X-ray powder diffractometric procedure for lattice parameter determination for long-chain molecules

An X-ray powder diffractometric procedure has been developed to determine the lattice parameters of long-chain hydrocarbons and mixtures thereof. Useful information was limited to low diffraction angles owing to the large size of the unit cell. Intensities were measured in the reflection mode for thick samples with low absorption. Centroids of the diffraction profiles were corrected for axial beam divergence, sample flatness, displacement and transparency and goniometer scale zero position. An internal Si standard was mixed with the sample to determine the displacement of the sample surface from the goniometer axis. The values of a, b, and c for the orthorhombic structures n-tricosane (n-C23H48) are 4.965(3), 7.462(4) and 62.12(2) A and for n-pentacosane (n-C25H52) are 4.964(3), 7.450(4) and 67.21(2) A. The values agree well with earlier single-crystal studies except for c for which a larger value is reported in the earlier work, probably due to sample impurity. (orig.)