H. LEVIN,C. C. BERGGREN, AND V. R. HONNOLD
4228
Thermal Activation Energies in Lithium Fluoride, Sodium Fluoride, and Sodium Chloride Crystals
by H. Levin, C. C. Berggren, and V. R. Honnold Hughes Aircraft Company, Culver Cdy, California (Received March 29, 1967)
Thermoluminescence measurements have been carried out on LiF, NaF, and NaCl single crystals X-irradiated in air at 293 and 373°K. Resultant glow curves have been analyzed to obtain thermal activation energy values corresponding to glow peaks representing thermal release of electrons from bound electron centers. Provisional identification of such electron trapping levels was based on computation of electron-trapping cross sections. These computations indicate that the F’ level exists at 1.06 ev in LiF, 0.72 ev in NaF, and 0.62 ev in NaC1.
Introduction The literature pertaining to thermal activation energy studies in alkali halides is quite extensive except for the lithium and certain sodium salts. Because of obvious diffculties, seldom is any attempt made to associate such thermal depth data with a particular color center model. Podini’ recently reported thermal activation energies for the excited F center in NaF, and Swank and Brown12 similarly, for NaCl and other alkali halides. Halperin, et aL13have reported thermal activation energies for various traps in NaC1 based on extensive thermoluminescence studies. The thermoluminescence technique offers a reasonably simple method of obtaining thermal depth data for traps in a given crystal. Analysis of individual glow peaks by the initial rise technique continues to offer a fundamentally correct route to thermal-activation-energy determinations. This is based on the fact that, regardless of the type of kinetics involved, it has been shown3that for the initial part of a given glow peak, the logarithm of the glow intensity ( I ) is directly proportional to the reciprocal of the absolute temperature (2‘). The thermal activation energy ( E ) is then obtained from the slope of the straight line resulting from a plot of In I vs. 1/T corresponding to the initial part of a given glow peak. Studies of alkali halide thermoluminescence by Stoddard4 and later by Braner and Israeli3 demonstrated a method of restricting the thermoluminescence observed in a given crystal to that resulting from the The J O U T Mof~ I’hysieal Chemistry
thermal release of only one type of trapped carrier, e.g., a bound electron. This is accomplished by first carrying out an X-irradiation in the dark at a temperature considerably above that temperature, T,, corresponding to the glow peak of interest. The crystal is then cooled to a temperature below T,, e.g., liquid nitrogen temperature, and illuminated in the F band. Such illumination serves to photoionize some of the electrons from F centers produced during the initial X-irradiation. Some of these liberated electrons may be trapped a t other sites. In this way, the existing distribution of shallow electron traps may be populated by the electrons released from F centers. Such a process will produce no redistribution of trapped holes, thus all populated shallow levels will correspond to trapped electrons as desired.
Experimental Procedure Single crystals of LiF and NaF were obtained from Harshaw Chemical Co. One NaF specimen was treated with dried H F in a sintering furnace for 2 hr at -973°K in order to effect OH- and C1- removal. This HF-treated crystal was subsequently (1) P. Podini, Phys. Rev., 141, 574 (1956). (2) R. K.Swank and F. C. Brown, ibid., 130, 38 (1963). (3) A. Halperin, A. A. Braner, A. Ben-Zvi, and N. Kristianpoller, ibid., 117, 417, 421 (1960). (4) A. E.Stoddard, ibid., 120, 114 (1960). (5) A. A. Braner and M. Israeli, ibid., 132, 2503 (1963).
THERMAL ACTIVATION ENERGIES IN LiF, NaF, AND NaCl CRYSTALS
compared with an untreated NaF specimen. Single crystal NaCl was grown by a modified Stockbarger technique from Johnson, Matthey and Co. "specpure" NaC1, prefused in HF. Crystal specimens (-0.1 X 1 X 1 cm) were individually X-irradiated by a 1-Mev electron beam (General Electric Co. electron beam generator) stopped by an aluminum-lead sandwich converter. The incident dose, in every case, was 1.3 X lo6 r delivered a t a constant rate of 727 r/sec. X-Irradiation was performed in air, in the dark, at either 293 or 373'K. Normally, the crystals were held in the dark at temperature approx 10-15 min to permit ventilation of the X-ray room. Thereafter, each crystal was wrapped in aluminum foil and desiccated. Within 0.5 to 2 hr after X-irradiation, a given specimen was mounted in a cryostat which was then evacuated to torr. F-band photoionization, designed to bleach -1076 of the F centers, was then performed. A BH-6 air-cooled high-pressure mercury arc lamp in conjunction with Corning glass filters (CS 7-54 for LiF, CS 7-60 for NaF, and CS 5-61 for NaC1) was used to carry out this optical bleaching. Photoionization of the center in most alkali halides is considered to occur in two steps. The first step involves photoexcitation of the bound electron from the ground state to a relatively shallow excited state and is independent of temperature. 'The second step involves the thermal release from the excited state to the conduction band and is temperature dependent. It was previously pointed out thah these conduction band electrons may be captured in a series of shallow traps, from which thermal release may also occur. Selection of a temperature at which F-band photoionization is carried out thus represents a compromise in a effort to achieve substantial probability for thermal ionization out of the excited F level and minimum thermal release from the electron trapping levels of interest. A temperature of 150'K was selected as reasonable for the three salts of interest in this work. Crystal warming was accomplished by a nichrome heating element wrapped at the opposite end of the copper bar, upon which the specimen was mounted. Warming rate control of the order of 13.6 f 0.3"K/ min was maintained by a motor-driven cam operating an autotransformer. Glow emission was chopped by a tuning fork chopper (200 cps), amplified by a photomultiplier (RCA 4460, S-11 response), reamplified, passed through a synchronous detector to suppress noise, and finally recorded synchronously with temperature on a twochannel recording potentiometer.
4229
Results Typical glow curves obtained from single crystal LiF, and NaCl X-irradiated a t 293 and 373°K are presented in Figures 1, 2, 3, and 4. The glow peaks observed below the temperature of X-irradiation, as pointed out earlier, result from emptying bound electron centers only. Those observed above the X-irradiation temperature may presumably be associated with either bound hole or bound electron centers. Theoretical and experimental studies of the effect of heating rate, p, reveal that doubling p, e.g., from 12 to 24"K/min, increases the temperature a t the glow peak, T,, by -2%. Furthermore, conditions of X-irradiation, e.g., dose, dose rate, and temperature, and any prethermoluminescencephotoexcitation will profoundly affect the nature and distribution of the bound states c v)
ki
24-
z 8
16
6 bW z
V v)
z
52 w 0 5w
Jy
-
8 - 166 0
140
A
- 1200
I
I
260
300
I
180
220
340
- 800 400 I
380
0 420
TEMPERATURE T. O K
!-
Figure 1. Glow curve for LiF (Harshaw, as received) X-irradiated in air at 373OK and photostimulated at 150°K.
v)
60
3 W
5
50
I61
NaF
W J
a >- 40
t v)
6 Iz
30
W
0
5 % W
20
z
5
dz
8
IO
220
TEMPERATURE T,
OK
Figure 2. Glow curve for NaF (Harshaw, as received) X-irradiated in air at 373°K and photostimulated at 150OK.
H. LEVIN,C. C. BERGGREN, AND V. R. HONNOLD
4230
Table I : Selected Comparative Glow Maxima' 8, OK/min
Tm,
Irradiation
OK
E, ev
LiF
1-Mev X-ray at 373OK'
13.6
166 313 395
0.42 0.44 1.06
This work
NaF
1-Mev X-ray at 373OK'
13.6
171 224 283
0.53
This work
Crystal
50-kvp X-ray at LNT*
NaCl
' In air.
I60
I2O
1
LMev X-ray at 293OK 45-kvp X-ray at liq air tempb
13.6 10.
0.72
187 232 29 1
... ...
218 216
0.62
Podini
I . .
...
This work Braner and Israeli
Obtained by F-band photostimulation after X-irradiation.
Under vacuum.
W
n I?
I\
t II
-140
?
...
Sourae
180
283
220
260
300
340
380
426 TEMPERATURE T, O K
TEMPERATURE T, 'K
Figure 3. Glow curve for NaF (Hanhaw, HF treated) X-irradiated in air at 373°K and photostimulated at 150'K.
Figure 4. Glow curve for NaCl X-irradiated in air at 293OK and photostimulated at 150°K.
ultimately observed. For these reasons, care must be exercised in comparing glow data. Such data are presented in Table I. Thermal activation energies corresponding to given glow maxima, which were obtained by the initial rise method,6 are also included. Reasonable agreement is observed with the electron trap data obtained by Podini for NaF, and by Braner and Israeli for NaC1. The usefulness of thermal depth data is, of course, quite limited unless such values can be associated with individual trapping levels of some recognized identity. While many, including Podini, Pringsheim and Yuster? and Heckelsberg and Daniels: have noted the relationship between color centers and thermoluminescence peaks, seldom is such work extended to association of
specific glow peaks with given center models. An exception is the work of Dutton and Maurerg with KC1 and KBr. In that study, thermally stimulated current measurements in conjunction with absorption spectra measurements, made after selective thermal bleaching, were employed to associate given T, values with specific centers, both bound electron (e.g., F') and bound hole (e.g., VI and Vd). Related values of E were cal-
The Journal of Physical Chemistry
(6) See, for example, 542, 545 (1958).
w. Hoogenstraaten, Philips
Res. Repts., 13,
(7) P. Pringsheim and P. Yuster, Phye. Rev.,78, 298 (1950).
(8) L. F. Heckelsberg and F. Daniels, J . Phys. Chem., 61, 416 (1957). (9) D . Dutton and R. Maurer, Phys. Rev.,90, 127 (1953).
THERMAL ACTIVATION ENERGIES IN LiF, NaF, AND NaCl CRYSTALS
culated from a Randall and Wilkins'O expression using a frequency factor of approximate applicability. In the work reported herein, an attempt has been made to assign thermal depth values to F' centers in the various crystals. This has been done on the basis of using computed electron-trapping cross sections to differentiate between the types of electron with which excess centers dealt. Prior to discussing such identification attempts, it is worth noting that, for the most part, impurity defects play no role in the glew data. In this regard, studies by Lushchikl' on alkali halides doped with impurity ions, e.g., copper, silver, and thallium, indicated that most of the glow peaks are independent of the impurity ions and correspond only to the "thermal ionization of color centers observed in the pure crystals." While the crystals used herein were not deliberately doped, there was some variation in contained impurities. Despite this, the recurrence of the same peaks in given crystals from various sources is indicative of their association with carrier trapping by lattice defects. The following development is based on the assumption of monomolecular kinetics for recombination of conduction band electrons and negligible retrapping of these electrons. The assumption of monomolecular kinetics is justified by the fact that the maximum number of free electrons is less than '/lo the number of recombination centers. This follows since the number of recombinat,ion centers and the initial number of F centers produced during the X-irradiation are equal, and it has been pointed out earlier that w l / 1 0 of the F centers were photoionized, Values of the frequency factor s (which Bube12 terms the attempt-to-escape frequency for trapped electrons and which approximates "the number of times per second that a bound electron can absorb energy from crystal phonons") may be calculated from Hoogenstraaten's relationship as
from the value for NaCl on the basis of dielectric constaRts and the results were compared with trends noted in Pick's13 data. Electron-capture cross sections corresponding to specific centers may thus be computed from a knowledge of s and T . Such values are presented in Table 11. Table I1 : Calculated Electron-Capture Cross Sections Associated with Various Glow Peaks in Alkali Halides Single crystal
Tm,
E. ev
8,
U.
OK
sec-1
ern,
center
NaF"
161 283
0.50 0.73
2 . 3 X 10l4 2 . 4 x 10"
5.6 X 1 . 9 X 10-16
. .. F'
NaFb
171 283
0.53 0.72
7 . 9 X IOl4 1 . 6 X 10l1
1.9 X 1.3 X
.. .
165 283
0.62 0.72
1 X lo1' 1 . 6 x 10"
2.3 X
..
NaCl'
218
0.62
7 . 4 X loE2 2 . 1 X
F'
LIF"
166 313 395
0.42 0.44 1.06
2 . 3 X 10" 1 . 4 X 106 6 . 0 x 10"
...
NaF"
F'
1.3 x
F'
3.6 X 6.4 X 1 . 7 X 10-16
... F'
Harshaw, HF treated at 973'K. a Harshaw, as received. Modified Stockbarger grown.
Identification of the glow peaks shown as F' is based on the following arguments. 1. The F' center is formed by electron capture into a neutral center (F center). Rose14states that u ranges from cm2for Coulomb attractive capture to cm2 for Coulomb repulsive capture. It is concluded that the u values of the designated F' centers, an order of cm2, are Table 111: Thermal and Optical Depths for the F' Center in Several Alkali Halides
(1)
Single crystal
Knowing the thermal depth E , the heating rate 6, and T,, one obtains the value of s. Bube shows that the s value is directly proportional to the capture cross section, u, of the unoccupied trap as
LiF NaF NaCl
sE//3k = xmzezm= (E/kT,)2eE'kTm
423 1
a
Reference 15.
hv,
E,
ev
ev
Elhu
2.0" 2 . 76b 2.43
1.06 0.72 0.62
0.52 0.26 0.26
Reference 16.
s = ~ [ 8 ~ * ( ~ / h ~ ) > " ' ( ~ T , ) ~(2) ]
where m* = effective mass of electron: 2.53 X lom2'g for NaC1, 2.7 X g for NaF (estd), 4.17 X g for LiF (estd); h = Planck constant = 6.63 X erg sec; k = Boltzmann constant = 1.38 X lo-'* erg/*K; u = cross section, cm2; s = frequency factor, sec-1. Values of m* for NaF and LiF were estimated
(10) J . T.Randall and M.H. F. Wilkins, Proc. Rou. SOC.(London), A184, 366 (1945). (11) C. B. Lushchik, Dokl. Akad. Nauk SSSR, 101, 833 (1955). (12) R. H.Bube, "Photoconductivity of Solids," John Wiley and Sons, Inc., New Pork, N . Y., 1960,pp 50,51. (13) H.Pick, Nuovo Cimento, Suppl., 7 , 508 (1958). (14) A.Rose, "Concepts in Photoconductivity and Allied Problems," Interscience Publishers, Inc., New York, N. Y., 1963,p 118.
Volume 71, Number 19 December 1967
4232
A. MUKHERJEE AND W. F. GRAYDON
indicative of capture by a neutral center and may thus be associated with F’ centers. 2. Values of 8 , for the glow peaks designated as originating from F’ center release, vary over a range of about one order of magnitude. This agrees with Lushchik’s data for s values associated with the F center in various alkali halides which demonstrate a similar variation. Such association of selected glow peaks with the F’ center, while strongly indicative, should be considered provisional until optical confirmation is made. On the basis of the above identified F’ thermal depths, it is interesting to compare E with the optical depth, hv. These are shown in Table III.l5J8 It will be noted that the Mott and Gurney’’ approximation,
E =
l/&v, applies only in the case of LiF and may not be used indiscriminately. Acknowledgment. This work was supported by the Air Force Materials Laboratory and the Air Force Avionics Laboratory under Contract AF33(615)2050. The authors appreciate the assistance of Dr. A. L. Gentile and Mr. 0. M. Stafsudd in the crystalgrowth phase of this work. (15) C. J. Delbecq and P. Pringsheim, J . Chem. Phys., 21, 794 (1953). (16) K. Konrad, M.S. Thesis, Illinois Institute of Technology, Chicago, Ill., 1966, p 10. (17) N.F, Mott and R. W. Gurney, “Electronic Processes in Ionic Crystals,” Oxford University Press, London, 1940,p 162.
Heterogeneous Catalytic Oxidation of Tetralin
by A. Mukherjee and W. F. Graydon The Department of Chemical Engineering, University of Toronto, Toronto, Canada (Received March 29, 1967)
A study has been made of the liquid-phase oxidation of tetralin (l12,3,4-tetrahydronaphthalene) with insoluble catalysts, and the reaction rates have been compared with those of the soluble ones. It was found that catalysts such as oxides of nickel, manganese, and copper were extremely active while the rest, such as oxides of aluminum and zinc were inactive. Rates of oxidation were measured in the temperature range of 45-90’. The initial product of reaction was found to be tetralin hydroperoxide which decomposed further into tetralone and tetralol. A ketone: alcohol ratio of 2 : 1 was found with most of the catalysts. It was further observed that a critical hydroperoxide to catalyst ratio existed below which the reaction did not proceed. Kinetic studies have been made with four of the best catalysts, and a reaction mechanism has been proposed.
Introduction The liquid-phase oxidation of tetralin with soluble Catalysts has been studied extensively.’-5 The mechanism is well understood. Data on heterogeneous catalysts, on the other hand, are very meagre. The only work reported so far on insoluble catalysts for the oxidation of tetralin was that of George! The catalysts that he used did not bring about great increases in rate over the thermal rate. The Journal of Phy&
Chemistry
It is known that p-type semiconductors (oxides of cobalt, nickel, manganese, copper, etc.) are good cata(1) Y . Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, Can. J . Chem., 41, 2020 (1963). (2) Y. Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, ibid., 41,2034 (1963). (3) Y.Kamiya and K. U. Ingold, ibid., 42, 1027 (1964). (4) Y. Kamiya and K. U. Ingold, ibid., 42, 2424 (1964). (5) A. E. Woodward and R. B. Mesrobian, J . Am. Chem. Soc., 7 5 , 6189 (1953).
