Asymmetric Diffraction with Parallel-Beam Synchrotron Radiation

In this paper, the advantages and disadvantages of using the asymmetric 2θ scanning technique with a fixed incident angle α are described. Vertical-scan powder diffractometers with long horizontal parallel slits with an aperture of 0.05° and parallel-beam synchrotron radiation with λ = 1.54 Å and α = 10°, 2°, and 1° were used to collect α-Al2O3, silver behenate CH3(CH2)20COOAg, and Si powder diffraction patterns. The synchrotron radiation data were analyzed by profile fitting, and the results were compared with those obtained by the conventional θ-2θ scanning technique. As expected, significantly higher intensities were obtained from the asymmetric diffraction data with α = 10°. At smaller α = 2° and 1°, however, the intensities were reduced because of a smaller effective beam height. The peak positions remained practically unchanged for the data obtained with α = 10°, but displaced toward higher 2θ angles for α = 2° or lower, and, consequently a refractive-index correction was needed. Profile shape was slightly broadened and became more Lorentzian in asymmetric diffraction with highly oblique incidence of the beam. The change in shape was, however, negligibly small. The results showed that intensive and reliable powder diffraction data can be obtained from asymmetric diffraction by fixing the incident beam at a sufficiently large angle to fully illuminate the available sample surface.