Small Band Gap Azides

Early measurements of the rates of photodecomposition of lead azide powders were conducted at a single wavelength, 254 nm, under varying conditions of light intensity and temperature [231,119]. From the data, reaction paths and decomposition models were suggested. The general conclusions were  

At constant temperature and intensity, the rate of nitrogen evolution rises first to a maximum and then decays approximately exponentially to a constant value, 

On ceasing irradiation, a dark rate decays exponentially to zero, with the total amount of gas released increasing with the total exposure. 

The constant rate section of the curve, R, varies linearly with intensity at constant temperature. 

To explain the observed data, Groocock [231] assumed that electrons are excited to the conduction band, leaving holes in the valence band which become trapped at an impurity center decomposition occurs only when holes are trapped at neighboring impurity sites. Pai-Vernecker and Forsyth [119], on the other hand, assumed that photons create excited states of the azide ion, and the excitons become trapped on an impurity center decomposition occurs between the trapped excited molecule and a neighboring azide molecule in the ground state. In both cases it is assumed that electrons, freed from the azide anion, are responsible for neutralizing lead atoms which diffuse to form colloidal lead. 

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The phrases large band gap and small band gap as used here should not be confused with the same terminology as often used in semiconductor literature. A small band gap azide would be classified as a large band gap semiconductor. 

As noted in the introduction, the small band gap azides of lead, silver, and thallium exhibit many similar properties which differentiate them from the large band gap azides. Barium azide may be an intermediate case since with irradiation it shows properties similar to both groups of materials. The small band gap azides in question detonate while barium azide deflagrates but will not sustain detonation. When the small band gap azides, barium azide, and silver and lead halides are exposed to radiation, decomposition appears to take place in both the metal and anion sublattices. Apparently, colloidal metal is formed from the metal sublattice [7,8,81-84] and, in addition, nitrogen [85,86] or halogen gas [87,88] is liberated from the anion sublattice. The relationship... 

Decomposition as discussed in this section has been studied by optical methods, X-ray diffraction, X-ray photoelectron spectroscopy, and infrared absorption. Although this section is concerned to a large extent with disorder resulting from decomposition of the metal sublattice, i.e., metal colloids, all types of disorder remaining after irradiation are considered, and some attention is given to the decomposition of the anion sub lattice. The decomposition of the anion sublattice of small band gap azides is considered in much greater detail in Section E dealing with gas evolution (primarily N2). 

Evidence for Colloidal Disorder in the Small Band Gap Azides fl. AgNs... 

Properties of Colloidal Disorder in Small Band Gap Azides... 

In this section brief consideration is given to mechanisms whereby colloidal metal may be produced in the small band gap azides by exposure to radiation. The mechanisms considered are those thought to be active in other inorganic crystals. No attempt is made here to be complete. 

If this mechanism operates in the small band gap azides, interstitial metal ions must occur as intrinsic disorder in these materials, as in the silver halides, and be mobile at low temperatures, e.g., 12°K, because TIN3 and Pb(N3)2 can be photodecomposed at 12°K. While nothing is known about the properties of such defects in azides, it is unlikely that they are sufficiently mobile at such low temperatures. The silver halides are insensitive to coloration at low temperature because the silver ion is immobile. This mechanism could of course be operative at higher temperatures in the azides. 

Colloidal metal is produced in the alkali halides both by additive coloration and by irradiation [109]. In either case, the F center, an electron trapped at a negative ion vacancy, is the stable defect at room temperature. Clustering of F centers takes place during heat treatment or in some cases during irradiation. When a region consists almost exclusively of F centers, a coUapse of the lattice takes place and colloidal metal is formed. It is unlikely that colloids are formed in the small band gap azides in this manner. F centers have not been detected in these azides and are thus not dominant defects. In addition, to allow clustering, the mechanism requires F centers to be mobile at 12°K, which is unlikely. Colloids are not formed at low temperatures in the alkali halides, presumably because the F centers are not sufficiently mobile. 

Mechanisms requiring diffusion may nevertheless operate in the small band gap azides at higher temperatures. Colloidal metal has been observed in some of the alkali azides [26,42] however, irradiation and heat treatment are necessary in most cases to produce the colloids, indicating that diffusion is necessary. Since F centers have been observed in NaNs [17], it is possible that colloids are formed in the alkali azides by the clustering of F centers. Alternatively, alkali metal diffusion may take place. The properties of colloids in alkali azides are discussed elsewhere in this chapter (see Section C). 

In summary, it has been reasonably well established that colloidal metal is produced by irradiation of the small band gap azides. However, the mechanism by which the colloids are produced remains uncertain. 

Because colloidal metal (color) is produced by irradiation of small band gap azides, they may be useful as photographic materials. In fact, silver azide emulsions have been made and their properties compared with emulsions of silver halides [208,209]. No attempt is made here to review in detail the photographic properties of emulsions. Instead a few characteristics of crystalline material are discussed briefly. 

This section deals with the radiation-induced decomposition of the azide sublattice as detected by nitrogen-gas evolution from both large and small band gap azides. The two groups of azides, which are shown in other sections of this chapter to have different radiation-induced disorder, also have different gas-evolution properties. Another significant difference is the photocurrent which accompanies decomposition in the small band gap azides but has not been observed in the large band gap azides. 

DiBona, D.A. Wiegand J. Sharma, Studies of Surfaces and Thin Films of Small Band Gap Inorganic Azides , JVacSciTech 13 (1), (1976)... 

Recent XPS studies by Sharma and associates also indicate, in general, that the alkah azides are insensitive to X-ray exposure whereas the small band gap... 

The second class of azides consists of the small band gap ( 4 eV) materials such as lead, silver, and thallous azides. These materials are explosive, and irradiation results in the evolution of nitrogen gas at longer wavelengths and with greater quantum efficiencies than in the large band gap materials. Although ESR... 

Singh observed that doping lead azide with small amounts of (BiNa) (0.24 wt %) results in increased rate and decreased activation energy for thermal decomposition [101]. On this basis it could be theorized that the rate-controlling step is the transfer of an electron from the azide ion to the conduction band and that if this energy barrier is reduced, for example by adding an impurity which introduces new energy levels in the band gap, decomposition rate is enhanced. 

Mark and Gora [24], commenting on the results, considered a model in which initiation is associated with a critical interface field at the Schottky-barrier contact between the metal electrode and the azide. Interface fields depend on properties of the sample and on the work function of the electrode, and are larger than the applied voltage divided by sample thickness. The model predicted an effect for uniform samples which was qualitatively consistent with experiment, but whose magnitude was too small to observe. However, the experimental samples were pressed pellets composed of individual grains which are likely to be separated by potential barriers [25,26]. Taking this into account, the model was consistent with experiment if initiation occurs at a critical interface field of about 2 X 10 V/m. This is a plausible value, in that fields in excess of 10 -10 V/m applied to surfaces of wide band-gap semiconductors commonly result in destructive breakdown due to carrier emission into the bulk from interface states [27-29]. 

A number of explosives fit the latter mechanism. These include lead azide, ammonium nitrate and pentaerythritol nitrate.The critical strains needed to initiate explosion may be estimated. As expected, the sensitivity of explosives to shock can also be rationalized by estimating their initial HOMO-LUMO gaps, or E°g. For solids such as Pb(N3)2, the energy needed to excite an electron from the valence band to the conduction band can be found by spectroscopy. A small gap means high sensitivity. ... 

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