Band Gap Azides

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. 

Lead azide crystaUizes in four polymorphic forms, the common form, a-Pb(N3)2, having 12 molecules in an orthorhombic unit cell (see Chapter 3). The fundamental optical absorption occurs at smaller energies than in the alkah azides and is in the range 4-5 eV (Chapter 5). The smah band gap azides TIN3 and Pb(N3)2 are conspicuous in that there is almost no evidence for the formation by irradiation of ions such as NJ or Ni in them. The only indication of such... 

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... 

Barium azide differs from the other azides discussed in this section in at least two reported respects It is a large band gap azide (see Chapter 5) and deflagrates but does not sustain steady state detonation [182]. Slow decomposition, due to both irradiation and heat, has been investigated extensively with the primary emphasis placed on gas-evolution studies. In this subsection the emphasis is on the disorder remaining in the sample after partial decomposition, primarily by... 

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. 

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