Size-strain separation in diffraction line profile analysis.

Separation of size and strain effects on diffraction line profiles has been studied in a round robin involving laboratory instruments and synchrotron radiation beamlines operating with different radiation, optics, detectors and experimental configurations. The studied sample, an extensively ball milled iron alloy powder, provides an ideal test case, as domain size broadening and strain broadening are of comparable size. The high energy available at some synchrotron radiation beamlines provides the best conditions for an accurate analysis of the line profiles, as the size-strain separation clearly benefits from a large number of Bragg peaks in the pattern; high counts, reliable intensity values in low-absorption conditions, smooth background and data collection at different temperatures also support the possibility to include diffuse scattering in the analysis, for the most reliable assessment of the line broadening effect. However, results of the round robin show that good quality information on domain size distribution and microstrain can also be obtained using standard laboratory equipment, even when patterns include relatively few Bragg peaks, provided that the data are of good quality in terms of high counts and low and smooth background.

Ordering of nitrogen in nickel nitride Ni(3)N determined by neutron diffraction.

The metallic interstitial nitride Ni(3)N was prepared from Ni(NH(3))(6)Cl(2) and NaNH(2) in supercritical ammonia (p(NH(3)) approximately 2 kbar) at 523 K. Its previously reported crystal structure, as determined from X-ray powder data, was confirmed by neutron powder diffraction: Ni(3)N crystallizes in the hexagonal epsilon-Fe(3)N-type structure (P6(3)22, Z = 2, a = 4.6224 A and c = 4.3059 A at room temperature). The N atoms on the octahedral sites of an hcp arrangement of Ni show virtually complete occupational order at ambient temperatures, which is preserved up to its thermal decomposition at T approximately 600 K. This behavior is in marked contrast to that of the isotypic iron nitride, epsilon-Fe(3)N, which shows reversible partial disordering within the same range of temperatures. Possible reasons for the different behaviors of the two nitrides epsilon-Fe(3)N and Ni(3)N are discussed.