In the Bragg-Brentano X-ray powder diffractometer geometry the Automatic Divergence Slit (ADS) provides a fixed area of illumination on a flat specimen. For this case, the “constant volume” diffraction, appropriate for a Constant Divergence Slit (CDS) diffractometer, is not applicable and intensities must be corrected by a sinθ factor before comparison to CDS data.
It is shown that for thin layers the diffraction pattern may be treated as a “constant volume” diffraction case when the ADS is used. Moreover, the derivation of the unit cell dimensions using a common least-square procedure can result in excellent lattice parameters without using internal standards, because the transparency aberrations are minimized.
ADS data were obtained for a variety of reference materials including several NBS SRM powders. It was found that thin samples made of powder mixed with vaseline gave “constant volume” diffraction, less angular aberration, and yet had line intensities only somewhat less than bulk samples.
Three main benefits arise using a combination of thin layers and ADS: (a) The amount of material needed for routine chracterization is small, (b) The relative experimental intensities are approximately the same as those obtained from bulk specimens using a CDS. (c) The measured (Bragg) scattering angles are more accurate compared with those measured from bulk specimens.