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PHOTODECOMPOSITION OF SOLIDMETALAZIDES
Photodecomposition of Solid Metal Azides by V. R. Pai Vernekerl Explosives Laboratory, Feltman Research Laboratories, Picatinny Arsenal, Dover, New Jersey 07801 (Received November 15, 1967)
;Study is made of the general characteristics of the photodecomposition of metal aaides (NaNa, KNa, BaNa, AgNa, and PbNs). Rates of gas evolution from azides irradiated with light of different wavelengths are plotted against the time of irradiation. It is shown that photolysis of NaNa, BaNe, and AgN3 leads to the formation of nitride, whereas photolysis of KN3 and PbNe does not lead to nitride formation. Further, the nitrides are shown to decompose with light of long wavelength. The acceleration in the photolytic rate of metal azides in the later stages of decomposition is attributed to (1) the photodecomposition of the metal nitride, if it is formed, and (2) to the photoionization of metal atoms, if light of sufficient energy is present. The photoemission of electrons from the metal serves to explain why the initial photolytic rate does not go to zero but decelerates to a constant minimum value. Introduction The interest in the photochemical decomposition of metal azides arises from the fact that it forms an attractive point of entry into the field of radiation-induced reactions in solids and further gives us an insight as to why some metal azides are explosive whereas others are not. Evidence for this is found in the wealth of information resulting from the photolytic studies of a z i d e ~ . ~ - lThe ~ photodecomposition behavior of these materials varieti widely from sample to sample and is very dependent on the method of preparation, age of the sample, the method of storage, and the impurities p r e ~ e n t . ~ J Bowever, ~J~ despite these difficulties it is possible to study the general features of photodecomposition by following the gas evolution as a result of irradiation. Jacobs, Sheppard, and Tompkinsl' studied the photochemical decomposition of XaN3, KN3, SrNB, and BaN(3. Their results can be summarized as follows. The general features of the time dependence of the rate of plhotolysis at constant intensity are (1) an initial deceleratory reaction, followed by (2) an acceleratory phase to the reaction, leading to (3) a constant rate of photolysis. I n NaNs, process 2 is absent: in the other three azides, its appearance was shown to depend critically on the energy of the incident photons (photons of wavelength shorter than 2537 A). Absorption spectra of NaK8 and KN, are very similar and so the divergence in the behavior of NaN3 from the rest of the azides is possibly attributable to topochemical features. The increase in the rate of gas evolution, which occurs in the later stages of decomposition, is not present in all metal azides. The object of the present study was to investigate the conditions for the appearance of this latter feature in the decomposition behavior. Experimental Section Materials. Sodium, potassium, and barium azides were prepared by passing hydrazoic acid gas through
aqueous solutions of their respective carbonates and precipitating by the addition of acetone. In one set of experiments the carbonates were of reagent grade quality and in another set of experiments they were of spectroscopic grade. Lead azide was prepared by passing hydrazoic acid gas through an aqueous solution of lead nitrite, the latter solution being prepared by passing nitrous oxides through a suspension of lead oxide in water. Reagent grade and spectroscopic grade lead oxides were used as alternatives. Lead azide was also prepared by the reaction of sodium azide with lead acetate. Silver azide was prepared by the reaction of sodium azide with silver nitrate. Barium nitride was also required to investigate the effect of light on this decomposition product, and samples were obtained from K & K Laboratories Inc., New York, N. Y. (1) Address correspondence t o the author a t Research Institute for Advanced Studies, Baltimore, Md. 21227. (2) (a) P. W. M.Jacobs and F. C. Tompkins, Proc. Roy. Soc. (London), A215, 254 (1952); (b) J. G. Dodd, J . Chem. Phys., 35, 1815 (1961). (3) W. E. Garner and J. Maggs, Proc. Roy. Soc. (London), A172, 299 (1939). (4) J. A. N. Thomas and F. C. Tompkins, ibid., A209, 550 (1951). (5) P. W. M. Jacobs, F. C. Tompkins, and D. A. Young, Discussions Faraday SOC.,28, 234 (1959). (6) B. E. Bartellete, F. C. Tompkins, and D. A. Young, Proc. Roy. Soc. (London), A246, 197 (1958). (7) S.K. Deb and A. D. Yoffe, ibid., A256, 514 (1960). (8) S.K. Deb and A. D. Yoffe, ibid., A256, 528 (1960). (9) P. Gray and T. C. Weddington, Chem. Ind. (London), 1555 (1955). (10) P. W. M. Jacobs, F. C. Tompkins, and V. R . Psi Verneker, J . Phys. Chem., 66, 1113 (1962). (11) P. W. M. Jacobs. J. G. Sheppard, and F. C. Tompkins, Fifth International Symposium on Reactivity of Solids, Munich, 1964, p RS-45. (12) P. W. M. Jacobs and A. R. T. Kurishy, J. Chem. SOC.,911, 4723 (1964). (13) V. R. Pai Verneker and A. C. Forsyth, J . Phys. Chem., 71, 3736 (1967). (14) P. W. M. ,Jacobs and A. R. T. Kureishy, Can. J . Chem., 44, 703 (1966).
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Apparatus. I n the photodecomposition apparatus, the azide was spread on the bottom of a transparent silica cell with a flat window at the top. The cell was connected to a vacuum line consisting of a trap immersed in liquid nitrogeli, a standard volume flask, an Alphatron vacuum gauge, a second trap, a vacuum valve, a diffusion pump, a third trap, a second Alphatron gauge, a fourth trap, a McLeod gauge, a second valve which separated the line from a second diffusion pump, and a mechanical pump. With this sequence of components the nitrogen evolution rate could be measured either on the first Alphatron gauge (Le,, “a static system”) with the first valve closed or on the second Alphatron gauge with the first valve open and the second valve closed ( i e . , the so called “dynamic system”). I n the second method, the evolved gas was continuously removed from the vicinity of the sample mm during thus maintaining a pressure of less than photolysis. The temperature of the azide was measured with a copper-constantan thermocouple which was embedded in the sample. As will be shown later, three wavelength ranges were of interest. 1 . Wavelengths Shorter than do00 A. These wavelengths are present in the radiations from a lowpressure mercury lamp and they were filtered out by insertion of a water filter in the beam. By comparing two photolytic runs, one with the water filter and the other without the water filter, the effect of light of wavelength shorter than 2000 A on the decomposition could be studied. 2. Wavelengths Centered about the Mercury Resonance L i n e at $537 A. These wavelengths were selected from the spectrum of a high-pressure mercury lamp by the use of an interference filter. 3. Wavelengths Longer than 3000 A. These were obtained from a high-pressure mercury lamp with the use of appropriate dye filters such as CS7-54 (without the filters the high-pressure lamp provides some intensity in the 2000-3000-~region, in addition to the longer wavelength). Tests were made to determine whether the appropriate metal nitride was present in the decomposition products. Irradiated samples were dissolved in water and tested colorimetrically for ammonia using Nessler’s reagent. The results of the tests for the presence of the nitride were checked at different temperatures and found to be independent of the temperature.
Results and Discussion The general features of the rate-time plot for any particular azide were not affected by (1) any variations in the impurity content of the different preparations of the same azide and (2) the method of collecting the gas, Le., “static” or “dynamic.” Figure 1A shows the rate of nitrogen evolution from barium azide as a function of the time of irradiation The Journal of Physical Chemistru
V. R. PAIVERNEKER
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TIME IN MINUTES Figure 1. Photolysis of barium azide usin! light of wavelength: A, 1849 2537 A; B, 2537 A ; C, ranging essentially from 2000 to 3000 A; D, of barium nitride with light of wavelength ranging essentially from 2000 to 3000 A.
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with light of wavelengths 1849 2537 A. For a fresh salt the rate falls off , reaches a minimum, and then rises 10 attain a constant rate. Figure 1B shows a similar plot for a different sample which was irradiated with light of wavelength 2537 A. The acceleratory portion CDE in Figure 1A is absent in Figure 1B. The highpressure mercury lamp was always used with a water filter, and Figure 1C shows a plot obtained when light of wavelength ranging essentially from 2000 to 3000 was used as the radiation source. The general features of the plot are similar to those in Figure 1A. After each experiment, the irradiated barium azide was dissolved in water and tested for ammonia using Xessler’s reagent and a colorimeter. A significant amount of NH, was found in all cases indicating the formation of barium nitride during irradiation. Barium nitride, Ba3X2, was next exposed to radialions of wavelength ranging essentially from 2000 to 3000 8 and the results clearly show that photolysis occurs with an accelerating rate. (Figure ID). Figures 2A and 2B present plots of the rate of nitrogen evolution from fresh samples of potassium azide us. time of irradiation, using different radiation sources (Le., different wavelengths). I n all cases, the rate decreased to a minimum and then increased to a constant value. Tests for KH3 (nitride formation) were negative. Figures 3A, 3B, 3C, and 3D show similar plots for sodium azide. Because NaN3 is reported to undergo a phase transition at 1 9 O , these experiments were done at
PHOTODECOMP~OSITIOX OP SOLID METALAZIDES
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Figure 4. Photolysis of AgN3 using light of wavelength: A, 1849 2537 h;; B, ranging essentially from 2000 t o 3000 A.
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Figure 2. Photolysis of KN3 using: A, light of wavelength ranging essentially from 2000 to 3000 b; B, light of wavelength 2537 h; alone.
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Figure 5. Photolysis of PbN6 using light of wavelength: A, 1849 2537 A;oB, ranging essentially
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Figure 3. PhotoAysis of NaN3 using light of wavelength: A, 1849 2537 A;oB, 2537 h;; C, ranging essentially from 2000 to 3000 A ; D, in the range 2400-3000 A.
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4 and 30". 'The general characteristics remained unchanged. Light of wavelength 2537 8 alone (Figure 3B) did not bring about the acceleration in the decomposition rate which was seen if light of wavelength 1849 2537 (Figure 3A), 2000-3000 (Figure 3C), or 2400-3000 A (Figure 3D) was used. I n all cases tests for nitride were positive. Silver azide irradiated with light of wavelength 1849-2537 8 did not show the acceleration in the decomposition rat e (Figure 4A). However, irradiation with light of wavelength ranging essentially from 2000 to 3000 A produced a pronounced acceleration (Figure 4B). Tests for nitride were positive. I n the case of lead azide (Figures 5A and 5B) no
acceleration was observed no matter what wavelength was used. Tests for nitride were negative. Table I summarizes the foregoing results for the different azides. The common feature of the photolytic decomposition of NaN3, I
