86
F. I. METZ,R. N. SCHWEIGER, H. R. LEIDERAND L. A. GIRIFALCO
oxygen on the surface of the Su-60 than on the Graphon. Table I11 shows the doubling of the reaction rate resulting from increased surface oxidation over a period of six hours. The somewhat slower rate of reaction on the oxide surface than on
Vol. 61
the H2 treated surface of Su-60 indicates that with extended surface oxidation some of the reactive complex sites which are formed immediately after addition of NzO must be converted to unreactive complex sites.
STRESS ACTIVATED LUMINESCENCE I N X-IRRADIATED ALKALI HALIDE CRYSTALS BY F. I. METZ,' R. N. SCHWEIGER, H. R. LEIDERAND L. A. GIRIFALCO Lewis Flight Propulsion Laboratory, National Advisory Committee for Aeronautics, Cleveland, Ohio Received June 61,1966
The luminescence caused,by stressing crystals of potassium bromide, sodium chloride, and lithium fluoride containing F-centers produced by X-rays has been investigated in the region of plastic flow. Light output was measured under varying conditions of irradiation time and strain rate. Rate of light output shows P linear dependence on strain rate (from 12.1 to 116 per cent./min.) for all three materials. It is shown that a process by which F-center electrons are excited to the conduction band by thermal spikes associated with the movement of dislocations, with subsequent recombination with a vacancy or a halogen atom, can account for the experimental observations. for each strain-li ht output curve in order to average out individual crystaf variations. The specimens were packed in crushed salt from the same stock and heated for a t least four hours a t 100' below the melting point. This heat treatment was found to yield sufficiently reproducible data. The specimens were then quenched to room temperature in a stream of air, separated from the crushed material and placed in a desiccator. The crystals were used within a few hours after quench. The specimens were irradiated in a light-tight aluminum foil container (1 mil thickness) which was rotated by a four r.p.m. clock motor around an axis normal to the X-ray beam. Irradiations were made with a Machlett AEG-50 T tube with a tungsten target and a beryllium window. The tube was operated at 50 KV. and 30 ma. providing about 72,000 Roentgen per hour a t the point of irradiation. The irradiation times used in the investigation of variation of light output with strain rate were one minute for KBr, 2.5 minutes for NaCl and ten minutes for LiF. For experiments a t constant strain rate, crystals were irradiated from one minute to two hours for all three materials. After irradiation, the crystals were placed in the compression apparatus in room light. Each specimen was exposed to room light for 0.7 minute and kept in the dark for one minute before compression. This procedure was found to adequately eliminate any excitation effects due to room light. A small tensile machine was modified for compression tests. The upper head of the tester was replaced with a proving ring and linear differential transformer to allow measurement of force during compression. The linear differential transformer was connected to an electronic micrometer whose output was recorded on one channel of a recorder. The specimen was placed on the lower ram which protruded up into a reflecting chamber consisting of several front silvered mirrors arranged to direct the light from the specimen onto two 1P28 phototubes. These phototubes were Sodium chloride, potassium bromide and lithium fluoride connected to an amplifier and the output was recorded on the other channel of therecorder. A constant fraction of the specimens were cleaved to 4 X 4 X 10 mm. from larger o tical grade single crystals which were obtained from tl% light from a crystal was measured in every experiment since Harshaw Chemical Company. Six specimens were used the geometry was constant. The useful phototube range was 2300 to 5800 A.13 The approximate light intensity as (1) Chemistry Department, I o w a State College, Ames, Iowa. seen by the phototubes for the maximum luminescence ob(2) R. A. Lad and F. I. Metz, J . Mech. Phgs. of Solids, 4, 28 (1955). served was 1.8 p lumens for KBr, 0.16 p lumen for NaCl and (3) J. Trinks, Wien. BeT., [IIal 147,217 (1938). 0.07 p lumen for LiF. The meamrements were reliable to (4) F. Urbach, ibid., 189,353 (1930). a reading of 0.001 p lumen. (5) F. G.Wick, J . Opt. Soc. A m . , 19, 141 (1939). The total strain introduced during an experiment was (6) A. Smekal, 2. Ver. Deut. Ing., 7 1 , 887 (1928). about 15% in the direction of the long axis. (7) H.J. Schroder, 2. Physik, 7 6 , 608 (1932). The general characteristics of the luminescence spectra (8) I. 8. Jacobs, Phys. Rev., 98, 993 (1954). were determined by placing various gelatine Wrathen filters (9) 2. Gyulai and D. Hartley, 2. Physik, 61, 378 (1928). in front of the phototubes. The filters used were Wratten (10) 2. Gyulai and J. Boros, Math. Naturw. Ana. ung. Acad. Wiss., 0, l A , 2B, 8K2, 15G and 22. Calibration of the proving 69, 115 (1940). ring was done by loading with weight,s, and it was found to be
In the course of a study on the effect of irradiation on transient creep of alkali halides done a t this Laboratory,2 it was observed that relatively intense visible light was emitted from irradiated crystals during compression. Such a phenomenon has been observed previously in irradiated alkali halides when they were compressed or otherwise mechanically worked. -5 It has been shown that the application of stress to X-rayed crystals causes This would suggest that the rate of recombination of trapped halogen atoms and electrons is considerably increased by external stress. Studies of the effect of stress on the conductivity of sodium chloridegglolead to the conclusion that the observed increase is t,he result of either vacancy generation or the freeing of electrons by the motion of dislocations. l1.12 In this investigation the luminescence caused by compressing single crystals of sodium chloride, potassium bromide and lithium fluoride containing Fcenters produced by X-rays was studied in the region of plastic flow in order to elucidate the mechanism. The rate of light emission was measured as a function of strain, strain rate, irradiation time and temperature. I n addition, the general characteristics of the emission spectra were determined. Experimental
(11) W. W. Tyler, Phys. Rev., 86, 80 (1952). (12) F. Seitz, ibid., 80, 239 (1950).
(13) R. W. Engstrom, J . O p t . Soc. Am., 8'7, No.8,420 (1947).
'
Jan., 1957
LUMINESCENCE IN X-IRRADIATED ALKALIHALIDE CRYSTALS
linear up to 10 kilograms, the largest load reached in the experiments. The linearity of the phototubes and recording circuits was verified. The rate of straining was changed by shifting belts onto different steps of a set of cone pulleys. Experiments were performed a t strain rates from 12.1 to 116yo/min. Strain rate was found to be constant during a run by checking with a dial indicator and stop watch. For determination of the temperature dependence of the light output, crystals were maintained at various temperatures ( f 0 . 2 ’ ) during compression. This was achieved by passing helium through a coil submerged in a Dry Iceacetone bath and then electrically heating the gas to the required temperature. Several small jets of gas were directed over the faces of the crystals. The temperature range covered was from 10 to 30’. With the procedure outlined above the data were reproducible to &5%.
I
Results and Discussion The rate of light emission as a function of compressive strain is shown in Fig. 1 for potassium bromide, lithium fluoride and sodium chloride for several strain rates. After a small induction time, the rate of light emission for all three materials was found to increase rapidly up to a strain of about 5%. The induction period waa attributed to the time required for the crystal ends to adjust to the apparatus, and the subsequent rise was attributed to the time required for the transition from elastic to plastic deformation. I n order to avoid complications due to these effects, all the following discussion will be restricted to the experimental region beyond 5% strain. It can be seen that for potassium bromide and lithium fluoride the rate of light emission becomes constant in the range greater than 5% strain. However, in the case of sodium chloride, the rate decreased continuously after this point. No explanation of this difference in behavior is as yet available. It should also be mentioned that if a t any time during an experiment the compression was interrupted, the luminescence decayed to zero in less than one second. On resuming compression, the light output rose again to its original value with equal rapidity. The rate of light output is directly proportional to strain rate for all three materials. This is true for sodium chloride provided the rates are compared a t values of constant strain. These results are shown in Fig. 2. The dependence of the rate of light emission on irradiation time is shown in Fig. 3. Sodium chloride and potassium bromide show a saturation effect, while for lithium fluoride the rate is still increasing up to 120 minutes of irradiation. Experiments carried out between 10 and 30” indicated that the effect of temperature on the rate of light emission is small. The activation energies calculated from the plots of log (rate) against 1/T, as shown in Fig. 4 were less than 0.05 e.v. The luminescence spectra for all three materials showed two peaks with maxima in the vicinity of 3500 and 4500 A. I n each case, the shoulder on the short wave length side of the 3500 8. peak was considerably greater than the shoulder on the long wave length side of the 4500 A. peak. Because of the crudeness of these measurements the precise position of the two maxima are in doubt; however, the spectra suggest that a t least two processes occur during compression which result in luminescence. The ratio of the intensities of the high
87
PERCENVMI N, 6
116
m
=
PERCENT/MIN, €
(L
a
0e
3
.4
.e
I L -
;o ( b)
PERGE!T/MIN,
21.9 0
2
4
6 8 1 0 PERCENT STRAIN, c
1
2
(C)
Fig. 1.-Dependence of rate of light emission on strain: (a) potassium bromide; irradiated for one minute; (b) lithium fluoride; irradiated for ten minutes; (c) sodium chloride; irradiated for 2.5 minutes.
Iw
d
STRAIN RATE (PERCENT/MIN), 2 . (C)
Fig. 2.-Dependence of rate of light emission on strain rate: (a) potassium bromide; irradiated for ten minutes; (c) sodium chloride; irradiated for 2.5 minutes.
F. I. METZ,R. N. SCHWEICER, H. R. LEIDERAND L. A. GIRIFALCO
Vol. 61
The absorption spectra were also measured in the wave length rate of 2100 to 10,000 A. Comparison of irradiated crystals before and after compression showed no significant difference in the spectra. It may be concluded, therefore, that the average F(0) center concentration did not change measurably during compression. At most, only 3 X lo9 photons/sec. were measured during the compression experiment. If only one emitted photon in B thousand were counted, and if the experiment ran 100 seconds, an upper limit of 1014 photons were emitted. Since the F-center concentration is of the order of 10l5per cm.,8 the emission processes could [b) not significantly alter the F-center concentration. Reasonable processes which may result in the .8 emission of light are the recombination of electrons .6 with halogen atoms or vacancies. Three rnechanisms which may be involved in such processes are the following: (1) recombination of halogen atoms 4 and F-centers as the result of normal diffusion. The increased number of vacancies during strain would .2 increase the diffusion rate and thereby lead to an increase in luminescence. (2) Tunneling of elec0 20 40 60 80 100 120 I40 trons from F-centers to holes. The recombination IRRADIATION TIME, MIN. of F-centers and holes may be imagined to take (C) place by a tunneling process as desczbed by DexFig. 3.-Dependence of rate of light emission on irradia- ter.14 tion time: (a) potassium bromide; (b) lithium fluoride; Since the probability of tunneling increases with (c) sodium chloride. All cryytals were strained at 64%/ increasing vacancy concentration, it is expected min. that the excess vacancies produced by strain would enhance the tunneling rate. Thus the effective transport of F-centers through the crystal, and their recombination with holes, would be increased by plastic strain. (3) Excitation of F-center electrons to the conduction band. Excited electrons could then recombine with halogen atoms or combine with vacancies to re-form F-centers. The energy for the excitation could arise from thermal spikes resulting from the motion of dislocations during strain. n The experimental observations indicate that neither tunneling nor diffusion is important in contributing to the light emission. The fact that luminescence decays very rapidly when compression is interrupted and that the rate of emission is directly proportional to the strain rate shows that the rate-determining step is accelerated only while the crystal is being strained. Mechanisms 1 and 2 depend on the presence of vacancies produced by moving dislocations and should therefore lead to a PERCENT, rate of light emission that increases with increasing 6 strain and decays only slowly when the compression is interrupted. Also, if a diffusion process were operative, the activation energy required would be of the order of 0.5 e.v. The observed activation .do330 .00334 ,00338 ,00342 ,00346 ,00350 ,00354 ,00358 energy is less than this by a t least an order of magICT. nitude, so that a diffusion mechanism is unlikely. Furthermore, if mechanisms 1 and 2 were opera(C) Fig. 4.-Temperature dependence of the rate of light tive, the rate of light emission would be proporemssion: (a) potassium bromide irradiated for one minute; tional to the square of the F-center concentration (b) lithium fluoride irradiated for ten minutes; (c) sodium since the number of F-centers is equal t o the numchloride irradiated for 2.5 minutes. All crystals were ber of halogen atoms. However, if mechanism 3 strained at 64YJmin. is rate determining, the rate of light emission should wave length peak to the low wave length peak in- depend only on the F-center concentration. Ascreased by about 80% for KBr and NaCl during the suming a constant scale factor which takes into (14) D. L. Dexter, Phys. Rev., 93, 985 (1954). experiment. The reverse was true for LiF.
. .
'
TRIETHYL PHOSPHITE
Jan., 1957
TO
account differences in irradiation procedure, it is possible to compute the relative F-center concentration from irradiation time using the data of Mador, Wallis, Williams and Herman.I5 Then, if the rate of light emission is first order, the ratio of the rate to F-center concentration should be constant. This is shown to be the case for sodium chloride in Table I. The above-mentioned investigators a,lso found a linear increase in surface F-center concentration with irradiation time for lithium fluoride; Fig. 3 shows a similar linear relationship of the rate with irradiation time in these experiments. TABLE I Ratio of the rate of light emission, A., to the F-center concentration COin the first 0.2 mm. of the surface layer for sodium chloride strained a t G-l.%/min. Irradiationtiine, Inin.
5 10 20 GO
i./Co X 101s (arbitrary units) 6
= 6%
1.2 1.3
1.4 0.96
€
= 8%
€
= 10%
0.68 0.79 .84 .75
0.91 0.96 1.0 0.90
(15) I. L. Mador, R. F. Wallis, M. C. Williams and R. C. Herman, Phus. Rev., 96, 617 (1954).
DIETHYLETHYL PHOSPHONlTE
89
Mechanism 3, in which the rate-determining step is the excitation of an electron from an F-center to the conduction band by a thermal spike, is the only one of those proposed which can account for all the experimental observations. This process depends directly on the strain rate rather than the strain, and predicts that the luminescence would decay rapidly on interruption of compression. Also, since the probability of exciting an electron from the Fcenter to the conduction band depends on the thermal spike rather than on the average temperature of the crystal, the temperature coefficient of the excitation process should be quite small. This mechanism leads directly to a spectrum of emitted light containing two peaks. The excited electron in the conduction band can combine with either a vacancy or a halogen atom to give an F-center or a negative ion, thereby giving off light a t two different energies. . Acknowledgment.-The authors wish to express gratitude to Dr. R. A. Lad for many valuable suggestions made during the course of this investigation.
RATES OF ISOMERIZATION OF TRIETHYL PHOSPHITE TO DIETHYL ETHYL PHOSPHONATE IN THE PRESENCE OF ETHYL IODIDE BY A. F. ISBELL, G. M. WATSONAND R. E. ZERWEKH,JR. Department of Chemistry, The Agricultural and Mechanical College of Texas, College Station, Texas Received July 87, 1066
Rates of the isomerization reactions have been determined by continuous density measurements a t 90, 100 and 110’ over a concentration range from 10 to 56 mole yo ethyl iodide. The initial G0-70yO of the reaction was found to be zero order with respect to triethyl phosphite and first order with respect to ethyl iodide. Initial specific reaction rate constants have been expressed as functions of concentration. The over-all composition-time curves resemble in shape those for an autocatalytic process. The experimental results over the entire temperature composition range studied have been found to be satisfactorily expressed in terms of concentrations by means of a single equation proposed much earlier by Zawidzki and Staronka.‘
The reaction investigated is commonly indicated by the over-all equation
+
P(0CzHs)a f CzHd --f ~zHsP(O)(OCzHs)z CIHSI . The general reaction was first described by Mi-
chaelis and Kaehne2 and was studied extensively by Arbuzov3 who showed the general nature of the reaction and proposed a possible mechanism. The general mechanism proposed by Arbuzov postulates two steps for the reaction: the formation of a phosphonium halide intermediate followed by its decomposition into the products
+
P(0R)s R’X +R’P(OR)s+XR’P(OR),+X- +R’P(O)(OR)z RX
+
Intermediates, which are condensation products of triphenyl phosphite with alkyl halides, have been isolated by Arbuzovs and by other^.^ They have (1) J. Zawidzki and W. Staronka, Anzsiger Alcad. W t k . Kvalcau A . , 319 (1915); Abhand. Abad. Wkss. Krakau, 66, 101 (1915): via C. A., 11, 2294 (1917). (2) A. Michaelis and R. Kaehne, Ber., 81, 1048 (1898). (3) A. E. Arbuzov, J . Russ. P h y s . Chem. Soc., 42, 395 (1910); 88, 161,293,687 (1906); Ber., 38, 1171 (1905). (4) 8.R. Landauer and H. N. Rydon, J. Chem. 800.. 2224 (1953).
the expected empirical formulas and have been assigned structural formulas corresponding to phosphonium halides. Landauer and Rydon,* however, have indicated that these phosphonium halides are rather unusual since they precipitate silver iodide quite slowly from silver nitrate solution^.^ In reactions involving trialkyl esters of phosphorous acid, no intermediate has been isolated, presumably due to their very transient existence and low stability under experimental conditions. I n the absence of a better alternative mechanism, more recent investigatorsa apparently agree that the reaction follows the general course proposed by Arbuzov. Kinetic measurements on this reaction had been performed by Zawidski and Staronka’ and by Staronka.6 The original publication of Zawidski ( 5 ) Since this reaction wa8 carried out in a solution of ethanol, with which triphenyl phosphite methiodide was ohown t o reaot rapidly to form ethyl iodide, the slow precipitation of AgI may neither prove nor disprove the ionic character of the methiodide. (6) W. Staronka, Roczniki Chin.,7 , 42 (1927); via C. A . , a%, 1264 (1928).
