An experimental investigation of the effects of axial divergence on diffraction line profiles

An experimental investigation has been carried out to determine the extent to which axial divergence in a conventional powder diffractometer influences the measurement of peak profile parameters. A Siemens D5000 theta-2 theta diffractometer was used for this study along with the LaB6 line profile standard SRM 660. Eight unique levels of axial divergence were investigated either by removing one or more of the Seller slits, or by introducing different combinations of Seller slits in the incident and diffracted beams. The measure of axial divergence used throughout is based on the maximum axial divergences of the incident and diffracted beams, Psi(i) and Psi(d). Axial divergence produces a small and almost constant shift Delta 2 theta(p) in the peak angle which in a typical diffractometer would amount to a zero offset 2 theta approximate to 0.005 degrees. The integrated intensity of a profile increases almost linearly with the product Psi(i)*Psi(d). The increase in breadth in profiles in the angular region 2 theta < 120 degrees arises mainly from the change in asymmetry D-H = H-lo - H-hi in which one side of the profile broadens (i.e., either the high angle or low angle side) without any significant broadening of the other side. Moreover, the asymmetry D-H is linearly dependent on 2 theta for a fixed level of axial divergence, and linearly dependent on the total axial divergence Psi(i)+Psi(d) at fixed 2 theta. (C) 1998 International Centre for Diffraction Data. [S0885-7156(97)01804-6].