**2.3. The atom and nanovoid as structural units of an ideal crystal**

A number of types of coordination polyhedra is limited. Creating an image of one of them reveals voids in polyhedral models of the crystal [58]. There are no sites of crystal lattice in these voids. Thus, the appearance of the voids is assumed to be explained by the category of coordination simple forms which are building the coordination polyhedron [45]. In this case the voids in polyhedral models of the crystal are explained as coordination vacancies due to partial incomplete population of peaks in coordination simple forms. From outside it can be seen as reduce of peaks in the coordination polyhedron proposed by Belov N. V. as a geomet‐ rical puzzle. In reality the coordination polyhedron is a matter image of coordination simple forms or their combinations with taking into account the population of the peaks.

It was found a mathematical relation between the number of atoms in the formula unit, coordination number of the atom, number of the facets, and the populations of peaks in simple forms [45, 46]. This relation shows that the interaction of inside structure and outside shape of the crystal is to be extended by a statement saying the following: the ideal crystal structure there are coordination nanovoids forbidden for self-population. These nanovoids are func‐ tionally and periodically located in the crystalline structure [3, 51, 52, 65]. For example, they form well-known throughout-pipelines (channels) in diamond.

So that, the crystal has a level hierarchy of its organization as a system. The first level shows chemical composition and relationships of atomic particles. The second one presents a function of their inter-location in the space. The level presents an organization of the crystal as a physical object. Categories " formula unit", "cell", and "crystal individuum"build the basis of every level. The habitus of the crystal is supposed to be a fourth level of its structural organization.

The coordination nanovoids are to be considered as the special (zero) level of the system organization of the crystal. They reflect the fact that the empty space before its substance population is a same participant (as the atoms) in the process of crystal formation. Namely the kept primary space is built from coordination voids which may be called relic nanovoids.

The category "coordination nanovoid"is important in other scientific fields. The nanovoids may be populated by other particles, for example, by photons, electrons, positrons, atoms or molecules. In this case the description of the particles absorber is to be started from charac‐ terizing the rest of primary space which is forbidden to the self-population by crystal sub‐ stance. For example, describing the structure of periclas as a photon absorber should begin from stating the relic tetrahedral voids, and than one can highlight the atoms groups Mg, О, MgO, Mg4O4, MgO6, Mg6O and others. Further we present description of interaction between periclas and UV-radiation [49, 51, 54].
