Measurement of powder diffraction sample absorption coefficients using monochromated radiation and transmission geometry

Powder diffraction data collected in transmission mode require an accurate value of sample absorption factor (μt) if the data are to be used quantitatively, e.g. for Rietveld refinement. When a primary-beam monochromator is in use, the direct beam may not be truly monochromatic, but rather consist of a series of harmonics of the desired wavelength. The intensity of these harmonics depends on instrumental parameters, but is sample independent. The contributions of these harmonics will result in incorrect values of μt for the primary wavelength if this is measured simply by determining the absorption of the direct beam by the sample at a single value of sample angle ω. The true value of μt may be determined by measuring the absorption over a range of ω and then fitting the data to the expression I(ω) = I₀[F₁ exp (− μt/cos ω) + (1 − F₁) exp (− μt/cos ω) ], where μt and μ't are the absorption factors for the primary component and the higher harmonics, respectively, and F₁ is the proportion of the primary component in the (attenuated) direct beam. Neglect of this effect will result in serious underestimation of μt; Rietveld refinement of data using such an incorrect value of μt will result in (in particular) unrealistic values of the thermal vibration parameters.