To the Description of the Structural Second-Order Phase Transitions in Triclinic Organic and Organoelement Crystals

All 318 992 records of triclinic crystal structures, presented in the Cambridge Crystallographic Data Centre (CCDC-2023), were analyzed for presence of structural second-order phase transitions occurring without changing the crystal system. Four crystals (10 records) undergoing a classical second-order phase transition were found. The phase transition order was not established for 35 crystals (97 records). The low-symmetry phases of the structures undergoing a classical second-order transition revealed pseudosymmetry relative to the symmetry operators of the high-symmetry phase with large degrees of electron density invariance, η ≥ 0.896(1), with the group–subgroup relationship for symmetry groups retained. A technique for determining the most mobile structure fragment during a second-order phase transition is proposed.