Lattice model for ordering in two-dimensional plastic crystals

We study the properties of a two-dimensional square lattice model in which each lattice site is occupied by an elliptical molecule which may point eit her horizontally or vertically. Molecules on neighbouring lattice sites int eract via a potential function based on the ellipsoid contact function intr oduced by Perram and Wertheim. We show that this model has a set of second- order phase transitions as the lattice constant is varied. These transition s separate phases exhibiting both nematic and staggered ordering from orien tationally disordered phases. We present the phase diagram for the model an d discuss how it changes as the eccentricity of the ellipses is varied. (C) 2000 Elsevier Science B.V. All rights reserved.