LATTICE GAS AND SPIN ORDERING ON STACKED TRIANGULAR LATTICES

Mean field theory is used to determine the highest temperature ordered phases of lattice gas and spin systems on stacked triangular lattices. We include nearest neighbour repulsive or antiferromagnetic interactions and map out the ordered phases as a function of interactions out to fifth neighbours. We show that the type of stacking, ...AAA.., ...ABAB... (hexagonal dose packed), ...ABCABC.. (cubic closed packed), has a profound effect on the nature of the ordering. For ...ABAB... and ...ABCABC... stacking the inter-plane interactions can compound the frustration within the planes resulting in phase diagrams dominated by incommensurate order just below the critical temperature. Comparison with relevant experimental lattice gas and magnetic systems is made.