The structural phase change in s-triazine. III. Lattice energy calculations

Abstract

For pt.II see ibid., vol.11, p.1771 (1978). Previous calculations of the lattice energy of s-triazine have been extended to investigate the nature of the structural phase change. When the energy is plotted in a two dimensional space defined by the shear angle of the unit cell and the rotation angle of the molecules, the points corresponding to the high- and low-temperature structures lie along the foot of a valley. The inclusion of electrostatic interactions represented by a quadrupole-quadrupole potential results in the structure at absolute zero of temperature corresponding with a minimum in the lattice energy and the stabilisation energy of the low-temperature structure is found to be close to that predicted by other methods. A 'soft mode' associated with the phase transition is identified.