A high resolution X-ray scattering study of the phases and phase transitions in N-(4-n-butyloxybenzylidene)-4-n-heptylaniline (40.7)

Abstract

We report high resolution X-ray scattering measurements of the smectic phases and phase transitions in the material N-(4-n-butyloxy-benzylidene)-4-n-heptylaniline (40.7) (I → N → Sm A → Sm C → Sm B). For the nematic to smectic A transition we have measured the critical exponents γ = 1.45 ± 0.03, ν ∥ = 0.78 ± 0.02, and ν⊥ = 0.65 ± 0.02. The in-plane positional correlations in the smectic A phase are described by a nearly two dimensional azimuthally-averaged Lorentzian liquid lineshape with a correlation length of 15.8 A. Measurements of the molecular tilt and lattice spacing in the smectic C phase along with previously reported heat capacity and light scattering measurements indicate that the smectic A-smectic C transition is described by a mean-field Landau-Ginsburg theory with an anomalously large sixth order term. The smectic C to smectic B phase transition is first order with a coexistence region of approximately 0.5 °C. Within this region the smectic B phase may coexist with either a smectic A or a smectic C phase. Finally, in the smectic B phase the layers undergo a continuous restacking transition from an ABAB to ABCABC stacking sequence with decreasing temperature between 35 and 31°C.