Statistical Models of Orientational Phase Transition in Molecular Crystals

Orientational ordering and disordering in plane lattices of molecules with some orien-tational pairwise interaction are investigated, based on the theory of the Ising lattice for magnetic systems. It is found how the characteristics of the orientational phase transition depend on the lattice type and the pairwise interactions between molecules. There are cases of transitions of the second and first kinds as well as of no transition. In case some external force dependent on the orientation of molecule is applicable, the molecular lattice system can exhibit a transition induced by that external force. These statistical models are 'investigated to suggest certain prototypes of · the orientational phase transition in the system of molecular crystal