Positional order in the smectic-G, -F, and -C layers of HBHA

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Abstract
The positional order correlation length « ξ∥ » parallel to the smectic (Sm) layers in the G, F and C phases of HBHA (7O.6) has been measured by X-ray diffraction. In the Sm-G phase its value is limited by crystallite size and lies between 1200-1500 A. At the Sm-G to -F transition ξ ∥ changes from 300 to 180 A with a temperature dependence predicted by a dislocation mediated melting model of 2D lattice. At the Sm-F to -C transition ξ ∥ changes discontinuously. The Sm-C phase is found to have an unusually large correlation length (∼ 50 Å), almost double of that in the Sm-A phase