Thallium azide, decomposition

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

Kabanov, A. A. etal., Russ. Chem. Rev., 1975, 44, 538-551 Application of electric fields to various explosive heavy metal derivatives (silver oxalate, barium, copper, lead, silver or thallium azides, or silver acetylide) accelerates the rate of thermal decomposition. Possible mechanisms are discussed. 

TIN3 (c). Wohler and Martin1 found the heat of decomposition of thallium azide to be 54.7. 

It has long been assumed that since lead azide, for example, is more sensitive than sodium azide when impacted, the reactivity of the former is greater than that of the latter. That this is not so was demonstrated by Walker [58] and Fox [59], who compared the rates of slow thermal decomposition under identical experimental conditions for the azides of sodium, thallium, and lead. The results, summarized in Figure 7, show that over most of the temperature range studied lead azide is the least reactive, and that above about 560°K sodium azide reacts more rapidly than either lead or thallium azides. Moreover, above 590 K the rate of evolution of heat is greater in sodium than in lead azide. Alternatively, the application of data for the energetics of decomposition derived from slow reactions is not applicable to fast reactions, since in the latter thermodynamic equilibrium is not attained and the mechanism for slow decomposition discussed in Chapter 6 may not apply. There is some evidence that this is so [60]. 

Thallium(I) nitride is a highly explosive black solid,1 but the yellow azide T1N3 is more stable the physical properties suggest that there is some covalent T1—N bonding in the solid state. The mechanism of the decomposition has been reviewed.287... 

TIN3 is one of the few metal azides that melt (MP in vacuo, 330°C) prior to decomposition. At 340°C it begins to sublime, evolves gas at 370°C, and explodes at 430°C, leaving silver-white thallium metal as residue [2]. It is only slightly soluble in cold water. 

Figure 9. Arrangement of thallium ions in thallous azide. Dotted outline shows the unit cell, and the full line indicates the geometry of the pyramidal crystals which commonly form from solution. The arrow indicates the direction in which decomposition takes place.

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... 

Hall and Williams [96] doped thin films of lead azide with T1 and Bi. There was no marked effect on the photodecomposition efficiency at 330 nm as compared to undoped films. However, both the spectral dependence of the rate and the optical absorption were altered by thallium. The incorporation of T1 (10 mole fractions) removed the 375 nm peak from the optical absorption spectra while the incorporation of Bi left the peak unaltered. Partial decomposition of films (0.1%) also removed the 375 nm peak (dotted curve. Figure 32). The results are consistent with the fact that the Tl " impurities require anion vacancies for charge compensation. This is equivalent to partial decomposition. They concluded that the peaks in the optical absorption curve and spectral photodecomposition curves are probably a result of charge-transfer excitons. Furthermore, peak separations may arise because of differences in the interaction energies of inequivalent lead and azide ions in the unit cell. The selective removal with decomposition of the 375 nm peak may indicate selective decomposition of the azide site having the highest valence band energy. The selective decomposition would reduce the density of states and thus the extinction coefficient for electronic transitions from that particular azide band. 

The photolysis of pressed pellets of thallous azide was studied by Deb and Yoffe [239]. Decomposition led to the formation of a thin surface layer which was assumed to be thallium. No induction period in the rate was observed. The rate at constant temperature for the full output of mercury lamp was proportional to the light intensity. The activation energy for decomposition was found to be 0.33 eV (405 < X < 436 nm) or 0.14 eV (320 < X < 380 nm), with a quantum yield of 2 X 10" for X = 365 nm. Studies of the photo-induced colloidal bands in TIN3 are reviewed by Wiegand (see Section D). 

Figure 7. Rates of slow thermal decomposition for the azides of lead, thallium, and sodium as a function of temperature, determined under the same experimental conditions.

---