SHORT-RANGE ORDER OR MIXED MDO-PHASES IN LAYERED, POLYTYPIC MATERIALS

Some layered materials, like Cu(C6H4(OH)COO)(2) . 2H(2)O (DSC) and K(3 )M(CN)(6) (M = Fe, Cr, Mn, Co) show disorder in their layer stacking, as revealed by the presence of diffuse ridges of diffraction along the layer normal. The ridges are found to have fairly sharp Bragg-like pe aks superimposed, which have been attributed to ordered regions of two types in the material, denoted MDO(1) and MDO(2) (MDO = maximum degre e of order). Thus for DSC the diffraction pattern has previously been interpreted as consisting of three distinct components, implying three spatially separate phases in the sample: Disordered regions (ridges) and MDO(1) and MDO(2) regions (peaks). It is shown in the present work that this multiphase model is inappropriate for DSC, but that the dif fraction pattern may be fully interpreted in terms of a single phase, involving short-range ordering to nearest and next nearest neighbours. In fact the diffraction pattern is directly related to the crystal gr owth mechanism, which may be described in terms of a Markov chain.