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By X-ray powder diffraction methods the crystal structures of a-Tic!, and of the products obtained by dry ball-milling activation of this form were studied. TiCl, microcrystals show initially a structural disorder in the positions of titanium atoms and the successive mechanical activation introduces into the structure some stacking faults, mainly associated to t 60" rotations of the triple layers CI-Ti-CI.The structural investigation has been carried out by an accurate fit of observed X-ray patterns to those calculated in correspondence of structures containing well-defined disordered sequences and of likewise well-defined crystallite sizes. Like y-TiC1, and MgC12, a-TiC1, is very sensitive to mechanical activation, which improves the performances of these three compounds when they are employed as catalysts, or supports for catalysts, in Ziegler-Natta polymerization processes.

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