Structural disorder in microcrystalline MgCl₂

Structural disorder in severely ground MgCl₂ is interpreted on the basis of a Bernoullian model with four parameters, specifying both the size and the statistical sequence of the layers; their choice completely dictates the intrinsic shape of all the X-ray diffraction lines. The X-ray powder spectra are reasonably well reproduced over a wide range of Bragg distances (~1.7 < d < ~9 Å) for samples with various degrees of disorder. Practical criteria are given to derive the statistical parameters from the X-ray spectra. Average crystal thickness as small as 20 Å across the layers is observed. From the usual structure with cubic packing in the Cl atoms, with increasing disorder at first the probabilities of cubic and hexagonal-like interlayer shifts (P_cub and P_hex) tend to become equal; the ultimate degree of disorder also involves ± 60° rotations between adjacent layers, with about the same probability as P_cub and P_hex.