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

The temperature dependence of the square of the smectic order parameter, Ψ2, was determined from the integrated x-ray scattering intensity. The data are described very well by the form I(t)/I(0)=1A(±)tx, where t=(T-TNA)/TNA. 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.