The crystal structure of Bromopentamminocobalt bromide ([Co(NH~3) ~5Br]Br~2) has been ab initio determined from the X-ray powder diffraction data collected on a Rigaku D/MAX-rB diffractometer. The pattern was auto indexed using the trial-error and the successive- dichotomy methods. The orthorhombic unit cell and the possible space group according to extinction conditions were obtained. 329 integrated intensities were extracted by whole pattern fitting. The approximate structure with all 7 independent non-H atoms was solved by direct methods followed by interactive cycles of least-squares refinement and Fourier syntheses. The following Rietveld refinement was carried out over the angular range of 10°～80°(2θ) with 3495 intensities step and 45 parameters. The final orthorhombic unit-cell parameters are: a=1.3692(1)nm, b=1.0707(1)nm, c=0.6940(1)nm, V=1.0175 (1)nm^3, M~2~0=49, F~3~0=93(0.0075, 43), Z=4. Space group is Pnma. The structure agreement factors are: R~p=0.066, R~w~p=0.090, R~F=0.041, R~B=0.042. In this paper, the crucial steps, the difficuties and the authors' practical experience in ab initio powder diffraction crystal structure determination methods were summed up.

Key words: COBALT COMPLEX, AMMINE COMPLEX, BROMO COMPLEX, CRYSTAL STRUCTURE DETERMINATION, POWDER DIFFRACTION