Lattice parameter means the dimensions of the unit cell in any of the crystallographic arrangements. In case of a cubic symmetry, the size of the lattice is fixed by the length of the cubic unit.
Each atom has a site defined by its geometrical centre of relationship with the crystal lattice and this is the mean position of the atom. The position of the atom is less well defined, because it undergoes thermal vibrations within the space available to it between its neighbours.
The crystal structure can be regarded as an array of atoms built of layers one over the other, each layer consisting of simple arrangement of uniform rows of atoms, which mate or key with the atoms in adjacent layers. Such layers of atoms are called crystallographic planes and the inter atomic forces in a solid vary greatly with geometry of these planes.
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