Crystal structure

The crystal structure of sodium chloride

A crystalline structure is such a collection of atoms in which a specific group of atoms, called a motive unit, is associated with each point of the crystal lattice , and all such groups are identical in composition, structure and orientation relative to the lattice. We can assume that the structure arises as a result of the synthesis of the lattice and the motive unit, as a result of the propagation of the motive unit by the translation group ^[1] .

In the simplest case, the motive unit consists of one atom, for example, in crystals of copper or iron . The structure arising on the basis of such a motive unit is geometrically very similar to the lattice , but still differs in that it is composed of atoms, not dots. Often this circumstance is not taken into account, and the terms “crystal lattice” and “crystalline structure” for such crystals are used as synonyms, which is non-strict. In cases where the motive unit is more complex in composition - consists of two or more atoms, there is no geometric similarity of the lattice and structure, and mixing these concepts leads to errors. So, for example, the structure of magnesium or diamond does not geometrically coincide with the lattice: in these structures motive units consist of two atoms.

The main parameters characterizing the crystal structure, some of which are interconnected, are as follows:

• type of crystal lattice ( syngony , Bravais lattice );
• the number of formula units per unit cell ;
• space group ;
• unit cell parameters (linear dimensions and angles);
• the coordinates of the atoms in the cell;
• coordination numbers of all atoms.