears as a shoulder t o the 365 K peak. O n prolonged irradiation i.e., 20 t o 60 min the intensity decreases, the low temperat [ire TlKi Fig. 1 - z - T(K! Fig. 2 Fig. 1. Effect of irradiation time on glow curve of BaFCl. (a) t 20 min, (e) 1 h, (f) 15 min = 30 s, (b) 2 min, (c) 5 min. (d) Fig. 2. Glow curves of BaFCl crystal resolved into component peaks by the thermal cleaning technique Thermolnniincscence and Optical Absorption of BsFCl Crystals zyxw zy 739 zyxwv zyxwvu zyxwv zyxwvu woveiengfh(nmi Fig. 3 - Fig. 4 Fig. 3. Glow ctirves of y-irradiated BaPCI, irradiated for (a) 2 h, (b) 24 h, (c) 24 h and anneal~ng for 40 d a t R T Fig. 4. Optical absorption spectra of y-irradiated BaFCl crystal, irradiated for (a) 2 h, (b) 24 h, (c) 24 h and annealed for 6 month shoulder almost disappears, and only the 365 K peak occurs. No appreciable change is observed as regards the position of the 365 K peak with time of X-irradiation. I n Fig. 1, curve (f) represents the glow curve of a BaFCl crystal quenched from 600 "C before it is irradiated. Two additional shoulders situated a t 385 and 400 K can be seen. I n order t o isolate the peaks from the complex curve ( f ) of Fig. 1 a thernial cleaning technique is employed. Fig. 2 shows the glow curves obtained after successive thermal cleanings ultimately giving rise to an isolated peak a t 415 K. The glow curves are also recorded on y-irradiated as-grown BaFCl crystals. I n Fig, 3 curve (a) represents the glow curve obtained after exposing the crystal for 2 h. I n this curve two broad peaks positioned a t about 330 to 350,365 to 370 may be seen in addition to a small peak a t 415 K. Prolonged irradiation for 5 to 24 h gives rise to two peaks a t 380 and 415 K and these are shown in curve (b), Fig. 3(c) represents the glow curve of a 24 h y-irradiated crystal obtained after room temperature annealing for 40 days. It consists of only one peak a t 415 K. 3.2 Optical absorption The optical absorption spectra of BaFCl taken a t RT is shown in Fig. 4. Two broad absorption bands a t about 440 and 550 nm are obtained in a crystal irradiated for 2 h (curve a). When the crystals are irradiated for longer times (24 h) the bands a t 440 and 550 nni split and double bands are obtained (curve b). These results agree with those reported earlier [4]. Optical absorption spectra taken on a sample irradiated for 2.1 h and after 6 months has shown only one band at 440 nm with very much decreased height (curve c). zyxwvu zyxwvutsrqp 3.3 Analysis of glow czirves The TL peak shown in Fig. 3 (c) has been analyzed by various methods and the trap depths and frequency factors obtained are given in Table 1. The geometrical factor 4s phlsica (a) 56/1? zyxwvutsrqpo 740 zyxwvut zy zyxwvutsrq zyxwv zyxwvu K. SOMAIAH, P. VEERESHAM, K. L. N. PRASAD, and V. HARIBABU 1' 7------ Fig. 5. Fitting the 415 K glow curve to Randall and Wilkins first-order kinetics equation. experimental, a calculated zyxwvu zyxwvut rot- zyxwv pg = Sjw is found to be less t h a n 0.52 and first-order kinetics is therefore employed. The glow curves are also analyzed by the method of numerical curve fitting [7]. n this method the Randall-Wilkins function [8] is used t o fit the observed intensity a t various temperatures I ( T ) = hopoexp -e __ T exp using the following approximation: m zy Table 1 Calculated values of trap depth and frequency factor by various methods for the 415 K glow peak I First-order kinetics Half-width a t the lower tempera,ture side ( 5 ) a t the peak (Halperin and Branner equation [lo] and modified by Chen [ll]) E _- -exp 4 E (eV) peak values of Po (101's-l) equation method 3.6 E - 3.16 k T , EIkT, kT; I1 Half-width towards fall-off side E =0.976 (6) a t the peak (Lushchiks equation [l2] modified by Chen [ll]) I11 Shape of the glow curve total half intensity width (w) (Chen's equation [ll]) IV Randall-Wilkin's equation [8] kT2 6 kT" E = 2.29 2 1.11 12 1.13 20 W E = 25 k T , 0.9 2.1 zy zyxwvu (r: zyxw zyxwvut zyxwv zyxwvutsr 741 Therrnoluminescence and Optical Absorption of BaFCl Crystals Thus equation (1) reduces to { x 1 -2 (g)+ '):( 6 - 24 + 120 (Ti'}], (3) where 8 = E l k , E is the trap depth, E the Boltzmann constant, h, the nuniber of electrons a t temperatures To, T , the glow pcak temperature, Po the frequency factor, and 4 the heating rate. The numerical calculations were carried out by using an IBM 3701155 computer. The solution of the general equation (3) for the thermoluminescent intensity I ( T ) gave the theoretical estimates for the 415 "C glow peak Fig. 3 (c). The experimentally determined and calculated intensities for the above glow peak are shown in Fig. 5 . The agreement between the experimental and theoretical glow curve shape is generally good. The best fit t o the actual curve was obtained with the value of E = 1.15 eV and Po = 42 x 10 s-l. zyxwvuts zyx 4. Discussion In alkali halides the anion vacancy occupied by an electron is known as F-center. Since BaFCl crystallizes in the tetragonal system Ui,:) both anionic sites, fluorine and chlorine, have tetragonal symmetry with the fourfold axis parallel to the optical axis c of the crystal. So two types of F-centres could be created in such crystals, the F(F) and F(a)centers corresponding to trapped electrons a t fluorine and chlorine vacancies, respectively. Taking into account the analogy between the position of the 550 nni band in BaFCl and that of the F-band in BaF,, Nicklaus and Fischer [4] concluded that the 550 and 440 nin absorption bands might be attributed to F(F) and F ( a ) centers, respectively. On the other hand, Yuste et al. [ 5 ] from a careful analysis of ESR spectra have disagreed with this attribution and concluded that the correct assignment is the opposite of that suggested by Kicklaus and Fischer [4]. This has been supported by the theoretical study of F-centers in BaFCl made by Lefrant and Harker [9]. The results show that X-ray and y-ray irradiation of BaFCl crystals produces four glow peaks. The peaks are situated in the temperature ranges 330 t o 350 K, 365 t o 370 K, 380 t o 385 K, and 400 to 415 K and they are designated as TI, T,, T,, and T, peaks. The glow curves obtained immediately after y-irradiation and after annealing a t room temperature for 40 days show that the three low temperature glow peaks disappear after some time and only the T4 peak is obtained. The optical absorption spectra of y-irradiated and annealed crystals (Fig. 4,curve c) show only one absorption band a t 440 nm. These results suggest that the T4 peak corresponds to the thermal ionization of F(F) centers. The optical absorption spectra of BaFCl crystals taken immediately after y-irradiation for 24 h show two absorption bands situated a t 440 and 550 nm. The glow curve obtained under the above conditions has two glow peaks (T, andT,). Since the T4peak has been attributed to F(F)centers, the T, peak probably corresponds to F(Z1)centers. The two low temperature peaks TI and T, which are quite unstable as compared to T, and T4 peaks may be associated with impurities. centers. These results show that F(F) centers are more stable than F(a) 3, 4s* A.S.T.M. Card, Index 3-0304. 742 zyxwvu zy zyxwvutsrq zyxwvut K. SOMAIAH et al. : Thermoluminrscence and Optical Ahsorytion of BaFC’l Arlsiioicledgeiiieiits One of us (K. S.) thanks the University Grants Commission, New Dclhi, India for awarding hiin a Teacher Fellowship and Financial assistance. The authors wish t o t h an k Dr. T.V. G. S. Nurti, Department of Physics, Indian Institute of Technology, Madras for helpful discussions and Prof. G. Sivarania Sastrj., Head of the Department of Physics, Osinania University for his interest and encoiirageinent . zyxwvut zyxw zyxwvutsrqpo zyxwv References B. SICKLAUS and P.FISCIIEIL J . Crystal Growth 12, 337 (1972). I<. S O W ~ I Aand H V. HARIB m u . Indian J . pure appl. Phys. 14, 702 (1976). F. K. FONG and 1’. X. YUCOM, J. chein. Phys. 41, 1383 (1964). E. SIGKLALTS and F. FISCIIER, phys. btat. sol. (b) 32, 453 (1972). I S ] 31. PLJSTE, L. TAUREL,A I . RAIIXAXLand D. LEXOYKE,J. Phys. Chem. Solids 3 i , 961 [I] 121 131 [4] (1976). [6] 11. XETHA KEDDY,K. SONAIAH.a n d V. HARIBABU,phys. stat. sol. (a) 54,243 (1979). [7] N. h. MOHAXand It. CHEN, J. Phys. 3,243 (1970). [ 8 ] J . T. R A K U - ~ Land L M. H. F. WILICIXS. Proc. Roy. Soc. A18.1, 365 (1943). [9] S. LEFRAITand A. H. HARKER. Solid State Commim. 19, 853 (1976). [lo] A. HALPERIN and A. A. BRANVER, Phys. Rev. 117, 408 (1960). [ I l l K.CHEN, J. appl. Phys. 40,670 (1969). [12] C‘. B. LUSHCHIK, Dokl. Akad. Nark SSSR 101, 641 (1965). (Rrcrized J d ! j 27, 1.979)