Refinement of α-lead azide by neutron diffraction

Abstract

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