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Heat capacity associated with nematic-smetic- A 1 -smectic- A ¯-smectic- A (crenelated)-smectic- A …

1989; American Institute of Physics; Volume: 39; Issue: 3 Linguagem: Inglês

10.1103/physreva.39.1369

0556-2791

Kenji Ema, C. W. Garland, G. Sigaud, Nguyen Huu Tinh,

Nonlinear Dynamics and Pattern Formation

High-resolution heat-capacity measurements have been made on mixtures of alkylphenyl-cyanobenzoyloxy benzoate (${\mathrm{DB}}_{\mathrm{n}}$CN) and cyanobenzoyloxypentyl stilbene (${\mathrm{C}}_{5}$stilbene). The mixtures ${\mathrm{DB}}_{5}$CN+${\mathrm{C}}_{5}$stilbene (with mole fraction of ${\mathrm{C}}_{5}$stilbene X=0.492) and ${\mathrm{DB}}_{6}$CN+${\mathrm{C}}_{5}$stilbene (X=0.505) both exhibit the phase sequence N--Sm-${\mathit{A}}_{1}$--Sm-A\ifmmode \tilde{}\else \~{}\fi{}--Sm-${\mathit{A}}_{\mathrm{cren}}$--Sm--${\mathit{A}}_{2}$, where N and Sm denote nematic and smectic, respectively. The thermal behaviors of these two samples are in excellent agreement. The second-order N--Sm-${A}_{1}$ transition can be well represented by the XY model. The Sm-${A}_{1}$--Sm-\~A transition is weakly first order with a substantial and anomalous excess heat capacity over a considerable range above as well as below the transition. This thermal effect correlates well with the unusual viscosity behavior observed in ${\mathrm{DB}}_{5}$CN+${\mathrm{C}}_{5}$stilbene. The excess heat capacity associated with the Sm-\~A--Sm-${A}_{\mathrm{cren}--}$Sm-${A}_{2}$ transition sequence is similar to the inverted Landau behavior observed for the Sm-C\ifmmode \tilde{}\else \~{}\fi{}--Sm-${C}_{2}$ transition in other systems, but with the peak truncated due to the presence of the Sm-${A}_{\mathrm{cren}}$ phase.