Critical behaviour at the isotropic-nematic and nematic-smectic A phase transitions of 4-butyloxyphenyl 4′-decyloxybenzoate liquid crystal from refractive index data

We present new high-resolution experimental data on the temperature dependence of the refractive index in the isotropic (I), nematic (N) and smectic A (SmA) phases of the 4-butyloxyphenyl 4′-decyloxybenzoate (10O4) liquid crystal. From an analysis of the data, using both the Vuks and Neugebauer models for the internal electric field, information on the orientational order parameter S(T) was derived. From a fitting procedure consistent with mean-field theory and the first-order character of the N-I transition, we obtain a value of 0.246±0.06 for the exponent β describing the critical behaviour of S(T) at the N-I transition, which is in good agreement with the tricritical value β=0.25. In a temperature range of about 4 K above and below the N-SmA transition temperature, pre-transitional evidence for the coupling between the nematic and smectic order parameters was observed. We show, for the first time, that the temperature derivative of the S(T) curve below and above the N-SmA transition has the same power law behaviour as the specific heat capacity with an effective critical exponent of 0.23±0.01.