The heterogeneity of a solid must be taken into account if the property under consideration is determined not only by the atomic short-range order but also by the spatial extent of regions with equal short-range order, the shape of these regions, the interface area between different regions, the correlation of shape and size of adjacent regions, the topological arrangement, etc. Such phenomena appear in most materials, e.G. In polycrystalline metals and alloys, powders, ceramics, porous materials, glasses and random composites.
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