Peak shape and resolution in conventional diffractometry with monochromatic X-rays

Incident-beam monochromators have been available commercially for many years, but the significant advantages of their use in conventional divergent-beam diffractometry are not widely appreciated. The improvement in the quality of diffraction data which results from the removal of the Kα₂ component of Kα radiation at source has important consequences in most applications of powder diffraction. In particular, there is a simple well characterized instrumental function with a predictable angular variation. The parameters which define this function are independent of any sample and provide a basis for comparing instrumental configurations and for assessing the quality of reference materials. At low angles the profiles are predominantly Gaussian and at high angles they tend to be Lorentzian, owing to the dominant effect of wavelength dispersion. The main features of using a strictly monochromatic radiation in conventional powder diffractometry, namely improved resolution, simple instrumental shape function and better evaluation of the background level at higher angles, are described. The benefits of these properties in current applications of powder diffractometry, e.g. indexing, line-profile analysis and structure analysis, are discussed briefly.