Elastic constants at the smectic A–crystalline smectic B phase transition of the compound 50.8

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Abstract
We measured the velocity and absorption of the longitudinal sound waves in different phases of the compound 50.8 at 2 MHz. Attention is given particularly to the smectic A-crystalline smectic B phase transition. We found that the jumps in the elastic constants at this transition are: ΔC11 >0, ΔC33 >0 and ΔC13 <0. The velocity anisotropy reverses its sign throughout this transition. Concerning the absorption, we found, in addition to an anomalous peak at this transition temperature, a broad maximum in the crystalline smectic B phase which was interpreted as due to the Landau-Khalatnikov mechanism, for which the relaxation time is anisotropic. We present a Landau type free energy, which includes a new term linear in the order parameter and quadratic in the strain field, in order to account qualitatively for the results.