STERIC HINDRANCE AND PERCOLATION IN ORIENTATIONAL GLASSES

Using a very simple renormalized site percolation model, the rich phas e diagram exhibited by ACN(x)Mn(1-x) mixtures, where A stands for K, N a or Rb while Mn represents Br, Cl or I, is obtained as function of cy anide concentration, x. From a steric hindrance argument, some cyanide s are found to be randomly frozen at low temperatures being thus preve nted from propagating long range orientational order. Actual cyanide d ensity x should then be divided into a frozen-in part, x(f), and a fre e-to-reorient part, x(r), making only the last term relevant to quadru polar ordering. The first term x(f) can indeed be calculated from micr oscopic characteristics of the molecules involved. This leads to the e xistence of two critical thresholds, x(c) and x(d). While the former i s associated to the disappearance of the ferroelastic transition, the latter defines the concentration extension range for the existence of ferroelastic domains. When available, experimental data for the critic al thresholds are in excellent agreement with our calculated values. P redictions are made for a large variety of mixtures.