Correction of intensities for preferred orientation in powder diffractometry: application of the March model

Diffracted intensities from axially symmetric flat-plate or capillary specimens, composed of effectively rod- or disk-shaped crystallites, can be corrected for preferred orientation with a single pole-density profile. A convenient procedure is to approximate this profile with a function whose variable parameters are fit during least-squares structure refinement. Several functions have previously been suggested but without theoretical justification. The present study reviews the derivation of this method and examines its assumptions and applications. The several proposed functions are compared with each other and with the March function which describes the pole-density distribution produced by rigid-body rotation of inequant crystallites (i.e. crystallites with unequal sides) upon axially symmetric volume-conserving compression or expansion. For its basis, ease of use, single variable parameter, direct interpretability and good refinement test results, the March distribution is proposed as an advantageous pole-density profile function for general use.