Integrated x-ray-scattering intensity measurement of the order parameter at the nematic-to-smectic-Aphase transition

The temperature dependence of the square of the smectic order parameter, ‖Ψ${\mathrm{\Vert }}^2$, was determined from the integrated x-ray scattering intensity. The data are described very well by the form I(t)/I(0)=1∓${\mathrm{A}}^{(\pm )}$‖t${\mathrm{\Vert }}^{\mathrm{x}}$, where t=(T-${\mathrm{T}}_{\mathrm{NA}}$)/${\mathrm{T}}_{\mathrm{NA}}$. The determined values for x do not agree with the theoretically expected value x=1-α, where α is the heat-capacity exponent. This disagreement raises questions concerning the quantitative validity of the Landau–de Gennes free energy.