Α-lead Azide Research Articles

Refinement of α-lead azide by neutron diffraction

Pb(N3) 2, orthorhombic, Pnma; a = 6.63, b = 16.25, c = 11.31 A, Z = 12 [Azaroff (1956), Z. Kristallogr. 107, 362-3691. The four independent azide groups in the structure are all asymmetric, with N N bond distances, corrected by the riding model, ranging from 1-164 to 1.196 A, and N N N bond angles as small as 177.9 ° . Introduction. It is well known that the stability of azide compounds is strongly correlated with the degree of covalency of the azide bonds in crystals. The structure of a-Pb(N3) z, a well known primary explosive, was previously determined from a partial, threedimensional, neutron diffraction data set by Choi & Boutin (1969). A recent advance (Garrett, 1972) in techniques for growing lead azide crystals has provided us with additional crystals of a size and shape suitable for neutron diffraction. We have therefore redetermined the structure by refining a full, three-dimensional data set, in order to improve the accuracy of the structure parameters. Intensities were measured out to a limiting value of sin 0/2 = 0.65 /~-I by the procedure described by Prince (1972), on a computer-controlled four-circle diffractometer at the NBS reactor. Of 1466 independent reflections within the limiting sphere, there were observable intensities for 1328. Intensities were reduced to F values, and reflections were assigned weights according to W = [a~ + (0.005F)21 -~, where a~ is the estimated standard deviation of F due to counting statistics. The structure was refined by full-matrix leastsquares methods (Finger & Prince, 1975), including an isotropic secondary extinction parameter (Zachariasen, 1968) to a weighted R of 0.043 and an unweighted R of 0.038. The extreme secondary extinction correction (Fcorr/Fcalc) was 0.34 for the 004 reflection. The final least-squares atomic positional parameters are given in Table I.* Discussion. The four crystaltographically distinct azide groups, designated in the following discussion as azide I, azide II, azide III, and azide IV, are all distorted significantly from the linear, symmetric conformation found in more stable ionic azide compounds (Choi & Prince, 1976). Bond distances and angles are given in Table 1. Final least-squares atomic positional parameters ( x 104)for a-Pb(N 3)2

The effect of crystal size on the thermal explosion of α-lead azide

It is shown experimentally that the critical explosion temperature of α-lead azide crystals in air decreases with increase in size of the crystals. The mode of decomposition of the crystals was examined with optical and scanning electron microscopy, and it was found that surface decomposition took place preferentially at localized regions; crystal breakup also occurred. Calculations of explosion temperature for the case of parallelopiped shaped particles in air were made. The predicted decrease in the explosion temperature for an increase in crystal thickness from 4 to 34 μm (minimum dimension) was 6 K, whereas the experimental value was ∼ 30 K. It is suggested that the difference between the experimental and theoretical results is due to the mode of decomposition during the preexplosion period. For crystals of thickness less than ∼ 3 μm neither explosion nor melting took place even for temperatures as high as 825 K.

The effect of crystal size on the thermal explosion of α-lead azide

The effect of crystal size on the thermal explosion of α-lead azide

Note on the ESR Spectrum of Mn2+ in α-Lead Azide

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Infrared and Raman Spectra of Single-Crystal α-Lead Azide

The polarized transmission infrared spectra between 400 and 4000 cm−1 and the He–Ne laser excited Raman spectra between 30 and 1500 cm−1 of an oriented single crystal of α-lead azide (PbN6), have been measured and the long-wavelength vibrational modes have been assigned by factor-group analysis. The observed splittings are in essential agreement with the reported space group D2h16− Pnma. The Raman-active lattice modes are observed in the frequency range 30–210 cm−1. The appearance of peaks in the 1330–1360−cm−1 region due to the symmetric stretching vibration of the azide ion, in both the infrared and Raman spectra, confirms the presence of fairly asymmetric azide ions in the lattice. A probable relationship between a component of the bending mode frequency of the azide ion and the thermal instability of α-lead azide and related metal azides is pointed out.

Madelung energy and electronic structure of BaN 6 and α-PbN 6

The Madelung energies of barium azide and α-lead azide are calculated as a function of the azide ion's charge distribution, and compared to the known lattice energies. The results suggest that the Bonnemay-Daudel charge distribution of the free azide ion is not strongly perturbed by either lattice. The charge-transfer exciton energies and band gaps are calculated, and the effects of distinct azide ion sites considered.

The explosive sensitivity of the metal azides to impact

The thermal decomposition kinetics of single crystals of α-lead azide have been studied as a function of surface to volume ratio. As in the case of sodium and thallium azides previously reported by Walker, Gane, and Bowden, the decomposition is directly proportional to the surface area of the decomposing crystal and the temperature dependence of the rate is given by Rate of decomposition (molecules m −2 sec −1)=7.4 × 10 30 exp −(123 ± 6) × 10 3 R·T where the activation energy is in units of kilojoules per mole. These results together with those of Walker, Gane, and Bowden, allow a direct comparison of the relative rates of decomposition for all three materials. This is found to be Na > T1 > Pb over most of the temperature range studied which is in the reverse order of explosive sensitivity. In rationalizing this observation with the thermochemical hot spot theory of explosion, consideration is given to the alternative tribochemical mechanism of initiation and it is proposed that the release of elastic strain energy when a fast crack runs through the material, if conserved at the crack tip, may be used to initiate the decomposition reaction. The criterion for the effectiveness of this mechanism is taken to be the ductile-brittle transition temperature. From measurements of the elastic-plastic properties of the three azides, it is found that at low-strain rates (10 −5 sec −1), these temperatures are not high enough to sustain an explosive reaction. However, evidence is given that on increasing the strain rate to that normally associated with impact, lead azide becomes rapidly more brittle and hence more susceptible to fracture initiation. Finally, some implications of the fracture mechanism are discussed and it is shown that preliminary experimental results are in accord with the predictions which such a mechanism allows.

Optical and electrical properties of thin films of α-lead azide

The optical absorption spectrum of α-lead azide was measured in the region 185–700 mμ at room, liquid nitrogen and liquid helium temperatures. Absorption coefficients were determined from the diffuse reflectance of powders and from transmission measurements on thin films. The spectral response of photoconductivity of thin films was determined from room temperature down to 77°K. The thin films of lead azide were prepared by vacuum depositing a thin film of metallic lead on either quartz or sapphire substrates followed by exposure of the lead film to gaseous hydrazoic acid. The results indicate that the fundamental absorption is at shorter wavelengths than indicated previously. Structure not previously reported has been found in the optical absorption and photoconductivity data and has been interpreted in terms of fundamental exciton structure.

Optical and electrical properties of thin films of α-lead azide

Optical and electrical properties of thin films of α-lead azide

Photodecomposition of α-lead azide in the solid state

Photodecomposition of α-lead azide in the solid state

Photoconductivity in ?-lead azide crystals

The photocurrent produced in α-lead azide by continuous irradiation has been measured as a function of the wavelength and intensity of the light, of temperature, and of time. The peak at 4070 Å has been interpreted as a transition from the valence band to an exciton level, which, from temperature-coefficient measurements, lies 8.2 kcal/mole below the conduction band. The total thermal band gap is estimated to be 37 kcal/mole. The peak at 4800 Å is regarded as due to electron promotion from impurity levels to a pre-dissociation level close to the conduction band, and the concentration of these impurity levels has been measured.

Radiation-induced Changes in α-Lead Azide

Optical, gravimetric, and x-ray methods were used to measure changes in the lattice and decomposition products in x-irradiated alpha lead azide. Findings are discussed. (C. H.)

The effect of high energy X-rays and pile radiation on the thermal decomposition and thermal explosion of α -lead azide

α -lead azide has been irradiated in air with the X-rays from a 1 MV generator and with pile radiation. At dosages below 10 4 r the thermal decomposition and thermal explosion characteristics of the X-irradiated material are indistinguishable from those of the unirradiated material. At higher dosages decomposition rates are changed and explosion times reduced. Pile radiation gives very similar results for the same energy deposition. Samples irradiated at 10 4 and 10 7 r showed no difference from unirradiated material on a friction sensitivity test.

Sensitivity of Alpha- and Beta- Lead Azide

LEAD azide, PbN6, exists in two allotropic forms1,2, one (α) orthorhombic and the other (β) monoclinic. α-Lead azide is readily prepared by mixing in the normal way solutions containing azide and lead ions; β-lead azide crystallizes as needles when aqueous solutions of sodium azide and lead nitrate interdiffuse slowly1. Table 1 summarizes the crystallographic relations1,3. Both forms are explosive and α-lead azide is in common use as a detonator. It is said1,2 that β-lead azide is much more sensitive and dangerous than α-lead azide.