Indexing rhombohedral powder diffraction patterns

Based on statistical data of 21 rhombohedral structure types, a systematic procedure has been developed for the rapid unique indexing of rhombohedral powder diffraction patterns. The key to the graphical method is a novel semilogarithmic chart of log d versus 1 − tan² (α/2) with α ranging from 2 to 118°. Convenient tables and formulas are given to expedite the evaluation of the lattice constants of an unknown phase.