Lead azide allotropic forms

Lead azide can exist in two allotropic forms the a-form is orthorhombic, the / -form monoclinic. (Miles [75], Garner and Gomm [37]). The crystallographically stable modification is the a-form. It is prepared by rapidly stirring a solution of sodium azide with a solution of lead acetate or lead nitrate. 

Lead azide can exist in two allotropic forms the more stable a-form which is orthorhombic, and the /1-form which is monoclinic. The a-form is prepared by rapidly stirring a solution of sodium azide with a solution of lead acetate or lead nitrate, whereas the /1-form is prepared by slow diffusion of sodium azide in lead nitrate solutions. The /1-form has a tendency to revert to the a-form when its crystals are added to a solution containing either the a-form crystals or a lead salt. If the /1-form crystals are left at a temperature of 160 °C they will also convert to the a-form. Some of the properties of lead azide are presented in Table 2.3. 

It was originally thought that lead azide existed in two allotropic forms a and 0 (VoL 111, p. 169). Now it is accepted that tlie substance is polymorphic and exists in four forms o, 0, y and 6. The a-orthorhombic is the only one acceptable for technical application. It is the main product of precipitation with traces of the other forms present [89]. The monoclinic jS-fomi is stable when dr>. but recrystallizes as the a-form. Tlie presence of some organic dyes (e.g. eosin) enhances the fomiation of form and hydrophile colloids inhibit its formation. Breaking a needle of Torm may produce an explosion (contrary to the views of some authors Vol. Ill, p. 173). The monoclinic y-form is less stable than a and jS [90]. It can be obtained from pure reagents at pll 3.5—7.0 or in the presence of vinyl alcohol. The triclinic 6-form precipitates from pure reagents at pH values of between 3.5 and 5.5 [90]. Both forms yand 6 are usually precipitated simultaneously and can be separated (with care ) by hand. 

Lead azide forms white or yellowish crystals. It is known to form four allotropic modificatimis a, p, y, and 8 (older literature refers only to the first two of them). The orthorhombic a-form with crystal density reported from 4.68 to 4.716 g cm [12-14] is the main product of precipitation, with traces of other forms present, and is the only form acceptable for technical applications [15]. A variety of crystal... 

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