Orientational phase transitions in planar systems of nonpolar molecules

The dotted line in Fig. 2.19 enclosed by two markers x represents the ratio K3/Ki = 0.207 at the inclination angle 6 = 25° observed for the system CO/NaCl(100). The point at which this line intersects the solid one gives the critical temperature which proves equal to 20K if the value U = 1.63 meV is substituted. If dispersion and repulsion interactions estimated as in Table 2.3 are taken into account, the value Tc is shifted to 22 K, which agrees well with the experimental value Tc= 17.5+21.5 K.  

Orientational phase transitions in planar systems of nonpolar molecules 

A switch to double-angle vectors given by Eq. (2.3.12) not only significantly simplifies the treatment of orientation phase transitions in planar systems of nonpolar molecules but also leads to a number of substantial inferences on the transition nature. First of all note that the long-range-order parameter t] (vanishing in a disordered phase and equal to unity at T = 0) in a -dimensional space (specified by the orientations of long molecular axes) can be defined as  

Denote the self-consistent field exerted on a molecule j by neighboring molecules as rjV where V represents the absolute magnitudes of quantities (2.3.25) and (2.3.27) for various ground states of nonpolar molecules on square and triangular lattices. Then the equation for t] appears as  

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