Interpenetration of three-periodic networks in crystal structures: Description and classification methods, geometrical-topological conditions of implementation

This review describes the current state of studies on the phenomenon of interpenetration of framework groups in crystal structures. The generally accepted terminology used in the description of topology of interpenetrating motifs, symmetric and topological properties of interpenetrating systems is given. The main advances of crystal chemistry in the systematization of interpenetrating structures are elucidated. It is noted that the major trend in the crystal chemistry of interpenetration is the development of methods for the topological classification of the entanglements of interpenetrating groups and the search for the regularities of their implementation in crystalline substances. The main ways of the formation of interpenetrating structures, the appearing here geometrical-topological restrictions, and also the effect of stereochemical factors and synthesis conditions on the possibility of interpenetration are considered.