9477
J. Phys. Chem. 1991,95, 9417-9480
Pressure Dependence of the Thermoluminescence of X-ray- Irradiated Alkali Halidest B. J. Baer and H. G. Drickamer* School of Chemical Sciences, Department of Physics and Materials Research Laboratory, University of Illinois (Champaign-Urbana), Urbana, Illinois 61801 (Received: April 24, 1991; In Final Form: June 10, 1991)
The thermoluminescence glow curves of a number of X-irradiated alkali halides have been observed at pressures up to 3.5 GPa. The changes in trap depth and in the thermoluminescencefrom F centers due to pressure are shown to be quite significant for NaBr, KCI, and KBr. The effect of pressure on the thermoluminescenceof M centers in the high-pressure phase of KCI and KBr is also shown. Discontinuitiesin the trap depths and the temperature at which the thermoluminescence is at maximum intensity occur at the phase transitions of KCI and KBr.
Introduction There have been a number of studies done of the thermoluminescence of irradiated alkali halides over the past three decades.I-I6 These studies are concerned with the effects of irradiation time7-9 (both y- and X-ray), emission spectra,l0 and the effects of dopants.11-16 The goal, largely, is to explain the nature of the thermoluminescence of alkali halides and calculate the trap depths of the electron in the various color centers. Studies of the pure alkali halides mainly focus on the glow peaks from the creation of F and M centers. An F center is created when an electron is present at an anion vacancy. An M center results when there are two adjacent F centers in a (100) plane. It has also been s h ~ w nthat ~ * these ~ peaks are temperature inhibited, meaning that the glow curve peak at its maximum intensity reaches a temperature maximum, for a given heating rate, after a minimum radiation dose. There is also a peak in the glow curves that is assigned as due to plastic deformation of the ~amp1e.l.~ There are three competing explanations for the thermoluminescence phenomena: First, the F center acts as an electron trap, and the electron is released from the trap upon heatinge7 Second, the F center acts as a recombination site for thermally released ho1es.l' Third, halogen atoms are mobile and are stabilized at interstitial positions in the crystal lattice.* Light would be emitted somewhere along the path toward halogen recombination with F centers. Pressures generated by the diamond anvil cell on the alkali halides can significantly change their lattice constant and in some cases cause a phase transition. It was hoped that the effect of pressure on the thermoluminescence of various alkali halides would provide an insight to this phenomenon. No previous work of this kind is known to us. Experimental Section Diamond anvil cells of the type developed by Merrill and Bassett17 were used to generate pressure up to 4.0 GPa. The diamonds were of the modified brilliant design and had a 0.5" culets with 16 facets on both the table and culet. The alkali halides were powdered into small crystals and irradiated for approximately 24 h by using an X-ray tube with a Mo target operated at 35 kV and 20 mA. The irradiated crystals were kept in the dark and refrigerated until needed. This was done to prevent thermal and optical bleaching. Two or three crystals along with some ruby chips were placed inside the 0.3-mm sample hole of the metal gasket. Only the most deeply colored crystals are used since these crystals should all have the same temperature-inhibited glow peaks. A drop of silicone oil, which serves as a hydrostatic pressure medium, was placed on top of the hole, and then the cell was closed. The diamond cell was placed in an aluminum jacket. The jacket had an insulated wire heater wrapped around it. A small fiber optic cable was placed between the cell and a photomultiplier. The fiber optic line was shielded by a small metal pipe which prevented stray light, much of which 'This work was supported in part by the Materials Science Division of the Department of Energy under Contract DEFG02-91ER45439.
comes from the red hot heating wire, from affecting our results. The photomultiplier was connected to a photon counter. These counts were stored on an IMB PC/XT during the experimental runs. Unfortunately, the emission is too weak to obtain data on the spectral output of the sample with our equipment. An experimental run consisted of heating the sample at a constant heating rate up to approximately 600 K. Heating rates of 10, 20, and 30 K/min were used. The temperature was measured by using an iron-constantan thermocouple that are in contact with one of the diamonds on one side and with a water-ice bath on the other. The heating rate was maintained constant by a specially designed electronic controller made by the School of Chemical Sciences electronic shop. We estimate that the possible error in our temperature measurement is about 5 K a t 450 K. Since the crystals are bleached after each run, it is necessary to reload the cell for the next run. At the end of each run, after the cell cools, the pressure is obtained by using the ruby luminescence method.l*J9 Since the sample can be optically bleached, the pressure could not be obtained before the experimental run. This creates an additional 0.2-GPa uncertainty for the sample pressure. The ruby luminescence measurements were obtained with a 0.5-m monochromator by using the equation:*O
P (in GPa) = 380.8[X,(7')/X0(i")1~ - 1 where X,is the wavelength of the zero pressure of the R I line of ruby luminescenceand is experimentally determined. Excitation of the ruby is done by a d d laser using the 442-nm line. Glow curves were plotted by using an HP 7470A plotter. If the background from blackbody radiation was significant, then another
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(1) Ausin, V.; Alvarez Rivas, J. L. Phys. Reu. B 1972, 6, 4828. (2) Ausin, V.; Alvarez Rivas, J. L. J . Phys. C 1974, 7 , 2255. (3) Rao, D. R.; Das, B. N. Cryst. Latrice Defects 1976, 6, 243. (4) Ang, T. C.; Mykura, H.J . Phys. C 1977, 10, 3205. (5) Murti, Y . V. G. S.;Sucheta, N. Phys. Status Solidi B 1982,109,325. (6) Mariani, D. F.; Brito, F.; Oyarzun, C.; Vignolo, J. J . Phys. Chem. Solids 1987, 48, 37 1 . (7) Jain, S.C.; Mehendru, P. C. Phys. Reu. 1965, 140, A957. (8) Ausin, V.; Alvarez Rivas, J. L. J . Phys. C 1972, 5, 82. (9) Mariani, D. F.; Alvarez Rivas, J. L. J . Phys. C 1978, 11, 3499. (10) Aguilar, M.; Lopez, F. J.; Jaque, F. SolidStare Commun. 1978,28, 699. ( 1 1 ) Klick, C. C.; Claffy, E. W.; Gorbics, S. G.; Attix, F. H.;Schulman, J. H.;Allard, J. G. J . Appl. Phys. 1967, 38, 3867. (12) Rascon, A.; Alvarez Rivas, J. L. J . Phys. C 1978, 11, 1239. (13) Moharil, S. V. Crysr. Lorrice Defects 1980, 9, 35. (14) Rascon, A.; Alvarez Rivas, J. L. J . Phys. C 1981, 14, L741. (15) Sastry, S.B. S.;Nagarajan, S. Phys. StarusSolidi B 1983, 117, 171. (1 6) Dalal, M. D.; Sivaraman, S.;Murti, Y . V. G. S. Cryst. Lairice Dtfects Amorphous Mater. 1985, I I , 15. (17) Merrill, L.; Bassett, W. A. Reu. Sei. Instrum. 1974, 45, 290. (18) Forman. R. A.; Piermarini, G. J.; Barnett. J. D.; Block, S.Science 1912, 176, 284. (19) Barnett, J. D.; Block, S.;Piermarini, G. J. Reo. Sci. Instrum. 1973, 44. 1. (20) Mao, H.K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. J . Appl. Phys. 1918, 49, 3216.
0022-365419 112095-9411%02.50/0 0 1991 American Chemical Society
Baer and Drickamer
9478 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
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run was made immediately after the first so that the background could be subtracted.
Results and Discussion A large number of glow curves were obtained on NaBr, KCI, and KBr samples, at various heating rates, in order to calculate trap depth changes with pressure. These three alkali halides proved the best candidates for thermoluminescence given the pressure and temperature regime in which the glow curve peaks occurred. Some typical glow curves are shown in Figures 1 and 2 for KCl and NaBr, respectively. The F and M center peaks, as well as the plastic deformation peak, are assigned on the basis of previous thermoluminescence work in the literat~re.’~~*’.~ The potassium halides undergo a phase transition at approximately 1.9 GPa to the CsCl structure, which does not destroy their color centers. The transition is likely to create only line defects in the solid and not the point defects associated with the observed phenomena. Furthermore, no significant intensity changes were observed to account for any large-scale creation or destruction of color centers. The thermoluminescence of these compounds in their high-pressure phase was of particular interest to us since this phenomenon had not been previously observed. We also tried to obtain data on NaCI, but its glow peak moved beyond our experimental limit at relatively low pressures. Some data were obtained for RbBr, but F centers could not be easily created with Mo radiation. The glow curves of both CsCl and CsBr proved too complicated to be of much use, as was the case for RbBr and KBr when they transformed to the CsCl structure. This may be due to an increase in the number of possible types of defects that can exist for alkali halides in the CsCl structure. Previous electronic absorption results2’ on the X-irradiated alkali halides in the CsCl structure
generally exhibited more complex spectra. The glow curve peak temperature maxima versus pressure was plotted in Figures 3,4, and 5 for NaBr, KCI, and KBr, respectively. Linear regression was used to plot the lines and calculate the T, at zero pressure and the change of T,,, with pressure. The data scatter was usually low, except for KCI. On some runs the data were conflicting with the general results. These data were deleted, but did not significantly change the overall result. We believe that on some runs the pressure changed drastically upon heating and invalidated the results. NaBr and KBr had very few runs that were discarded. The pressure ranges for the experiment were dictated by the following considerations: (1) The thermoluminescence intensity decreased below our detection limit as the pressure increased; (2) the glow curve moved beyond our temperature range; or (3) the plastic deformation peak interfered with the F center peak. What is apparent in all three plots is the substantial increase in the temperature of the intensity maximum versus pressure. This indicates a large effect on the thermoluminescence with a small change in the lattice constant. This is particularly true of NaBr, which is shown in Figure 2. There is also a very large drop in T, (the temperature at which the thermoluminescence intensity reaches a maximum) at the transition. This can be seen for KCI by comparing panels b and c in Figure 1. The glow curve characteristics for some of the alkali halides we tried are listed on Table I. It is observed that an alkali bromide has a significantly lower T, than its corresponding alkali chloride at any given pressure while in the NaCI structure. (21) Maisch, W. G.; Drickamer,H.G.J . Phys. Chem.Solids 1958,5,328.
The Journal of Physical Chemistry,Vol. 95,No. 23, 1991 9419
Thermoluminescence of X-Irradiated Alkali Halides
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By obtaining glow curves at different heating rates, trap depths can be calculated. The trap depths calculated in Table I were obtained by using the formula by Hoogenstraated2
Figure 5. T, versus pressure for KBr. The lines are drawn by using a least linear squares fit. The symbols 0 and 0 represent heating rates of 10 and 20 K/min heating, respectively. Note the large decrease in T, at the phase transition (1.9 GPa). The symbol A represents a second peak observed, possibly also from F centers, with 20 K/min heating. The symbol X represents the M center peak at heating rate of 20 K/min. The symbols * and 0 represent the plastic deformation peak at heating rates of 10 and 2 0 K/min, respectively.
The trap depths of alkali halides with the NaCl structure, which we could measure, show a significant increase in trap depth with increasing pressure. KCl and KBr have similar percentage increases in their trap depth over the same pressure range. For some glow curves we could apply the initial rise methodz3 to calculate trap depths. This method usually resulted in trap depths 20-30% greater than for Hoogenstraaten’s method, but showed the same trends. It is suggested by K i v i t ~ , through ~~,~~ extensive computer analysis, that Hoogenstraaten’s method yields more accurate results. It should be noted that our calculated trap depths are shallower than those previously r e p ~ r t e d and ~ . ~ our values of T , are greater than those previously r e p ~ r t e d . ~ - ~ * ~ ~ Unfortunately, only the KC1 samples had a glow curve with just one peak that was clearly the F center when in the CsCl structure. Consequently, we could not calculate the trap depth for any other
E (in K) = In [(T~,/T~z)~(w~/wI)~[T~IT~z/(T~I - TmJ1 (23) Garlick, G. F. J.; Gibson, A. F. Proc. Phys. Soc. A 1948, 60, 574. (24) Kivitz, P.; Hagebeuk, H. J. L. J . Lumin. 1977, IS, I . where w Iand w 2are the first and second heating rates, respectively. (25) Kivitz, P. J . Lumin. 1978, 16, 119. (22) Hoogenstraaten, W. Phirips Res. Rep. 1958, 13, 515.
(26) Veersham, P.; Subba Rao, U. V.; Hari Babu, V. Crysf.Lorrice Defecrs
1980, 9, 23.
9480 The Journal of Physical Chemistry, Vol. 95. No. 23, 1991
Baer and Drickamer
TABLE I: Characteristics of Some of the Compounds Tested Using Various Hefting Rates4 10 K/min heating
compound NaCl NaBr KCI (LP) KBr (LP) KCI (HP) KBr (HP)8 KCI M center (HP) KBr M center (HP)
T,)K 389 465 404 411
dTm/dP, K/GPa 82 51 35 54
20 K/min heating dTm/dP, T,)K K/GPa 505 139 405 74 486 50 433 33 51 492 51, 5 5 432, 456 36 1 17 365 17
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30 K/min heating dT,/dP, Tm,bK K/GPa 419 499
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1.31d 0.88 0.44 0.58
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'Errors in dT,/dP are approximately 5%. Errors in trap depths are approximately 20%. T,,, at 0.0 GPa for LP phase; T, at 1.9 GPa for HP phase. 'Trap depths for this compound calculated by comparing the 20 and 30 K/min data. dExtrapolated from data. cTrap depths for this compound calculated by comparing the IO and 20 K/min data. 'Trap depths for this compound calculated by comparing the 20 and 30 K/min extrapolated data. 8Two glow peaks observed. alkali halide used in our experiments which has that structure. The trap depth decreased significantly for KCI at the transition and is substantially constant thereafter. The decrease at the transition is most probably due to the increase in anion to cation distance at the transition, which goes from 3.06 to 3.19 A; however, the coordination increases from 6 to 8. The zero pressure distance is 3.1 5 A. Interestingly, this change in the anion to cation distance to near the zero pressure value at the transition also corresponds to the change in T , to near its zero pressure value at the transition. The T , decreases sharply at the transition for KBr and RbBr, as well, although the RbBr data scatter was so great that we cannot confidently quantify the amount. The glow peak due to plastic deformation',' was present in almost every run on the KCI, KBr, NaBr, CsCI, CsBr, and RbBr samples. This should not be surprising given the fact that under high pressure even a small strain on the sample, as the pressure was being increased, could create some kind of deformation. It is interesting to note that there is no discontinuity in the T , of this peak at the transition. The low-pressure phases of the KBr and KCI samples had relatively low scatter in the data so that trap depths could be calculated. The KCI samples showed that this peak was nearly pressure independent and the trap depth was approximately 1.O eV. However, the trap depth of the plastic deformation peak for KBr decreased from 1.35 to 0.72 eV over the pressure range of the low-pressure phase. The thermoluminescence of M centers were generally too weak and too broad to permit any analysis. However, when the potassium halides had undergone a phase transition to the CsCl structure, the glow peak due to presence of M centers was significantly more intense. This could be due to (1) a change in the emission wavelength to a more sensitive part of the spectrum for the photomultiplier, possibly due to the large change in size of the M center or (2) the aggregation of F centers to form M centers. The M center glow peaks of KBr and KCI displayed similar temperature characteristics, but it was felt that the data we obtained are insufficient to calculate trap depths. The theoretical model that best fits our data is the one suggested by Ausin and Rivas.8 If the halogen atoms are mobile, then increasing the pressure would make it more difficult for those atoms to move from one interstitial site to the other. The phase
transition would change the relative location of all the interstitial sites. It was shown that KCl has an increase in the cation to anion distance at the transition, and this may account for the decrease in T,. The other alkali halides in the CsCl structure may have several pathways that lead to thermoluminescence, hence their more complicated glow curves, due to the larger size of the interstitial sites. The two other theoretical models mentioned in the introduction involve thermally releasing electrons or holes from their traps. However, this would not explain the wide range of trap depth changes. In NaBr the trap depth nearly triples over an increase of 1.6 GPa, but T , increases by less than 30%. Additionally, the high-pressure phase of KCI displays a slightly decreasing trap depth as the pressure is increased. This suggests that the mechanism is more complicated than releasing electrons or holes from their traps.
Summary We have observed the thermoluminescence of a number of X-irradiated alkali halides. The temperature dependence of the thermoluminescence was shown to be greatly affected by pressure. There is a large decrease in the T,,,and trap depth of the potassium halides at their transition pressure which is approximately 1.9 GPa. We have also shown that there are significant increases in the calculated trap depths of alkali halides in the NaCl structure. It is believed that these data support the model proposing mobile halogen atoms at interstitial sites as the mechanism for thermoluminescence. The glow peak due to plastic deformation is common, but not well understood. Note Added in Proof. A very recent study indicates that the diamonds used in this work exhibit a weak thermoluminescent peak at -290 OC at a heating rate of 30 K/min. This could contribute significantly to the intensity of the plastic deformation peak mentioned above. Acknowledgment. We greatly appreciate the assistance of Dr. Scott R. Wilson in getting our samples irradiated. We would like to thank C. J. Hawley for designing and building the temperature control unit. We would also like to thank our machine shop for building the heating jacket for the diamond cell.
