Mise en évidence et étude, par diffraction des rayons X sur poudre, d'une transition ordre–désordre pour le nickelocène Ni(C₅H₅)₂

About forty X-ray powder diagrams were recorded, in the range 5 to 295 K, with a prototype diffractometer and a high-efficiency cryostat. The temperature dependence of the cell parameters and the principal thermal expansion coefficients presents anomalies (below 240 K) which are due to the existence of an order–disorder transition in the range 240–170K. There is a correlation between the temperature dependence of the thermal expansion coefficients α_hkl and the asymmetric broadening of corresponding peaks. The existence of domains and frontier zones and the important reorganization of the molecular packing, in the ordered phase, explain the asymmetrical broadening of peaks. The nickelocene molecular configuration is probably eclipsed (D_5h) in the ordered phase.