Structure determination of a steroid directly from powder diffraction data†

We report the determination of the crystal structure of a new polymorph of the pharmaceutical material fluticasone propionate, which is obtained as a microcrystalline powder by a supercritical crystallization procedure; the structure was solved directly from powder diffraction data using our Genetic Algorithm technique (in which a population of trial structures evolves through well-defined procedures for mating, mutation and natural selection) and refined using Rietveld refinement techniques.