To the Description of the Structural Second-Order Phase Transitions in Organic and Organoelement Crystals of Middle and Highest Syngonies

All 65 863 records for cubic, hexagonal, trigonal, and tetragonal crystals, presented at the Cambridge Crystallographic Data Centre (CCDC-2023), have been analyzed for the existence of structural second-order phase transitions, occurring without a change in the crystal syngony. Based on the analysis of the data obtained, two indicators of possible susceptibility of a crystal to a second-order phase transition are proposed. The first indicator is the degree of invariance of electron density relative to the group symmetry operators of the high-symmetry phase ηtot for the atomic structure on the whole and ηsubl for the atomic sublattices of his structure. The threshold values (ηtot ≥ 0.487(1) and ηsubl ≥ 0.982(1)), at which a crystal can be subjected to a second-order phase transition, are found for at least one atomic sublattice. The second indicator is the group–subgroup relationship for the lattice translations of the high- and low-symmetry phases. Among 39 crystals (180 records) with unestablished transition order, four crystals are found, in the structures of which ηtot and ηsubl exceed the threshold values, and the translations of the bases of high- and low-symmetry cells follow the group–subgroup relationship.