= Crystal packing
<Crystal packing> describes how constituents are arranged and make contact within a <crystal>. It includes both the repeated geometry and, for molecular crystals, the orientations and intermolecular contacts of the molecules. Composition alone does not determine this arrangement: the same substance can have different <crystal polymorphs> with different packing.
A useful geometric model replaces constituents by equal hard spheres. <Hexagonal close packing> and <face-centered cubic> packing put each sphere next to twelve nearest neighbours but arrange successive close-packed layers differently. Their local contact geometry can therefore agree while their longer-range order differs. Real molecular shapes and directional <intermolecular forces> supply further constraints. Packing affects <mass density>, available slip geometry and directional response, making <diffraction> a way to connect structural measurements with macroscopic properties rather than merely assign a repeating pattern.
Back to article page
