Orientational Phase Transition in Molecular Crystal N2

Abstract

The orientational phase transition of the solid N 2 is studied on the basis of the statistical theory for orientational rotators with multiple directions. The orientational field hindering free rotation of the nonpolar diatomic molecule N 2 is assumed to be caused by the 6–12 Lennard-Jones potential. The molecule N 2 can point to one of four orientations along the cube body diagonals. It is shown that the four molecules in a unit cell take the α-form structure with the crystal symmetry Pa3 at the ground state, which forms the sublattices consisting of four simple cubic lattice. The temperature dependence of the orientational order parameter is obtained in the molecular field approximation. It is shown that the first order phase transition takes place in the solid N 2 .