N14nuclear-quadrupole-resonance study of orientational ordering in the smectic phases of achiral TBBA and chiral TBACA

Abstract

The ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ quadrupole coupling constant and asymmetry parameter $\eta$ have been measured (using a double-resonance technique) as a function of temperature in the smectic $A$, smectic $C$, and smectic $H$ phases of achiral TBBA and chiral TBACA, as well as in the smectic $\mathrm{VI}$ and the solid phase of TBBA. In the low-temperature smectic $H$ and smectic $\mathrm{VI}$ phases of achiral TBBA the results clearly demonstrate the existence of bipolar orientational ordering $〈cos2\varphi 〉\ne 0$ whereas they show the existence of polar orientational order $〈cos\varphi 〉\ne 0$ in the smectic $H$ phase of chiral TBACA. Here $〈cos\varphi 〉\propto {(T_c-T)}^{\beta }$ with $\beta =0.33\mathrm{}\pm 0.02$. In the smectic $C$ phase of both systems the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ nuclear-quadrupole-resonance spectrum is mainly determined by anisotropic fluctuations in the long molecular axis and the orientational order parameters can be only roughly estimated.