Cluster Phenomena and the Displacive to Order-Disorder Crossover

Renormalization-Group methods are used to calculate the critical distribution functions for large groups of ordering coordinates in two- and three-dimensional systems undergoing structural phase transitions. In two dimensions the distribution function shows clear evidence of well-defined clusters of precursor order, and cluster walls: The asymptotic collective behavior is always that of an order-disorder rather than a displacive system. In three dimensions the order-disorder component of the asympototic behavior is markedly weaker.