Centrosymmetric phases

However, recent theoretical studies [90] indicate that systems exhibiting attractive interactions of the A and D blocks generally should favour an -ABCDABCD- sequence leading to a non-centrosymmetric lamellar phase. As the theoretical phase diagram (Fig. 28) predicts that the annealing process goes through a mixed centrosymmetric phase (MCS BC) before the fully separated centrosymetric one (CS) is reached, the prediction may not be at... 

The ruthenium analog 7 is isomorphous and isostructural with 8, and has almost identical behavior. The high-temperature phase change occurs at a slightly lower temperature, between 313 and 323 K. This centrosymmetric phase is isomorphous... 

The structure of FeRu2(CO)i2 " has proved more interesting than expected. The cluster also undergoes a phase transition, from a centrosymmetric disordered phase at temperatures above 228 K, to a non-centrosymmetric phase below this temperature. The non-centrosymmetric phase is disordered in the metal atom positions immediately below the phase-transition temperature, but becomes perfectly ordered at 173 K. As discussed above, the cluster has approximate C2 symmetry, but the overall geometry is that of the Dt, type structure with some small distortions. 

If the liquid crystal is composed of chiral molecules, then one linear term is allowed in the free energy. This is the term that changes sign when the axes are inverted, which is allowed in a chiral (non-centrosymmetric) phase. Thus for chiral liquid crystals, the proper free energy expression is... 

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