The low-temperature phase of sodium azide has been studied at a number of temperatures between 12 and 293 K using a combination of single-crystal and powder X-ray techniques. The structure is monoclinic with space group C2/m. The unit cell undergoes a discontinuous shear of 0.5 ° in the ac plane from the rhombohedral (R3m) structure at the transition temperature (292.2 K), and this is followed by a continuous change amounting to a total shear of 5-3 ° at 12 K. The unit-cell dimensions at 12 K are a= 6.1654 (5), b=3.6350(3), c=5.2634(6) A, /3=107.543 (5) °, and Rwp = 14.7. It is concluded that the transition is of the ferroelastic type and is nearly, but not exactly, second order.

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A comparison of different methods of X-ray diffraction analysis for the determination of crystallite size and microstrain; namely, line profile analysis, Rietveld refinement, and three approaches based on the variance method, is presented. The analyses have been applied to data collected on a ceria sample prepared by the IUCr Commission on Powder Diffraction. In the variance method, split Pearson VII, the Voigt function, and its approximation pseudo-Voigt function were fitted to X-ray diffraction line profiles. Based on the fitting results, the variances of line profiles were calculated and then the crystallite size and root mean square strain were obtained from variance coefficients. A SS plot of Langford as well as a Fourier analysis and Rietveld refinement have been carried out. The average crystallite size and microstrain were determined. The values of area-weighted domain size determined from the variance method are in agreement with those obtained from line profile analysis within a single (largest) standard uncertainty, and the volume-weighted domain sizes derived from the SS plot, Fourier size distribution, and Rietveld refinement agree within a single standard uncertainty. The results of rms strain calculated from variance and Pearson VII shape function and those from Rietveld refinements fall within a single esd. However, the variance method in conjunction with pseudo-Voigt and Voigt functions produce rms strains substantially larger than those determined from line profile analysis and Rietveld refinements.

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