Revised crystal data for magnesium dichloride, MgCl₂

A refinement of the cell parameters of MgCl₂ has been achieved by powder X-ray diffraction. The diffraction pattern was obtained using a standard PW-1050 Philips diffractometer operating in the step-scan mode and equipped with a graphite monochromator and a Xe-filled proportional counter. Cu radiation with a wavelength of 1.54056 Å was used. The specimen was placed in an air-tight sample holder in an N₂ atmosphere. An emission spectrographic analysis revealed a purity of > 99%, while the XRD pattern showed that only a small amount of Mg(OH)Cl was present. Unit-cell dimensions of a = 3.640(4) and c = 17.673(15) Å have been found with V = 234.2(7), Z = 3, D_x = 2.339 Mg m^-3. These values are different from those found by Ferrari, Braibanti & Bigliardi [Acta Cryst. (1963). 16, 846] using a photographic method. The difference is probably due to the higher accuracy obtained for the present data with the diffractometer.